Surface Replacement Arthroplasty of the Shoulder
3. Rationale for Surface Replacement
Going right back to basics we have to reconsider how arthritis effects the joint itself. Obviously there is roughening and distortion of the normal smooth articular surfaces and therefore what is required is a new surface to reconstitute the geometry of the smooth bearing surface. It would only seem reasonable to fix this new bearing surface in the most minimally invasive manner possible. This rationale was attempted at the hip and unfortunately was a failure. However it is successful at the knee. The femoral prosthesis of most knee replacements are in fact a surface replacement of the distal femur.
It is interesting to review why surface replacement of the hip has previously failed. This was essentially a materials problem. If only the surface is being replaced then this is a large weight-bearing surface area of plastic exposed to wear hence the production of plastic debris is greater. This plastic then gained access to the bone cement junction under the cap of the femoral prosthesis, osteolysis ensued, notching of the neck and then failure of the prosthesis. Also to dislocate the hip to gain access to the femoral head destroys part of the blood supply to the femur and reaming of the femoral head destroyed the retinacular vessels. Hence it is not surprising that there was some degree of subsidence and avascular necrosis of the remaining femoral head, although this was not the primary cause. The blood supply to the humeral head is entirely different, with no retinacular vessels and the blood supply arriving via tuberosity muscle attachments. It appears that the main reason that surface replacement of the hip failed is because of increased particulate plastic wear. Amstutz , one of the main proponents of surface replacement of the hip notes in 1994 that in his series of hemi-arthroplasties treated by surface replacement of the femoral side, there was no loosening, hence the assumption was made that the cause of loosening was plastic debris access and secondary osteolysis. Partial resurfacing of the femoral head has also been reported by Siguier et al in ?Clinical Orthopaedics and Related Research, 2001? . This appears to be a promising way forward for avascular necrosis of the femoral head. As it is only used on one side of the joint and there is no plastic debris problem.
With respect to surface replacement of the shoulder, Steffee & Moore  reported the use of the Indiana hip cup used as a interposition arthroplasty for the humerus. No follow up was recorded for these cases. Rydholm & Sjogen  from Sweden reported the results of the Scan cup in 1993 in which they used a hemi-spherical cemented cup. This had good clinical results, but they reported a 25% loosening rate at an average of 4.2 years. It was noted that no central fixation peg was used for this cup. top