Biomechanical comparison of tuberosity-based proximal humeral locking plate compared to standard proximal humeral locking plate in varus cantilever bending.PURPOSE
 In a prior biomechanical study using a tuberosity-based proximal humeral locking plate (TBP) an improvement in greater tuberosity (GT) fixation strength with the TBP compared to a standard proximal humeral locking plate (PHLP) was demonstrated.  The purpose of this study is to compare the TBP to the PHLP with a simulated calcar gap fracture under cyclic varus cantilever forces.


 Seven matched pairs of cadaveric humeri were studied and 11A2.3 proximal humerus fractures were created by a 1 cm gap osteotomy at the surgical neck.  Matched pairs were randomized for fixation using either a PHLP or a TBP.  The proximal articular aspect of the humerus was potted and secured to the base of a load frame.  The shaft was subjected to cyclic varus cantilever loading with a roller positioned 8 cm from the osteotomy.  Change in vertical displacement of the diaphyseal fragment was monitored and digital images were obtained.  Failure was defined as vertical displacement greater than 20 mm.  Specimens not exhibiting failure over the course of 10,000 cycles were then loaded to 20 mm of vertical displacement.  Reactant forces of the specimens at these displacements were recorded.


 Four/seven TBP specimens and four/seven PHLP specimens survived 10,000 cycles.  The average cycles to failure for TBP specimens was 7325 cycles and 5715 cycles for PHLP specimens (p = 0.525).  For the specimens that survived 10,000 cycles, the decrease in calcar gap was superior in the TBP specimens (p = 0.018).  A similar trend was seen when these specimens were loaded to failure where the percent calcar gap recovery was higher for the TBP at 74.71 ± 10.07% versus 53.22 ± 30.35% for the PHLP (p = 0.072).  In specimens that were loaded to failure after survival of 10,000 cycles the average stiffness of the TBP construct was 20.51 N/mm, and 11.74 N/mm for the PHLP construct (p = 0.024).


 CONCLUSION
 In addition to superior GT fixation shown in a prior study, the TBP construct demonstrates significantly greater stiffness at the neck fracture compared to the PHLP, when loaded to failure.  In addition, there was a trend towards less collapse in this calcar gap model.
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