Assessment of coxarthritis risk with dimensionless biomechanical parameters

Unfavourable distribution of contact stress over the load bearing area is considered a risk factor for early coxarthritis and it is of interest to outline respective biomechanical parameters for its prediction. The purpose of the work was to develop a transparent mathematical model which can be used to assess contact stress in the hip from imaged structures of pelvis and proximal femora, in large population studies and in clinical practice.

We upgraded a previously validated three-dimensional mathematical model of the human hip in the one-legged stance HIPSTRESS by introducing parameters independent from the size of the structures in the images. We validated a new parameter - dimensionless peak stress normalized by the body weight and by the radius of the femoral head (pmaxr²/WB) on the population of 172 hips that were in the childhood subjected to the Perthes disease and exhibited increased proportion of dysplastic hips.

The dimensionless parameter pmaxr²/WB exhibited smaller number of indecisive cases of hip dysplasia predicted by the model than the previously used parameter pmax/WB (6% vs. 81%, respectively). A threshold for an increased risk of early coxarthritis development by the HIPSTRESS parameter H = pmaxr²/WB was found to be 2.

We proposed a dimensionless peak stress on the load bearing area with the border value of 2 as a decisive parameter over which hips are at risk for early development of degenerative processes and presented a method for determination of biomechanical parameters with the use of nomogram.