1. Wirrall University Teaching Hospital, NHS Foundation Trust, Arrowe Park Road, Upton, Wirral CH49 5PE, UK.
2. Centre for Hip Surgery, Wrightington Hospital, Appley, Bridge, Lancashire WN6 9EP, UK.
3. The Lower Limb Arthroplasty Unit, Northern General Hospital, Herries Road, Sheffield S5 7AU, UK.