Reduced shock breakaway set screw for use with a surgical construct
Contents
1. Aug 21 2014 Sheet 8 of 10 US 2014 0236237 A1 e x Bo 2s 2 Patent Application Publication Aug 21 2014 Sheet 9 of 10 US 2014 0236237 A1 Fig 13A Fig 13B Patent Application Publication Aug 21 2014 Sheet 10 of 10 US 2014 0236237 A1 Fig 14 Stresa I Sirsin Curve US 2014 0236237 A1 REDUCED SHOCK BREAKAWAY SET SCREW FOR USE WITH A SURGICAL CONSTRUCT CROSS REFERENCE TO RELATED APPLICATIONS 0001 This application claims the benefit of U S provi sional patent application Ser No 61 543 405 entitled Implant Structures that Deform at a Designed Torque and Minimize Shock to Patients and Physicians filed Oct 5 2011 and PCT US2012 058696 entitled Reduced Shock Breakaway Set Screw for Use with a Surgical Construct filed Oct 4 2012 both of which are incorporated herein by refer ence in their entirety FIELD OF THE INVENTION 0002 The present invention relates to an improved break away set screw for use with surgical constructs for the spine having an improved geometry which reduces the shock to the patient and the surgeon during the set screw break off proce dure BACKGROUND OF THE INVENTION 0003 Spinal surgery using one or more surgical implants to stabilize manipulate and or repair the spine is well known in theart Onetype of common spinal surgery involves fusing or stabilizing two or more vertebra by application ofa surgical construct to the posterior surfaces ofthe vertebra by me2. 0 00025 9926807 662 0 3 1E 005 4E 005 Fracture Strain Stress Triaxiality 0 3 0 0 2 0 33 0 1 0 8 Aug 21 2014 APPENDIX B continued AISO83 H116 Parameters Used in the Model 0 1 1 Yield Stress psi Plastic Strain 30000 0 0 000000000 31069 9 0 000870102 31948 7 0 001781570 32685 2 0 002707380 33609 7 0 004114250 34571 9 0 005917320 35825 8 0 008891000 37076 5 0 012713700 38087 2 0 016560700 38939 1 0 020423600 39677 6 0 024297900 40330 9 0 028180900 40917 5 0 032070500 41450 6 0 035965500 41939 6 0 039865000 42529 3 0 045015700 43402 1 0 053627800 44434 4 0 065523800 45526 0 0 080413800 46460 3 0 095319700 47536 8 0 115321000 48875 8 0 145076000 50665 4 0 194896000 53299 4 0 294631000 55251 0 0 394434000 56813 8 0 494277000 58123 4 0 594145000 59254 3 0 694031000 60251 6 0 793930000 61145 2 0 893840000 61955 8 0 993759000 What is claimed is 1 A reduced shock breakaway set screw for use with a surgical construct comprising a threaded lower portion and an upper head portion separated by a substantially annular groove having circular and non circular geometry 2 The reduced shock breakaway set screw of claim 1 wherein said substantially annular groove further comprises an upper radius a lower radius and a substantially flattened portion separating said upper radius and said lower radius 3 The reduced shock breakaway set screw of claim 1 wherein said reduced shock breakaway set screw is made of metal 4 Th
3. 0145 4 CREATE SECTION 0146 a from top menu Section Create 0147 b Name the section as desired For this thesis tube sec select Solid select Homogeneous 0148 c Click Continue 0149 d Select the proper material For this thesis A15083 0150 e Check Plane stress strain thickness and leave value as 1 0151 f Click OK 0152 5 ASSIGN SECTION 0153 a from top menu Assign Section 0154 b Select the regions of the part from the viewport that must be associated with the section just created For this thesis there are 3 regions 0155 c Hit Enter on keyboard 0156 d Select the desired section For this thesis tube sec 0157 e Inthe Thickness section of the window leave the Assignment selection as From section 0158 f Click OK 0159 g Hit Esc on keyboard 0160 MESH MODULE from pull down window just above view port Select Module Mesh 0161 6 MESHING THE PART 0162 a from top menu Seed Edges 0163 i HOLD THE SHIFT KEY and select all edges bordering the region of interest including the nearby partition borders For LOR2 groove this should be 7 straight edges and one semi circular edge 0164 ii Hit Enter on keyboard 0165 iii Keep defaults and define Approximate ele ment size under Sizing Controls For this thesi
4. the reduced shock breakaway set screw 10 may be machined out of a solid piece of a titanium alloy stainless steel or cobalt chromium alloy In accordance with at least one aspect of the present invention the reduced shock set screw may undergo any of the conventional or otherwise appropriate surface treatments 0067 The reduced shock breakaway set screw 10 may be adapted to be used in securing a spinal rod or other elongated member within a pedicle screw head connector ring band clamp bone screw cap orother portion ofa surgical construct in such a way as to substantially eliminate translational or rotational movement of the rod with respect to the vertebra or Aug 21 2014 other parts ofthe surgical construct As used herein a surgical construct for use with the spine is a multicomponent device constructed from stainless or titanium based steel consisting of solid grooved or slotted plates or rods may be metal or PEEK that are longitudinally interconnected and anchored to adjacent vertebrae using bolts hooks or screws 0068 The reduced shock breakaway set screw 10 has an outer surface 16 and can be divided into an upper head portion 11 and a lower threaded set screw portion 12 separated by a substantially annular groove 13 Referring now to FIG 3 the reduced shock breakaway set screw 10 may have an internal opening 14 that runs axially down the center of the reduced shock breakaway set screw from the upper head portion 1
5. 3 3 of FIG 2 of a reduced shock breakaway set screw according to one embodiment of the present invention 0052 FIG 4 is a fragmented cross sectional view taken from FIG 3 of the substantially annular groove of a reduced shock breakaway set screw according to one embodiment of the present invention 0053 FIG 5A is a stress strain curve wherein area under the curve is the indicative of a materials toughness 0054 FIG 5B is a stress strain curve showing energy dissipation 0055 FIG 6A is a graph showing crack growth rate as a function of crack size 0056 FIG 6B is an illustration of the competing intrinsic promoting and extrinsic impeding forces in crack propa gation 0057 FIG 7 is a partial frontal view of a reduced shock breakaway set screw according to one embodiment of the present invention having apertures between the substantially annular groove and an internal opening 0058 FIG 8 is a partial frontal view of a reduced shock breakaway set screw according to one embodiment of the present invention having ribs running across the substantially annular groove 0059 FIG 9 is a partial frontal view of a reduced shock breakaway set screw according to one embodiment of the present invention having both apertures between the substan tially annular groove and an internal opening and ribs running across the substantially annular groove 0060 FIGS 10A and 10B shows the axisymmetric cross sections of the tw
6. In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein Aug 21 2014 said threaded lower portion further comprises a recess sized to mate with a tool for removing screws 0023 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein the upper radius is from about Y64 inches to about 1 4 inches and said lower radius is from about Ys4 inches to about 1 4 inches 0024 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein the substantially flattened portion has a length of from about 0 inches to about inches 0025 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein the substantially flattened portion has a length of 0 inches 0026 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein the reduced shock set screw may undergo a surface treatment 0027 Ina second aspect the present invention provides a reduced shock breakaway set screw for use with a surgical construct comprising a threaded lowe
7. LOR2 groove which is useful because more elongation means more plastic dissipation as discussed above The displacement at which the structure completely fails for the LOR2 model was 0 54 of the rotation cycle and the displacement at which the structure completely fails for the L3R8 model was 0 94 of the rotation cycle This is a 74 increase in rotation before failure The researchers consider this entire increase to be due to plastic strain since only 3 09E 7 ofthe rotation cycle is completely elastic for the LOR2 model and 0 0142 is elastic for L3R8 Using the geometric extremes the behavior ofthe structure in relation to the curve profiles is shown in FIGS 13A B It should be noted that complete failure was defined by the researchers to be the moment when the last element was deleted and this point is not the same moment when the ALLPD curve becomes level 0112 Although the recoverable elastic energy is slightly higher for L3R8 than for LOR2 FIG 11B this increase is negligible considering the overall magnitude of the work done as shown by the output graphs of LOR2 and L3R8 in FIGS 13A 13B As discussed above this is the reason why the ratio of recoverable elastic strain to plastic energy dissi pation was used Numerically the ALLSE ALLPD ratio for the smallest groove LOR2 was 0 0274 and the same ratio for the largest groove L3R8 was 0 0174 This is over a 3696 decrease in the ratio showing that the geometric structure in
8. a single piece of titanium alloy and have a hexagonal top portion that mates with a tightening device a lower threaded set screw portion that mates with a threaded bore of a pedicle screw construct to secure a stabilizing rod and an annular v shaped notch separating the two portions 0008 In these prior art systems the surgeon uses an extended counter torque tool that holds the top ofthe pedicle screw and stabilizing rod to try to limit or prevent transmis sion ofthe rotational torque used to tighten the set screw from being transmitted to the construct as a whole or to the vertebra of the patient The shaft of the counter torque tool is hollow and sized to receive the shaft of a break off driver The break off driver is longer than the counter torque tool and slides through the shaft of the counter torque tool to mate with the hexagonal head of the set screw As set forth above the hexagonal heads of these breakaway set screws are designed to shear offthe threaded body ofthe set screw once the proper torque has been achieved The surgeon simply turns the break off tool while keeping the counter torque tool still until the hexagonal head shears off the threaded body of the set screw at the pre determined torque This set screw break off SSBO procedure is repeated for all of the set screws in the construct The SSBO procedure is performed 6 times for the average spinal implant construct and many more times for larger constructs in pat
9. all of the elastic strain energy as kinetic energy in the form of vibrations i e shock 0075 Therefore it is believed that as the elastic strain energy decreases relative to plastic dissipation energy the shock will also decrease As would be clear to one of ordinary skill in the art from simple geometry as the upper head portion of the breakaway set screw undergoes increased rota tion before failure then more elements must be experiencing deformation assuming that the plastic strain limit of each element is identical As more elements experience deforma tion then more energy is dissipated plastically 0076 The improved groove geometries of the present invention act to increase the proportion of the energy that is released as heat from the plastic deformation of the material in the substantially annular groove thereby reducing the Aug 21 2014 amount of kinetic energy released from the elastic deforma tion of the metal in the substantially annular groove and accordingly the shock transmitted to the patient and surgeon 0077 Given the same pre set shearing torque for the same material with identical bulk and surface properties the geo metric changes in the substantially annular groove 13 of the set screw 10 can influence the energy release behavior of a crack such that the maximum shock released upon breaking shearing is reduced It has been found based on the theory of Linear Elastic Fracture Mechanics LEFM that wher
10. and lower radius may be from Va inches to about Ya inches and the substantially flattened portion 28 may have a length I of from 0 inches to about Ys inches 0073 In an alternative embodiment of the present inven tion the substantially annular groove is a single arc having a radius that is longer than the groove wall thickness 25 In this embodiment the length I of the substantially flattened portion 28 may be 0 0074 The external work performed on the device by applying torque to the reduced shock breakaway set screw 10 is primarily converted into either plastic dissipation energy or recoverable elastic strain energy that manifests itself as shock FIG 14 is a generic stress strain curve showing areas of elastic and plastic deformation Plastic deformation for most materials is caused when the structure undergoes so much stress that the bonds between individual atoms break and reform to an adjacent atom Plastic deformation happens in the direction that these atoms move Essentially this con cept relies on the material s toughness or energy absorption potential before failure Visually one can see a material s toughness by observing the area under the engineering stress strain curve See FIG 5A When a metal is elastically deformed there is no such breaking and reforming of bonds within the metal and if the material breaks shears under only elastic deformation the material will snap back to its original shape releasing essentially
11. axisymmetric with twist model was created is set forth below in Appendix A 0108 By lengthening the flattened bottom section of the groove in a tube section and reducing the stress concentra tions from the fillet radii in a 2D axisymmetric model the percent of work that went into plastic energy dissipation was increased by 36 In addition the radians that the structure rotated before failure was shown to increase by 74 along with the area experiencing plastic yielding All these results were consistent showing that the plasticity of a structure is dependent on the specific geometry surrounding the failure region 0109 In all cases the maximum torque required for the structure to experience torsional failure was almost identical Aug 21 2014 This result indicates that the failure strength of a tube under torsion is primarily a function of the failure region s mean thickness which remained constant and therefore was not affected by surrounding geometries 0110 The general trend of geometry to plastic dissipation can best be seen by the pivot table graph in FIG 11A It also shows the two geometries that have the lowest and highest values of plastic dissipation As expected due to what is known about stress risers the smaller groove with the small est radius has the lowest plastic dissipation and the widest groove with the largest radius has the greatest plastic dissi pation The relatively sharp groove in LOR2 has only a s
12. substantially flattened portion separating the upper radius and the lower radius 0043 In another aspect the present invention may include a method of reducing shock to the patient and physician created when breaking off the upper portion of a breakaway set screw for use with a surgical construct having the method steps of 1 placing the threaded lower end of a reduced shock breakaway set screw according to the present invention designed to break at a pre defined torque into a threaded bore of a surgical construct sized to receive it and 11 tightening the reduced shock breakaway set screw to the pre defined torque witha torque applying tool until the upper head portion ofthe reduced shock breakaway set screw shears away from the threaded lower set screw portion of the reduced shock breakaway set screw 0044 In another aspect the present invention may include a method of reducing shock created during set screw break off of a breakaway set screw for use with a surgical construct having a threaded lower portion and an upper head portion separated by a substantially annular groove comprising increasing the ratio of energy released from the plastic defor mation of the material in the substantially annular groove to the amount of energy released as kinetic energy from the Aug 21 2014 elastic deformation of the material in the substantially annu lar groove during set screw break off 0045 In another aspect the present invention m
13. which corresponds to the actual measured and designed for break off torque This yield strength is the quasi static yield strength as higher strain rates result in higher yield strength The damage model utilized was ductile damage i e failure strain as a function of triaxi ality since the validity of material models can be judged based on their ability to correctly determine the failure strain throughout all loading conditions as identified by triaxiality values Therefore since the damage model used was defined by maximum equivalent strain at a particular triaxiality equivalent plastic strain PEEQ is used to visualize plastic deformation Additional information may be found in Abaqus CAE User s Manual 12 9 3 Defining Damage the disclosure of which is hereby incorporated by reference in its entirety 0106 The method used generally involved creating a 2D axisymmetric sketch of the part creating a material with damage conditions assigning the material to the part creat ing a mesh on the part using CGAX3 element that allows twist assigning node regions to which boundary rotational conditions were applied FIG 10 and setting the conver gence behavior The element type utilized CGAX3 was par ticularly important to the entire model and simulations because it provided the 2D model an additional degree of freedom Traditional 2D axisymmetric analysis only allows in plane movement Per Abaqus Analysis User s Manual 25 1 6 Axi
14. 1 4V was determined to be 950 MPa Therefore c yield is about 548 MPa TABLE 1 ID and OD measurements of prior art set screw Validation from From Digital Image Analysis a micrometer 0 005235 m 0 0061111 m 0 0053 m 0 0061 m Inner diameter ID Outer diameter OD 0095 In thin wall torsion the stress in the wall of a tube is assumed to be independent of the radius Therefore the stan dard torsional stress equation is combined with the circular tube polar moment of inertia Tar P zs 0D ID E 32 J 0096 Break off occurs when there is full plasticity at v yield since the stress is considered constant throughout The mean radius is used T yield 7 a OD ID Tyreak off 322 mem 0097 This break off torque is 11 24 N m 0098 Fully plastic torque ofa hollow shaft is calculated by subtracting what the torque carrying capacity would have been of a rod of the same material with a diameter equal to the ID from the torque carry capacity of a solid bar with the diameter equal to the OD Ttube 7solid 7hollow 0099 The torque of each section Tsolid and Thollow is calculated by m Pun Jry D3 0100 Distortion energy is again used for vy The value of Tsolid is 32 56 N m and Thollow is 21 36 N m Thus the FPT break off torque is Ttube 32 56 21 36 11 2 N m Aug 21 2014 0101 Finally a torque wrench valid from 3 6 N m to 29 N m validated the maximum torque reach
15. 1 to or into lower set screw portion 12 In one embodiment of the present invention the internal opening 14 is machined into the set screw blank as a blind bore running through what will be the upper head portion into what will become the lower threaded set screw portion of the reduced shock breakaway set screw 10 In another embodiment the blind bore stops at the top of what will become the lower threaded set screw portion of the reduced shock breakaway set screw 10 The internal opening 14 is further defined by inner surface 15 The distance between inner surface 15 and outer surface 16 defines a wall thickness 17 0069 Upper head portion 11 is configured to mate with a torque applying tool not shown Upper head portion 11 may be hexagonal in cross section for some or all of its length and sized to fit within and mate with a drive socket of a socket wrench or torque wrench a manual torqueing instrument or other torque generating tool having a hollow end portion that is hexagonal in cross section and intimately fits over the upper head portion 11 As will be appreciated by those of skill in the art upper head portion 11 may have any cross sectional shape so long as it mates with the torque applying tool in such a way as to permit the torque applying tool to apply an amount of torque sufficient to cause the upper head portion 11 to shear offofthe threaded lower set screw portion 12 The upper head portion 11 may also have a solid upper portio
16. 115 2D axi symmetric CGAX3 element gives additional twist angle degree of freedom 0116 Disadvantage of axisymmetric is that one cannot use explicit methods 0117 Units of values must be kept consistent by the user PART MODULE from pull down window just above view port Select Module Part 0118 1 CREATING THE PART from top menu Part Create 0119 a name the part tube on the text input box select Axisymmetric select Deformable select Shell check Include twist creates a generalized axisymmetric model that allows rotational movements and force calculations specify Approximate size to at least twice as big as the greatest cross section length or the radius whichever is greatest For this thesis 0 012 click Continue 0120 b Draw desired cross section with a defined radius For LOR2 groove in this thesis 0121 1 0 00265 radius length from center line to left most straight edge of tube wall 0 0058 tube height continuous vertical dimensioned length on the left side two non dimensioned vertical lengths on the right where one extends from the base and one extends from the top but neither touch one another 0 0008 regular wall thickness on the upper and lower horizontal boundaries of the tube cross section Maximum moment for shear failure 6 307E 04 6 293E 04 Approximately the same 0122 ii Define the groove by 0123 1 D
17. F THE INVENTION 0011 In general the present invention relates to an improved break off set screw for use with surgical constructs for the spine having an improved geometry which reduces the shock to the patient and the surgeon during the set screw break off procedure 0012 In a first aspect the present invention provides a reduced shock breakaway set screw for use with a surgical construct comprising a threaded lower portion and an upper head portion separated by a substantially annular groove hav ing circular and non circular geometry 0013 In another embodiment the substantially annular groove of the reduced shock breakaway set screw of the first aspect of the present invention further comprises an upper radius a lower radius and a substantially flattened portion separating said upper radius and said lower radius 0014 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein said reduced shock breakaway set screw is made of metal 0015 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein said reduced shock breakaway set screw is made from a metal selected from the group consisting of titanium and stainless steel 0016 In another embodiment the reduced shock break away set screw of the first aspect of the pre
18. US 20140236237A1 as United States a2 Patent Application Publication o Pub No US 2014 0236237 Al Mahajan et al 43 Pub Date Aug 21 2014 54 71 72 73 21 22 86 60 REDUCED SHOCK BREAKAWAY SET SCREW FOR USE WITH A SURGICAL CONSTRUCT Applicant The University of Akron Akron OH US Inventors Ajay Mahajan North Canton OH US Jason King North Canton OH US Assignee The University of Akron Akron OH US Appl No 14 349 708 PCT Filed Oct 4 2012 PCT No PCT US12 58696 371 OA 2 4 Date Apr 4 2014 Related U S Application Data Provisional application No 61 543 405 filed on Oct 5 2011 Publication Classification 51 Int Cl A61B 17 70 2006 01 A61B 17 86 2006 01 52 U S CL CRE A61B 17 7032 2013 01 A61B 17 863 2013 01 A61B 17 866 2013 01 USPE os nonnen 606 270 57 ABSTRACT A reduced shock breakaway set screw for use with medical implants and constructs for the spine having improved geom etry of the groove area between the upper head portion and lower threaded portion of the set screw The groove area of the set screw has circular and noncircular portions which in at least one embodiment comprise upper and lower radii sepa rated by a flattened groove bottom Other embodiments have apertures running between the groove area and an internal cylindrical bore and or ridges or ribs running across the groove area The im
19. a being sheared and therefore result in an identical release of energy compared to the prior art systems However this energy release is over a longer period and may be continuous or occur in observable spikes at inter mittent times within the time period of shearing failure Regardless ofthe shape ofthe profile curve the area under the curve should remain relatively constant if the cross section remains the same since the same energy will be required to shear it EXAMPLES 0090 The following examples are offered to more fully illustrate the invention but are not to be construed as limiting the scope thereof Further while some of examples may include conclusions about the way the invention may func tion the inventor do not intend to be bound by those conclu sions but put them forth only as possible explanations More over unless noted by use of past tense presentation of an example does not imply that an experiment or procedure was or was not conducted or that results were or were not actu ally obtained Efforts have been made to ensure accuracy with respect to numbers used e g amounts temperature but some experimental errors and deviations may be present Example 1 Experimental and Evaluation Validation Of Manufacturer Specified Set Screw Break Off SSBO Torque for Prior Art Set Screw 0091 As set forth above the proper torque for the set screws is critically important in pedicle screw systems Not enough
20. all thickness 25 These ridges or ribs 29 may be evenly spaced along the substantially annular groove 13 In one embodiment the ridges or ribs 29 may be contiguous with the substantially flattened portion 28 some orall ofthe upper and lower radii 26 27 In one embodi ment there are both a plurality of apertures 30 and a plurality of ribs 29 alternately arranged and evenly spaced along the substantially annular groove 13 as in FIG 9 In one embodi ment ridges or ribs 29 are variations in the groove wall thickness 25 0087 The purpose of the apertures 30 is to provide a geometry that initiates crack propagation at multiple points so that there are no an extremely long cracks since the crack speed is influenced by crack size FIG 6B The multiple holes are intended to increase the amount of energy dissipated through plastic deformation since each crack initiation site must undergo a certain amount of intrinsic toughening Therefore in order for the part to fail complete shear cannot rely on the brittle kinetic energy of one crack spreading cir cumferentially At the tip of crack propagation plastic defor mation dissipates energy and this design causes plastic defor mation around the entire circumference since the plastic zone will always be leading the propagating crack It has been found that tubes with two holes have more ductility before failure than tubes with one hole The holes are a geometric stress riser that initiates crack p
21. anced materials International Journal of Solids and Structures 37 311 329 the disclosure of which is incorporated herein by reference Intrinsic forces stimulate crack growth and are dependent on the material properties while the extrinsic forces hinder propagation and are primarily a function of crack size geom etry Ductile materials such as metals predominantly toughen intrinsically whereas brittle materials toughen through extrinsic forces Material and process variabilities such as strain rate strain hardening surface irregularities surface processing e g Shot peening electro polishing and grain structure all affect a part s macroscopic behavior through their influence on the microscopic intrinsic and extrinsic properties 0083 As a general rule however it is clear that 1 the crack propagation rate increases as the crack grows 2 crack propagation will become brittle when the growth rate is too fast because R is smaller than the energy release rate result ing in an abundance of kinetic energy i e shock and 3 metals primarily toughen due to intrinsic crack initiation forces 0084 The improved groove geometry of the present invention has been found to slow the rate of crack growth thereby dissipating more of the stored energy for plastic deformation and eventually less as shock By broadening the US 2014 0236237 A1 substantially annular groove 13 and reducing or eliminating the sharp corners or acute angles in
22. ans of pedicle screws 0004 There is a large market for pedicle screws and there are numerous designs and manufacturers ofthis type of fusion device The spine market in the U S is 6 8 billion and 34 of this market over 2 billion involves pedicle screw sys tems These systems are usually placed bilaterally and the system on each side is typically composed of a minimum of one stabilizing rod a pedicle screw for each vertebra and a set screw at each pedicle screw to secure the stabilizing rod Sometimes the securing feature at the head of the pedicle screw is a separate connector Each company has a slightly different design of the components but generally all pedicle screw constructs require a set screw to be tightened to a specific torque to ensure a proper connection between the pedicle screw and stabilizing rod and thus a rigid fixation It has been found that if the torque applied to the set screw is insufficient the construct will loose integrity and the stabi lizing rod will not be rigidly fixed as required and could slide or rotate Additionally an application of too much torque it has been found can result in a fracture of the vertebra or a loosening of the bone implant connection Too much torque can also severely deform the screw threads causing them to loose strength and to slip when the patient later puts a load on the spine or surgical construct 0005 Initially a surgeon using these types of set screws would si
23. attened portion has a length of from about 0 inches to about inches 0038 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein the upper radius is about Y64 inches to about 4 inches and said a lower radius is from about Ys4 inches to about 1 4 inches 0039 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein the sub stantially flattened portion has a length of 0 inches 0040 In another embodiment the substantially annular groove of the third aspect of the present invention includes any ofthe embodiments described above wherein the upper or lower radii is greater than the groove wall thickness 0041 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein the sub stantially annular groove may undergo a surface treatment 0042 The present invention also includes a method of reducing shock to the patient and physician created when breaking off the upper portion of a breakaway set screw for use with a surgical construct at a defined torque using a reduced shock breakaway set screw having a threaded lower portion and an upper head portion separated by a substantially annular groove having an upper radius a lower radius and a
24. away from the start of the groove on both sides such that the region of interest does not extend into either box For this thesis the plastic deformation will not extend into either box if both lengths are 0 0002 0135 e Hit Esc on keyboard 0136 f Click Done just below the view port PROPERTY MODULE from pull down window just above view port Select Module Property 0137 3 DEFINING MATERIAL from top menu Material Create see APPENDIX B herein for the AL5083 H116 values 0138 a Name the material as desired For this thesis A15083 0139 b General Density 0140 c Mechanical Elasticity Elastic E Poisson s ratio 0141 d Mechanical Plasticity Plastic tabular input of Yield Stress and Plastic Strain US 2014 0236237 A1 0142 e Mechanical Damage for Ductile Metals Duc tile Damage 0143 i Tabular input of Fracture Strain and Stress Triaxiality zero for strain rate 0144 ii Suboptions Damage Evolution Type Dis placement Softening Linear Degradation Maximum Displacement at Failure should be 25 or less of your element seeded length for this thesis 0 25 seeded length of 4E 005 1E 005 This value is the amount of elongation an edge will have after the failure criteria is met and is similar to defin ing the amount of strain occurring in the necking region of an engineering stress strain curve
25. ay include a method of reducing shock to the patient and physician created when breaking off the upper portion of a breakaway set screw for use with a surgical construct by increasing rotational distance traveled by the torque applying tool from the time the pre defined torque is reached to the time that the upper head portion of the reduced shock breakaway set screw shears completely away from the threaded lower set screw portion of the reduced shock breakaway set screw 0046 In another aspect present invention may include a method of increasing the amount of energy released from the plastic deformation of a ductile material in a substantially annular groove of a breakaway set screw during set screw break off using the reduced shock breakaway set screw described herein BRIEF DESCRIPTION OF THE DRAWINGS 0047 For a full understanding of the invention reference should be made to the following detailed description and the accompanying drawings wherein 0048 FIG LA isa printout of an oscilloscope output ofthe initial accelerometer signal recorded during SSBO of a com mercially available prior art breakaway set screw 0049 FIG 1B is a graph of the test data of FIG 1A wherein the data has been converted to show acceleration g forces per unit time 0050 FIG 2 is a frontal view of a reduced shock break away set screw according to one embodiment of the present invention 0051 FIG 3 is a cross sectional view taken along lines
26. e reduced shock breakaway set screw of claim 3 wherein said reduced shock breakaway set screw is made from a metal selected from the group consisting of titanium and stainless steel 5 The reduced shock breakaway set screw of claim 4 wherein the metal is titanium 6 The reduced shock breakaway set screw of claim 1 further comprising an internal opening extending from said upper head portion into said threaded lower portion 7 The reduced shock breakaway set screw of claim 1 wherein said internal opening extending from said upper head portion to the top of said threaded lower portion 8 The reduced shock breakaway set screw of claim 2 further comprising an internal opening extending from said upper head portion into said threaded lower portion 9 The reduced shock breakaway set screw of claim 2 wherein said internal opening extending from said upper head portion to the top of said threaded lower portion US 2014 0236237 A1 10 The reduced shock breakaway set screw of claim 6 further comprising a plurality of apertures extending between said internal opening and said substantially annular groove 11 The reduced shock breakaway set screw of claim 7 further comprising a plurality of apertures extending between said internal opening and said substantially annular groove 12 The reduced shock breakaway set screw of claim 8 further comprising a plurality of apertures extending between said internal opening and said substantially annular g
27. e the stress at the moving crack tip is considered linear elastic with two dimensional stress the crack undergoes a rapid brittle propagation through the structure s thickness when it exceeds a critical stress intensity At this critical stress intensity the energy release rate G energy per unit length along the crack tip ofthe separating material potential energy release of the elastic strain is greater than the crack resistance The excess ofenergy becomes kinetic energy which controls the crack tip speed through the material with the total kinetic energy equal to Er G R da 0078 Where 0079 E kinetic energy 0080 G energy release rate 0081 R crack resistance force Assuming that 1 the stress during crack propagation is constant 2 G is independent of crack speed and 3 R is constant 0082 Crack resistance and propagation forces are actually a complex combination of a variety of forces depending on things such as environment material and crack structure geometry The preceding equation is only a rough guide since for example crack resistance does not remain constant because as crack growth rate is controlled by crack size FIG 6A along with other factors The primary groupings of these forces that influence crack growth are intrinsic and extrinsic as shown in FIG 6B See Ritchie R O Gilbert C J amp McNaney J M 2000 Mechanics and mechanisms of fatigue damage and crack growth in adv
28. ed before break off as approximately 11 3 N m comparison of all the failure torques is in Table 2 TABLE 2 Comparison of Torque Values Method Torque Value N m Design break off torque as supplied by 11 00 N m Medtronic Thin Wall Torsion TWT 11 24 N m Fully Plastic Torque FPT 11 2 N m Torque wrench 11 3 N m Example 2 Axisymetric with Twist Computer Modeling Experiments 0102 Axisymetric with twist computer modeling experi ments have been conducted using commercially available modeling software manufactured by Dassault Syst mes Sim ulia Corp and sold under the tradename Abaqus version 6 10 2 These experiments confirmed that geometric manipulations of the groove significantly affects the plastic behavior of the structure while still allowing the maximum torque to be separately controlled 0103 An explanation of the different components of energy that were tracked forthe whole model may be found in the Energy Balance section 1 5 5 of the Abaqus Theory Manual the disclosure of which is incorporated herein by reference These were referred to by the software as field variables with ALLPD referring to the plastically dissipated energy and ALLSE referring to the recoverable elastic strain energy ALLWK refers to all of the external energy added to the model as it is turned Another critical variable was rota tional strain before failure The rotation of each test was constant and took exactly one time incr
29. ement to reach the final rotation location specified at a constant velocity 0104 The law of conservation of energy states that energy can neither be created nor destroyed In addition since basic material laws state that plastic strain is irreversible deforma tion of a material then the amount of plastically dissipated energy accumulated can never be decreased However since the Abaqus model being utilized deletes elements when they have surpassed the strain limit defined by the material the plastically dissipated energy contained in each deleted ele ment is also deleted Therefore since every shear model run showed that the geometry sheared all the way through the point at which fracture completed 1s also the point at which the last element was deleted and the last decrease in ALLPD was observed 0105 Modeling decisions were made based on recom mendations given in Classical Metal Plasticity of the Abaqus 6 10 Analysis User s Manual of the disclosure of which is incorporated herein by reference The progressive damage and failure models in Abaqus are able to model both quasi static and dynamic situations It was determined that the manually applied strain rates in the set screw break off are not high enough to significantly affect the failure stress thus quasi static modeling was used This assumption is due to the Ti 6Al 4V yield strength used in the initial calculations de US 2014 0236237 A1 scribed above in Example 1
30. erally directed to a reduced shock breakaway set screw for use with medical implants having improved geometry of the groove area between the upper and lower portions of the set screw The geometry serves to slow down the fracturing process during shearing thereby increasing the proportion of energy dissi pated as heat from plastic deformation of the material to the amount of energy released as kinetic energy from elastic deformation The amount of energy released as shock to the patient or the surgeon is determined by the amount of energy released as kinetic energy of vibration as material elastically deformed and then snaps back to its previous condition 0066 Referring now to FIG 2 a reduced shock break away set screw is shown generally indicated by the numeral 10 The reduced shock breakaway set screw 10 can be made of any ductile metal or other material that may be safely implanted in the human body and will not deform at or about the torque required for shearing In one or more embodi ments the metal may be selected from titanium alloys stain less steel and cobalt chromium alloys In one embodiment the reduced shock breakaway set screw 10 is made of a commer cially available Ti 6A1 4V Titanium alloy In one embodi ment the set screw 10 is made of a commercially available Ti 6A 4V ELI Titanium alloy In one embodiment the reduced shock breakaway set screw 10 is made of a commercially available 316L stainless steel In one embodiment
31. fluences the plastic behavior of a material A numerical summary of results for LOR1 and L3R8 is found in Table 3 US 2014 0236237 A1 TABLE 3 Aug 21 2014 Comparison of the two geometric extremes LOR2 and L3R8 Maximum Rotation Maximum Plastic Total before Elastic dissipation work complete energy energy ALLSE done shear ALLSE ALLPD ALLPD ALLWK failure LOR2 2 442E 06 8 902E 05 0274 7 430E 05 9 280 L3R8 2 796E 06 16 073E 05 0174 14 352E 05 16 169 36 74 Decrease Increase 0113 In summary geometric changes to the groove pro file around the outer circumference of the tube caused plas ticity to increase as indicated by a decrease in the ratio of elastic energy to plastic dissipation energy and an increase in radians revolved before complete shear failure Through all geometries the tube was a constant material A1 5083 H116 and the maximum moment before failure remained approxi mately constant 0114 Itis thus evident that the reduced shock breakaway set screw constructed as described herein substantially improves the art Only particular embodiment s have been presented and described in detail and the invention should not be limited by the drawings or the description provided For an appreciation of the true scope and breadth of the invention reference should be made only to the claims that follow APPENDIX A Abaqus Version 6 10 2 Steps Used to Build the 2D Axisymmetric Model with Twist Notes 0
32. for Nlgeom so large displacements can be considered 0201 e Select the Incrementation tab 0202 i Leave default Type as Automatic 0203 ii Increase maximum number of increments so this does not limit the calculations For this thesis 10 000 0204 iii Leave everything else as default to start with If later runs produce errors then Increment size values may need adjusted For this thesis Ini tial was adjusted to 0 001 0205 iv Click OK US 2014 0236237 A1 11 0206 10 MODIFY OUTPUT VARIABLES TRACKED 0207 a from top menu Output Field Output Requests Edit F Output 1 0208 b Leave already selected values alone 0209 c Expand Forces Reactions select RM Reac tion moments 0210 d Expand Failure Fracture select DMICRT Damage initiation criteria 0211 e Expand State Field User Time select STA TUS Status some failure and plasticity models VUMAT LOAD MODULE from pull down window just above view port Select Module Load 0212 11 CREATE FIXED BOUNDARY CONDITIONS 0213 a from top menu BC Create 0214 b Name as desired change Step to Initial 0215 c Category is Mechanical select Displace ment Rotation 0216 d Click Continue 0217 e Click Sets at the bottom right of the view port 0218 f Select
33. ients with severe deformities such as scoliosis 0009 Unfortunately each SSBO imparts an immense if short lived shock to both the patient and the surgeon due to the energy released during the catastrophic failure of the metal at the V shaped notch when the hexagonal head sepa rates from the lower threaded set screw portion Bench top studies of a prior set screw using accelerometers at various points on and around the pedicle screw have recorded a shock of from about 200 g to about 800 g depending upon a variety of human factors including how the tool was being held by the surgeon FIG 1A B This shock creates significant prob lems for both the patient and the surgeon It can lead to the pedicle screw breaking through the side of the vertebra or fracturing the vertebra The shock can also reduce the pull out strength of the pedicle screw in the patient thus increasing the chance of a later revision surgery being required These risks are particularly high for patients suffering with osteoporosis Further the repeated shock may also cause premature wear and or injury to the surgeon s hands and significantly increases the chance that the tools could slip in the surgeon s hands causing pain or injury to the patient US 2014 0236237 A1 0010 Accordingly there is a need in the art for a breakoff set screw for use within a spinal surgery construct wherein the shock to the patient and physician from the SSBO is reduced SUMMARY O
34. mall area of influence while the widest groove of L3R8 spans and therefore affects much more material Since more material is influenced by the stress riders of L3R8 there will be more elements experiencing plastic deformation 0111 Despite the change in stress concentrations the maximum moment required to shear each cross section expe rienced negligible variation FIG 12A Therefore since the minimum cross section did not change this moment remains primarily a function ofthe thinnest cross section ofthe groove as predicted by the equations above in Example 1 However the increased amount of displacement over which the moment must be applied causes the external work done on the struc ture to increase since work is force times displacement FIG 12A illustrates the relative shapes of the moment curves for LOR2 and L3R8 Additionally whereas FIG 11A shows the relative maximums of the plastic dissipation ALLPD FIG 12B shows the curve profile for the trials for LOR2 and L3R8 The decreasing plastic dissipation energy ALLPD term in FIG 11A is believed to be an inaccurate artifact due to element deletion When the elements were deleted after they had reached the complete failure criteria defined by the mate rial model the energy terms associated with those elements are also deleted This also shows that the deformation occurred at a slower rate and over a longer time when com paring the wider L3R8 groove to the narrow
35. mply tighten them by hand until the surgeon judged that the proper tightness had been achieved The problem with this approach was that there was no objective way for the surgeon to determine whether the set screw had been tight enedto the required torque and the surgeon could easily apply toolittle ortoo much torque And ifthere were a problem with the construct either during surgery or later it was impossible for the surgeon to prove that the proper amount of torque had been applied Aug 21 2014 0006 To address these issues a variety of systems were developed that utilized torque wrenches of various designs These systems either required the surgeon to read the torque off the instrument during surgery or provided an audible sound and rotational slip when the proper torque had been reached One problem with these prior art systems was the difficulty involved in reading the torque measurements or hearing and identifying the sound during surgery In addition the torque wrenches used in these systems could loose their precision with use and fail to undergo rotational slip at the target torque 0007 In another prior art system the problems of the torque wrench based systems were avoided by means of breakaway set screws having a head designed to shear off the threaded body of the set screw once the proper torque has been achieved While there are a variety of configurations known in the art breakaway set screws are ordinarily made from
36. n with a recess shaped to receive within it the end portion of a torque apply ing tool or a drive bit so long as the arrangement permits the torque applying tool to apply the necessary amount of torque to cause the upper head portion 11 to shear off of the threaded lower set screw portion 12 0070 Threaded lower set screw portion 12 has threads 18 and a set screw end 19 The inner surface 15 at the threaded lower set screw portion 12 may contain a set of reverse threads not shown to facilitate removal of the set screw with an easy out tool or other screw removal tool after it has been broken off from the upper head portion 11 Alternatively removal may be facilitated by providing a shaped recess in the top of the lower threaded set screw portion 12 sized to mate with any conventional drive bit or drive end used with a torqueing instrument to impart counter rotations for removal of the set screw portion 12 The threaded lower set screw portion 12 is sized to fit in a threaded bore located in a pedicle screw head connector ring band clamp bone screw cap or other similar portion of a surgical construct for use with the spine to anchor a rod or other elongated member to a pedicle screw 0071 When reduced shock breakaway set screw 10 is tightened as described above the set screw end 19 comes into engagement with the rod or other elongated member holding it in place The set screw end 19 can be any shape or configu ration that can sec
37. o most dissimilar geometries used for the axisymetric with twist computer modeling trials FIG 10A shows example no LOR2 LOR2 which has a simple US 2014 0236237 A1 groove and 0 0002 inch fillet radius while FIG 10B shows example no L3R8 L3R8 which has a 0 0003 inch flat at the bottom of the groove with a 0 0008 inch fillet radius In both the lower edge is fixed while a moment about the central axis is applied to the top edge 0061 FIGS 11A C reports pivot graph results ofthe para metric Abaqus model testing showing from top to bottom Abaqus variables ALLPD 11A ALLSE 11B ALLWK 11C 0062 FIGS 12A and 12B provide graphs comparing the output of axisymetric with twist computer modeling trials showing The maximum moment remains relatively con stant while increasing in duration and B The plastic dissi pation of energy and total displacement strain increases significantly 0063 FIGS 13A and 13B provide graphical results of the energy and moment values for the plastic dissipation shown in FIGS 12A and 12B respectively The thick line shows the point where first yield occurs the circle at the peak shows where the first element is deleted and the second circle shows where the final element is deleted 0064 FIG 14 is a generic stress strain curve showing the elastic and plastic deformation regions DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 0065 The present invention is gen
38. o the Y X plane 0184 i from top menu View Toolbars Views must be checked 0185 ii Look for the icon in the upper toolbar that has an up ward and a right ward facing arrow labeled Y and X respectively 0186 iii Click this icon 0187 b Create the boundary set 0188 i from top menu Tools Set Create 0189 ii Name it BOUNDARY select Node click Continue 0190 iii Click amp drag to box select desired nodes For this thesis only the nodes located on the bottom edge of the tube HOLD THE SHIFT KEY and select addition nodes if desired 0191 iv Hit Enter on keyboard 0192 c Create the loading set 0193 i from top menu Tools Set Create 0194 ii Name it LOAD select Node click Continue 0195 iii Click amp drag to box select desired nodes For this thesis only the nodes located on the top edge of the tube HOLD THE SHIFT KEY and select addition nodes if desired 0196 iv Hit Enter on keyboard STEP MODULE from pull down window just above view port Select Module Step 9 CREATE LOADING STEP 0197 a from top menu Step Create 0198 b Leave name as Step 1 leave defaults i e General amp Static General 0199 c Click Continue 0200 d On the initial Basic tab select On
39. proved geometry serves to slow down the fracturing process during shearing thereby increasing the pro portion of energy dissipated as heat from plastic deformation of the material to the amount of energy released as kinetic energy from elastic deformation thus reducing shock to the patient and physician without changing the preset break off torque for the set screw Patent Application Publication Aug 21 2014 Sheet 1 of 10 US 2014 0236237 A1 IB Pri s El 2 5 ce E 2 re E Tangential Acceleration Patent Application Publication Aug 21 2014 Sheet 2 of 10 US 2014 0236237 A1 Patent Application Publication Aug 21 2014 Sheet 3 of 10 US 2014 0236237 A1 Resilience Patent Application Publication Aug 21 2014 Sheet 4 of 10 US 2014 0236237 A1 Fig 6A tees i ceavage Fracture Minden n 3 microvoid ii m ioomlescencej behind crack tip ahead of crack tip Patent Application Publication Aug 21 2014 Sheet 5 of 10 US 2014 0236237 A1 Patent Application Publication Aug 21 2014 Sheet 6 of 10 US 2014 0236237 A1 FIG 10A FIG 10B US 2014 0236237 A1 Aug 21 2014 Sheet 7 of 10 Patent Application Publication Trend ion ipat LS s ic Diss Plast ig H r i Fig HB v Trend ic Energ st Ela rable Recove end amp X des 5 E ze ee Ri zt x p Patent Application Publication
40. r portion and an upper head portion separated by a substantially annular groove wherein said substantially annular groove is an arc having a radius greater than the groove wall thickness 0028 In a third aspect the present invention provides a substantially annular groove for use with a breakaway set screw having improved geometry wherein the amount of energy released from the plastic deformation of the material in the substantially annular groove during set screw break off is increased 0029 In another embodiment the substantially annular groove of the third aspect of the present invention further comprises an upper radius a lower radius and a substantially flattened portion separating the upper radius and said lower radius 0030 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein the upper radius is from about Y 4 inches to about 4 inches and the lower radius is from about V64 inches to about 4 inches 0031 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein the sub stantially flattened portion has a length of from about 0 inches to about inches 0032 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein said sub stan
41. raw a short straight edge extending from the ends of the nearby right vertical lines toward the left side constrain these lines as hori zontal 0124 2 Draw a semi circle defined by two points between the ends of the two new straight lines defining the semi circle radius LOR2 groove is 0 0002 create a tangent constraint between the semi circle and the two new horizontal lines 0125 3 Draw an Isolated Point midway on the circumference of the semi circle location marker will indicate when this point is being selected by turning from a cross to a filled in circle 0126 4 Define the length between this new point and the left edge For this thesis 0 0004 0127 iii Confirm the sketch created is a closed struc ture otherwise it will be invalid 0128 c Hit Esc on keyboard 0129 d Click Done just below the view port 0130 2 PARTITIONING THE PART from top menu Tools Partition select Face select Sketch 0131 a from top menu Add Line Rect angle 0132 b Click the left top corner of the tube wall to start the first box and click somewhere just above the groove on the right side of the tube wall 0133 c Click the left bottom corner ofthe tube wall to start the second box and click somewhere just below the groove on the right side of the tube wall 0134 d from top menu Add Dimension define the length slightly
42. roove 13 The reduced shock breakaway set screw of claim 9 further comprising a plurality of apertures extending between said internal opening and said substantially annular groove 14 The reduced shock breakaway set screw of claim 1 further comprising a plurality of ridges running across said substantially annular groove 15 The reduced shock breakaway set screw of claim 2 further comprising a plurality of ridges running across said substantially annular groove 16 The reduced shock breakaway set screw of claim 6 further comprising a plurality of ridges running across said substantially annular groove 17 The reduced shock breakaway set screw of claim 7 further comprising a plurality of ridges running across said substantially annular groove 18 The reduced shock breakaway set screw of claim 8 further comprising a plurality of ridges running across said substantially annular groove Aug 21 2014 19 The reduced shock breakaway set screw of claim 9 further comprising a plurality of ridges running across said substantially annular groove 20 The reduced shock breakaway set screw of claim 2 further comprising a plurality of ribs running between said upper radius and said lower radius 21 The reduced shock breakaway set screw of claim 7 wherein said threaded lower portion further comprises a recess sized to mate with a tool for removing screws 22 The reduced shock breakaway set screw of claim 9 wherein said threaded lower por
43. ropagation At the intrinsic tip of slow crack propagation plastic deformation dissipates energy The multiple holes are intended to increase the amount of energy dissipated through this plastic deformation since each crack initiation site must undergo a certain amount of intrinsic toughening and slow propagation Therefore the brittle energy release at the crack tip cannot be the main driver ofcrack propagation in these geometries as the critical stress intensity must be reached independently in each section between the holes 0088 The ridges or ribs 29 are meant to provide added support so that the crack propagation does not initiate until the desired initial torque is reached and further to delay and Aug 21 2014 guide crack propagation The ridges or ribs 29 should force additional cracks to require initiation and or add resistance to reduce crack growth rate thereby reducing kinetic energy release The ridges orribs 29 should force additional cracks to require initiation and or add resistance to reduce crack growth rate thereby reducing energy release rate Ideally this design will cause significant plastic deformation around the entire circumference since the plastic zone will always be leading the slowly propagating crack while the process will still seem instantaneous to the surgeon because of the greatly reduced length that each crack must travel 0089 The embodiments shown in FIGS 7 9 may have the same cross sectional are
44. s 4E 005 0166 iv Click OK 0167 v HOLD THE SHIFT KEY and select the remaining faces For this thesis there should be 6 straight edges 0168 vi Hit Enter on keyboard 0169 vii Keep defaults and define Approximate element size under Sizing Controls For this the sis 0 0001 0170 viii Click OK 0171 ix Hit Esc on keyboard 0172 b from top menu Mesh Controls 0173 i HOLD THE SHIFT KEY and select all part regions For this thesis there are 3 regions 0174 ii Hit Enter on keyboard 0175 iii Select Tri leave default technique as Free 0176 iv Click OK Aug 21 2014 0177 c from top menu Mesh Part Hit Enter on keyboard 0178 d If the mesh does not exactly match up to the seeded edges and this condition is important to the model ex Mesh convergence analysis then repeat all steps in 6 b EXCEPT un check the Use mapped mesh ing where appropriate under the Algorithm section in the pop up window ASSEMBLY MODULE from pull down window just above view port Select Module Assembly 0179 7 CREATE ASSEMBLY 0180 a from top menu Instance Create 0181 b Select desired part s For this thesis just tube click OK will automatically assume assigned mesh 0182 8 CREATE NODE SETS 0183 a Line up the viewport t
45. sent invention includes any of the embodiments described above wherein the metal is titanium 0017 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above wherein further comprising an internal opening extending from said upper head portion into said threaded lower portion 0018 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above further comprising an internal opening extending from said upper head portion to the top of the threaded lower portion 0019 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above further comprising a plurality of apertures extending between said internal opening and said substantially annular groove 0020 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above further comprising a plurality of ridges running across said substan tially annular groove 0021 In another embodiment the reduced shock break away set screw of the first aspect of the present invention includes any of the embodiments described above further comprising a plurality of ribs running between said upper radius and said lower radius 0022
46. symmetric solid element library the disclosure of which is hereby incorporated by reference in its entirety the element type CGAX3 also allows elements the freedom to twist about the axis Movement moment and stresses due to torsion on the modeled structure could not have been obtained without this additional degree of freedom The overall length ofthe modeled sections remained the same however the fillet radius and length of the substantially flattened portion of the bottom of the groove was varied The fillet radii values were 0 0002 in R2 0 0004 in R4 and 0 0008 in R8 with the length of the flattened bottom section ranged from 0 0 in LO i e simple semicircle groove to 0 0003 in in incre ments of 0 0001 in L1 L2 and L3 respectively These measurements were only used to determine relative performance in Finite element modeling and they are not the intended measurements of the invention herein 0107 Constants were the minimum and maximum wall thicknesses and total height of the model A parametric study was conducted with every combination ofthese variables 1 e 12 total models to show how these geometric changes affected plasticity These combinations were labeled by their length LO L1 L2 L3 followed by their radius R2 R4 R8 e g L3R8 The two extreme cases of these models LOR2 and L3R8 are illustrated in FIGS 10A and 10B for clarity Detailed information on how the 2D
47. the BOUNDARY set click Con tinue 0219 g Check U1 U2 UR2 and UR3 click OK 0220 12 CREATE ROTATING BOUNDARY CONDI TIONS 0221 a Since the torsion of the tube top is displace ment controlled as opposed to force controlled it must be defined as a boundary condition from top menu BC Create 0222 b Name as desired change Step to Step 1 0223 c Category is Mechanical select Displace ment Rotation 0224 d Click Continue 0225 e Click Sets at the bottom right of the view port 0226 f Select the LOAD set click Continue 0227 g Check UR2 set value to radians of rotation for the loaded set For this thesis 0 3 click OK JOB MODULE from pull down window just above view port Select Module Job 0228 13 CREATE JOB 0229 a from top menu Job Create 0230 b Name as something OTHER than Job 1 this name causes an error for some reason leave defaults click Continue 0231 c Leave defaults click OK 0232 14 RUN JOB from top menu Job Sub mit select the desire job 0233 15 VIEW RESULTS from top menu Job Results select the desire job APPENDIX B AIS083 H116 Parameters Used in the Model Density lbf Young s Poisson s Displacement Seed size in s in Modulus psi Ratio at Failure shear region
48. the groove 13 the improved groove geometry ofthe present invention slows the crack propagation by providing increased pathways along the grain boundaries for the crack to propagate wherein dissipat ing more energy in plastic deformation thereby less as shock 0085 By contrast the prior art SSBO breakaway set screws having a substantially V shaped notch with straight angled sides and a very sharply angled bottom provide a sudden release of energy as a result ofthe instantaneous shear failure experienced when the surgeon achieves the designed torque This is illustrated in FIGS 1A and 1B by the high acceleration peak at the start of the signal which indicates that crack initiation and propagation is rapid This SSBO crack speed together with the set screw geometry greatly influence the failure behavior of the prior art set screws and make the normally ductile material response of the metal become primarily a brittle rupture 0086 Inthe embodiment best seen in FIG 7 there are one or more apertures 30 running between the substantially flat tened portion 28 and the inner surface 15 ofthe inner opening 14 These apertures 30 may be evenly spaced along the sub stantially annular groove 13 In an embodiment best seen in FIG 8 there are one or more ridges or ribs 29 running from the upper radius to the lower radius ofthe substantially annu lar grove 13 Ridges or ribs 29 may be any shape and may be any periodic increase in groove w
49. tially annular groove further comprises an internal open ing 0033 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein said sub stantially annular groove further comprises a plurality of apertures extending between said internal opening and said substantially annular groove 0034 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein said sub stantially annular groove further comprises a plurality of ridges running across said substantially annular groove US 2014 0236237 A1 0035 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein said sub stantially annular groove further comprises a plurality of ridges or ribs running between said upper radius and said lower radius 0036 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein the sub stantially annular groove is a single arc having a radius that is longer than the groove wall thickness 0037 In another embodiment the substantially annular groove of the third aspect of the present invention includes any of the embodiments described above wherein said sub stantially fl
50. tion further comprises a recess sized to mate with a tool for removing screws 23 The reduced shock breakaway set screw of claim 2 wherein said upper radius is from about amp 4 inches to about 1 4 inches and said lower radius is from about 64 inches to about Ya inches 24 The reduced shock breakaway set screw of claim 2 wherein said substantially flattened portion has a length of from about 0 inches to about Ys inches 25 The reduced shock breakaway set screw of claim 2 wherein said substantially flattened portion has a length of 0 inches 26 A reduced shock breakaway set screw for use with a surgical construct comprising a threaded lower portion and an upper head portion separated by a substantially annular groove wherein said substantially annular groove is an arc having a radius greater than the groove wall thickness qe caes o k k k
51. torque can allow the stabilizing rod to slip Over torque ofthe set screw can severely deform the screw threads causing them to lose strength and possibly allowing the rod to slip when the patient later puts load on the spine Materials engineering and machine design theories as well as perform ing a test using a torque wrench validated that the prior art set screw tested does break at the torque specified by the manu facturer 0092 Theshear failure during SSBO was explored experi mentally and by applying distortion energy theory to thin wall torsion TWT and fully plastic torque FPT equations SSBO design torque of 11 00 N m as supplied by the manu facturer was compared to an experimental torque wrench measurement and to the torque required to reach the shear strength calculated by distortion energy theory in both TWT and FPT 0093 The basic information requirements of TWT and FPT were obtaining the inner diameter ID and outer diam eter OD of the set screw and calculate the shear strength First digital imaging and micrometer readings were used to US 2014 0236237 A1 determine the inner and outer diameters of the set screws at the point of failure Table 1 Second From the Distortion Energy Von Mises principles C yield T yield 0094 Using material data of titanium alloy 6 Aluminum 4 Vanadium Ti 6AL 4V which is the primary material from which many implants are made Quasi static yield stress for Ti 6A
52. urely hold the rod or other elongated mem US 2014 0236237 A1 ber in place and prevent either translational or rotational movement of the rod or other elongated member Possible configurations for set screw end 19 may include a v shaped point coaxial with the set screw portion 12 sharpened ring with or without a v shaped point coaxial with the set screw portion 12 or any other conventional or otherwise suitable configuration 0072 Substantially annular groove 13 runs around the circumference of the reduced shock breakaway set screw 10 between the upper head portion 11 and threaded lower set screw portion 12 ofthe set screw In the embodiment shown in FIGS 2 4 the bottom of the substantially annular groove 13 is elongated defining an upper radius 26 and a lower radius 27 joined by a substantially flattened portion 28 having a length of e the substantially flattened portion 28 being tan gential to both upper radius 26 and lower radius 27 The substantially flattened portion 28 need not be perfectly flat but is generally non circular and not part of either the upper radius 26 or the lower radius 27 The precise dimensions of the upper radius 26 lower radius 27 and substantially flat tened portion 28 may dependon the size ofthe set screw being used In one aspect of the present invention the upper radius 26 or the lower radius 27 is greater than the groove wall thickness 25 In one embodiment ofthe present invention the upper radius
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