Factors influencing maxVB, BMM and BMD and the validity of this study
The incidence of DISH increases with age and the number of bone cross-links [10]. Although BMM, BMD, and CT from the thoracic to the lumbar spine were not necessarily taken at the same time, the average interval between inspection dates was 1–2 months, and all inspections were performed within a year. The number of bone bridges reported to develop over the mean course of the five-year period suggests that a gap of approximately one year is not so significant [10]. In addition, studies have reported that age, renal function, postmenopausal osteoporosis, history of hyperparathyroidism, rheumatism, and history of steroid and osteoporosis medication are factors affecting P1NP and TRACP-5b [8, 9]. Thus, the eligible patients were eliminated according to the patient’s medical records or subsequent telephonic interviews. Since we are dealing only with male patients and do not need to consider postmenopausal osteoporosis, a rapid decline in bone mineral density is unlikely to occur within a year or so. Furthermore, since no patients were treated for osteoporosis, the effect of bone metabolism markers was considered small. Finally, in a previous study, BMM did not increase until the first week following a fracture but remained elevated up to 1 year following the fracture [7, 24]. Hence, patients with fractures occurring within one year were also excluded from this study, suggesting that we eliminated as many factors as possible that could affect BMM.
Differences in BMM between DISH and ankylosing spondylitis (AS)
AS and DISH involve bone cross-linking between the vertebrae, but show completely different levels of BMD and BMM. Among BMM, bone resorption markers, such as serum C-telopeptides, which belong to type I collagen (CTX), are high in AS. Of note, higher CTX signifies lower bone density values. Both AS and DISH are similar in terms of the formation of bone bridges in the spine when bone resorption markers increase, but they differ in bone density and P1NP [25–27]. The elevated bone resorption markers in both diseases are different, and the augmented bone resorption in AS induces the decreased bone density, whereas bone resorption in DISH increases at the expense of accelerated bone formation. First, it is essential to distinguish AS as a prerequisite for assessing bone healing in DISH; for this, the modified New York criteria evaluated by clinical and imaging items and human leukocyte antigen B27 (HLA-B27) have been used previously [28, 29]. In this study, all tests were conducted within insurance, but HLA-B27 was not tested. Nevertheless, as the incidence of DISH is very high compared with AS, which is remarkably low (0.48/100,000), in the Japanese population, and because patients without SI joint fusion were examined in this study, the population is considered AS-free [30]. The results of this study further showed that bone density was higher in the DISH group and that only P1NP increased in proportion to max VB, with TRACP-5b showing a compensatory rise. These increases were also considered to represent DISH as they were different from the trends of BMD and BMM in AS.
Effect of max VB on P1NP
To the best of our knowledge, no paper has yet examined the effect of the number of bone cross-links on BMM. Moreover, as mentioned above, studies about BMM comparing DISH and non-DISH have not eliminated confounding factors [17–19]. First, age must be adjusted because it affects both the number of bone cross-links and P1NP. As age increases, the number of bone cross-links increases while P1NP decreases [10, 31]. Given these findings, we would expect P1NP to decrease with increasing max VB. Unexpectedly, P1NP was positively correlated with max VB after adjustment for other confounders. The formation of bone cross-links is caused by ectopic ossification, in which the anterolateral ligament component of the vertebral bodies becomes an osteophyte [32]. Ectopic ossification has been proposed with the involvement of mesenchymal cells and osteoblastic progenitor cells [33]. P1NP, which is produced from the early stage of osteoblast differentiation, acutely reflects early bone formation; high P1NP indicates high osteogenic potential, which may be due to ectopic ossification and bone bridging after migration of osteoblast progenitor cells [34]. Therefore, max VB can be considered an indicator of bone formation.
Effect of BMM on BMD
In the present study, BMD showed a negative correlation with both bone formation and resorption markers, suggesting that high BMD and bone formation markers are incompatible. An interesting paper examined the correlation between P1NP levels and hip bone density in teriparatide-treated patients, reporting that approximately 60% of patients presented with less hip BMD, which was independent of the changes in P1NP levels [35]. This fact may support the result that when both bone formation and resorption markers as well as bone metabolic turnover are low, bone density is high. Conversely, when BMM are elevated, bone metabolic turnover is increased and bone density is low. Subsequently, we considered max VB and BMD. Regarding bone density in DISH, there have been reports of higher or unchanged bone density compared with controls [11–16]; however, bone density is higher for max VB from 4 to 8 and unchanged for max VB from 9 to 18 compared with the non-DISH group. Therefore, bone density results in cases of DISH will vary depending on the max VB [21]. This is because when max VB is between 4 and 8, bone formation and compensatory bone resorption are moderate, resulting in high BMD. Conversely, when max VB is between 9 and 18, bone formation and compensatory bone resorption are accelerated, resulting in lower BMD due to increased bone metabolic turnover.
Can max VB be a predictive indicator of bone healing?
High bone density is reported to be favorable for bone healing, while high levels of bone formation markers also promote bone healing [36, 37]. Basically, bone density and bone formation are positively correlated, but exceptions exist. For example, if bone formation is remarkably enhanced compared with bone resorption, one’s density will be high and bone healing will not be too bad. However, excessive bone formation produces excessive bone resorption, which is good for bone healing because it increases bone metabolic turnover, but bone mass is moderate. Inoue et al. used BTR (TRACP-5b/P1NP) as an index of bone remodeling and a factor that can predict bone healing in posterior lumbar interbody fusion [22]. Notably, this index was devised because low P1NP and high TRACP-5b levels are risk factors for poor bone remodeling. Although the mechanism of bone healing in posterior lumbar interbody fusion and posterolateral fusion remains unclear, the rate of bone healing is fast-tracked in osteoinductive cages and demineralized bone fibers, which could be considered as ectopic ossification [38–40]. The results of this study, in which BTR was inversely proportional to maxVB, suggested that the increase in maxVB, which may have promoted ectopic ossification, enhanced bone healing.
Limitations
This study has some limitations that are worth acknowledging. First, this was a retrospective study, with a small sample size. Second, this study was analyzed using P1NP, which reacts at the early stage of bone formation, rather than osteocalcin or bone alkaline phosphatase, which is a marker of the final stage of bone formation, because the study was conducted within insurance. In the future, it will be necessary to investigate bone healing after surgery for chronic spinal diseases and fractures with the degree of bone cross-linking in a prospective study.