1. Field of the invention
The present invention relates to an apparatus for external fixation and stabilization of a fractured bone, and, more particularly, to such an apparatus having a fixation rod interconnected to the fractured bone at a plurality of locations via a plurality of respective fixation pins.
2. Description of the related art
External fixation apparatus of known design are utilized for fixating and stabilizing fractured bones. While fixation apparatus and systems have undergone considerable evolutionary changes over the years, they all rigidly hold the sections of a broken bone in alignment throughout the healing process. Fixation devices may be in the form of a relatively crude splint or cast, or a more modern and sophisticated system involving surgical fixation pins secured to an external fixation rod, or the Ilizarov system well known to those skilled in the art.
Conventional fixation systems currently on the market may include a hexagonal fixation bar used to interconnect and rigidly secure a plurality of fixation pins inserted into the fractured bone at various points, with each fixation pin being retained within a clamp secured to the fixation rod. Each clamp is installed onto the fixation rod by sliding the clamp over one end or the other and tightening one or more nuts when the clamp is in its desired longitudinal position on the rod. An example of such a system is disclosed in U.S. patent application Ser. No. 08/017,933, entitled "EXTERNAL FIXATION APPARATUS" filed Feb. 16, 1993, the disclosure of which is expressly incorporated herein by reference.
A problem with conventional designs is that they are constructed of a metal having a known modulus of elasticity. The modulus of elasticity does not change for a particular metal, regardless of whether the selected metal is a commercially pure or alloy material. For example, stainless steel has a modulus of elasticity of about 30.times.10.sup.6 PSI; commercially pure titanium and titanium alloy have a modulus of elasticity of about 16.times.10.sup.6 PSI; aluminum has a modulus of elasticity of about 10.times.10.sup.6 PSI; and magnesium has a modulus of elasticity of about 8-9.times.10.sup.6 PSI. The modulus of elasticity in the axial direction of the fixation rod relates to the amount of elastic movement possible within the fixation rod in an axial direction for a given axial load. Movement of the fixation rod in the axial direction, in turn, relates to the amount of axial movement at the fracture site of the bone. It is generally accepted in the art that an increased axial movement at the fracture site theoretically improves the fracture healing process. However, for a material such as stainless steel having a high modulus of elasticity, the loads experienced at the fracture site are not sufficient to elastically deform the fixation bar. Accordingly, no movement of the bone in an axial direction at the fracture site occurs, with resultant detrimental effects on the healing process.
One known method of increasing the load on the bone at the fracture site is to loosen the clamp assemblies attached to the fixation rod and slide the clamp assemblies in an axial direction along the fixation rod in a direction toward the fracture site. However, this results in a constant load being placed on the bone at the fracture site rather than inducing axial movement of the bone sections on each side of the fracture site, since the fixation rod does not elastically move under such typical loads.
Another problem is that the step between each respective modulus of elasticity for conventional fixation bars constructed of different metals is quite large. For example, the step between the modulus of elasticity for stainless steel and titanium is about 14.times.10.sup.6 PSI (30-16). Since stainless steel and titanium are the two most commonly used materials for conventional fixation rods, the modulus of elasticity is either 30.times.10.sup.6 PSI or 16.times.10.sup.6 PSI, without the ability to select a material having a modulus of elasticity disposed therebetween.
A still further problem is that clamp assemblies of conventional design are made to accept a fixation rod of a particular exterior geometric configuration. When a fixation rod constructed of one metal is substituted for a fixation rod constructed of another metal, the exterior geometry must remain the same in order to attach to the clamp assemblies.. However, as the axial compression stiffness of the fixation rod changes as a result of the change in the modulus of elasticity, the bending stiffness and torsional stiffness also change as a result of using a different material. Therefore, using a metal which has a lower modulus of elasticity to improve the axial loading on the bone may result in other detrimental affects such as twisting or bending of the fixation rod, thereby allowing movement of the bone sections in a radial direction at the fracture site.
What is needed in the art is an external fixation apparatus which includes a plurality of fixation rods having an axial compression stiffness which may be varied, while maintaining a relatively constant torsional and bending stiffness.