Abstract
Humeral head size is defined by the radius of curvature and the thickness of the articular segment. This ratio of radius to thickness is within a narrow range with an average of 0.71. The articular surface of the normal humeral head measured within the AP plane is defined by three landmarks on the non-articular surface of the proximal humerus. The perfect circle concept can be applied for assessment of the anatomic reconstruction of the post-operative x-rays and more importantly can be used intra-operatively as a guide when choosing the proper prosthetic humeral head component. The humeral head is an elliptical shape with its AP dimension being approximately 2 mm less than the SI dimension. This shape contributes to the roll and translation of the normal shoulder but is not replicated by the spherical shape of the prosthetic humeral head.
The glenoid vault has a consistent 3D shape and use of the vault model within 3D planning software can define the patient's pre-morbid anatomy, specifically the location of the joint line and patient specific version and inclination. Use of this tool can assist the surgeon in defining the optimal implant and its location. In patients with little or no bone loss, a symmetric glenoid implant is often ideal for resurfacing. When there is asymmetric bone loss, often seen posteriorly with osteoarthritis, an asymmetric posteriorly augmented component can improve the ability to correct the deformity while maintaining the native joint line. It is suggested that these augmented implants in selected patients will help restore and maintain humeral alignment and lessen the risk for residual posterior humeral head subluxation and eccentric loading of the glenoid component.
