The Role of K-Line and Canal-Occupying Ratio in Surgical Outcomes for Multilevel Cervical Ossification of the Posterior Longitudinal Ligament: A Retrospective Multicenter Study.

OBJECTIVE

To evaluate the impact of the K-line and canal-occupying ratio (COR) on surgical outcomes in patients with multilevel cervical ossification of the posterior longitudinal ligament (OPLL).

METHODS

Patients with cervical myelopathy due to multilevel OPLL who underwent decompression surgery (anterior or posterior) from 2013 to 2022, with 2-year minimum follow-up, were enrolled. Radiological evaluations included K-line, COR, OPLL type/level, and cervical parameters (C2 slope [C2S], T1 slope [T1S], K-line tilt). Clinical outcomes included Japanese Orthopaedic Association (JOA) score and neck-pain visual analogue scale. Patients were categorized by K-line status (+/-) and COR (<50% or ≥50%).

RESULTS

Among 575 patients, JOA recovery was significantly better in the K-line (+) and in low COR (<50%). In high COR (≥50%), K-line (-) was associated with poorer recovery. In low COR, outcomes were similar regardless of K-line. Anterior decompression with fusion (ADF) yielded the best outcomes. Laminoplasty (LP) was optimal for COR ≥50% and/or K-line (+), while laminectomy with fusion (LF) was better for COR ≥50% and K-line (-). In high COR, K-line was influenced by cervical alignment, C2S, and T1S, while in low COR, it was mainly affected by COR percentage.

CONCLUSION

Combining K-line and COR is essential for surgical planning in multilevel OPLL. When COR is high, K-line plays a significant role in predicting neurological recovery. ADF led to superior recovery, whereas for patients with K-line (-) and high COR, LF offered better results than LP. Cervical parameters at high COR influence the K-line more.