**1. Introduction**

Ossification of the posterior longitudinal ligament (OPLL) is a heterotopic ossification of spinal ligaments and a unique degenerative spine disease that causes neurologic disorders in middle and old age [1,2]. Although the prevalence of OPLL in Asian countries is reported to range from 1.9% to 4.3% [2], a majority of patients with OPLL seem to have

**Citation:** Hirai, T.; Yoshii, T.; Sakai, K.; Inose, H.; Yuasa, M.; Yamada, T.; Matsukura, Y.; Ushio, S.; Morishita, S.; Egawa, S.; et al. Anterior Cervical Corpectomy with Fusion versus Anterior Hybrid Fusion Surgery for Patients with Severe Ossification of the Posterior Longitudinal Ligament Involving Three or More Levels: A Retrospective Comparative Study. *J. Clin. Med.* **2021**, *10*, 5315. https:// doi.org/10.3390/jcm10225315

Academic Editor: Emmanuel Andrès

Received: 5 October 2021 Accepted: 12 November 2021 Published: 15 November 2021

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no neurologic symptoms [3]. However, ossified lesions that develop in the spinal canal compress the spinal cord and nerve roots, causing myelopathy or radiculopathy [4]. Surgical treatment should be provided for patients with progressive myelopathy [5,6], whereas conservative treatment is suitable for minimally symptomatic patients [7].

Surgical strategies have evolved as options for treating OPLL based on various studies. Posterior decompression with fusion was demonstrated to provide indirect decompression and stabilize the spinal structure in multiple segments [8]. However, it is also known that sufficient decompression cannot be achieved via a posterior approach in patients with massive OPLL who have kyphosis, and outcomes are relatively poor in posterior decompression with fusion even after eliminating the dynamic factor [9]. Anterior cervical corpectomy and fusion (ACCF) is a key strategy for achieving adequate decompression in patients with cervical OPLL [6]. However, implant failure often occurs in patients treated with anterior cervical corpectomy, and other complications also frequently occur, such as respiratory problems and dysphagia [10]. It was also reported that ACCF results in higher perioperative complication rates compared with anterior cervical discectomy and fusion (ACDF) [11]. Graft dislodgement immediately after ACCF requires emergen<sup>t</sup> salvage surgery; therefore, posterior surgery is widely used by spine surgeons and neurosurgeons even for the treatment of patients with massive ossification or sagittal malalignment [12]. To overcome these issues, an anterior hybrid technique of combined ACCF and ACDF was developed for patients with multiple segmental OPLL [13,14]. This technique allows for more screws to be placed to stabilize the anterior strut, and it is thought to provide better postoperative stability of the fused segments compared with ACCF for patients with multi-level OPLL [15]. However, no investigations have compared traditional ACCF and hybrid ACCF to perform a detailed verification of the structural stability of constructs in these two surgeries, and no studies have focused exclusively on patients with severe OPLL involving three or more segments. Therefore, we conducted this retrospective study to compare the clinical and radiologic outcomes of anterior decompression with traditional ACCF and hybrid ACCF in patients with severe OPLL.

#### **2. Materials and Methods**

#### *2.1. Patients and Methods*

This single-center retrospective cohort study was carried out in accordance with the STROBE guidelines [16] and compared ACCF and hybrid fusion for treatment of patients with OPLL involving ≥3 levels. Patients with a history of previous cervical spine surgery or injury were excluded. The study involved consecutive patients in whom anterior surgery was required to treat severe myelopathy due to a compressive lesion involving at least 3 segments, regardless of the duration of symptoms, in our hospital from 2007 to 2018. We previously performed traditional ACCF for all patients until 2011, and thereafter we performed anterior hybrid fusion where possible for patients in whom the vertebral body in the lesion could be preserved. In principle, anterior cervical surgery was performed in patients with OPLL occupying 50% or more of the anteroposterior diameter of the spinal canal. The level to be decompressed was decided based on the neurologic findings and the presence of spinal cord compression. In addition, we performed corpectomy in the levels that had the most compressive OPLL lesion and applied ACDF in the most proximal or distal segments that had relatively small compressive lesions in the anterior hybrid operation.

#### *2.2. Operative Technique*

#### 2.2.1. Anterior Cervical Corpectomy with Fusion (ACCF Group)

The operative technique for this procedure was described previously [1]. The anterior decompression with fusion procedure includes partial removal of vertebral body and discs with a strut graft. Segments to be operated were diagnosed based on preoperative radiographic and clinical findings. The length of the bone graft was measured intraoperatively using X-calipers between the upper and lower endplates of vertebral bodies operated

in a neutral cervical position. A strut graft collected from the iliac crest or made using artificial bone made from hydroxyapatite (Boneceram®; Olympus Corporation, Tokyo, Japan) was used for 2 corpectomies (3 segments), and fibula strut grafts were used for 3 or more corpectomies (4 or more segments). A semi-rigid plate was inserted in all cases. In principle, fixed screws were placed for the distal vertebrae and variable screws for the proximal vertebrae (VENTURE™ Anterior Cervical Plate System; Medtronic Sofamor Danek Inc., Memphis, TN; Figure 1a). This technique was performed by five senior spine surgeons. Patients basically wore a neck collar for 2–3 months postoperatively.

**Figure 1.** Long semi-constrained plate fixation with artificial bone graft. (**a**) Preoperative radiograph showing the C2–7 lordotic angle, C-SVA, and T1 slope. Postoperative radiographs (**b**) after dual-level corpectomy (C4–5) and ossification floating decompression and (**c**) with dual artificial bone graft after discectomy (C3/4) and single corpectomy (C5). (**d**) Postoperative radiograph showing fused segmen<sup>t</sup> angle and fused segmen<sup>t</sup> height in the ACCF group. (**e**) Postoperative radiograph showing fused segmen<sup>t</sup> angle and fused segmen<sup>t</sup> height in the hybrid group.

#### 2.2.2. Anterior Hybrid Procedure (Corpectomy-Discectomy with Fusion, Hybrid Group)

The ≥2 levels that caused relatively severe cord compression were treated with corpectomy (as in the ACCF group), and the remaining disc level was treated with discectomy. Autograft or artificial bone graft (Boneceram®; Olympus Corporation, Tokyo, Japan) for

segments treated with corpectomy and cervical fusion cage or artificial bone graft were placed with a plate and 6-screws fixation (Figure 1b). This technique was performed by four senior spine surgeons. Patients basically wore a neck collar for 2–3 months postoperatively.

#### *2.3. Clinical Evaluations*

Most of the patients visited at 3, 6, 12, 18, and 24 months for postoperative clinical and radiologic follow-up. All patients were followed up for 2 years at our institution. The degree of cervical myelopathy before and after surgery was assessed using the Japanese Orthopaedic Association (JOA) scoring system [5]. Briefly, this score comprises four items, including upper extremity motor function, lower extremity motor function, sensory function, and bladder function (Table 1). The JOA score is the sum of these items (I + II + III + IV in Table 1). The recovery rate of the JOA score was calculated to compare pre- and postoperative JOA scores as follows: Recovery rate (%) = (Postoperative score—Preoperative score) × 100/(17—Preoperative score). These clinical findings were recorded using electronic data capture (Claris FileMaker Pro 19; Claris International, Cupertino, CA) with security systems in place. The presence of dysphagia was defined as moderate or severe symptoms according to Bazaz score. The incidence of segmental paralysis (so-called C5 palsy), aspiration pneumonia, delirium, and deep venous thrombosis were recorded.

**Table 1.** Scoring system for cervical myelopathy (JOA score).


#### *2.4. Radiologic Evaluations*

Cervical sagittal alignment (C2–7 lordotic angle) was assessed using tangential lines drawn on the posterior edge of the C2 and C7 vertebral bodies on lateral radiographs acquired in a neutral standing position [10]. Preoperative center of the head—C7 sagittal vertical axis (C-SVA) [17] and T1 slope [18]—were also measured (Figure 1a). The fused segmen<sup>t</sup> angle (FSA) and fused segmen<sup>t</sup> height (FSH) were also determined. Briefly, FSA is the angle between lines drawn parallel to the cranial endplate of the cranial vertebrae of the fused segmen<sup>t</sup> and the caudal endplate of the caudal vertebrae of the fused segment, and FSH was determined as the mean value of the anterior and posterior vertebral body heights at the fused segments (Figure 1d,e) [19,20]. In the hybrid group, these parameters were independently calculated in the ACCF and ACDF segments. Additionally calculated were changes in both FSA (ΔFSA) and FSH (ΔFSH) between before and immediately after the operation, |ΔFSA| and |ΔFSH|. Graft migration was defined as subsidence >3 mm. Solid fusion was defined as the presence of continuous bone connecting the Luschka joints at the operated segments on X-ray. Radiologic measurements were performed by an independent assessor (M.H.). Formal analysis was performed by another independent assessor (T.H.). These two doctors are certified by the Japanese Society for Spine Surgery and Related Research to perform spine surgery.

#### *2.5. Statistical Analysis*

Differences between the two groups were assessed using one-way analysis of variance, the Mann Whitney *U* test, or the Chi-square test. Multivariate logistic regression with a forward stepwise procedure was performed to identify key risk factors for postoperative implant migration (*p* < 0.1 for entry), with occurrence of graft migration as the objective variable and age, sex, and radiographic parameters as explanatory variables. All statistical analyses were carried out using SPSS for Windows (version 20.0; IBM Corp., Armonk, NY, USA). A *p*-value of less than 0.05 was considered statistically significant.
