Wednesday, January 2, 2013

Inability to replicate humeral anatomy

Humeral head arthroplasty and its ability to restore original humeral head geometry

It is of interest that shoulder surgeons feel compelled to "restore normal glenohumeral anatomy" whereas hip and knee surgeons are comfortable with their ability to install functional prosthetic anatomy that bears only a slight relationship to that of the normal joint.

The authors of this report investigated twenty-four human cadaveric arms without degenerative changes   (14 female and 10 male, ranging in age from 68 to 99 years.

The neck shaft angle averaged 135.0 ± 4.4 degrees with a range of 127.6-141.4
Retroversion averaged 18.5 ± 9.0 with a range of 2.7-37.4 
Coronal diameter (mm) averaged 48.8 ± 3.2 with a range of 42.7-55.1 
Axial diameter (mm) averaged 43.6 ± 2.5 with a range of 38.9-49.0 
Head height (mm) averaged 16.9 ± 1.5 with a range of 14.4-20.1
Coronal radius of curvature (mm) averaged 24.6 ± 1.3 with a range of 22.2-27.6  
Axial radius of curvature (mm) averaged 23.0 ± 1.1 with a range of 21.3-25.2 

Even this small sample of normal humeri demonstrate that no prosthetic system could hope to 'replicate normal anatomy'. 

With the simulated osteotomy and prosthetic implantation, there was a 4.8° decrease in inclination (P < .01) and 11.3° increase in retroversion (P < .001). The radius of curvature in the coronal plane was not significantly different (P = .284). However, in the axial plane, the prosthesis was significantly larger than the original head for both head diameter (P < .001) and radius of curvature (P < .05).

The authors conclude " that the humeral head is not a perfect segment of a sphere and an osteotomy along the anterior cartilage-metaphyseal interface does not remove only the proximal humeral articular surface. Even with a fully adaptable prosthetic implant, replacement arthroplasty is not able to restore original head geometry."

On page 186 of Practical Evaluation and Management of the Shoulder we reported yet another variable of critical importance in humeral arthroplasty, the offset of the center of the humeral articular surface in relation to the center of humeral medullary canal into which the prosthetic stem is press fit.

From all this, we can only conclude that prosthetic anatomy will different than normal anatomy. As is the case for our hip and knee arthroplasty colleagues, our goal is to insert durable components in a manner that enables patients to improve their shoulder mobility, smoothness, stability, strength and function without worrying too much about whether we are replicating the 'real thing'. Furthermore, it has yet to be demonstrated that the clinical results with more recent 'generations' of humeral implants that have more variability in geometry are superior to those of their predecessors.

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