Surgical Effect and Clinical Characteristics

In the present study, we investigated the postoperative progression of T-OPLL and evaluated the long-term results after CD using the posterior approach. This 5-year follow-up study indicated that the length and width of the maximum region of ossification in patients with T-OPLL still tend to increase after total resection by CD. We also found that although T-OPLL progressed, all patients achieved a satisfactory recovery at the last follow-up, and no patients exhibited any neurological deterioration due to T-OPLL progression.

T-OPLL is an ectopic ossification of the posterior longitudinal ligament at the thoracic spine with an unknown cause and is one of the leading causes of thoracic myelopathy. OPLL is most prevalent in East Asians, especially in Japanese, South Koreas, and Chinese. Although T-OPLL is less common than cervical OPLL, it has an insidious onset, high disability rate, high surgical risk, and high postoperative paralysis rate12. CD is a promising but invasive surgical approach, removing the compressive force directly. Nevertheless, the long-term results of this approach are unclear. Progression of cervical OPLL has been well-documented in the literature. However, the progression of T-OPLL remains to be elucidated.

Ossification Progression

Multiple studies have indicated that cervical OPLL progression is common. Yoshimura et al. showed that cervical OPLL progressed in all affected subjects in a group of 30 patients after a follow-up of 3 years8. Byung-wan et al. evaluated the progression of 60 patients with cervical OPLL by CT with a minimum follow-up of 2 years and found that progression of cervical OPLL is associated with younger age, involvement of multiple levels, and mixed type morphology13. One study reported the progression of T-OPLL after posterior compression and fixation. Shurei et al. analyzed nine consecutive patients with T-OPLL who underwent posterior decompression and fixation for at least 3 years of follow-up and concluded that the size of the T-OPLL still increased after spinal stabilization14.

This study is among the first to investigate the progression of T-OPLL after CD. In the present study, most T-OPLL cases were of continuous and mixed types. In addition, progression of T-OPLL was observed in all cases after more than 5 years follow-up. Previous studies have shown that progression is more frequent in patients with continuous and mixed types of cervical OPLL. This was, to some extent, in accordance with previous studies. However, it depends on whether CD was performed. Further studies are required that examine whether continuous and mixed types of T-OPLL are more common.

Mechanism Research

To date, the etiology and pathomechanism of OPLL remain unclear. Multiple studies have suggested that OPLL is a multifactorial disease influenced by numerous genetic and non-genetic factors15, 16. Non-genetic factors include mechanical stress, degeneration process, diet, and biological rhythm. Compared to the cervical spine, the thoracic spine has a smaller range of motion limited by the thorax. It is universally acknowledged that the thoracic spine is more stable, experiences less mechanical stress than the cervical spine, and is less susceptible to degeneration. Genetic studies of T-OPLL revealed that deleterious mutations in several genes might contribute to the development of T-OPLL by whole genome sequencing.

Previous studies reported that progression of cervical OPLL was much more common in patients who had undergone surgical treatment than in those who received nonsurgical treatment due to biological stimulation after decompression, such as changes in the microcirculatory environment within the spinal canal. Histological studies of OPLL suggest that OPLL develops through the process of endochondral ossification, initiating from the differentiation of mesenchymal stem cells into chondrocytes. In most of the studies on the progression of cervical OPLL and T-OPLL, the surgical patterns were decompression and stabilization via a posterior procedure. T-OPLL continues to grow after spinal stabilization because the OPLL mass still exists after surgery. However, in the current study, all T-OPLL masses were completely resected by CD. As a result, we speculated that the following factors caused progression. First, the OPLL mass was not completely removed, and the remnant of the OPLL mass progressed postoperatively. Second, after the OPLL was resected, there were changes in the microcirculatory environment within the spinal canal. The mechanism of progression of OPLL is similar to that of myositis ossificans traumatica. Third, the adjacent longitudinal ligament continued to gradually expand into the original site.

The security and efficacy of CD have been discussed in several previous studies. Multiple studies have reported that CD is a safe and effective procedure3, 7. However, there are debates regarding indications for CD. Lee et al. advocated that CD through a posterior approach is safe, effective, and less invasive to ventral dural compressive lesions in the lower thoracic region17. Ma et al. retrospectively investigated the clinical outcomes of 23 patients with T-OPLL circumferential spinal cord decompression and concluded that indications of CD are limited to thoracic myelopathy caused by severe anterior impingement of various etiologies from T4–T1218. The results of this study are highly reliable because it has the longest follow-up time to date in evaluating the long-term results of patients with T-OPLL after CD. Although T-OPLL progressed in all affected subjects in our study, myelopathy was not influenced by the progression in the current 5-year follow-up study. CD can provide adequate reserve ventral space to cope with postoperative T-OPLL progression. In the present study, CD was performed in seven patients in the upper thoracic spine. All patients recovered well after long-term follow-up. We considered that the outcomes of thoracic surgery are closely related to the surgeon's experience, such as careful performance.

There are several limitations to this study. First, the number of patients enrolled was relatively small. A multicenter study with a larger sample size should be performed to examine T-OPLL. Second, the measurement process was semi-automatic. Therefore, human errors may have occurred. However, we believe that the evaluation of the ossification area was accurate and valid, as indicated by the high intra-observer and inter-observer intra-class correlation coefficients. Third, the follow-up duration was short. T-OPLL progression may result in neurological dysfunction after a longer follow-up period. Our findings need to be fully proven over a more prolonged follow-up period.

In conclusion, the present study demonstrated that the size of the T-OPLL increased after CD. Although the OPLL mass was mostly removed at the CD level, OPLL progression did not decrease or stop. According to the long-term follow-up results, CD is a safe and effective procedure that can provide adequate reserve ventral space to cope with postoperative OPLL progression.