The most common long-term complication of adult degenerative scoliosis patients who receive corrective surgery is ASD. According to previous studies, the reasons and risk factors for ASD after long segmental spinal surgery remain controversial. Some reports have indicated that aging and mechanical and biomechanical changes in the fused segment were risk factors [14,25,26], but unrandomized controlled patient characteristics, preoperative surgical data, and mixed short and long segmental spinal fusions generated doubtful results.
Whether the possibility of ASD increases as more motion segments are included in spinal fusion remains unclear. Dehnokhalaji et al. [27] found that the length of fusion was not significant in developing ASD, but only the distal intervertebral disc was discussed. Bassani et al. [28] proposed that patients with single-level fusions were more likely to have clinical ASD than those with multilevel fusions [29]. Gillet et al. included patients with 5 or more fusion levels and found that the risk of ASD was not increased [30]. However, in our studies, no significant difference in the number of fusion levels suggested that the adjacent degeneration was not affected by the fusion levels.
The rib cage provides a stabilizing effect and may balance the detrimental effect of long-level fusion in the development of ASD. These findings may be attributed to the bracing effect of the rib cage. Thus, the recommended length of fusion should be extended to the thoracic area when treating patients with ASD proximal to a prior fusion [30]. In our study, extending the fusion level to the thoracic spine did not increase the risk of ASD (p>0.05); thus, “rib cage protection” had no effect because sagittal alignment has a more profound influence than structural anatomical protection.
PI is a morphological parameter that plays a key role in the regulation of positional pelvic and spinal factors [14,31]. The potential for variations in spinal curves is associated with variations in pelvic positional parameters. For the same PI, SS and PT can vary, with the following relation among them: PI (morphological) = SS (positional) + PT (positional) [14]. PI creates different lordosis values to allow conditions conducive to standing posture and gait, according to the principle of biomechanical economy [14]. In contrast, we found that lower PIA patients may be a contributing factor to the development of ASD after long spinal fusion (Table 2). Many studies [32–38] have focused on how to correct the sagittal profile, including increased LLA, reduced normal C7 plumb line and PTA, but our results show that there was no significant significant difference between the incidence of ASD with preoperative and postoperative spinopelvic parameters, except for PIA. The PI, or pelvic base angle, is a useful descriptive terminology and an extremely important parameter for determining the global spinal balance of an individual [39,40]. The PI thus determines the relative position of the sacral plate with respect to the femoral head and the amount of lumbar lordosis required to maintain an erect posture [39–42]. In summary, higher PIA was always accompanied by higher LLA due to higher SSA, which decreased the adjacent segment facet pressure and reduced the energetic consumption of erect muscle. The accepted postoperative sagittal profile did not indicate proper preservation of an erect back muscle. One study [43] found significant geometrical reductions of erector spinae by approximately 26 and 14% at the L5-S1 and L4-L5 levels, respectively, after posterior lumbar surgery. Another study [44] found that fusion generated a 12% reduction in the total multifidus muscle force during erect standing, and 10.5% reductions were produced during 20° flexion
In our study, muscle weakness may be one of the major causes of adjacent level degeneration.
Although the PIA is an anatomical indicator of sagittal balance and is simple to estimate, the correlation between the PIA and ASD has not been determined. Our study revealed that the lower preoperative PIA contributed to the development of ASD after long segmental fusion maybe result from the suspicious of lower amount of back erect muscle. A decreased LLA resulting from the lesser postoperative and final follow-up PIA may lead to an anterior shift of the upper body trunk. The patient would have to spend more energy in maintaining sagittal balance, which may cause the adjacent level to sustain more compressive load [45]. Eventually, the back muscle fatigues, and ASD develops.