The effect of necrotic lesion size and rotational degree on the stress reduction in transtrochanteric rotational osteotomy for femoral head osteonecrosis--a three-dimensional finite-element simulation.

Background: The prospect for success in the efficacy of osteotomy for precollapse stage of femoral head osteonecrosis depends on the ability to predict reliably the stress changes derived from specific osteotomies. A three-dimensional finite-element analysis was thus designed to compute necrotic femoral head stress changes with different extent of necrocrosis that accompany anterior or posterior rotational osteotomies.

Method: Computed tomography images of a standard composite femur were used to create the three-dimensional finite-element intact femur model. Based on the intact model, 27 models simulating three different levels of necrotic region together with nine different rotational osteotomies were created. The von Mises stress distributions of each model were analyzed and compared for a loading condition simulating single-legged stance.

Findings: (1) The stress reduction in anterior rotational osteotomy is more effective as compared to that of the posterior rotational osteotomy for various necrotic lesion sizes. (2) Von Mises stress on the necrotic zone decreased with increasing rotational angle. The decreasing rate was higher for the femoral head with a narrow lesion. (3) Femoral head with a wider necrotic lesion had a higher risk for developing collapse due to high local stress on the surface of necrotic region; whereas the necrotic region tended to expand in size instead of collapse for femoral head with a narrow lesion due to high local stress on the interface between necrotic region and healthy bone.

Interpretation: Transtrochanteric rotational osteotomy is a technically demanding procedure and associated with high complication risks, a more scrupulous planning including the finite-element analysis should be considered before doing surgery in clinical subjects.