Abstract
Introduction
Posterior lumbar interbody fusion (PLIF) is a common treatment for nerve root disease associated with lumbar foraminal stenosis or lumbar spondylolisthesis. At our institution, PLIF is usually performed with high-angle cages and posterior column osteotomy (PLIF with HAP). However, not all patients achieve sufficient segmental lumbar lordosis (SLL). This study determined whether the location of PLIF cages affect local lumbar lordosis formation.
Methods
A total of 59 patients who underwent L4/5 PLIF with HAP at our hospital, using the same titanium control cage model, were enrolled in this cohort study. The mean ratio of the distance from the posterior edge of the cage to the posterior wall of the vertebral body/vertebral length (RDCV) immediately after surgery was 16.5%. The patients were divided into two groups according to RDCV <16.5% (group P) and ≥16.5% (group G). The preoperative and 6-month postoperative slip rate (%slip), SLL, local disk angle (LDA), ratio of disk height/vertebral height (RDV), 6-month postoperative RDCV, ratio of cage length/vertebral length (RCVL), and ratio of posterior disk height/anterior disk height at the fixed level (RPA) were evaluated via simple lumbar spine X-ray. The preoperative and 6-month postoperative Japanese Orthopedic Association (JOA) and low back pain visual analog scale (VAS) scores were also evaluated.
Results
Groups G and P included 31 and 28 patients, respectively. The preoperative %slip, SLL, LDA, RDV, JOA score, and low back pain VAS score were not significantly different between the groups. In groups G and P, 6-month postoperative %slip, SLL, LDA, RDV, RDCV, RCVL, and RPA were 3.3% and 7.9%, 18.6° and 15.4°, 9.7° and 8.0°, 36.6% and 40.3%, 21.1% and 10.1%, 71.4% and 77.0%, and 56.1% and 67.7%, respectively. The 6-month postoperative SLL, LDA, RDV, RDCV, RCVL, and RPA significantly differed (p=0.03, 0.02, 0.02, <0.001, <0.001, and <0.001, respectively).
Conclusions
Anterior PLIF cage placement relative to the vertebral body is necessary for good SLL in PLIF.
Keywords: posterior lumbar interbody fusion, lumbar lordosis, high-angle cage, anterior cage placement
Introduction
Posterior lumbar interbody fusion (PLIF) is one of the most common surgical treatments for lumbar foraminal stenosis and lumbar spondylolisthesis1,2). PLIF can improve spinal alignment by correcting slippage and applying compression force to screws to achieve segmental lumbar lordosis (SLL)2,3). We routinely perform PLIF with high-angle cages and posterior column osteotomy (PLIF with HAP); some patients achieve sufficient SLL, whereas others do not (Fig. 1). Insufficient segmental correction of lumbar lordosis alignment reportedly increases the incidence of adjacent segmental disorders during the postoperative course4), causes postoperative lower back pain, and results in social and psychological disabilities5). Thus, we believe that it is important to perform single PLIFs considering the segmental lumbar alignment of the site.
Figure 1.
A: Preoperative SLL is 4.0°, postoperative SLL is 5.0°. Postoperative local lordosis is poor.
B: Preoperative SLL is 9.0°, postoperative SLL is 21.0°. Postoperative local lordosis is good.
SLL, segmental lumbar lordosis
Although there are scattered reports detailing that an anterior placement of the cage is effective for SLL alignment6-8), the only study focusing on PLIF is that by Priyan et al.9) Their report concluded that anterior placement of PLIF cages achieves SLL. However, they acknowledged multiple study limitations, including the nonrandom selection of several cages, evaluation of multiple fusion levels, and lack of preoperative evaluation between good and poor lordotic groups.
We hypothesized that anterior placement of cages would favor SLL and that a longer cage length relative to the vertebrae would make it difficult to achieve SLL. Therefore, our study aimed to determine whether the location of the cages in the PLIF procedure affects SLL.
Materials and Methods
This research was approved by the Institutional Review Boards of the authors' affiliated institutions, and patients agreed to participate in the study with the option to opt-out at any stage of the study.
The participants were patients who underwent PLIF with HAP at L4/5 between February 2020 and July 2022. The exclusion criteria were previous spinal surgery, fusion levels other than L4/5, fusion of more than two vertebrae, use of implants from companies other than our preferred company, and cage subsidence 1 week following surgery.
The surgical procedure was as follows: Osteotomy with total resection of the inferior articular process and a portion of the superior articular process and placement of a large amount of local bone graft in the interbody space. Cages were bilaterally inserted and fixed with percutaneous pedicle screws. Furthermore, a compressive force was applied between the screws in all cases. Total resection of the superior articular process, which would have resulted in total facetectomy, was not performed due to excessive bleeding from the lateral muscles at the time of removal. Instead, a portion of the superior articular process was resected to facilitate lateral cage placement.
In this study, we used only one type of PLIF cage: The titanium control cage (CoRoentⓇ Large MASⓇ; NuVasive, San Diego, CA, USA). The cage angle used was 12.0°; anteroposterior cage length, 23 mm; and cage height at the highest point, 11.5 mm. This was the smallest of the CoRoent titanium cages.
Evaluation using lateral lumbar spine X-ray imaging was completed preoperatively and 6 months postoperatively. At 6 months postoperatively, we evaluated the bone union rate via computed tomography (CT) based on the Yanai classification10). According to this classification, bone fusion is classified into four categories: Grade 1, complete fusion is achieved, with bone bridge formation between the upper and lower vertebral bodies; grade 2, a bone bridge is not formed, no translucency is observed around the cages, and thick fusion mass formation is noted; grade 3, fusion is not achieved, and translucency is observed around the cages; and grade 4, the cage sinks into the vertebral body, or there is bone resorption around the cage, indicating pseudoarthrosis. We defined grade 1 as bone fusion. In this study, the bone union rate following PLIF was 98.3% (58/59). From this result, the PLIF section was determined to be stable, and each item was evaluated 6 months postoperatively.
The evaluation parameters were the preoperative/6-month postoperative slip rate (%slip), SLL, local disk angle (LDA), ratio of disk height/vertebral height (RDV), Japanese Orthopedic Association (JOA) score, low back pain visual analog scale (VAS) score, 6-month postoperative ratio of the distance from the posterior edge of the cage to the posterior wall of the vertebral body/vertebral length (RDCV), ratio of cage length/vertebral length (RCVL), and ratio of posterior disk height/anterior disk height at the fixed level (RPA).
Cage subsidence was defined as displacement of the cage by more than 2 mm to the superior or inferior endplate on CT performed 1 week following surgery11).
The cage was inserted on both sides. If there was rotation or scoliosis in the vertebral body, there might have been a shift in the position of the left and right cages on the lateral X-ray image. At that time, each evaluation was measured using the posteriorly located cage to avoid underestimation. Each point to be measured on the vertebral body was also measured at the more posteriorly located point if any rotation was present. The average SLL immediately after surgery was 17.7°, indicating good lordosis formation. Subsequently, we decided to divide the patients into two groups based on the mean RDCV immediately after surgery. Because the mean immediate postoperative RDCV was 16.5%, the patients were divided into two groups: Group G was defined as those with immediate postoperative RDCV ≥16.5%, and group P was defined as those with immediate postoperative RDCV <16.5%.
The two groups were compared using the t-test. All statistical analyses were conducted using IBM SPSS version 27.0 (IBM Corp., Armonk, NY, USA), and a p-value of <0.05 was considered statistically significant.
The formulas of the evaluation items are based on a previous study12) and are presented in Fig. 2, 3, 4.
Figure 2.
(1) %slip: S/W×100.
(2) Segmental lumbar lordosis (SLL): ∠AB-CD.
%slip: Percentage of the slip distance to the longitudinal diameter of the vertebral body.
Figure 3.
(3) Ratio of disc height (DH)/vertebral height (VH) (RDV): DH/VH×100.
DH: (anterior disk height [ADH]+posterior disk height [PDH])/2.
Vertebral height (VH): (cranial VH at fusion level+caudal VH at fusion level)/2.
(4) Local disk angle (LDA): ∠EF-GH.
(5) Ratio of the posterior disk height/anterior disk height at the fixed level (RPA): PDH/ADH×100.
Figure 4.
(6) Ratio of the length from the posterior edge of the cage to the posterior wall of the vertebral body (LCPV)/vertebral length (VL) (RDCV): LCPV/VL×100.
LCPV: (cranial LCPV+caudal LCPV)/2.
VL: (cranial VL+caudal VL)/2.
(7) Ratio of cage length (CL)/vertebral length (VL) (RCVL): CL/VL×100.
Results
From February 2020 to July 2022, 133 patients underwent PLIF with HAP. After excluding those who met the exclusion criteria, 59 patients (mean age at surgery 69.2 [43-82] years, male/female ratio=22:37) were included in the final analysis (Fig. 5).
Figure 5.
Flowchart of patient selection.
Preoperative diagnoses included 50 patients with degenerative spondylolisthesis, 3 with spondylolisthesis, 3 with lumbar foraminal stenosis, and 3 with lumbar disk herniation. The G and P groups had 31 and 28 patients, respectively. Demographic data are presented in Table 1.
Table 1.
Patients’ Demographic Data.
| Group G | Group P | ||
|---|---|---|---|
| Sex | Male | 17 | 5 |
| Female | 14 | 23 | |
| Mean age (years) | 70.5 | 67.7 | |
Patients were divided into two groups according to RDCV<16.5% (group P) and ≥16.5% (group G).
RDCV, ratio of the distance from the posterior edge of the cage to the posterior wall of vertebral body/vertebral length
On the preoperative lumbar lateral X-ray images, %slip was 16.9% and 18.3%; SLL, 13.0° and 11.9°; LDA, 5.1 and 3.3°; RDV, 29.5% and 25.7%; JOA score, 14.5 and 13.9; and low back pain VAS score, 61.9 and 59.2 in the G and P groups, respectively. The results of the preoperative assessments did not significantly differ between the two groups (Table 2). In the G and P groups, the postoperative %slip was 3.3% and 7.9%; SLL, 18.6° and 15.4°; LDA, 9.7° and 8.0°; RDV, 36.6% and 40.3%; RDCV, 21.1% and 10.1%; RCVL, 71.4% and 77.0%; RPA, 56.1% and 67.7%; JOA score, 21.7 and 23.2; and low back pain VAS score, 15.2 and 13.1, respectively. The postoperative results indicated significant differences between the two groups in the SLL, LDA, RDV, RDCV, RCVL, and RPA values (p=0.03, 0.02, 0.02, <0.001, <0.001, and <0.001, respectively) (Table 3).
Table 2.
Results of Preoperative Evaluation.
| *Group G (SD) | *Group P (SD) | P-value | |
|---|---|---|---|
| %slip (%) | 16.9 (24.7) | 18.3 (10.3) | 0.78 |
| SLL (°) | 13.0 (5.3) | 11.9 (4.2) | 0.40 |
| LDA (°) | 5.1 (3.6) | 3.3 (3.4) | 0.05 |
| RDV (%) | 29.5 (10.4) | 25.7 (8.5) | 0.13 |
| JOA score | 14.5 (4.1) | 13.9 (5.9) | 0.70 |
| Low back pain VAS score | 61.9 (30.8) | 59.2 (28.7) | 0.73 |
*Patients were divided into two groups according to RDCV<16.5% (group P) and ≥16.5% (group G).
%slip, slip rate, SLL, segmental lumbar lordosis; LDA, local disc angle; RDV, ratio of disc height/vertebral height; RDCV, ratio of the distance from the posterior edge of the cage to the posterior wall of vertebral body/vertebral length; JOA, Japanese Orthopedic Association; VAS, visual analog scale; SD, standard deviation
Table 3.
Results of Postoperative Evaluation.
| *Group G (SD) | *Group P (SD) | P-value | |
|---|---|---|---|
| %slip (%) | 3.3 (4.8) | 7.9 (16.2) | 0.13 |
| SLL (°) | 18.6 (5.7) | 15.4 (4.9) | 0.03 |
| LDA (°) | 9.7 (2.8) | 8.0 (2.7) | 0.02 |
| RDV (%) | 36.6 (5.6) | 40.3 (5.8) | 0.02 |
| RDCV (%) | 21.1 (6.6) | 10.1 (5.5) | <0.001 |
| RCVL (%) | 71.4 (6.5) | 77.0 (5.8) | <0.001 |
| RPA (%) | 56.1 (11.5) | 67.7 (13.7) | <0.001 |
| JOA score | 21.7 (5.0) | 23.2 (4.7) | 0.24 |
| Low back pain VAS score | 15.2 (22.8) | 13.1 (20.2) | 0.72 |
*Patients were divided into two groups according to RDCV<16.5% (group P) and ≥16.5% (group G).
%slip, slip rate; SLL, segmental lumbar lordosis; LDA, local disc angle; RDV, ratio of disc height/vertebral height; RDCV, ratio of the distance from the posterior edge of the cage to the posterior wall of vertebral body/vertebral length; RCVL, ratio of cage length to vertebral length; RPA, ratio of posterior disc height/anterior disc height; JOA, Japanese Orthopedic Association; VAS, visual analog scale; SD, standard deviation
Discussion
Regarding PLIF, there are reports indicating that SLL can be increased by increasing the cage angle13,14) and that lordotic cages are more advantageous for lordosis formation than are nonlordotic cages15). Total facetectomy in PLIF is reportedly advantageous for lumbar lordosis formation9,16), and we have performed PLIF with HAP, focusing on obtaining bone fusion and lordosis formation. However, in some cases, lordosis is not achieved as expected, which is the reason this study was conducted. Herein, we identified that the cases with better postoperative SLL and LDA in PLIF with HAP were those with anteriorly placed cages relative to the vertebral body along with a high RPA. No significant difference was observed in %slip. The outcome of anterior cage placement was consistent with our hypothesis.
It has been reported that lordosis formation should be achieved by raising the anterior disk height and retracting the posterior disk as much as possible9). Our findings are consistent with those of this previous study, and we additionally demonstrated that cases with a large RPA also had a larger lumbar lordosis formation.
There are two ways to increase the anterior disk height: Anterior placement of the cage and the use of a tall cage. It is theoretically advantageous to place the cage anteriorly to increase the anterior disk height and compress the posterior interbody space to achieve lordosis. Although there have been several reports of anterior placement of the cage being involved in lordosis formation6-9), to the best of our knowledge, the study by Priyan et al. is the only report of the use of a PLIF cage9). Their study demonstrated that similar to our results, anterior cage placement favors SLL acquisition.
It has been reported that the more anteriorly the cages are placed in the interbody space, the further forward the center of rotation for deformity correction moves, and the larger the gap where compression can occur behind the cage. Thus, we also evaluated the degree of postoperative %slip correction and RCVL. If the postoperative %slip correction is good, the posterior gap space for applying compressive forces will be larger. In addition, the longer the cage length relative to the vertebral length, the smaller the posterior gap when compression is enforced, which may make it more difficult to influence the compressive force. For this reason, we believe that both %slip and RCVL influence lordosis formation. In the present study, no significant difference was observed in postoperative %slip, but there was a significant difference in RCVL between the two groups. This may be because the %slip was adequately corrected in all cases during PLIF. As for RCVL, the results were consistent with our expectations. The longer the cage length relative to the vertebral body length, the lower the compressive force applied, making it difficult to achieve good lordosis formation. However, shorter cages may be beneficial in obtaining further lordosis, but this may also decrease the area of placement with the endplate, which may be disadvantageous in terms of bone fusion. The cage length used in this study was 23 mm, which was the same for all cases, but further research is warranted to determine how lordosis formation and bone fusion are affected by different cage lengths.
Regarding cage height, Konrad et al. reported normal disk height using CT17), with the anterior disk height at the L4/5 level ranging from 9.8 to 11.0 mm; we used a cage height of 11.5 mm anteriorly, which was almost the same. Excessively tall cages reportedly increase the force applied to the endplate, causing cage subsidence and correction loss17), increasing anterior longitudinal ligament (ALL) tension, and limiting the application of posterior compression and resistance of posterior shortening, all of which are detrimental to lordosis formation9). The use of a cage that is too large is counterproductive to lordosis formation, and it is important to select an appropriate cage size. However, placing a cage with as much height as possible will help lift the anterior aspect of the interbody space. Furthermore, no research has been conducted on how increasing the height of the cage affects lordosis formation, causes an increase in ALL tension, or leads to ALL rupture. Because disk height also varies between individuals and with age17,18),we believe that anatomical studies should be conducted in the future. In addition, as regards the disk height, RDV was greater when the disk was placed posteriorly, probably because the cage immediately contacts the endplate when compression force is applied, thus preventing posterior shortening, making it difficult to apply a compression force, and creating a disadvantageous situation for lordosis formation. Further research is needed to determine the extent to which disk height reconstruction is necessary.
No significant differences were observed in postoperative clinical outcomes between the two groups based on the JOA and VAS scores. Although there was a significant difference in the postoperative SLL between the G and P groups, we suggest that there was no difference in clinical outcome as both groups achieved good alignment improvement of >15°19). However, we suggest that better correction of local lumbar spinal alignment will affect the global spinal alignment as the postoperative course lengthens, which may lead to differences in clinical outcomes. In addition, we emphasize on the correction of local lumbar spinal alignment. We believe that long-term postoperative follow-up is required to examine the effect of the procedure on global spinal alignment and clinical outcomes.
The present study has some limitations. First, the disk level was restricted to L4/5: Vertebral levels L4/5 and L5/S1 both contribute to lordosis in lumbar spine alignment20,21). In this study, the average preoperative SLL of L5/S1 was 17.6°. Lordosis of the lower lumbar spine is reportedly 15°-20°;19) the preoperative SLL of L5/S1 was within the normal range and was excluded from this study. Second, measurements were performed via plain radiography, which is sometimes difficult to accurately evaluate because sagittal X-ray images are subject to rotation. For measurements, we selected the more posterior of the two cages to avoid selection bias, considering that anterior cage placement is reportedly more favorable for kyphoplasty. Thus, we believe that our selection did not lead to an overestimation. We also expressed the distance as a ratio for error elimination in the fluoroscopic distance. Third, the postoperative follow-up period was short. In the present study, intraoperative cage subsidence was evaluated via CT at 1 week postoperatively, and subsidence cases were excluded, but cage subsidence may gradually occur after surgery22).
However, the bone union rate at 6 months postoperatively was very high, at 98.3%, and even if there was loss of correction due to postoperative cage subsidence, we considered that fixation site stability was achieved at 6 months postoperatively. Therefore, we believe that the local lumbar alignment, such as %slip, SLL, and LDA, did not change further after 6 months and that improvement and stability were achieved. Also, to prevent intra- or post-operative endplate injury and cage subsidence, we tried to be cautious when performing endplate scraping and placed the cage on the hard outer side of the endplate. Currently, we do not know which cases should not be treated with PLIF with HAP. Fourth, only one type of cage was used. We also suggest that the cage in this study is a bit large for a petite woman. A previous study showed that the average L4/5 disk height is 9.8 and 6.0 mm anteriorly and posteriorly, respectively, in women17). However, in this study, we examined the influence of the cage placement position on postoperative lumbar lordosis formation. Therefore, we unified the size of the cage to eliminate other factors. We used the smallest size among the CoRoent cage series used in this study. We suggest taking body size, gender, and age into consideration when selecting a cage to reduce intra and postoperative complications. In addition, when examining the size of the cage, it is necessary to increase the variation in the angle, height, and length, and this is an issue to be addressed in the future.
Finally, although we focused on the cage in our examination of lordosis formation, other factors also contribute to the acquisition of lordosis. Previous studies have reported that a wide posterior osteotomy19) tends to apply compressive forces easily to the posterior region, and patients with large preoperative local lordosis angles have larger angles postoperatively6). We also perform PLIF in combination with posterior column osteotomy under conditions favorable to the acquisition of local lordosis. Because various factors, such as the preoperative local angle and degree of disk narrowing, affect lordosis formation in PLIF, surgery should be performed considering lordosis formation according to individual characteristics. Multivariate analysis should also be conducted to examine the factors contributing to a good SLL; however, we were unable to do this due to the small number of cases.
In conclusion, we examined the factors that led to a larger postoperative disk angle in PLIF with HAP, focusing on cage features. For good postoperative lumbar lordosis formation, it is important that the PLIF cage is placed anteriorly and that it is not too long relative to the vertebral body. Because the postoperative large disk angle is directly related to postoperative local alignment, the location and size of the cage are considered important factors for obtaining good local lordosis in PLIF. Further research is warranted to determine what cage size relative to the vertebral body is best for lordosis formation.
Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.
Sources of Funding: None.
Author Contributions: H.M. and Y.U. designed the study; T.M., S.K., Y.S., and M.I. performed the experiments and analyzed the data; H.S. and Y.T. supervised the experiments; and D.I. wrote the manuscript.
Ethical Approval: This study was approved by the Ethics Committee of Kashiba Asahigaoka Hospital (Approval code: 08-1-013).
Informed Consent: Informed consent for participation was obtained in an opt-out manner.
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