Humeral Head Geometry: An Analysis of Global Offset against Intramedullary Shaft Diameter and Humeral Head Size: A CT Scan Study.

BACKGROUND: Establishing normal native humeral head anatomy is essential for understanding pathology and maintaining biomechanics. Although humeral head size and medullary canal dimensions vary, the relationship between the centers of the head and the medullary canal has not been defined. We aim to determine the relationship between humeral head diameter, humeral canal size and global offset. Global offset is defined as the perpendicular distance between the geometric center of the best-fitting sphere of the humeral head and the axis of the humeral shaft. To our knowledge, no prior computed tomography (CT) study has assessed these parameters across both age and sex.10,21,27,28 METHODS: This retrospective observational study reviewed age- and sex-matched shoulder CT scans obtained from the Office of the Medical Investigator. Three-dimensional models were generated using Checkpoint software. Surface markers were placed manually on the humeral head and shaft. Best-fit spheres and cylinders were overlaid on each humeral head and shaft. From these models, humeral head size and shaft diameter were measured, and global offset was subsequently calculated.

RESULTS: Healthy shoulder CT scans from 50 male and 50 female patients aged 18-93 years were analyzed. Male patients demonstrated larger average medullary canal diameter (15.38 mm, range 9.56-20.20 mm vs 13.14 mm, range 8.28-18.98 mm; P < 0.001), humeral head diameter (51.02 mm; range, 45.20-58.60 mm vs 44.88 mm, range 38.60-50.20 mm; P < 0.001), and global offset (6.41 mm, range 2.12-10.72 mm vs 5.53 mm; P = 0.006) compared with female patients. A positive correlation between humeral head diameter and medullary canal diameter was observed in male patients only (male Rˆ2 = 0.32; female Rˆ2=0.12). No statistically significant relationship was observed between age and global offset in either sex.

DISCUSSION/CONCLUSION: Findings suggest weak to moderate relationships between humeral head size and shaft diameter, with the strongest correlation between humeral head diameter and medullary canal size in male patients. Global offset demonstrated either weak or no correlation with humeral head size or medullary canal diameter. This lack of association suggests that global offset varies independently of humeral head and canal size. Global offset is an essential morphometric parameter in stemmed humeral arthroplasty because it defines the location of the native humeral head center relative to the humeral canal. Because stemmed implants achieve fixation within the canal, recreating the native global offset is necessary to restore the physiologic center of rotation and shoulder anatomy.