Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
2001-02-02
2003-01-21
Philogene, Pedro (Department: 3732)
Surgery
Instruments
Orthopedic instrumentation
C606S062000, C606S064000, C606S104000
Reexamination Certificate
active
06508820
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to bone fracture fixation devices, and more particularly to systems that involve positioning of an intramedullary nail within the intramedullary canal, followed by cross-locking for fixation of the intramedullary nail to achieve bone fixation.
Intramedullary fixation is a well-accepted technique for internal fracture fixation of long bones, typically the femur or the tibia, although humeral and forearm (radial or ulnar) applications also are possible. This fixation technique involves inserting an intramedullary nail, usually a hollow shaft having a slight bend or curvature, into the intramedullary or marrow canal. Once inserted and properly positioned within the bone, the intramedullary nail is fixed to the bone by cross-locking, with screws extended transversely with respect to the elongated nail through the bone, and through holes in the intramedullary nail, or in the case of hollow nails, through diametrically opposed holes in the nail wall.
The cross-locking fixation technique is shown in U.S. Pat. No. 5,122,141 (Simpson, et al.), which also illustrates an inclined disposition of bone screws through the nail at the proximal end of the femur. U.S. Pat. No. 5,112,333 (Fixel) also illustrates an intramedullary nail secured in the femur using fasteners directed transversely of the nail.
FIG. 1
illustrates an intramedullary nail
1
within the intramedullary canal
2
of a long bone
3
, for example the femur. The nail is fixed by two bone screws
4
and
5
. Bone screw
4
, extended through a wall
6
of the bone on opposite sides of the intramedullary nail, also extends through diametrically opposed holes
7
and
8
through the nail wall to secure the intramedullary nail within the intramedullary canal. Bone screw
5
extends in similar fashion through the bone wall and through openings
9
and
10
through the nail wall, to further secure the nail.
FIG. 2
is an enlarged view showing a portion of bone screw
4
extending through hole
7
. The bone screw has an elongated shank
11
having a shank diameter constituting a “minor” diameter of the screw. An external thread
12
surrounds the shank, with the radial extremity of the thread determining a major diameter of the screw. The diameter of hole
7
closely approximates the major diameter of the bone screw, so that the thread extremity establishes a substantially helical contact or interface with the intramedullary nail along the wall defining hole
7
.
Although this arrangement has in general been satisfactory, several difficulties arise due to the amplitude and direction of stresses at the intramedullary nail/screw interface. More particularly, both the tibia and the femur are required to support substantial body weight, and thus are subject to substantial axially directed compressive forces and substantial shock in the axial direction. The muscles also can exert twisting forces upon the bone. An intramedullary fixation system is subject to these same forces.
Fasteners such as bone screw
4
are designed primarily to bear loads in the axial direction with respect to the fastener, and thus are well suited for certain uses, e.g. securing bone plates. However, when used to interlock an intramedullary nail, the bone screw is subject to the aforementioned axial compressive stresses and twisting, which operate as sheer forces directed laterally or transversely with respect to the screw. In some cases, the sheer forces are of sufficient magnitude to fracture or break the bone screw at a point near the intramedullary nail hole that accommodates the screw.
One attempt to solve this problem involves using larger-diameter bone screws. A consequence of using larger screws is that the holes through the intramedullary nail needed to accommodate the screws must also be larger, which compromises the integrity of the nail. Accordingly, although larger screws may reduce the risk of screw failure due to sheer, they are likely to increase the risk of nail failure.
Another approach is to form the bone screws from a material selected for a high resistance to fracture, for example stainless steel. The materials selected to form the intramedullary nail and bone screws, however, must have a high degree of biocompatibility as well. Titanium and certain titanium-based alloys are highly preferred for their biocompatibility, despite their notch sensitivity characteristics as compared to stainless steel. Steel components lack the degree of biocompatibility desired in many applications. A “partial solution” of using a titanium intramedullary nail in combination with steel bone screws would not be satisfactory, due to galvanic corrosion at the nail/screw junctions.
Another approach addressing this problem is seen in U.S. Pat. No. 5,814,047 (Emilio, et al.). The Emilio patent describes a fixation system in which the intramedullary nail is secured by several flexible screws with distal end portions slightly inclined relative to the longitudinal nail extension, as opposed to more rigid, transverse screws. This arrangement, however, requires elongate flexible screws of different lengths, and structure within the nail for channeling these screws and diverting the tips at a slant relative to the nail.
Another problem caused by stresses laterally of the bone screws is a risk of plastic deformation of the screw threads, the interior of the holes through the nail wall accommodating the screws, or both as a result of the forces involved. For example with reference to
FIGS. 2 and 3
, as threads
12
and the internal surface of hole
7
are urged against one another, there is a high stress concentration along the thread/wall interface which can tend to flatten the external threads, or lead to depressions in the hole wall, or both, as indicated by the broken lines in FIG.
3
. In any of these events the integrity of fixation is compromised. Any transverse loads can cause further plastic deformation, and may further compromise fixation.
Therefore, it is an object of the present invention to provide an interlock screw for securing an intramedullary nail, with an improved capacity to withstand forces directed laterally with respect to the screw, i.e. in directions perpendicular to the screw length.
Another object is to provide an intramedullary interlock screw with an external thread providing a larger area of contiguous surface contact at the interface with an intramedullary nail secured by the screw in a bone fixation application.
A further object is to provide a bone fixation system in which the components can be formed from a wider variety of materials, and yet maintain desired levels of strength and resistance to fatigue.
Yet another object is to provide an intramedullary interlock screw that has a reduced major diameter such that openings in intramedullary nails to accommodate the screws can be reduced in size, while maintaining in the screw a desired resistance to bending under laterally applied forces.
SUMMARY OF THE INVENTION
To achieve these and other objects, there is provided a fastener for securing a fixation member with respect to osseous material. The fastener includes an elongate shank formed of a biocompatible material and extended in an axial direction. The shank has a maximum shank diameter and a shank outer surface. An external thread, formed of a biocompatible material, is disposed helically about the shank. The external thread has a substantially uniform thread pitch in the axial direction and defines a thread outer surface substantially parallel to the shank outer surface and spaced apart from the shank outer surface by a thread height. The width of the thread outer surface in the axial direction is at least twenty percent of the thread pitch, and the maximum shank diameter is at least six times the thread height.
As compared to the previously known bone screw shown in
FIGS. 1-3
, the width of the thread outer surface, i.e. the crest length, is considerably larger in proportion to the pitch length. Also as compared to the known screw, the maximum shank diameter is at least six times the threa
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