Association of Subacromial Bursa Characteristics With Preoperative Nocturnal Pain and the Efficacy of Surgical Treatment for Pain Relief: A Prospective Cohort Study

Abstract

Background:

Nocturnal pain is a common symptom in patients with subacromial bursitis (SAB), yet its underlying determinants and optimal management remain unclear.

Purpose:

To evaluate the impact of bursal pathological and morphological features on nocturnal pain and the effectiveness of arthroscopic bursectomy.

Study Design:

Cohort Study; Level of evidence, 2.

Methods:

A total of 130 patients were initially enrolled for arthroscopic surgery between January and July 2025. Five patients were lost to follow-up, leaving 125 patients for analysis. Preoperative baseline characteristics and intraoperative bursal features—including morphological classifications (filamentous, reticular, septate, and fullness type) and pathological types (hyperemic and fibrofatty)—were recorded. Nocturnal pain was assessed using the visual analog scale preoperatively and at 4 and 12 weeks postoperatively. Data were analyzed using Friedman and Gamma regression tests.

Results:

Nocturnal pain declined substantially after arthroscopic bursectomy, with clear improvements from preoperative assessment to 4 and 12 weeks postoperatively. Fullness-type bursal morphology was associated with significantly greater pain burden, showing a 22% increase compared with the filamentous reference type, whereas reticular and septate morphologies did not differ significantly. Pathological findings were strongly related to symptom severity. Compared with patients exhibiting no pathological features, those with “both positive” pathology reported 29% higher pain scores, followed by fibrofatty (16% higher) and hyperemic pathology (13% higher). No meaningful associations were observed for patient age, sex, or body mass index.

Conclusion:

Hyperemic, fibrofatty, and “both positive” pathological features, as well as fullness-type bursal morphology, are strongly associated with higher nocturnal pain in patients with SAB. Arthroscopic bursectomy yields rapid and sustained symptom relief, supporting its role as an effective treatment option for patients who do not respond to conservative management.

Subacromial bursitis (SAB) is a major cause of shoulder pain and functional limitation, constituting a key component of the subacromial pain syndrome spectrum.^11,12 It is particularly prevalent among individuals engaged in repetitive overhead activities, such as athletes, laborers, and teachers.^19,24 SAB typically arises from repetitive mechanical irritation caused by overuse, trauma, subacromial impingement, or rotator cuff pathology, leading to synovial hyperemia, edema, and inflammatory exudation within the bursal cavity.^16,30 Clinically, patients often present with a painful arc between 60° and 120° of abduction, weakness during overhead movement, tenderness over the lateral acromion, and marked nocturnal pain that frequently disrupts sleep and quality of life.^3,7 Notably, nocturnal pain may partly reflect local inflammatory activity, as suggested by studies demonstrating increased subacromial blood flow in patients with nocturnal shoulder pain, and it may also be influenced by structural or pathological alterations in the bursa, which remain to be fully elucidated.^2,14 This suggests its potential utility as a clinical indicator of disease severity, warranting further investigation.

Nonoperative management—including rest, activity modification, physical therapy, and nonsteroidal anti-inflammatory drugs—is generally the first-line treatment. Subacromial corticosteroid injections are also widely employed to relieve inflammation and nocturnal pain, demonstrating short-term efficacy in many cases. However, pharmacological interventions may lead to adverse effects, and a considerable proportion of patients experience persistent or recurrent symptoms despite adequate conservative therapy, eventually requiring surgical management.^1,6,8

Arthroscopic subacromial bursectomy has proven effective for refractory cases, providing both diagnostic and therapeutic value. Nevertheless, the determinants of persistent nocturnal pain and the prognostic significance of specific bursal features remain insufficiently defined. Most existing studies have emphasized mechanical impingement or rotator cuff pathology, whereas the contribution of bursal pathological and morphological variations—such as hyperemia, fibrofatty degeneration, and structural subtypes—has rarely been explored in relation to pain severity.^5,17,29

Therefore, this prospective study aimed to elucidate the relationship between bursal pathological and morphological characteristics and the severity of nocturnal pain in patients with SAB, and to evaluate the efficacy of arthroscopic intervention in achieving sustained postoperative pain reduction. By integrating intraoperative morphological observations with postoperative outcomes, this study seeks to clarify the underlying mechanisms of nocturnal pain and guide more individualized surgical decision-making for patients who are unresponsive to conservative treatment.

Methods

Study Design

This study was designed as a prospective cohort study. A total of 130 patients were consecutively recruited over 7 months. For each patient, pre- and postoperative pain scores at 4 and 12 weeks were recorded. In addition, arthroscopic assessment documented the pathological and morphological features of the subacromial bursa (Figure 1).

Figure 1.

CONSORT flow diagram of participants’ enrollment, follow-up, and analysis. CONSORT, Consolidated Standards of Reporting Trials.

Participants

This prospective cohort study was conducted at the Orthopedic Department of The Second Affiliated Hospital, Yuying Children's Hospital of Wenzhou Medical University between January and July 2025. A total of 130 patients with SAB were consecutively enrolled. The inclusion criteria were as follows: (1) failure of conservative management; (2) persistent nocturnal pain, defined as a visual analog scale (VAS) score >4 at night; (3) unilateral shoulder involvement; (4) positive Hawkins-Kennedy and Neer impingement signs; (5) magnetic resonance imaging (MRI)-confirmed ³⁰ SAB without significant rotator cuff tear; (6) age between 30 and 75 years; and (7) absence of comorbidities—including diabetes mellitus, cardiovascular disorders, rheumatoid arthritis, or thyroid dysfunction.

Ethical Approval

The study protocol was reviewed and approved by the institutional review board of The Second Affiliated Hospital, Yuying Children's Hospital of Wenzhou Medical University. All participants provided written informed consent before enrollment.

Sample Size Calculation

Sample size was determined based on the primary study objective, which was to evaluate the association between bursal pathological features (morphology, hyperemia, and fibrofatty change) and nocturnal pain scores using multiple linear regression. An effect size of f² = 0.15 (medium effect) was assumed according to the Cohen guidelines. The significance level was set at .05, with a power of 80% (1 −β = 0.80). The regression model included 8 predictors: 5 main variables (3 dummy variables for bursal morphology, hyperemia, and fibrofatty change) and 3 covariates (age, sex, and body mass index [BMI]). Using G*Power software, the minimum required sample size was 109 patients. ²¹ To account for potential loss to follow-up, approximately 20% more patients were recruited, resulting in a total sample of 130 patients.

Baseline Characteristics and Intraoperative Assessment

Preoperative baseline characteristics—including age, sex, and BMI—were recorded for all patients.

Intraoperatively, the subacromial bursa was systematically evaluated under arthroscopic visualization for both pathological features and morphological subtypes. Morphological subtypes were defined based on the distribution and organization of intrabursal fibrous structures observed during arthroscopy:

• • Filamentous type: Delicate, freely mobile synovial villi within the bursal cavity.

• • Reticular type: Multiple fibrous strands interwoven to form a more organized mesh-like structure.

• • Septate type: A well-defined internal septum partitioning the bursal cavity into multiple compartments; cysts often form in the anterior compartment adjacent to the subscapularis tendon.

• • Fullness type: Markedly thickened synovium or fibrous tissue nearly obliterating the bursal cavity; corresponding MRI often shows a bursal complex exceeding 2 mm, indicating synovial proliferation rather than simple fluid accumulation (Figure 2).

Figure 2.

Arthroscopic images of different morphological classifications of the subacromial bursa. (A) Filamentous type. (B) Reticular type. (C) Septate type. (D) Fullness type.

To our knowledge, no standardized or widely accepted intraoperative classification system for subacromial bursa morphology currently exists. Therefore, the morphological subtypes used in this study were based on systematic arthroscopic observations and descriptive intraoperative characteristics. Pathological features—including hyperemic type and fibrofatty type—were similarly assessed according to intraoperative appearance.

All intraoperative assessments were performed by a single experienced orthopaedic surgeon to ensure consistency and minimize interobserver variability.

Surgical Intervention and Rehabilitation

All patients underwent standard arthroscopic subacromial bursectomy under general anesthesia using posterior and lateral portals. The inflamed subacromial bursa was completely excised while carefully preserving the rotator cuff tendons, and hemostasis was achieved using radiofrequency ablation as needed. Intraoperatively, the subacromial bursa was systematically evaluated for morphological subtypes based on arthroscopic observations. To minimize potential confounding effects of acromial morphology on postoperative outcomes, decisions regarding additional acromial bony procedures were guided by acromial type according to the Bigliani classification ¹⁵ : Type I (flat) generally required no intervention; type II (curved) could be managed with soft tissue decompression; and type III (hooked with prominent spurs) may necessitate careful bony resection and smoothing. In this study cohort, the majority of patients exhibited type I or type II acromions, and no patient demonstrated type III morphology requiring bony intervention. Therefore, no acromioplasty or spur resection was performed, ensuring that postoperative outcomes primarily reflect the effects of subacromial bursectomy and intrinsic bursal pathology.

Postoperative rehabilitation followed a standardized protocol, including gradual range-of-motion exercises, activity modification to avoid early overload, and progressive strengthening over subsequent weeks. Patients were instructed to avoid heavy lifting or overhead activities for the first 4 to 6 weeks, with follow-up visits at 4 and 12 weeks to monitor recovery and functional outcomes.^10,18

Outcome Measures and Follow-up

The primary outcome was nocturnal pain intensity, assessed using the VAS. Pain scores were recorded preoperatively and at 4 and 12 weeks postoperatively. Missing data were addressed using multiple imputation to minimize bias (Figure 3).

Figure 3.

Box plots of nocturnal pain scores at baseline (preoperative) and at 4- and 12-week postoperative follow-ups. Numbers indicate median and interquartile range.

Statistical Analysis

Repeated measures of nocturnal pain at baseline (preoperative) and at 4-week and 12-week follow-up were analyzed using the Friedman test. Post hoc pairwise comparisons were performed using the Wilcoxon signed-rank test and Bonferroni correction.

Preoperative nocturnal pain scores were analyzed using Gamma regression with a log link function, selected due to the continuous, strictly positive, and right-skewed distribution of the scores. Coefficients were exponentiated to yield relative risks (RRs).

To examine the independent associations of bursal morphology and pathology, 2 nested models were fitted:

• • Model 1 (Main Effects Model): This model included morphological and pathological classifications as primary exposures, mutually adjusted.

• • Model 2 (Interaction Model): This model extended Model 1 by adding an interaction term to test for effect modification.

A likelihood ratio test indicated no significant interaction (P > .05); therefore, model 1 was used as the final model. Common demographic variables (age, sex, and BMI) were evaluated as potential confounders. Still, they showed no significant association with preoperative pain, and their inclusion did not materially alter the effect estimates. For parsimony and statistical power, these variables were excluded from the final model.

Model robustness was assessed through standard diagnostics—including residual-fitted value correlation, scale-location slope, Cook's distances, and estimation of the dispersion parameter—confirming that model assumptions were reasonably met.

Results are presented from the final model 1, reporting adjusted RRs and 95% CIs. All analyses were performed in R, Version 4.3.0 (R Foundation for Statistical Computing), with 2-sided P < .05 considered statistically significant.

Results

Of the 130 patients initially enrolled, 5 were lost to follow-up due to loss of contact, leaving 125 patients for analysis. The cohort included 66 women (52.8%) and 59 men (47.2%), with a median age of 57 years (interquartile range [IQR], 52-63) and a median BMI of 21.2 kg/m² (IQR, 19.7-23). The mean preoperative nocturnal pain score was 6.01 ± 1.13 (median, 6; IQR, 5-7).

Nocturnal pain demonstrated a significant change over time based on the Friedman test (χ² = 248.53; P < .001). Median pain scores decreased from 6 (IQR, 5-7) preoperatively to 2 (IQR, 1-2) at 4 weeks and 0 (IQR, 0-0) at 12 weeks. Post hoc Wilcoxon signed-rank tests with Bonferroni correction confirmed significant reductions from preoperative to 4 weeks (V = 7875; P < .001; Cohen's d = 4.06), from preoperative to 12 weeks (V = 7875; P < .001; Cohen's d = 6.24), and from 4 to 12 weeks (V = 7503; P < .001; Cohen's d = 2.22). These findings indicate a progressive and sustained improvement in nocturnal pain after surgery (Table 1).

Table 1.

Baseline Characteristics and Nocturnal Pain Scores of Patients (n = 125) ^a

Variable	Preoperative	4-Week Postoperative	12-Week Postoperative
No. of patients	125	125	125
Women	66 (52.8)	–	–
Men	59 (47.2)	–	–
Age, years	57 (52-63)	–	–
BMI, kg/m2	21.2 (19.7-23)	–	–
Pain score	6.01 ± 1.13	1.78 ± 0.86	0.16 ± 0.37
Pain score	6 (5-7)	2 (1-2)	0 (0-0)

^aData are presented as mean ± SD, median (IQR), or n (%). Baseline demographic variables (sex, age, and BMI) are reported only for the preoperative assessment; follow-up columns for these variables are not applicable. BMI, body mass index.

Gamma regression analysis identified significant associations between specific bursal morphological and pathological characteristics and preoperative nocturnal pain scores. Among morphological features, fullness-type bursae—characterized by markedly thickened synovium nearly obliterating the bursal cavity (Figure 2D)—were significantly associated with increased pain scores (risk ratio [RR], 1.22 [95% CI, 1.12-1.33]; P < .001), corresponding to a 22.4% increase compared with the filamentous reference type, which exhibited delicate, freely mobile synovial villi within the cavity (Figure 2A). Reticular and septate morphologies showed no significant differences from the filamentous reference. All pathological subtypes were also significantly associated with elevated pain scores compared with the “no pathological feature” reference group. The strongest association was observed for the “both positive” pathology (RR, 1.29 [95% CI, 1.19-1.41]; P < .001), representing a 29.4% increase, followed by fibrofatty pathology (RR, 1.16 [95% CI, 1.08-1.25]; P < .001) and hyperemic pathology (RR, 1.13 [95% CI, 1.05-1.22]; P = .002), corresponding to 15.9% and 13% increases, respectively (Figure 4).

Figure 4.

Forest plot of RRs with 95% CIs for factors associated with preoperative pain. The vertical dashed line represents the null (RR = 1). Points indicate RRs, horizontal bars show 95% CIs, and numbers on the bars indicate RR (95% CI). Significance: ***P < .001, **P < .01, *P < .05, NS P≥ .05. RR, risk ratio.

Model diagnostics indicated excellent fit and robustness. The correlation between fitted values and residuals was near zero (r = 0.012; P = .899), confirming the adequacy of the linear predictor. The slope of the scale-location plot was nonsignificant (β = −0.032; P = .467), suggesting acceptable homoscedasticity. Visual inspection of the scale-location plot revealed minor fluctuations that were not statistically significant and did not affect parameter estimates or inference. No high-influence outliers were observed (maximum Cook's D = 0.118; no cases with Cook's D > 0.5 or > 1), and the estimated dispersion parameter was 0.0205, further supporting the stability and reliability of the model (Figure 5).

Figure 5.

Diagnostic plots summarizing model adequacy, including assessments of linearity (Residuals vs Fitted), residual normality (Q–Q Residuals), variance homogeneity (Scale–Location), and influential observations (Cook’s distance).

These results indicate that specific bursal morphological and pathological features—relative to the filamentous and “no pathological feature” reference groups—are strongly associated with preoperative nocturnal pain and that the Gamma regression model provides robust and reliable estimates.

Postoperative complications and adverse events were recorded at each follow-up. Among the 125 patients, 9 (7.2%) experienced postoperative complications. The most common events were transient shoulder stiffness (n = 5) and superficial wound erythema or bruising (n = 4). All complications resolved with conservative management within 4 weeks. No major complications, such as deep infection, neurovascular injury, or rotator cuff damage, were observed. None of the reported complications affected the overall improvement in nocturnal pain scores.

Discussion

To the best of our knowledge, this is the first study to comprehensively investigate the associations between arthroscopic morphological subtypes and pathological features of the subacromial bursa with nocturnal pain, as well as to evaluate the therapeutic efficacy of arthroscopic bursectomy in patients with SAB.^5,9,17 Our results demonstrated that nocturnal pain decreased significantly after surgery, with sustained improvement observed at both 4 and 12 weeks postoperatively. Importantly, specific morphological and pathological subtypes—namely, the hyperemic, fibrofatty, and fullness types—were significantly associated with higher nocturnal pain scores, whereas demographic variables such as age, sex, and BMI were not significantly related to pain severity. These findings highlight that intrinsic bursal pathology and morphology are key determinants of symptom severity and may inform the expected outcomes of arthroscopic intervention.

Among the morphological subtypes, the fullness type showed the strongest association with nocturnal pain. The markedly thickened synovium may increase intrabursal tension and magnify mechanical impingement during nighttime shoulder compression, thereby heightening nociceptor sensitivity.^11,22 Conversely, the filamentous, reticular, and septate types possess comparatively thinner synovial membranes and less cavity-obliterating tissue, corresponding to lower structural load and milder symptoms.

In parallel with these morphological patterns, 4 distinct pathological features were identified arthroscopically. The normal type displays a thin, smooth synovial lining without inflammatory or adipose alterations. The hyperemic type reveals pronounced vascular engorgement and increased perfusion, reflecting acute inflammatory activation that promotes effusion and nociceptor sensitization. ³ The fibrofatty type reflects chronic bursal remodeling, characterized by fibrotic thickening of the bursal wall with patchy adipose deposition within the synovial layer and fatty lobulations protruding toward the cavity. These changes represent fibroadipose replacement of the normal synovial lining rather than rotator-cuff–related fatty change. The hyperemic with fibrofatty change type represents a mixed phenotype with concurrent vascular congestion and adipose deposition (Figure 6). Fibrofatty change, in particular, replaces the normal synovial architecture with fibroadipose tissue capable of producing proinflammatory mediators such as interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α), thereby perpetuating local inflammation.^3,4,27 Similar adipokine-driven inflammatory responses—including leptin- and adiponectin-mediated pathways—have been reported in other synovial bursae, suggesting shared mechanisms of inflammation-induced nociception. ²⁸

Figure 6.

Arthroscopic images of different pathological features of the subacromial bursa. (A) Normal subacromial bursa. (B) Fibrofatty type. (C) Hyperemic type. (D) Combined fibrofatty type and hyperemic type.

Notably, the production of key inflammatory mediators such as TNF-α, IL-1, and IL-6 exhibits circadian rhythmicity, with peak secretion typically occurring at night and early morning. This temporal pattern may exacerbate nocturnal pain by enhancing synovial inflammation and nociceptor sensitization during periods of rest, contributing to sleep disruption and impaired quality of life.^20,23,25 Taken together, these data suggest that vascular hyperemia and fibrofatty change act synergistically to intensify nociceptive input, particularly at night, in patients with SAB.

Our findings align with previous reports highlighting the role of bursal inflammation in shoulder pain. Unlike earlier studies that included patients with rotator cuff pathology, our analysis focused exclusively on isolated SAB. By integrating arthroscopic morphology, pathology, and MRI features, this study provides a unified classification that helps explain the variable presentations of SAB and offers objective indicators of pain severity. ¹³ The more significant nocturnal pain seen in the hyperemic and fullness types likely reflects the combined effects of inflammatory sensitization and increased mechanical compression during rest.

Clinically, identifying hyperemic, fibrofatty change, or fullness-type bursae may help predict which patients are more likely to experience severe nocturnal pain and benefit from surgical intervention after failure of conservative therapy. Our results confirm that arthroscopic bursectomy provides rapid and sustained pain relief, with most patients achieving minimal or no nocturnal discomfort by 12 weeks. Postoperative MRI showed marked reductions in synovial thickness and intrabursal fluid accumulation, further supporting the efficacy of arthroscopic bursectomy in removing inflamed bursal tissue (Figure 7).

Figure 7.

Comparison of preoperative and 12-week postoperative MRI findings. (A) Preoperative MRI showing bursal inflammation and effusion. (B) Postoperative MRI at 12 weeks showing significant improvement (arrows).

Strengths and Limitations

This study has several strengths. The prospective cohort design and systematic intraoperative documentation of bursal morphology and pathology enhance data reliability and are consistent with recommendations for high-quality observational studies. ²⁶ Use of standardized outcome measures, consistent follow-up intervals, and sensitivity analyses—accounting for missing data, outliers, and alternative regression models—further strengthen the robustness of the findings.

Nevertheless, several limitations should be acknowledged. First, the single-center design may limit the generalizability of the results, and the 12-week follow-up period precludes evaluation of long-term outcomes. Second, other potential contributors to nocturnal pain, such as rotator cuff integrity, capsular stiffness, or systemic inflammatory status, were not directly assessed. Third, the classification of bursal morphological subtypes is inherently subjective, and inter- and intraobserver reliability has not yet been formally evaluated. Future studies should aim to validate this classification system and assess its reproducibility across multiple observers and centers. Finally, the incorporation of multimodal imaging, histopathological analysis, and multicenter recruitment will help clarify the mechanistic basis of nocturnal pain in SAB.

Finally, these results suggest directions for future research. Long-term, multicenter studies could evaluate the durability of surgical pain relief and investigate whether preoperative imaging can predict high-risk bursal subtypes. Exploring interactions among bursal pathology, rotator cuff integrity, and patient-specific factors such as activity level may inform individualized treatment strategies. Integrating molecular analyses could further elucidate the biological mechanisms of nocturnal pain and identify therapeutic targets beyond surgery.

Conclusion

Hyperemic type, fibrofatty change, and fullness-type bursal morphology are closely associated with nocturnal pain severity in patients with SAB. Arthroscopic bursectomy provides rapid and sustained pain relief, underscoring its value in cases refractory to conservative management. A comprehensive understanding of inflammatory, structural, and circadian mechanisms offers a rational framework for managing nocturnal pain and guiding clinical decision-making in SAB.