Despite recent diagnostic advances with MRI, femoral head osteonecrosis (ON) remains an important unsolved problem from the standpoint of surgical management. About 30,000 new cases present annually, with ON patients making up 10% to 20% of most total hip series (usually second only to osteoarthrosis). Because of their relative youth, as well as other factors, osteonecrosis patients have an alarming rate of prosthesis loosening. Of the limited alternatives for head-preserving surgical intervention, the clinical results from vascularized fibular grafting are among the most promising. As currently practiced, however, the crucial issue of maximizing the structural efficacy of the construct has been given relatively little attention. Recent data from the applicant laboratory and elsewhere indicate that the details of graft placement can be a major determinant of whether or not the procedure achieves its goal of substantially lowering stresses in the lesion. Since lesion involvement patterns and their associated structural compromise are highly variable, it is difficult to know intuitively what grafting position is most desirable for a given patient. In recent NIH-supported work, the applicants developed and validated a patient-specific finite element model that allows interactive study of the three-dimensional positioning of a bone graft in a necrotic femoral head. Using this model, they propose to conduct a series of clinically- grounded finite element stress analysis studies which they believe will result in practical guidelines for improved graft surgery. In multi- center prospective and retrospective series (Aim 1), they will build patient-specific finite element models of both ungrafted and grafted necrotic femoral heads, and will examine how clinical outcome correlates with the stress-to-strength ratio (SSR). To test whether or not contemporary surgical grafting practice consistently achieves reasonably maximal structural reinforcement (Aim 2), they will compare SSR's for surgically obtained graft positioning, versus those for ostensibly improved positioning identified intuitively and by formal mathematical optimization. Finally, to identify practical surgical guidelines which can result in optimal or near-optimal graft placement (Aim 3), they will conduct parametric studies of graft insertion depth, tilt, tip shape, circumferential orientation, and other factors, and they will undertake evaluations and refinements of a new drill guide designed to facilitate accurate graft placement intraoperatively. At the conclusion of the study, the applicants expect to have elucidated the major factors governing structurally efficacious cortical bone grafting in osteonecrosis.
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