Ideally the hip arthroplasty should not be subject to bony or prosthetic impingement, in order to minimise complications and optimise outcomes. Modern 3d planning permits pre-operative simulation of the movements of the planned hip arthroplasty to check for such impingement. For this to be meaningful, however, it is necessary to know the range of movement (ROM) that should be simulated. Arbitrary “normal” values for hip ROM are of limited value in such simulations: it is well known that hip ROM is individualised for each patient. We have therefore developed a method to determine this individualised ROM using CT scans.

CT scans were performed on 14 cadaveric hips, and the images were segmented to create 3d virtual models. Using Matlab software, each virtual hip was moved in all potential directions to the point of bony impingement, thus defining an individualised impingement-free 3d ROM envelope. This was then compared with the actual ROM as directly measured from each cadaver using a high-resolution motion capture system.

For each hip, the ROM envelope free of bony impingement could be described from the CT and represented as a 3d shape. As expected, the directly measured ROM from the cadaver study for each hip was smaller than the CT-based prediction, owing to the presence of constraining soft tissues. However, for movements associated with hip dislocation (such as flexion with internal rotation), the cadaver measurements matched the CT prediction, to within 10°.

It is possible to determine an individual's range of clinically important hip movements from a CT scan. This method could therefore be used to create truly personalised movement simulation as part of pre-operative 3d surgical planning.

Background

Over 10% of total hip arthroplasty (THA) surgeries performed in England and Wales are revision procedures¹. Malorientation of the acetabular component in THA may contribute to premature failure due to mechanisms such as edge loading and prosthetic impingement. It is known that the pelvis flexes and extends during activities of daily living (ADLs), and excessive pelvic motion can contribute to functional acetabular malorientation. Preoperative radiographs can be performed to measure changes in pelvic tilt during ADLs to identify high risk individuals and inform surgical decision making. However, radiographs require time-consuming radiation exposure, and are unable to provide truly dynamic 3-dimensional analysis. The purpose of this study was to develop and evaluate a motion capture method using inertial measurement units (IMUs). This would provide a rapid, non-invasive analysis of pelvic tilt which could be used to support surgical planning.

Background

It is not always clear why some patients experience recurrent dislocation following total hip arthroplasty (THA). In order to plan appropriate revision surgery for such patients, however, it is important to understand the specific biomechanical basis for the dislocation. We have developed a novel method to analyse the biomechanical profile of the THA, specifically to identify edge loading and prosthetic impingement, taking into account spinopelvic mobility. In this study we compare the results of this analysis in THA patients with and without recurrent dislocation.

Background

Over 10% of total hip arthroplasty (THA) surgeries performed in England and Wales are revision procedures¹. Malorientation of the acetabular component in THA may contribute to premature failure. Yet with increasingly younger populations receiving THA surgery (through higher incidences of obesity) and longer life expectancy in general, the lifetime of an implant needs to increase to avoid a rapid increase in revision surgery in the future.

The Evaluation of X-ray, Acetabular Guides and Computerised Tomography in THA (EXACT) trial is assessing the pelvic tilt of a patient by capturing x-rays from the patient in sitting, standing and step-up positions. It uses this information, along with a CT scan image, to deliver a personalised dynamic simulation that outputs an optimised position for the hip replacement. A clinical trial is currently in place to investigate how the new procedure improves patient outcomes².

Our aim in this project was to assess whether accurate functional assessment of pelvic tilt could be further obtained using inertial measurement units (IMUs). This would provide a rapid, non-invasive triaging method such that only patients with high levels of tilt measured by the sensors would then receive the full assessment with x-rays.