What is the Birmingham Mid Head Resection (BMHR) and why do we need it? Derek McMinn
The Birmingham Mid Head Resection (BMHR) or 'mini hip replacement' as some patients call it, fills the void between conservative hip arthroplasty like the Birmingham Hip Resurfacing (BHR) and traditional Total Hip Replacement (THR). Aimed at patients who want a hip resurfacing and all the benefits that come with it, but whose bone stock and quality isn't good or strong enough to support it. The BMHR has advantages similar to hip resurfacing including that it is easily revisable to a hip replacement if needed.
The BMHR can deal with bone loss in the top part of the femoral head and restore peculiar anatomy much better than the BHR. Derek McMinn remarks that there is a varied failure rate for patients with osteonecrosis or avascular necrosis (AVN) with THR, also noting that THR patients with osteonecrosis have a significantly worse outcome than those with osteoarthritis. Mr McMinn shows several examples of patients with osteonecrosis where he has performed a Birmingham Hip Resurfacing and achieves a good outcome. The difficulty of performing a hip resurfacing in a patient with osteonecrosis is they are prone to further collapse.
A survivorship graph shows a 10% failure rate at 10 years in Mr McMinns series – something that makes him feel uncomfortable about doing hip resurfacing in patient with AVN. Dr Sugano from Osaka, Japan, shows despite using a conservative neck-retaining THR stem with large Birmingham modular head that the femoral neck bone melts away. This is to do with how the joint is loading and the fact that the load is going through the stem into the femoral shaft creating proximal stress shielding.
McMinn establishes that a conical shaped implant with a short stem would be an ideal shape for transferring the load and reducing proximal stress shielding. The BMHR cannot be used with a small headed component and must be used with a large modular head. X-rays at one year post operation show the neck of the femur is still there and has not melted away as shown in Dr Sugano's series with a hip replacement. RSA studies show no measurable migration of the components out to two years.
The complex procedure used a curved stem originally however Mr McMinn felt this was too technically challenging for wide use so the design was altered to become a straight stem. Anti-rotational flutes were added and the implant is HA coated. McMinn shows examples of how well the BMHR can deal with bizarre anatomy and the patient will still achieve a good outcome.