CHRONIC STRESS TOLERANCE LEVELS FOR HUMAN ARTICULAR-CARTILAGE - 2 NONUNIFORM CONTACT MODELS APPLIED TO LONG-TERM FOLLOW-UP OF CDH

Two computer models of nonuniform-contact stress on the articular surf ace of the human hip were used to study the relationship between chron ically excessive articular cartilage contact stress and long-term clin ical outcome in a series of patients with congenital dislocation of th e hip (CDH). The analyzed database consisted of 409 stylus digitized r adiographs 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 st ress distribution function whose pole was coincident with the resultan t force acting through the hip, and which acted over a contact area wh ose borders were determined soley by bony landmarks. In the second mod el (Brinckmann), the direction of the pole of the contact stress distr ibution function was initially unknown; one bother of the contact regi on was determined by radiographic landmarks, while the other border de pended upon the pole of the iteratively determined contact stress dist ribution function. In both models, the contact stress distributions we re converted to area engagement histograms, corresponding to the fract ional areas of cartilage experiencing specific ranges of stress (0.5 M Pa increments). These histograms were integrated over time to calculat e a cumulative contact stress overdose, which was then compared to cli nical outcome. Reasonable correlations (Spearman rho=0.63-0.66) with p atient outcomes were obtained for optimally chosen damage thresholds, although these thresholds were appreciably different (2.0 versus 4.5 M Pa) due to the respective modelling assumptions.