Abstract
The relationship between excessive articular contact pressure, aseptic necrosis, and the longโterm outcome with unilateral congenital dislocation of the hip (CDH) was studied in a series of 84 patients treated by closed reduction and followed for an average of 29.2 years. Contact stress was estimated from archived radiographs taken at the time of maturity and at several followโup visits. At a recent review, each patient was rated both clinically for pain and function and radiographically for deformity, degeneration, and aseptic necrosis. For each of 431 films, articular contact stress (force/area) was estimated mathematically, based upon a frontal plane equilibrium (force) analysis and a landmarkโbased inference of threeโdimensional head surface (area). Good correlation with final deformity (Spearman ฯ = 0.78) was obtained when the hips were ranked in terms of a new cumulative overpressure index Pฬ, defined as a timeโpressure product involving years of pressure exposure beyond a 2 MPa pressure damage level. An unsatisfactory outcome occurred in 90.4% of the hips experiencing Pฬ > 10 MPaโyears (most of which had aseptic necrosis involvement), whereas the outcome was satisfactory in 80.9% of hips with Pฬ < 10 MPaโyears.