Evaluation of X-ray, Acetabular Guides and CT in THR

Total hip replacement is one of the most successful surgical procedures of modern times, with over 80,000 performed each year in the UK. The vast majority of patients experience dramatic pain relief and improvement in function for many years. Despite this, however, there remains a significant risk of complications, including dislocation, leg length discrepancy, squeaking, and premature wear and failure of the implant. It is known that such complications are more likely to occur if the acetabular component is incorrectly positioned during the surgery. Up to 5% of all primary hip replacements need to be revised within the first 10 years, and in many cases malposition of the acetabular (hip socket) component is implicated in the early failure.

The Corin OPS™ offers such a custom-made acetabular alignment guide. Prior to the patient's surgery, a CT scan of the patient's pelvis \& legs is performed, and the images are used to produce a 3D computer model of the patient as they stand with a "virtual" hip replacement in place. Four additional X-rays of the pelvis and lumbar spine are also taken, with the patient adopting various predetermined "functional" positions (e.g. sitting in a chair, about to stand up). From these X-rays, the changes in the pelvic and femoral orientations can be measured for these functional positions. This data is then used to drive a simulation of the movement of the 3D computer model. The orientation of the components of the virtual hip replacement are then adjusted to optimise the biomechanical function of the joint. Once the proposed implant positions have been reviewed and approved by the surgeon, a custom-made acetabular orientation guide is 3-D printed and sterilised. During the surgery, this guide is fitted into the patient's acetabulum prior to implanting the components. Using a simple system of two sterile laser pointers, the orientation indicated by the guide can then be reproduced when the definitive acetabular component is implanted. There is no published data, however, on the accuracy of acetabular component positioning using this particular method. The standard of care in the UK for acetabular component positioning is the "freehand technique", where the surgeon is to be guided by a combination of the visible anatomical landmarks within the surgical field, and the wider environment of the operating theatre. It is important that the impact of this acetabular guide on acetabular component positioning be assessed in order to determine whether this intervention is likely to improve patient outcomes and reduce the risk of complications of total hip replacement.