Total knee arthroplasty involves implantation of new articulating surfaces for the tibia, femur and patella. The most common technique for providing new articulating surfaces for the tibia involves resecting an upper portion of the tibia then attaching a prosthetic implant to the tibia over the resected surface. A typical tibial implant includes a tibial tray adapted to abut the resected surface of the tibia and a bearing portion which includes the new tibial articulating surfaces. The tibial tray has a shape that is adapted to generally approximate the shape of the resected surface of the tibia so that the implant generally aligns with the resected surface when the tray is properly attached to the tibia.
The tibial implant may also include a stem and a keel. The keel extends from an inferior surface of the tray into the patient's bone so as to provide both fixation and positional stability. The stem extends from the inferior surface of the tray or from an inferior surface of the keel into the central canal of the patient's tibia.
In many tibial prostheses the stem and keel extend from a central position in the base. For some patients however the central canal of the tibia is not located centrally with respect to the tibial articulating surfaces. With these patients, orthopedic surgeons must undertake additional positioning steps to ensure that the tibial prosthesis is properly seated and that it articulates properly with the femoral prosthesis. This problem can be particularly acute in revision surgery where an existing tibial prosthesis is being removed and a new tibial prosthesis is being implanted. When the original tibial prosthesis is removed in revision surgery, often portions of the tibia to which the prostheses are attached become damaged during the removal process or lack viable bony support for the replacement tibial prosthesis. It therefore becomes necessary to attach the new prosthesis farther down into the central canal of the patient's tibia than the original prosthesis. In revision surgery a long stem is typically used with the tibial prosthesis to ensure good distal fixation of the prosthesis within the canal. The use of long stems however exacerbates positioning problems for patients whose anatomic canal is not centrally located with regard to the tibial articulating surfaces, potentially forcing the surgeon to compromise coverage of the resected tibia.
Accordingly, a need exists for tibial prostheses with keel and stem portions that can be variably positioned in the medial-lateral direction in order to provide the best possible fit for tibial prostheses for patients whose anatomic canal deviates from standard locations with respect to the tibial articulating surfaces, particularly for use in revision surgery. U.S. Pat. No. 5,271,737 discloses a tibial prosthesis with an offset stem. This stem however is fixed in position and is not adjustable. Accordingly, it won't provide the best possible fit for patients whose tibial canal deviates from the dimensions of this tibial prosthesis. There still exists a need for a tibial prosthesis with an adjustable keel that will allow surgeons the flexibility to treat patients whose tibial canal may not only be not centered but which might be offset in the medial-lateral direction by an unknown amount.