Sacroiliac screw

An improved joint fusion screw for transiliac fixation has an elongate hollow shaft. The hollow shaft has an externally threaded end portion extending to a tip end and a non-externally threaded shank portion having openings. The tip end has at least two bone cutting flutes at the bottom of the shaft. Each bone cutting flute has a cutting edge on a circumferential exterior of the threaded tip to cut bone and direct the cut bone internally into the hollow shaft toward the shank.

TECHNICAL FIELD

The present invention relates to an improved spinal fixation screw for transiliac fixation and a method of use.

BACKGROUND OF THE INVENTION

Many complaints of lower back pain and leg pain have been attributed to herniated discs or other injuries to the spinal column. Extensive therapy and treatment has often been unsuccessful in alleviating such pain. It has been established that some of this lower back and leg pain can be attributed to symptomatic sacroiliac dysfunction or instability. Normally, the sacroiliac joint which spans between the sacrum bone and ilium bone has nutation of one to two degrees. “Nutation” is the medical term which describes the relative movement between the sacrum and ilium. A patient's sacroiliac joint can become damaged resulting in hypermobility of the joint. Because of the small range of motion in the sacroiliac joint, hypermobility is very difficult to diagnose. Therefore, lower back pain or leg pain caused by sacroiliac dysfunction often goes misdiagnosed or undiagnosed.

Accordingly, it is an objective of this invention to provide a device for correcting symptomatic sacroiliac dysfunction or instability. It is another aspect of this invention to provide a device which enhances stability and compression for purposes of immobilizing a joint, and for fusing two opposed bone structures across the joint.

SUMMARY OF THE INVENTION

An improved joint fusion screw for transiliac fixation has an elongate hollow shaft. The hollow shaft has an externally threaded end portion extending to a tip end and a non-externally threaded shank portion having a plurality of openings. The tip end has at least two bone cutting flutes at the bottom of the shaft. Each bone cutting flute has a cutting edge on a circumferential exterior of the threaded tip to cut bone and direct the cut bone internally into the hollow shaft toward the shank. Each cutting edge lies in a plane parallel to an axis of the elongate hollow shaft.

In each embodiment, the hollow shaft has a bone chamber for receiving the cut bone fragments. The bone chamber extends to at least the openings of the shank portion. Autograft cut bone fragments are directed to the openings to enhance new bone growth and rapid fusion of the fusion screw. Preferably, the openings of the shank portion are elongated slots.

The screw has an enlarged flat head affixed or integral to an end of the shank. The end of the shank portion has internal or female threads for receiving a threaded driver cap. The threaded driver cap has a cannulated opening or aperture for passing a guide wire and a torquing tool receiving cavity to thread the screw into the bone. The drive cap is affixed into the threaded end of the shank. The driver cap can be removably attached to allow bone packing material to be packed into the hollow shaft after screw insertion into the bone. The at least two cutting flutes are preferably diametrically opposed.

In one embodiment, each bone cutting flute has an arcuate ramp extending from the cutting edge toward an inside diameter of the hollow shaft. The cut bone fragments are directed internal along the arcuate ramps upon implantation of the screw into the hollow shaft. Each of the cutting edges form spiral cut autograft bone upon screw implantation. Each of the formed spiral cut autograft bone remains connected by tissue to increase osteoconductivity.

In a second embodiment, the at least two cutting flutes extend starting from the tip end longitudinally through two or more threads.

In a third embodiment, the tip end can have a web or bridge extending across the hollow shaft. The web or bridge has an aperture for receiving a guide wire. The aperture is coaxial with an axis of the screw and the aperture of the cap driver.

A method of transiliac fixation using the improved screw comprises the steps of pre-drilling an opening in the sacrum and the ilium bones to be fixed with a pilot hole opening and inserting a joint fixation screw with a hollow shaft onto the pre-drilled opening while cutting autograft bone fragments directed into the hollow shaft. The hollow shaft has a bone receiving chamber extending to a plurality of openings further in the hollow shaft and the step of threading of the screw directs the autograft bone fragments to the openings to enhance fusion. The screw can have apertures at the tip end and at the driver cap and the method may further comprise the steps of inserting a guide wire to create a drill path, inserting a cannulated drill over the guide wire to pre-drill the pilot hole, and then inserting the screw onto the guide wire to direct the path for insertion into the bone.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIGS. 1, 2 and 3, three versions or embodiments of an improved joint fixation screw10A,10B and10C for a transiliac fixation are shown. Each embodiment has common features with variations on the cutting flutes and tip end design.FIG. 4shows the proximal head end opposite to the bone cutting tip end. The head30at this end is a common feature to all three embodiments.

Each screw10A,10B and10C has a hollow elongated shaft20. The shaft20has an externally threaded end portion21and a smooth shank portion25. The smooth shank portion25has a plurality of openings24open to a chamber12inside the hollow shaft20. At a proximal end of the screws10A,10B and10C is an enlarged head30. The center of the head30is a threaded opening32open to the chamber12. The threaded end portion21of the hollow shaft20has threads29. All these features are common to each screw10A,10B and10C.

In a first embodiment ofFIG. 1, the screw10A has two cutting flutes11. Each flute11is diametrically opposed from the other and each has a cutting edge13formed from a leading thread29at the tip end21. The cutting edges13lie in a plane parallel to the axis of the screw shaft and each had an arcuate ramp14for directing bone fragments into the hollow chamber12. The cut fragments spiral into the chamber12along the ramped surfaces14.

In a second embodiment ofFIG. 2, each of the cutting flutes11extend deeper across two or more threads29of the end portion21to a bottom16. The cutting edges13are still circumferentially in a plane parallel to the axis, but the deep longitudinally extending opening of the flutes11captures the cut bone fragments and directs them into the chamber12in pieces that are broken on threading.

With reference toFIG. 3, a third embodiment, a bridge or web23extends across the hollow shaft20at the tip end21defining a pair of openings15between the bridge or web23and the flutes11, as shown. The bridge or web23has an aperture27for receiving a guide wire. This third version screw10C has the same flutes11as shown inFIG. 2.

FIG. 4, shows a threaded driver cap40inserted and threaded into the threads32of the enlarged head30. This driver cap40has a torque receiving cavity41with projections42to receive a torquing tool to implant the screw10A,10B or10C. Centrally, there is an aperture47to allow the screw10C to pass over a guide wire along a directional pre-drilled path.

As the screw10A,10B or10C is torqued into the pre-drilled pilot hole, the cutting flutes11create autograft bone fragments that are delivered directly into the chamber12. In this way, the patient's bone fragments are made available to enhance new bone growth to fuse the screw10A,10B or10C in place.

One purpose of this invention is to direct bone that is cut by the self-tapping threads and cutting edges13at the tip of the bone screw10A,10B or10C or otherwise gathered by the flutes11into the internal chamber12of the screw to serve as additional autograft material. Previously, this material would be compressed into the bone around the outside of the screw. The screw would be filled with previously harvested autograft material which could be packed into the screw from an opening in the head end of the screw. The screw is used to secure two bones together, in this case the sacrum and the ilium. When preparing the bone to accept the screw, a hole will be drilled and tapped to a size smaller than the actual screw. The screw can be packed with graft material prior to implantation. The self-tapping edge of the screw will cut additional autograft material as it is installed and the flute will direct this freshly cut autograft material to join the existing material in the inner chamber12of the screw. Some of this material will be pushed out of the plurality of openings24or fenestrations in the shaft20of the screw as it is tightened to aid in fusion around and into the body of the screw. Many variations of similar flute shapes will produce a similar result. The screw material can be anything hard and strong enough to cut and direct bone chips and withstand the biomechanical loads of the application, preferably titanium, stainless steel or alloys of these materials or metals will work satisfactorily.

The present invention SI (Sacro-iliac) screw described herein has shown several important features of this screw. These features include: the lagging where the head30is pulled down by the coarse threads29and the lagged portion is within the smooth shank portion25and not engaged by threads29; the screw has a large bore or chamber12for inserting bone graft with the option to cap this bore which communicates to the SI space after insertion with the driver cap40; the shaft20has only the single threaded end portion21to engage only the sacrum bone, the ilium bone is positioned on the smooth shank portion25; an optional anti-back out feature under the head30in the form of a series of wedge-shaped teeth33to engage the iliac bone surface can be used as shown inFIG. 5. A wedge shaped washer (not shown) could be used under the screw head30to accommodate the surface angle of the ilium with respect to the screw axis and also employ the anti-back out feature shown inFIG. 5, but as a separate washer piece.