INTRODUCTION
Posterior cervical fusion (PCF) is an important method for degenerative cervical spine disease, cervical ossification of posterior longitudinal ligament (OPLL), cervical trauma and tumor disease. Historically, PCF was developed at the beginning of the 20th century from the methods of Hibbs [1] and Albee [2] who used only an autologous bone graft. Following the description of the wiring technique by Rogers [3] in 1942, new methods such as translaminar screws, lateral mass screws (LMS), and pedicle screws were introduced [4-12]. These instrumentations provide immediate stability, and patients benefit from earlier mobilization and rehabilitation while waiting for bone fusion. However, long-term stabilization through instrumental fixation should achieve final bone fusion such as posterolateral fusion (PLF) and proper preparation of the fusion bed is inevitable. However, conventional PLF has some disadvantages including bone chip migration to the spinal cord, which is exposed for decompression, posterior neck pain due to far lateral muscle detachment to make a sufficient fusion bed, or poor fusion rate due to bone chip resorption which has only been reported to be approximately 60%–80% [13-15]. Another procedure for PCF is cervical intrafacetal fusion (cIFF) using bone chip insertion into well-dissected and prepared facetal joint spaces. Therefore, cIFF technique added with minimal PLF could prevent disadvantage of conventional PLF and increase bone fusion rate; however, this is not currently common in cervical spine surgery. In this study, we hypothesized that cIFF achieves a higher rate of bone fusion compared to cervical PLF and demonstrated that adding cIFF to minimal PLF helps maintain cervical alignment and promotes bone fusion.
MATERIALS AND METHODS
This study was approved by the Institutional Review Board (IRB) of Asan Medical Center (IRB No. 2023-1087), ensuring compliance with ethical standards. Given the retrospective nature of the study, the requirement for informed consent was waived by the IRB.
1. Study Design
In this single-center study, patients who underwent posterior cervical laminectomy and screw fixation accompanied by cIFF with minimal PLF and conventional PLF for cervical myelopathy from March 2012 to December 2023 were investigated retrospectively. Among 179 patients, 46 patients were finally selected for the study: 15 patients had posterior cervical fixation accompanied by conventional PLF and 31 patients had cIFF with minimal PLF. Inclusion criteria were a diagnosis of degenerative cervical myelopathy or OPLL. Exclusion criteria consisted of (1) fusion level equal to or less than 2, (2) fusion including occipito-cervical or C1 level, (3) cervical spine tumor, (4) infectious disease, (5) congenital disease, (6) traumatic spine disease, (7) patients followed-up for less than 6 months.
2. Surgical Procedure of cIFF With Minimal PLF
The patient was placed in a prone position with the head secured in a 3-point Mayfield skull clamp (Integra Life Science Corporation, Cincinnati, OH, USA). A midline skin incision was conducted. For visualization of the LMS insertion point, paravertebral muscles were dissected to the lateral margin of the lateral mass. Bilateral gutters were made to remove the lamina for cord decompression using high-speed drilling (Fig. 1A). Then, the lamina was removed en bloc to minimize spinal cord damage (Fig. 1B–D). Drilling with 2- and 3-mm diamond burs was performed to widen the facetal cavity (Fig. 1E, F). Next, curettage was conducted for the meticulous decortication of the intrafacetal space (Fig. 1G, H). In this step, to avoid ventral penetration beyond the facetal space, drilling was kept within a 10-mm depth. Then, bone chips harvested from the removed lamina were inserted into the intrafacetal space using a small impactor (Fig. 1I, J). Afterwards, 2 different screw insertion techniques were used according to the targeted surgical areas: LMS was applied for C3 to C6, and a cervical pedicle screw was used for C2, C7, and T1. Alternatively, for small lateral masses, a pedicle screw was inserted. After cervical screws were inserted, rods were placed and tightened. Remaining bone chips mixed with demineralized bone materials were applied along the lateral side of the lateral mass, which was decorticated for PLF. However, the volume of bone chips for PLF was applied limitedly to avoid bone chip migration onto the spinal cord, and lateral surface decortication in lateral mass was not performed to avoid postoperative neck pain which we named minimal PLF. Finally, muscles and skin were closed tightly. The patient was instructed to wear a Philadelphia collar (Ossur Orthopedics, Reykjavik, Iceland) for 5 months after the surgery for solid bone fusion and a computed tomography (CT) scan was conducted to assess whether bone fusion had been achieved at the 6 months after surgery.
3. Clinical and Radiological Analysis
The patients were divided into 2 groups, conventional PLF group and cIFF with minimal PLF group, and patients’ medical and radiographical records were reviewed. Clinical outcomes were assessed by examining the changes in Nurick grades before and after the surgery. Radiological parameters including C2–7 Cobb angle and C2–7 sagittal vertical axis (SVA) were measured preoperatively, immediate postoperatively, and at the 1-year follow-up.
In cIFF with minimal PLF group, cIFF location and PLF location were divided, and the fusion rates of each location were compared in postoperative 6 months. The achievement of fusion was decided by the presence of trabecular bone bridging on a CT scan. To analyze the successful fusion, the fusion bridge was carefully evaluated at each facetal level. The first step was examination of the CT sagittal image to determine whether IFF or PLF had occurred. This evaluation was performed by drawing the facet outline (Fig. 2A, B) and identifying a direct bone bridge inside the facet joint was decided to intrafacetal fusion, whereas a circumferential fusion bridge outside the facet joint was considered as evidence of PLF (Fig. 2C, D). If a direct bone bridge was not observed at a specific level, the CT coronal image at that same level was examined (Fig. 2E, F). The levels in which the CT sagittal and coronal images showed no direct bone bridge were classified as nonfusion.
RESULTS
1. Demographics
The demographic data of the enrolled patients is presented in Table 1. The 46 patients included 31 males (67%) and 15 females (33%), with a mean age at surgery of 63.4 years (range, 38–85 years). The average clinical follow-up was 19.5 months (range, 6–106 months). In cIFF with minimal PLF group, 6 patients had pedicle screws instead of LMS in the segment of the small lateral mass between C3 and C6 levels. Two patients experienced wound dehiscence from superficial infection. Among them, 1 patient underwent a revision procedure and 1 patient received hyperbaric oxygen therapy without revision. During the screw insertion procedure, 1 patient experienced a vertebral artery injury when trying to insert the pedicle screw, and then, this was converted to LMS. Two patients developed C5 palsy: in 1 patient, this was caused by the ventral extrafacetal impaction of a bone chip into the neural foramen, and they underwent revision surgery to remove the bone chip, resulting in a gradual recovery of motor function (Fig. 3A, B). The other C5 palsy was of unknown origin and recovered spontaneously. In conventional PLF group, 13 patients had pedicle screws instead of LMS in the selected segment dependent on surgeon’s decision. One patient underwent a revision surgery due to postoperative hematoma.
The comparison of demographic data between cIFF with minimal PLF group and conventional PLF group is presented in Table 2. In cIFF with minimal PLF group, 15 patients were diagnosed with CSM, and 16 patients were diagnosed with OPLL. In conventional PLF group, 9 patients were diagnosed with CSM, and 6 patients were diagnosed with OPLL. The Nurick score improvement after operation was 0.55 ± 0.49 in cIFF with minimal PLF group which was significantly lower than conventional PLF group (0.53 ± 0.71, p = 0.040). In cIFF with minimal PLF group, the mean bone marrow density (BMD) was -0.41 ± 1.39 and in conventional PLF group, BMD was -0.74 ± 1.45. In all patients, 7 patients had osteopenia, and 3 had osteoporosis. However, all osteopenia and osteoporosis patients did not receive any additional medications for osteoporosis.
2. Comparison of Cervical Parameters Between Selected cIFF With Minimal PLF Group and Conventional PLF Group
Comparison of cervical parameters of preoperative, postoperative, 1-year follow-up, and postoperative change (1-year follow-up – immediate postoperative) between cIFF with minimal PLF group and conventional PLF group is presented in Table 3. Among 31 patients of cIFF with minimal PLF group, 16 patients whose follow-up period was above 1-year were selected and sagittal parameter was calculated. The average Cobb angle before surgery for the patients was 10.5° ± 10.0° and 8.6° ± 8.2° in immediate postoperative and 8.8° ± 7.6° in 1-year follow-up. In conventional PLF group, the average Cobb angle before surgery was 16.3° ± 7.2° and 13.3° ± 8.4° in immediate postoperative and 3.7° ± 8.8° in 1-year follow-up. The change (1-year follow-up – immediate postoperative) in cIFF with minimal PLF group was 0.1° ± 4.0° which was significantly lower than conventional PLF group (-9.7° ± 8.4°, p = 0.022). In the cIFF with minimal PLF group, the average C2–7 SVA before surgery for the patients was 34.8 ± 18.9 and 36.2 ± 12.3 in immediate postoperative and 34.5 ± 15.3 in 1-year follow-up. In conventional PLF group, the average C2–7 SVA before surgery was 29.8 ± 13.8 and 32.5 ± 13.2 in immediate postoperative and 42.6 ± 20.3 in 1-year follow-up. The change (1-year follow-up – immediate postoperative) in cIFF with minimal PLF group was -1.7 ± 10.5 which was not significant but lower than conventional PLF group (10.0 ± 16.7, p = 0.430).
3. Comparison of Fusion Rate According to cIFF and PLF Locations in cIFF With Minimal PLF Group
In cIFF with minimal PLF group, cIFF location and PLF location were divided and the fusion rates of each location in postoperative 6 months were carefully compared (n = 31). The total number of facets among the cIFF with minimal PLF group was 272. Among them, fusion success in cIFF location was observed in 267 facets (98.1%), and in PLF location, 244 facets (89.7%) were successful, which was significantly different (p < 0.001). When analyzed by level, the fusion rate of cIFF in C2–3, 3–4, and 6–7 was significantly higher than that of PLF (Table 4).
DISCUSSION
In the situation where wide cervical laminectomy for spinal cord decompression is performed, placing bone chips around a decorticated lateral mass for PLF can potentially induce the migration of bone chips onto the spinal cord, resulting in unfortunate spinal cord compression. The concerns of bone chip migration make it insufficient for the application of bone chips in the PLF bed [13,14]. Moreover, the bone fusion rate at the area treated with PLF was reported to be approximately 60%–90% and tended to be poorly fused where preoperative instability was found [16-22]. Additionally, extensive lateral muscle dissection to create a PLF bed provokes uncomfortable postoperative neck pain [13,14]. Therefore, we utilized a technique that involves the direct placing of bone chips into the intrafacetal space to induce cIFF, which applies as additional methodology to limited PLF to overcome the disadvantages of PLF alone. When we conduct cIFF combined with minimal PLF, it is possible to perform less invasive PLF as an adjuvant procedure by applying a small volume of bone chips as well as less lateral muscle detachment and avoiding lateral surface decortication of lateral mass. After removing the soft tissue within the intrafacetal space and utilizing the autologous bone chips obtained during laminectomy, we can safely and conveniently impact bone chips without concerns of their migration towards the spinal cord. Because the bone chips are fixed within the intrafacetal tight space, they are expected to promote effective bone fusion. Intrafacetal fusion, unlike onlay grafts that performed in PLF, is expected to allow a higher rate of direct fusion considering the transmission of axial loading and Wolff’s law [23].
There have been a few reports of the intrafacetal technique in PCF. Cofano et al. [24] reported the insertion of cervical interfacetal spacer allograft during PCF in patients who had developed pseudarthrosis after anterior cervical discectomy and fusion. The paper suggested that the fusion rate could be improved, and foraminal stenosis could be addressed by increasing the foraminal height and width. However, their study targeted patients who had undergone the anterior approach. Although Kansal et al. [25] used the terminology of intrafacetal fusion in bilateral C1–2 transarticular screw insertion, this was approached by anterior high cervical retropharyngeal approach. Therefore, this present study is the first report to address intrafacetal fusion in the posterior approach of a general degenerative cervical spine and OPLL. In the previous reports, the terminology “intrafacetal” and “interfacetal” were intermingled, but we considered “intrafacetal” might be more appropriate terminology to describe a fusion method that inserts bone chips into the cervical facetal space.
Based on the data, when comparing the follow-up results on the sagittal alignment of cIFF with minimal PLF group and conventional PLF group, it was noted that the Cobb angle and C2–7 SVA were observed as having better maintenance in lordotic curve during the follow-up in cIFF with minimal PLF group rather than conventional PLF group. In comparison with previous study using LMS fixation with PLF only, this result of present study showed better outcome. Lee et al. [10] reported the difference of Cobb angle between final follow-up and immediate postoperative was -0.70° after LMS with PLF only, which is higher in postoperative loss of lordotic curve than our results (0.1° ± 4.0°). And, in C2–7 SVA, the difference between final follow-up and immediate postoperative was 2.06 which higher than our results (-1.7 ± 10.5). Inose et al. [26] documented alterations in cervical lordosis after posterior cervical decompression and PLF between preoperative and postoperative 1-year results which were 9.9° ± 11.3° and 4.6° ± 10.4°. Additionally, the C2–7 SVA from 21.8 ± 20.2 to 29.3 ± 20.2. Comparing to our preoperative and postoperative 1 year data of cIFF with minimal PLF group, in which the preoperative Cobb angle was 10.5° ± 10.0° and reduced to 8.8° ± 7.6° 1 year postoperative, and the C2–7 SVA measured 34.8 ± 18.9 preoperatively and 34.5 ± 15.3 1 year postoperatively, the present novel technique demonstrated a better preservation of cervical sagittal alignment.
The previous literatures which studied about PCF rates used dynamic x-ray and the absence of instability was used as the indication of fusion [9,16-18,27-31]. Highsmith et al. [18] reported a fusion rate of 92% in 24 months and Heller et al. [21] 61.5% in 25.5 months in which they used absence of pseudoarthrosis in simple x-ray as a criteria of fusion. However, Hong et al. [9] reported 100% of fusion in 18.9 months. Therefore, the evaluation of fusion by simple x-ray has a variable range of fusion rate. In our study, the CT scan observing bone bridge or trabecular bone formation enabled a more objective comparison of fusion rates by directly evaluating the cIFF location and PLF location in the fusion bed in patients and we evaluated them in relatively early follow-up period than previous studies. In cIFF with minimal PLF group, when counting the number of fused facets in cIFF location, the fusion rate 98.1% in 6 months was significantly higher when compared to PLF location (89.7%, p < 0.001). Because of the minimal PLF procedure leading to a relatively small fusion bed space due to limited muscle dissection and the use of fewer bone chips, the lower fusion rate observed may be attributed to these factors. However, at just 6 months postoperative state, the cIFF demonstrated a significantly higher fusion rate of 98.1%, suggesting it a promising option to improve the fusion success in PCF surgery. The locations of the worse facet fusion rate in PLF compared to cIFF were C2–3, C3–4, and C6–7 levels, which is supposed to be the results of micromotion in the uppermost and lowermost portion of the cervical region. Therefore, the need of cIFF is emphasized in these levels.
In cIFF with minimal PLF group, C5 palsy was observed in 2 cases postoperatively, and in one of them, we considered it was caused by bone chips which were deeply inserted and penetrated beyond ventral facetal surface resulting in nerve root compression. The penetrated bone chips were confirmed on postoperative CT. Therefore, we performed a revision procedure to remove the penetrated bone chips. The patient recovered gradually from C5 palsy during the 19-month follow-up. It is important to note that during cIFF procedures, there might be a risk of nerve root or vertebral artery compression from the penetration of bone chips beyond the ventral facetal margin. This complication could be prevented by avoiding drilling more than 10-mm depth from the dorsal facet surface and aggressive bone chip impaction during cIFF.
This study had several limitations. First, the retrospective design of the present study might have the potential selection bias and limited data; therefore, we need to perform a more precise study design such as prospective study including patient-reported outcomes with Japanese Orthopaedic Association score or visual analogue scale score to demonstrate the advantage of cIFF compared to PLF. Second, the data of fusion rate did not directly compare cIFF and PLF using double-arm study. Instead, we analyzed fusion results based on patients who underwent both procedures simultaneously, making an independent comparison difficult. Third, the comparison of fusion rates at the 6 months postoperatively may not have shown sufficient fusion. Comparing the long-term follow-up CT-based fusion rates with the conventional PLF group, would provide a more accurate analysis. Fourth, factors that can influence fusion outcomes, such as BMD and types of screw fixation were not taken into consideration: in cIFF with minimal PLF group, pedicle screws were inserted in 6 patients (19%) because of a small lateral mass between the C3 and C6 levels, and in conventional PLF group, pedicle screws were inserted in 13 patients (86%) because of surgeon’s preference. Finally, the cases enrolled in this study were chosen with a specific focus on degenerative spine conditions and OPLL. Further validation in extended indications, for example trauma or spine metastasis, is required to determine the effectiveness of cIFF.
CONCLUSION
In this study, to overcome the limitations of PLF, which has a relatively lower fusion rate, risks of bone chip migration to the spinal cord, and can induce neck pain due to wide muscle dissection and extended decortication, we introduced an alternative technique that directly inserts bone chips into the intrafacetal space combined with minimal PLF. Postoperative sagittal alignment was more preserved in cIFF with minimal PLF group rather than conventional PLF group. The fusion rate according to location, intrafacetal space (98.1%) was significantly higher than PLF bed (89.7%) in cIFF with minimal PLF technique. Considering the concerns of disadvantages in the PLF procedure as well as the convenience of the intrafacetal fusion technique, the additional cIFF combined with minimal PLF might be a beneficial alternative for posterior cervical decompression and fixation.
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