Shoulder

What it is

A ball-and-socket joint between the head of the humerus and the glenoid cavity of the scapula.

The trade it makes is the whole story. The head of the humerus is large; the glenoid is small and shallow — it covers only about a third of the head at any moment. A deep socket would hold the ball securely and permit little movement. A shallow one permits enormous movement and holds nothing.

What holds it together instead is soft tissue: the rotator cuff compressing the head into the socket, the joint capsule and its ligaments, and the glenoid labrum, a rim of fibrocartilage deepening the socket a little.

The shoulder is really a complex of joints working together — this one, the acromioclavicular and sternoclavicular joints, and the scapula gliding on the ribcage. Raising your arm overhead uses all of them.

What it does

Moves in every direction: flexion, extension, abduction, adduction, internal and external rotation, and circumduction. No other joint in the body has the range.

Full elevation of the arm is not this joint alone. Roughly two-thirds of the movement happens here and the remaining third comes from the scapula rotating on the ribcage — a relationship called scapulohumeral rhythm. The commonly quoted 2:1 ratio is an average that varies through range and between people.

What loads it

Every upper-body pressing, pulling, carrying and throwing movement.

What is commonly got wrong about it

"Shoulder impingement" as a settled mechanical diagnosis. The traditional model — bone pinching tendon, treated by shaving the bone — was substantially undermined when the CSAW trial compared subacromial decompression against a placebo operation and against no treatment, and found no advantage for the decompression itself.¹ Many clinicians have abandoned the term.

That scans explain shoulder pain. Rotator cuff abnormality is present in around 10% of people aged 20 and under and 62% of those over 80, at a similar rate whether the shoulder is painful or not.²

That the shoulder is inherently unstable and needs protecting. It is the least bony-stable joint in the body and it functions perfectly well for most people for a lifetime.

"Shoulder joint" as one joint. Four articulations contribute to moving your arm.

Sources

1. Beard DJ, Rees JL, Cook JA, et al. Arthroscopic subacromial decompression for subacromial shoulder pain (CSAW): a multicentre, pragmatic, parallel group, placebo-controlled, three-group, randomised surgical trial. The Lancet 2018; 391(10118):329–338. doi:10.1016/S0140-6736(17)32457-1 PMID 29169668 313 patients across 32 UK hospitals. Decompression showed no benefit over arthroscopy-only placebo (mean difference −1.3 points on the Oxford Shoulder Score, 95% CI −3.9 to 1.3). Both surgical groups beat no treatment by a margin the authors judged not clinically important.
2. Teunis T, Lubberts B, Reilly BT, Ring D. A systematic review and pooled analysis of the prevalence of rotator cuff disease with increasing age. Journal of Shoulder and Elbow Surgery 2014; 23(12):1913–1921. doi:10.1016/j.jse.2014.08.001 PMID 25441568 30 studies, 6,112 shoulders. Rotator cuff abnormality rose from 9.7% in people aged 20 and under to 62% in those 80 and over, with a similar rise regardless of symptoms. The authors conclude cuff degeneration is common enough to be considered a normal aspect of ageing.

Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.