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 [16], 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? [17]. 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
[18] 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 [19] 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.
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