Subtalar arthrodesis stabilisation with screws in an angulated configuration is superior to the parallel disposition: a biomechanical study

The purpose of this study was to compare the stability of two established screw configurations (SC) for subtalar arthrodesis using a cyclic loading model. Eight paired human cadaver hindfoot specimens underwent subtalar arthrodesis with either parallel or angulated SC. The instrumented specimens were subjected to a cyclic loading protocol (1000 cycles: +/- 5 Nm rotation moment, 50 N axial force). The joint range of motion (ROM) was quantified before and after cyclic loading, in the three principal motion planes of the subtalar joint using pure bending moments of +/- 3 Nm. After instrumentation, the angulated SC showed significantly less mean ROM compared to the parallel SC in internal/external rotation (1.4A degrees A +/- 2.2A degrees vs. 3.3A degrees A +/- 2.8A degrees, P = 0.006) and in inversion/eversion (0.9A degrees A +/- 1.4A degrees vs. 1.5A degrees A +/- 1.1A degrees, P = 0.049). After cyclic loading, the angulated SC resulted in significantly less mean ROM compared to the parallel SC in internal/external rotation (3.3A degrees A +/- 4.6A degrees vs. 8.8A degrees A +/- 8.0A degrees, P = 0.006) and in inversion/eversion (1.9A degrees A +/- 2.3A degrees vs. 3.9A degrees A +/- 3.9A degrees, P = 0.017). No significant differences in the mean ROM were found between the angulated and parallel SC in dorsal extension/plantar flexion. The angulated SC resulted in decreased ROM in the subtalar arthrodesis construct after instrumentation and after cyclic loading compared to the parallel SC. The data from our study suggest that the clinical use of the angulated SC for subtalar arthrodesis might be superior to the parallel SC.