Anterior cervical fusion - A finite element model study on motion segment stability including the effect of osteoporosis

Study Design. Three-dimensional, nonlinear finite element models were used to evaluate the stability of the mid cervical spine after anterior fusion i n patients with and without osteoporosis. Objectives. The objective of this study was to compare the change in flexib ility of C5-C6 after anterior discectomy with both loose-fitting and tight- fitting fusion graft. Summary of Background Data. Many factors such as surgical technique, osteop orosis, and excess neck motion during the postoperative period may contribu te to fusion failure. Knowledge about changes in biomechanical properties a fter the surgical procedure is important for selection of grafts with appro priate strength and for guiding patients in postoperative care and rehabili tation. Methods. Analyses with anterior fusion models with both loose-fitting and t ight-fitting graft were performed in a normal and an osteoporotic spine. Th e motion of the C5 vertebra in relation to the C6 vertebra were calculated, after multidirectional moment loads of 0.5 Nm combined with a compressive preload of 105 N. Results. Loose-fitting graft produced both an increase and a decrease in mo tion under various external moment loads, with graft compressive stress bel ow the compressive strength of the graft material. A reduction in motion wa s observed under all moment loads when a tight-fitting graft was used. The compressive stress in the tight-fitting graft was higher than the strength of the graft material. Osteoporosis increased the principal motions with bo th the loose-fitting graft and tight-fitting graft. Maximum increase in mot ion with a loose-fitting graft construct was observed under extension and a xial torsion moment loads. Conclusions. Anterior discectomy and insertion of the loose graft resulted in increased motion. A tight-fitting graft is beneficial in reducing motion , but the stress within the graftincreases beyond the graft strength. The p resence of osteoporosis was nominally significant when the graft was tight fitting.