Accuracy of templating the acetabular cup size using digital films

Purpose To determine the reliability and accuracy of templating the acetabular cup, using digital Xray films in patients undergoing total hip replacement. Methods Patients undergoing total hip replacement were subjected to preoperative pelvic digital xrays. A 3 cm spherical ball was placed over the greater trochanter as a digital marker to gauge the magnification of the digital film. The acetate template provided by the implant companies was superimposed over the digitally magnified X-ray films on the computer screen and size of the cup( Y) determined. The size of the spherical ball on the same digital films was also measured (X) mm using the scale provided on the acetate templates. The actual size of the acetabular cup to be used was determined using the formula 30YX. Results The templating of 40 acetabular cups were done by 2 independent observers on two occasions. There was good intraobserver and interoberserver reliability with intraclass correlation coefficient values greater than 0.8. Using this technique of templating, the actual size of the acetabular cup was predicted to the same size in 75 of cases to within 1 size in 97, and to within 2 sizes in 100.Conclusion It is possible to accurately and reliably predict the size of the acetabular cup, using acetate templates on digital films, using the technique described.