Chronic stress tolerance levels for human articular cartilage: two nonuniform contact models applied to long-term follow-up of CDH.

Two computer models of nonuniform contact stress on the articular surface of the human hip were used to study the relationship between chronically excessive articular cartilage contact stress and long-term clinical outcome in a series of patients with congenital dislocation of the hip (CDH). The analyzed database consisted of 409 stylus digitized radiographs from 83 patients with unilateral CDH, who had been treated by closed reduction, and whose average follow-up time was 29.2 y. The first model (nonuniform Legal) involved a three-dimensional contact stress distribution function whose pole was coincident with the resultant force acting through the hip, and which acted over a contact area whose borders were determined soley by bony landmarks. In the second model (Brinckmann), the direction of the pole of the contact stress distribution function was initially unknown; one border of the contact region was determined by radiographic landmarks, while the other border depended upon the pole of the iteratively determined contact stress distribution function. In both models, the contact stress distributions were converted to area engagement histograms, corresponding to the fractional areas of cartilage experiencing specific ranges of stress (0.5 MPa increments). These histograms were integrated over time to calculate a cumulative contact stress overdose, which was then compared to clinical outcome. Reasonable correlations (Spearman rho = 0.63-0.66) with patient outcomes were obtained for optimally chosen damage thresholds, although these thresholds were appreciably different (2.0 versus 4.5 MPa) due to the respective modelling assumptions.