1. Field of the Invention
The invention generally relates to a screw for joining parts of bone.
Many types of bone fractures, particularly fractures of the fibula are lateral fractures occurring near the distal end of a long bone. In such a case, if reinforcement is necessary, it may be desired to provide such reinforcement parallel to the length of the bone. In particular, this invention is directed to the provision of reinforcement, with a compressive force, along the medullary canal of the long bone, with access to the medullary canal being achieved by drilling along the center of the bone from the end of the bone.
While most fractures of long bones are transverse fractures, such fractures usually do not occur along lines perpendicular to the length of the bone, but rather occur along fracture lines which cross the bone in a diagonal direction. Thus, direct compressive force in many cases tends to cause the segments of the bone to shift laterally, thus defeating the purpose of intentionally applied compressive force. For this reason, any device effecting longidutinal compression at a fracture site must also provide a means for aligning the two segments of the bone so as to avoid any lateral shift.
2. Description of the Prior Art
Many types of screws are known in the prior art. For example, Russell in U.S. Pat. No. 146,023 describes a wood screw with a thread of varying pitch gradually decreasing from the point of the screw.
Fracture screw adjusting means are also known in the prior art as described by Charnley in U.S. Pat. No. 2,801,631.
Anchoring arrangements for joining two dissimilar materials are disclosed in Canadian Pat. No. 731,381, issued to Fischer.
None of the above-disclosed attaching structures have been particularly applied to use as a distal fibular screw.
A screw described by Herbert in U.S. Pat. No. 4,175,555 has two threads, separately located at different locations along the length of the screw at distal and proximal ends, respectively, the threads being separated by a shank portion. The distal threaded portion has a diameter which is smaller than the proximal threaded portion, while the thread pitch of the (narrow) distal threaded portion is greater than the thread pitch of the (wide) proximal threaded portion. This thread pitch differential causes the distal end of the screw to attempt to advance through bone tissue for each clockwise turn of the screw at a rate greater than the rate of advance of the proximal end of the screw. The difference in advance rates results in the compression of bone at the distal end against bone at the proximal end. It can therefore be seen that Herbert effects a compression which changes in accordance with the number of turns that the screw is threaded when the proximal threads are engaging bone.