FIELD OF THE INVENTION
The present invention relates to a device for attaching a suture, tissue, or implant to a bone.
BACKGROUND OF THE INVENTION
The use of blind rivets in non-medical fields is wide spread. Such blind rivets, which typically have a closing head consisting of separate and comparatively widely spread legs, are known for example from U.S. Pat. No. 4,696,610 to Wright and U.S. Pat. No. 4,580,936 to Francis. These known blind rivets have longitudinal slits or cuts near the end of the shaft which form the closing head when the blind rivet is closed. The closing heads so formed rest on a large area. These known blind rivets incur the drawback that the closing heads are formed by bending the longitudinal legs and, as a result, the leg ends are not sharp tines and cannot be pressed into the material to which the rivet is being attached. Thus, these types of blind rivets do not prevent the blind rivet from rotating in the boreholes of the materials to be connected.
A different non-medical blind rivet is known from British Application No. 2,054,082 of Tucker Fasteners. This fastener includes a pyramidal shaped head that passes through a cylindrical bore.
Other fasteners operate in a manner analogous to a blind rivet. Examples of these fasteners include screws, marrow pins, and hip screws. The Seidel marrow pin for the humerus is fitted with slits at the tip and the slits are spread apart by a central pin with a ballhead. In this manner, the legs are bent outward like wings. However, these legs are bent only slightly away from the nail. Use of these fasteners to anchor an implant in porotic bone is problematic. Often an implant can be affixed to such bone only by injecting bone cement into the bone. This procedure can further damage a bone due to heat necrosis as the cement hardens. Moreover, the cement no longer can be removed as would be required in case of infection.
Another drawback of these known fasteners is that the anchoring strength is determined by its diameter. If there is axial overload, a cylinder of bone equal to the fastener diameter will be torn out. In healthy bone, the anchoring strength of bone screws will be sufficient. But in the case of osteoporosis, in joint zones, or as regards thin, shell-like bones, screw affixation often will be inadequate. Larger screws cannot be used because of lack of space, or else they destroy the remaining bone even more.
Thus, there exists a need for an improved fastener.
SUMMARY OF THE INVENTION
The present invention relates to an orthopedic fastener for fixing an implant, suture, or tissue to a bone. The fastener comprises a blind rivet and a closing element. The blind rivet includes a head at a proximal end, a shank connected to the head, and an anchoring portion near a distal end. A bore extends through the head and shank from the proximal end to the distal end and the bore has a polygonal cross-section through the anchoring portion. The closing element has a body with an outer surface and is configured and dimensioned to be received in the anchoring portion. The outer surface of at least a portion of the closing element distal end has a polygonal cross-section flaring along the longitudinal axis toward the closing element distal end. The anchoring portion of the blind rivet severs into a plurality of anchoring legs when the closing element is pulled in the bore toward the blind rivet proximal end to thereby prevent rotation of the fastener.
The anchoring legs can be spread by the closing elements into a semi-circular shape relative to the longitudinal axis to thereby form the closing head of the blind rivet. This feature provides a broad rest for instance on the inside bone surface and, because the semi-circular shape of the anchoring leg ends rest almost perpendicularly on the bone surface, this design offers jaw-like anchoring the closing head. The blind rivet can be made of a plastically deforming material, such as pure titanium, a titanium alloy, or implant-steel, to enhance the formation of the semi-circular anchoring legs.
In another embodiment of the fastener of the invention, two mutually opposite ends of two mutually opposite anchoring legs of the closing element subtend a distance “L” which is two to three times the blind rivet's diameter “D”. Again this considerable widening of the anchoring legs provides a broad rest on the inside bone surface.
The polygonal cross-section of the bore can include a plurality of corners that form rupture sites between the corner and the outer diameter of the shank to facilitate severing of the anchoring portion into the anchoring legs. In an exemplary embodiment, the rupture sites have a wall thickness between about 1% and 9% of the outside diameter of the shank.
The head of the blind rivet can be integral with the shank. Alternatively, the head of the blind rivet is detachable from the shank. If detachable, the rivet head can include an inside thread and the shank includes a matching outside thread for detachable coupling of the head and shank.
The closing element distal end can have a conical cross-section. The distal end of the closing element can also have a tip (like a gimlet or awl) for facilitating insertion of the closing element in bone. In order to facilitate severing, the anchoring portion can have a wall thickness that is between about 1% and 20% of the outer diameter of the shank. This design allows spreading the rivet shank in simple manner into the separate anchoring elements by means of the polygonal cone at the closing element. In one embodiment, the distal end has a wall thickness that is greater than that of the proximal end of the blind rivet.
In an exemplary embodiment, the fastener includes a closing pin operatively associated with the closing element. The closing pin has a first end extending through the bore to pull the closing element through the blind rivet toward the proximal end. The closing element can be detachably connected to the closing pin. For example, the closing pin can be connected to the closing element by a design rupture site which allows the closing pin to separate from the closing element. Alternatively, the proximal end of the closing element includes a thread and an end of the closing pin includes a matching thread.
The fastener according to the present invention results in palliation. In use, the fastener is insertable through a small borehole and, following affixation, includes a broad resting surface on the inner bone surface due to the spread out anchoring legs. These anchoring legs can dig into the inner bone surface and thereby prevent the blind rivet from rotating.
One advantage offered by the present invention is that the fastener of the invention requires only a small borehole in the bone. However, after implantation, the blind rivet of the invention broadly rests on the inner bone surface. Moreover and illustratively in a manner different from the case relating to a hip screw, the semicircular anchor-like design of the anchoring legs and their anchoring in the bone preclude rotation by the fastener head. With respect to porotic bones, the spongiosa in joint heads—if still present at all—will not be mechanically stressed. In other words, the hip screw is situated in a cavity, and this feature entails a dislocation of the hip head relative to the hip screw. The hip screw only can become functional after it makes contact with the inner bone surface. In such cases, however, the interface between screw and bone often is inadequate, so that the bone screw may penetrate the hip joint. In the invention on the other hand, the inner head surface is used as the interface between implant and bone when using the blind rivet of the invention with the semi-circular anchoring legs. In this manner, the surface of contact with the bone is larger and matches optimally on account of the anatomically matching anchoring legs. If the blind rivet of the invention is used to anchor a marrow spindle into the femur head, the size of the borehole receiving the blind rivet of the invention will only be about 8 mm. This feature offers the advantage that in comparison with conventional systems of marrow-spindles/hip-screws, the diameter of the marrow spindle can be substantially reduced in the application of the invention.
In the medical field, the fastener of the invention is applicable almost universally in the treatment of bone fractures for which a bone screw or similar fastener is presently used. Specifically:
for porotic bones partly comprising only a very thin cortex, the fastener of the invention can replace screws in fastening plates;
in the spongiosa, the fastener of the invention can serve as an anchor for plates, sutures or to reattach tendons and ligaments (the claws formed when spreading the closing head will anchor well into the trabecular structure of the spongiosa); and
in joint heads such as the femur head or the humerus head, the fastener of the invention can be used as the anchor of a side plate or of an intramedullary support (heretofore large screws have been used to anchor the longitudinal supports, however these screws anchor less than optimally in the joint heads' porotic bones).