The Normal Shoulder Capsule
A loose elastic sleeve lets the humeral head glide freely.
- 1–2 mm: Resting thickness (thin fibrous sleeve)
- ~90°: External rotation (full range)
- Loose: Axillary fold (redundant tissue)
- Normal: Synovial fluid (low viscosity)
A sleeve built for glide
The capsule is a slack, redundant envelope.
The axillary pouch
A loose lower fold unfolds during arm elevation.
Collagen at rest
Fibers run in loose parallel bundles, not cross-linked.
Synovial Inflammation Begins
Cytokines flood the capsule lining and trigger swelling.
- +40%: Synovial hypertrophy (lining thickness)
- TGF-β, IL-1: Key cytokines (drive fibrosis)
- Night pain: Pain onset (classic early sign)
- ~15%: Rotation loss (external rotation)
Cytokine trigger
Inflammatory mediators recruit fibroblasts into the capsule.
Vascular reaction
New capillaries appear beneath the synovial layer.
Painful arc
Motion becomes painful before it becomes limited.
Progressive Collagen Deposition
Disorganized type III collagen thickens the capsule wall.
- 4 mm+: Wall thickness (triples baseline)
- III → I: Collagen type shift (stiffer matrix)
- Elevated: Myofibroblasts (contractile cells appear)
- ~40%: Rotation loss (external rotation)
Matrix remodeling
Fibroblasts convert to contractile myofibroblasts.
Wall stiffening
Cross-linked collagen replaces elastic tissue.
Stiffness before pain fades
Motion loss now outpaces the original pain.
Adhesion Across the Axillary Fold
Capsule surfaces stick together, erasing the loose pouch.
- ~50%: Adhesion density (of fold surface)
- Obliterated: Axillary fold (redundancy lost)
- Contracted: Rotator interval (thickened, taut)
- ~65%: Rotation loss (external rotation)
Self-adhesion
Opposing capsule folds fuse into scar bridges.
Pouch collapse
The loose lower recess flattens and locks shut.
Rotator interval involvement
Coracohumeral ligament thickens and tightens further.
Frozen Shoulder — Contracted Capsule
A thick, adherent capsule severely restricts humeral glide.
- 7–8 mm: Wall thickness (5–6× baseline)
- <10 mL: Capsule volume (vs 20–30 mL normal)
- >70%: Global motion loss (all planes)
- 12–24 mo: Course (to spontaneous thaw)
Global stiffness
Every plane of motion is now restricted, not just rotation.
Capsule volume loss
Arthrographic volume falls to a third of normal.
Slow natural course
Freezing, frozen, and thawing phases span years.