Three-dimensional analysis of the proximal part of the humerus: Relevance to arthroplasty

Background: Recreation of normal anatomical relationships may be important to optimize the outcome of proximal humeral arthroplasty. With use of compu terize tomographic data and three-dimensional computer modeling, we concurr ently studied both extramedullary and intramedullary humeral morphology, in cluding canal shape, and related these findings to the design of proximal h umeral prostheses. Methods: Sixty cadaveric humeri (thirty pairs: fifteen from male donors and fifteen from female donors) were studied, Three-dimensional computer model s were built from canal and periosteal contours extracted from computerized tomographic data and multiple measured anatomical parameters, including hu meral canal axis, humeral:head center, and hinge point offset; greater tube rosity and bicipital groove offset; humeral head center, radius, thickness, retroversion, and inclination; and size and torsion of sections of the can al. Results: On the average, the humeral head center was offset both medially ( seven millimeters) and posteriorly (two millimeters) from the humeral axis. The humeral head hinge point did not line up with the axis but instead was laterally offset by an average of seven millimeters. The average humeral h ead thickness was nineteen millimeters. The humeral head thickness and leng th were proportionately linked. There was marked variability in all of thes e parameters. Humeral head inclination averaged 41 degrees but was less var iable than previously described, with 95 percent of our sample within the r ange of 35 to 46 degrees. The proximal section of the humeral canal was ret roverted, and the retroversion was found to be similar to that of the humer al head on statistical analysis. Version of the middle and distal sections of the canal, however, was dissimilar to that of the proximal section of th e canal. Proximal humeral retroversion was found to be extremely variable a nd averaged 19 degrees. The accuracy, reliability, and repeatability of the computer-based-model measurements were found to be excellent. Conclusions: Measurements of external proximal humeral morphology made with three-dimensional computer models of cadaveric specimens derived from the Midwestern United States agreed, in general, with those described for diffe rent populations evaluated with different measuring techniques, Proximal hu meral morphology was extremely variable as highlighted by the large ranges of measurements seen for all variables. Examination of the intramedullary m orphology showed that there is an internal version, with measurements depen dent on the canal distance distal to the anatomical neck. Clinical Relevance: Because of the marked variabilities seen in proximal hu meral morphology, newer prosthetic designs are now allowing surgeons to con trol multiple prosthetic variables, An understanding of the normal values f or proximal humeral morphology can serve as an important guideline for comp onent selection, especially when the normal anatomy is distorted. Additiona lly, variations in intramedullary version mag have important consequences f or future designs of press-fit proximal humeral replacement.