Search Results (74)

Background/Objectives: Surgical repair of the rotator cuff tendons can lead to unsatisfactory results and the requirement for further surgical treatment. Development of repair techniques is limited by a lack of appropriate functional pre-clinical testing, especially over extended motion cycles. The purpose of this study was to demonstrate the efficacy of a novel shoulder simulator by assessing changes in internal muscle forces following a rotator cuff tear and double-row surgical repair. Methods: The developed shoulder simulator used motors to apply controlled movements/displacements to tendons (supraspinatus, infraspinatus, subscapularis, teres minor, anterior deltoid and middle deltoid) of a cadaveric human shoulder to produce cyclic abduction motion representative of normal shoulder function. The required displacement for each muscle was determined using a musculoskeletal model. The resultant force applied to each tendon during the cycles was measured using a compression load cell. Results: The developed simulator in this proof-of-concept study enabled the contribution of the different muscles involved in the shoulder during abduction to be assessed for the intact shoulder. The shoulder was also tested with a 50% supraspinatus tear and a double row surgical repair of the supraspinatus to assess the change in internal muscle forces. Conclusions: The study indicated that successful cyclic testing of cadaveric samples could be achieved using the simulator and changes in the internal muscle forces of the shoulder were identified following a supraspinatus tear and double-row surgical repair. Full article

Background/Objectives: Shoulder pain or instability have different causes and therefore different treatment approaches. The most common are surgical approaches, ranging from more conservative to more radical, such as arthroplasty. In the presence of irreparable rupture or tears of the supraspinatus and subscapularis muscles, particularly in younger patients without arthritis or osteoarthrosis, it becomes a surgical challenge to avoid early arthroplasty. Despite the development and proposal of various surgical techniques to restore stability and function, and eliminate pain of the glenohumeral joint, there remains a lack of consensus on the most effective approach in the absence of arthritis. This work aims to systematically review the available evidence on surgical procedures for treating shoulder joint instability and pain due to irreparable subscapularis and supraspinatus tendon tears, excluding cases with coexisting arthritis or arthrosis. Methods: This systematic review was conducted following PRISMA 2020 guidelines. Search was performed across the major databases PubMed, Scopus, and Web of Science, up to 2025. Inclusion criteria comprised clinical studies (randomised controlled trials, cohort studies, case series), reviews and systematic reviews reporting on surgical interventions for shoulder instability and pain caused by irreparable tears of the subscapularis and supraspinatus in patients without degenerative joint disease. As shoulder surgery techniques have been continually evolving since the early procedures, no lower limit was set for the time interval. The related literature that was not identified in the mentioned databases was added manually through parallel searches of associated themes and suggestions from the websites of those databases. To be eligible, papers had to describe shoulder instability surgery, surgical techniques, indications, and patients’ recovery outcomes. The key tasks, such as title and abstract screening, were performed by V. M., L. R., and M. A. N. to ensure thoroughness and reduce bias. Results: From an initial search result of 98 works, a total of 66 titles and abstracts were analysed, resulting in a final selection of 24 studies that fulfilled the mentioned criteria and were therefore included in this review. The oldest paper mentioning a shoulder surgery in this context was published in 2003. The most frequently described procedures included total joint replacement, tendon transfers, Superior Capsule Reconstruction, partial rotator cuff repair and others. Across studies, improvements were noted in shoulder stability, range of motion, and functional scores. However, heterogeneity in surgical techniques and outcome measures limited direct comparison. Conclusions: The conducted systematic review reveals an important gap and highlights the need to evolve beyond traditional shoulder surgery techniques and, if possible, to provide a single solution for the different origins of shoulder instability. Several surgical options demonstrate promising outcomes in managing shoulder instability and pain due to irreparable subscapularis and supraspinatus tears in patients without glenohumeral arthritis. However, those procedures are associated with anatomical changes in the shoulder joint, compromising the joint’s full function and prolonging recovery time. Nevertheless, this conclusion is based on a small sample size of the current literature and a lack of high-level evidence. Further comparative and long-term studies are needed to establish optimal treatment strategies. Full article

The canine supraspinatus insertion tendon (SIT) is frequently involved in shoulder disorders; however, objective ultrasonographic reference data remain limited. This study aimed to describe the ultrasonographic appearance of the SIT in clinically non-lame dogs and to develop a reproducible protocol for the quantitative analysis of tendon thickness and relative echogenicity. Thirty-five clinically non-lame dogs (70 shoulders) underwent prospective B-mode ultrasonography. On ultrasonographic examination, the SIT appeared as a structure composed of two clearly differentiated regions: a proximal region, which was thinner and more hyperechoic, and a distal region, which was thicker, relatively more hypoechoic, and attached to the bone surface. These findings were consistent with previous descriptions of the SIT ultrasonographic appearance reported in the literature. Tendon thickness and mean echogenicity were evaluated in both regions. Mean echogenicity was quantified using region-of-interest analysis and pixel intensity histograms, with trapezius muscle echogenicity used for background normalisation. Paired statistical tests and Pearson correlation analyses were performed. No significant differences were identified between the left and right SIT measurements for either thickness or relative echogenicity (p > 0.05). The distal region was significantly thicker than the proximal region (p < 0.001). Relative echogenicity also differed significantly between regions: the proximal region was more hyperechoic and heterogeneous, whereas the distal region was more homogeneous and hypoechoic (p < 0.001). Body weight and height were positively associated with tendon thickness, whereas associations with mean echogenicity were reduced after normalisation to trapezius muscle echogenicity. In conclusion, the canine SIT comprises two ultrasonographically distinct regions that differ in thickness and echogenicity. The proposed protocol provides an objective and standardized method for the quantitative ultrasonographic evaluation of tendon structure in dogs. However, this study represents a preliminary assessment of the SIT ultrasonography, and a larger sample size is required to confirm the statistical results obtained. Full article

Background/Objectives: Rotator cuff tears (RCT) are one of the most common causes of shoulder pain and loss of function. The aim of this study was to evaluate shoulder morphometric and soft tissue parameters using magnetic resonance imaging (MRI) and to compare them between patients diagnosed with RCT and symptomatic controls without MRI-evident RCT. Methods: In this retrospective study, shoulder MRI images of 64 patients diagnosed with RCT and 63 control subjects without RCT aged 20–80 years were analyzed. The critical shoulder angle (CSA), lateral acromial angle (LAA), acromial index (AI), coracohumeral distance (CHD), acromiohumeral distance (AHD), coracoacromial ligament (CAL) thickness, and supraspinatus tendon (SST) thickness were measured. Binary logistic regression analysis was applied to identify independent factors associated with RCT. Results: CSA was significantly higher in the RCT group compared to the symptomatic control group without RCT (36.18 ± 4.0 vs. 32.61 ± 2.6, p < 0.001). In contrast, CHD (7.06 ± 1.13 vs. 8.05 ± 1.30 mm, p < 0.001), AHD (6.15 ± 1.27 vs. 7.23 ± 0.99 mm, p < 0.001), and SST thickness (5.17 ± 0.69 vs. 6.89 ± 0.64 mm, p < 0.001) were significantly lower in the RCT group. No significant differences were found between the groups in terms of LAA, AI, and CAL thickness (p > 0.05). In the logistic regression analysis, increasing age (OR = 1.159; p = 0.044) and CSA (OR = 1.808; p = 0.005) were identified as independent risk factors, while an increase in SST thickness was identified as inversely associated with RCT (OR = 0.35; p = 0.003). ROC analysis demonstrated that SST thickness had the highest discriminatory performance (AUC = 0.963), while the combined multivariable model achieved an AUC of 0.979. Conclusions: CSA, CHD, AHD, and SST thickness are important morphometric and soft tissue parameters associated with RCT. In particular, the assessment of CSA and SST thickness may provide a more robust and clinically meaningful approach to RCT-associated changes. The combined evaluation of CSA and SST thickness may contribute to the understanding of morphometric associations related to RCT. Full article

Acute or chronic rotator cuff tears are major causes of shoulder dysfunction, motivating the development of scaffolds with tailored thickness and mechanics for supraspinatus tendon regeneration. This study aimed to investigate the effect of bilayer poly(butylene adipate-co-terephthalate) (PBAT) scaffold thickness on the tenogenic differentiation of rat adipose mesenchymal stem cells (r-AdMSCs) and supraspinatus tendon regeneration. Aligned fibers with a diameter of approximately 476 nm were deposited onto randomly oriented layers at different times (4 h; 4S, 6 h; 6S, 8 h; 8S), and scaffolds with increasing thicknesses from 441 µm (4S) to 1132 µm (8S) were produced. Mechanical testing showed comparable tensile strength for 4S and 6S (≈1.9–2.0 MPa) and modulus (5.5–7.3 MPa), while 8S exhibited markedly reduced stiffness (0.5 MPa) and hyper elastic deformation. Mechanical performance across degradation conditions remained strongly thickness-dependent: thinner scaffolds retained integrity and strengthened, with modulus increases during hydrolytic and enzymatic degradation, whereas thicker matrices showed limited remodeling and instability. Rat-AdMSCs’ were cultured on the scaffolds for 21 days. Cell-free and cell-laden mechanical responses further reflected thickness effects: cell-free samples stiffened due to media-induced passive matrix tightening, whereas cell-laden scaffolds showed extracellular matrix (ECM)-driven reinforcement, most prominently in 4S, which reached 2.1 MPa tensile strength with improved elasticity and balanced deformation. The 4S scaffold exhibited the highest tensile strength and significantly increased collagen-1 (col1), tenomodulin (tnmd) and scleraxis (scx) expression compared with the other groups. In conclusion, among all groups, 4S scaffolds demonstrated the most favorable mechanical and biological performance, suggesting that scaffold thickness plays a critical role in regulating tendon regeneration and will become even more suitable when matured in bioreactors. Full article

Background: Sugaya type III rotator cuff re-tears are characterized by preserved tendon continuity with reduced thickness and are often associated with persistent pain and functional impairment. Bioinductive collagen implants may enhance tendon healing, but clinical evidence in this population remains limited. This study aimed to evaluate the clinical and radiologic outcomes of arthroscopic repair with bioinductive collagen implant augmentation in patients with Sugaya type III re-tears. Methods: This retrospective case series (Level IV) included 15 patients (mean age 61.7 years) with MRI-confirmed Sugaya type III re-tears. An a priori power analysis based on a large effect size (Cohen’s d = 0.80) indicated that a sample size of 15 would provide 80% power to detect clinically meaningful changes in the primary endpoint. Clinical outcomes were assessed preoperatively and at 6 and 12 months postoperatively using VAS, ASES, SANE, and WORC scores. MRI was used to evaluate changes in supraspinatus tendon thickness. Non-parametric statistical analysis with Bonferroni correction was applied. Results: The median VAS pain score improved from 6.5 (IQR, 6.0–7.0) preoperatively to 2.8 (IQR, 2.0–3.5) at 6 months and to 2.1 (IQR, 1.5–2.8) at 12 months (adjusted p < 0.001). The median ASES score increased from 45.0 (IQR, 39.0–51.0) to 78.0 (IQR, 72.0–85.0), with a median improvement of 33 points. SANE and WORC scores also showed significant improvements. Supraspinatus tendon thickness increased from 4.8 mm (IQR, 3.7–5.7) to 6.9 mm (IQR, 5.4–8.3) at 12 months (adjusted p < 0.001). No graft failure was observed on follow-up MRI. Conclusions: Arthroscopic repair with bioinductive collagen implant augmentation may be associated with short-term improvements in pain, function, and tendon thickness in patients with Sugaya type III re-tears. Given the small sample size and lack of a control group, these findings should be interpreted cautiously, and further prospective comparative studies are needed. Full article

Background and Objectives: This study aimed to assess the association between the aponeurotic expansion of the supraspinatus tendon (AEST) and supraspinatus tendon tears utilizing magnetic resonance (MR) arthrography in an extensive patient cohort and to determine its clinical relevance. Materials and Methods: All MR arthrography images were retrospectively assessed by two radiologists with expertise in arthrography. In both the AEST group and the comparison group consisting entirely of patients with rotator cuff tendon tears, the location and extent of the tear were documented, along with the distance between the tear and the AEST in the study group. In patients with AEST, it was also noted whether the tear extended to or structurally involved the AEST. Results: AEST was identified in 61 (7.4%) of the 827 MR arthrograms. The isolated supraspinatus tendon tear emerged as the most prevalent, representing 33.3% of patients with AEST. The triple combination of tears involving the supraspinatus, subscapularis, and infraspinatus accounted for 25.9% of patients with AEST. AEST tears alone were observed in just 3.3% of cases. The incidence of triple combination tears in patients with AEST was found to be significantly lower at 25.9% compared to 59.3% in the control group without AEST (p = 0.028). Conclusions: In this study, the authors found that the incidence of combined tendon tears in patients with AEST was significantly lower compared to the control group. This result suggests that the AEST likely acts as a structural barrier limiting the anterior progression of rotator cuff tears, particularly toward the subscapularis tendon. Full article

Large rotator cuff tendon injuries pose a dual clinical challenge: poor inherent healing capacity and high mechanical demands. To address this, we have developed a bifunctional scaffold that combines a slow-degrading, mechanically robust polyurethane core coated with tendon-derived extracellular matrix (ECM) extract to provide both structural support and regenerative cues. In a rabbit model of supraspinatus tendon injury, this ECM-polyurethane scaffold facilitated healing of critical-sized defects, resulting in aligned, tendon-like tissue with improved biomechanical properties. This study further explored the tendon healing mechanisms of the ECM-polyurethane scaffold in a rabbit model of large supraspinatus tendon injury using transcriptomic and qPCR analyses. At one month post-surgery, while both ECM-coated and uncoated polyurethane scaffolds initially provoked similar inflammatory responses when compared to healthy tendon, their healing pathways diverged significantly. The control polyurethane scaffold activated pathways associated with adipose tissue development, a non-functional outcome, whereas the ECM-coated scaffold actively directed healing toward tendon regeneration. These results demonstrate that the ECM coating is the critical factor driving divergent healing responses in polyurethane scaffolds, even though their underlying biomechanical properties are similar. This underscores the importance of combining biomechanical reinforcement with biologically active regenerative signals for effective regeneration of tendon and other load-bearing tissues. Full article

Background/Objectives: Shoulder involvement is common in rheumatoid arthritis (RA), yet periarticular pathology may remain subclinical. Musculoskeletal ultrasonography enables detection of early inflammatory and structural changes; however, data on asymptomatic shoulder involvement in RA are limited. This study aimed to evaluate ultrasonographic shoulder findings in asymptomatic RA patients, compare them with healthy controls, and assess their association with disease activity. Methods: This prospective case–control study included 31 patients with RA without shoulder pain and 33 asymptomatic healthy controls. Bilateral shoulder examinations were performed using standardized ultrasonographic protocols. Periarticular findings were assessed using the Ultrasound Shoulder Pathology Rating Scale (USPRS), and acromiohumeral distance (AHD) was measured. Disease activity was evaluated using the Disease Activity Score in 28 joints (DAS28). Group comparisons, correlation analyses, receiver operating characteristic (ROC) analysis, and multivariate logistic regression were performed. Results: There were no significant differences in demographic characteristics or comorbidities between groups. Biceps tendinopathy scores were significantly higher in RA patients than in healthy controls (1.0 ± 0.79 vs. 0.5 ± 0.56, p = 0.01), whereas other ultrasonographic parameters, including supraspinatus tendinopathy, dynamic impingement findings, AHD, and total USPRS score, did not differ significantly. DAS28 showed significant positive correlations with biceps tendinosis (r = 0.37, p < 0.05) and supraspinatus tendinosis (r = 0.36, p < 0.05). Total USPRS demonstrated acceptable discrimination for identifying moderate–high disease activity (AUC = 0.73). In multivariate analysis, DAS28 was independently associated with biceps tendinopathy (OR = 1.62, 95% CI: 1.05–2.49). Conclusions: Asymptomatic patients with RA exhibit a higher burden of biceps tendon involvement compared with healthy individuals, and subclinical shoulder ultrasonographic findings are associated with disease activity. Asymptomatic patients with RA demonstrated a significantly higher biceps tendinopathy score compared with healthy controls, whereas most other ultrasonographic parameters, including total USPRS score and AHD, did not differ significantly. These findings suggest that the long head of the biceps tendon may represent a relatively more sensitive site of subclinical periarticular involvement in RA; however, the overall ultrasonographic differences were limited and should be interpreted cautiously. Full article

Background: Asymptomatic structural joint abnormalities are prevalent among athletes, yet studies on their multi-joint distribution and comparisons with low-activity controls remain lacking. This article evaluated the prevalence and characteristics of asymptomatic structural abnormalities across joints in collegiate athletes compared with controls using 3.0-T MRI. Methods: The cross-sectional study enrolled 53 asymptomatic elite collegiate athletes (high physical activity, HPA) and 84 healthy volunteers (low physical activity, LPA) aged 18–25 years. All participants were asymptomatic with no history of joint trauma or surgery. Generalized estimating equation (GEE) logistic regression was employed to identify independent risk factors for joint abnormalities after evaluation. Results: A total of 666 joints were analyzed. Participants with at least one joint abnormality were significantly more common in the HPA group than LPA group (49.1% vs. 6.0%, p < 0.001). At the joint level, overall abnormality prevalence was 13.5% versus 2.2%, respectively. In the HPA group, knee joints were the most frequently affected (24.2%), predominantly involving meniscal lesions. Shoulder pathologies consisted exclusively of supraspinatus tendon lesions (6.8%), while ankle abnormalities were primarily bone marrow edema (5.9%). GEE analysis identified high physical activity (adjusted OR = 5.23; 95% CI: 1.55–17.71; p = 0.008) and elevated BMI (adjusted OR = 1.09 per kg/m²; 95% CI: 1.03–1.15; p = 0.001) as independent risk factors. Conclusions: Asymptomatic abnormalities are highly prevalent and demonstrate intra-individual clustering across multiple joints. MRI-based surveillance represents a promising strategy for early risk identification and injury prevention. Full article

Platelet-rich plasma (PRP) is widely used as a second-line treatment for chronic tendinopathy that persists despite structured conservative care, yet outcomes and imaging correlates remain heterogeneous. This review outlines PRP biology and preparation, summarises quantitative imaging techniques for monitoring tendon response, and presents the experience of a single centre integrating these methods into routine supraspinatus and lateral elbow PRP workflows. PRP is described as an autologous platelet concentrate with variable leukocyte and fibrin content, with leukocyte-rich formulations commonly selected for chronic tendinopathy. Quantitative approaches—including ultrasound shear-wave elastography and radiomics, MRI T2/T2* mapping, CT-based bone metrics, PET/CT, and optical techniques—offer numerical biomarkers of tendon structure, mechanics, and inflammation but are rarely implemented in PRP trials. At the authors’ centre, leukocyte-rich PRP is injected under ultrasound guidance after failed physiotherapy, and follow-up combines validated questionnaires with grey-level run-length matrix texture analysis of ultrasound and 3.0 T MRI T2 distribution profiling. A pilot ultrasound study in supraspinatus and common extensor tendinosis showed uniform short-term clinical improvement and significant changes in most texture features, with selected parameters correlating with symptom relief. A prospective supraspinatus cohort demonstrated significant six-month clinical gains in both tendinosis and small partial-thickness tears, whereas only the tendinosis group exhibited T2 profile convergence toward asymptomatic patterns. These data indicate that quantitative ultrasound radiomics and whole-length T2 profiling are feasible imaging biomarkers that capture PRP-induced tendon remodelling beyond qualitative imaging and may help tailor PRP protocols to specific tendon phenotypes. Full article

The in vivo effect of platelet-rich plasma (PRP) on supraspinatus tendon morphology and subacromial bursa cell gene expression in degenerative rotator cuff tears remains unclear. This randomized controlled trial evaluated the effect of preoperative leukocyte-poor PRP (LP-PRP) subacromial injection on supraspinatus tendon histology and subacromial bursa gene expression. Sixteen patients with full-thickness supraspinatus tears were randomized to receive an ultrasound-guided LP-PRP injection (n = 8) or no injection (n = 8) six weeks before arthroscopic repair. Tendon biopsies were assessed using the modified Movin score. Gene expression of collagen type I, II and III, metalloproteinase 3 and 13, and interleukin 1β and 6 genes from subacromial bursa cells was quantified using quantitative real-time PCR. The results of the two groups were compared to determine any statistically significant difference regarding all the examined parameters. The PRP group demonstrated a significantly lower total modified Movin score than controls (6.5 vs. 12.1, p = 0.002), with lower scores for fiber structure, fiber arrangement, nuclear rounding, inflammation and cell density (all p < 0.003), while angiogenesis did not differ (p = 0.149), indicating an architecture closer to that of normal tendon. No statistically significant differences in gene expression were observed (all p > 0.05), although collagen II and metalloproteinase 3 and 13 showed biologically relevant downregulation [fold change 0.23 (95%CI 0.05–1.09), 0.24 (95%CI 0.002–26.10), and 0.26 (95%CI 0.02–2.76), respectively]. The LP-PRP injection was associated with improved supraspinatus tendon histological characteristics and biologically relevant reductions in selected bursal genes, in the setting of supraspinatus tendon tear. Full article

Background/Objectives: Rotator cuff repair (RCR) is a common orthopedic procedure, with healing outcomes strongly influenced by patient-specific factors such as tissue quality, tear characteristics, and biological healing potential. Bioinductive collagen implants have shown great results in enhancing tendon healing and reducing retear rate. This study aimed to evaluate the clinical and imaging outcomes of RCR augmented with a xeno-derived collagen membrane over 24 months and to assess complications or implant failures. Methods: Patients underwent arthroscopic RCR using anchors (single or double-row) with additional xeno-derived matrix augmentation. The study included patients older than 40 years with full-thickness supraspinatus and/or infraspinatus tendon tears (DeOrio–Cofield grade 3–4) who were candidates for arthroscopic rotator cuff repair and provided informed consent. Clinical outcomes were assessed using the Constant–Murley Score (CMS), Disabilities of the Arm, Shoulder and Hand score (DASH), and Visual Analogue Score (VAS) at baseline, 3, 6, 12, and 24 months. MRI was performed preoperatively and at 24 months to assess tendon thickness. Results: All scores improved significantly. CMS increased from 16.3 ± 4.1 to 82.9 ± 5.8, VAS decreased from 7.8 ± 1.0 to 1.5 ± 0.8, and DASH improved from 70.3 ± 6.4 to 12.4 ± 4.5 (p < 0.05). Tendon thickness in the supraspinatus (T3) increased from 4.2 ± 0.9 mm to 6.8 ± 1.2 mm (p < 0.05). Retear rate was 7.55%, with no major complications. Conclusions: The bioinductive collagen implant showed notable results in improving tendon thickness, healing, and excellent clinical outcomes in RCR, without membrane-related complications. The study was designed as a prospective single-arm case series without a control group and that was the main limitation; The absence of adverse reactions in this cohort further supports the favorable safety profile of this implant in the present study population. Full article

Background/Objectives: This study aimed to characterize macro- and micro-morphological properties of the supraspinatus tendon (SST) in para swimmers during the chronic stage of rotator cuff tendinopathy, integrating ultrasound assessments of tendon thickness, peak spatial frequency radius (PSFR) for collagen organization, acromiohumeral distance (AHD), and occupation ratio to evaluate subacromial impingement risk. Methods: In a cross-sectional design, 43 elite para swimmers (aged 18–30 years, S7–S10 classes with lower extremity impairments) from Para Swimming Team Poland were divided into rotator cuff tendinopathy (RC; n = 22) and asymptomatic control (CON; n = 21) groups. Measurements on the dominant shoulder utilized B-mode ultrasound (Alpinion X-CUBE 90) to assess SST thickness at 5, 10, and 15 mm proximal to the greater tuberosity, PSFR via MATLAB-analyzed spatial frequency spectra, AHD, and occupation ratio. Two-way and one-way ANOVAs assessed group and measurement effects (p < 0.05); Pearson correlations examined the relationships between thickness and PSFR. Results: Para swimmers with tendinopathy exhibited greater SST thickness across sites (p < 0.001, η² = 0.63), higher PSFR at all intervals (p ≤ 0.009, η² = 0.53) peaking at 10 mm, wider AHD (p = 0.002, η² = 0.21), and lower occupation ratio (p < 0.001, η² = 0.44) versus controls. Strong positive correlations linked thickness and PSFR proximally (r = 0.75–0.79, p < 0.001). Conclusions: Chronic tendinopathy in para swimmers manifests as thickened SST with collagen disarray, altered subacromial space, and impingement risk, distinguishing pathological from healthy tendons. Integrated ultrasound metrics aid diagnosis and inform interventions for overhead athletes with locomotor disorders. Full article

Backgrounds/Objectives: Fatty infiltration (FI) of rotator cuff (RC) muscles is a critical prognostic factor after surgical repair. While the Goutallier–Fuchs grading system is widely used, its reproducibility is often debated. This study aimed to validate a previously reported three-dimensional (3D) magnetic resonance imaging (MRI)-based volumetric quantification method by comparing it with histologic findings in a chronic rotator cuff tear (RCT) rabbit model. Methods: Eighteen shoulders from nine rabbits were randomly assigned to three groups (n = 6 each): repair (A), chronic tear (B), and control (C). In groups A and B, a chronic RCT model was established by detaching the supraspinatus tendon, with group A receiving repair after six weeks. At 12 weeks after repair, 7.0T MRI was performed, and volumetric quantification of intra-muscular fat was performed using semi-automated 3D Slicer software. Histologic fat proportion was measured via Oil Red O staining and ImageJ analysis. Results: The muscle weight and MRI-based muscle volume were significantly lower in group B than group C (p < 0.05). The radiologically measured fat proportion was significantly higher in groups A (1.8 ± 0.8) and B (2.8 ± 0.7) compared to group C (0.5 ± 0.4, p < 0.001). Histologic analysis showed a corresponding pattern (3.0 ± 1.2%, 5.2 ± 1.0%, 1.7 ± 1.0% for groups A, B, and C, respectively; p < 0.001). A strong positive correlation was identified between the radiologic and histologic measurements of FI (r = 0.784, p < 0.001). Conclusions: Direct histologic comparison validates the reliability of 3D MRI-based volumetric quantification for evaluating FI of the RC muscle in a chronic RCT rabbit model. This objective approach may address the inherent limitations of the conventional qualitative grading system. Full article