Assessment of a synthetic anterior cervical ligament in a spinal fusion model in sheep.

Abstract

The anterior cervical ligament is routinely excised during cervical interbody fusion. Loss of this ligament may predispose to instability at the fusion site. Anterior plating restores stability but leads to a risk of stress shielding. An alternative approach would be to use a less rigid fixation system that would provide anterior support while allowing micromotion that could enhance bone healing within the fusion site. To determine whether augmentation of an interbody fusion with a synthetic ligament enhances fusion. Prospective randomized study in a large animal model of cervical fusion. The primary outcome was evidence of interbody fusion, as determined by radiography, computed tomography (CT), histology and biomechanical testing. Twelve skeletally mature sheep underwent single level (C2-3) anterior discectomy and interbody fusion using fresh frozen allograft. In six animals, the fusion was augmented with a braided polyethylene device rigidly fixed to C2 and C3 with screws. Sheep were euthanized 12 weeks postoperatively. Specimens were radiographed and then examined by CT. Six fusion sites (three control, three augmented) were used for nondestructive biomechanical testing to assess the stability of the fusion site. The remaining specimens were processed for undecalcified histology. As determined by radiography, the augmented group had 83% solid union as compared with 67% in the control group (p<.05). There was no difference regarding bone graft dislodgment between the two groups. The extension stiffness of the augmented group was significantly higher (p<.05) than that of controls, but there was no significant difference between the two groups in flexion stiffness. There was no significant difference in bone formation in the two groups as determined by CT and histology, although there was a trend for increased endochondral ossification in the augmented repairs. There was no evidence of significant adverse tissue reactions to the ligament. Use of a synthetic ligament was associated with a moderate increase in fusion rate and a statistically significant increase in fusion site stiffness in extension. The use of an augmentation device, such as this synthetic ligament, may be beneficial in cervical fusion, especially when multilevel surgery is being contemplated.