Joints

Part 1 · General anatomy

How joints are classified, what makes them stable, and what stops them moving

The trade-off that defines every joint

Figure 1 · The stability and mobility trade-off

Every joint in the body is a solution to the same engineering problem, and every joint solves it differently:

Mobility and stability are inversely related. A joint that moves freely is easy to displace; a joint that is hard to displace does not move much.

The shoulder chose mobility and pays for it with dislocation. The hip chose stability and pays for it with a restricted range that the lumbar spine must compensate for. The sacroiliac joint chose stability almost entirely. The knee, uniquely, was asked for both — a large sagittal range with weight-bearing stability in a joint with almost no bony congruence — and the price is the highest ligament and meniscal injury burden in the body.

When you assess any joint, you are asking where on that spectrum it sits, and whether disease or injury has moved it.

Learning outcomes

By the end of this chapter you should be able to:

• Classify joints structurally and functionally, and give examples of each subtype.
• Describe the fibrous, cartilaginous and synovial classes in detail, with examples and clinical relevance.
• List and describe the essential and accessory features of a synovial joint.
• Describe the synovial membrane, its two cell types, and the composition and function of synovial fluid.
• Classify synovial joints by articular surface geometry and state the degrees of freedom of each.
• List the factors contributing to joint stability and rank their relative importance at named joints.
• State Hilton’s law and describe the four types of articular receptor and their rehabilitation significance.
• Define close-packed, loose-packed and resting positions, and the capsular pattern, and use them clinically.
• Apply arthrokinematic principles and the convex–concave rule to mobilisation.
• Distinguish laxity from instability, and describe the assessment of generalised hypermobility.
• Describe the principal joint pathologies and the mechanisms of joint contracture.

Classification

Figure 2 · Classification of joints by structure

Two classifications run in parallel. Learn both, and learn how they map onto each other.

4.2.1 Structural classification — by the material uniting the bones

4.2.2 Functional classification — by the range of movement permitted

The mapping is not perfect — the interosseous membrane of the forearm is a fibrous syndesmosis but permits substantial movement, and the first sternocostal joint is a synchondrosis that behaves as a synarthrosis. State the structural class first and the functional consequence second.

Fibrous joints

Cartilaginous joints

Clinical note — the pubic symphysis in pregnancy. Relaxin and progesterone increase symphyseal and sacroiliac laxity, allowing physiological widening (normally up to ~10 mm; beyond this suggests symphyseal diastasis). Pregnancy-related pelvic girdle pain is best managed with load-management, pelvic-floor and gluteal training and, where indicated, a pelvic belt — not with rest.

Synovial joints

Figure 3 · The structure of a synovial joint

Figure 4 · The six types of synovial joint

4.5.1 Essential features

Every synovial joint has all five:

• Articular cartilage — hyaline (fibrocartilage at the sternoclavicular, acromioclavicular and temporomandibular joints, which develop in membrane). Avascular, aneural, no perichondrium (Chapter 3).
• Articular (joint) capsule — two layers:
• Outer fibrous layer: dense irregular connective tissue attached to the periosteum beyond the articular margins. Locally thickened into capsular ligaments. Poorly vascular but richly innervated — it is a principal source of joint pain and the source of proprioceptive afference.
• Inner synovial membrane: lines everything inside the capsule except the articular cartilage, menisci and discs.
• Joint (synovial) cavity — a potential space containing a thin film of fluid.
• Synovial fluid.
• Ligaments — capsular (thickenings of the capsule, e.g. glenohumeral ligaments), extracapsular (e.g. fibular collateral ligament of the knee), or intracapsular (e.g. cruciates, which are intracapsular but extrasynovial).

4.5.2 Accessory features (present in some joints)

4.5.3 The synovial membrane and synovial fluid

The synovium is a specialised connective tissue lining, 1–3 cells deep, with no basement membrane and no tight junctions — it is a discontinuous layer, not an epithelium, which is why it is freely permeable to solutes.

Beneath lies the subintima — areolar, adipose or fibrous — which carries a rich capillary network, lymphatics and nerve endings. Synovium is one of the most vascular tissues in the body relative to its mass, which is why it is a target in systemic inflammatory disease and why haemarthrosis follows even minor synovial injury.

Synovial fluid is a dialysate of plasma with added hyaluronan and lubricin.

The thixotropic property has a direct clinical meaning: fluid is thick and resistant at rest, and thins as movement speeds up. That is the physiological basis of morning stiffness and of the “warming up” of an arthritic joint, and part of the rationale for gentle early-range movement before loaded activity.

Functions of synovial fluid: lubrication, nutrition of articular cartilage and menisci, shock absorption, and waste removal.

Synovial fluid analysis — worth recognising

A hot, exquisitely painful joint with fever and a refusal to move is a septic arthritis until proved otherwise — a surgical emergency, not a physiotherapy referral. Do not mobilise; escalate.

4.5.4 Classification of synovial joints by surface geometry

Two joints resist classification and are worth naming: the knee is a modified hinge (bicondylar) with obligatory axial rotation, and the temporomandibular joint is a modified hinge with both rotation and translation, divided by a disc.

Joint stability

Figure 5 · What stops a joint moving further

Stability is never a single structure. Rank the contributors for the joint in front of you.

Concavity–compression, and why the rotator cuff matters

The glenoid fossa accommodates only about a third of the humeral head, and its passive ligaments are lax through most of the mid-range. Stability there is produced by concavity–compression: the rotator cuff compresses the humeral head into the labrum-deepened glenoid concavity, generating a stabilising force proportional to the compressive load. It follows that (a) shoulder stability is dynamic and trainable, (b) cuff weakness or inhibition produces instability without any structural lesion, and (c) rehabilitation of the unstable shoulder is largely a matter of restoring cuff and scapular control. This is a rare instance where the anatomy directly prescribes the treatment.

Laxity versus instability

• Laxity = increased passive accessory movement on examination. It may be entirely normal for that person.
• Instability = laxity plus symptoms — giving way, apprehension, recurrent subluxation, functional failure.

Generalised joint hypermobility is screened with the Beighton score (0–9): passive fifth MCP extension >90° (1 each side), thumb to forearm (1 each side), elbow hyperextension >10° (1 each side), knee hyperextension >10° (1 each side), and forward flexion with palms flat on the floor (1). A score of ≥5/9 in adults (≥6 in children, ≥4 over 50) suggests generalised hypermobility; the diagnosis of hypermobile Ehlers–Danlos syndrome or hypermobility spectrum disorder requires the full 2017 criteria, including a five-part questionnaire, systemic features and exclusion of other heritable connective tissue disorders. Clinically these patients need proprioceptive and strength training through range, load management and pacing, and they are typically made worse by the end-range stretching they instinctively seek.

Nerve and blood supply

Hilton’s law

The nerve supplying a joint also supplies the muscles that move the joint and the skin overlying their insertions.

Examples: the hip is supplied by branches of the femoral, obturator and sciatic nerves — which is why hip pathology refers to the knee via the obturator nerve, and why a child with a limp and knee pain must have the hip examined. The shoulder is supplied by the suprascapular, axillary and lateral pectoral nerves, hence pain referral over the deltoid.

Blood supply comes from periarticular anastomoses (the genicular anastomosis at the knee, the cruciate and trochanteric anastomoses at the hip, the anastomoses around the elbow), whose redundancy allows collateral flow when a main vessel is occluded or when the joint is held in an extreme position.

Articular mechanoreceptors

Four types, classified by Wyke, and the reason proprioceptive rehabilitation exists.

Why this matters clinically. Ligament rupture destroys type III receptors and part of the afferent supply, producing a measurable proprioceptive deficit and altered reflex muscle timing that persists after the ligament is reconstructed. This is the anatomical justification for balance and perturbation training as a mandatory component of ligament rehabilitation, and for the evidence that such training reduces recurrent ankle sprain and ACL injury rates.

Arthrogenic muscle inhibition is the same story in reverse: joint effusion, pain and abnormal afferent input reflexively inhibit motor drive — most famously quadriceps inhibition after knee injury or effusion, which can be produced experimentally by injecting as little as 20–30 mL of saline into a healthy knee. Effusion control is therefore not cosmetic; it is a prerequisite for strength gain.

Joint positions and the capsular pattern

These four concepts are the practical vocabulary of manual assessment.

Selected positions and patterns

The ankle example is worth dwelling on. The talar trochlea is wider anteriorly than posteriorly. In dorsiflexion the wide part engages the mortise, the syndesmosis splays slightly, and the joint is close-packed and stable. In plantarflexion the narrow part sits in the mortise, the joint is loose, and it is vulnerable — which is precisely why the classic inversion sprain occurs in plantarflexion and inversion, and why the anterior talofibular ligament is the first structure to fail.

Arthrokinematics and joint mobilisation

Chapter 1 introduced roll, glide and spin and the convex–concave rule. Applied to treatment:

• Distraction (traction) separates the surfaces along the perpendicular to the treatment plane; glide is applied parallel to it. The treatment plane (Kaltenborn) lies in the concave surface and moves with it, not with the convex bone.
• Kaltenborn grades of traction/glide: I (loosen — nullify compressive force), II (tighten — take up slack), III (stretch — beyond slack, into resistance). Grades I–II for pain; grade III for restriction.
• Maitland grades of oscillatory mobilisation: I (small amplitude at beginning of range), II (large amplitude within range), III (large amplitude into resistance), IV (small amplitude at end of range), V (high-velocity low-amplitude thrust — manipulation). Grades I–II are pain-modulating; III–IV address stiffness.
• The resting position is the starting point for assessment and for pain-relieving techniques; progressively less-loose positions are used as tolerance improves.

Two honest caveats. First, the convex–concave rule is a heuristic, and in-vivo imaging at the shoulder shows translations that do not always follow it; treat it as a starting hypothesis tested against the patient’s response. Second, the mechanism of manual therapy is now understood to be substantially neurophysiological — descending inhibition, temporary changes in mechanosensitivity, and non-specific contextual effects — rather than a durable structural change in capsular length. That does not make it useless; it makes it an adjunct that should be paired with active loading, not a treatment in itself.

Joint pathology: what a physiotherapist must recognise

Joint contracture: know which tissue is short

The distinction is not academic: capsular contracture responds to mobilisation and prolonged low-load stretch; myogenic contracture to positioning and active lengthening; neurogenic contracture to tone management alongside both; and bony ankylosis to neither.

The cost of immobilisation

Within days to weeks of immobilisation a joint shows:

• proliferation of intra-articular fibrofatty connective tissue and adhesion formation
• loss of water and glycosaminoglycan from the capsule with loss of the interfibrillar distance that lets collagen fibres glide
• random new collagen cross-linking
• cartilage proteoglycan loss, softening and thinning
• ligament weakening at the insertion site (the bone–ligament junction fails before the ligament substance does)
• disuse osteopenia
• and sarcopenia with sarcomere loss when the muscle is held short. Recovery takes far longer than the loss — often months for what was lost in weeks. Immobilise only as much as necessary, for as short a period as necessary, and move everything else

Where students consistently go wrong

• Giving only one classification. Say the structural class and the functional consequence.
• Forgetting the cruciates are intracapsular but extrasynovial. The synovial membrane reflects around them.
• Believing the synovial membrane covers the articular cartilage. It lines everything inside the capsule except cartilage, discs and menisci.
• Treating stability as ligamentous by default. At the shoulder and the lumbar spine it is overwhelmingly muscular.
• Confusing close-packed with resting position. Close-packed = maximum congruence and stability; resting = maximum laxity and volume, and the position you mobilise in.
• Ignoring the capsular pattern. A non-capsular restriction points away from arthritis toward a mechanical block.
• Confusing laxity with instability. Laxity is a sign; instability is a symptomatic diagnosis.
• Missing septic arthritis. Hot joint + fever + refusal to move = emergency.
• Ignoring effusion because “it is only swelling”. Effusion reflexively inhibits muscle. Control it first.
• Forgetting Hilton’s law. Knee pain in a child may be a hip.

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 4 of 24 · Human Anatomy · Physiotherapist India Next: Chapter 5 — Muscle: from whole muscle to sarcomere, fibre types, and the motor unit.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents