BIOMECHANICAL COMPARISON OF THE DEWAR AND INTERSPINAL CERVICAL-SPINE FIXATION TECHNIQUES

Study Design. This study evaluates and compares the stiffness of two c ervical spine fixation techniques. Objectives. This biomechanical stud y was carried out to compare the interspinous and Dewar cervical spine fixation techniques. Summary of Background Data. Interspinous wiring is a commonly used method of fixation in the cervical spine. The Dewar technique is less commonly known and practiced, and clinical experien ce has suggested that it maybe a more stable technique, Methods. Cervi cal spine specimens stabilized with the interspinous and ''Dewar'' tec hniques were biomechanically tested inflexion and in torsion. Stiffnes s and energy absorption under moderate loads were compared, The Dewar technique uses contoured double corticocancellous iliac grafts as inte rnal graft/splints fixed to the spine with threaded pins and wire, The interspinous technique is a single. interspinous wire loop. Eleven fr esh human cervical spines were harvested from cadavers. The spines wer e destabilized at C4-C5 by sectioning all tissue except the anterior l ongitudinal ligament. Each fixation technique was applied alternativel y and tested on each spine. Results. In torsion testing (n = 5), the D ewar fusion was 61% stiffer than the interspinous technique (P < 0.02) , Dewar: 11.3 N/mm (s.d, 4.9 N/mm) and interspinous: 8.4 N/mm (SD 3.3 N/mm). Inflexion testing (n = 6), the Dewar technique was 35% stiffer than the interspinous technique (P < 0.10). Dewar: 655.4 Nmm/degree (S D 293 Nmm/degree) and interspinous: 406.8 Nmm/degree (SD 113.0 Nmm/deg ree). Energy absorption with the interspinous technique was greater in flexion (P < 0.10) and in torsion (P < 0.005), indicating more deform ation with the interspinous technique. There was no statistically sign ificant difference between the means of specimens tested first and tho se tested second independently of the fixation technique. Conclusions. These tests indicate that the Dewar cervical spine fixation is stiffe r than the single interspinous wire in both flexion and particularly t orsion, This project is the only biomechanical study of the Dewar tech nique that we are aware of, and the results support the clinical findi ngs regarding the effectiveness of this technique.