Scientific Review

OPTIMAL HIP ARTHROPLASTY

ANTONIO MORONI, MD
BOLOGNA, ITALY
OFC RESEARCH CHAIR

Meta-analyses of 15 trials comparing cemented and uncemented prostheses, suggest the superiority of cemented fixation; however, in these trials older generation implants were used so the uncemented components were often Moore’s prostheses, which are known for sub-optimal performance. The introduction of modern generation uncemented prostheses could lead to results at least as good, if not superior, to cemented prostheses.

In osteoporotic bone, biological fixation, a special type of uncemented fixation consisting of implants coated with calcium phosphates, is particularly promising. Prospective randomized studies of both hip and knee arthroplasty featuring RSA analysis showed that biological fixation performs as well as cemented fixation. This is always superior to standard uncemented fixation.

Clinical studies of screws coated with hydroxyapatite used to fix wrist and hip fractures show that biological fixation is also effective with porous bone. As cemented fixation needs good cancellous bone in order to achieve primary stability, porous bone is not the ideal bone substrate for cemented prostheses. Indeed, in the osteoporotic patient very little cancellous bone is found in the proximal femur.

An interesting surgical concept for these patients is the surgical technique developed by Furlong. With this technique, the cancellous bone of the proximal femur is not removed when broaching the canal but is fragmented and impacted. This technique produces a “biological bone cement” which is useful in providing primary stability. There is evidence that even in elderly patients with weak bone, osteointegration will occur postoperatively.

Head Diameter

In the femoral neck fracture population, a large diameter head is a solution to increasing stability thus reducing the rate of dislocation. This is the reason why several surgeons prefer the use of bipolar or unipolar prostheses over total hip arthroplasty. Another reason to choose a large diameter head is that the elderly do not tolerate changes in their joint anatomy.

The correct size of the prosthetic head is the size which replicates the patient’s original head. In the recent past, this was not possible because of technological limitations. Now, there are innovative bearings such as metal/metal and metal/polyurethane that permits the use of a prosthetic head similar in size to the patient’s original head.

The use of metal-on-metal bearings with large diameter heads used to be popular in the past but was abandoned because of the introduction of metal-polyethylene bearings. Recently it has been reintroduced, since metal-on-metal hip resurfacing has been further developed.

Another interesting solution which has the advantage of no elevation of metal ions in the serum is metal-polyurethane bearings. In the past, prostheses have provided poor shock absorption. Because the hip is a synovial joint, it also has excellent tribological characteristics of low friction, high load carrying capabilities, high shock absorption and long endurance. A newly-developed implant consisting of a buffer made of a polycarbonate urethane-compliant material replacing the cartilage in the acetabulum has the advantage of minimal reaming. Furthermore, by ensuring that microelastohydrodynamic lubrication is maintained, friction can be nearly eliminated, thus decreasing the potential for wear. Because of the relatively large femoral head sizes (44-55mm) that are able to be used with this system, complications such as dislocation, subluxation, or leg length discrepancy are rare. The potential benefits include minimal bone removal, increased ROM, faster rehabilitation, and overall a more anatomical reconstruction of the hip which is so important for elderly hip patients with osteoporosis.

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