To the best of our knowledge, this 34-year longitudinal cohort study is the first to examine the relationship between ASD and height loss. We observed that height loss was significantly correlated with changes in the sagittal modifiers of the SRS-Schwab classification . Our results showed that height loss was more common in female subjects and was related to coronal and sagittal deformities observed in DLS and ASD.
There was no significant change in the values for TK over 34 years, which is similar to the findings of Kobayashi et al. who also reported small changes in TK over their study period of 12.3 years . On the other hand, Diebo et al. reported a wider range for TK, with spinal malalignment ranging from hyper- to hypokyphosis , which might have been because of the wide range of spine flexibility, disc degenerations, and age observed in their cohort. Kamimura also showed that spinal kyphosis was significantly associated with height loss in elderly Japanese women . Their report used self-reported categories of kyphosis, which though simple, was not objective.
Aging-related spinal changes begin with degenerative inter-vertebral disk changes . Later, height loss may occur because of several causes, such as progression of intervertebral disk degeneration, vertebral deformity or fracture, and decreased muscle strength, all of which are found in subjects with ASD. Hence, radiographic parameters are useful tools for subjective evaluation of ASD, since changes in SVA, PT, and PI-LL might also lead to height loss.
The main cause for DLS is disc degeneration, leading to decreased disc height and the progression of DLS. Faraj et al. provided strong evidence that increased disc degeneration in DLS, the first sign of height loss, leads to progression of the lumbar scoliosis curve . In the current study, more subjects were diagnosed with DLS compared to the general population. Such a high incidence might be due to the fact that most people living in this area are agricultural workers in the area, and the inherent requirement of the job is that they should remain in a forward bending position for long periods of time. This position increases the load on the lumbar spine, which can be a cause for the higher prevalence of degenerative scoliosis in this cohort.
Furthermore, it is reported that LL decreases and SVA increases as degenerative scoliosis progresses , with preceding coronal and sagittal deformities also influencing each other . Hence, scoliosis is reportedly one of the factors associated with height loss of 3 cm or more .
In our study, height loss was more pronounced in women, which might be because of the differences in the skeletal structure between the sexes. It should be noted that these differences may be observed over time; however, to date, no prospective studies have confirmed this change, although Takemitsu et al. have reported a higher prevalence of lumbar degenerative kyphosis in women in a previous study , which might explain the higher prevalence of height loss in women than in men. Besides, women are reportedly more likely to experience degenerative disk changes . Furthermore, while vertebral deformity is expected to occur in elderly subjects , women were at a higher risk of vertebral fractures associated with aging. Reduced muscular strength due to advanced age and vertebral fractures because of osteoporosis mainly occur in women, and the female sex is a risk factor for DLS . Therefore, ASD and height loss are more commonly seen in women. In a 10-year prospective study, Yoshimura reported that in men, height loss did not differ significantly between those in the 40-49 years age group and the 70-79 years age group, but in women, height loss was more common in the 70-79 years age group than in other age groups (40-49 and 60-69 years) . These results suggest that height loss progresses rapidly with age in women.
The complication rate of ASD surgery is high [35,36], and early care for maintaining sagittal alignment is one of the most important treatments. The use of bisphosphonate and denosumab as an early treatment to maintain spinal alignment and to prevent height loss [37,38], especially when combined with the reportedly effective physical treatment for spinal deformity, may result in delaying surgical treatment [39,40].
One of the strengths of our study is that height was measured accurately in both times, while in previous reports, participants recalled their previous heights from memory, which might have led to inaccurate height recording . Our study also had several limitations. Firstly, the number of subjects was low, which affected the findings of our study. However, follow-up periods were longer than previously reported studies. Secondly, our study did not examine QOL score, or living status in relation to height loss. Thirdly, subjects were living in an agriculture area and trunk bending positions were a requirement of their job. Therefore, we cannot generalize our results to other populations. Large longitudinal studies including subjects from more diverse backgrounds are required to address these limitations.