Measurement of polyethylene wear in metal-backed acetabular cups. I. Three-dimensional technique.

A new technique, the 3-dimensional technique, has been developed. Using a digitzer, points taken from anteroposterior and lateral radiographs are input into a personal computer. A 3-dimensional solid model of the prosthesis is created using custom software. Tilt and anteversion of the acetabular cup are measured, and the model is rotated to a standard frontal view, allowing measurement of femoral head displacement from the center of the acetabular cup. Comparing serial radiographs of the same patient gives the direction and distance of femoral head displacement over time, and this value is used to calculate the minimum volume of polyethylene debris generated. The 3-dimensional technique has been validated by construction of a precision acrylic phantom, and milling of a metal-backed acetabular cup to simulate polyethylene wear. Three-dimensional measurement of femoral head displacement using this technique has an accuracy of +/- 0.15 mm, and volume calculations are within 8% of the true amount of polyethylene removed from the cup. The 3-dimensional technique gives previously unavailable information, and can be used for clinical evaluation of polyethylene wear and evaluation of new prosthetic designs and bearing surfaces.