Sunday, May 4, 2014

Is it important to restore humeral head anatomy in an 'anatomic' shoulder replacement?

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

These authors mapped a sphere to specific preserved nonarticular landmarks of the proximal humerus to estimate the native humeral head radius of curvature and head height from the osteoarthritic, deformed humeral head. They compared their fit on the arthritic shoulder to the 'normal side control' in cases of unilateral arthritis.

The mean side-to-side difference in normal shoulders was 0.2 mm radius of curvature and 0.6 mm head height.

They concluded that a sphere mapped to preserved nonarticular bone landmarks can be used for accurate preoperative measurement of premorbid humeral head size and therefore the selection of an anatomically sized prosthetic head.

Comment: There are occasions when it is important to replicate the anatomy of the native humeral head, such as when hemiathroplasty is used to treat avascular necrosis of the humeral head without glenoid involvement. However, in most situations surgeons are dealing with glenohumeral arthritis affecting both sides of the joint. Thus, as is the case for total hip arthroplasty, the goal is not 'anatomic' arthroplasty in the sense of duplicating the anatomy of the normal joint, but rather the goal is the creation of a prosthetic arthroplasty that provides mobility and stability to the joint. In performing a total shoulder we select a glenoid component that covers the prepared glenoid bone surface and then match that glenoid with a humeral head that provides a mobile and stable joint as indicated by the "40, 50, 60 rules". Similarly in the ream and run procedure, we usually ream the glenoid to a diameter of curvature of 58 mm and use a humeral head of 56 mm diameter of curvature. So in neither case is an attempt made to duplicate the preoperative or 'normal' anatomy. 
To preserve time, money and radiation exposure, we avoid routine preoperative CT scans to generate a surgical plan.
Finally, as these authors point out, even if the 'anatomic' head size were determined by complex preoperative calculations, the optimal head size and shape is influenced by variability in the location of the humeral osteotomy and by the need to manage soft tissue tightness or laxity and glenoid version (for example using an eccentric humeral head).

The term 'anatomic' arthroplasty is often used to distinguish the placement of a humeral head on the humerus, in contrast to a 'reverse' arthroplasty in which the ball is placed on the glenoid, However, 'anatomic' does not mean that the surgeon should necessarily use a humeral head of 48 mm diameter of curvature and a height of 18 mm just because that is what the calculated 'normal' anatomy is estimated to be.

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