Correlation of computed finite element stresses to bone density after remodeling around cementless femoral implants.

Fundamental to the development of a durable, uncemented femoral component is an understanding of the remodeling process that goes on after implantation. Predicting the bone remodeling that results from the use of a given hip implant would facilitate the design of a prosthesis that would optimize bone adaptation. This report combines the results of finite element stress analysis of the AML prosthesis implanted in vitro into a proximal femur with quantitative bone mineral density measured in vivo in the medial and lateral aspects of human femora at periods after implantation. Unimplanted femora were also analyzed for comparison purposes. Bone density measurements were obtained using dual energy x-ray absorptiometry. Absolute values of the maximum principal stress and maximum shear stress calculated in the femur at the time of implantation accurately predict bone density resulting from remodeling caused by the prosthesis. The calculated initial strain was not found to correlate with resultant bone density. These findings suggest that the results of stress analyses using three dimensional models of femora implanted in vitro can predict bone remodeling around prostheses and may be used to quantitate appropriate design criteria for total hip replacements.

Dual-energy X-ray absorptiometry measurement of bone mineral density around porous-coated cementless femoral implants. Methods and preliminary results.

The measurement of bone mineral density in defined areas around metal implants has improved with the development of dual-energy X-ray absorptiometry. We used this technique to compare the bone mineral density adjacent to metal cementless femoral implants with that of identical regions of bone in normal proximal femora. We studied the anteroposterior views only of 72 femora which contained total hip implants and 34 non-operated femora. We compared the regional bone mineral density of bone adjacent to proximally porous-coated and distally porous-coated implants of one design, to measure the relative differences in the remodelling changes induced by different amounts of porous coating. We also measured differences in bone density with time and with variations in implant size (and therefore stiffness). The greatest decrease in bone mineral density (34.8%) occurred in the most proximal 1 cm of the medial femoral cortex around relatively stiff, extensively porous-coated implants. The next most severe decrease (20% to 25%) was in the next most proximal 6 cm of the medial femoral cortex. Small, progressive decreases in bone mineral density continued for five to seven years after implantation.

Catastrophic wear of tibial polyethylene inserts.

A number of factors play an important role in the wear-resistance of tibial polyethylene inserts. Among these are manufacturing processes that adversely affect the wear-resistance of polyethylene (such as heat treatments to the articular surface or gamma irradiation used for sterilization), tibio-femoral articular geometry, polyethylene thickness, knee alignment, femoral-component-bearing surface material, modularity of the tibial inserts and tibial trays, and quality of the polyethylene itself. The authors report an unusually high rate of failure by wear of tibial polyethylene inserts from a series of 176 Porous Coated Anatomic (PCA) knees in which there were eight revisions (4.5% of the series) performed for tibial polyethylene wear at an average of 60 months. Nine additional knees (5.1%) had thinning of greater than 30% of the initial polyethylene thickness. Four of the unsuccessful knees revealed areas of osteolysis filled with membranes containing large amounts of particulate polyethylene. In addition to the 176 knees from a series from a Los Angeles university, the cases of five other knees in four patients who came for treatment from outside hospitals with full-thickness wear of the tibial polyethylene are discussed. One of these five knees was a cementless PCA knee that developed massive osteolysis in response to the particulate polyethylene debris.