Introduction

Ossification of the posterior longitudinal ligament (OPLL) is a degenerative disorder of the cervical spine that may lead to neurologic impairment. Many clinical studies have demonstrated satisfactory neurologic recovery following cervical laminoplasty [1,2,3]. However, the poor outcomes after surgery for patients are due to (1) the alignment of the cervical spine being kyphotic and (2) the large size of the OPLL [4, 5]. In 2008, an index named K-Line was proposed by Fujiyoshiet [6]. K-Line, a straight line joining the midpoints of the spinal canal at C2 and C7 on a lateral radiograph, is an index used for judging the severity of OPLL. The K-Line (+) OPLL was defined as the ossified mass not exceeding the K-Line, whereas the K-Line (−) OPLL was defined as the OPLL mass growing beyond the K-Line. Posterior decompression was recommended as a better approach for patients with K-Line (+) OPLL, and posterior decompression with instrumented fusion (PDF) was reported as a surgical option for K-Line (−) patients [7].

In our clinical practice, the satisfaction of K-Line (−) patients with OPLL was not the same after posterior approach surgery. Based on the different postures the patients assumed under evaluation by X-ray and in daily life, we speculated that the clinical outcome of K-Line (−) patients with OPLL may have some relationship with the standing posture. To our knowledge, there are few reports on the K-Line in different standing postures. Therefore, this retrospective study tried to investigate whether the standing position is related to the different postoperative outcomes in K-Line (−) patients with OPLL.

Methods

Informed consent was obtained from all patients. All methods were carried out in accordance with relevant guidelines and regulations.

In total, 74 K-Line (−) OPLL patients who underwent laminoplasty operations at our institution were included in this study from January 2017 to December 2018. Patients with acute traumatic cord injuries; patients with other confirmed neurologic disorders, such as Parkinson’s disease, multiple sclerosis and polio; and patients with ossification of the yellow ligament or a history of cervical spine surgery were excluded. The K-Line was defined as positive(+) when the OPLL peak did not exceed the K-Line and negative(–) when the OPLL peak exceeded the K-Line. OPLL patients with a negative K-Line in the upright standing position but a positive K-Line in the natural and relaxed standing positions were assigned to Group A. OPLL patients with a negative K-Line both in the upright standing position and in the natural and relaxed standing positions were assigned to Group B.

All the patients were asked to undergo an X-ray in two different standing positions: First, they were asked to learn how to keep the upright position first through pictorial charts and were requested to “stand as straight as possible and do not lean forwards, backwards or to the sides before taking the X-ray.” Next, all the patients were told to stand in a comfortable upright position that made them feel comfortable and relaxed just as in daily life and then to maintain this position during imaging.

All 74 patients who finished the 1-year follow-up were divided into 2 groups according to the K-Line in different standing positions. Thirty-four patients were placed into Group A due to a negative K-Line in the upright standing position and a positive K-Line in the comfortable standing position, and the remaining forty patients were arranged into Group B with a negative K-Line in both the upright standing position and the comfortable standing position.

An open-door laminoplasty operation was performed in both groups as follows. A posterior midline incision was made according to the required level of laminoplasty. High-speed blur was used to create a complete laminotomy on one side and an incomplete laminotomy on the contralateral side after C3-C6 (or C7) were fully exposed. The elevation of the lamina at the open side provided sufficient spinal cord decompression. The open side was stabilized with laminoplasty baskets and screws. In each case, appropriate basket sizes were chosen for adequate enlargement of the spinal canal. Philadelphia collars were applied for 2 months postoperatively. The patients were followed up at 1, 3, 6 and 12 months postoperatively. Pre- and postoperative JOA scores for cervical myelopathy and recovery rates at 1, 3, 6 and 12 months were collected.

Data were analyzed using SPSS software (version 24.0; SPSS, Chicago, IL). Continuous variables were expressed as the mean ± standard deviation (SD), and categorical variables were expressed as frequencies or percentages. An independent t test was used to analyze the difference in continuous variables between the 2 groups. A Chi-square analysis and Fisher’s exact test were used to examine the differences among categorical variables. P < 0.01 was considered statistically significant.

Results

The mean age was 61.9 ± 2.9 years in Group A and 60.4 ± 3.5 years in Group B. There were no statistically significant differences among the demographic data between the groups in terms of sex, social drug use (smoking and alcohol drinking), axial neck pain, myelopathy, or the clinical findings for the Hoffman sign, the Spurling test, dyskinesia and paresthesia. Patient characteristics and baseline functions are shown in Table 1.

Table 1. Comparisons of patient characteristics and baseline functions
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The postoperative outcome during the entire follow-up period was assessed by the JOA score, as demonstrated in Fig. 1. The average Hirabayashi recovery rates (%) for patients in the 2 groups were 58.6 ± 10.1% in Group A and 31.5 ± 9.9% in Group B (p < 0.001). There was a significant difference between the groups.

Fig. 1
figure 1figure 1

Comparison of JOA scores in the K-Line (−) (+) and K-Line (−) (−) groups. “*” Means statistical significance

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Discussion

In 2008, a concept named K-Line was proposed by Fujiyoshi, who recommended anterior decompression surgery as the first choice for K-Line (−) patients because sufficient posterior shift of the spinal cord and neurologic improvement were not obtained after posterior decompression surgery [6]. In 2013 and 2014, Taniyama considered the modified K-Line in magnetic resonance imaging to predict insufficient decompression and clinical outcome [8, 9] because it was a better predictor of the space for the spinal cord. In the process of selecting surgical methods for ossification of the longitudinal ligament, the key factor for posterior surgery was enough space for the spinal cord. Sufficient space for the spinal cord to ward off ventral compression could be achieved in two different ways: enough posterior space or sufficient cervical lordosis.

Laminoplasty was developed in Japan and is now used in patients worldwide. Laminoplasty has been recognized as an effective method to obtain posterior space, and motor function can be maintained to the greatest extent. However, sometimes this does not bring about enough indirect decompression, resulting in unfavorable outcomes [10]. To solve these problems, Matsuoka et al. concluded that titanium basket implantation is a safe procedure with satisfactory clinical results that has the potential to promote bone union between the spacer and both laminae, as well as lateral mass, just as we observed in our clinical practice. Laminoplasty with a titanium basket is a useful alternative as an improved method compared with the conventional procedure [11].

It has been reported that cervical local kyphosis is a predictive factor for poor outcomes after laminoplasty [12]. Therefore, some methods need to be taken to increase the lordotic nature of the cervical spine. Based on its better clinical outcome, PDF was regarded as a proper surgical option for OPLL with the effects of increasing the recovery rate, improving the recovery effect and significantly reducing the JOA score [13,14,15]. However, posterior cervical fusion surgery also has its own complications, such as acute blood loss anemia, surgical site infection, C5 palsy, incidental durotomy, adjacent segment degeneration, junctional kyphosis and pseudoarthrosis [16]. Ha et al. reported that, compared to posterior fusion, laminoplasty was recommended for the treatment of multilevel cervical OPLL because it could maintain lordosis and reduce the risk of kyphosis due to minimal damage to the musculo-ligamentous complex [17].

To our knowledge, there has been no study investigating the relationship between a patient's recovery and standing posture after laminoplasty with a titanium basket. Different standing postures showed different spine profiles, and compared to upright standing postures, a more lordotic global cervical angle (GCA), a more kyphotic global thoracic angle (GTA) and a less kyphotic global lumbar angle (GLA) could be obtained during natural and relaxed standing positions [18]. In other words, cervical lordosis could be increased during natural and relaxed standing postures, which provides enough space for the spinal cord. Hey et al. reported an approximately 10° increase in lordosis in the GCA angle in natural and relaxed standing positions compared with upright standing positions in a study of sixty young and healthy adults. The natural and relaxed standing position was a better choice than the upright standing position in order for postoperative patients returning to their daily activities to maintain their balance with minimal effort [19, 20]. Although better cervical lordosis could be attained after posterior fusion surgery, it was not necessary to restore the cervical curvature intentionally. In a study of cervical alignment variations in different postures, Hey et al. found that lordotic correction of the cervical spine is not always physiological and may not prove beneficial in certain patients [21]. We believe that the normal cervical spine has a wide morphology and varies according to the posture assumed by an individual, and the optimal cervical curvature should be determined by individualized postures in daily life rather than by a standard.

The limitations of the present study are as follows. First, the sample size was relatively small, which could have led to false positives in the statistics. Second, there was a short follow-up period of 1 year, which may have been too short for changes in OPLL after cervical laminoplasty to appear, and this may have affected the surgical outcomes. Despite these limitations, to our knowledge, this is the first study to investigate the relationship between the K-Line (−) OPLL patients’ recovery and standing posture after laminoplasty with a titanium basket.

Conclusion

Different standing postures are the influencing factors for the efficacy of laminoplasty in the treatment of K-Line (−) patients with OPLL. Natural and relaxed standing postures should also be focused on before devising a surgical plan for OPLL patients.