Mechanical strength of the femur following resurfacing and conventional total hip replacement procedures.

A series of static and fatigue loading tests was performed on fresh frozen cadaver femora which had undergone THARIES surface replacement and conventional T-28 stem-type replacement. The strength, stiffness, and energy absorbed during static loading tests were comparable for femora containing the 2 designs. Superior notching of the femoral neck during valgus reaming reduced the ultimate static fracture load in 2 specimens. The large scatter in fatigue life observed was most likely due to variations in bone quality and geometry, techniques of reaming and positioning of the prosthetic components, and the initial selection of the cyclic load for each individual specimen. Significant differences in cycles to failure were observed between paired specimens tested under similar conditions. Based upon tests performed upon surface replacement specimens that did not fail in fatigue, and which were subsequently tested to static failure, it appears that a cyclic load roughly one-fourth of ultimate can be tolerated without fatigue fracture.