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
(R2 = 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.