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Hybrid locked plating of osteoporotic fractures of the humerus.

BACKGROUND
Locked plating techniques recently have gained popularity and offer a different biomechanical approach for fracture fixation compared with traditional compression plating. In certain clinical situations, it may be preferable to employ a "hybrid" construct, in which an unlocked screw is used to assist with reduction and locked screws are subsequently used to protect the initial reduction. In the present study, we used an unstable osteoporotic fracture model of the humerus to determine (1) whether a hybrid construct behaved more like a locked construct or a conventional unlocked construct and (2) whether there was a difference between locked and unlocked constructs.


METHODS
Thirty third-generation Sawbones humeri were divided into three groups of ten humeri each. A locking plate with combination holes was applied to each bone with use of either a locked construct, an unlocked construct, or a hybrid construct. To simulate purchase in osteoporotic bone, all screw-holes were drilled to 0.3 mm less than the diameter of the screw used. Each specimen was then osteotomized in the middle part of the shaft, and a 5-mm segment was removed. Oscillating cyclic torsion testing was performed to +/-10 N-m for 1000 cycles, torsional stiffness was determined at periodic cyclic intervals, and the groups were compared.


RESULTS
The locked and hybrid constructs demonstrated similar behavior. The initial stiffness was similar in these two groups. At ten cycles, the locked and hybrid constructs retained 96.3% and 95.4% of their initial stiffness, respectively. During the remainder of cycling the stiffness of the locked and hybrid constructs decreased in a linear fashion (R(2) = 0.89 and 0.88, respectively), and at 1000 cycles the stiffness of the locked and hybrid constructs averaged 80.0% and 79.2% of the initial values, respectively (p = 1.0). In contrast, the unlocked constructs initially were significantly less stiff than both the locked and hybrid constructs (p < 0.001). At ten cycles the unlocked constructs retained 80.4% of their initial stiffness, and at 1000 cycles they retained only 22.3% of their initial stiffness.


CONCLUSIONS
Hybrid constructs are mechanically similar to locked constructs, and both are significantly more stable than unlocked constructs under torsional cyclic loading.


CLINICAL RELEVANCE
Combining screws in the hybrid configuration used in the present study did not compromise the mechanical performance of the construct. Hybrid constructs may decrease cost and may provide additional clinical value when treating fractures in osteoporotic bone.