Subjects and Surgical Procedure
Adjacent segment disease (ASD) after L4-S1 PLIF for degenerative disease of lower lumbar spine was developed in 17 of 200 patients (8.5%) at the final follow-up. Segment lesions of them were identified at the level above the fusion and all of them were enrolled in the ASD group. In the ASD group, 6 male and 10 female subjects were included, with an average age of 66.1±5.1.The mean follow-up period was 62.8±24.6 months. BMI at admission was 27.04±3.00 kg/m2 at average. In the Non-ASD group, 82 male and 101 female subjects were included, with an average age of 64.7±8.7.The average follow-up duration was 85 months (72 months～99 months). BMI at admission was 23.87±3.33 kg/m2 at average. 101 patients had BMI≥25 kg/m2, 13 patients (12.87%) of which finally developed ASD. 121 patients had BMI＜25 kg/m2, 4 patients (3.31%) of which were diagnosed as having ASD in the end. Table 1 show the comparisons of basic characteristics of patients between ASD and Non-ASD group. Statistical analysis identified the significant difference in BMI (23.87 vs 27.04, p＜0.001) , BMI intergroup (23.5% vs 76.5%, P=0.001) and the average follow-up period (85.0 vs 62.8, P＜0.001). But there was no statistically significant difference between the two groups in such basic variables as age, sex , comorbidities, hospital stay, intraoperative blood loss and operation time. Figure 2 illustrates the frequency distribution of follow-up time of ASD patients after L4-S1 PLIF.
Radiographic Evaluation and Clinical Data Collection
In the radiographic evaluation, measured lower lumbar spinal parameters using pre and postoperative radiographs are shown in table 2.LL and LLL increased by 3.96°(38.71° vs 42.67°, P＜0.001) and 3.60°(26.22° vs 28.82°, P＜0.001) after lower lumbar fusion surgery, respectively. Moreover, LDI increased by 0.03 (0.66 vs 0.69, P=0.004) postoperatively. Table 3 demonstrates the comparisons of lower lumbar spinal parameters and LDI with and without ASD groups. There was no statistical significance in radiographic parameters of pre LL(37.17° vs 38.85°,P=0.341), pre LLL(23.71° vs 26.36°,P=0.177), the value of pre LDI(0.65 vs 0.66,P=0.682), post LL(40.35° vs 42.88°,P=0.241), post LLL(27.69° vs 28.93°,P=0.410), and the value of post LDI (0.71 vs 0.68, P=0.578). When comparing pre and postoperative LDI subgroups between ASD and Non-ASD groups, the patients with low LDI pre and postoperatively in the ASD group was 2 (11.8%) and 4 (23.5%), respectively. The patients with moderate LDI pre and postoperatively in the ASD group was 13 (76.4%) and 6 (35.3%), respectively. The patients with high LDI pre and postoperatively in the ASD group was 2 (11.8%) and 7 (41.2%), respectively. Consequently, significant difference was found in the postoperative LDI subgroups (P=0.001) but none in the preoperative LDI subgroups (P=0.252).
Relationship Between Postoperative LDI and ASD
With respect to the abnormal range of LDI that could be associated with the development of ASD, we attempted to integrate the low LDI subgroup and the high LDI subgroup into a single group named abnormal LDI intergroup. The patients with normal LDI pre and postoperatively in the ASD group was 13 (76.5%) and 6 (35.3%), respectively. The patients with abnormal LDI pre and postoperatively in the ASD group was 4 (23.5%) and 11 (64.7%), respectively. Statistical analysis revealed a significant difference between normal and abnormal LDI intergroup postoperatively (P=0.001), but none was found in the preoperative status (P=0.267). A typical case is presented in figure 3. When comparing the incidence of ASD among different postoperative LDI subgroups, we find statistical significance between low and moderate LDI subgroup (25% vs 4.1%, P=0.006), high and moderate LDI subgroup (18.4% vs 4.1%, P=0.007) but none was observed between low and high LDI subgroup (25% vs 18.4%, P=0.859)(table 4).
Kaplan-Meier analysis survivorship by LDI subgroups was performed to assess the rate of Non-ASD survival for ASD in patients undergoing L4-S1 PLIF (Figure 4). In low LDI subgroup, ASD-free survival was estimated to be 87.5% at 3 years and 75.0% at 6 years. In moderate LDI subgroup, ASD-free survival was estimated to be 99.3% at 3 years and 97.6% at 6 years. In high LDI subgroup, ASD-free survival was estimated to be 97.4% at 3 years and 84.5% at 6 years. The Kaplan-Meier curves showed a marked difference in ASD-free survival among the three kinds of LDI subgroups (Log Rank test, P=0.001). When comparing survival proportions of every two kinds of LDI groups, we found that the survival rate differ significantly between low and moderate LDI subgroup (Log Rank test, P=0.0012) , high and moderate LDI subgroup (Log Rank test, P=0.0005).However, no statistical significance was observed between low and high LDI subgroup(Log Rank test, P=0.7223).