Risk of ossification of posterior longitudinal ligament (OPLL) volume progression following laminoplasty.

Background context: Progression of ossification of posterior longitudinal ligament (OPLL) after laminoplasty (LP) can lead to recurrent compression of the spinal cord, neurological progression, and possibly revision surgery. Continuity of OPLL across segments, termed true continuous segments (TCS), has been previously found to help maintaining lordosis after LP. However, the impact of TCS on post-LP OPLL volume progression remains unknown.

Purpose: This study aimed to investigate the influence of TCS in post-LP OPLL volume progression.

Study design: Retrospective cohort study.

Patient sample: Patients who underwent LP during 2006-2017 and had preoperative computed tomography (CT) and a minimum five year CT follow-ups.

Outcome measures: Progression of OPLL volume as measured on CT.

Methods: Preoperative CT images were assessed, dividing patients into TCS and non-TCS groups. The demographics, preoperative segments of OPLL were recorded. As per the previously described classification, TCS was recorded as I (continuous type OPLL on disc space spanning the upper and lower adjacent vertebral bodies for more than half of their height without any bony crack regardless of bridge formation between vertebral bodies), II (OPLL adherent to both upper and lower adjacent vertebral bodies by bridging) or III (obvious interbody auto-fusion), and the TCS II and III were defined as true bridging segments (TBS). The number of disc levels which have a TCS was documented as the TCS counts. OPLL volume was calculated by manually segmenting images using 3D slicers (Figure 2). OPLL annual volume progression rate (AVPR) was calculated by the volume change of OPLL divided by follow-up period. An AVPR greater than 5% was defined as OPLL progression. The AVPR of both groups were compared. Multivariable logistic analysis was conducted to account for confounders.

Results: A total of 56 patients (33 males and 23 females) were included in this study, with a mean age of 53.2±8.7. The average CT follow-up durations were 95.1±33.8 months. The preoperative OPLL segments of non-TCS and TCS groups were 2.9±1.0 and 4.4±1.3 (p<0.001) and the preoperative volume of non-TCS and TCS groups were 986.3±603.7 and 3512.1±1909.8mm³ (p<0.001), respectively. At final follow-up, the AVPR of non-TCS and TCS groups were 12.7%±18.2% and 5.9%±4.7% (p=0.393), respectively. Nineteen (54.3%) patients in the non-TCS group and 10 (47.6%) patients in the TCS group showed OPLL progression (p=0.632). Correlation analysis revealed that the TCS Type I counts did not present significant correlation with AVPR, while preoperative CL (p=0.021), TBS counts (p=0.029) and age (p=0.001) significantly negatively correlated with AVPR. Multivariable logistics analysis results revealed that more TBS counts (OR=0.095, p=0.026) and higher age (OR=0.894, p=0.019) are associated with lower risk of post-LP OPLL progression, and the AUC of model was 0.816. Subgroup analysis showed patients with TBS have significant lower OPLL progression ratio and AVPR than the patients with TCS type I.

Conclusion: TBS counts were negatively associated with OPLL annual volume progression rate and OPLL progression risk following LP, while TCS type I counts did not show significant impact on post-LP OPLL progression. TBS should be identified and considered when planning LP for OPLL.