Interference screw fixation of cervical grafts: A biomechanical study of a new method of cervical fixation

The dislodgement of anterior bone graft in the cervical spine is a frequent complication of attempted fusion following discectomy or corpectomy. It has been hypothesized that fixation augmented with interference screws may increase the pull-out strength of the construct and decrease the rate of these complications. Mechanical tests were conducted to compare interference screw fixation methods for enhancing the fixation between the bone graft and the adjacent vertebra. The anterior pull-out strengths of cervical bone grafts were compared using fixation with and without the addition of interference screws. Both discectomy and corpectomy graft models were examined in vitro. The mean pull-out force for a Smith-Robinson type bone graft alone was 58.1 N (SD ± 11.4 N); for the graft augmented with two 3.5 mm cancellous bone screws, 153.9 N (±58.9 N); for the graft with four 3.5 mm screws, 217.1 N (SD ± 69.9 N). The pull-out strengths of the two and four 3.5 mm screw constructs were significantly greater than the strength of the graft alone (p < 0.05). Similarly placed 2.7 mm cortical screws of the same length provided increased pull-out strength (123.7 N ± 38.6 N and 142.5 N ± 38.2 N for two and four screws, respectively): however, in comparison to the graft alone, these differences were not statistically significant. For both screw types, the four screw fixations were stronger than the two-screw fixations, although these differences were not statistically different. For the corpectomy model, the interference screw fixation (two 3.5 mm cancellous screws) provided a significantly better failure load in comparison to the graft alone (p < 0.01). This study demonstrates that interference screw fixation can augment the stability of the anterior bone graft construct following anterior cervical grafting.