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Surface Replacement Arthroplasty of the Shoulder

4. Development of Copeland Shoulder

In 1979 work was started to develop a surface replacement arthroplasty of the shoulder. Cadaveric anatomical specimens were measured for size and sphericity of the humeral head. Three prostheses were developed with a 22 mm radius of curvature and three depths of skirt was enough to cover the anatomical variation range. Many different designs were tried on dry bones to try and determine the best method of fixation for this surface cap. From the outset it was designed as a cementless prosthesis. The Mark 1 prosthesis was fixed with a central smooth round peg and a screw passed from the lateral side of the humerus to screw into the prosthesis to act as an anti-rotation bar. This was used clinically on 19 patients but it was soon realised that the fixation was adequate with the impaction stem alone and that the screw was unnecessary. It was also realised that if the prosthesis were to loosen then toggling of this anti-rotation bar would de-associate the tuberosities and this would present a difficult reconstruction revision problem. Therefore the use of the screw was abandoned at an early stage.  top

The prosthesis was first used clinically in 1986. When the fixation screw was abandoned a better design of central peg was made. The fluted taper fit central fixation peg is critical to the success of the design. The volume of bone removed by the central tapered drill in the humeral head is the same as the volume of bone in the grooved tapered peg. Therefore the outside diameter of the drill is slightly less than the outside diameter of the peg. When the peg is offered up to the bone, the cap has to be impacted home to achieve a tight fit. Initially bone is pushed into the groove and later biological remodelling takes place. From 1993 the whole of the bony surface of the glenoid and humeral components have been hydroxyappetite coated so there is an initial mechanical fix and then later a biological fix with bony ingrowth in to the hydroxyappetite coating. The original design concept was that the surface replacement would mimic as closely as possible the original anatomy. Therefore very simple instruments were developed to allow anatomical placement of the humeral head. In essence these instruments allow determination of the centre of the sphere based on the angle of the surgical neck as seen at operation. No complicated instrumentation is required to do this and it is never put in a fixed degree of version offset or angulation. Rather an attempt is made to replicate the original anatomical bearing surface.  top

Development of a Glenoid Prosthesis

Measurements of glenoid anatomy were made on anatomic and cadaveric specimens. A pear shaped glenoid was developed that mimicked the normal anatomy. In the early 1980s a popular method of fixation of plastic components of joint replacement was by finned interference fit peg. For example, the tibial component of the total knee replacement and the acetabular component of the Ring plastic hip prosthesis. This plastic peg was initially used to fix the Copeland Mark 1 glenoid in 1986. A central drill hole was made and the peg fitted by impaction. The use of all-polethylene impaction fit pegged prosthesis was eventually abandoned for theoretical reasons rather than practical considerations. It was becoming obvious that the main reason for the long-term failure of prostheses was due to polyethylene debris. With the finned design of prosthesis there was a large surface area of plastic presented in direct contact with bone and hence the potential for fretting and debris formation was high. Recent long-term review of these prostheses showed this to be the case in 6 of 19 prostheses that were inserted [20]. When the grooved taper peg fixation of the humeral component was developed the same principal was applied to the glenoid side once again for cementless impaction fitting. From 1993 this has also been covered with hydroxyappetite coating for secondary bony ingrowth fixation.  top

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