Bone plate and screws are commonly used for fixation of humeral shaft fractures.A hybrid construct with unlocked screws and locked screws has gained popularity for
fixation of comminuted fractures. Unlocked screws are used to reduce fracture gap while locked screws with better fixation strength helps maintain and protect the fixation. It is currently unclear if configurations with different number of unlocked and locked screws would perform the same biomechanically. The purpose of this study was to carry out biomechanical tests to elucidate if performance of a hybrid construct was dependent on screw pattern.
A total of 18 humeral shaft fracture models divided into 3 groups were plated with
a 6-hole straight shaft plate under 3 different screw configurations. Comparisons were made between 1 unlocked and 2 locked screw (Technique A) and 2 unlocked and 1
locked screw (Technique B) against the Control which is an all locked construct. All fracture models were first tested dynamically under 4-point bending to fluctuate between bending moments of 1.2Nm and 12Nm. Tests were carried out at 1Hz for 10,000 cycles. Fracture gap and plate stiffness monitored during tests were used for comparison between the 3 configurations. Static failure tests were subsequently carried out to measure the construct stiffness, yield and failure strengths. Failure test was carried out in displacement control at the rate of 0.5 mm/sec.
The control and the hybrid constructs demonstrated similar behavior for both
dynamic tests and static failure tests. There were no significant difference between the configurations in stiffness (p = 0.153) and fracture gap micromotion (p = 0.563) in dynamic tests. There were also no significant differences between 3 configurations in yield strength (p = 0.854), failure strength (p = 0.076) and stiffness (p = 0.409) in static failure tests.
The current study showed no significant difference between all-locking, double
locking and single locking configurations of screw-plate constructs used in fixation of humeral shaft fractures. The results of this study provided biomechanical evidence to orthopedic surgeons in using hybrid plated constructs.