Where do locking screws purchase in the humeral head?

One of the limiting factors in finding the best osteosynthesis approach in proximal humerus fractures is the current lack of information on the properties of the cancellous bone regions engaged by the implants fixing the epiphysis. The aim of this study is to assess the densitometric and mechanical characteristics of these regions when using a proximal humerus locking plate (PHLP).

Nineteen PHLPs were mounted on cadaveric humeri using only their three most distal screws. Subsequently, the plates were removed and the bones were scanned using high-resolution peripheral quantitative computed tomography. Bone mineral density (BMD) was determined in the intact proximal epiphysis and in the exact locations where the six proximal screws would have been positioned concluding the instrumentation. Each plate was then repositioned on its bone and a minimally destructive local torque measurement was performed in the same six locations. A statistical analysis was performed to detect significant differences in the investigated parameters between screw positions, and to test the ability of local torque values to discriminate the bone mineral density of the entire humeral head (BMD(TOT)).

Novel data about the cancellous bone engaged by the screws of a PHLP are provided. Different epiphyseal locations showed statistically significant different properties. A local torque measurement was a good predictor of the BMD(TOT).

Position and direction of the epiphyseal screws on a locking implant are determinant to engage bone regions with significantly better bone quality. A breakaway torque measurement in a given screw position can distinguish between humeral heads with different densitometric properties.