Mechanical Testing of Epiphysiodesis Screws

Epiphysiodesis is performed to treat leg-length discrepancies and angular deformities in children. However, when placed across a physis to modulate growth, screws can bend or break postoperatively. This study evaluated the mechanical properties of 3 different screw designs commonly used when performing an epiphysiodesis. Six 4.0-mm cannulated, fully threaded; six 4.0-mm cannulated, partially threaded; and six 4.0-mm noncannulated, partially threaded cancellous screws underwent cantilever bending and tension testing in a simulated physis. All screws were tested in simulated cancellous bone foam blocks. All testing was performed using a servo-hydraulic testing machine to determine stiffness and ultimate load. For statistical analysis, one-way analysis of variance with Tukey's honestly significant difference test in post hoc analysis was used to assess significant differences among groups (P<.05). The noncannulated, partially threaded screws had a significantly lower stiffness than the 2 cannulated screw types in the tension test (P<.001) and bending test (P<.001). Additionally, the noncannulated, partially threaded screws had significantly higher ultimate load to failure than the 2 cannulated screw types in the tension test (P<.001) and the cannulated, partially threaded screws in the bending test (P=.045). The results indicate that noncannulated, partially threaded screws have a higher ultimate load capacity and are less stiff than both cannulated, partially threaded screws and cannulated, fully threaded screws. Surgeons should take into consideration that noncannulated, partially threaded screws are less likely to fail following epiphysiodesis. [Orthopedics. 2018; 41(2):e240-e244.].