Supraspinatus occupation ratio

The supraspinatus occupation ratio is used to classify supraspinatus muscle atrophy and fatty degeneration, predict prognosis, and guide surgical repair.

Usage

The supraspinatus occupation ratio is one method used to assess the degree of volume loss of the supraspinatus muscle. Similar methods, such as the tangent sign and Goutallier classification, assist in assessing the degree of degeneration for pre-therapeutic work-up.

Similar to the Goutallier classification, which determines the amount of fatty infiltration and muscle belly atrophy of rotator cuff muscles ¹, the supraspinatus occupation ratio is the cross-sectional area of the supraspinatus muscle compared to the supraspinous fossa to grade the severity of muscle atrophy ².

Classification

Supraspinatus occupation ratio is assessed on the oblique view of the supraspinous fossa on T1WI sequences as a Y-view to visualize the bony landmarks of the scapular spine, coracoid process and distal clavicle ⁴.

• grade 1 (minimal to mild atrophy): >60% occupation

• grade 2 (moderate atrophy): 60-40% occupation

• grade 3 (severe atrophy): <40% occupation

Grades 1 and 2 suggest a higher likelihood of problem free and reliable direct repair and grade 3 demonstrating surgically challenging and potentially unreliable repair with direct suturing ⁵.

History and etymology

First described in MRI of the shoulder in 1996 by Thomazeau et al. ³, this technique is reproducible through oblique-sagittal projections across the scapula and through the medial border of the coracoid process such that the supraspinatus fossa is in plane ³. Further studies have demonstrated the application of the Thomazeau et al. method of measuring supraspinatus occupation ratio through the use of ultrasound ⁴.

Practical points

Rotator cuff tears are common injuries to the shoulder and lead to muscle atrophy and fatty degeneration ⁵. Fatty degeneration refers to intramuscular fatty accumulation and muscle atrophy refers to a decrease in volume of muscle belly which is a negative prognostic factor that influences decision making surrounding surgical repair options ⁵.

See also

• tangent sign

• Goutallier classification