Intramedullary reaming for press-fit fixation of a humeral component removes cortical bone asymmetrically.

Periprosthetic humeral fractures are major complications of shoulder arthroplasty. Bone removal during surgical reaming is a risk factor for these fractures. Although it is recognized that the endosteal surface of the humerus is asymmetrical whereas the reamers are symmetrical, to our knowledge, the effect of cylindrical reaming on the pattern of cortical bone removal during reaming has not been previously studied. The medullary canals of 10 cadaveric humeri (mean age, 73 years) were reamed in a manner similar to that used during humeral arthroplasty. Cortical dimensions were obtained from computed tomography scans before and after reaming. In unreamed humeri, the anterior-posterior endocortical diameter was 20% smaller than the medial-lateral diameter. The average medial-lateral diameter (15.6 +/- 2.3 mm) was significantly greater than the anterior-posterior diameter (12.5 +/- 1.9 mm) at 13 cm distal to the tuberosity (P < .00005). Successive cylindrical reaming preferentially thinned the anterior and posterior cortices. This bone loss would not be apparent on anterior-posterior radiographs. Intramedullary reaming to obtain substantial cortical contact asymmetrically removes cortical bone in a manner that may increase the risk of periprosthetic fracture.