Progression rate of OPLL after laminoplasty has been reported to be up to 70%. The rate of ossified mass growth in conservatively managed OPLL patients is much lower, with 24% in vertical growth and 13% in thickness growth groups.[2,3,12,16,17] The present study demonstrated a 26.8% progression rate of vertical growth and 22.7% rate of thickness growth, which is similar to the results of previous studies.
Previous studies have reported that OPLL progresses more often in continuous and mixed types compared to segmental type. Younger age has also been reported to be a risk factor of ossified mass growth in patients who have undergone laminoplasty.[2,13,18,19] Hori et al reported that thickness progression after laminoplasty for cervical myelopathy induced by OPLL occurred in 21% of patients and progression was more frequently observed in ossified mass involving the posterior aspect of the C3 vertebral body.[11,13] The present study demonstrated that risk factors of OPLL progression reported in post-laminoplasty patients, such as general morphology of mixed or continuous type, young age, and C2-C3 involvement, also can apply to conservatively managed patients.
Similar to the growth of the ossified mass as a whole, growth in each segment is important as a risk factor for cervical myelopathy, as cord compression in a single motion segment by OPLL can trigger myelopathy. However, the classification focusing on general morphology of the ossified mass cannot be used to analyze OPLL in each motion segment. From this aspect, present authors have previously suggested a novel classification system based on the morphology of the ossified mass involving each motion segment. In this novel classification, type 1 usually corresponds to segmental type, type 4 to continuous type, and type 2 and 3 cannot be clearly matched to the classic classification based on the general morphology of OPLL. The results of the present study demonstrated that OPLL thickness progression was most common in type 3. Furthermore, segmental progression of ossified mass was more frequently found with increased segmental ROM ≥5°. It has been reported that increased ROM of the cervical spine increases the risk of myelopathy in patients with OPLL and that prognosis is poor after operation in such patients.[20–22] Lee et al also reported that patients who had undergone laminoplasty, which preserves segmental motion, present progression of ossified mass and aggravation of clinical symptoms more often than patients who have undergone fusion surgery that eliminates motion in the operated segments. The present authors' previous study also showed that OPLL progression was more frequently observed in segments with increased segmental motion. The result of the present study corresponds to those of above-mentioned previous studies, which demonstrates that increased segmental ROM ≥5° is a risk factor for ossified mass growth. Increased ROM in cervical spine seems to be a risk factor of myelopathy and for OPLL progression. Based on these results, it would be better to consider fusion operation in progressive OPLL patients rather than motion preserving techniques such as laminoplasty. The present study also demonstrated that thickness progression is more common in segments with initial thickness >5 mm, which has not been reported before.
In their study analyzing CT images of OPLL patients, Choi et al reported that connection of ossified mass to vertebral body and trabeculation formation within ossified mass is a sign of stable state of OPLL, suggesting a lower risk of ossified mass growth. Present study also evaluated the relationship between trabeculation within ossified mass and OPLL progression. No ossified mass with trabeculation formation showed thickness progression, but the result did not reach statistical significance. Definite trabeculation could only be observed in wide ossified masses with some space within cortical border of ossification. As all patients received conservative management and most of the ossified masses were relatively small, the space within ossified mass could not be clearly identified in many cases. Therefore, trabeculation within ossified mass was very rarely observed in the present study population (6.6%), which could be the reason for statistical insignificance.
Only two (2.1%) patients included in this study showed progression of myelopathic symptoms and underwent surgery. However, previous studies have demonstrated that myelopathy can occur in 16.7% to 48.7% of conservatively managed OPLL patients in long-term follow-up.[7–10] The average follow-up period of present study was relatively short, <4 years, which could be the reason of lower rate of patients showing symptom aggravation. To evaluate the rate of radiologic ossified mass progression causing clinical symptom aggravation, a long-term follow-up study would be needed.
Our study is not without limitations. First, the cutoff value for determining thickness progression was 1 mm, which was lower than that used in most previous studies, which used a 2-mm cutoff value.[11,13,14] However, those studies were conducted on patients who had undergone laminoplasty. It has been reported that ossified mass progression is slower in conservatively managed patients compared to patients who have undergone laminoplasty because of myelopathy. In the same context, segmental growth of >2 mm was very rare in present study (2 segments). Second, as a retrospective observational study, the follow-up period was heterogeneous. However, the follow-up period was not significantly different between patients with and those without OPLL progression. Finally, as previously noted, short follow-up period has limited the ability of this study to describe the rate of OPLL progression causing cervical myelopathy.
In conclusion, the results of the present study provide few clues that can identify high-risk patients and segments in conservatively managed patients with OPLL. Young age, mixed type, and C2-C3 involvement are risk factors of OPLL progression. Special attention is necessary for segments with morphology of crossing the segment, but not fusing (type 3), segmental ROM ≥5°, and initial segmental thickness >5 mm, as these factors represent a higher risk of ossified mass growth in the segment.
No funds were received in support of this work.
No relevant financial activities outside the submitted work.
Spine. 2020;45(4):234-243. © 2020 Lippincott Williams & Wilkins