Cartilage contact pressure elevations in dysplastic hips: a chronic overload model

BACKGROUND Developmental dysplasia of the hip (DDH) is a condition in which bone growth irregularities subject articular cartilage to higher mechanical stresses, increase susceptibility to subluxation, and elevate the risk of early osteoarthritis. Study objectives were to calculate three-dimensional cartilage contact stresses and to examine increases of accumulated pressure exposure over a gait cycle that may initiate the osteoarthritic process in the human hip, in the absence of trauma or surgical intervention. METHODS Patient-specific, non-linear, contact finite element models, constructed from computed tomography arthrograms using a custom-built meshing program, were subjected to normal gait cycle loads. RESULTS Peak contact pressures for dysplastic and asymptomatic hips ranged from 3.56 - 9.88 MPa. Spatially discriminatory cumulative contact pressures ranged from 2.45 - 6.62 MPa per gait cycle. Chronic over-pressure doses, for 2 million cycles per year over 20 years, ranged from 0.463 - 5.85 MPa-years using a 2-MPa damage threshold. CONCLUSION There were significant differences between the normal control and the asymptomatic hips, and a trend towards significance between the asymptomatic and symptomatic hips of patients afflicted with developmental dysplasia of the hip. The magnitudes of peak cumulative contact pressure differed between apposed articular surfaces. Bone irregularities caused localized pressure elevations and an upward trend between chronic over-pressure exposure and increasing Severin classification.