ALTERED LOAD HISTORY AFFECTS PERIPROSTHETIC BONE LOSS FOLLOWING CEMENTLESS TOTAL HIP-ARTHROPLASTY

Dual energy x-ray absorptiometry was used to measure periprosthetic, d istal femoral, and proximal tibial bone mass in the affected and contr alateral limb of eight patients 10 years after unilateral total hip ar throplasty with a cementless, porous-coated titanium alloy femoral ste m. Gait analyses to assess the presence of asymmetries in loading of t he lower extremities were also performed 10 year postoperatively. The patients had excellent clinical results and no other significant lower extremity pathology. On the basis of comparison of the affected and u naffected proximal femora, bone loss adjacent to the proximal medial a spect of the femoral stem was determined to be 34% (p < 0.001). Howeve r, the patients also had 16% less bone in the ipsilateral proximal tib ia (p = 0.003) and 15% less bone in the ipsilateral femur 3 cm distal to the prosthesis (p = 0.007) compared with the contralateral limb. Wh en normalized to the asymmetry in tibial bone mineral content, the est imated proximal medial periprosthetic bone loss was still statisticall y significant, but the magnitude was reduced from 34 to 17% (p = 0.009 ). The gait analyses indicated that several measures that influence th e loads at the hip and knee joints were reduced in the involved limb c ompared with the contralateral limb. Furthermore, the bilateral differ ence in the vertical component of the external force acting on the pro ximal tibia was correlated with the bilateral difference in tibial bon e mineral content (r = 0.80, p = 0.02). These data suggest that two me chanical factors, the local stress-shielding effect of the prosthesis and the global effect of decreased loading of the limb, can both make significant contributions to periprosthetic bone loss. It is apparent that the magnitude of the periprosthetic bone loss related to stress-s hielding has been overestimated by as much as 50% in retrospective stu dies.