A spontaneous anterior fusion of lumbar spine after posterolateral lumbar fusion with pedicle screw-plate system

doi:10.21203/rs.3.rs-4860858/v1

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Research Article

A spontaneous anterior fusion of lumbar spine after posterolateral lumbar fusion with pedicle screw-plate system

https://doi.org/10.21203/rs.3.rs-4860858/v1

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Background

Lumbar posterolateral fusion (PLF) with Pedicle Screw-Plate (PSP) fixation without interbody fusion found spontaneous anterior fusion (SAF). No study has reported the SAF of lumbar spine in patients who underwent PLF. This study reports the rate of SAF of lumbar spine after PLF with PSP system compared with the rate of SAF after PLF with pedicle screw-rod (PSR) system.

Methods

Retrospective reviews of charts and radiographs were performed in 111 patients who underwent PLF with PSP system and PSR system for treatment of degenerative lumbar spine. Demographic data, diagnosis, Myerding grading, level of fusion, Pfirmann grading, disc height, pedicle screw depth, follow up time and SAF were compared between PSP and PSR groups.

Results

Fifty-five patients in the PSP group and 56 patients in the PSR group were included with similar baseline characteristics. There were more degenerative spondylolisthesis patients in the PSP group compared with patients in the PSR group. However, there was no significant difference of Pfirmann grading in both groups. Fifteen patients (27.3%) in the PSP group developed SAF while no patient in the PSR group had SAF. When patients in the PSP group were analyzed, the mean follow-up time in patients with SAF was significantly longer than the no SAF group.

Conclusion

This study has reported SAF of lumbar spine in patients who underwent PLF with the PSP system. The SAF increasingly occurred with longer follow-up time. In this study, no SAF was found in patients who underwent PLF with the PSR system.

Posterolateral fusion

Lumbar spine

Spontaneous anterior fusion

Pedicle screw

Degenerative lumbar spondylosis is a major cause of chronic low back pain and radiculopathy. In general population approximately 60–85% experienced low back pain during some point in their lives. Fortunately, chronic low back pain persisting more than 3 months was 15%-45% [1]. With failure of conservative management, lumbar spinal fusion is the main and effective treatment options for degenerative lumbar spondylosis such as lumbar disc degeneration, lumbar instability and lumbar deformity [2].

Among lumbar spinal fusion, several studies have reported that lumbar interbody fusion (LIF) provided significant pain relief and function improvement more than posterolateral lumbar fusion (PLF). Patients who had previously undergone lumbar spinal fusion by PLF technique but continued to experience back pain, even though solid fusion was achieved. Subsequently, revision surgery and LIF were performed, resulting in a notable reduction of their previous symptoms [3–5]. This finding is consistent with the principles of spinal biomechanics, as the anterior and middle columns of the spine bear a significant portion of the total load, up to 80% [6]. However, higher blood loss, longer operative time and more technical demand have been reported in LIF compared to PLF [7, 8].

From the clinical observation of this study, patients who underwent PLF with pedicle screw-plate (PSP) system found spontaneous anterior fusion (SAF) of lumbar spine. This phenomenon is quite different with patients who underwent PLF with pedicle screw-rod system (PSR). The cause of this phenomenon remains unclear due to lack of definitive evidence. When the SAF takes place, it provides anterior and posterior spinal stability. It could be the best surgical options, if PLF was performed with resulting SAF. Not only surgical complications were reduced but also the spinal devices such as PEEK cage and bone graft substitutes were unnecessary.

The objective of this study was to report the rate of SAF of lumbar spine after PLF with PSP system compared with the rate of SAF after PLF with PSR system. Additionally, to determine the factors that caused SAF after PLF.

Retrospective study was performed after obtaining approval from the Committee

on Human Rights Related to Research Involving Human Subjects, ..Hospital,… University. This study reviewed charts and radiographs of 111 consecutive adult patients who underwent PLF for treatment of degenerative lumbar spine between January 2008 to December 2017. Fifty-five patients underwent PLF with PSP system and fifty-six patients underwent PLF with PSR system.

All the patients included in the study were aged over 40 and had been diagnosed

with degenerative lumbar spinal stenosis, degenerative or isthmic lumbar spondylolisthesis. Their follow-up duration was more than 18 months. To assess their condition, all patients underwent various imaging scans, including plain film lumbosacral spine, dynamic radiograph, and MRI scans. These scans involved obtaining sagittal T1-weighted, T2-weighted, coronal T2-weighted, and axial images throughout the lumbar spine, with angles adjusted to align with the disc spaces.

The study had specific exclusion criteria. Patients with underlying traumatic, infective, or neoplastic etiology, as well as those with inflammatory diseases such as ankylosing spondylitis, were excluded. Additionally, individuals who had previously undergone lumbar spine surgery, concomitant discectomy procedures, interbody fusion or patients who underwent revision surgery within 6 months after their initial surgery were excluded from the study.

Demographic data of the patients, including age, sex, height, body weight, BMI, diagnosis, and level of fusion, were collected. Radiographic parameters were evaluated as follows; the degree of slip in spondylolisthesis was determined according to the Meyerding classification [9], degree of intervertebral disc degeneration was determined according to Pfirrmann grading system [10], the average screw depth in the vertebral body was measured by divided depth of screw in vertebral body by length of vertebral body (Fig. 1), grading of SAF was determined by degree of anterior fusion whether the fusion mass completely or partially involved the intervertebral disc (Fig. 2), and preoperative anterior intervertebral disc height and posterior intervertebral disc height were measured. All radiographic parameters were measured by using the Synaps Radiology PACS program (FUJIFILM Medical Systems, U.S.A., Inc., version 5.7.100) accessed through the hospital intranet.

Statistical analysis

For baseline characteristics, this study used mean/median and standard deviation/range for continuous variables. Frequency (n) and percentage (%) were calculated for categorical data. This study applied Kaplan-Meier method for survival analysis for presence of SAF (complete or partial) and the Kaplan-Meier curve was constructed. Logistic regression test for predictors of SAF was done for variables that were suspected based on clinical concept, and odds ratios were estimated. This study used STATA 16.0 Program, College Station, Texas, USA for all statistical analysis. P-value < 0.05 was defined as a significant level.

Sample size

Sample size was estimated by using STATA 16.0 Program based on alpha error 0.05, beta error 0.2 by two independent proportions formula: P1 = 0.15 (according to pilot study), P2 = 0.01. The calculated sample size was 58 per group. The final required total sample size was 116 patients.

A total of 111 patients (55 patients in PSP group and 56 patients in PSR group) were included in this study. Patients in both PSP group and PSR group had similar baseline characteristics (Table 1). Both groups showed with statistical significance (p < 0.05) higher percentage of female patients than male patients; 47 female patients (85.5%) for PSP group and 37 female patients (66.1%) for PSR group. There were no significance differences in age and BMI of both groups.

Table 1

Baseline characteristics of the patients
	Total	PSP (n = 55)	PSR (n = 56)	P-Value
Parameters	Total	PSP (n = 55)	PSR (n = 56)	P-Value
Sex				0.002
Male, n(%)	27 (24.3)	8 (14.5)	19 (33.9)
Female, n(%)	84 (75.7)	47 (85.5)	37 (66.1)
Age, years, mean(SD)	61.8 (7.3)	63.3 (6.8)	60.9(7.4)	0.105
BMI, kg/m², mean(SD)	25.6 (4.3)	26.9 (4.5)	25.1 (4.3)	0.113
Underlying disease, n(%)
Diabetes mellitus	30 (27.0)	15 (27.3)	15 (26.8)	0.859
Hypertension	47 (42.3)	29 (52.7)	18 (32.1)	0.084
Dyslipidemia	28 (25.2)	18 (32.7)	10 (17.9)	0.054
Disease, n(%)				< 0.001
Lumbar spinal stenosis	77 (69.4)	29 (52.7)	48 (85.7)
Degenerative spondylolisthesis	31 (27.9)	23 (41.8)	8 (14.3)
Isthmic spondylolisthesis	3 (2.7)	3 (5.5)	0 (0)
Meyerding grading, n(%)				< 0.001
No listhesis	77 (69.4)	29 (52.7)	48 (85.7)
Grade 1	32 (28.8)	24 (43.6)	8 (14.3)
Grade 2	2 (1.8)	2 (3.6)	0 (0)
Level of fusion, n(%)				0.199
1 level	35 (31.5)	18 (32.7)	17 (30.4)
2 levels	48 (43.2)	22 (40.0)	26 (46.4)
3 levels	22 (19.8)	14 (25.5)	8 (14.3)
4 levels	6 (5.4)	1 (1.8)	5 (8.9)
Pfirmann grading, n(%)				0.095
Grade 2	3(2.7)	1 (1.8)	2 (3.6)
Grade 3	12 (10.8)	5 (9.0)	7 (12.5)
Grade 4	41 (36.9)	21 (38.2)	20 (35.7)
Grade 5	55 (49.5)	28 (50.9)	27 (48.2)
Disc height, mm.
Anterior, mean(SD)	7.84 (2.4)	7.62 (2.6)	8.05 (2.2)	0.044
Posterior, mean(SD)	6.94 (1.8)	6.65 (2.1)	7.23 (1.7)	0.021
% of screw depth, mean(SD)	82.18 (2.9)	80.17 (5.2)	86.3 (3.2)	0.125
Follow-up time, month mean (range)	29.9 (18.0–86.0)	32 (18.0–69.0)	26.8 (18.0–86.0)	0.096
PSP: Pedicle screw-plate, PSR: Pedicle screw-rod

Three common comorbid diseases including diabetes mellitus, hypertension and dyslipidemia were observed. Out of the total of 111 patients, 27.0% of patients had diabetes mellitus, 42.3% of patients had hypertension and 25.2% of patients had dyslipidemia. There were no significant differences between both PSP and PSR groups in terms of these underlying diseases.

Most patients were diagnosed with lumbar spinal stenosis (69.4%). Others were degenerative spondylolisthesis (27.9%) and isthmic spondylolisthesis (2.7%). There were significant differences in diagnoses between PSP group and PSR group (P < 0.001). Most common diagnosis in both groups were lumbar spinal stenosis (85.7% in PSR group and 52.7% in PSP group). In the PSP group 41.8% of patients were diagnosed degenerative spondylolisthesis while only 14.3% of patients in PSR group were similarly diagnosed.

In the same way, Mayerding grade for magnitude of listhesis were significantly different between the PSP group and the PSR group (P < 0.001).

Patients of both groups underwent PLF from 1 level to 4 levels and most of them underwent PLF 2 level (43.2%). There was no significant difference of level of PLF in both groups (P = 0.199). The most common Pfirmann grading for degenerative disc in both groups were grade 5 which found 50.9% in the PSP group and 48.2% in the PSR group respectively. There was no significant difference of Pfirmann grading in both groups (P = 0.095).

The mean anterior disc height and the mean posterior disc height of both groups were significantly different P = 0.044 and P = 0.021 respectively. In the PSR group the mean anterior disc height and the mean posterior disc height were higher than those of the PSP group.

There was no significant difference in percentage of screw depth in vertebral body per length of vertebral body in both groups (P = 0.125).

Overall, the mean follow-up time was 29.9 months. The PSP group had longer follow-up time (32.0 months) than the PSR group (26.8 month). However, there was no significant difference of follow-up time in both groups (P = 0.96)

From overall 111 patients who underwent PLF, SAF presented in 15 patients. All

of them were in the PSP group (Fig. 3); 6 patients with complete SAF and 9 patients with partial SAF. No SAF was observed in any patients who underwent PLF with the PSR system. The presence of SAF (complete SAF and partial SAF) in the PSP group was found at 27.3% (Table 2).

Table 2

Presence of spontaneous anterior fusion
Results	Total (n = 111)	PSP (n = 55)	PSR (n = 56)
Complete SAF, n(%)	6 (5.4)	6 (10.9)	0 (0)
Partial SAF, n(%)	9 (8.1)	9 (16.4)	0 (0)
Total SAF, n(%)	15 (13.5)	15 (27.3)	0 (0)
PSP: Pedicle screw-plate, PSR: Pedicle screw-rod

A total of 55 patients in the PSP group were analyzed for correlation factors (Table 3). SAF presented in 15 patients whereas 40 patients had no SAF. There was no significant difference in ratio of female patients and male patients in both SAF group and no SAF group. Also, there was no significant difference in the mean age and the mean BMI of both groups P = 0.786 and P = 0.810 respectively. In terms of underlying diseases, diabetes mellitus, hypertension and dyslipidemia, there were no significant differences between SAF group and no SAF group (P = 0.376, P = 0.920, P = 0.244 respectively).

Table 3

Comparison of the clinical characteristic and radiographic findings of the patients in PSP group between patient who had SAF and No SAF
	SAF (n = 15)	No SAF (n = 40)	OR (95%CI)	P-Value
Parameters
Sex
Male, n(%)	2 (25.0)	13 (27.6)	1.25 (0.22,5.57)	0.752
Female, n(%)	6 (75.0)	34 (72.3)	1.16 (0.31, 3.44)	0.854
Age, years, mean(SD)	63.3 (6.7)	63.4 (7.6)	1.01 (0.92,1.11)	0.786
BMI, kg/m², mean(SD)	26.5 (3.7)	27.2 (4.2)	0.97 (0.82,1.17)	0.810
Underlying disease, n(%)
Diabetes mellitus	5 (33.3)	10 (25.0)	1.03 (0.60,4.45)	0.376
Hypertension	8 (53.3)	21 (52.5)	0.93 (0.21,3.96)	0.920
Dyslipidemia	4 (26.7)	14 (35.0)	0.40 (0.08,1.87)	0.244
Disease, n(%)
Lumbar spinal stenosis	9 (60.0)	20 (50.0)	1.04 (0.32, 2.43)	0.640
Degenerative spondylolisthesis	6 (40.0)	17 (42.5)	1.17 (0.27, 5.24)	0.829
Isthmic spondylolisthesis	0 (0)	3 (7.5)	-	-
Meyerding grading, n(%)
No listhesis	9 (60.0)	20 (50.0)	1.04 (0.32, 2.43)	0.640
Grade 1	6 (40.0)	18 (45.0)	0.94 (0.21,4.17)	0.932
Grade 2	0 (0)	2 (5)	-	-
Level of fusion, n(%)
1 level	5 (33.3)	13 (32.5)	1.02 (0.87, 1.92)	0.324
2 levels	7 (46.7)	15 (37.5)	1.31 (0.54,2.87)	0.133
3 levels	3 (20.0)	11 (27.5)	0.95 (0.85,1.95)	0.167
4 levels	0 (0)	1 (2.5)	-	-
Pfirmann grading, n(%)
Grade 2	0 (0)	1 (2.5)	-	-
Grade 3	1 (6.7)	4 (10.0)	0.77 (0.75,2.35)	0.689
Grade 4	6 (40.0)	15 (37.5)	0.88 (0.11,3.02)	0.911
Grade 5	8 (53.3)	20 (50.0)	0.69 (0.43, 2.39)	0.577
Disc height, mm
Anterior, mean(SD)	7.57 (2.8)	7.23 (2.7)	1.10 (0.84,1.45)	0.475
Posterior, mean(SD)	6.20 (1.9)	6.83 (1.8)	0.90 (0.60,1.35)	0.615
% of screw depth, mean(SD)	78.5 (7.6)	81(8.4)	0.86 (0.48,3.62)	0.455
Follow up time, month mean(SD)	52.3 (8.57)	26.5(11.1)	1.21 (1.08,1.44)	0.010
SAF: Spontaneous anterior fusion, PSP: Pedicle screw-plate

The most common diagnosis in both SAF and no SAF group was degenerative spinal stenosis. There was no significant difference in the number of patients who were diagnosed with degenerative spinal stenosis and degenerative spondylolisthesis in both groups. However, no patients who were diagnosed isthmic spondylolisthesis presented SAF. Similarly, those Meyerding classification, number levels of fusion and Pfirmann grading, disc height and screw depth were not significant differences.

This study also found that SAF was not seen in isthmic spondylolisthesis, grade 2 spondylolisthesis and 4 levels fusion. However, because of the small sample size of isthmic spondylolisthesis this might have produced inconclusive results.

The mean follow-up time in patients with SAF (52.3 ± 8.57 months) was significantly (P = 0.01) longer than the no SAF group (26.5 ± 11.11 months).

From Kaplan-Meier the SAP analysis (Fig. 4) estimated that 50% of SAP was found at follow-up time 58.8 months (95% CI, 40.27 to 77.73). The incidence rate was 0.009 per month. The SAP estimates were 29 months, 58.8 months and 69 months for 25%, 50%, 75% of the survival time respectively.

LIF is an effective and common treatment option for a number of lumbar spine disorders. Interbody fusion is a mainstay in treatment of disabling low back pain and leg pain. LIF can be performed through posterior, transforaminal, anterior and lateral approaches. Despite the advances in surgical technology and instrumentation, several studies reported the complications associated with LIF. Nonunion after lumbar interbody fusion has ranged from 7–20%. [11] Okuda et al, reported incidental durotomy in 19 patients out of 251 patients during posterior LIF surgery and 17 of them subsequently developed neurological deficits. [12]

Recently, lateral LIF technique has been reported with many advantages. One of the important advantages is using a large footprint cage placed on cortical bone at lateral apophyseal rings which provide rigid construction in load sharing manner [13]. The operation combined a retroperitoneal approach with posterior pedicle screw fixation for the stability of the construction. However, from the previous reports, the pseudoarthrosis rate of lateral LIF was 2.6%-19% [14]. Additionally, many surgical complications were reported such as vascular complication, nerve injury, urologic, cardiac and pulmonary complication [15].

Certainly, getting the interbody fusion may be one of the most important goals for treatment of degenerative spinal disorder. Many innovations such as new devices, new cages, new bone substitutions have been developed for this purpose. In this study, all of the patients underwent lumbar PLF without discectomy or interbody fusion. This study found that 27.3% had SAF. This SAF occurred only in the PSP group and the rate of SAF increased significantly with time of follow-up.

Very few literatures have studied spontaneous fusion of lumbar spine and the definite pathophysiology has remained unclear. Most of the reported spontaneous fusions of lumbar spine occurred at the facet joints or bridging osteophyte of the adjacent vertebral bodies. Huang et al, studied radiographs of 86 spondylolisthesis patients and they found 18 patients had spontaneous lumbar fusion. The most common site of fusion was facet joints. They found spontaneous spinal fusion directly in the intervertebral disc in only 1 case. In that case, the intervertebral disc of L5-S1 was very narrow which caused almost bone to bone contact of vertebral endplates [16]. Another two case reports of SAF were also found in the severe narrowing of intervertebral disc spaces [17, 18]. In this study, the SAF of lumbar spine might have occurred with different pathophysiology. In this study’s cases, the intervertebral disc space still remained and there was no bone-on-bone fusion as in case reports. Computed tomography scan of SAF found the fusion masses were created inside intervertebral discs which starting from the periphery to the center of the intervertebral disc (Fig. 5).

From the literature review, no study has reported the SAF in patients who underwent PLF of lumbar spine. The research team proposed the causes of the SAF in the PSP group might come from the progressive intervertebral disc degeneration after PSP fixation. Several studies have shown degeneration of intervertebral disc was associated with the possibility to find the calcification, endochondral ossification, and hypertrophic differentiation. From biochemistry analysis many substances which related to osteoblast activity such as collagen type X, Runt-related transcription factor 2, osteoprotegerin and alkaline phosphatase were found in degenerative intervertebral disc [19–22].

The PSP system provided more rigid stability which resulted in limit motion of the intervertebral disc. Mechanical loading at the endplate was reduced then nutrient transport mechanism to provide nutrition to the intervertebral disc was disrupted [23]. This process rapidly increased degeneration of the intervertebral disc, then the process of calcification of the intervertebral disc was initiated.

From the biomechanical study the PSP system showed more reduction in range of motion compared with the PSR system, especially in axial rotation [24]. The difference of stability between the PSP system and the PSR system constructs came from several reasons according to the study of Crawford et al such as in the PSP system, the plate was placed closer to the long axis of the spine. Second, in the PSR system pedicle screw needs a connector between rod and screw but in the PSP system the screw inserts directly into the plate. Finally, the distance between the longitudinal element and long axis of the spine in the PSP system was closer than in the PSR system [25]. Results from this study found there was no SAF in patients who underwent PLF with the PSR system. The stability of the PSR construct may be less rigid than the construct of the PSP. Then maybe there are some motions of the intervertebral disc in the PSR construct.

When this study looked at the other factors between the PSP group and the PSR group, there was no significant difference between groups except for follow-up time. In this study the average follow-up time of the PSR group was shorter than the average follow-up time in the PSP group, although it was not significantly different. When this study performed subgroup analysis in the PSP group, patients who developed SAF had significant longer follow-up time more than patients who had no SAF (P = 0.01). Therefore, probably in longer follow-up time period, SAF may be found more in patients who had solid PLF.

Radical discectomy and replacement with cage or allograft was the standard surgical

technique for LIF. These procedures need more tissue destruction, more surgical steps, longer operative time and higher cost of devices and instrumentations. PLF with pedicle screw fixation is a simple operation. With more biomechanical advantages of plating, SAF without discectomy can occur. This may change the approach for the fusion of anterior lumbar spine.

There were some limitations to the present study. Firstly, it was a retrospective study and

the population in both groups did not reach the target due to limitations in the database. Secondly, this study relied on plain radiograph and MRI assessment, and there was no evaluation

of the correlation with clinical outcomes of the patients. Thirdly, this study could not obtain

tissue histopathology and biochemistry testing of the patients who developed SAF which could

explain more about the pathophysiology of the lumbar SAF.

This study has reported SAF of lumbar spine in patients who underwent PLF with the PSP system. The SAF was found in 27.3% of patients in the PSP group and the SAF increasingly occurred with longer follow-up time. In this study, no SAF was found in patients who underwent PLF with the PSR system.

LIF: Lumbar interbody fusion

PLF: Posterolateral lumbar fusion

PSP: Pedicle screw-plate

PSR: Pedicle screw-rod

SAF: Spontaneous anterior fusion

Acknowledgements

None.

Authors’ contributions

Author contributions to the study and manuscript preparation include the

following. Conception and design: CK. Acquisition of data: IC, PR, GK. Analysis and

interpretation of data: IC, CK and TL. Drafting the article: IC, CK, TL. Critically revising the

article: CK and TL. Approved the final version of the manuscript on behalf of all authors: CK.

Statistical analysis: UU. Administrative/technical/material support: PC and PL.

Funding

The authors declare that no funding was received for this study.

Availability of data and materials

The datasets and materials supporting the results of this study are

included within the article, further inquiries can be directed to the corresponding author.

Ethics approval and consent to participate

The study was approved by the Human Research Ethics Committee, Faculty of Medicine, Ramathibodi Hospital, Mahidol University. ID: COA.MURA 2023/179 Ref. 2024/551.

All procedures and methods were carried out in accordance with a Declaration of Helsinki

regulation and guidelines. Written informed consent was obtained from all participants and/or their legal guardian(s) before participation into the study.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

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No competing interests reported.

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A spontaneous anterior fusion of lumbar spine after posterolateral lumbar fusion with pedicle screw-plate system

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Materials and Methods

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