Development
Surface Replacement Arthroplasty of the Shoulder
2. Development of Surface Replacement
If a step forward in shoulder replacement is proposed then we have to review what is wrong with what we?ve got.
1. Problems with cement
1a. Loosening
High incidence of glenoid loosening leaves major problems on
the glenoid side with unpredictable bony erosion [6-11]. So much
bone may be eroded that revision may be impossible [11, 12]. On the
humeral side, although loosening is rare, if it does occur then bony
loss and erosion may be catastrophic such that revision may be
impossible.
1b. Revision Difficulties
If a stemmed prosthesis is inserted in the wrong degree of
version to remove this and revise it is a major surgical procedure,
which may fracture the humeral stem and require a longer stem
prosthesis to gain stability. When cement is used in the humeral
shaft of an elderly rheumatoid patient the shaft may crack on
impaction of the prosthesis and leakage of bone cement can cause
radial nerve problems. The use of cement without a cement restrictor
can allow cement to go all the way down to the elbow and preclude
ipsilateral elbow replacement, eg in the rheumatoid [13, 14].
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2. Problems with a Stem
If an intra-medullary stem is used then a stress riser must arise at the tip of the prosthesis. Shoulder replacement patients are often in the elderly group with fragile osteoporotic bones, particularly in the rheumatoid. If they are elderly and infirm with poor balance then falls are much more likely and hence a fracture at the tip of the prosthesis becomes a much more likely event. This occurs in both the hip and the knee and is now seen in the shoulder in increasing numbers. Fractures around the stem of a prosthesis are very difficult to treat and may cause major problems with revision of surgery.
If arthritis is secondary to a malunited fracture, sometimes it is impossible to pass a stem down the humeral shaft without corrective osteotomies. This has prognostic implications for the result of the shoulder replacement [15]. On occasion the medulla of the humeral shaft may already be compromised by cement and an intra-medullary stem coming upwards from below ie intra-medullary fixation of fracture or a stemmed elbow replacement.
In cementless stem designs, reaming of humeral shaft can also cause fracture.
We have to question why a stem is used at all to fix the humeral prosthesis in arthritis. The majority of stemmed humeral prostheses extend half way down the length of the humerus. There is no scientific evidence to support why this should be so. If a fracture is not present there is no requirement for such a length of intra-medullary fixation. Obviously if there is a fracture at the proximal humerus as in the four-part fracture then a central intra-medullary stem is required as a scaffold to rebuild the shoulder around.
3. Problems with Materials
We know from long term hip and knee studies that the major long
term problem with loosening is due to particulate HDP wear debris
causing macrophase response and osteolysis. The same is true in the
shoulder. Although the surface area of plastic in the shoulder is
smaller, because the shoulder is not a captive joint with a fixed
centre of rotation, the range of motion of the humeral head upon it
is much greater and translational and sheer forces much more
important. If one could avoid using HDP at all this would be of
benefit for the long term outlook of the shoulder (i.e. hemi-arthroplasty).
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