Assessing the supraspinatus-to-trapezius cross-sectional thickness ratio as a quantitative classification for rotator cuff injury

INTRODUCTION

Rotator cuff injuries affect up to 18% of the UK population annually,^[1] with supraspinatus tendon tears being the most common.^[2] These injuries often lead to shoulder pain, functional limitation, and reduced quality of life. Chronic tears are frequently associated with muscle atrophy and fatty infiltration,^[3] both of which negatively impact surgical repair outcomes and post-operative recovery.^[3,4]

At present, the most widely used risk stratification tool is the Goutallier classification.^[5] [Figure 1], assessing fatty infiltrates in magnetic resonance imaging (MRI). While this tool offers aid in classifying rotator cuff injuries, it remains subjective due to the absence of truly quantitative thresholds, limiting its applicability. The inter- and intra-observer variability in classification is well-documented, particularly in differentiating adjacent grades.^[6] Furthermore, the Goutallier classification relies on MRI for assessment, which adds cost^[7] and time for classification^[8] and poses issues when considering lower-resource settings.^[9]

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Figure 1:

Schematic showing supraspinatus-to-trapezius ratio, A:B.

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Given these challenges, there is growing interest in developing a more accessible and quantitative method for assessing rotator cuff muscle fatty atrophy. One such approach is the measurement of the supraspinatus-to-trapezius (SS:TRAP) muscle thickness ratio. The trapezius muscle, unaffected in isolated rotator cuff pathology and easily visualized in the same imaging plane, serves as a stable internal reference. Importantly, this ratio can potentially be assessed through ultrasound, which is widely available, cost-effective, and non-invasive.^[10,11]

This study evaluates the correlation between the SS:TRAP ratio measured on MRI and the established Goutallier grades. Demonstrating such a relationship would support the SS:TRAP ratio as a surrogate marker for muscle quality and pave the way for developing ultrasound-based diagnostic tools for rotator cuff assessment.

MATERIALS AND METHODS

Image analysis

All MRIs were performed using standardized procedures for shoulder imaging. Coronal sections at the spinoglenoid notch were obtained and used as the reference for all data extraction. This section was chosen due to its ability to display the muscle bulk of the supraspinatus and due to the orientation, which can be replicated using ultrasound images.

To calculate the maximum thickness of both the supraspinatus and trapezius muscles, a digital tracing tool was used to increase consistency and clinical applicability. All measurements were recorded from the same coronal proton density (PD) view. For each patient, the maximum linear muscle thickness was measured perpendicular to the long axis of each muscle using digital calipers. The SS:TRAP ratio was calculated as: Maximum thickness of supraspinatus/maximum thickness of trapezius [Figures 1-3]. Two fellowship-trained musculoskeletal radiologists with 1 year and over 10 years of experience, respectively, performed the measurements.

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Figure 2:

Examples (a-c) of supraspinatus-to-trapezius ratio on magnetic resonance imaging.

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Figure 3:

Longitudinal ultrasound image of the shoulder showing supraspinatus-to-trapezius ratio.

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For each subject, Goutallier grade was assessed using the standard 5-point scale [Figures 4 and 5] by two fellowship-trained musculoskeletal radiologists with 1 year and over 10 years’ experience, with consensus to resolve conflicts. Grading was done from the same coronal images from SS:TRAP was assessed to minimize confounding factors.

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Figure 4:

Goutallier classification. ISP: Infraspinatus, SSP: Supraspinatus, SUB: Subscapularis

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Figure 5:

Schematic of Goutallier classification.

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RESULTS

This study had a cohort of 150 patients. The 150 patients were divided across the Goutallier grades I–V in populations of 70, 33, 32, 6, and 9, respectively [Figure 6].

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Figure 6:

Demographics of various grades of Goutallier.

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The mean SS:TRAP ratio for each Goutallier grade was also calculated at 2.46, 2.09, 1.25, 1.19, and 0.92, respectively [Figure 3]. While a general decreasing trend in SS:TRAP ratio was observed from Grade I to III and again from Grade IV to V, the value for Grade IV was unexpectedly elevated. This anomaly is likely due to the very small sample size (n = 6) and large standard deviation (SD = 2.13) in this group.

One-way ANOVA analysis of variance was conducted to assess whether there were statistically significant differences in the calculated mean SS:TRAP ratios between the five Goutallier classes. This testing resulted in an F ratio = 18.74 (P < 0.00001). Since the results are considered statistically significant at P < 0.05, this testing highlights that there is a significant difference between the mean SS:TRAPs across the five Goutallier grades. Further studies with larger samples in higher grades should include post hoc testing to clarify specific group differences. The mean SS:TRAPs and the associated SDs have been further demonstrated in a graphical view [Figure 7 and Table 1].

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Figure 7:

Mean supraspinatus-to-trapezius ratio and standard deviations in each Goutallier Grade.

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DISCUSSION

Rotator cuff tears are a common and disabling condition, often associated with muscle atrophy and fatty infiltration. The Goutallier grading system, while widely used for prognostication and treatment planning, is limited by its subjectivity, variability, and dependence on MRI – a resource-intensive modality. This score was calculated by assessing fatty infiltration to show muscle fatty infiltration as measured on an MRI scan. Due to the technological logistics and associated costs of MRI, this limits the utility of the grading system, hence the need for a simpler and cheaper method. Coinciding with fatty infiltration, muscle bulk atrophy is a common finding in tears and is a measurement that can be taken on ultrasound, providing a potential solution to the issue. This offers promise to reduce time to diagnosis, prompting quick and effective treatment to improve patient outcomes. Due to the wider access to ultrasonography compared to MRI, this allows effective injury classification in resource-limited settings.

As demonstrated in the results section and confirmed by the ANOVA testing, there is a significant difference in the mean SS:TRAP ratios between different Goutallier grades. This shows the correlation between the cross-sectional areas of supraspinatus and trapezius as a proxy for Goutallier grade. The observed trend showed a general decline in SS:TRAP ratio with increasing Goutallier grade, aligning with expected muscle atrophy.

In the context of the diagnosis of rotator cuff muscle injuries, ultrasound offers many advantages compared to conventional MRI. Most notably, ultrasonography is significantly cheaper than MRI and is therefore an appropriate imaging modality in resource-limited settings. This improves wider access to healthcare and reduces health disparities in scarce situations. Second to this is the access to ultrasonography compared to MRI. Most healthcare settings, including primary care, have access to ultrasonography as cost and spatial logistics can both be accommodated in a range of settings. Conversely, MRI often requires large separate rooms, precise temperature control, and environmental homeostasis.

Importantly, while all measurements in this study were obtained from MRI, the rationale for this approach was to simulate a setting where ultrasound could later be applied. The choice of the spinoglenoid notch as the measurement plane allows for future reproducibility using sonographic techniques, which are more accessible and cost-effective in resource-limited environments. However, clinical validation using ultrasound is necessary before such claims can be confirmed. Due to the aforementioned ease of ultrasound in comparison to MRI, this offers both the routine screening of patients with shoulder injury as well as the monitoring of muscle atrophy, rehabilitation status, and surgical success to inform prognosis and future interventions. These benefits will reduce the time to diagnosis, consequently improving patient outcomes. The wider access to this new diagnostic model will also reduce local and global health inequalities through better prognostic information.

MRI still remains superior compared to ultrasonography when considering fat infiltration. Due to the technical differences between the two modalities is limited in fat detection, and must be taken relative to the deltoid muscle, so it can only be used as a proxy for Goutallier grade.

Despite these limitations, ultrasonography to assess SS:TRAP clearly remains a useful tool as a proxy for Goutallier grade, reducing resource burdens, better access to diagnostic tools, and ultimately reducing time to diagnosis with the ultimate goal of improving patient care.

CONCLUSION

This study demonstrates a statistically significant correlation between the SS:TRAP muscle thickness ratio measured on MRI and the Goutallier grade of fatty infiltration in the supraspinatus muscle. The SS:TRAP ratio offers a simple, quantifiable, and potentially reproducible marker of muscle atrophy, which may address the limitations of the subjective and MRI-dependent Goutallier classification.

While the measurements in this study were obtained using MRI, the SS:TRAP ratio was selected with the intent of future translatability to ultrasonography, which could offer a faster, more accessible, and cost-effective alternative for rotator cuff assessment – particularly in resource-constrained settings.

To validate this approach, prospective studies using ultrasound should be undertaken to define diagnostic thresholds, evaluate interobserver reliability, and assess clinical utility across different stages of rotator cuff pathology. If validated, the SS:TRAP ratio could serve as a practical tool for risk stratification, treatment planning, and monitoring of muscle recovery in rotator cuff injuries.