Effects of Cervical Paravertebral Extensors in Patients with Cervical Ossification of the Posterior Longitudinal Ligament Grouped According to mK-Line

Objectives: The objective of this study was to quantify the morphology, composition, and asymmetry of the paravertebral extensor muscles (PSEMs) in patients with cervical ossification of the posterior longitudinal ligament (OPLL) who had different modified K-line (mK-line) and the minimum interval between the mK-line and OPLL (INTmin) values and to investigate the relationship between PSEMs and symptoms and outcomes following laminoplasty. These original findings elucidated that the atrophy of PSEMs could predict decompression outcomes and provided a theoretical basis for paraspinal muscle rehabilitation.Methods: A total of 94 consecutive patients who underwent laminoplasty for OPLL between January 2020 and January 2022 were enrolled in this retrospective study. The relative cross-sectional areas (rCSA), functional cross-sectional areas (rFCSA), and FCSA/CSA ratio of the multifidus (MF), semispinalis cervicalis (SSCe), semispinalis capitis (SSCa), and splenius capitis (SpCa) were measured at the C3-C7 segments on cervical magnetic resonance imaging (MRI). This study compared the differences between the mK-line (+) group and the mK-line (-) group, as well as between the INTmin <4 mm group and the INTmin >= 4 mm group, using the independent t-test or Mann-Whitney test for continuous variables and the chi(2)-test for categorical variables. The correlations between the PSEMs and symptoms were analyzed using either the Pearson or Spearman correlation coefficient.Results: The relative total CSA (rTCSA) of the PSEMs, especially the MF, was significantly smaller in the mK-line (-) group. However, the FCSA/CSA of the right deep extensor muscle (DEM) was larger. The asymmetry of the MF TFCSA/TCSA showed a significant difference between the mK-line groups. In the INTmin <4 mm group, the PSEMs rCSA and rFCSA were significantly smaller, while the bilateral MF TFCSA/TCSA and right SSCe TFCSA/TCSA were larger. The asymmetry of the superficial extensor muscle rCSA was significantly lower in the group with INTmin <4 mm. The postoperative modified Japanese Orthopedic Association score (mJOA) and mJOA recovery rate were positively correlated with the INTmin and DEM rCSA and negatively correlated with the asymmetry of MF FCSA/CSA.Conclusions: In patients with mK-line (-) or INTmin <4 mm, the PSEMs were smaller, and the DEM atrophy and composition changes were predominant. The MF asymmetry was higher in patients with mK-lines (-), whereas the SEM atrophy and asymmetry were more prevalent in patients with INTmin <4 mm. The DEM was related to the preoperative and postoperative mJOA scores. DEM-preserving surgery or DEM-specific rehabilitation exercises can improve the recovery of patients with OPLL during the perioperative period. In addition, attention should be paid to the evaluation of the SEM, especially the SpCa at the C3 and C5 levels.