Last updated: October 2026
What is the shoulder made of? Shoulder anatomy is three bones — humerus, scapula and clavicle — meeting at four joints, wrapped by four rotator cuff muscles. The main joint, the glenohumeral, is the most mobile joint in the human body.
Key takeaways
- Three bones, one socket: AAOS describes the shoulder as “a ball-and-socket joint made up of three bones” — the humerus, scapula and clavicle.
- The ball is far bigger than the socket: StatPearls puts the humeral head against the shallow glenoid fossa at “a 4:1 surface area ratio.”
- The labrum makes up the difference: AAOS states that it “deepens the socket by up to 50%.”
- Four muscles keep the ball centred. The cuff’s primary role is “to stabilize the glenohumeral joint through compression of the humeral head against the glenoid,” per StatPearls.
- The supraspinatus starts the lift and takes most of the damage: it abducts the shoulder for the first 0° to 15°, and AAOS says “most tears occur in the supraspinatus tendon.”
- The underside is the weak point. StatPearls notes the cuff covers the front, top and back of the joint “while leaving the inferior aspect relatively unsupported.”
- Nights are where the anatomy becomes audible: the clinical picture of impingement includes “nocturnal pain when lying on the” operated side.
Why does shoulder anatomy matter after shoulder surgery?
Because every instruction you are given afterwards is written about one named structure. AAOS describes rotator cuff repair as “re-attaching (stitching) the tendon back to its original site on the head of the humerus,” while a Bankart repair reattaches “the labrum and ligaments with suture anchors.” Different tissue, different rules, different sling.
The vocabulary also changes what you can ask at a follow-up. A restriction on external rotation reads differently once you know the infraspinatus and teres minor are the muscles that produce it. Our overview of shoulder surgery types sets out which procedure addresses which structure.
What 3 bones make up the shoulder?
Three, and only one ties the arm to the rest of the skeleton. AAOS lists the humerus (upper arm bone), the scapula (shoulder blade) and the clavicle (collarbone), with the head of the humerus resting “in a shallow socket in the shoulder blade.”
The scapula carries the landmarks surgeons name: the glenoid, which is the socket itself; the acromion, the bony roof above the joint and the surface an acromioplasty takes spurs from; and the coracoid process at the front, anchor for several ligaments. The clavicle is the bridge outward — StatPearls calls the sternoclavicular joint “the only synovial articulation between the upper limb and axial skeleton.” Everything else holding your arm on is soft tissue.
What are the 4 joints of the shoulder?
One does most of the moving and three support it. StatPearls calls the glenohumeral joint “the shoulder girdle’s major articulation” and groups the sternoclavicular, acromioclavicular and scapulothoracic joints as the “minor shoulder articulations.”
| Joint | What it connects | What it contributes | Published detail |
|---|---|---|---|
| Glenohumeral | Humeral head to the glenoid of the scapula | Most of the arm’s movement; the joint most surgeries address | “Exhibits the highest degree of mobility among all human joints” |
| Acromioclavicular | Acromion to the clavicle | Lets the scapula rotate as the arm goes overhead | Vertical stability comes from the coracoclavicular ligament — conoid plus trapezoid |
| Sternoclavicular | Clavicle to the sternum | The only synovial link from arm to torso | 35° of range in abduction, 70° front to back; injuries are uncommon |
| Scapulothoracic | Scapula gliding on the ribcage | Carries abduction past 90° | Scapular rotation driven by trapezius and serratus anterior |
Raising your arm is shared work, in a published sequence. StatPearls puts normal abduction at 150°, with the supraspinatus moving the shoulder through the first 0° to 15°, the deltoid’s middle fibres taking it from 15° to 90°, and scapular rotation enabling anything beyond 90°. Normal external rotation is 90° and internal rotation 70° to 90° — the two numbers post-op restrictions are written against.
What do the 4 rotator cuff muscles do?
They hold the ball centred while the arm moves. The four are easy to name with the mnemonic SITS — subscapularis, infraspinatus, teres minor, supraspinatus — and StatPearls states their job plainly: stabilising the joint “through compression of the humeral head against the glenoid.”
| Muscle | Origin on the scapula | Insertion on the humerus | Primary job | Nerve |
|---|---|---|---|---|
| Supraspinatus | Supraspinous fossa | Greater tuberosity | Initiates abduction, 0° to 15° | Suprascapular (C5–C6) |
| Infraspinatus | Infraspinous fossa | Greater tuberosity, just below the supraspinatus | Powerful external rotation | Suprascapular (C5–C6) |
| Teres minor | Lateral border, below the infraglenoid tubercle | Greater tuberosity, below the infraspinatus | External rotation | Axillary (C5–C6) |
| Subscapularis | Subscapular fossa | Lesser tubercle | Internal rotation; resists forward displacement of the humerus in abduction | Subscapular (C5–C7) |
Two details in that table explain a lot of post-op caution. The cuff tendons blend into the capsule around the front, top and back of the joint but leave the underside relatively bare, which StatPearls links to the risk of inferior dislocation. And the supraspinatus is “the only rotator cuff muscle not primarily responsible for humeral rotation” — it starts the lift, so losing it shows up as trouble raising the arm rather than turning it. Our page on rotator cuff function takes the four one at a time.
What do the labrum, capsule and bursae add to shoulder anatomy?
They turn a shallow dish into a working socket. AAOS: “The head of the upper arm bone is much larger than the socket, and the labrum surrounds the socket to help stabilize the joint. The labrum deepens the socket by up to 50%.” StatPearls adds that the labrum is continuous with the long head of the biceps tendon at its upper edge — why a tear at the top of the socket can involve the biceps.
The capsule is the sleeve around all of it: a fibrous sheath running from the anatomical neck of the humerus to the rim of the glenoid, lined by a synovial membrane that secretes lubricating fluid. Inside it sit thickened bands, the glenohumeral ligaments, which “become taut at varying degrees of abduction and humeral rotation.” Which ligament restrains the joint therefore depends on where your arm is — including where it is overnight.
Bursae are the friction pads. StatPearls names the subacromial (subdeltoid), subcoracoid and subscapular bursae, with the subacromial lying between the deltoid and the capsule, superficial to the supraspinatus tendon. AAOS describes the same structure from the patient’s side: “a lubricating sac called a bursa between the rotator cuff and the bone on top of the shoulder (acromion),” which lets the tendons glide and can itself become inflamed. Our labrum anatomy page covers the rim in more detail.
How does shoulder anatomy map onto the common injuries?
Each structure fails in a characteristic way, which is why a diagnosis names a part rather than a region.
| Structure | What typically goes wrong | Source note |
|---|---|---|
| Supraspinatus tendon | The usual site of a rotator cuff tear | “Most tears occur in the supraspinatus tendon” (AAOS) |
| Cuff and bursa under the acromion | External impingement, with subacromial bursitis | Compression of the cuff “by extrinsic structures, including the acromion” (StatPearls) |
| Labrum, upper edge | SLAP lesion; may involve the biceps origin | “Often seen in athletes due to repetitive stress on the biceps tendon” (AAOS) |
| Labrum, front and bottom | Bankart lesion, usually after a forward dislocation | AAOS |
| Labrum, back of the socket | Posterior labral tear — traumatic or wear-related | AAOS |
| Sternoclavicular joint | Dislocation, uncommon; posterior ones are a surgical emergency | StatPearls |
Most cuff tears are not a single accident. AAOS attributes the majority to wear “that occurs slowly over time,” notes they are more common in the dominant arm, and says tendon blood supply decreases with age, impairing repair. The volume is substantial: “almost 2 million people in the U.S. visit their doctors because of rotator cuff tears” each year.
The anatomy also predicts when it hurts. StatPearls describes the impingement picture as “a painful arc during flexion and abduction from 60° to 120°, along with nocturnal pain when lying on the” operated side — the anatomical reason the night half of this subject exists. We cover the forces involved in sleep position biomechanics.
How does the repaired structure shape the sling?
It sets both the length of the immobilisation and the movement that is barred. The figures below are what AAOS publishes for each repair; your own protocol governs.
| Procedure | Structure repaired | Sling, per AAOS | Other published limits |
|---|---|---|---|
| Rotator cuff repair | Tendon back onto the head of the humerus | “The first 4 to 6 weeks,” arm largely unused | Passive motion usually begins within those weeks; strengthening at 8 to 12 weeks |
| Labrum repair (Bankart or posterior) | Labrum and ligaments, with suture anchors | “2 to 6 weeks, depending on the type of repair” | Often nothing heavier than 5 pounds for up to 12 weeks |
| SLAP repair | Upper labrum, sometimes the biceps origin | 2 to 6 weeks | Sport-specific exercises usually from 12 weeks |
Both AAOS pages put the end of recovery in months. After cuff repair, “most patients have a functional range of motion and adequate strength by 4 to 6 months”; after labrum repair, the shoulder “should be fully healed in 4 to 6 months.”
One surgical detail explains why external rotation limits appear so often. StatPearls describes the rotator cuff interval — the gap between the supraspinatus and subscapularis tendons — and notes that closing its lateral portion restricts external rotation of the arm held at the side, so the closure is made with the shoulder turned outwards to protect the range of motion that follows. Where that lands week by week is in our rotator cuff sleep phases, labrum repair types, shoulder replacement 12-week plan and summary of 11 sling protocols.
Methodology: this article is assembled from the AAOS OrthoInfo and NIH StatPearls pages linked above and the makers’ production specs for the pillows named. It is general information, not medical advice, and no claim here rests on examining any individual shoulder. Where anything differs from your own surgeon’s instructions, follow theirs.
Frequently asked questions
What are the three bones of the shoulder?
The humerus, the scapula and the clavicle. AAOS describes the shoulder as a ball-and-socket joint made of those three, with the head of the humerus sitting in a shallow socket on the scapula called the glenoid.
Why does the shoulder dislocate so easily?
Because the socket is small and the capsule is loose. StatPearls attributes the joint’s high mobility and its “susceptibility to dislocation” to the loose capsule and the humeral head’s size relative to the shallow glenoid — a 4:1 surface area ratio.
What does the rotator cuff actually do?
It compresses the humeral head into the socket so the arm can move without the ball sliding off. StatPearls describes coordinated contraction of the four muscles as what “maintains humeral head centering within the glenoid during shoulder motion.”
Which rotator cuff muscle tears most often?
The supraspinatus. AAOS says “most tears occur in the supraspinatus tendon,” and StatPearls notes that tendon is “uniquely susceptible to compressive forces from subacromial impingement.” No source reviewed here gives a reliable percentage, so none is quoted.
What is the difference between the labrum and the rotator cuff?
The labrum is cartilage, the cuff is muscle and tendon. AAOS calls the labrum “a rim of cartilage tissue” that deepens the socket and anchors ligaments and a biceps tendon; the cuff is four muscles running from the scapula to the humerus.
Why does shoulder pain get worse at night?
Position is part of it. StatPearls lists “nocturnal pain when lying on the” operated side among the typical findings in impingement, alongside a painful arc from 60° to 120°. Our night pain timeline covers what published cohorts report after surgery.
How long is the sling after a rotator cuff repair?
AAOS says most patients use a sling and avoid using the arm for the first 4 to 6 weeks, and that the length depends on “the severity of your injury, your tissue quality, and the type of repair you had.” After labrum repair the published range is 2 to 6 weeks.
Where shoulder anatomy meets the night
Knowing the parts changes what you can read. A restriction makes sense once the structure behind it has a name, and the figures above — 0° to 15° of supraspinatus abduction, a 4:1 socket ratio, an underside with no cuff over it — are why AAOS describes the arm as largely unused at first.
Once you are lying down that becomes a positioning problem: an incline that keeps the shoulder from dropping backwards, and a firm support under the forearm so the arm is not hanging from a strap all night. The Flexicomfort Shoulder Pillow measures 56 × 40 × 15 cm and carries two adjustable straps for that second job. Our reclined sleeping setup guide covers the arrangement as a whole.
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Flexicomfort Shoulder Pillow
56 × 40 × 15 cm arm-abduction sling wedge with two adjustable straps.
See the Flexicomfort Shoulder Pillow on AmazonProducts mentioned in this guide
Aeris Wedge Pillow — 65 × 65 cm base, 22 cm apex, for the reclined position these protocols describe