TOTAL HIP REPLACEMENT IN YOUNG PATIENTS
The increasing number of total hip replacements in young patients in recent years, is accompanied by an increased number of revision operations due to excessive wear of the polyethylene in the young active patient, leading to osteolysis and loosening of the prosthesis. For this reason, there has been extensive research to find alternative bearings with better wear properties to cope with this tremendous devastating problem. At this moment, ceramic-on-ceramic and metal-on-metal seem to be the most suited alternatives for the metal-on-polyethylene bearing in the young active patient.
The number of patients suffering from an disabling orthopaedic hip problem is continually increasing. In 1998, 13,5 % of patients undergoing a primary or revision hip replacement at our department, were younger than 40 years, 33,4% were younger than 60 years. These numbers are still increasing up to 14,6% and 40% respectively in 1999. This has lead to an increased use of alternative bearings such as ceramic-on-ceramic and metal-on-metal in hip replacement surgery.
Especially in young active patients, the wear of polyethylene is a tremendous problem, leading to early aseptic (without infection) loosening of the prosthesis, implicating an increased number of revision surgery. For young patients, this is extremely dramatic, especially when loosening happens already a few years after implantation. This means that the young patient would need a large number of revisions during lifetime. Migration of the ceramic or metal ball into the polyethylene socket, leading to osteolysis with loosening of the prosthesis is frequently seen.
Scientific results from numerous studies indicate that wear is inevitable in all kind of prostheses. The production of wear particles and ions of orthopaedic implant devices is inevitable. The design of the device, the material properties, the surgical technique and the activity level of the patient are crucial. Wear occurs in all types of joint implant devices, however with different extensiveness in relation with the former mentioned factors. 90 % of the released particles are smaller than 1,0 µm in diameter. The volume and form of the particles are related to different patterns of wear, e.g. the wear particles in total knee replacement are much larger. The wear particles mainly consist of polymers and, in lesser amount, of metal particles. There are also “third body” particles present related to the fabrication or implantation of the prosthesis, such as cement, hydroxyapatite and corrosion particles. The surrounding tissues of a loosening prosthesis contain about 108 up to 109 particles per gram dry weight. The local host response of the human immunity system to these particles is responsible for initiating the osteolysis (brake down of the bone) with consecutive loosening of the prosthesis.
The human macrophage (cell of the immunity system) plays a central role in the biological reaction to the wear particles of the prosthesis. Macrophages will take up these particles intracellular (a proces called phagocythosis) and will join together to form large multinuclear foreign body cells. These cells are able to take up larger particles. The uptake of particles initiates the release of cytokines, produced by those cells. Cytokines are mediators who stimulate the osteoclasts (eat the bone), leading to extensive bone resorption (osteolysis). Several other cellular mechanisms involving fibroblasts, osteoblasts and stem cells are involved in this process.
To reduce the clinical implications of the wear particles, one need to try to reduce the production of these particles by improvement of the design, materials and surgical technique. As long as there is no specific drug to inhibit this biological response, the only way to deal with the problem is looking to alternative friction couples (ceramic-on-ceramic and metal-on-metal). If polyethylene is still used, the minimal thickness should be 8 mm (even 10 mm)! When thinner liners are used, one should do it in combination with smaller balls of only 22-26 mm in diameter. It is however remarkable that the international standard (ISO-norm) for the fabrication of polyethylene liners is only 5 mm thickness! All clinical studies indicate that there is an increased wear when thin polyethylene liners are used. In vitro results of cross-linked poly’s are not yet proven !
Alternative bearings :
There are many indications that other materials should be used in the young patient. Metal-on-metal, such as used in the BHR resurfacing prosthesis, and ceramic-on-ceramic are preferable nowadays. This evolution will hopefully cope with the increasing numbers of revision surgery in the new millennium. Polyethylene is practically completely abandoned in our service. Time only will determine if this is the correct way to go.