INTRODUCTION
Ossification of the posterior longitudinal ligament (OPLL) is a pathology of the posterior longitudinal ligament of the spine that is caused by fibrosis and characterized by the progression of calcification to ossification with unclear etiology that can compress the spinal cord and cause neurological symptoms [1,2]. Surgical treatment of OPLL includes anterior direct decompression and posterior indirect decompression [3]. Anterior surgery tends to provide better decompression of the spinal cord and nerve roots than posterior surgery [4,5]. However, anterior surgery is relatively challenging and risky, especially in patients with combined dural ossification (DO), which can easily lead to complications such as cerebrospinal fluid leakage (CFL) and neurologic deficits [6-8]. Mizuno et al. [9] classified OPLL-DO into 3 types based on morphological features. Among them, en bloc type DO is fused with OPLL and the vertebral body and is often associated with severe spinal cord compression. Our previous study proposed that the “hook sign (HS)” was considered a characteristic imaging finding of en bloc type OPLL-DO [10].
The anterior controllable antedisplacement and fusion (ACAF) technique is a new procedure for the treatment of OPLL that can better reduce complications such as CFL and displacement of the implant than the traditional anterior approach [11,12]. The goal of the ACAF technique is to free the vertebrae-OPLL complex (VOC) to so that it can move forward. In ACAF, direct treatment of the VOC is not needed; however, the disc adjacent to the VOC is treated, thereby requiring relatively more fixed segments than ACCF surgery. Hence, we designed a novel technique, the “Klotski technique.” The antedisplacement space is designed according to the shape of the vertebrae-OPLL-DO complex (VODC) only within the OPLL-involved segments, and then the VODC is antedisplaced as in Klotski, a sliding block puzzle (Fig. 1).
In this study, we aimed to present the details of the Klotski technique and investigate the safety and effectiveness of the technique for patients with en bloc type OPLL-DO. The clinical data of patients with short-segment en bloc type OPLL-DO who were treated with the ACCF or ACAF using the Klotski technique were included in a retrospective analysis to compare neurological recovery and complications.
MATERIALS AND METHODS
1. Study Population
The study was reviewed and approved by the local ethics committee with waiver of informed consent from patients given its retrospective nature. Between January 2020 and January 2022, the clinical data of a total of 25 patients with short-segment en bloc type OPLL-DO and symptoms of cervical myelopathy who underwent ACAF using the Klotski technique or ACCF were reviewed. All procedures were performed by the same surgeon.
The inclusion criteria were as follows: (1) en bloc type OPLLDO, with the presence of a HS on the sagittal bone window of the computed tomography (CT) image; (2) cervical myelopathy with short segmental compression, OPLL involving ≤4 segments, ≤2 segments requiring antedisplacement or resection.
The exclusion criteria were as follows: (1) patients with myelopathy caused by other diseases, such as disk herniation; and (2) patients with a history of previous surgery, infection, or tumor.
All patients signed an informed consent form for the procedure after being informed of the surgical protocol and the expected benefits, risks and complications. The diagnosis of en bloc type OPLL-DO was confirmed via CT scan before surgery. All patients were followed up for at least 6 months.
2. Clinical Assessment
The number of fused segments, duration of surgery and intraoperative blood loss volume were recorded. The Japanese Orthopedic Association (JOA) score was used to assess the neurological status before surgery, within 2 weeks after surgery, and at the last follow-up. The improvement rate (IR) was calculated as IR=(postoperative JOA score–preoperative JOA score)/(17–preoperative JOA score)×100%. Complications such as CFL, spinal cord and nerve root injury, and wound infection were recorded.
3. Radiologic Assessment
All patients underwent x-ray, CT, and magnetic resonance imaging (MRI) of the cervical spine before surgery, within 2 weeks after surgery, and at the last follow-up.
A diagnosis of en bloc OPLL-DO was established for all patients according to the preoperative sagittal CT image. The “hook sign” sign is characterized by a bony high signal along the posterior wall of the vertebral body and resembles a hook extending cephalad or caudal along the dura. The extent of the OPLL was identified. The relationship between en bloc type DO and vertebrae was analyzed. We classified en bloc OPLL-DO into 3 types: type A, OPLL fusion to the vertebrae extending cephalad; type B, OPLL fusion to the vertebrae extending caudal; and type C, OPLL fusion to the adjacent vertebrae (Fig. 2).
The spinal canal occupying rate (OR), decompression width and spinal canal area at the narrowest part of the spinal canal were calculated from CT axial images. The OR was defined as the thickness of OPLL-DO divided by the anteroposterior diameter of the bony spinal canal on the axial CT image. MRI was used to evaluate the compression of the neural elements.
4. Surgical Technique
After successful anesthesia in the supine position, a conventional Smith-Robinson incision was made to reveal the anterior vertebral space, and an intraoperative radiograph was used to locate the surgical segment. In addition, intraoperative neuromonitoring using somatosensory evoked potential and motor evoked potential is done routinely.
1) ACAF using the Klotski technique
The process of ACAF has been reported previously [13]. Diskectomies were performed on the operated segments. According to the thickness of the ossification, the anterior part of the vertebral body of the VODC was removed. According to the type of OPLL-DO, different surgical strategies are used, and the posterior margin of the cephalad or caudal vertebral body is removed using a high-speed grinding drill based on the length of the calcification. A cage filled with cancellous bone is placed in each intervertebral space. Piezosurgery was used to perform osteotomies in the left hook joint of the vertebral body until the posterior wall of the vertebral body was cortically broken. A precurved titanium plate is fixed at the caudal and cephalad vertebrae of the antedisplaced segment, and a self-tapping screw is screwed into each vertebral body. Then, osteotomies were performed in the right hook joint until the cortical fracture of the posterior wall was reached, at which time the VODC was completely free, and the screws were slowly tightened to lift the VODC, thus expanding the space of the spinal canal and achieving decompression.
2) ACCF procedure
Diskectomies and vertebrectomy were performed in the operated segments. A high-speed drill is used to thin the OPLL as much as possible. For DO, the “floating method” will be used to avoid causing dural defects and CFL [14]. For dural defects with CFL, autologous fascia and bioadhesives were used to seal the dural defect. After decompression, an artificial vertebral body filled with autologous bone, a titanium plate and screws were placed to reestablish the structure and stability of the operated segment.
The fascia and skin were then sutured layer-by-layer, and a wound drain remained. The drainage tube was removed after 24 hours, when the drainage volume was <15 mL. The cervical brace was fixed for 6 weeks.
5. Statistical Analysis
The data were statistically analyzed using IBM SPSS ver. 18.0 (IBM Co., Armonk, NY, USA). The results of the measurement data were recorded in the form of mean±standard deviation, the Mann-Whitney U-test and Chi-square test were used to compare clinical and radiologic outcomes between groups, and Student t-test was used to compare differences between preoperative and postoperative clinical and radiologic outcomes. Differences with a p-value of less than 0.05 were considered significant.
RESULTS
1. General Data
Perioperative details, including operative duration, blood loss volume, number of fixed segments and length of hospital stay for each group, are presented and compared between groups in Table 1. There was no significant difference in the length of hospital stay or the blood loss volume between the 2 groups, but the duration of ACAF was significantly longer (p<0.05).
2. Clinical Assessment
At the final follow-up, the JOA scores of the patients in both groups were significantly better than those before surgery. The rate of improvement in JOA score was 82.4%±8.8% in the ACAF group and 71.9%±12.4% in the ACCF group, and there was no significant difference in the IR between the 2 groups.
In the ACCF group, there were 3 patients with postoperative deterioration of neurological function. The incidence was 21.4%, and all of them had decreased muscle strength and sensory loss in the extremities immediately after surgery, which was considered to be caused by an intraoperative spinal cord injury. Two patients’ condition were improved after rehabilitation exercises at the 3-month postoperative follow-up, and 1 patient’s condition was improved at the 6-month postoperative follow-up. In the ACAF group, there was one case of postoperative neurological deterioration, with an incidence of 9.1%. A decrease in unilateral C5 motor function occurred immediately after surgery, which was considered to be related to an intraoperative nerve root injury that improved after 2 weeks of rehabilitation.
In the ACCF group, there were 5 cases of intraoperative CFL, all of which were diagnosed intraoperatively, and clear cerebrospinal fluid (CSF) was seen to flow from the dura, with an incidence of 35.7%. The incidence of intraoperative CFL was 9.1% in the ACAF group. If CFL occurred, autologous muscle fascia and biologic adhesives were used intraoperatively to seal the dural defect. A wound drain was placed, and if CSF continued to drain, a lumbar pool drain was placed for 5 days and then removed. In the 6 patients with CFL, no fluid accumulation or infection occurred.
3. Radiologic Assessment
The spinal canal occupancy rate, decompression width, and spinal canal area before the operation and at the final follow-up in both groups are shown in Table 1. The decompression width and postoperative spinal canal area in the ACAF group were greater than those in the ACCF group, and the difference was statistically significant (p<0.05). Magnetic resonance showed that the spinal cord was successfully decompressed in both groups.
4. Case Presentation
1) Case 3
A 42-year-old man presented with a 2-year history of numbness in both hands and unstable walking that had become more severe for more than 6 months. Preoperative images showed OPLL from C3 to C5 and type A en bloc DO at C4. He underwent the ACAF of C3–6 using the Klotski technique. Postoperative CT scan and MRI demonstrated the enlargement of the spinal canal and the restoration of the morphology of the compressed spinal cord (Fig. 3).
2) Case 7
A 71-year-old man presented with a 6-month history of numbness and pain in the left upper limb that had become aggravated with weakness in the left lower limb for 2 months. Preoperative images showed OPLL from C4 to C6 and type B en bloc DO at C5. He underwent ACAF of C3–6 using the Klotski technique. Postoperative CT scan and MRI demonstrated the enlargement of the spinal canal and the restoration of the morphology of the compressed spinal cord (Fig. 4).
3) Case 8
A 57-year-old man presented with a 4-month history of numbness and weakness of the limbs. Preoperative images showed OPLL from C3 to C6, type C en bloc DO at C4, and type B en bloc DO at C5. He underwent ACAF of C3–6, and the Klotski technique was applied at C4 and C5. Postoperative CT scan and MRI demonstrated the enlargement of the spinal cord and the restoration of the morphology of the compressed spinal cord (Fig. 5).
DISCUSSION
OPLL is a heterotopic ossification of the posterior longitudinal ligament of the spine, and in cases of combined neurological impairment, surgery is the only effective treatment [1,2,15,16]. The anterior cervical approach can directly decompress the spinal cord by removing the OPLL. For patients with severe ossification occupying >50% of the posterior longitudinal ligament, direct decompression through the anterior cervical approach is more effective than posterior laminoplasty [17,18]. However, the neurologic deficit and CFL after anterior direct decompression were notable challenges [19].
DO is an important reason for the poor outcome of anterior surgery. In these cases, the boundaries between the posterior longitudinal ligament and the dura are not easily separated, and an improper operation may lead to complications [6,7]. Severe OPLL is more likely to be accompanied by DO, and often the doublelayer sign is not obvious, probably due to the progressive fusion of the dura with the OPLL as ossification progresses. Mizuno et al. [9] proposed that en bloc type DO fused with the VOC is often associated with severe spinal cord compression. All cases in this group were en bloc type DO, with a mean OR up to 64%. For en bloc type DO, the authors’ research team presented the ‘hook sign,’ which is seen more frequently in cases of severe OPLL when the OPLL has fused with the dura mater and there is no layered structure [10].
In patients with en bloc type DO, the incidence of post-ACCF CFL will be higher [6], and the incidence of CFL in the ACCF group in our study was 35.7% (5 of 14). Despite capacity of the surgical microscope to illuminate and magnify the surgical field, removing the ossified material safely is still challenging. Yamaura et al. [20] used the “floating technique” to “float” the “eggshell” ossified material on the dural surface forward with the dural sac under the tension of CSF. However, for en bloc type OD, ossification has often invaded the entire layer of the dura mater, and it is often impossible to separate the DO from the dura mater. Frequent intraoperative touching and squeezing of the dural sac puts extra pressure on the spinal cord, so it is prone to neurologic deficits [9]. In this study, 3 patients (21.4%, 3 of 14) in the ACCF group experienced neurologic deficits.
In 2018, Sun et al. [13] first reported that the application of ACAF for the treatment of cervical OPLL had satisfactory results. In the same year, Lee et al. [21] introduced vertebral body sliding osteotomy (VBSO) for the treatment of cervical spondylotic myelopathy caused by OPLL. The ACAF procedure entails a systematic process involving anterior cervical plating followed by complete liberation of the ossified vertebral body complex. Subsequently, traction is applied using screws to achieve realignment. In contrast, VBSO begins with the liberation of the ossified vertebral body complex, followed by traction using ALLIS forceps, and concludes with anterior cervical plating. The ACAF technique provides enhanced stability and greater traction force due to its sequential nature, which allows for minimal slotting and a smaller surgical field. However, there exists a potential risk of traction failure. Conversely, VBSO offers improved visualization and a higher tolerance for errors, rendering it a more controllable procedure.
In ACAF, the VOC is moved entirely anteriorly rather than directly removing the ossification, thus effectively reducing the risk of CFL and spinal cord injury by reducing the intrusion of the ossification-dural interface. However, conventional ACAF usually requires treatment of the normal disc adjacent to the responsible segment. The number of fused segments is often greater than that in ACCF surgery. Sun et al. [22] proposed the “shelter technique” in the treatment of OPLL involving the C2 segment. On this basis, to reduce the number of fusion segments in ACAF, we propose the “Klotski technique” for en bloc type DO because the antedisplacement space is designed according to the shape of the VODC within the OPLL-involved segments and the location where DO may occur is not involved. Then, the VODC is completely antedisplaced as in Klotski. This technique does not involve normal discs in adjacent segments and requires fewer fused segments.
There are 2 key prerequisites for the implementation of the Klotski technique. First, the OPLL must be fused to the posterior wall of the vertebral body and extend to the adjacent vertebral body. In this study, en bloc OPLL-DO was classified into 3 types. All OPLLs are fused to the posterior wall of at least one vertebral body, which makes it possible to simultaneously antedisplace the vertebrae and extend the OPLL to the adjacent segment. It is noteworthy that on some sagittal reconstructed CTs, fusion of the OPLL with the vertebral body appears to be inadequate, with some fractures, but this does not affect the antedisplacement of extended OPLL, provided that it is adequately released. Second, the design space in the adjacent vertebrae should be sufficient to accommodate the OPLL. With the assistance of a microscope, total resection of the adjacent posterior wall of the vertebral body via the intervertebral space is possible. In actual surgery, there is often an obvious soft tissue gap between the posterior wall of the vertebral body and the OPLL. The process of grinding away the posterior wall of the vertebral body is safe because the dura is protected by the OPLL. Care is taken not to break the OPLL at the point of heaviest compression at the intervertebral disc level, as this may affect the antedisplacement effect or cause CFL. A suitable space to accommodate the OPLL was able to be created in patients with OPLL extending towards the adjacent endplate (Fig. 3J). In this group of cases, the OPLL extended to 2–4 vertebral bodies in the ACAF group, and satisfactory decompression was achieved in all cases by antedisplacing 1–2 vertebrae and applying the Klotski technique, which did not increase the number of fixed segments when compared with the ACCF group, and the risks of nerve injury and CFL were reduced. Compared to traditional ACAF, grinding the posterior edge of the adjacent vertebrae is easier than performing a bilateral osteotomy in the vertebral body.
This study demonstrates that it is feasible to reduce the number of fused segments in ACAF using the Klotski technique; however, the study has some limitations in that the size of the patient sample was small and the follow-up period was relatively short. Studies with more cases and longer follow-up should be conducted. In addition, this was a single-center retrospective study, so a multicenter prospective randomized controlled study should be conducted.
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