The angle between the humeral neck and shaft varies among shoulders.

Implant manufacturers are marketing humeral prostheses with variable neck shaft angles with the goal of matching the native humeral anatomy.

A simpler and less costly approach is to use a prosthesis with a fixed 135 degree neck shaft angle

inserted after making a humeral neck cut at a 45 degree angle with the medullary canal as shown in these drawings by Steven Lippitt from The Shoulder.

The same cut is made whether the native head is in some valgus

or some varus

with the same anatomical result.

The authors of Fixed- vs. variable-angle humeral neck cut in anatomic total shoulder arthroplasty tested this concept in a trial of 50 patients with osteoarthritis having a primary anatomic TSA randomized preoperatively to receive either a variable- or fixed- neck shaft angle (NSA) prosthesis. Humeral neck cut in the variable-NSA group matched the patient’s anatomic neck, with prostheses having a NSA of 127.5, 132.5, or 137.5 degrees. Fixed-NSA cuts were made at 132.5 degrees using an intramedullary guide.

The average preoperative neck shaft angle for all patients was 134 ± 7 degrees.

When comparing average difference in preoperative and postoperative measurements in the fixed-NSA group, the humeral head offset from the humeral shaft axis decreased by 1.4 mm, and the COR moved superiorly by 3.0 mm without significant medialization or lateralization.

The results were essentially the same for the variable angle group: the humeral head offset decreased by 1.2 mm, and the COR moved superiorly by 2.9 mm without significant medialization or lateralization.

The authors concluded that both fixed- and variable-NSA anatomic TSA humeral components demonstrate adequate restoration of premorbid anatomy radiographically.

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