A Commentary on “Flexion K-Line Status Predicts Surgical Strategy in Multilevel Cervical Ossification of the Posterior Longitudinal Ligament: A Multicenter Comparison of Laminoplasty and Laminectomy With Fusion”

Yutaro Kanda

doi:10.14245/ns.2551834.917

Ossification of the posterior longitudinal ligament (OPLL) is not rare in East Asian countries such as Korea and Japan, with prevalence rates ranging from 1.9% to 4.0% [1]. The K-line is an imaginary line drawn connecting the midpoint of the spinal canal at C2 and C7 in patients with cervical OPLL, helping surgeons decide the surgical approach and procedure. Cervical laminoplasty (LP) has demonstrated favorable neurological outcomes in patients with K-line (+) and multilevel OPLL, although K-line (-) patients could experience insufficient decompression and poor neurological recovery when treated by LP. Therefore, anterior cervical decompression and fusion (ACDF) or laminectomy with fusion (LF) is frequently selected in patients with K-line (-) and multilevel OPLL because of favorable neurological recovery [2]. However, perioperative complications, such as dysphagia and graft complications, were more common when treated by ACDF compared to LF [2]. Consequently, surgeons are more likely to choose posterior surgery for high-risk patients, such as patients of advanced age and/or comorbidities.

The optimal surgical procedure for the cervical OPLL is still a matter of debate. Some patients with K-line (+) and multilevel OPLL after LP experience limited neurological recovery [3]. There is a limit to the decision-making by the use of K-line alone. This may be because of the lack of dynamic factors. In fact, a retrospective study of 72 patients demonstrated large preoperative segmental range of motion (ROM) at the peak of the OPLL as an independent risk factor for poor surgical outcome after LP for K-line (+) OPLL [3]. Another study of 45 patients with OPLL also demonstrated a preoperative C2–7 ROM of greater than 20° as the risk factor of poor clinical outcome [4].

The authors [5] conducted a multicenter study of 426 patients with multilevel cervical OPLL with a focus on the K-line at the flexion position (FK-line). As expected, FK-line (-) patients tended to have a more kyphotic alignment and a more severe spinal cord compression, which would result in poorer neurological recovery compared to FK-line (+) patients. These findings are consistent with prior reports [6,7]. In contrast to these reports, the authors [5] compared clinical outcomes between LP and LF in a significant number of patients with FK-line (-), which should be commended. LF achieved better correction of sagittal alignment and greater posterior cord shift, leading to superior neurological outcomes compared with LP. Considering that nearly half of the patients with K-line (+) were FK-line (-) in this study, spine surgeons should more aggressively select LF, although LF is associated with longer operative times and increased blood loss with decreased ROM in the cervical spine.

C5 palsy, a major complication of cervical spine surgery, is a major concern for spine surgeons. An in vivo study using rats by Yokota et al. [8] and several clinical studies [9,10] demonstrated that greater posterior spinal cord shift is associated with C5 palsy. Additionally, Kurakawa et al. [11] demonstrated a correction angle exceeding 20° and C4–5 foraminal stenosis as the risk factors for developing C5 palsy. Although the authors did not show a greater incidence of C5 palsy in LF compared to LP despite a greater spinal cord shift, these findings should be crucial points of attention to achieve better clinical outcomes when using posterior instrumentation.

Although the optimal surgical procedure for the cervical OPLL is still controversial, the authors [5] provided valuable insights and more plausible evidence in posterior surgery for cervical multiple OPLL. I look forward to reading a next valuable research article from the authors soon.

NOTES

Conflict of Interest

The author has nothing to disclose.

REFERENCES

1. Boody BS, Lendner M, Vaccaro AR. Ossification of the posterior longitudinal ligament in the cervical spine: a review. Int Orthop 2019;43:797-805.

2. Yoshii T, Morishita S, Egawa S, et al. Prospective investigation of surgical outcomes after anterior decompression with fusion and laminoplasty for the cervical ossification of the posterior longitudinal ligament: a propensity score matching analysis. J Clin Med 2022;11:7012.

3. Saito J, Koda M, Furuya T, et al. Segmental motion at the peak of the ossification foci is independent risk factor except for mal-alignment and thick ossification foci for poor outcome after laminoplasty for cervical ossification of the posterior longitudinal ligament: analyses in patients with positive Kline, lordotic alignment, and lower canal occupying ratio. J Orthop Surg Res 2020;15:407.

4. Maruo K, Moriyama T, Tachibana T, et al. The impact of dynamic factors on surgical outcomes after double-door laminoplasty for ossification of the posterior longitudinal ligament of the cervical spine. J Neurosurg Spine 2014;21:938-43.

5. Shin JJ, Yoo SJ, Park SJ, et al. Flexion K-line status predicts surgical strategy in multilevel cervical ossification of the posterior longitudinal ligament: a multicenter comparison of laminoplasty and laminectomy with fusion. Neurospine 2025;22:937-48.

6. Nori S, Aoyama R, Ninomiya K, et al. K-line (-) in the neck-flexed position affects surgical outcomes in patients with ossification of the posterior longitudinal ligament after muscle-preserving selective laminectomy. J Orthop Sci 2020;25:770-5.

7. Takeuchi K, Yokoyama T, Numasawa T, et al. K-line (-) in the neck-flexed position in patients with ossification of the posterior longitudinal ligament is a risk factor for poor clinical outcome after cervical laminoplasty. Spine (Phila Pa 1976) 2016;41:1891-5.

8. Yokota A, Fujishiro T, Usami Y, et al. An experimental rat model of C5 palsy following posterior decompression surgery of the cervical spine. Spine (Phila Pa 1976) 2022;47:E124-31.

9. Imagama S, Matsuyama Y, Yukawa Y, et al. C5 palsy after cervical laminoplasty: a multicentre study. J Bone Joint Surg Br 2010;92:393-400.

10. Takase H, Murata H, Sato M, et al. Delayed C5 palsy after anterior cervical decompression surgery: preoperative foraminal stenosis and postoperative spinal cord shift increase the risk of palsy. World Neurosurg 2018;120:e1107-19.

11. Kurakawa T, Miyamoto H, Kaneyama S, et al. C5 nerve palsy after posterior reconstruction surgery: predictive risk factors of the incidence and critical range of correction for kyphosis. Eur Spine J 2016;25:2060-7.