Total Hip Replacement (Posterior approach): Uncemented Mathys TwinSys stem & Vitamys cup
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The rectangular cross-section of the ‘Corail-type’ gives excellent rotational stability – the main force on the stem to resist is rotational. The Corail had excellent 20-year outcomes. Mathys have continued the principle with a titanium alloy stem and hydroxyapatite coating.
The cup is based on the Mathys one-piece cup which also has excellent 20-year outcomes. The addition of Vitamin E to the cross-linked polythene of the Vitamys has added benefits of reducing late oxidation. This has very low wear rates in laboratory and in clinical studies. My active patients have 0.05mm linear wear a year and normal patients are at 0.03mm. This is with a 36mm head so the patients have the added benefit of low dislocation rates.
Intra-operative fracture remains the highest concern reducing this risk needs the surgeon to study the shape of the femur during digital planning and adapt technique appropriately. Some patients have bone that is hard but brittle, such as high level athletes or those who have been on long-term bisphosphonate use. It is still the opinion of most that the stem should fill the proximal femur.
I do advise restricting activity levels though in weeks 3 and 4 following operation. This is in accordance with studies on fracture healing that support early micromovement to stimulate callus growth, followed by reduced load to minimise movement of the consolidating new bone, termed ‘reverse dynamisation’. So in week 3 and 4 patients continue to weight-bear, but reduce walking distance to around 2,000 steps a day by staying at home.
The Twinsys does not depend on a collar to avoid migration, and this makes subsequent revision easier should it ever be needed. The original design of the Mathys cup had good results out to 20 years. The Vitamys is demonstrating very low wear rates and as linear wear is only 0.05mm a year in active patients, it should last a long time. therefore at 2 months if all is well I discharge the patient to full and un-restricted activity.
Author: Professor James Richardson MBChB, FRCS, MD
Institution: Keele University, The Robert Jones & Agnes Hunt Orthopaedic Hospital, Oswestry, UK.
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