Conus hip prosthesis.

Abstract

50 years ago, prosthetic replacement of the hip joint ushered in a new epoch in orthopaedics. Total hip replacement made it possible to remove a severely diseased, painful hip and restore normal function and a normal quality of life to the afflicted patient. The early results of total hip replacement are almost all spectacular and hip replacement has become the most successful type of orthopaedic surgery. These good results using an approach that was technically relatively simple resulted in a temptation to implant prosthetic hip joints with ever increasing frequency in ever younger patients. This led to the emergence of new problems, which were not so clearly recognised at the outset: it emerged that the stability of prosthetic hip joints was of limited duration. This had the following consequence: If a total hip prosthesis is implanted in an elderly person whose remaining life-expectancy is shorter than the longevity of the prosthesis, hip replacement is a life-long solution. We can therefore say that, for a patient who has only 10 to 15 years left to live, their hip problem is solved by total hip replacement. For young people, who still have a long life expectancy in front of them, it is different. They will experience failure of the artificial joint and require further surgery. The commonest and most important type of failure in total hip prostheses is aseptic loosening, which is associated with resorption of bone at the site of the prosthesis. The cause of this phenomenon has only gradually been recognised in the course of the years. Initially, the unanimous opinion was that the methacrylate cement, used to fix the components of the prosthesis in the bone, was the definitive cause of aseptic loosening because fissures and fractures of the cement were almost always found during surgical revision of loosened joints. There was talk of "cement disease" and great efforts were made to improve the quality of the cement and the cementing technique. Moreover, even today, there is no established answer to the question whether, over the course of many years, cement ages and becomes friable, a process that may have major implications for young patients. For this reason, ways of reliably fixing the prosthesis in the bone without methacrylate cement were also explored at the same time. Valuable pioneering work in this field was carried out with uncemented dental implants made of titanium and with a roughened surface. With these implants, the phenomenon of osseointegration, i.e. the deposition of bone directly on the roughened metal surface without any intervening connective tissue, was observed. This phenomenon has also been utilised successfully in hip prostheses: if artificial hips made of titanium alloy with a coarse-blasted surface and with a high primary mechanical stability are placed in the bone, osseointegration also occurs. In parallel with this development, Willert, from Göttingen, identified the most important cause of loosening of the prosthesis: he established that, when an artificial joint articulates, very fine particles of polyethylene are eroded from the prosthetic cup when the surfaces of the joint glide over one another and that these are only partially removed by the lymphatic system. A large proportion of the particles accumulates in the artificial joint and in the gap between the prosthesis and the bone, giving rise to foreign body granulomata, which resorb bone thus leading to loosening. The number of eroded particles is considerable. In 1998, Patricia Campbell, from Los Angeles, showed that 470,000 particles per step were produced from an articulation between a metal head and a polyethylene cup. This huge number gives an indication how small these particles are, since the linear erosion of the polyethylene surface only amounts to about 0.1 to 0.2 mm a year. This relatively recent recognition of "particle disease" has led to the investigation of other materials, which produce fewer erosion particles, for artificial joint articulations. Three possible options are available today, but it is not yet possible to decide for certain which of them is superior to the others: the longest experience has been with metal/metal articulation with articulatory pairings of cobalt/chromium/molybdenum alloy. In recent years, ceramic/ceramic articulations of aluminium oxide-ceramic and pairings of ceramic with highly crosslinked polyethylene have also been used. With these modern articulations, particle erosion can be reduced about 200 fold. If the erosion particles are an important cause of loosening of the prosthesis, it is reasonable to expect that, with these new joint pairings, the durability of an artificial joint can be substantially prolonged. The cone prosthesis described here has been developed on the basis of the knowledge and experience of the last 20 years. The conical anchoring of the stem, involving the placing of a conical implant in the conically reamed medullary cavity, results in continuous contact between the stem of the prosthesis and the bone, with a high degree of primary stability. The sharp longitudinal ribs on the stem, which cut a little into the bone, provide a high degree of rotational stability. In the case of conical fixation, the mechanical transmission of force varies with the diameter of the stem: the weight bearing surface of a cone in relation to its length is greater where the stem has a larger diameter than at the tip of the stem where the diameter is smaller. Therefore, for geometrical reasons, the conical fixation of the stem results in a transmission of force that is predominantly proximal and avoids proximal stress protection. The round cross-section permits free adjustment of the angle of anteversion and avoids any forced rotation as a result of bony deformity, which is very important in the case of dysplastic hips.