Günther Heimke

Knee arthoplasty: recently developed ceramics offer new solutions

Knee arthroplasty has reached a high degree of reliability. Many of the remaining long-term loosening cases are due to the polyethylene wear debris. The introduction of the alumina ceramic to polyethylene bearing couple had been a promising means for its reduction. The discussion of its applications in total knee replacements shows the possibilities and limitations of this bearing couple. The complete avoidance of the polyethylene debris can be expected from the articulation of alumina ceramic against itself, as long-term follow-up studies of total hip replacements clearly indicate. However, this combination can only be considered with a design of the prostheses in which any incongruity of the articulating surfaces can be excluded. As this necessitates the introduction of an additional component into the relatively narrow space between the femoral condyle and the tibia replacement, a further condition is the availability of a material which, whilst maintaining the tribological advantages of the alumina ceramic, has a much higher mechanical strength than the medical grade alumina ceramic available before. As the specifications of a recently developed modification of an alumina ceramic are in the desired range, with an improvement by a factor of two for nearly all relevant properties, a total knee replacement in which all articulating surfaces are in complete congruity and consist of this extremely wear resistant modified alumina ceramic could be designed, manufactured, and tentatively implanted in a human cadaver knee.

An investigation of skin deformation around percutaneous devices

According to the literature one of the primary failure models of percutaneous devices (PDs) in soft tissue implantation sites is that of mechanically induced trauma along the implant/tissue interface. In order to avoid such avulsion, subcutaneous flanges have been incorporated into PD designs to provide a better distribution of stresses along the implant/tissue interface. Tissue necrosis and inflammation were then observed to be most pronounced around the rim of the subcutaneous flange. It was the goal of this study to develop and test PDs with subcutaneous flanges of varying stiffnesses in order to allow the flange to distribute stresses into the surrounding tissues more evenly, thereby reducing the likelihood of failure due to avulsion. PDs were tested with flanges of constant thickness or varying linearly, reducing toward the flange rim. They were evaluated using an in-vitro testing method which was designed to simulate the situation of an implanted PD subjected to external loading. The results indicated that the stiffness characteristics of the subcutaneous flange had a substantial effect on the behavior of the devices under loading. A PD having a radial stiffness decrease would improve the likelihood of implant survival in the clinical situation due to the reduction of mechanically induced trauma along the implant/tissue interface.

Follow-up-Study—Based Wear Debris Reduction with Ceramic—Metal—Modular Hip Replacements

Total hip replacements (THRs) have reached higher levels of reliability during the past three decades. Recently, some clinics reported success rates of well above 90% after 10 yr, and more than 80% after 15 yr (1,2). The chief cause of the remaining failures after mid- and long-term service times, nearly independent of the implant material and design, is late aseptic loosening (3).