Abstract
Introduction
It is important to measure 2-dimensional (2D) polyethylene wear in total hip arthroplasty (THA) accurately in order to estimate value of wear performance. However, wear vector direction is usually defined in a coordinate system specific manner, which sometimes leads to confusion and makes it difficult to compare measures between techniques. We systematically evaluated the influence of four different coordinate system definitions for the measurement of 2D wear in acetabular cups form radiographs.
Materials and Methods
We performed 2D wear measurement of 152 hips that underwent THA using a 26mm CoCr femoral head with the same design of cementless acetabular shell between September 2003 and March 2005. All hips received either a CPE liner (76 hips, gamma sterilized at 25 kGy) or a XLPE liner (76hips, 10Mrad e-beam irradiation, EtO sterilized) randomly during the surgery. The average follow-up was 81.6±8.2 months.
Supine AP radiographs obtained postoperatively at 6 months and final follow-up were assessed for each femoral head penetration and angle using Roman v1.70 software (http://www.cookedbits.co.uk/roman/). The wear magnitude and angle between the two follow-ups was calculated using four coordinate system definitions:
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#1; the line tangent to both ischiums was defined as 0 degrees, with wear directed medially defined as positive and wear detected laterally defined as negative. (Martell; JBJS Vol79-A No11 p1635–41)
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#2; wear directed toward the liner (Zone 1 and Zone 2) defined as positive and away from it as negative (Zone 3 and Zone 4). (Wan; CORR No 449 p267–224)
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#3; wear vector magnitude angle ranging between 0 and 360, starting from the medial part of the line (x axis). (Greedink; JBJS Vol90-B p839–46)
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#4; wear vector between 0 and 180, directed towards the liner, was identical with coordinate #3. The vector between 180 and 360, directed away from the liner, was analyzed into the positive cosine magnitude. (Modified #3)
Results
There were 74 (48.7%) negative wear cases using coordinate definition #1 and 49 (32.2%) using coordinate definition #2. There were no statistically significant differences between average penetration rates for CPE and XLPE groups using the 4 coordinate definitions, except when negative values were included [Fig.1].
Discussion and Conclusions
Wan et al reported nearly 50% of 2D wear measurement showed negative values as a sequel to error in radiographic quality and measurement. In some studies, negative values were eliminated from the average penetration, or only wear vector magnitude was used and reported as the median of penetration. However, negative values of wear result simply from differences of coordinate system definition. For example, seven cases in the CPE group showed small focal osteolysis at final follow up in this study. All osteolysis cases were detected as negative wear using coordinate definition #1, and as positive wear with the other coordinate definitions [Fig.2]. Our coordinate system definition (#4) is more sensible when the wear vector is directed partially outward because it eliminates negative values and correctly reports the head displacement as wear.
