Abstract
The nature of human anatomy necessitates a continuum of implant sizes to recreate near-normal joint mechanics and also afford adequate fixation. Nowhere is this clearer than in the very constrained space required for design of shoulder implants. The effects of muscles acting about the humeral head clearly determine the shoulder’s mechanics. Also standard cement fixation may be undesirable due to difficulties in revision surgery be it required. To emphasize this, two recent developments will be discussed in the evolution of a shoulder design: a) adaptation of the prosthesis to the bony shaft for cement fixation and b) position adaptation of the humeral-head to recreate normal gleno-humeral kinematics.
Humeral stems are generally inserted undersized to the shaft and made ‘analog’ by the use of cement. We have studied this fixation biomechanically to find how little cement was required. Our fixation appeared satisfactory with about the proximal 4-cm of cemented stem. We also looked at shortening the stem but found indeed that stem-length was beneficial. Finally we have sought adaptability in design rather than in cement. We have achieved this by a tri-flanged design for the distal stem. This allows stem compression for intimate contact. In addition, its out of round shape, afforded more rotational stability in cement sheath.
For kinematics, Wallace et al discovered that the head could be displaced a variable distance from the center of the shaft and unique to each patient. Later studies showed that a mismatch could lead to improper mechanics with glenoid impingement. The solution proved to be a variable displacement humeral-head, which would allow the surgeon to select the direction and magnitude of displacement during surgery. Thus, this evolution of prosthetic shoulder design allows a smaller number of prostheses to be adapted satisfactorily to the continuum of humeral anatomy and also provide superior joint kinematics.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
