Musculoskeletal System
Rotator cuff tears: supraspinatus and weak initiation of abduction
Rotator cuff tears damage any of four muscles that hold the humerus in its shoulder socket; a tear of supraspinatus, the cuff muscle that starts abduction, classically weakens that movement.
Remember
Overhead work, pain at the front and outer side of the shoulder and a weak lift out to the side suggest a rotator cuff tear, most often of supraspinatus, the cuff muscle that starts abduction.
The rotator cuff is a group of four small muscles: supraspinatus, infraspinatus, teres minor and subscapularis. Their tendons (the tough cords that join muscle to bone) wrap around the head of the humerus, the ball of the shoulder joint, and keep it centred in its socket. A tear is damage to one of these muscles or their tendons. Infraspinatus and teres minor turn the arm outwards, and subscapularis turns it inwards.
Supraspinatus is the cuff muscle that tears most often. Its tendon runs under the acromion (the bony point at the top of the shoulder), so years of repeated overhead work can wear it down. It also starts abduction, lifting the arm out to the side, and produces roughly the first 0 to 15 degrees. The middle fibres of deltoid then take over for the next 15 to 90 degrees. When a supraspinatus tear does cause symptoms, it therefore makes the first part of the movement weak and painful, so brushing hair becomes difficult.
A typical story is a person whose work involves repeated overhead activity, such as painting ceilings, with months of anterolateral shoulder pain (felt at the front and outer side of the shoulder) that can spread down the arm. Smoking is a further risk factor, as is a family history of cuff tears. Many tears cause no symptoms at all.
Management usually combines pain relief with physiotherapy. Surgery is an option for large acute tears, or for young people with full-thickness tears (tears that go right through the tendon).
Mnemonic
SITS
- S Supraspinatus
- I Infraspinatus
- T Teres minor
- S Subscapularis
Use the four letters to list the four rotator cuff muscles.
Key facts
- Torn most often
- supraspinatus, the cuff muscle that starts abduction.
- Start of abduction
- supraspinatus produces about the first 0 to 15 degrees of abduction, then the middle fibres of deltoid take over.
- Typical features
- anterolateral shoulder pain radiating down the arm, with weakness of abduction.
- Risk factors
- repetitive overhead activity, smoking and a family history of cuff tears.
- Weak turning outwards
- points to infraspinatus or teres minor rather than supraspinatus.
- Weak turning inwards
- points to subscapularis.
- Symptoms
- many tears cause no symptoms at all.
Exam traps
- Infraspinatus is a genuine cuff muscle that can tear, but it turns the arm outwards, so a tear weakens that movement and not the start of abduction. It also tears less often than supraspinatus.
- Deltoid does abduct the arm, but it is not one of the four cuff muscles, and it takes over only after supraspinatus has started the movement.
- Subscapularis is a cuff muscle, but it turns the arm inwards, so a tear weakens inward turning and not abduction. It is also not the muscle most commonly torn.
- Adhesive capsulitis (frozen shoulder) is a thickened, scarred joint capsule (the sleeve of tissue around the joint), so the main problem is loss of movement and not reduced power when lifting the arm.
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