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Biomechanical analysis of lateral humeral condyle fracture pinning
Tamir Bloom1, Linda Y Chen, Sanjeev Sabharwal
1Department of Orthopaedics, University of Medicine, New Jersey, USA.bloomta@umdnj.edu
Background:
The purpose of this study was to determine the optimum pin configuration and the number of pins needed to stabilize the Milch type II lateral humeral condyle fractures in a pediatric bone model.
Methods:
Forty synthetic pediatric humeri were sectioned through the lateral distal humerus to simulate a Milch type II lateral condyle fracture. Each fracture was stabilized with 0.062-in K-wires in 1 of 5 configurations: 2 convergent pins, 2 parallel pins, 2 divergent 30-degree pins, 2 divergent 60-degree pins, and 3 divergent pins (n=8/group). Models were tested in extension, flexion, varus, and valgus by applying a translational force through the distal fragment at 0.5 mm/sec oscillating between 5 N and 50 N for 10 cycles. For internal and external rotation, constructs were tested at 0.5 degree/sec between ±1 Nm more than 10 cycles. The maximum force and torque values were also recorded. For bending loads, stiffness was calculated between 0.5 and 5 mm of displacement, whereas torsional stiffness was calculated between 1 degree and 10 degrees of rotation. Data for stiffness were analyzed with a 1-way analysis of variance and a 2-sample t test (P<0.05).
Results:
Among 2-pin configurations, divergent (60 degrees) pins provided statistically greater stability than less divergent pins in torsional loading, and greater stability than parallel pins in valgus loading. Three divergent pins had statistically greater stability than all the 2-pin configurations in valgus and torsional loading, and tended to provide more secure fixation in varus loading.
Conclusions:
For 2-pin constructs, maximizing pin divergence at the fracture site provided greater stability in torsional loading and valgus loading. The addition of a third pin in a divergent orientation increases stability compared with 2-pin constructs in valgus, internal, and external rotation loading.
Clinical Relevance:
Bicortical pins placed with maximum divergence and spread at the fracture site maximizes stability for 2-pin constructs in Milch type II lateral condyle fractures. If the stability of the fracture is questionable after 2 pins are inserted, the addition of a divergent third pin enhances the stability.
