Subjects and Surgical Procedure
ASD after L4-S1 PLIF for the degenerative disease of lower lumbar spine was developed in 17 out of 200 patients (8.5%) during the final follow-up. The segment lesions were identified at the level above the fusion and all of them were assigned to the ASD group. In the ASD group, 7 male and 10 female patients were included, with an age of 66.1±5.1 on average. The mean follow-up period was 62.8±24.6 months. The average BMI at admission was 27.04±3.00 kg/m2. In the Non-ASD group, 82 male and 101 female patients were included, with an average age of 64.7±8.7.The average follow-up duration was 85 months (72 months～99 months). The average BMI at admission was 23.87±3.33 kg/m2. In 79 patients, BMI≥25 kg/m2, of whom 13 patients (12.87%) ended up developing ASD. In 121 patients, BMI＜25 kg/m2, of whom 4 patients (3.31%) were finally diagnosed as having ASD. Table 1 presents the comparisons of basic characteristics of patients between ASD and Non-ASD groups. Statistical analysis was conducted to identify 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). Nevertheless, there was no statistically significant difference observed between the two groups in such basic variables as age, gender, comorbidities, hospital stay, intraoperative blood loss and operation time. Figure 2 shows the frequency distribution of follow-up time for ASD patients after L4-S1 PLIF.
Radiographic Evaluation and Clinical Data Collection
In the radiographic evaluation, the lower lumbar spinal parameters as measured using both 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) following the lower lumbar fusion surgery, respectively. Moreover, LDI rose by 0.03 (0.66 vs 0.69, P=0.004) postoperatively. In Table 3, the comparisons of spinopelvic parameters and LDI with and without ASD groups is demonstrated. There is no statistical significance found 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) and PI (55.49±8.22° vs 53.26±7.13°,P=0.251).When the pre and postoperative LDI subgroups were compared between ASD and Non-ASD groups, the number of patients with low LDI pre and postoperatively in the ASD group was 2 (11.8%) and 4 (23.5%), respectively. The number of patients with moderate LDI pre and postoperatively in the ASD group was 13 (76.4%) and 6 (35.3%), respectively. The number of patients with high LDI pre and postoperatively in the ASD group was 2 (11.8%) and 7 (41.2%), respectively. Consequently, a significant difference was observed in the postoperative LDI subgroups (P=0.001) but not in the preoperative LDI subgroups (P=0.252).
Relationship Between Postoperative LDI and ASD
With regard 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 as abnormal LDI intergroup. The number of patients with normal LDI pre and postoperatively in the ASD group was 13 (76.5%) and 6 (35.3%), respectively. The number of 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 not in the preoperative status (P=0.267). A typical case is presented in Figure 3. When the incidence of ASD among different postoperative LDI subgroups was compared, statistical significance was found not only between low and moderate LDI subgroups (25% vs 4.1%, P=0.006),but also between high and moderate LDI subgroups (18.4% vs 4.1%, P=0.007). However, no statistical significance was observed between low and high LDI subgroups (25% vs 18.4%, P=0.859, as shown in Table 4.
Kaplan-Meier analysis survivorship by LDI subgroups was performed to assess the rate of Non-ASD survival for ASD among the 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 exhibited a noticeable difference in ASD-free survival among the three types of LDI subgroups (Log Rank test, P=0.001). When the survival rates of every two kinds of LDI groups were compared, it was found out that this figure varied significantly not only between low and moderate LDI subgroups (Log Rank test, P=0.0012), but also between high and moderate LDI subgroups (Log Rank test, P=0.0005). Nevertheless, no statistical significance was observed between low and high LDI subgroups (Log Rank test, P=0.7223).