Two-dimensional versus three-dimensional radiographic measurements of polyethylene wear

Because polyethylene wear adversely affects the outcome of total hip arthroplasty, reliable techniques for in vivo radiographic evaluation of femoral head penetration into the polyethylene liner are needed. With the recent development of software to measure three-dimensional femoral head penetration, the adequacy of two-dimensional head penetration measurements to estimate a three-dimensional process has been questioned. Thus, the purpose of the current study was to compare directly two-dimensional and three-dimensional radiographic measurements of femoral head penetration. Using two computer assisted radiographic techniques, the authors of this study measured the two-dimensional and the three-dimensional penetration of the femoral head into the polyethylene liner in 202 hips implanted with porous coated acetabular components. A comparison of two-dimensional and three-dimensional measurements showed that, on average, the mean amount of three-dimensional head penetration was larger than the mean amount of two-dimensional head penetration. However, linear regression analysis showed that the two measurements were highly correlated (r2 = 0.87, slope = 0.99, intercept = 0.08 mm). For most patients (95%), two-dimensional and three-dimensional measurements of femoral head penetration were nearly equal. However, there was a small subset of patients (5%) for whom three-dimensional radiographic penetration measurements were three times greater than corresponding two-dimensional radiographic penetration measurements. No common risk factor among this small subset of patients was identified. The current study provides practical information for the orthopaedic surgeon trying to assess polyethylene wear in hip replacement cases. It shows that for most patients head penetration can be measured sufficiently from anteroposterior radiographs alone. However, in a small percentage of patients (5% in the current study) the femoral head also moves perpendicular to the plane of the anteroposterior radiograph. For these cases, accurate analysis of head penetration requires three-dimensional analysis using anteroposterior and lateral radiographs.