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

Because polyethylene wear adversely affects the outcome of total hip arthro plasty, reliable techniques for in vivo radiographic evaluation of femoral head penetration into the polyethylene liner are needed. With the recent de velopment 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 tw o-dimensional and the three-dimensional penetration of the femoral head int o the polyethylene liner in 202 hips implanted with porous coated acetabula r components. A comparison of two-dimensional and three-dimensional measure ments showed that, on average, the mean amount of three-dimensional head pe netration was larger than the mean amount of two-dimensional head penetrati on. However, linear regression analysis showed that the two measurements we re highly correlated (r(2) = 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 subs et of patients (5%) for whom three-dimensional radiographic penetration mea surements were three times greater than corresponding two-dimensional radio graphic penetration measurements. No common risk factor among this small su bset of patients was identified. The current study provides practical infor mation for the orthopaedic surgeon trying to assess polyethylene wear in hi p replacement cases. It shows that for most patients head penetration can b e 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. Fo r these cases, accurate analysis of head penetration requires three-dimensi onal analysis using anteroposterior and lateral radiographs.