Stems are used in prosthetic joint implants to anchor the implant in a bone cavity. The bone is typically prepared to receive the stem by milling, drilling, or reaming the bone to form a cavity sixed and shaped for receiving the stem of the implant. The stem is driven into the prepared cavity of a bone, with its proximal end extending out of the cavity and carrying a joint bearing surface, which may be part of an assembly separately attached or coupled to the proximal stem. Many stems are fluted, i.e., they have distally extending longitudinal splines or ridges that cut their own force fit and provide greater stability and anchoring when the stem is inserted into the prepared cavity. Other stems may rely on cement fixation for a major portion of their length, but these also generally have at least some portion in the upper region that has a muting fit to the adjacent prepared bone surface. This upper portion must be placed in a precisely defined rotational orientation in order to seat properly when the stem is fully inserted.
Typically, once a fluted force-fitted stem is driven a certain distance into a prepared cavity, it becomes extremely difficult to rotate the stem to reorient the position of the upper portion. Instead, the stem must be withdrawn and reinserted, a process which can damage the bone and complicate, or increase the duration of, the operation. The stem may be aligned by marking the bone and the stem, and then matching or sighting along the respective marks to align the stem with respect to the bone as it is driven in. For example, to install an artificial hip prosthesis, the surgeon may measure the mechanical axis of the femur, perform a resection of the proximal femoral end, and then mark the plane of the mechanical axis on the periphery of the resected surface to indicate the required angular orientation of the ball joint component. The mark is then used to orient the stem as it is inserted. A drawback of this method is the imprecision of the alignment process. Because the stem seating surface is at the top of the stem, and the relevant bone markings lie at the distal end, the respective landmarks on the stem and bone are initially not in close proximity to each other. Thus, parallax and other problems associated with alignment by eye may result. Also, the stem may wander from its initial aligned position as it is being advanced into the prepared cavity.
The above-referenced patent application addresses this problem by providing prosthesis stem components having an axially-running flute or slot, or external splines, and providing a separate alignment body which nestles in the prepared cavity in a defined orientation, and keys to flute or slot such that the stem is held in the desired rotational plane as it is inserted. The alignment body or tool must be withdrawn before the stem is fully inserted, but after the stem is sufficiently well engaged to hold its own alignment. Various embodiments may control the degree of purchase of the stem and the phase where removal of the tool is effected, or may adapt those guides to different prosthesis systems. This may be done by employing a somewhat flexible polymer for the flute-engaging body, forming it with a jaw that flexes open for removal, by employing only a narrow key or guide, or by adjusting the length or diameter of the fluted region or length of slot and depth of bite to achieve dependable insertion depth and grip before tool removal is necessary.
However, there remains a need to provide tools and methods for aligning the stem of an orthopedic implant during insertion into a prepared bone cavity.