Does pre-operative psoas muscle cross-sectional area predict post-operative outcomes following lumbar spine fusion surgery?—a systematic review and narrative synthesis

Abstract

Background

Low back pain remains a leading cause of disability globally, with surgical interventions commonly utilised as treatment. Rates of lumbar fusion surgery continue to rise, but poor outcomes remain common. Preoperative psoas muscle morphology has been proposed as a potential predictor of postoperative success. Evidence supporting a link between psoas morphology and outcomes post-fusion surgery is growing yet remains unclear. This systematic review and narrative synthesis aimed to evaluate whether pre-operative psoas muscle morphology predicts clinical and radiographic outcomes following lumbar fusion surgery.

Methods

Five databases (MEDLINE, PubMed, CINAHL, Web of Science, and SportDiscus) were searched from inception to June 2025. Included studies assessed pre-operative psoas morphology [cross-sectional area (CSA), volume] and reported post-operative clinical (pain, disability, quality of life) or radiographic outcomes (fusion rates, alignment, complications). Risk of bias was assessed using Quality in Prognostic Studies (QUIPS), and certainty of evidence evaluated using Grading of Recommendations, Assessment, Development, and Evaluations (GRADE).

Results

Sixteen retrospective cohort studies encompassing 2,250 patients were included. Overall, data indicates that psoas morphology is not a consistent predictor of outcomes following fusion procedures. A limited number of low-quality studies, however, suggest that the surgical approach may influence this relationship. Posterior fusion techniques showed some association where a larger psoas was linked to early improvements in pain and function, while lateral and anterior approaches demonstrated no relationship.

Conclusions

Based on very low to low certainty evidence, routine pre-operative psoas assessment is not currently justified for predicting lumbar fusion outcomes. Higher-quality prospective studies are needed to clarify the prognostic value of psoas morphology in spine surgery.

Highlight box.

Key findings

• Current evidence does not support a consistent association between pre-operative psoas muscle morphology and outcomes after lumbar fusion.

• A modest, short-term association was found for posterior approaches, but not for lateral or anterior techniques.

• The overall quality of evidence is low, with a high risk of bias in most included studies.

What is known and what is new?

• Preoperative muscle quality is a topic of interest for predicting surgical outcomes.

• This is the first review to stratify findings by surgical approach, revealing approach-specific patterns that should be considered. However, due to the low quality and heterogeneity of available evidence, these findings should be interpreted cautiously.

What is the implication, and what should change now?

• Routine pre-operative assessment of the psoas muscle is not currently justified for predicting outcomes in lumbar fusion surgeries.

• Surgical technique and intraoperative factors may be more critical than pre-operative psoas size for lateral approaches. Future high-quality, prospective research with standardized methodologies is essential.

Introduction

Low back pain is the leading cause of disability worldwide and imposes a substantial socioeconomic burden (1,2). This condition leads to substantial healthcare costs, lost productivity, and reduced quality of life for millions of individuals (2). While most cases of low back pain are managed conservatively, surgical interventions such as fusion procedures are increasingly utilized (3). Indeed, spinal fusion rates have increased substantially in many countries over the past two decades (4-6). However, despite the growing use of fusion surgeries, the efficacy of these interventions remains variable, with a considerable proportion of patients experiencing post-surgical complications, including persistent pain, reoperation, and functional limitations (3).

Multiple factors influence outcomes following lumbar spine surgery, including condition-related characteristics such as symptom duration and severity, as well as patient-related factors like social determinants of health such as socioeconomic status, education level and physical health status (7-12). For example, lower socioeconomic status has been associated with longer post-operative recovery times and higher rates of disability, potentially due to limited access to resources, supportive care, and rehabilitation services. Additionally, structural factors such as pre-surgical paraspinal morphology and fatty infiltration have been linked to post-surgical recovery (13). Among these, the role of muscle quality and function has gained increasing attention, with evidence suggesting that pre-surgical physical status may be a critical determinant of surgical success (14-18).

Accumulating evidence suggests that pre-surgical cross-sectional area (CSA) of the psoas muscle may be a potential predictor of post-surgical outcomes following lumbar spine fusion surgeries (19,20). The psoas muscle has been investigated as a radiological marker of muscle mass and is commonly used as a surrogate for sarcopenia and overall physiological reserve (21,22). Indeed, computed tomography (CT) measurement of the psoas CSA at the level of the third or fourth lumbar vertebra is recognized as a valid and efficient method for estimating whole-body skeletal muscle mass, as muscle mass at these levels is minimally affected by patient movement (23). A recent systematic review found that a smaller pre-surgical multifidus and erector spinae CSA was associated with worse functional status after lumbar spine surgery (24). While the psoas muscle specifically was not explored in detail in this review, the findings suggest that psoas alone may serve as a spine-specific biomarker that has potential prognostic value in lumbar fusion surgery. It should also be noted that the previous review did not evaluate key radiographic and clinical outcomes such as fusion success, spinal alignment, pain, motor weakness, and quality of life, limiting the clinical applicability of the findings. A clearer understanding of how psoas morphology influences surgical recovery could enhance patient-specific care, including targeted prehabilitation and rehabilitation strategies. Therefore, this systematic review aimed to evaluate whether pre-operative psoas muscle morphology (e.g., CSA) is associated with clinical (pain, disability, motor weakness and quality of life) and radiographic (fusion metrics and alignment correction) outcomes post lumbar fusion surgery. We present this article in accordance with the PRISMA reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-2026-0087/rc).

Methods

The review was prospectively registered with PROSPERO (registration No. CRD42022304122).

Search strategy and study selection

A comprehensive search strategy was implemented in the following electronic databases from inception until June 2025: MEDLINE, PubMed, CINAHL, Web of Science, and SportDiscus. Keywords and Medical Subject Headings related to the characteristics of the psoas, surgical intervention, and surgical outcome measures were used to create the advanced search. The core search terms used are presented in Figure 1. This core strategy was developed in conjunction with a librarian, experienced in reviews of biomedical literature. Results of the literature searches were uploaded to EndNote, then imported into Rayyan software (2016, Qatar Computing Research Institute, Doha, Qatar) (25), where duplicates were removed. Two independent reviewers (D.T.B. and C.R.F.) conducted title and abstract screening using inclusion and exclusion criteria. The full-text of all selected articles were screened for eligibility and the reasons for study exclusion recorded. If agreement could not be reached between reviewers, a third reviewer (G.M.M.) was consulted. The reference lists of included studies were also reviewed to identify additional articles.

Figure 1.

Advanced search strategy used in systematic review.

Eligibility criteria

Studies included were peer-reviewed articles investigating the association between pre-surgical psoas morphology and post-surgical outcomes following lumbar spine fusion surgery. Surgical interventions were defined as invasive procedures under general anaesthesia conducted on the lumbar spine, fusing one or more motion segments of the lumbar spine, such as lateral lumbar interbody fusion (LLIF), oblique lumbar interbody fusion (OLIF), anterior lumbar interbody fusion (ALIF) posterior/transforaminal lumbar interbody fusion (PLIF/TLIF) and posterolateral fusion (PLF). Eligible studies were also required to report clinical and/or radiographic outcomes. Clinical outcomes after surgery included pain, disability, motor weakness and quality of life. Radiographic outcomes included fusion metrics and alignment correction. Studies required a quantifiable measure of the morphological dimensions of the psoas major muscle before lumbar spine fusion surgery, for example, CSA, functional CSA (fCSA), and volume. CSA is defined as the total area of a muscle measured along the axial plane. Whereas fCSA refers to the muscle content with the removal of measured fat infiltration that can be present in the muscle mass. Psoas volume is calculated by summing CSAs across consecutive axial slices multiplied by slice thickness. Exclusion criteria included studies involving paediatric populations, large deformity surgeries, non-English publications, review articles, conference abstracts, and grey literature.

Data extraction

Data was extracted by two independent reviewers (D.T.B. and C.R.F.) using a bespoke Excel spreadsheet. The collected data included study design, sample size, patient characteristics (e.g., age, sex), type of surgery, as well as data pertaining to outcomes of psoas major morphology (e.g., CSA) and post-surgical outcomes. The reviewers compared extracted data and resolved any discrepancies through consultation with a third reviewer (G.M.M.). When studies did not report data in sufficient detail, authors were contacted via email to request the information. Follow-up emails were sent if no response was received within two weeks and a final email attempt was sent one week later if unsuccessful. If data could not be obtained, the article was excluded.

Data analysis

In line with Cochrane guidelines, meta-analyses with a random-effects model were planned if at least two studies shared the same surgical approach, outcome measure at comparable follow-up timepoints, psoas metric (e.g., CSA) at the same anatomical landmark using the same imaging modality, and reported effect estimates with variance measures (26). A P<0.05 was deemed statistically significant, with heterogeneity assessed via I² and interpreted as follows: 0–40% might be unimportant; 30–60% could represent moderate heterogeneity; 50–90% could represent substantial heterogeneity; and 75–100% represents considerable heterogeneity. If criteria were not met for any outcome, the findings were reported as a narrative synthesis as per synthesis without meta-analysis (SWiM) guidelines (27).

Subgroup analysis

Where possible, subgroup analyses were conducted in accordance with the registered study protocol. Specifically, subgroups were created based on the surgical approach described in the included study [anterior (ALIF), posterior (PLIF and PLF) and lateral (LLIF and OLIF)] as each approach has a different interoperative interaction with the psoas muscle.

Subgrouping justification

Given the surgical interaction with the psoas muscle varies fundamentally by surgical approach, subgroup analyses were planned to examine whether the prognostic value of pre-operative psoas morphology differs by approach to spine surgery. In posterior approaches, the psoas is not directly manipulated; therefore, psoas size was hypothesized to act primarily as a surrogate for systemic physiological reserve (i.e., sarcopenia or frailty), potentially influencing early functional recovery, pain, and quality of life through systemic rather than local biomechanical mechanisms (28). In LLIF procedures, the transpsoas corridor requires direct retraction of the psoas; a larger psoas CSA could theoretically better tolerate retraction, reducing post-operative motor weakness, whereas a smaller or atrophic psoas might be more vulnerable to injury (28). For OLIF procedures, which traverses the interval between the psoas and the aorta, the psoas is retracted but not traversed; here, psoas morphology was hypothesized to influence post-operative hip flexion recovery and cage subsidence risk due to differential retraction forces on a more lateralized muscle (14). In anterior approaches, the psoas is not directly encountered, and therefore no association between psoas morphology and outcomes was anticipated.

Risk of bias assessment

The risk of bias in the included studies was assessed using the Quality in Prognostic Studies (QUIPS) tool, which evaluates six key domains: study participation, study attrition, prognostic factor measurement, outcome measurement, confounding, and statistical analysis. Each domain identifies potential sources of bias critical to ensuring the validity of prognostic research. Overall risk of bias is not averaged but weighted towards key domains. A study is low risk if most domains are rated low, moderate risk if at least one is moderate but not high, and high risk if any key domain is high risk (29).

Quality of evidence assessment

The overall certainty of evidence for the association between psoas morphology and post-operative outcomes was evaluated using the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) approach. As the included studies precluded meta-analysis, GRADE was applied to the narrative synthesis (30). Initial evidence from observational studies was rated as low quality due to inherent limitations in study designs (e.g., retrospective cohorts, risk of confounding). Evidence was evaluated further for inconsistency (heterogeneity in measurement protocols and outcomes), indirectness (variability in surgical techniques), and imprecision (small sample sizes in subgroup analyses). Where applicable, evidence was upgraded for large effect sizes (e.g., posterior approaches showing transient benefits) or dose-response gradients (e.g., larger psoas CSA correlating with early functional improvement).

Results

Study selection

The database search identified 316 studies. After removing duplicates (n=73), a total of 243 studies were included for title and abstract screening. Title and abstract screening eliminated 184 studies, leaving 59 studies eligible for full-text screening. Of the 59 studies, three full texts could not be obtained. After screening the remaining full texts, 16 studies met the eligibility criteria for inclusion in this review, with reasons for exclusion detailed in the PRISMA flowchart (Figure 2).

Figure 2.

PRISMA flow diagram.

Study characteristics

Cohort demographics

The sample size, study design, approach, and surgical type undertaken are reported in Table 1. Data from 2,250 patients were evaluated across 16 cohort studies. All studies were retrospective cohort studies. Studies were conducted in China (20,31,34,36,39,43), India (40), Japan (35), Korea (32,37), Thailand (44), Romania (19) and the USA (33,38,41,42). The most common surgical interventions were variations of lateral lumbar fusions: LLIF (n=5), PLF (n=5), PLIF (n=3), and TLIF (n=4), followed by OLIF (n=1) and ALIF (n=1) (Table 2). No studies examining total disc replacement were identified in this review.

Table 1. Study characteristics of the included studies.

References	Study design	Country	Sample size (n)	Approach	Surgery performed	Mean follow-up time
Caprariu [2023] (19)	Retrospective cohort	Romania	86	Posterior	TLIF	12 m
Chen [2023] (31)	Retrospective cohort	China	113	Posterior	TLIF	41.56 m
Choi [2017] (32)	Retrospective cohort	Korea	192	Posterior	PLF or PLIF	56 wk
Giraldo [2025] (33)	Retrospective cohort	USA	100	Anterior or lateral	ALIF, LLIF	12 m
Han [2022] (34)	Retrospective cohort	China	351	Posterior	PLIF	12 m
Hiyama [2021] (35)	Retrospective cohort	Japan	60	Lateral	LLIF	16.3 days
Hou [2024] (36)	Retrospective cohort	China	126	Posterior	PLF	17.9 m
Jung [2021] (37)	Retrospective cohort	Korea	123	Lateral	LLIF	46.8 m (M), 45.8 m (F)
Moser [2023] (38)	Retrospective cohort	USA	95	Lateral	LLIF	>6 m
Song [2023] (39)	Retrospective cohort	China	89	Oblique	OLIF	16.9 m
Thakar [2016] (40)	Retrospective cohort	India	120	Posterior	PLIF	†
Urakawa [2023] (41)	Retrospective cohort	USA	212	Posterior	PLF (n=4); TLIF (n=88)	6 m
Verla [2016] (42)	Retrospective cohort	USA	257	Posterior	TLIF	15.7 m
Xu [2024] (43)	Retrospective cohort	China	149	Posterior	PLIF	25.71 m
Yingsakmongkol [2021] (44)	Retrospective cohort	Thailand	81	Lateral and posterior	LLIF +/− PSF	24 h–32 m
Zhou [2024] (20)	Retrospective cohort	China	212	Posterior	PLIF	>12 m

^†, not defined in text. ALIF, anterior lumbar interbody fusion; F, female; h, hour; LLIF, lateral lumbar interbody fusion; m, month; M, male; OLIF, oblique lumbar interbody fusion; PLF, posterolateral fusion; PLIF, posterior lumbar interbody fusion; PSF, posterior supplementary fixation; TLIF, transforaminal lumbar interbody fusion; wk, week.

Table 2. Outcomes measured in the included studies.

References	Surgery conducted	Outcomes
		Clinical						Radiographic
		Pain	Disability	Neurological complications	Quality of life	Muscle weakness		Degree of canal stenosis	Adjacent segment degeneration	Spine alignment	Fusion outcomes	Cage subsidence
Caprariu [2023] (19)	TLIF		•					•
Chen [2023] (31)	TLIF								•
Choi [2017] (32)	PLIF or PLF										•
Giraldo [2025] (33)	LLIF or ALIF	•	•	•
Han [2022] (34)	PLIF									•	•
Hiyama [2021] (35)	LLIF					•
Hou [2024] (36)	PLF		•
Jung [2021] (37)	LLIF	•		•
Moser [2023] (38)	LLIF											•
Song [2023] (39)	OLIF	•
Thakar [2016] (40)	PLIF				•					•
Urakawa [2023] (41)	PLF; TLIF	•	•		•
Verla [2016] (42)	TLIF	•	•
Xu [2024] (43)	PLIF								•
Yingsakmongkol [2021] (44)	LLIF +/− PSF	•		•		•
Zhou [2024] (20)	PLIF	•	•							•

• indicates outcome measured/assessed in this study. ALIF, anterior lumbar interbody fusion; LLIF, lateral lumbar interbody fusion; OLIF, oblique lumbar interbody fusion; PLF, posterolateral fusion; PLIF, posterior lumbar interbody fusion; PSF, posterior supplementary fixation; TLIF, transforaminal lumbar interbody fusion.

Psoas measurement

The CSA of the psoas muscle was measured at various locations within the lumbar spine, including at the disc level and vertebral endplates spanning from L1 to S1 (Table 3). Each study utilized anatomical landmarks for consistent measurement points within the study cohort, such as the lower and upper endplates of L3, the L3 pedicle level, and the disc spaces at L2/3, L3/4, L4/5, and L5/S1.

Table 3. Image outcomes.

References	Image modality			Psoas measurements			Location of measurement
	CT	MRI		Total cross-sectional area	Functional CSA	Psoas muscle volume	Disc level						Pedicle			Whole muscle
							L1/L2	L2/L3	L3/L4	L4/L5	L5/S1		L3	L4	L5
Caprariu [2023] (19)		•			•					•
Chen [2023] (31)		•								•
Choi [2017] (32)		•						•	•	•
Giraldo [2025] (33)		•								•
Han [2022] (34)		•		•					•
Hiyama [2021] (35)		•		•						•
Hou [2024] (36)	•				•					•
Jung [2021] (37)		•		•					•
Moser [2023] (38)				•									•	•	•
Song [2023] (39)		•		•					•
Thakar [2016] (40)		•		•					•	•
Urakawa [2023] (41)	•			•		•										•
Verla [2016] (42)		•		•			•
Xu [2024] (43)		•			•				•	•	•
Yingsakmongkol [2021] (44)	•	•		•		•										•
Zhou [2024] (20)		•			•	•		•	•	•	•

• indicates outcome measured/assessed in this study. CSA, cross-sectional area; CT, computed tomography; MRI, magnetic resonance imaging.

Reporting of psoas measurements varied across the included literature. The CSA measurements were utilized to quantify the muscle’s total area at a transverse slice, including total CSA (tCSA), which encompasses both muscle and fat, and the fCSA, which encompasses muscle only. Additionally, the statistical technique of normalizing raw data was conducted in a number of studies (Table 4). The vertebral body CSA or height² were standardizing measures used to produce a normalized tCSA (ntCSA) or normalized fCSA (nfCSA) to account for individual anatomical differences. One study evaluated psoas volume as an outcome. Three studies utilized CT scans to measure psoas characteristics (36,41,44) and fourteen used magnetic resonance imaging (19,20,31-35,37,39,40,42-44). One study used both CT and magnetic resonance imaging (MRI) modalities to assess psoas characteristics (44).

Table 4. Fusion studies.

References	Psoas outcome	Normalized too	Post surgical outcomes	Results
				Univariant analysis	Multivariant analysis	Summary of main findings
LLIF
Jung [2021] (37)	ntCSA	Unnormalized data	Pain	✕		There was no relationship between psoas outcomes and pain
	ntCSA	Height2 of the patient	Pain	√		The ntCSA was a positive predictive factor for achieving a substantial clinical benefit, in back pain, in men (OR 1.251, P=0.04) and women (OR 1.795, P=0.02)
			Neurological complications	✕		ntCSA not correlated with neurological complication rates (P>0.05)
Hiyama [2021] (35)	tCSA, deltatCSA	Unnormalized data	Motor weakness	✕		There was no relationship between psoas outcomes and motor weakness
Yingsakmongkol [2021] (44)	Left PS volume	Unnormalized data	Pain	✕		There was no relationship between PS volume and asymptomatic and symptomatic thigh pain (asymptomatic pain 411.48 vs. symptomatic thigh pain 419.04, P=0.90)
			Numbness	✕		There was no relationship between asymptomatic and symptomatic numbness (421.76 vs. 397.96, P=0.47) and in PS volume
			Motor weakness	✕		There was no relationship between PS volume and weakness (411.40 vs. 419.67mm2, P=0.89)
Giraldo [2025] (33)	tCSA; fCSA	Unnormalized data	Pain	✕		PS size/quality (tCSA/fCSA) did not predict preoperative disability or postoperative pain improvement
			Disability			PS size/quality (tCSA/fCSA) did not predict preoperative disability or postoperative pain improvement
Moser [2023] (38)	fCSA	Unnormalized data	Cage subcidence	✕		Greater psoas FCSA/VBA at L3 and L4 are independent risk factors for severe cage subsidence in men after SA-LLIF, suggesting higher compressive forces from the PS may contribute to subsidence
		Vertebral body or disc at the location of measurement	Cage subcidence	✕		Greater psoas fCSA at L3 and L4 are independent risk factors for severe cage subsidence in men after SA-LLIF, suggesting higher compressive forces from the PS may contribute to subsidence
Posterior fusion spine surgery—PLIF, PLF, TLIF
Caprariu [2023] (19)	nfCSA	Vertebral body or disc at the location of measurement	Disability	✕		Lumbar stenosis severity (Schizas criteria) had a weak correlation with ODI but no predictive value for 1-year outcomes; facet joint OA and neural foramen area did not correlate with ODI
Han [2022] (34)	fCSA	Unnormalized data	Fusion	✕		There was no relationship between fusion outcomes and PS fCSA
	nfCSA	Vertebral body or disc at the location of measurement	Fusion	✕		PS nfCSA was not found to have a relationship with fusion outcomes (Union, 0.58±0.16 mm2 vs. non-union 0.55±0.15 mm2, P=0.44)
Hou [2024] (36)	fCSA, muscle attenuation	Unnormalized data	Disability	✕		There was no relationship between disability outcomes and PS fCSA or muscle attrition reported
	nfCSA	Vertebral body or disc at the location of measurement	Disability		✕	There were no significant differences for nfCSA. PS attenuation was an independent predictor for MCID attainment, exhibiting an OR of 1.141 (95% CI 1.054–1.236, P=0.001); the OR was non-significant for rfCSA (P=0.40)
Choi [2017] (32)	tCSA	Unnormalized data	Fusion	√		At surgical level L4-5 the PS tCSA had a negative impact on nonfusion status (OR =0.812, P=0.03) but not at surgical level L3/4 (P=0.09)
Chen [2023] (31)	fCSA	Unnormalized data	ASD	√		Higher preoperative PS-FI was significantly correlated with L5/S1 foraminal stenosis degeneration after L4/5 TLIF
Xu [2024] (43)	rFCSA, rTCSA, FI of PM	Vertebral body or disc at the location of measurement	ASD	√	√	Postoperative decrease in FI of the PS at the adjacent level (L3/4) was a protective factor against ASD (OR =3.497, P=0.001). An increase in PM FI and higher BMI were independent risk factors for ASD
Thakar [2016] (40)	tCSA	Unnormalized data	Time to recovery	✕		There was no relationship between time to recovery and PS tCSA (P>0.05)
			Disc degeneration	✕		There was no relationship between disc degeneration outcomes and PS tCSA (P>0.05)
			Alignment	✕		There was no relationship between spine alignment outcomes and tCSA (P>0.05)
	ntCSA	Vertebral body or disc at the location of measurement	Time to recovery	✕		There was no relationship between PS ntCSA and duration of symptoms (P=0.76)
			Disc degeneration	✕		There was no relationship between PS ntCSA and disc degeneration seen on CT (P=0.89)
				✕		There was no association between PS ntCSA and spine alignment specifically the grade of lithesis (P=0.05)
Urakawa [2023] (41)	tCSA	Unnormalized data	Pain	✕		No relationship reported
			Disability	✕		No relationship reported
			QOL	✕		No relationship reported
	ntCSA	Height2 of the patient	Pain	√		Both VAS Back (P=0.001) and VAS Leg (P=0.04) reported significant difference between low (females <529 mm2/m2 and males <529 mm2/m2) and high PS ntCSA
			Disability	✕		There was no significant difference reported in post operative ODI for patients who underwent TLIF or PLF and ntCSA
			QOL	√		SF-12 PCS [6 mo (39.6 vs. 44.2, P=0.01)]
			Function	√		Post operative PROMIS PF was significantly improved in patients with larger ntCSA (P=0.03)
			Time to recovery	√		Lower ntCSA was a risk factor for failure to reach MCID in ODI (OR =2.68, P=0.01) and VAS Leg (OR =2.43, P=0.02)
Verla [2016] (42)	Left side PS muscle thickness on axial image slice	Unnormalized data	Muscle weakness	√		Thinner axial slices of the PS was associated with decreased strength
			Pain	√		PS axial thickness was correlated with improvement in VAS pain 6-weeks post operatively (NUMBER)
			Disability	√		PS axial thickness was correlated with improvement in ODI scores 6-weeks post operatively
Zhou [2024] (20)	tCSA, fCSA	Unnormalized data	S1 pedicle screw loosening		√	36.32% of patients experienced S1 screw loosening. Postoperative hypertrophy of PS at L3/4, L4/5, and L5/S1 was confirmed to be a protective factor of S1 screw loosening (P<0.001). Preoperative measurements were not a predictive factor
		Vertebral body or disc at the location of measurement	S1 pedicle screw loosening		√	36.32% of patients experienced S1 screw loosening. Postoperative hypertrophy of PS at L3/4, L4/5, and L5/S1 was confirmed to be a protective factor of S1 screw loosening (P=0.01). Preoperative measurements were not a predictive factor
OLIFs
Song [2023] (39)	Muscle CS shape	Unnormalized data	Post operative complications	√		Post operative PS swelling was significantly associated with increased complications (L2/3, P=0.03; L3/4, P=0.04; L4/5, P=0.02)
			Cage subsidence	✕		There was no relationship between PS shape and post-operative OLIF cage subsidence
Anterior lumbar interbody fusions
Giraldo [2025] (33)	tCSA, fCSA	Unnormalized data	Pain	✕		Psoas size/quality (tCSA/fCSA) did not predict postoperative pain improvement
			Disability	✕		Psoas size/quality (tCSA/fCSA) did not predict postoperative disability improvement

√ indicate the relationship reported and ✕ indicate no relationship found. ntCSA, normalized tCSA (normalized to height or vertebral body CSA, also known as psoas muscle index). nfCSA, normalized fCSA (normalized to height or vertebral body CSA). nFCSA, normalized fCSA (normalized to height or vertebral body CSA). LLIF, lateral lumbar interbody fusion (insert delta sign), change in (pre versus post-op imaging). ASD, adjacent segment degeneration; BMI, body mass index; CI, confidence interval; CS, cylindrical spirals; CT, computed tomography; fCSA, functional cross-sectional area; FCSA/VBA, functional cross-sectional area normalized to the vertebral body area; FI, fatty infiltration (a measurement of the fCSA divided by the tCSA); MCID, minimally clinically important difference; mo, month; NS, not significant; ODI, Oswestry Disability Index; OLIF, oblique lumbar interbody fusion; OR, odds ratio; PLF, posterior lateral fusion; PLIF, posterior lumbar interbody fusion; PROMIS-PF, Patient-Reported Outcomes Measurement Information System Physical Function; PS, psoas muscle; PSF, pedicle screw fixation; QOL, quality of life; SF-12 PCS, 12-Item Short Form Health Survey Physical Component Summary; tCSA, total cross-sectional area; TLIF, transforaminal lumbar interbody fusion; VAS, Visual Analog Scale.

Clinical outcomes

Post-operative clinical outcomes included pain (n=7) (16,20,33,39,41,42,44), assessed using the Visual Analog Scale, and disability (n=6), measured with the Oswestry Disability Index (ODI) (n=6) (19,20,33,41,42,45). Other patient-reported outcomes included quality of life (n=2) (40,41), evaluated with the Short Form Health Survey (SF-36), physical function (n=1) (40) assessed by the Patient-Reported Outcomes Measurement Information System (PROMIS) as well as motor weakness (n=2) (35,44) and neurological complications (n=3) (33,37,44) (Tables 2,4).

Radiographic outcomes

The included studies assessed various radiographic outcomes, including, fusion status and cage subsidence (n=2) (34,38), degree of canal stenosis (n=1) (19), spinal alignment (n=2) (20,34) which was evaluated through imaging techniques such as X-ray or CT scans to confirm bone union.

Meta-analysis preclusion

Applying the pre-specified criteria for meta-analysis, none of the 16 included studies met the requirements for meta-analysis due to methodological heterogeneity. For example, the six studies investigating posterior approaches varied in post-surgical outcomes and follow-up timepoints (range: 6 weeks to 24 months). Similarly, disability (ODI) was reported by four studies across three different surgical approaches (posterior, lateral and anterior) with varying follow-up durations. With respect to psoas measurement techniques, these varied widely across studies, including different anatomical levels (ranging from L1 to S1), metrics (CSA vs. fCSA vs. volume), normalisation methods (none vs. vertebral body CSA vs. height²), and imaging modalities (MRI, CT, or both). Therefore, subsequent findings are presented as a narrative synthesis.

Relationships between psoas and surgical outcomes

All analyses examining relationships between the psoas and post-surgical outcomes are summarised in Table 3. A total of 16 studies examined fusion surgeries, with the results split by surgical approach. Six studies focused on posterior surgical fusions, including TLIF, PLIF, and PLF none of which had direct contact with the psoas. Six studies examined LLIF procedures (31,33,35,37,38,44) and one study investigated OLIF (39). One study described a single-stage LLIF with supplementary posterior fixation (LLIF + PSF) involving two spinal access incisions (44). The results are presented for both the univariate and multivariate analyses, with a narrative synthesis of each post-surgical outcome.

LLIF

Pain

Three studies examined the relationship between psoas morphology and post-operative pain following LLIF (33,37,44). No study found a significant association between pre-operative psoas CSA and post-operative pain outcomes. All studies reported no significant correlation between total psoas area index and VAS scores at final follow-up (mean 24.8 months).

Disability

One study assessed the relationship between psoas morphology and disability following LLIF (37). This study found no significant correlation between total psoas area index and ODI scores at final follow-up (mean 24.8 months, r=0.064, P=0.64).

Motor weakness

Two studies examined motor weakness by measuring thigh function post-operatively. Neither study found a significant relationship between pre-operative psoas CSA and volume on measures of motor weakness (35,44).

Cage subsidence

One study demonstrated that greater pre-surgical psoas fCSA at L3 (non-severe group =1,848.7 mm² vs. severe group =2,367.8 mm²) and L4 (non-severe group =2,700.6 mm² vs. severe group =3,200.9 mm²) are independent risk factors (P=0.005 and 0.03, respectively) for non-severe vs. severe cage subsidence in men after SA-LLIF (38). This difference was also significant when using nfCSA (normalised to vertebral body area (P values respectively =0.03, 0.022). Notably, this finding was specific to male patients and was not observed in females.

Disc degeneration

Disc degeneration of the disc superior to the fusion was assessed via MRI by one study (40). There was no correlation between the psoas CSA and degeneration of the disc.

PLIF, TLIF and PLF

Pain

Two studies examined the relationship between psoas morphology and post-operative pain following a PLIF/TLIF (41,42). Both found significant relationships between psoas morphology and pain outcomes (41,42). Specifically, one study found that a thicker axial psoas profile was associated with reduced pain at six weeks post-surgery (L2/3, P=0.03; L3/4, P=0.04; L4/5, P=0.02) (42); however, this effect did not persist at 3- and 6-month follow-up.

Disability

Four studies analysed disability outcomes (19,36,41,42), with mixed reports. Two studies found that a thicker pre-surgical psoas was associated with significantly improved disability outcomes at six weeks post-operatively (42,45), with one study demonstrating a stronger effect in females compared to males (42). However, the remaining two studies reported no significant relationships between psoas characteristics and disability outcomes (36,41).

Pedicle screw loosening

One study reported on S1 screw loosening, defined as 1 mm haloing around the S1 screw at 1-year follow-up (20). However, no relationship was found between pre-operative psoas CSA and this outcome.

Fusion status

One study assessed fusion status one year post-operatively using dynamic X-ray, defining non-union as motion exceeding 3 mm or 3 degrees, or the presence of bone mass growth. No significant differences were observed in fCSA or nfCSA of the psoas between the union and non-union groups (34).

Spinal alignment

One study reported on global alignment of the spine, defined as sagittal balance, pelvic incidence and angles of lordosis (20). However, no relationship was found between pre-operative psoas CSA and any of these outcomes.

Quality of life

One study assessed quality of life after posterior fusion surgeries using the Short Form-12 Physical Component Summary (SF-12 PCS) (41). Patients with lower psoas CSA normalised to height had significantly worse physical quality of life at 6 months (39.6 vs. 44.2, P=0.007). No differences were observed in mental quality of life scores.

OLIF

Motor weakness

One study examined the relationship between pre-operative psoas morphology and functional recovery following OLIF (39). Patients with larger pre-operative psoas CSA demonstrated faster recovery of hip flexion strength, with most returning to baseline function by 3 months post-surgery. Conversely, patients with smaller pre-operative psoas CSA showed prolonged weakness that in some cases persisted beyond 6 months.

Cage subsidence

One study demonstrated no relationship between OLIF cage subsidence and psoas CSA at last follow-up (16.9 months) (39).

ALIF

Pain

In the ALIF subgroup of a single study (n=9), no relationship between pre-operative psoas muscle morphology and post-operative back or leg pain was found (33). Despite the anatomical proximity of the surgical approach to the psoas major, neither CSA or fat infiltration percentage (FI%) demonstrated any predictive value for post-operative pain outcomes.

Disability

No significant association was observed between psoas morphology and disability outcomes in the ALIF subgroup (33).

Sarcopenia

Six studies specifically recruited sarcopenic patients and investigated the relationship between pre-operative psoas CSA and post-operative outcomes. In posterior fusion procedures (PLIF/TLIF/PLF), lower pre-operative psoas CSA was associated with poorer short-term functional outcomes, delayed ambulation, and reduced quality of life at ≤6 weeks post-operatively (41,42). In lateral approaches (LLIF/OLIF), no consistent relationship was found between sarcopenia status and post-operative pain, disability, or motor weakness (37,44).

Risk of bias

Among the 16 included studies, 10 (62.5%) were classified as having a high risk of bias, primarily due to heterogeneity of psoas measurement techniques, insufficient adjustment for confounders (e.g., age, comorbidities, and sex), and notable attrition rates. Four studies (25%) demonstrated a moderate risk of bias, while only two studies (12.5%) were rated as low risk (Table 5). The retrospective design of all studies further contributed to potential selection bias.

Table 5. Risk of bias QUIPS.

References	Study participation	Study attrition	Prognostic factor measurement	Outcome measurement	Study confounding	Statistical analysis & reporting	Overall risk
Caprariu [2023] (19)	Moderate	Moderate	Low	Low	Moderate	Low	Moderate
Chen [2023] (31)	Low	Low	Low	Low	Low	Low	Low
Choi [2017] (32)	Low	Moderate	Moderate	Low	Moderate	Low	Moderate
Giraldo [2025] (33)	Low	Moderate	Low	Low	Moderate	Low	Moderate
Han [2022] (34)	Moderate	Moderate	High	Moderate	High	Moderate	High
Hiyama [2021] (35)	Low	Moderate	Moderate	Low	Moderate	Low	Moderate
Hou [2024] (36)	Moderate	Low	High	Moderate	High	Moderate	High
Jung [2021] (37)	Low	Low	Moderate	Low	Moderate	Low	Low
Moser [2023] (38)	Moderate	Low	High	High	Moderate	High	Moderate
Song [2023] (39)	Low	Moderate	Moderate	Low	Moderate	Low	Moderate
Thakar [2016] (40)	Moderate	High	High	Moderate	High	Moderate	High
Urakawa [2023] (41)	Moderate	Moderate	High	Moderate	High	Moderate	High
Verla [2016] (42)	Low	Moderate	Moderate	Low	Moderate	Low	Moderate
Xu [2024] (43)	Moderate	Moderate	Moderate	Moderate	High	Moderate	Moderate
Yingsakmongkol [2021] (44)	Low	Low	Moderate	Low	Moderate	Low	Low
Zhou [2024] (20)	Low	Moderate	Moderate	Low	Moderate	Low	Moderate

QUIPS, Quality in Prognostic Studies.

GRADE assessment

The GRADE evaluation revealed very low to low certainty evidence for the prognostic role of psoas morphology (Table 6). Key outcomes of pain, disability, and fusion rates were supported by retrospective studies (downgraded for risk of bias), with inconsistent findings across surgical approaches (downgraded for inconsistency). For posterior approaches, a modest association between larger psoas CSA and early functional recovery was noted (upgraded for dose-response), but the transient nature of these effects and lack of adjusted analyses limited confidence. No approach demonstrated moderate or high certainty evidence.

Table 6. GRADE score.

Outcomes	Anticipated absolute effects (95% CI)	No. of studies	Certainty of the evidence (GRADE)	Comments
Pain (e.g., VAS)	Larger psoas size may be associated with improved pain scores, but the evidence is very inconsistent	8	⊕◯◯◯	Downgraded for risk of bias (all retrospective), inconsistency (highly variable findings), and imprecision (wide CIs, some null effects)
Disability (e.g., ODI)	Larger psoas size may be associated with improved early disability scores, but effects may be transient	7	⊕⊕◯◯	Downgraded for risk of bias (retrospective) and inconsistency. Upgraded for a dose-response gradient in some posterior approach studies
Fusion success	The impact of psoas size on fusion rates is unclear, with studies reporting positive, negative, and null associations	4	⊕◯◯◯	Downgraded for risk of bias (retrospective), serious inconsistency, and imprecision
Neurological complications	Psoas size likely has little to no consistent association with neurological complications like thigh symptoms	4	⊕⊕◯◯	Downgraded for risk of bias (retrospective). Not downgraded for inconsistency as findings are consistently null
Cage subsidence (LLIF)	In men undergoing LLIF, larger psoas functional CSA may be a risk factor for subsidence	1	⊕◯◯◯	Downgraded for risk of bias (retrospective), imprecision (single study, limited sample), and indirectness (only men, single approach)
Screw loosening	Postoperative psoas hypertrophy may be a protective factor against S1 pedicle screw loosening	1	⊕◯◯◯	Downgraded for risk of bias (retrospective) and imprecision (single study)

⊕◯◯◯: very low; ⊕⊕◯◯: low. CI, confidence interval; CSA, cross-sectional area; GRADE, Grading of Recommendations, Assessment, Development, and Evaluations; LLIF, lateral lumbar interbody fusion; ODI, Oswestry Disability Index; VAS, Visual Analog Scale.

Discussion

This review investigated the relationship between pre-surgical psoas major morphology and outcomes following lumbar spine fusion surgery. Overall, the available data suggest that the morphology of the psoas does not appear to independently predict clinical and radiographic outcomes following lumbar fusion procedures. However, a small number of studies found that this relationship may depend on the surgical approach applied. Posterior fusion techniques demonstrated some association, such that a larger psoas was related to improvements in pain, early functional outcomes, and quality of life while lateral and anterior approaches showed no relationship with post-operative outcomes. Notably, findings are based on low to very low certainty, and there remains a paucity of primary research exploring the influence of psoas morphology on outcomes following spine fusion surgery.

While a prior review has examined the role of paraspinal muscles on postsurgical outcomes (13), the review did not evaluate the full spectrum of clinical and radiographic measures or consider how surgical technique may modify this relationship specifically in the context of psoas morphology. As such, the present review adds a novel contribution to the literature by systematically analysing the association between pre-operative psoas morphology and a broad range of post-operative outcomes, stratified by surgical approach. The findings reveal that while the overall relationship between psoas characteristics and post-surgical outcomes is variable, approach-specific patterns emerge such that posterior techniques demonstrate modest associations with improved pain, early functional recovery, and quality of life, whereas lateral and anterior approaches show no consistent relationship. The approach-specific associations suggested in this review are derived from a small number of retrospective studies with inconsistent results and should be considered exploratory, not definitive.

In posterior fusion procedures (PLIF/TLIF/PLF), where the psoas muscle is not directly manipulated, limited studies report that larger pre-operative psoas CSA was associated with reduced pain, improved disability and quality of life in the early post-operative period (≤6 months) (41,42). However, in lateral approaches (LLIF/OLIF), where the psoas is directly manipulated, the review found no consistent relationship between pre-operative psoas muscle morphology and post-operative outcomes. This null finding challenges the conventional assumption that a larger psoas would better tolerate surgical retraction. Instead, the single OLIF study in the analysis identified post-operative psoas muscle swelling as a more significant predictor of complications than baseline morphology (39), implying that surgical technique and intraoperative factors may outweigh pre-operative muscle characteristics in determining neurological and function outcomes. While interesting, this finding is based on limited retrospective data, and larger prospective studies are required to determine whether approach-specific relationships exist between perioperative psoas morphology and post-surgical recovery.

Only one small study (n=9) examined the association between pre-operative psoas morphology and outcomes following ALIF (33). This study found no significant relationship between pre-surgical psoas CSA and post-operative pain or disability. While this suggests limited prognostic value, due to the small sample size, caution should be taken in drawing conclusions. Notably, compared to posterior and lateral access to spine surgery, the anterior approach avoids disruption of the paraspinal musculature, which may reduce the compensatory importance of the psoas for post-operative stability. As such, the lack of association may reflect differences in surgical approach and the fact that psoas remains unaffected during this procedure. While plausible, this suggestion is speculative and clearly more ALIF studies with larger samples are required to confirm the prognostic value of pre-surgical measurements of psoas.

Notably, most studies (93%) included univariate analyses, with only two (12.5%) applied multivariate analyses to post-operative outcomes. Univariate analyses allow for simple associations between a single psoas metric (e.g., CSA) and an outcome, but they cannot account for confounding factors. In contrast, multivariate analyses isolate the independent effect of psoas muscle characteristics while adjusting for known factors that influence recovery such as older age, comorbidities like diabetes, and obesity, and surgical factors (duration of surgery and number of levels) (46). Therefore, the unadjusted associations presented in this review should be interpreted with caution. Future research should prioritize multivariate modelling, incorporating machine learning or hierarchical regression to better understand the relationship between psoas and post-surgical outcomes. Additionally, most studies analysed only segments of the psoas (e.g., single-slice measurements), neglecting the whole-muscle structure and function. Future work should model the full muscle in three-dimensional (3D), leveraging emerging imaging techniques like MRI-based volumetry or artificial intelligence (AI)-assisted segmentation (47,48). This could provide deeper insights into the psoas muscle’s role in spine surgery outcomes, including regional variations in atrophy, fat infiltration, and dynamic functional contributions to post-operative recovery.

Strengths and limitations

A study protocol was registered prospectively, and the results of this review have been reported in accordance with PRISMA and SWiM guidelines. Comprehensive database searches were undertaken, providing an extensive and thorough review of the literature. The analysis approach was also consistent with the prospectively registered protocol. However, the findings of this review should be interpreted considering limitations with the design and characteristics of included studies. First, substantial methodological heterogeneity was evident across studies, precluding a quantitative meta-analysis. While this limited our ability to statistically evaluated the magnitude of associations, this is not considered a detraction from the key findings of the review. Second, most included studies were retrospective in design, which introduces potential selection bias and means the reliability of results may be affected by the quality of medical record-keeping. Third, because 93% of included studies did not adequately control for confounders [age, sex, body mass index (BMI), comorbidities, social determinants of health], limited conclusions can be drawn about the independent prognostic value of psoas morphology and should be addressed in future studies. Finally, variability in psoas measurement methods across studies should be considered when interpreting the findings. Although CSA and fCSA were the most commonly reported measures, studies differed in their use of raw versus normalised values, the anatomical level at which psoas measurements were taken, and the imaging modality used. Future studies should adopt more consistent approaches to anatomical landmark selection, normalisation, and reporting of psoas morphology to improve interpretability and synthesis across the literature.

Clinical implications

Based on very low to low certainty evidence, routine pre-operative assessment of psoas morphology currently offers limited predictive value for outcomes following lumbar fusion procedures. This recommendation is conditional on the current evidence base and may be revised as new high-quality data emerge. The absence of clear associations in lateral approaches suggests that surgeons should focus more on optimizing intraoperative techniques (such as minimizing time and extent of retraction) rather than relying on pre-operative muscle assessments to predict complications. This perspective aligns with emerging evidence emphasizing the importance of surgical technique over patient factors in preventing worse post-operative symptoms following transpsoas approaches (49). Lastly, only one ALIF study was identified, and no total disc replacement studies were found during this review. This highlights an important area for future research, given that these procedures are becoming increasingly popular and retains motion of the spine segment.

Conclusions

This review provides a comprehensive evaluation of the relationship between pre-operative psoas muscle morphology and outcomes following lumbar fusion surgery. Current evidence does not support the routine use of pre-operative psoas assessment as a prognostic tool due to the predominantly very low to low certainty of evidence and the high risk of bias of the included studies. Higher-quality prospective studies with standardized measurement protocols are needed to clarify the prognostic value of psoas morphology in spine surgery.

Supplementary

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