Influence of screw type on obtained contact area and contact force in a cadaveric subtalar arthrodesis model.

The purpose of this study was to compare the compression effect of the 7.0-AO screw and the 6.5 mm Ideal Compression Screw (I.CO.S.) screw in an in vitro subtalar arthodesis model. Six fresh-frozen, human cadaver foot specimens were obtained for analysis. The subtalar joint was opened laterally without affecting the articular surfaces. ATekscan 5051 sensor with a maximum pressure of 250 PSI and a sensel-density of 62 sensel/sq-cm was placed into the joint, which allowed for continuous measurement of the contact area and contact forces achieved by one 7.0 AO-screw, and thereafter by one 6.5 I.CO.S.-screw. When tightening the screw, mean contact area increased by 0.21 cm2 for the AO-screw (p<0.05), and by 0.27 cm2 for the I.CO.S.-screw (p>0.05). When comparing the tightened AO-screw and I.CO.S.-screw, mean contact area increased from 1.40 cm2 to 1.97 cm2 (p<0.05). The mean contact force also increased when tightening the screws. This increase was 7.6 N for the AO-screw (p<0.05) and 14.8 N for the I.CO.S.-screw (p>0.05). When comparing the tightened AO-screw and I.CO.S.-screw, mean contact force increased from 54.9 N to 81.7 N (p<0.05). The obtained results have shown that the design of the screw influences the achieved compression force. The superior compression of the I.CO.S.-screw might be explained by the better gripping and additional compression mechanism of its head. The shape of the head of the cannulated AO-screw, in contrast, may be critical to resist against the weak cortical bone of the calcaneus, i.e. it can sink into soft bone resulting in a loss of compression force.