Between 2010 and 2011, 24 consecutive patients with single level cervical disc disease undergoing single level ACDF at our hospital were included. After the approval from the Institutional Review Board of our hospital, 12 men and 12 women participated and completed this study. The age of the patients at the time of surgery ranged from 31 to 61 years (mean, 47.4±10.1 years). The inclusion criteria was single level degenerative disc disease between C3-C4 or C6-C7. All the patients had only radicular pain without neck pain refractory to a minimum 6 weeks of conservative treatment. The exclusion criteria included : multi-level disc disease; cervical stenosis; severe spondylosis; previous cervical surgery; deformity; and patient age more than 70 years. All operations were performed by D.A.S. using the same surgical protocol. Data were collected and analyzed by S.M.H.

The patient was positioned in a physiological neutral position. A standard Smith-Robinson approach was used to expose the symptomatic level in all the patients. The surgical technique used was basically the same in all the patients. The examination was performed during the intraoperative procedure. We used the Caspar retractor (Aesculap, PA, USA) in order to open up the disc space. The Caspar retractor consists of a rack-and-pinion and a two armed (one fixed and the other moveable) compartment. Each Caspar pin was placed in the center of the upper and lower vertebral bodies. After confirmation of pin placement, the Caspar retractor was attached to the pins. A square incision was made along the border of both the endplate and bilateral uncus using a number 11 blade. Anterior annular tissue was completely removed along with anterior longitudinal ligament using a pituitary rongeur. Then, the disc space was distracted by an average of additional 2 to 3 mm over the preoperative disc height by turning the lever on the movable arm using a torque meter (Tohnichi, Tokyo, Japan), which had a maximum capacity of 10.0 kgf·cm and had an accuracy of less than 3 percent. In order to turn the lever using the torque driver, we made a linear groove on the top of the lever (Fig. 1). We stopped turning the lever when sufficient intervertebral space was exposed. The maximum torque just before the arm was suspended by the ratchet mechanism was recorded. After the measurement, a standard discectomy was performed. According to preliminary data, the subjects were divided into two groups based on the torque force (high torque group : >6 kgf·cm; low torque group : ≤6 kgf·cm).

Clinical evaluation was performed by a nurse specializing in pain management who was blinded to the patients' treatment allocations. The patients were asked to check the neck disability index (NDI) and grade their pain intensity before surgery, and at routine postoperative intervals of 1, 3, 5 days and 1, 3, 6 months. The NDI scores were expressed in a range from 0 (no disability) to 50 (maximum disability). Pain intensity was reported on a scale of 0 to 10 using the subjective visual analogue scale (VAS; 0=no pain; 10=the worst pain imaginable). The intervertebral disc height was obtained from preoperative and postoperative cervical X-rays. The subject lay in a supine neural position and scans were performed. The middle disc height of images was measured by S.M.H.

The results were expressed as mean±standard deviation. Linear regression analysis was used to evaluate the relationship between the torque force and the VAS scores for neck pain. Statistical analysis was performed with SPSS Version 16.0 for Windows (SPSS, Inc., Chicago, IL, USA).

In the low torque group, the mean age of 12 patients (6 males and 6 females) undergoing ACDF was 47.5±10.6 years. The mean operation time was 45.8±6.5 minutes and the mean amount of bleeding was 25.8±12.6 mL. Meanwhile, in the high torque group, the mean age of 12 patients (6 males, 6 females) undergoing ACDF was 48.1±8.4 years. The mean operation time was 43.3±5.7 minutes and the mean amount of bleeding was 28.2±13.8 mL. From these data, it is apparent that there were no significant differences between the two groups with regard to age, sex, operation time, and amount of bleeding (Table 1).

The VAS scores for posterior neck pain had a linear correlation with torque at postoperative 1st and 3rd days (y=0.99×-1.1, r²=0.82; y=0.77×-0.63, r²=0.73, respectively Fig. 2). After 5 days postoperatively, there was no correlation between VAS scores for posterior neck pain and torque. Nine (37.5%) among twenty-four patients had VAS scores of more than 6 for neck pain. All cases with significant VAS pain scores of more than 6 had the torque value of more than 6 kgf·cm. The mean VAS scores for posterior neck pain in the low torque group were at 0.8±0.8 on admission, 3.1±1.3 on day 1 postoperatively, 2.6±1.0 on day 3 postoperatively, 2.5±1.2 on day 5 postoperatively, 0.8±1.1 at one month postoperatively, 0.7±0.8 at three months postoperatively, and 0.4±0.5 at six months postoperatively. In the high torque group, the equivalent values were 0.7±1.2, 6.0±0.6, 4.9±0.8, 2.9±0.7, 1.3±1.1, 1.2±1.0, and 0.6±0.7. VAS scores for posterior neck pain were lower in the low torque group than in the high torque group on both 1 and 3 days postoperatively (3.1±1.3, 2.6±1.0 compared with 6.0±0.6, 4.9±0.8, p<0.01) (Fig. 3).

While VAS scores for posterior neck pain showed a linear correlation with torque at postoperative 1st and 3rd days, NDI scores did not show any correlation with torque in all postoperative periods. The mean NDI scores in the low torque group were 46.8±2.2 on admission, 39.8±2.8 on day 1 postoperatively, 30.8±3.6 on day 3 postoperatively, 23.7±4.3 on day 5 postoperatively, 16.3±3.8 at one month postoperatively, 13.8±3.0 at three months postoperatively, and 12.3±2.3 at six months postoperatively. In the high torque group, the equivalent values were 47.5±2.2, 40.5±2.0, 32.0±3.9, 25.2±3.2, 16.1±3.4, 14.1±2.7, and 12.2±1.8. The difference in NDI scores between the low torque group and the high torque group was not statistically significant in all postoperative periods (Fig. 4).

There was a mean increase of 2.1 mm in disc height at 24 operated segments. The mean disc height increased from 4.1±1.4 mm to 6.2±1.9 mm. In the low torque group, eleven of the 12 subjects gained height postoperatively. The mean gain in height was 1.9 mm while the standard deviation was 1.2 mm. In the high torque group, all subjects gained height after surgery (2.2±0.9). The difference in the intervertebral disc height between the low torque group and the high torque group was not statistically significant. In addition, there were not significant relationships among the intervertebral disc height, VAS scores, NDI scores and torque.