Open the humeral shaft
Expose the humeral shaft by extending and adducting the shoulder. Open the medullary canal and gently enlarge the humeral canal with rasps of increasing sizes.
Correct rasp size
Insert rasps of increasing size until the end of the rasp protrudes above the bone an amount equal to the previously measured posteromedial metaphyseal extension (h). If the rasp fits snugly in this position, the optimal size has been found. If not, increase the rasp size and progress until you reach the appropriate height. Be careful not to exert too high of a force as the shaft is at risk of fracturing.
Size of the humeral shaft component
The correct size of the humeral shaft component equals the last rasp size used.
Pearl: cancellous bone removal
In order to insert the maximal size of suitable rasps, it is often helpful to remove some of the medial cancellous bone as illustrated. Use a curette or rongeur.
Determine humeral head retroversion
The humeral head should be retroverted approximately 25° relative to the distal humeral epicondylar axis. As illustrated, the retroversion guide shows the perpendicular to the humeral head axis. The forearm is perpendicular to the epicondylar axis. The angle between the guide and the forearm equals the retroversion of the humeral head.
The humeral head retroversion can additionally be checked using the distance between the deepest point of the bicipital groove and the centerline of the rasps or prosthesis. This distance should be approximately 8 mm.
Pearl: Prosthesis with pronounced calcar design
If a prosthesis with a pronounced calcar design is used, this type of prosthesis will center itself if the maximal stem size is used.
This technique acknowledges the patients individual retroversion.