Abstract
Large heads offer substantial advantages over small ones in hip arthroplasty, as they are far less likely to dislocate. This feature is of particular benefit in very dysplastic females who often have a degree of joint laxity making dislocation a real possibility. Large metal heads have a range of problems, so registries report that they are now being substituted by large ceramic heads, typically reducing in diameter by 15% or more from the native size.
All current designs of the femoral ball heads, whether for resurfacing of replacement share a unique design characteristic: a subtended angle of 120° defining the proportion of a sphere that the head represents. A novel design has recently been proposed that might reduce conflict between the femoral ball head rim and the iliopsoas tendon. This paper explains the problem of iliopsoas impingement on femoral heads of native diameter, and the consequences.
Material and Methods
Using MRI, we measured the contact area of the Iliopsoas tendon on the femoral head in sagittal reconstruction of 20 hips with symptoms of FAI. We also measured the Articular extent of the femoral head on 40 normal hips and 10 dysplastic hips. We then performed virtual hip resurfacing on normal and dysplastic type hips, attempting to avoid the overhang of the rim inferomedially.
Results
The contact area of the Iliopsoas tendon on the femoral head in extension is well visualized (Figure 1). The femoral head articular surface has a subtended angle of 120° anteriorly and posteriorly, but only of 100° medially. Virtual surgery in a femoral head of a dysplastic hip showed that when the femoral head is resurfaced with an anatomic sized component, the femoral ball head has a 20° skirt of metal protruding medially where iliopsoas articulates (figure 2). Reducing this by 15%, (eg to put a 40mm ball head onto a hip that had a 46mm femoral diameter), completely avoids any chance of iliopsoas tendon using the femoral head as a fulcrum. MRI of a dysplastic hip with a 40mm ball shows that iliopsoas impingement is hard to substantiate (figure 3).
Discussion
The excessive extent of the femoral components of anatomic proportions may contribute to the pain felt by many following their use. However, the 15% reduction in head size undertaken in total hip arthroplasty completely defunctions the femoral head as a fulcrum. Groin pain is a real issue following hip arthroplasty, but edge loading of the iliopsoas probably only occurs in hip resurfacing or large head metal on metal hip replacement. This may provide another explanation for these bearings are symptomatic in general. Hip resurfacing needs to be undertaken with great care in dysplasia to avoid iliopsoas impingement. In ceramic bearing hip replacement, where ball heads are typically reduced by 15% or more from the native diameter, iliopsoas edge loading on the head rim is unlikely.
