Introduction Dedicated software has been developed to allow computerised pre-operative planning for joint replacements where digital x-rays of known magnification are placed on a computer screen and templates are matched to produce an accurate plan. Improving efficiency, reducing error and improving patient outcomes are the goals.

Methods Two methods for determining magnification of x-rays have been considered. 1: a metal marker of known size is placed on the limb in the same plane as the joint and the software matches the template to the x-ray. Several problems arise including unrecognizable markers in the obese or when taking full pelvis x-ray. 2: a computer tomogram scout film provides a digital format, accurate magnification, a film of adequate quality and the capacity to plan on the normal hip in unilateral disease. The new software was developed using digital radiology to provide; certainty of patient identification, tools for measurement, tools for drawing and to allow linear and angular movement of components of the image. Risk management was undertaken. Data concerning component details completed the program.

Results The phase one trial demonstrated that the tool achieved the goals set. A digital image could be matched to implant component data. The tools allowed; measurement of relevant anatomy, consideration of leg length difference, and angular deformity, identification of landmarks such as center of rotation of the socket and offset, and comparison between the normal and pathological sides. Components could be chosen and errors detected, such as incompatible components or wrong side, and an order generated for forwarding to the supplier. Choice of stem was always within one size of pre-operative templating. Choice of socket was within two sizes.

Conclusions The explanation for discrepancy may be the intra-operative desire to judge size according to “feel” at surgery. A surgeon may wish to demonstrate accuracy by “forcing” intra-operative decision towards verifying pre-operative planning. What is surprisingly is that surgeons do not have uniform views as to the “ideal” socket position or stem position.

In relation to the conduct of this study, one or more the authors have received, or are likely to receive direct material benefits.