In the present study, spinal malalignment in AS patients was observed. AS patients presented higher PT, TK, TLK, SVA, and TPA, and lower SS, LL, SPA and SSA, which indicated retroversion of the pelvis, more local deformity and greater global imbalance. These results were consistent with previous published studies[14, 15, 17]. We also noticed that in our study the mean value of PI(47.4˚) in AS patients with kyphosis was statistically higher than that of controls(43.2˚). Debarge et al. found the mean value of PI was 61.9 ˚ in AS group and 50.6 ˚ in the control group by analyzing 28 AS patients with fixed kyphosis and 154 asymptomatic volunteers. However, Lee et al.[15, 17] found that AS patients had a lower PI compared to normal controls. The difference might be mainly due to different AS and control population included. A previous study confirmed that Chinese population had lower PI compared with Caucasian individuals. Besides, extremely wide variations have been showed in the value of PI, which ranged from 35˚ to 85˚. Therefore, whether the value of PI was higher or lower in AS still needs more investigations to be confirmed furtherly.
To date, correlation between sagittal spinopelvic parameters and the development of spinal disorders in patients with AS is not clearly confirmed yet. Debarge et al. proposed that the patients with a high PI would have more kyphotic deformity than those with a low PI if they had the same C7 plumb line position. However, Lee et al. confirmed that AS patients with a lower PI had a greater risk of sagittal imbalance. In their both investigations, only SVA and SSA were analyzed as parameters assessing the whole deformity. So, we wondered what was the real potential role of PI in the pathology of AS with kyphosis. To clarify this question, we evaluated the relationship between PI and the spinopelvic parameters describing local deformity or global sagittal balance in AS patients with thoracolumbar kyphosis.
In our study, the parameters indicating global sagittal balance included TPA, SPA, SVA and SSA. The results showed that in AS patients with thoracolumbar kyphosis increasing value of PI was significantly correlated with more global sagittal imbalance(larger TPA, lower SPA) and there was no association between PI and the local deformity(Fig. 2). However, we also found that PI in AS was not significantly correlated with SVA or SSA, which are also the spinopelvic parameters usually used to measure the whole sagittal deformity. There might be several reasons for this. First, the value of SVA is greatly influenced by pelvic compensation or patients’ postures[22, 25]. For this reason, Van Royen et al. suggested that it was not suitable to use SVA to measure the sagittal imbalance in AS from a standing full-length spine radiograph. Second, SSA strongly correlates with SS in a healthy population. Normal individual with higher PI may has higher SSA, which was confirmed by the dates of control group in our study. Third, TPA or SPA is independent of position and pelvic retroversion. TPA also combines the information from both SVA and PT. Besides, SSA or SPA decreases as the deformity increases, while TPA increases as the deformity increases[19, 22]. Therefore, it may be better to use TPA or SPA to evaluate the global sagittal alignment of spinal deformity. However, the reason why increasing PI was associated with more global sagittal malalignment was not confirmed in this study. We believe that there would be other risk factors of global sagittal malalignment for AS patients with kyphosis, which need more investigations.
It had been confirmed that PT、SS and LL were correlated with PI in normal population[26, 27]. However, there was no such relationship between LL or SS and PI in our AS patients. We believe that it is the spinal kyphotic deformity that changes the relationship between PI and other parameters. The exact reason and mechanism were still unconfirmed in our study. The spinal deformity of AS patients is more complex and there are still some unknown things about the pathogenesis and development of deformity. On the other hand, TPA was not correlated with PI in controls in our study,. However, Zhou et al.found that TPA was significantly associated with PI in 218 asymptomatic adults. The reason might be the difference of population included in both studies and the number of our control group was smaller. The association between increasing PI and larger TPA in AS patients with thoracolumbar kyphosis might be significant for the surgical design. Protopsaltis et al.also found that TPA was significantly correlated with PI in adult spinal deformity and suggested that the targets for sagittal spinal alignment increase with increasing PI.
There are some limitations in our study that require consideration. First, the present study is essentially a retrospective review. Therefore, a prospective study with a long follow-up may be needed to further confirm the role of PI in the development of deformity of AS. Second, AS patients were examined by X-ray in a natural standing position, however some patients with severe kyphosis might stood with the knees in natural flexion in order to compensate for sagittal imbalance. Thus, some parameters depending on position, such as SVA, were not accurate. Furthermore, the number of normal controls was relatively small and only AS patients with severe kyphosis were included. As such, selection bias may exist in our study. The result in our study should not be generalized to the whole AS population.