Clinical assessment of tibial polyethylene thickness: Comparison of radiographic measurements with as-implanted and as-retrieved thicknesses

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Abstract

Sixty-six posterior cruciate-retaining inserts of one fixed-bearing design were revised because of wear or osteolysis. Tibial polyethylene thickness was estimated from standard anteroposterior radiographs and compared with the original or final minimum thicknesses. The most accurate method involved measuring the shortest distance from each femoral condyle to a transverse line through the middle of the baseplate’s superior surface. On radiographs acquired 6 weeks after arthroplasty, the mean error was −0.1 mm (underestimate) ± 0.6 mm (standard deviation), with 72% of the measurements within 0.5 mm of the original minimum thickness and 87% within 1.0 mm. The method was less accurate for radiographs made before revision, for which the mean error was 0.6 mm (overestimate) ± 1.0 mm. In this study, 41% and 70% of the prerevision measurements were accurate to within 0.5 and 1.0 mm, respectively.

Section snippets

Methods

We studied 66 posterior cruciate-retaining Anatomic Modular Knee (DePuy, Johnson & Johnson, Warsaw, IN) primary total knee arthroplasties that required revision of the tibial component for wear or osteolysis (Fig. 1). Single-leg standing anteroposterior radiographs (35 × 43 cm) were taken at 6 weeks after arthroplasty and again at less than 7 months (mean, 3 months) before revision.

The minimum polyethylene thickness of each compartment was measured on each radiograph via 2 methods and adjusted

Measurement reproducibility

Radiographic polyethylene thickness measurements were made to the nearest 0.1 mm. Mean differences between the first and second measurement within the same compartment were 0.1 ± 0.1 mm for each method. Caliper measurements of explanted polyethylene thickness were made to the nearest 0.01 mm. The mean difference between each pair of caliper observations was 0.08 ± 0.08 mm.

Comparison of methods

The metal-to-middle method proved to be significantly more accurate than the metal-to-metal method on the 6-week radiographs

Discussion

In this investigation, the metal-to-middle method proved to be a more accurate approach for estimating tibial polyethylene thickness than the more commonly used metal-to-metal method. The metal-to-middle method is more accurate because its accuracy is less compromised by anteroposterior tilt of the baseplate shadow. However, medially eccentric wear with the studied design closed the gap between the accuracy of the 2 methods. As the magnitude of medial compartment wear increased, the point of

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No benefits or funds were received in support of this study.

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