Saturday, April 16, 2016

Is the normal humeral head round?

An anthropometric analysis to derive formulae for calculating the dimensions of anatomically shaped humeral heads

These authors investigated the elliptical shape of the humeral head using three-dimensional computer models of 79 proximal humeri derived from CT scans of white subjects, 47 male and 32 female; ages, 17-87 years. 

Formulae for calculating humeral head height, diameters of the base of the humeral head in the frontal and sagittal planes, and radii of curvature in the frontal and sagittal planes were derived from the linear regression plots that were found to have strong (1 ≥ R ≥ 0.50) correlations. By Welch t-test, differences between 'small', 'medium' and 'large' head sizes were statistically significant in each case (P ≤ .022). The elliptical shape of the base of the humeral head was found to elongate with increasing humeral head size.
The authors suggest that the findings of this study may have implications for future prosthetic shoulder design in which the goal is to replicate normal anatomy.

These results are interesting. Below is a scatter plot with linear trend lines that demonstrate the mathematical relationship between the length difference between the head axes in the frontal and sagittal planes (DF − DS) and the diameter of the base of the head in the frontal plane (DF).
While the trends for smaller heads in women and greater elliptically in larger heads are apparent, we note the high degree of variance among individual subjects. It would be difficulty for "future prosthetic shoulder design" to accommodate this variability. 

Similar to the goals of the total hip surgeon, our goal in shoulder arthroplasty is not to 'replicate normal anatomy' but rather to achieve a reconstruction that optimizes shoulder mechanics. Not infrequently, this is best achieved by using a head size and shape different than the 'normal anatomy. See for example, this link.


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