Variable angle bone fixation device

A bone fixation element includes a threaded head and a shaft extending along a longitudinal axis from a proximal end to a distal end, an outer surface of the head being one of carburized and nitrided and including a first groove extending into an outer surface of the head along a path interrupting the threading and extending along an angle counter to an angle of the threading.

BACKGROUND

Bone fixation plates are often positioned over a fractured or otherwise damaged portion of bone and secured thereto using bone screws inserted through screw holes of the bone fixation plate. The screw holes extend transversely through the bone plate and are sometimes formed with threads to lockingly engage a threaded head of the bone screw. Variable angle screws are often employed which permit a user to insert the screw through the plate at a user-selected angle relative to an axis of the plate hole. However, the engagement threads of the head of such variable angle screw heads with the threading of the plate hole may burr threads of one or both of the bone screw and the bone plate, causing a loss in bone fixation strength. Damage to the bone plate or bone screw in this manner may cause the bone fixation procedure to lose efficacy. Those skilled in the art continue to search for ways to increase the strength of the screw-plate interface in variable angle systems.

SUMMARY OF THE INVENTION

The present invention is directed to a bone fixation element comprising a threaded head and a shaft extending along a longitudinal axis from a proximal end to a distal end, an outer surface of the head being one of carburized and nitrided and including a first groove extending into an outer surface of the head along a path interrupting the threading and extending along an angle counter to an angle of the threading.

DETAILED DESCRIPTION

The present invention may be further understood with reference to the following description and the appended drawings, wherein like elements are referred to with the same reference numerals. The present invention relates to the stabilization of bones and, in particular, to the stabilization of a fractured or otherwise damaged bone using a bone screw inserted through a bone fixation device (e.g., a bone plate). Exemplary embodiments of the present invention describe a variable angle bone screw having a threaded head and a threaded shaft and having a carburized or nitrided outer surface configured to increase a surface hardness thereof to a desired level. The threaded head comprises one or more grooves extending into an outer surface thereof at an angle relative to a longitudinal axis of the bone screw to aid in alignment of the threads of the head with threads of a variable angle screw hole of the bone fixation device. The shaft comprises one or more notches extending into an outer surface thereof at any angle relative to the longitudinal axis within a permitted range of angulation, as will be described in greater detail later on. In one embodiment, the bone plate may be formed of a metallic alloy exhibiting a hardness within a predetermined range. The bone screw may be carburized or nitrided such that an outer surface of the bone screw has a hardness greater than a hardness of the bone plate. Thus, the exemplary bone screw according to the invention prevents burring of the screw during insertion into the bone plate while providing a consistent connection strength to the bone and bone plate. Furthermore, the exemplary system according to the invention reduces galling during use while also providing an increased overall strength when compared to standard screws including increased yield strength, ultimate tensile strength and fatigue strength, as those skilled in the art will understand. It should be noted that the terms “proximal” and “distal” as used herein, are intended to refer to a direction toward (proximal) and away from (distal) a user of the device.

As shown inFIGS. 1-6, a bone screw100according to an exemplary embodiment of the invention extends from a proximal end102comprising a head104along an elongated shaft106to a distal end108. In an exemplary embodiment, an outer surface of the head104is substantially spherical to permit variable angle insertion of the bone screw100into a bone fixation device200, as will be described in greater detail later on. It is noted, however, that the head104may be formed in any other shape without deviating from the scope of the invention (e.g., to permit a single-angle insertion of the bone screw100into the bone fixation device200). The outer surface of the head104is provided with threading110having a pitch configured to lockingly engage threading212formed on a walls of an opening202extending through the bone fixation device200, as will also be described in greater detail later on. One or more grooves112may be provided on the head104, each groove112extending at least partially into the threads110and extending along an axis substantially angled with respect to a longitudinal axis114of the bone screw100. The grooves112are configured to interrupt the thread110, thus creating a plurality of thread starts which aid in alignment of the thread110with the threads212of the hole202in an operative configuration especially when the bone screw100is inserted into a bone plate hole angled with respect to an axis of the bone plate hole (i.e., when the threading of the head104is misaligned with the threading of the bone plate hole). The grooves112further permit the bone screw100to advance distally into the bone when rotated via a driving mechanism (not shown).

Each of the grooves112may be angled, for example, at an angle of approximately 8.5±1° relative to the line B-B, although any other angle may be used without deviating from the scope of the invention. In an exemplary embodiment, the grooves112are angled counter to a direction of the threading110. For example, as seen inFIG. 1, the line B-B is perpendicular to the path of the threading110and the groove112is angled relative to the line B-B so that, traveling along the threading110from a proximal end110A thereof toward a distal end110B, the angle between the threading110and the groove112is greater than 90° on the proximal side of the thread and less than 90° on the distal side of the thread. In another embodiment, the grooves112extend at an angle of approximately 5-85° relative to the line B-B (i.e., 95° to 175° relative to the threading110). In yet another embodiment, the grooves112may extend substantially parallel to the line B-B. The grooves112according to this embodiment extend along substantially a complete length of the threading110. In another embodiment (not shown), the grooves112may extend for only a partial length of the threading110.

In a first exemplary embodiment of the invention, the bone screw100may be formed with five grooves112disposed evenly circumferentially about the head104and equidistant from one another, as shown inFIG. 5. Specifically, each of the grooves112in this embodiment is separated from adjacent grooves112by approximately 72°. In another embodiment (not shown), the bone screw100comprises six grooves112separated from one another by approximately 60°. In yet another embodiment, as shown inFIG. 6, the bone screw100may comprise eight grooves112separated from one another by approximately 45°.

The head104may further comprises a recess116extending thereinto from the proximal end102. The recess116is configured to permit engagement with a distal end of a driving mechanism (not shown) for applying torque to the bone screw100as would be understood by those skilled in the art. The embodiment ofFIGS. 1-6is depicted with a torx-shaped recess116. It is noted, however, that any other shape may be employed without deviating from the scope of the invention (e.g., slotted, phillips, square, hexagonal, etc.), as those skilled in the art would understand.

The shaft106is provided with threading118having a pitch substantially the same as the pitch of the threads110. In another embodiment of the invention (not shown), the pitch of the threading118may be greater than or smaller than the pitch of the threads110. The threading118of the shaft106may be formed with two leads, as those skilled in the art will understand. The multi-lead configuration of the threading118aids in linear advancement of the bone screw100into the bone, as those skilled in the art will understand. As would be understood by those skilled in the art, the length of the shaft106is generally selected to conform to requirements of a target procedure. A distal portion of the shaft106may comprise one or more notches120configured to create a gap in the continuity of the threads110and permit self-tapping of the bone screw100, as those skilled in the art will understand. The distal portion of the shaft106may taper to a smaller diameter at the distal end106to, for example, aid in insertion. The distal end106may be sharpened or blunt as desired.

The bone screw100may be formed of a material selected to have a greater hardness that a material of a bone fixation device200with which it is to be employed. Specifically, the bone screw100may be formed of one of stainless steel and CCM (Co-28Cr-6Mo Alloy). The bone screw100may then be carburized or nitrided to further increase a surface hardness thereof to approximately 68 HRC or more, as those skilled in the art will understand. In an exemplary embodiment, the hardness of the bone screw100may be approximately 67-74 HRC and, more particularly, 67.5-70.3 HRC. In contrast, the bone fixation device200may be formed of commercially pure Titanium grades 1, 2, 3 and 4, Ti-6Al-7Nb, Ti-6Al-4V, Ti-6Al-4V ELI, Ti-15Mo, CCM (Co-28Cr-6Mo Alloy), stainless steel or another material different than the material of the bone screw100. As those skilled in the art will understand, a hardness of the bone fixation device200may be between approximately 75 HRB (e.g., for a CP1 material) and approximately 45 HRC (e.g., for a CCM material). This configuration prevents burring of the threads110of the bone screw100as they are inserted into the bone fixation device100while also increasing a holding strength of the bone fixation system in the bone.

FIGS. 7-10depict the exemplary bone fixation device200according to the invention. Although the device200shown is a bone plate, it is submitted that any other bone fixation device may be used without deviating from the scope of the invention (e.g., an intramedullary nail, etc.). The bone plate200may, for example, be a 4.5 mm broad variable angle compression plate including eight holes202extending through a body204. Any or all of the holes202may be formed as variable angle combination holes comprising a first variable angle hole portion206and a second compression hole portion208open to the first hole portion. The first hole portion206may comprise a first relief cut210formed adjacent a first surface203, a second cylindrical threaded portion212extending distally therefrom and a third relief cut214formed adjacent a second surface205configured to contact the bone in an operative configuration. The relief cut210may extend at an angle of approximately 15° relative to a longitudinal axis of the hole202, although other angles may be used without deviating from the scope of the invention. The first hole portion206further comprises one or more slots207provided on an outer wall thereof, the slots207extending substantially perpendicular to a screw hole axis. As those skilled in the art will understand, the slots207interrupt the threads of the threaded portion212to provide multiple thread starts which aid in alignment of the threaded portion212with the bone screw100. The second hole portion208may comprise a first tapered hole portion216and a second tapered hole portion218extending distally therefrom. It is noted that although the bone fixation device200is depicted with eight holes, any other number of holes may be used without deviating from the scope of the invention and these holes may include any variety of know bone screw mounting holes. The bone fixation device200may also comprise any number and combination of variable angle holes, single holes and combination holes without deviating from the scope of the invention. The second surface205may further comprise a plurality of undercuts220configured to reduce a contacting surface area between the bone fixation device200and the bone to, for example, reduce impairment of blood supply after implantation, as those skilled in the art will understand.

In an operative configuration, the bone screw100is inserted through the bone fixation device200and into the bone. As those skilled in the art will understand, a physician or other user may select a desired angle of insertion to conform to the requirements of a particular procedure. Multiple thread starts provided by the grooves112provided on the head104and the slots207provided in the hole202aid in alignment of the threads110of the head with the threaded portion212of the hole202. As the bone screw100is screwed through the bone fixation device200and into the bone, the carburized or nitrided outer surface of the bone screw100prevents burring of the threads110. The increased rigidity of the bone screw100relative to the bone fixation device200also permits removal and reinsertion of the bone screw100into the bone (e.g., to correct a position thereof within the bone) without causing a burring thereof.