Bone screws and particular applications to sacroiliac joint fusion

Abstract

Procedures for the fusion of the sacroiliac joint advantageously make use of an implant selected to distract the joint upon insertion and to maintain or increase tension upon insertion. The implant can have a varying structure along its length. In some method described herein for fusing the sacroiliac joint using an implant, an implant is screwed into the sacroiliac joint between the sacrum bone and the iliac bone. The implant comprises a shaft, a tool engagement flange at top end of the shaft, a pointed tip comprising no more than about 20 percent of the length of the screw, and threads spiraling around the shaft. For screws of particular interest, the volume displacement perpendicular to the shaft increases at least about 5 percent from a point adjacent the tip to a point near the top of the shaft. Some of the desirable screw designs can be used in other orthopedic application, especially situations involving varying bone hardness. Useful filler material can be formed from a blend of bone powder and bioactive agents.

1 . An orthopedic implant suitable for insertion into the extra-articular recess of the sacroiliac joint, the implant comprising a tapered shaft that comprises a head at a first end of the shaft, a tip at a second end of the shaft that is opposite the head at the first end of the shaft, a core connecting the head and the tip, and threads extending along the core and the tip,
 wherein the top surface of the head comprises a driver engagement element,   wherein a channel extends throughout the entire length of the tapered shaft through the head, core, and the tip, and   wherein at least some of the threads comprises an asymmetrical cross-sectional shape.   
   
   
       2 . The orthopedic implant of  claim 1  wherein the tip comprises a cutting flute. 
   
   
       3 . The orthopedic implant of  claim 1  wherein the implant surface is porous. 
   
   
       4 . The orthopedic implant of  claim 1  wherein the tip is a bottom section of the shaft below a boundary at which a change in structure occurs. 
   
   
       5 . The orthopedic implant of  claim 4  wherein the change in structure comprises a change in lateral extent of the threads, core, or shaft. 
   
   
       6 . The orthopedic implant of  claim 1  wherein the driver engagement element comprises a depression for engaging with a tool. 
   
   
       7 . The orthopedic implant of  claim 1  wherein the shaft comprises an opening through the core that opens into the channel. 
   
   
       8 . The orthopedic implant of  claim 1  wherein all of the threads comprises an asymmetrical cross-sectional shape. 
   
   
       9 . The orthopedic implant of  claim 1  wherein the asymmetrical cross-sectional shape is characterized by a difference in the cross-sectional shape of the top of the thread relative to the bottom of the thread. 
   
   
       10 . The orthopedic implant of  claim 1  wherein the asymmetrical cross-sectional shape is characterized by a flatter top surface relative to a more angled lower surface. 
   
   
       11 . The orthopedic implant of  claim 1  wherein the core is tapered. 
   
   
       12 . The orthopedic implant of  claim 1  wherein the asymmetrical cross-section shape is characterized by threads near the tip comprising a shape different than threads near the head of the shaft. 
   
   
       13 . The orthopedic implant of  claim 1  wherein the lateral extent of the threads varies along the length of the shaft. 
   
   
       14 . The orthopedic implant of  claim 1  wherein the threads extends along the core and the tip discontinuously. 
   
   
       15 . The orthopedic implant of  claim 1  wherein the threads spiral around the core and tip continuously. 
   
   
       16 . A method for inserting an implant into a patient's sacroiliac joint, the method comprising:
 tightening a first sizing element selected from a sizing system comprising a set of exchangeable sizing elements that correspond to differently sized implants into a prepared location for the implant to evaluate the size of appropriate implant for implantation into the prepared location.   
   
   
       17 . The method of  claim 16 , wherein each of the exchangeable sizing elements comprises an outer shape approximates the outer shape of each of the available implants. 
   
   
       18 . The method of  claim 16 , wherein the prepared location is prepared by a cutting instrument. 
   
   
       19 . The method of  claim 16 , wherein the prepared location is prepared by inserting the implant through a cannula. 
   
   
       20 . The method of  claim 16 , wherein the prepared location is prepared by cutting through a cannula.