One method of bone fracture repair comprises securing screws across the fractured portions of the bone. The purpose of the screw or screws is to tightly secure the bone fragments together while the bone heals. Typically, the screws comprise lag design, each being a solid member having an externally threaded section on one end and a head on the other end. The head bears against the exterior surface of the bone when the screw is secured to a desired torque to place the bone fragments in compression.
During the process of revascularization, the edges of the fracture commonly will resorb, typically about 1 to 3 mm. This movement reduces or eliminates the initial compression that the screw has provided. A bone lag screw that could provide continuous dynamic compression across the fracture while accommodating resorption as the bone heals would provide improved long-term stabilization until revascularization is completed.
A number of patents disclose a spring section within a lag screw for this purpose. The spring section would allow the screw to elongate slightly as it is being inserted. If sufficient initial compression existed, the spring section would provide dynamic compression during the repair process. The spring sections shown in these patents, however, could be damaged during insertion because the torque applied to the head of the screw passes through the spring section. This torque would result in torsional deformation of the spring section during insertion, which could create stress risers within the spring that could lead to early mechanical failure. Furthermore, during the process of removal, the spring could distort and fail mechanically, requiring extraction methods that could be more destructive to the bone.