A visual marker for early atrophy of the supraspinatus muscle on conventional MRI: introduction of the blackbird sign

Abstract

Objectives: The aim of this study was to introduce the blackbird sign as a fast, qualitative measure of early supraspinatus (SSP) muscle atrophy and to correlate the sign with quantitatively assessed muscle volume and intramuscular fat fraction (FF) in patients with full-thickness SSP tears.

Materials and methods: The blackbird sign describes the asymmetric pattern of early SSP atrophy: on sagittal MR images, the supero-posterior contour of the muscle becomes concave, resembling the shape of a blackbird. MRIs of patients with full-thickness SSP tears were retrospectively reviewed for the presence of the blackbird and tangent signs. Patients were then divided into group 1: negative tangent sign and negative blackbird sign (n = 67), group 2: negative tangent sign and positive blackbird sign (n = 31), and group 3: positive tangent sign (n = 32). A 2-point Dixon sequence was acquired in all patients from which quantitative FF and muscle volumes were calculated.

Results: In total 130 patients (mean age 67 ± 11 years) were included. Mean SSP volume was significantly smaller in group 3 (15.8 ± 8.1 cm³) compared to group 2 (23.9 ± 7.0 cm³, p = 0.01) and group 1 (29.7 ± 9.1 cm³, p < 0.01). Significantly lower muscle volumes were also found in group 2 compared to group 1 (p = 0.02), confirming that the blackbird sign is able to identify early SSP atrophy. Mean FF in the SSP was significantly higher in group 3 (18.5 ± 4.4%) compared to group 2 (10.9 ± 4.7%, p < 0.01) and group 1 (6.1 ± 2.6%, p < 0.01).

Conclusion: Visual assessment of early muscle atrophy of the SSP is feasible and reproducible using the blackbird sign, allowing the diagnosis of early SSP atrophy.

Clinical relevance statement: In routine clinical practice, the blackbird sign may be a useful tool for assessing early muscle degeneration before the risk of postoperative rotator cuff re-tears increases with progressive muscle atrophy and fatty infiltration.

Key points: Quantitative measurements of rotator cuff injuries require time, limiting clinical practicality. The proposed blackbird sign is able to identify early SSP atrophy. Reader agreement for the blackbird sign was substantial, demonstrating reproducibility and ease of implementation in the clinical routine.

Keywords: Magnetic resonance imaging; Muscular atrophy; Rotator cuff.

Fig. 1

Patient selection flow chart. Patients with surgically confirmed supraspinatus (SSP) tears were divided into three groups according to the results of the MR-based assessments. Group 1: patients with a negative tangent sign and negative blackbird sign (n = 67). Group 2: patients with positive blackbird sign but a negative tangent sign, i.e., early SSP muscle atrophy (n = 31). Group 3: patients with a positive tangent sign, i.e., advanced SSP muscle atrophy (n = 32)

Fig. 2

Sagittal oblique T1-weighted images of a 59-year-old patient with early SSP muscle atrophy resulting in a positive blackbird sign (A, B). The blackbird sign is assessed at the centre of the SSP muscle. The atrophy of the supero–posterior SSP muscle subregion creates an inwardly curved muscle contour, giving the impression of an anterior-facing blackbird. In contrast, the medial and inferior SSP muscle subregions appear unchanged, while the atrophied region is filled with fatty tissue (asterisk)

Fig. 3

Sagittal oblique T1-weighted images of one patient from each group at the level of the middle of the SSP muscle belly (A–C) with their corresponding coronal fat-saturated proton density-weighted images showing the full-thickness SSP tears in the lower row (D–F). The normal convex shape of the SSP muscle is readily visible in the patient in group 1 (A) compared to the patient in group 2 with a positive blackbird sign (B) and the patient in group 3 with advanced muscle atrophy (C). Note also the advanced fatty SSP muscle infiltration and greater tendon retraction in the patient in group 3 (C, F)

Fig. 4

Multiple sagittal oblique T1-weighted images from one patient in each group, arranged from lateral (left) to medial (right). Group 1: normal SSP muscle. Group 2: early SSP muscle atrophy, i.e., positive blackbird sign (outline arrow). Group 3: advanced muscle atrophy, i.e., positive tangent sign. Note, that the change in the supero–posterior contour of the superior SSP muscle is due to early atrophy in the patient in group 2 with a positive blackbird sign. In addition, the patient with a positive tangent sign in group 3 also shows a substantial increase in fat infiltration compared to groups 1 and 2

Fig. 5

Sagittal oblique FF map of a 55-year-old patient calculated from the 2-point Dixon sequence (A). Semi-automated segmentation was performed by drawing regions of interest (ROIs) on every five slices around the SSP muscle (B). C Shows the 3D model of the segmented SSP muscle (red)

Fig. 6

Box plots of SSP muscle volume (A) and intramuscular FF (B) calculated from the quantitative 2-point Dixon sequence for each group. Bonferroni adjusted intergroup assessment revealed significant differences between each group for the muscle volume, as well as the intramuscular FF (asterisks)