Does the Lumbar Paraspinal Muscle Status Have a Role in Predicting Mechanical Complications After Adult Spinal Deformity Surgery?

Abstract

Study design

Retrospective analysis.

Objectives

To assess the role of paraspinal muscle morphology in predicting complications after adult spinal deformity (ASD) surgery.

Materials and Methods

A total of 93 patients who underwent surgery for ASD from 2017 to 2022 were enrolled. Using early postoperative X-ray, they were divided into proportioned (P), moderately proportioned (MP), and severely proportioned (DP) groups based on the Global Alignment and Proportion (GAP) scores. Further, they were classified into two groups: Group A (presence of mechanical complications) and Group B (no mechanical complications). In addition, other parameters including preoperative BMI, smoking status, cross-sectional area (CSA), and grades of paraspinal muscle fatty infiltration (FI) were calculated in all patients using preoperative MRI. These parameters were compared across the groups using a one-way analysis of variance (ANOVA). Post-hoc pairwise testing was done using Bonferroni’s method. These were also compared between groups A and B using a 2-sample t-test.

Results

The mean follow-up period was 32.7 months (24-64 months). 27 (29%) of 93 patients developed mechanical complications following ASD surgery. Of the 27 patients, 6 (22.2%) were proportioned, 10 (37%) were from MP and 11 (40.7%) were from the DP group. Group A had low CSA (P = 0.014), and high FI (P = 0.003) grades compared to group B. Further, 22.2% (6/27) had a history of smoking before surgery (OR = 6.57).

Conclusions

We recommend consideration of preoperative smoking, CSA, and FI of paraspinal muscles in addition to the GAP score to minimize mechanical complications in patients undergoing ASD surgery.

Keywords

adult spinal deformity spine sarcopenia postoperative complications smoking

Introduction

Adult spinal deformity (ASD) is a complex and multifaceted condition that encompasses a wide range of structural and functional abnormalities affecting the spine in the adult population. Though the exact etiology for this is still uncertain – the reason behind being called “de novo” – multiple hypotheses exist in the literature. The prevalence of ASD in people aged 60 years or older is upto 68%¹ and of which 60% of them require surgical intervention.² The goal of managing such patients is primarily pain relief and improving quality of life while minimising complications. However, the incidence of patients requiring revision surgery in their follow-up ranges from 6.1% to 46.5%^3–5 with the most common cause being proximal junctional kyphosis/failure.⁶ This is superadded by the average cost of ASD revision surgery, which is approximately $ 55,000 per case with the cost of revision surgery exceeding that of index surgery by a factor of 2.1.⁷

These failures are attributed to inadequate fixation, long construct, inadequate deformity correction, and decreased bone quality.⁸ Recently, much attention has been given to sagittal spinopelvic parameters as the primary cause of these failures. Following this, the Global Alignment and Proportion (GAP) score, a pelvic incidence (PI) based proportional method was developed to predict mechanical complications in patients undergoing surgery for ASD.⁹ According to their observation, patients with moderately or severely disproportioned spinopelvic parameters had mechanical complication rates of 47% and 95% respectively. However, the GAP score had overall poor predictive ability for the occurrence of mechanical complications, based on a systematic review and meta-analysis of external validation of the score which included 1206 patients (AUC = 0.69).¹⁰

These conflicting results of previous literature raise serious questions about the predictability of mechanical complications following ASD surgery. Further, studies considering other known risk factors for the development of complications after ASD surgery, such as body mass index (BMI), sarcopenia, and smoking, present a knowledge gap till date. Hence, our hypothesis is that ASD patients with poor preoperative paraspinal muscle morphology, irrespective of their GAP score are associated with a higher risk of mechanical complications after ASD surgery.

Materials and Methods

Study Design and Participants

This retrospective study was conducted in compliance with the principles of the Declaration of Helsinki. The data were collected from patients who had undergone surgery for adult spinal deformity (ASD) in a tertiary care centre.

We included patients aged >18 years who had undergone surgery for ASD (at least one of the following: coronal Cobb angle of >20°, sagittal vertical axis of >5 cm, or pelvic tilt of >25°) with >3 levels of posterior instrumented fusion with more than 2 years of follow-up. We excluded patients with tandem canal stenosis; a history of previous spine surgery; inappropriate radiographs; history of postoperative surgical site infection; neuromuscular or syndromic deformity; and uncontrolled medical comorbidities such as malignancy, diabetes, or sepsis that can cause disability or worsen general health.

From January 2017 to December 2022, 114 patients underwent adult spinal deformity correction surgery in our institute. Of these, 93 patients fulfilled our inclusion criteria and were included in our study. We collected patients’ demographic details, such as age, gender, BMI, and smoking status at the time of surgery from their medical records.

Radiographic Evaluation

Using anteroposterior (AP) standing X-rays Cobbs angle (CA) was calculated. Sagittal parameters including, sacral slope (SS), pelvic tilt (PT), pelvic incidence (PI), L1-S1 lumbar lordosis (L1-S1 LL), L4-S1 lumbar lordosis (L4-S1 LL), global tilt (GT), and sagittal vertical axis (SVA) were calculated in the lateral whole spine standing X-ray. Routine radiologic follow-up was carried out at the immediate postoperative period, 3, 6, 12, and 24 months after surgery. With the above parameters, an immediate postoperative GAP score was calculated. The patients were followed up at regular intervals to look for mechanical complications. As per clinical significance, mechanical complications were defined as proximal junctional kyphosis or failure, distal junctional kyphosis or failure, rod breakage, and other implant-related complications (not related to proximal junction failure such as screw loosening, breakage, or pullout or interbody graft, or set screw dislodgement). Mechanical revision was defined as revision surgery due to mechanical complications.

Using a preoperative T2WI axial MRI image at the level of the upper endplate of the apex of deformity, the grade of fatty infiltration (FI), and cross-sectional area (CSA) of paraspinal muscles were calculated. FI of the paraspinal muscles was graded based on a 4-grade fat infiltration grading system (Figure 1).¹¹ CSA was measured by drawing a region of interest around the muscles, excluding epimuscular fat. An example of a measurement method for CSA is shown in Figure 2. All measurements were done using integrated measurement tools in an archiving communication system (PACS).

Figure 1.

Muscular location and fat infiltration grades on T2 axial MRI slice (evaluation with 4-grade system) (A) Grade 1 = < 10% fat infiltration, (B) Grade 2 = 10%-30% fat infiltration, (C) Grade 3 = 30%-50% fat infiltration, (D) Grade 4 = > 50% fat infiltration¹¹.

Figure 2.

Axial T2-weighted MRI obtained at the level of the upper endplate of L3. CSA of paraspinal (ES and MF) (in cm²) muscles is measured by drawing a region of interest around the muscles, excluding epimuscular fat.

Three fellowship-trained spine surgeons conducted all the measurements, and the final measurements were averaged with Fleiss kappa values for inter- and intra-observer reliability of 0.76 and 0.62 respectively.

Patient Classification

Patients were divided into three groups based on their GAP score: proportioned (P), moderately disproportioned (MP), and severely disproportioned (DP). The study population was further divided into Group A (presence of mechanical complications) and Group B (no mechanical complications).

Statistical Analysis

All statistical analyses were performed with R language and environment (version 4.4.1). Age, BMI, sagittal parameters, and follow-up in months were summarized as mean (SD) or median (range); other characteristics were presented as frequency and percentage. The data were assessed using the Shapiro–Wilk test for normality. The FI and CSA were compared across P, MP, and DP groups using a one-way analysis of variance (ANOVA). Post-hoc pairwise testing was done using Bonferroni’s method after the ANOVA/Kruskal Wallis test. All the radiological parameters were also compared between groups A and B using a 2-sample t test. A Chi-square test was performed to test the association between gender/smoking status/FI/CSA of paraspinal muscles and patient groups (P/MP/DP and Group A/B). A multiple logistic regression model was used to determine the association between complications with smoking status and CSA of paraspinal muscles. All tests were tested for P < 0.05 for significance.

Results

The mean age was 68.3 years (42-88 years). The mean follow-up period was 32.7 months (24-64 months). Of 93 patients, 67 (72%) were female. The mean BMI was 24.7 (18.4-37.8). According to the GAP score, 19.4% (18/93), 31.1% (29/93), and 49.5% (46/93) of patients were classified into P, MP, and DP categories, respectively (Figure 3). There were no significant differences in mean age, gender ratio, mean BMI, and smoking status between the P, MP, and DP groups. Of all sagittal radiological parameters, we found that SVA was significantly higher in the DP group than in the other 2 groups (P = 0.041). However, on comparison within groups, we found that SVA and CSA of paraspinal muscle were significantly different between the P and DP groups. Further, the SVA was lower in group P patients than in group MP. FI grades were not significant in comparison between each group (P/MP/DP). The demographic and radiological parameters of P, MP, and DP groups are given in Table 1.

Figure 3.

The overall distribution of the GAP score in the present study. 0-2: proportioned (P), 3-6: moderately disproportioned (MP), >7: severely disproportioned (DP).

Table 1.

Comparison of Demographics and Radiological Parameters Between Proportioned, Moderately Disproportioned, and Severely Disproportioned Groups.

	Proportioned (N = 18)	Moderately Dis-Proportioned (N = 29)	Severely Dis-Proportioned (N = 46)	P-value
Age (years), mean ± SD	60.4 ± 8.3	62.0 ± 6.5	64.1 ± 8.1	0.320
Current smoking, n (%)	4 (22.2)	7 (36.8)	4 (8.6)	0.462
Cobbs angle (in degrees), mean ± SD	24.3 ± 3.6	25.7 ± 4.2	28.3 ± 7.6	0.630
Pelvic incidence (in degrees), mean ± SD	48.04 ± 8.55	46.96 ± 10.30	67.25 ± 9.12	0.88
Pelvic tilt (in degrees), mean ± SD	24.99 ± 4.30	17.80 ± 3.62	29.28 ± 8.25	0.120
Sacral slope (in degrees), mean ± SD	23.86 ± 3.16	31.29 ± 4.35	37.7 ± 6.54	0.250
Global tilt (in degrees), mean ± SD	15.97 ± 5.40	30.25 ± 7.25	30.38 ± 7.40	0.075
SVA (in cm), mean ± SD	5.4 ± 2.10	7.35 ± 2.25	13.40 ± 2.65	0.041*
GAP score, mean ± SD	1.5 ± 0.3	4.7 ± 2.1	8.3 ± 3.5	0.007
FI, n (%)				0.080**
Grade 1	7 (38.9)	11 (37.9)	12 (26)
Grade 2	5 (27.7)	7 (24.1)	6 (13.1)
Grade 3	3 (16.7)	4 (13.8)	13 (28.3)
Grade 4	3 (16.7)	7 (24.2)	15 (32.6)
CSA of paraspinal muscles (in cm²), mean ± SD	21.050 ± 3.110	20.517 ± 2.070	18.415 ± 4.310	0.062***

* Kruskal wills test. Bonferroni test: The SVA value of Group P and MP is statistically significant (P-value <0.001). The SVA value of Group P and DP is statistically significant (P-value <0.001). The other group (MP and SP) is not significant. **FI grades were not significant between any groups (P, MP, and DP) after Bonferroni calculation.

***ANOVA test. Bonferroni test: The CSA value of group P and DP is statistically significant (P value 0.016). Other groups are not significant.

27 (29%) of 93 patients developed mechanical complications following ASD surgery (group A). Of the 27 patients, 22.2% (6/27) were proportioned, 37% (10/27) were from MP and 40.7% (11/27) were from the DP group. The incidences of mechanical complications are shown in Table 2. The proximal junctional kyphosis or failure was the most common complication and was found in 66.6% (18/27) patients. 29.6% (8/27) of patients underwent revision surgery for mechanical complications and all these patients had grade 3 or 4 FI and a mean CSA of 10.360 cm² of paraspinal muscle.

Table 2.

The Incidence of Complications (Two or More Mechanical Complications (eg, Screw Loosening and Rod Breakage) Occurred Simultaneously in 4 out of 27 Patients).

Complications	N (%)
Proximal junctional kyphosis or failure	18 (66.6%)
Distal junctional kyphosis or failure	5 (18.5%)
Rod breakage	4 (14.8%)
Other implant-related complications	12 (44.4%)

Compared to patients without mechanical complications (66/93) (group B), group A had low CSA of paraspinal muscles (12.310 cm² vs 18.622 cm²) (P = 0.014). In group A, 77.7% (21/27) of patients belonged to either grade 3 or 4 fatty infiltration of paraspinal muscles (Figures 4 and 5). While in group B, only 27.3% (18/66) of patients belonged to either grade 3 or 4 fatty infiltration of paraspinal muscles (P = 0.003) (Figure 6). Further, group A had a higher mean SVA than group B (15.20 cm vs 8.24 cm) (P = 0.035). 22.2% (6/27) had a history of smoking before surgery in group A compared to 13.6% (9/66) in group B (P = 0.020). The comparison between group A and group B is shown in Table 3. The comparison of SVA and CSA of paraspinal muscles based on FI grades is shown in Table 4. The box and whisker plot showing the distribution of SVA and CSA of paraspinal muscles based on grades of FI is shown in Figure 7.

Figure 4.

A 70-year-old man with the T2WI axial image showing grade 3 FI of paraspinal muscles, low CSA (11.310 cm²) (A), and a whole spine standing lateral X-ray showing ASD (B). During the postoperative period, the GAP score was 3 (C). PJK was noted at the 3-year follow-up (D), Which required revision and extension of fixation up to D7 (E).

Figure 5.

A 65-year-old woman with T2WI axial image showing grade 4 FI of paraspinal muscle, low CSA (9.56 cm²) (A), and a whole spine standing lateral X-ray after ASD surgery at 8-month post-op period (B). Underwent revision and extension of fixation up to D9 (C, D).

Figure 6.

A 60-year-old woman with T2WI axial image showing grade 1 FI of paraspinal muscle, CSA of 13.322 cm²) (A), and a whole spine standing lateral X-ray (B). Underwent ASD surgery showing immediate post-op (C) and 5-year post-op (D) requiring no revision.

Table 3.

Comparison of Demographics and Radiological Parameters Between Group a (With Mechanical Complications) and Group B (Without Mechanical Complications).

	Group A (N = 27)	Group B (N = 66)	P-value
Age (in years), mean ± SD	62.1 ± 7.3	59.8 ± 8.2	0.342**
Gender (M/F), n	9/18	17/49	0.220*
BMI, mean ± SD	23.7 ± 3.5	25.6 ± 4.1	0.070**
Smoking status, n (%)	6 (22.2)	9 (13.6)	0.020*
Cobbs angle (in degrees), mean ± SD	30.65 ± 10.20	26.73 ± 5.70	0.120**
Pelvic incidence (in degrees), mean ± SD	56.50 ± 17.24	47.85 ± 13.70	0.225**
Pelvic tilt (in degrees), mean ± SD	26.30 ± 6.36	23.33 ± 7.35	0.345**
Sacral slope (in degrees), mean ± SD	32.65 ± 7.20	27.50 ± 8.15	0.125**
Global tilt (in degrees), mean ± SD	27.34 ± 5.21	25.54 ± 6.25	0.260**
SVA (in cm), mean ± SD	15.20 ± 3.5	8.24 ± 2.7	0.035**
GAP score, mean ± SD	8.5 ± 4.2	5.1 ± 4.5	0.095**
FI, n			0.003*
Grade 1	2	31
Grade 2	4	17
Grade 3	8	11
Grade 4	13	7
CSA of paraspinal muscles (in cm²), mean ± SD	12.310 ± 2.110	18.622 ± 3.012	0.014**

*Chi-square test, **Two independent t test. Bold letters signify the statistically significant values.

Table 4.

Comparison of SVA and CSA of Paraspinal Muscles Based on FI Grades.

	FI Grade				P Value
	1 (N = 33)	2 (N = 21)	3 (N = 19)	4 (N = 20)	P Value
SVA, (in cm²), median (range)	4.1 (0, 12.8)	5.15 (0, 16.5)	5.5 (1.5, 18.8)	8.9 (1.5, 19.7)	0.004*
CSA, (in cm²), mean (SD)	22.3 (4.44)	21.3 (4.73)	20.3 (4.35)	17.9 (4.44)	0.010**

*Kruskal Wallis test. Bonferroni test: FI grades 1 and 4 are statistically significant (P-value 0.003).

**ANOVA. Bonferroni test: FI grades 1 and 4 are statistically significant (P-value 0.005). FI grades 2 and 4 are statistically significant (P-value 0.022). Other groups are not significant.

Figure 7.

The box and whisker plot show the distribution of SVA and CSA of paraspinal muscles based on grades of FI.

Multiple logistic regression analysis was performed, which showed that smoking (OR 6.57) (P = 0.033) and low CSA of paraspinal muscles (OR 0.57) (P < 0.001) were the most significant factors for predicting mechanical complications after ASD surgery (Table 5).

Table 5.

The Odds Ratio (OR) for Smoking Status and CSA.

	Group A (N = 27)	Group B (N = 66)	OR (95 % CI)	P Value
Smoking status
Yes, n (%)	6 (22.2%)	9 (13.6%)
No, n (%)	21 (77.8%)	57 (86.4%)	6.57 (1.24 – 42.9)	0.033
CSA, (in cm²), mean (SD)	12.310 (2.110)	18.622 (3.012)	0.57 (0.34 – 0.70)	<0.001

Bold letters signify the statistically significant values.

Discussion

The effect of paravertebral muscle degeneration on spinopelvic parameters has been extensively studied earlier.^12–15 However, its role in degenerative scoliosis concerning long-term outcomes and its complications following surgery has not been described elsewhere. Our study finds an association between paraspinal muscle morphology with mechanical complications following adult spinal deformity surgery. Lower CSA and higher FI grades are seen in patients with mechanical complications (group A) than those without mechanical complications (group B). Further, all patients who had undergone revision surgery had grade 3 or 4 FI with very low mean CSA of paraspinal muscles compared to other groups.

The GAP score initially described on behalf of the European Spine Study Group (ESSG), is a pelvic incidence-based proportional method of analysing the sagittal plane that predicts mechanical complications in patients undergoing surgery for ASD.⁹ They reported that patients with a proportioned group as per GAP score had a mechanical complication rate of 6%, while the MP and DP groups had complications of 47% and 95%, respectively. However, following this several external validations of the GAP score were done which showed minimal-to-moderate predictive capability for mechanical complications associated with ASD correction.^10,16,17 This is further highlighted by the fact that 22.2% of patients who developed mechanical complications in our study were from the proportioned group. This demonstrates that radiological sagittal characteristics are not the only factor influencing mechanical complications after ASD surgery.

The most common complication and cause for revision surgery was proximal junctional kyphosis/failure (PJK and PJF) accounting for 66.6%. This finding is similar to those reported in the recent literature.⁶ Proximal Junctional Kyphosis (PJK) is defined as a proximal junctional sagittal Cobb angle of ≥10° or a proximal junctional sagittal Cobb angle at least 10° greater than the preoperative measurement.¹⁸ Proximal junctional failure, instead, refers to the fracture of the UIV or one vertebra above the UIV, the pullout of instrumentation at the UIV, and/or sagittal subluxation.⁹ Many variables, which include the patient’s age, pre-existing low bone mineral density, preoperative hyper-kyphosis, inappropriate sagittal alignment, and surgery-related factors (eg, extensive para-spinal muscle dissection at the UIV, disruption of the supraspinous and interspinous ligaments, improper end-vertebra selection, facet violation, pelvic fixation, magnitude of correction), are likely contributing to these phenomena.^19–24

CSA and FI of paraspinal muscles are associated with many degenerative spinal pathologies such as lumbar canal stenosis,²⁵ prolapsed intervertebral disc,²⁶ ASD,²⁷ and even osteoporotic fractures.²⁸ Further, FI in paraspinal muscles has been associated with reduced muscle strength, impaired functional outcomes, and increased risk of postoperative complications, such as adjacent-segment degeneration, persistent LBP, and interbody fusion non-union.²⁹ However, this association between paraspinal muscle morphology and complications following ASD surgery is the first of its kind. We measured both these parameters using conventional T2WI MRI images which is the most used imaging modality in clinical settings. However, there are multiple imaging modalities and assessment techniques for assessing the morphology of paraspinal muscles.³⁰ Hence, we recommend promoting uniform measurement techniques to increase the study’s reliability and decrease the heterogenicity in assessing paraspinal muscle morphology.

Regarding radiological sagittal parameters, recent literature demonstrated that sagittal spinopelvic alignment in ASD patients plays an important role in pain and disability and it’s a primary determinant of patient-related outcome measures (PROM).1,31 –34 However, in our study patients who had mechanical complications differed significantly in sagittal vertical axis (SVA). This aligns with the previous finding.¹⁷ However, we found that there was no statistically significant difference in other sagittal parameters between groups A and B (Table 3).

ASD surgery frequently results in extended hospital stays, significant blood loss, and a correspondingly elevated risk of postoperative complications. The causes of complications following such surgeries are always multifactorial. Limiting the cause of these complications only to sagittal alignment may not be sufficient. Hence, the GAP score should be used with caution as it may not explain or predict mechanical failure based on alignment in all populations as originally expected.¹⁷ Further, the calculation of the GAP score requires third-party software making these methods less practical in everyday clinical practice.

The strength of our study includes patients with no differences in demographic and clinical baseline characteristics among all the groups. Another advantage is that they do not need special applications or devices to calculate these parameters. However, this study has the following limitations. Firstly, prone to selection bias inherent to retrospective studies. Secondly, the sample size is different across different groups. Thirdly, influences on clinical outcomes and osteoporosis were not considered.

Conclusions

Our study demonstrates preoperative active smoking status, low CSA, and high FI of paraspinal muscles are associated with mechanical complications after ASD surgery. Hence, we recommend the consideration of these preoperative parameters as well in addition to radiological sagittal parameters to predict the mechanical complications following ASD surgery.

Footnotes

Author contributions

All authors made substantial contributions to the conception or design of the work, the acquisition, analysis, or interpretation of data, drafted the work or revised it critically for important intellectual content, approved the version of the manuscript, and agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Ethical Statement

ORCID iDs

Ganesh Kumar

Vikas Tandon

Rajat Mahajan

Jitesh Manghwani

Data Availability Statement

All data generated or analyzed during this study are included in this manuscript. Data will be provided on request.*

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