Interference screw fixation of cervical grafts: A combined in vitro biomechanical and in vivo animal study

The dislodgement of an 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 pullout strength of the construct and decrease the rate of these complications. In vitro mechanical tests and in vivo sheep studies were conducted to compare interference screw fixation methods for enhancing the fixation between the bone graft and the adjacent vertebra. Using human cadavor cervical spines, the anterior pullout strengths of cervical bone grafts were compared using fixation with and without the addition of interference screws for the in vitro mechanical testing. The mean pullout forces for a Smith-Robinson type bone graft alone was 53.1 N (SD 11.4 N); for the graft augmented with two 3.5 mm cancellous bone screws, 153.9 N (58.9 N);and for the graft with four 3.5 mm screws, 217.1 N (SD 69.9 N). The pullout 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 pullout strength (123.7 N 38.6 N and 142.5N 38.2 N for two- and four-screw constructs, 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 significant. For the in vivo portion of the study, a singie-level anterior cervical discectomy and fusion were performed on 20 sheep. The fixation was augmented i n ten following surgery. At the end of 12 weeks, all nondisplaced cervical graft/vertebra constructs were harvested, radiographs were taken, and histology was performed. A very high rate of cervical graft dislodgement was found in this sheep model of anterior cervical fusion. The graft survival curve demonstrates that, for both groups, the high-risk period of graft dislodgement is during the first week following surgery. There was statistically-improved graft survival at 1, 2, 6, and 12 Weeks for the grafts augmented with interference screws compared with that seen in the control group (P< 0.04). All nondisplaced grafts resulted in solid fusion by observed in direct visual examination, roentgenogram, and histology of the specimen. No adverse effect to graft incorporation was found for the interference screw group. This study demonstrates that interference screw fixation augments the stability of the anterior bone graft construct and decreases the dislodgement following anterior cervical grafting without compromising the graft incorporation.