A biomechanical evaluation of graft loading characteristics for anterior cervical discectomy and fusion. A comparison of traditional and reverse grafting techniques.

STUDY
DESIGN: A biomechanical study of graft loading characteristics for anterior cervical discectomy and fusion comparing the amount and location of transmitted forces.
OBJECTIVES: To evaluate the difference between traditional iliac grafting and reverse iliac grafting used for anterior cervical discectomy and fusion in the amount and location of forces applied to the grafts. SUMMARY OF BACKGROUND DATA: Traditional fusion after anterior cervical discectomy involves placing a tricortical iliac crest strut into the disc space with the cortical portion facing anteriorly and the cancellous portion posteriorly. Recently, reverse iliac grafting has been introduced in which the cortical portion is placed in the posterior disc space and the cancellous portion in the anterior disc space. There is no biomechanical or clinical study showing an advantage of using one technique over the other. This study is the first to produce data supporting one technique as biomechanically superior.
METHODS: Five fresh cadaveric cervical spines were tested using pressure-sensitive film placed between the bone graft and the vertebral endplate after an anterior discectomy was performed. A 10-pound load was applied to the cervical spine at predetermined sagittal positions. Recordings were made at neutral, 10 degrees of flexion, and 10 degrees and 20 degrees of extension after traditional and reverse iliac grafting.
RESULTS: Graft forces were identical in both traditional and reverse grafting in the location and amount of force applied. Total force increased to the maximum in flexion and gradually decreased in more extended positions. The location of the forces was completely anterior with flexion, moving to the posterior portion of the graft with positions of extension. With 10 degrees of flexion, the load applied to the grafts was 20.4 N. In the neutral position, the load was 12 N. The loads decreased further with extension with forces of 11 N in 10 degrees extension, and 4 N in 20 degrees of extension.
CONCLUSIONS: The optimal position of the tricortical iliac graft for an anterior cervical fusion is with the stronger cortical portion placed in the anterior disc space and the weaker cancellous portion placed in the posterior disc space. In this traditional position, the graft will best resist the loads applied to the cervical spine, preventing graft collapse.