Calcium sulphate is now a proven adjunct to the replenishment of bone stock in joint replacement surgery. Alone and as a composite, it has been used successfully for many years in both dental and orthopaedic applications. OsteoSet (Wright Medical Technology), a processed, purified material, has been used as a bone void filler in 51 revision total hip arthroplasty (THA) procedures. Follow-up of these cases ranges from 3 to 48 months. Radiographs show that the calcium sulphate has disappeared in all cases. In all but three patients, all of whom had failure of the acetabular component or infection, the calcium sulphate has been replaced with what appears to be trabeculated cancellous bone. Clinical results for cases that did not have mechanical failure or infection are indistinguishable from any revision THA in which the acetabular component is well fixed. Implantation of the calcium sulphate pellets calls for preparation of a well vascularised bed. The pellets are placed in such a way that load is not transferred to them from the implanted acetabular component. Rather, the load should be transferred from the acetabular component directly to host bone. Postoperatively, load bearing is limited for at least eight weeks and for longer of the quality of the supporting bone is poor.
For many years, it has been taught that the human knee is a ‘hinge’ joint and that the motion of the knee is controlled by a ‘four-bar link’. This classic view of the motions of the knee suggests that there is a prescribed path for the knee as it proceeds from extension to flexion and flexion to extension. This prescribed motion includes ‘rollback’, a term used for the progressive posterior displacement of the femur on the tibia as the knee moves from extension to flexion, Most of the total knee prostheses available today have been designed to permit the movements that are required by this model of knee motion. The design features necessary to permit this motion are a lack of constraint between the tibial and femoral components, and a ‘J’ curve of the posterior part of the femoral component such that the radius of curvature is smaller on the posterior portion of the component than on the distal part. Studies of the anatomy of the knee date back to the 1800s, before radiological studies were possible. Radiological evidence does not support the four-bar link and rollback theories or indicate that a ‘J’ curve is necessary. Rather, radiographs suggest that the knee is more of a ball-in-socket joint on the medial side with little or no rollback in normal function. Three-dimensional studies of the moving human knee both in vitro and in vivo also demonstrated that the knee joint moves as a ball-in-socket joint on the medial side, and that the lateral side displaces posteriorly or anteriorly as necessary to accommodate the rotational position of the tibia relative to the femur. These kinematic findings have led to the design of a pros-thesis that mimics the normal knee. The femoral prosthesis has a single radius of curvature to each condyle both in the sagittal and coronal planes. The mating tibial component has an exactly conforming geometry on the medial side leading to ball-in-socket type of kinematics. The lateral side of the tibial component allows anterior or posterior displacement of the femur, mining the normal changes that take place with internal and external rotation. Initial clinical results total knee arthroplasty procedures performed with this prosthesis are just passing the three-year follow-up interval. There have been no reports of catastrophic problems, and surgeons have been pleased with the stability, the rapidity with which function is regained, and the excellent range of motion following arthroplasty. Patients who have a more traditional total knee arthroplasty in one knee and the medial pivot prosthesis in the other prefer the medial pivot because of the feeling of stability.
