Saturday, November 12, 2016

Humeral head osteotomy in shoulder arthroplasty - how do you slice it?

Humeral head osteotomy in shoulder arthroplasty: a comparison between anterosuperior and inferoanterior resection techniques

These authors proposed an approach to humeral osteotomy based on landmarks on the anatomic neck in an attempt to restore 3-dimensional humeral head morphology. They digitized the anatomical neck of 30 normal human cadaver shoulders.

They compared the virtual performance of two different osteotomy techniques: the traditional, following the anterosuperior anatomic neck; and a new technique, defined by the inferoanterior anatomic neck. 

Length-width difference of the anterosuperior resection area was higher than in the inferoanterior osteotomy (6 ± 2 mm vs. 3 ± 1 mm; P < .001).
Retroversion of the anterosuperior resection plane was higher than the native head (50° ± 12° vs. 37° ± 11°; P < .001), whereas retroversion after the inferoanterior osteotomy (32° ± 12°) did not differ from native (P = .057).
Inclination differed after the anterosuperior osteotomy (129° ± 5°) and the inferoanterior osteotomy (127° ± 4°) compared with the native head (134° ± 4°; P ≤ .001).

They concluded that inferoanterior referenced osteotomy generated a more circular resection area, matching the native humeral head retroversion more closely than in the anterosuperior technique.

Comment: The goal of humeral implant positioning is not to reproduce 'normal' anatomy, but rather to optimize glenohumeral stability and kinematics. This may require non-anatomic positioning of the humeral joint surface, such as in the use of anteriorly eccentric humeral heads to manage functional decentering observed at surgery.

The pathoanatomy encountered at shoulder arthroplasty varies widely, but rarely looks like the normal cadaver shown above. Here are some examples in which cases finding anteroinferior landmarks would be a challenge.

The positioning of the humeral articular surface is dictated by (a) the positioning of the prosthetic stem in the humeral canal and (b) the geometry of the humeral prosthesis placed on the prosthetic stem. In our technique, the humeral neck cut does not control the final head position.

The orthopaedic axis is defined by the largest cylindrical sizer that fits in the canal.

The cut is made at 45 degrees with this orthopaedic axis, irrespective of the proximal humeral anatomy

and in 30 degrees of retroversion.

the prosthesis is seated so that its lateral edge is just below the berm.


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