Objective: To explore the correlation between the inserting angle of vertebral screws and the extent of post-operative lateral angulation instantly in surgery via anterior approach for thoracolumbar fractures.
Methods: The clinical data were from 172 patients consecutively treated with surgery via anterior approach in thoracic and lumbar fractures between May 2004 and January 2008. These cases included 124 males and 48 females at the age of 15-70 years old (mean 39 years old). One fracture were located at T11, 37 at T12, 88 at L1, 30 at L2, 15 at L3, 3 at L4. One segment was involved in 170 cases and two segments in 2 cases. According to Frankel assessment for neurological status, there were 19 cases of grade A, 24 cases of grade B, 45 cases of grade C, 53 cases of grade D, and 31 cases of grade E. The time from injury to operation was 2-30 days (median 8 days). According to the coronal Cobb angle instantly after surgery, the patients were divided into three groups: 0-5 degrees group, 5-10 degrees group and over 10 degrees group. Every group was further divided into four subgroups according to the type of the internal fixation instruments: Z-plate subgroup, Antares subgroup, Profile subgroup and single screw rod (SSR) subgroup. Radiograph images were used to evaluate the coronal Cobb angle and inserting angle between the screws and end-plates. The screws were named as A, B, C and D in vertebral bodies from the cephalic to the caudal portion. The angles between the superior endplate and the screws A, B were named as angles A, B, and the angles between the inferior endplate and the screws C, D were named as angles C, D. The differences were compared between the screw's inserting angle A+D (or/and B+C) and the post-operative coronal Cobb angle instantly, and linear regression analysis was done. The satisfaction survey was accomplished.
Results: Surficial infection occurred in 1 patient at 7 days and incision healed well after debridement; other incisions healed by first intention. A total of 172 cases were followed up for 6-49 months (mean 39 months). The degree of satisfaction was 3-10 points, median 8.5 points. Various degrees of neurological function recovered in final follow-up except patients for Frankel A grade. The mean coronal Cobb angles were (0.75 +/- 3.91) degrees for pre-operatively, (3.17 +/- 4.07) degrees for instantly post-operatively and (3.46 +/- 4.21) degrees at last follow-up; showing statistically differences between pre-operatively and instantly post-operatively, between pre-operatively and at last follow-up (P < 0.05). Comparing the screw's inserting angle A+D (or/and B+C) and the coronal Cobb angle, there was statistically significant difference between Z-Plate subgroup and other subgroups in 0-5 degrees group (P < 0.05), and there were no statistically significant differences between other subgroups in each group (P > 0.05). Except the screw's inserting angle A+D (Z-Plate and SSR subgroups) and angle B+C (Antares subgroup) in 0-5 degrees group, the post-operative coronal Cobb angle correlated closely with the screw's inserting angle A+D (or/and B+C) in other subgroups of 3 groups.
Conclusion: Nonparallelism between the vertebral screws and the correlative end plate is one of the main causes of post-operative spinal lateral angulation.