Bone properties of the humeral head and resistance to screw cutout

Surgical treatment of fractures involving the proximal humeral head is hampered by complications. Screw cutout is the major pitfall seen in connection with rigid plating. We have exploited a bony explanation for this phenomenon.

We examined the convex surface of the humeral head looking at the density and the topographical strength of the subchondral bone using mechanical testing of bone cylinders harvested from the humeral head. We also studied the osseous architecture of the subchondral bone and thickness of the boneplate of the humeral head using a 3-dimensional serial sectioning technique.

The bone strength and bone density correlated well and revealed large regional variations across the humeral head. Bone strength and stiffness of the trabecular bone came to a maximum in the most medial anterior and central parts of the humeral head, where strong textural anisotropy was also found. We found in particular a lower bone strength and density in the posterior and inferior regions of the humeral head. A rapid decline in bone strength within a few mm below a relatively thin subchondral plate was also reported.

We have in this paper explored some of the most important factors connected with screw stability at the cancellous bone level. We discovered large variations in bone density and bone strength across the joint surface rendering certain areas of the humeral head less suitable for screw placement. The use of rigid plate constructs with divergent screw directions will predictably place screws in areas of the humeral head comprising low density and low strength cancellous bone. New concepts of plates and plating techniques for the surgical treatment of complex fractures of the proximal humerus should take bone distribution, strength, and architecture into account.