Friday, December 13, 2013

Humeral head size in anatomic shoulder replacement

Determination of humeral head size in anatomic shoulder replacement for glenohumeral osteoarthritis

These authors suggest that a goal of shoulder arthroplasty is to match the pre-arthritic anatomy of the humeral head. They point out that in the presence of advanced glenohumeral osteoarthritis, the humeral head is distorted by loss of height and by peripheral osteophytes, "making it difficult to preoperatively
define the anatomically correct prosthetic humeral head size". They suggest that "inaccurate" selection and positioning of the humeral prosthesis may result in shoulder stiffness, rotator cuff tearing, poor subscapularis tendon healing, and increased glenoid component wear or loosening and instability.

They defined consistent nonarticular landmarks in 31 normal cadaveric humeri. They determined side-to-side differences in radius of curvature and head height in 22 pairs of normal shoulders. They found a minimal side-to-side difference in curvature and height for normal shoulders. In shoulders with unilateral osteoarthritis, the curvature and height, defined by the sphere, showed small mean side-to-side differences between the normal and pathologic sides.

The authors have validated the concept that a sphere, superimposed on specific preserved extra-articular
landmarks of the proximal humerus  can be used to accurately predict humeral head size and height. They suggest that this method can help to more reliably restore native anatomic relationships in total shoulder arthroplasty. They state that the humeral head should form a circle that includes the head,
lateral cortex, and medial calcar. Actually, as pointed out by Codman, the radius of curvature of the proximal humeral convexity normally equals that of the humeral articular surface plus the thickness of the cuff and tuberosity, as shown in his figure below.

Comment: While restoring the premorbid anatomy of the humeral head may be a goal of humeral hemiathroplasty in cases where the glenoid anatomy is unaffected, in most cases of shoulder arthroplasty the choice of head size, shape and eccentricity is determined by the geometry of the glenoid arthroplasty (whether prosthetic - as in the case of a total shoulder, or non-prosthetic - as in the case of a ream and run) as well as the range of motion and stability achieved at surgery as shown here by the "40, 50, 60 rules". Just as one does not plan a total hip to restore the native size and position of the femoral head, one does not often use the premorbid anatomy of the humeral head to determine the size and shape of the humeral head in glenohumeral arthroplasty when, as is usually the case, both sides of the joint are affected. 

In glenohumeral arthroplasty, it is the glenoid arthroplasty that determines the curvature of the humeral prosthesis and it is the soft tissue laxity and stability that determines the head height and the need for eccentricity.
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