Little known facts about the Orthopedic Hip Replacement
The History of the Hip Replacement
The hip replacement has been one of the most successful and reliable orthopedic procedures since its
first implant in the USA in 1940. An incredible 97% of patients having undergone the operation report improved function of the hip. Death rates are much less than 1%. With the modern hip replacement, the patient should expect to be able to walk immediately after the operation. The modern joint owes its design to the work o
f John Charnley at Wrightingdon Hospital; his own design almost entirely replaced all of the designs in
From UHMWPE to CoCr and PMMA!
The ball (femoral component) and socket (acetabular component) arrangement of the hip replacement implant has been experimented with a wide range of materials in the hip replacement's long history. This includes ivory, ceramics, cobalt chromium alloy and ultra high molecular weight polyethylene(UHMWPE). Ivory was used due to its favourable interface with human tissues, allowing for a good, stable recovery of the hip joint. UHMWPE also has fantastic properties for a biomaterial; it has a very low coefficient of friction, is self lubricating and is highly resistant to abrasion. UHMWPE also has the highest impact strength of any thermoplastic and is odourless, tasteless and non-toxic. Some implants are cemented into position with PMMA (bone cement) and/orscrews, and some are not, relying on positive bone growth around the femoral part of the implant in the total hip replacement. The materials that are used in hip replacement are also commonly used in total and partial knee replacements.
Hip Replacements for the more active patient
An alternative to total hip replacement (THR) is femoral head resurfacing, where the femoral head is not entirely removed. The surface layer of bone is removed and replaced by a cup (usually manufactured from cobalt chromium alloy, CoCr). The advantages of hip resurfacing over total hip replacement include less bone removal and fewer complications if revision surgery is needed in the future. The total hip replacement is generally 28mm in diameter, whereas the hip resurfacing cup can by up to 56mm in diameter. The larger diameter of the resurfacing generally leads to better stability. For these reasons hip resurfacing is often preferable for younger, more active patients.
One possible complication in hip replacement joints is stress shielding. This is the term used to describe the removal of normal stress from the femur when the femoral component of a total hip replacement is implanted. Usually when we walk, or run, our femur is stressed along its entire length, and actually deforms slightly under the load. When the femoral component, often manufactured from titanium, is implanted it may prevent the top third of the femur from undergoing this stress. Wolff's Law states that bone in a healthy person will remodel in response to the loads it is placed under. This means that if the load on a bone decreases the bone will become less dense, and therefore more likely to fracture under high stresses.