García-Ramos, C¹; Pérez-Oliva, J¹; Carreón-Cerda, C²; Solorio-Pineda, S¹; Hernández-Moctezuma, D¹; Álvarez-Ramírez, C¹; Alpízar-Aguirre, A¹; Reyes-Sánchez, A¹

Language: English
References: 22
Page: 164-171
PDF size: 1167.46 Kb.

ABSTRACT

Introduction: the multifidus muscle is a key stabilizer of the lumbar spine. While its deterioration has been associated with degenerative spinal conditions, the specific impact of its preoperative morphology on surgical outcomes remains unclear. The objective of this study is to evaluate the relationship between preoperative multifidus muscle morphology (cross-sectional area and fatty degeneration) and clinical-surgical outcomes following lumbar fusion surgery. Material and methods: a retrospective cohort study of 99 patients aged 40-65 years with L4-L5 degenerative spondylolisthesis undergoing lumbar fixation and arthrodesis (2022-2024). Preoperative MRIs assessed cross-sectional area multifidus (CSAM) and fatty degeneration. Three spine surgeons performed measurements with substantial interobserver reliability (κ = 0.80). Clinical outcomes (ODI, Roland Morris, SF-36, VAS), surgical variables, and complications were analyzed. Results: the cohort (31.31% male, 68.69% female; mean age 59.15 ± 8.78 years) showed significant positive correlations between CSAM and intraoperative blood loss (r = 0.249, p = 0.013) and postoperative complications (r = 0.217, p = 0.031). Fatty degeneration demonstrated no significant association with complications (p = 0.214) or clinical improvement. All patient groups showed significant clinical improvement at 12 months postoperatively regardless of fatty infiltration severity. Conclusions: larger multifidus muscle volume correlates with increased surgical complexity, while fatty degeneration does not predict complications or clinical outcomes. These findings highlight the importance of comprehensive muscle morphology assessment in preoperative planning. A retrospective cohort study was conducted.

INTRODUCTION

Degenerative spondylolisthesis affects 10-15% of patients with chronic low back pain, and surgical intervention is often required when conservative measures fail.¹ The lumbar paraspinal muscles, particularly the multifidus, play a crucial role in maintaining spinal stability, posture, and protecting structural elements.^2,3,4,5 Understanding preoperative predictors of surgical complexity becomes crucial for optimizing outcomes in this specific population.

Age-related degeneration of the lumbar paravertebral muscles follows a predictable pattern, with systematic reviews demonstrating progressive fatty infiltration and muscle atrophy that correlates with advancing age.² This degenerative process significantly impacts spinal sagittal alignment, as muscle quantity and quality are critical determinants of spinal biomechanics.³ The accuracy of MRI signal intensity ratio measurements in evaluating multifidus muscle injury and atrophy has been well-established, providing reliable non-invasive assessment tools for clinical practice.⁴

Changes in paraspinal muscles have been consistently associated with low back pain and spinal degeneration, with CT studies demonstrating clear relationships between muscle degeneration and clinical symptoms.⁵ Fat infiltration of paraspinal muscles is particularly associated with both low back pain and disability in community-based adults,⁶ highlighting the clinical relevance of muscle quality assessment. Advanced 3D analysis techniques using T2 images have improved our ability to quantify fatty infiltration of paravertebral lumbar muscles,⁷ offering more precise evaluation methods.

The correlation between multifidus fatty atrophy and lumbar disc degeneration in low back pain patients has been demonstrated,⁸ while the effects of spinal stabilization exercises on muscle cross-sectional areas show the potential for therapeutic intervention.⁹ Sedentary lifestyle represents a significant risk factor for low back pain development,¹⁰ emphasizing the modifiable nature of some muscle-related risk factors. CT measurements have consistently shown reduced trunk muscle areas in patients with chronic low back pain,¹¹ supporting the importance of muscle preservation.

Preoperative paraspinal and psoas major muscle atrophy, along with paraspinal muscle fatty degeneration, have been identified as factors influencing surgical outcomes in lumbar disc disease.¹² Severe lumbar intervertebral disc degeneration associates with Modic changes and fatty infiltration in paraspinal muscles at most lumbar levels,¹³ illustrating the interconnected nature of spinal degeneration processes. CT imaging studies confirm significant differences in trunk muscles between chronic low back pain patients and healthy controls,¹⁴ while local denervation atrophy of paraspinal muscles contributes to postoperative failed back syndrome.¹⁵

Given this comprehensive background, our study aims to precisely evaluate how both morphological characteristics of the multifidus muscle influence intraoperative parameters and clinical outcomes following lumbar fusion surgery for degenerative spondylolisthesis, building upon existing literature while addressing gaps in surgical outcome prediction.

MATERIAL AND METHODS

STUDY DESIGN AND POPULATION

A retrospective cohort study was conducted following STROBE guidelines. The study received approval from the institutional ethics committee (reference #2021-45-SPINE), and informed consent was waived due to the retrospective nature. Sample size calculation was performed a priori using G¹Power software, indicating that 97 patients would provide 90% power to detect a moderate correlation (r = 0.30) with α = 0.05.

Inclusion criteria comprised:

• • Patients aged 40-65 years.
• • Single-level L4-L5 degenerative spondylolisthesis (Meyerding grade I-II).
• • Primary lumbar fixation and arthrodesis performed between January 2022 and June 2024.
• • Complete preoperative MRI studies including axial T2-weighted sequences.
• • Minimum 12-month postoperative follow-up.

Exclusion criteria were rigorously applied:

• • Previous lumbar spine surgery.
• • Traumatic or isthmic spondylolisthesis.
• • Active spinal infection or malignancy.
• • Inflammatory spondyloarthropathies.
• • Neuromuscular disorders affecting paraspinal muscles.
• • Incomplete imaging or clinical data.
• • Revision surgery cases.

DATA COLLECTION PROTOCOL

CLINICAL VARIABLES

Data abstraction followed a standardized protocol using our institution's electronic medical records. Demographic and clinical variables included:

• • Age, sex, body mass index (BMI).
• • Comorbidities.
• • Smoking status and pack-year history.
• • Duration of symptoms prior to surgery.
• • Preoperative and 12-month postoperative patient-reported outcomes:
  ­­– Oswestry disability index (ODI) version 2.0.
  • – Roland-Morris disability questionnaire.
  • – SF-36 Health Survey (physical and mental component summ
  • – Visual analog scale (VAS) for lumbar pain (0-10).

SURGICAL VARIABLES

Intraoperative data were extracted from anesthesia records and surgical reports:

• • Operative time (skin incision to closure).
• • Intraoperative blood loss (quantified by suction canister volume minus irrigation).
• • Need for blood transfusion.
• • Surgical approach (posterior vs transforaminal lumbar interbody fusion).
• • Fusion levels and instrumentation details.

COMPLICATION ASSESSMENT

Postoperative complications were systematically categorized and monitored for 12 months, and those included were: surgical site infection, hardware failure, pseudarthrosis, symptomatic adjacent segment disease, superficial infection, transient neurological deficits, dural tear, reoperations.

MRI ACQUISITION AND ANALYSIS PROTOCOL

IMAGING PARAMETERS

All patients underwent preoperative lumbar spine MRI using 3.0 Tesla scanners (Philips Achieva) with standardized protocols:

• • Axial T2-weighted turbo spin-echo sequences.
• • Slice thickness: 3 mm with 0.5 mm gap.
• • Field of view: 200 × 200 mm.
• • Matrix size: 320 × 256.
• • Repetition time/echo time: 3,000-4,000/90-110 ms.

MUSCLE MORPHOLOGY ASSESSMENT

Three fellowship-trained spine surgeons (interobserver variability κ = 0.80) with 10-15 years of experience, blinded to clinical outcomes, independently performed all measurements using DICOM viewer (version 3.3.5). Measurements were conducted at the L4-L5 intervertebral disc level following a standardized protocol:

Cross-sectional area measurements:

• • Multifidus muscle boundaries defined:
  • – Medial: spinous processes and interspinous ligament.
  • – Lateral: plane between multifidus and longissimus muscles.
  • – Posterior: thoracolumbar fascia.
  • – Anterior: vertebral body and lamina.

• • Manual tracing excluded visible fat and fibrous tissue.
• • Three consecutive slices centered at the disc level were measured and averaged.
• • Vertebral body area (VBA) was measured at the same level for normalization.
• • Muscle-to-bone ratio calculated as CSAM/VBA (Figure 1).

Fatty degeneration classification:

Fatty infiltration was graded using a modified four-grade scale adapted from Faur et al., classification (Figure 2):⁸

Grade 1 (mild): < 25% fatty infiltration, normal muscle bulk.

Grade 2 (moderate): 25-50% fatty infiltration, some muscle atrophy.

Grade 3 (severe): 50-75% fatty infiltration, marked muscle atrophy.

Grade 4 (very severe): > 75% fatty infiltration, severe muscle replacement.

For statistical analysis, grades were grouped as "mild degeneration" (grades 1-2) and "severe degeneration" (grades 3-4).

QUALITY CONTROL MEASURES

To ensure measurement consistency:

• • Initial training session with reference images and standardized measurement protocol.
• • Intra-observer reliability was assessed by repeating 20 random measurements after four weeks.
• • Regular calibration sessions during the data collection period.
• • Consensus meeting for measurements with > 15% interobserver variation.
• • All measurements were performed in standardized window settings (window width 400 HU, level 40 HU).

STATISTICAL ANALYSIS

Data analysis was performed using SPSS Statistics version 28.0 (IBM Corp., Armonk, NY). Continuous variables were expressed as mean ± standard deviation for normally distributed data or median with interquartile range for non-normal distributions. Categorical variables were reported as frequencies and percentages.

Primary analyses:

• • Normality assessment using the Kolmogorov-Smirnov test with Lilliefors correction.
• • Interobserver reliability for morphological measurements using intraclass correlation coefficients (ICC) for continuous variables and Cohen's kappa (κ) for categorical variables.
• • Correlation analyses using Pearson's correlation for normally distributed continuous variables and Spearman's rank correlation for ordinal or non-normal data.
• • Group comparisons using independent t-tests or Mann-Whitney U tests for continuous variables, and chi-square or Fisher's exact tests for categorical variables.

Secondary analyses:

• • Multiple linear regression to adjust for potential confounders (age, sex, BMI, comorbidity index).
• • Logistic regression for binary outcome variables (complication occurrence).
• • Analysis of covariance (ANCOVA) for group comparisons of clinical improvement.
• • Sensitivity analyses excluding outliers (> 3 standard deviations from the mean).

Statistical significance:

A two-tailed p-value < 0.05 was considered statistically significant. For multiple comparisons, the Bonferroni correction was applied to maintain the family-wise error rate. All statistical assumptions were tested, including linearity, homoscedasticity, and multicollinearity. Missing data were handled using multiple imputation with 5 imputed datasets when the missingness was < 5%.

RESULTS

PATIENT CHARACTERISTICS

From an initial screening of 134 patients, 99 met all inclusion criteria. The cohort comprised 31 men (31.31%) and 68 women (68.69%) with a mean age of 59.15 ± 8.78 years. Mean preoperative clinical scores demonstrated significant disability: ODI 45.43 ± 17.66, Roland-Morris 14.64 ± 5.40, and lumbar VAS 7.15 ± 1.44.

Comorbidities were present in 43.4% of patients, with hypertension (20.8%) and diabetes mellitus (13.2%) being the most common. The mean presence of comorbidity was 1.2 ± 1.1. Spondylolisthesis was grade 1 in 54.7% and grade 2 in 45.3% of cases. Demographic and baseline characteristics are detailed in Table 1.

MUSCLE MORPHOLOGY ASSESSMENT

All morphological measurements showed substantial interobserver agreement (κ = 0.80). Intra-observer reliability was excellent (ICC = 0.92, 95%CI 0.88-0.95). Fatty degeneration distribution was: grade 1: 6 patients (6.1%), grade 2: 49 patients (49.5%), grade 3: 33 patients (33.3%), grade 4: 11 patients (11.1%).

The distribution of fatty degeneration grades is presented in Table 2.

The mean CSAM was 4,403.3 ± 856.7 mm² and mean VBA was 1,281.0 ± 245.3 mm², resulting in a muscle-to-bone ratio of approximately 3.44:1. Detailed morphological measurements are presented in Table 3.

SURGICAL OUTCOMES AND CORRELATIONS

A strong positive correlation was observed between operative time and intraoperative blood loss (r = 0.754, p < 0.001). Mean operative time was 185.4 ± 45.2 minutes, and mean blood loss was 450.8 ± 156.7 ml. CSAM showed significant positive correlations with both blood loss (r = 0.249, p = 0.013) and overall complication incidence (r = 0.217, p = 0.031).

Blood loss also correlated with postoperative complications (r = 0.374, p < 0.001). The overall complication rate was 18.2%, with major complications occurring in 8.1% and minor complications in 10.1% of patients. Notably, fatty infiltration showed no significant association with postoperative complications (p = 0.214).

CLINICAL IMPROVEMENT

All patient groups demonstrated significant improvement in clinical outcomes at 12-month follow-up (Table 4). The mean improvement in ODI was 25.5 ± 8.3 points (p < 0.001), representing a 56.1% improvement from baseline. Roland-Morris improved by 8.2 ± 3.1 points (p < 0.001), and lumbar VAS decreased by 4.3 ± 1.2 points (p < 0.001). SF-36 physical component summary scores improved from 28.4 ± 6.7 to 41.2 ± 8.3 (p < 0.001).

Statistical analysis confirmed no significant correlation between fatty degeneration grade and the magnitude of improvement in ODI (r = 0.08, p = 0.43), Roland-Morris (r = 0.05, p = 0.62), or VAS (r = 0.09, p = 0.38). Subgroup analysis comparing "mild degeneration" (grades 1-2) versus "severe degeneration" (grades 3-4) showed no significant differences in clinical improvement (p > 0.05 for all outcome measures) (Table 5).

Multiple regression analysis adjusting for age, sex, BMI, and comorbidity index confirmed that CSAM remained independently associated with blood loss (β = 0.231, p = 0.018) and complications (β = 0.205, p = 0.035), while fatty degeneration showed no independent associations.

DISCUSSION

This study provides compelling evidence that distinct aspects of multifidus muscle morphology have specific implications for lumbar fusion surgery outcomes. The substantial interobserver reliability (κ = 0.80) strengthens the validity of our morphological assessments and subsequent conclusions, building upon established MRI measurement accuracy for multifidus muscle evaluation.⁴

The most significant finding was the association between larger multifidus muscle volume and increased surgical complexity. This finding extends previous research demonstrating that paraspinal muscle degeneration correlates with degenerative lumbar spondylolisthesis.^16,17 While Da et al.¹⁶ found quantitative relationships between paraspinal muscle degeneration and spondylolisthesis degree, our study advances this knowledge by demonstrating the surgical implications of muscle volume. Similarly, Liu et al.¹⁷ established associations between paravertebral muscle parameters and single-level degenerative spondylolisthesis, but our work specifically addresses surgical outcomes.

The paradoxical finding that larger muscle volume predicts increased surgical complexity, while fatty degeneration shows no such association, challenges conventional surgical wisdom. This suggests that the mechanical challenges posed by robust muscular anatomy may outweigh the metabolic implications of fatty infiltration in determining surgical outcomes. This distinction becomes particularly relevant when considering that sarcopenia significantly impacts degenerative lumbar spinal stenosis outcomes¹⁸ and associates with low back pain in community studies.¹⁹

From a technical perspective, larger CSAM likely indicates: 1. greater muscle bulk requiring more extensive retraction and potentially compromising surgical exposure, 2. richer vascularization increasing bleeding risk during dissection, and 3. potentially correlating with higher body mass index, though this requires further investigation. Our findings align with Stanuszek et al.¹² who emphasized preoperative muscle assessment, but refine the approach by distinguishing between volume and quality implications.

Our findings align with the systematic review by Dallaway et al.² showing age-related paravertebral muscle degeneration, yet we demonstrate that among surgical candidates, age-related changes may have different implications than previously understood. The relationship between muscle quality and spinal sagittal alignment established by Jun et al.³ provides context for understanding why muscle morphology matters, though our results suggest its impact differs between conservative and surgical management.

The lack of association between fatty degeneration and surgical outcomes challenges some assumptions derived from community-based studies.^6,20 While Kjaer et al.²⁰ found MRI-defined fat infiltrations associated with low back pain, and Teichtahl et al.⁶ demonstrated relationships with disability in community adults, our surgical population appears to respond differently to intervention. This may reflect the fundamental difference between mechanical decompression/fusion versus conservative management for pain originating from different sources.

Notably, all patient groups experienced significant clinical improvement regardless of preoperative muscle morphology. This finding is particularly encouraging in the context of studies showing severe degeneration associations with poor outcomes.^13,21,22 Kim et al.²¹ identified paraspinal muscle problems as risk factors for adjacent segment degeneration, but our results suggest this may not affect short-term surgical success. The potential for postoperative denervation atrophy identified by Sihvonen et al.¹⁵ underscores the importance of surgical technique in preserving muscle function.

CLINICAL IMPLICATIONS

Based on our findings, we propose the following clinical algorithm for preoperative assessment:

• 1. Routine CSAM measurement on preoperative MRI for all patients undergoing lumbar fusion.
• 2. Stratification of surgical complexity based on CSAM values:
  
  
  • a. CSAM > 5,000 mm²: anticipate increased bleeding risk and plan accordingly.
  • b. CSAM 4,000-5,000 mm²: standard surgical planning.
  • c. CSAM < 4,000 mm²: consider potential for easier dissection but assess bone quality.

• 3. Patient counseling emphasizing that fatty degeneration on MRI does not predict surgical outcomes.
• 4. Resource allocation optimization based on anticipated surgical complexity.

LIMITATIONS AND FUTURE DIRECTIONS

This study has several limitations that should be acknowledged. Its retrospective design introduces inherent selection biases, though we employed rigorous inclusion criteria and statistical adjustments to mitigate these effects. The single-center nature may limit generalizability, though our cohort characteristics align with typical degenerative spondylolisthesis populations. While our visual grading scale showed excellent reliability, future studies would benefit from quantitative fat fraction measurements using chemical-shift imaging as employed by Hoppe et al.⁷

Additionally, we did not account for all potential confounders, such as specific surgical techniques or surgeon experience, though all procedures were performed by fellowship-trained spine surgeons using standardized approaches. The 12-month follow-up period provides robust short-to-medium term outcomes, but longer-term assessment is needed to evaluate the sustainability of clinical improvements.

Future research should focus on:

• • Multicenter validation of our findings across diverse populations.
• • Development of automated CSAM measurement tools for clinical implementation.
• • Investigation of whether preoperative rehabilitation can modify surgical risk factors.
• • Long-term assessment of muscle morphology changes following surgery.
• • Exploration of genetic and metabolic factors influencing muscle degeneration patterns.

CONCLUSIONS

In patients undergoing L4-L5 fusion for degenerative spondylolisthesis, multifidus muscle volume correlates with surgical complexity parameters, including blood loss and complication rates. In contrast, fatty degeneration does not predict surgical outcomes or clinical improvement. These findings highlight the importance of comprehensive muscle morphology assessment in preoperative planning while challenging conventional assumptions about the prognostic significance of fatty infiltration.

From a clinical perspective, this distinction is crucial: surgeons can now use CSAM measurements to anticipate technical challenges while reassuring patients that fatty changes on MRI do not necessarily portend poorer surgical outcomes. The integration of muscle morphology assessment into routine preoperative evaluation represents a practical advancement in personalized surgical planning for lumbar degenerative disorders.

REFERENCES

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AFFILIATIONS

¹ Spine Surgery Department, National Institute of Rehabilitation. Mexico City, Mexico.

² Clínica Hospital ISSSTE Mérida, Mexico.

Conflict of interests: the authors declare that they have no conflicts of interest to disclose.

Ethical considerations: No funding was received for the development of this study. No data has been manipulated. All participants provided informed consent before their participation.

CORRESPONDENCE

Alejandro Reyes-Sánchez. E-mail: alereyes@inr.gob.mx

Received: 07-14-2025. Accepted: 01-09-2026.