These authors sought to define the relationship between the humeral neck-shaft angle and the entry point for insertion of the humeral component. They used three-dimensional images of 36 cadaveric humeri.
They found that the intramedullary axis line was located a mean of 9 ± 2 mm posteriorly and 11 ± 3 mm medially from the bicipital groove.
The axis line was 9 ± 2 mm posterior and 11 ± 2 mm medial with a standard NSA.
The axis line in humeri with a varus NSA was 8 ± 2 mm posteriorly and 9 ± 2 mm medially.
The axis line was 10 ± 3 mm posteriorly and 14 ± 3 mm medially with a valgus NSA.
They recommended that the location of the entry point be moved toward the center of the humeral head to align with the centerline of the intramedullary canal in humeri with a valgus NSA in particular.
They recommended that the location of the entry point be moved toward the center of the humeral head to align with the centerline of the intramedullary canal in humeri with a valgus NSA in particular.
Comment: The 'varus' or 'valgus' orientation of the neck shaft angle is not a major concern in shoulder arthroplasty. Looking at the two examples shown above, each could be well served by a humeral cut at 45 degrees with the long axis of the shaft. In that the position of the component is guided by the endosteal anatomy of the humerus, the starting point may not be of critical importance.
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