Evidence-based physiotherapy and rehabilitation resources
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Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · General anatomy
A joint is where two bones meet, and every one of them is a compromise. Build it for stability and you lose movement; build it for movement and something has to hold it together. Understanding which compromise a joint has made tells you how it will fail.
Joints are grouped in two different ways, and examiners will ask for either. It is worth being clear that these are two lenses on the same joints, not two sets of joints.
| By structure — what lies between the bones | By function — how much movement is allowed |
|---|---|
| Fibrous — joined by fibrous tissue | Synarthrosis — immovable |
| Cartilaginous — joined by cartilage | Amphiarthrosis — slightly movable |
| Synovial — separated by a fluid-filled cavity | Diarthrosis — freely movable |
The two mostly line up: fibrous joints tend to be immovable, cartilaginous ones slightly movable, synovial ones freely movable. But not always — a syndesmosis is fibrous yet permits real movement, and the structural classification is the more reliable one to work from.
Figure 1 · Classification of joints by structure
The bones are joined directly by fibrous tissue. There is no cavity and usually very little movement.
| Type | What it is | Examples | Notes |
|---|---|---|---|
| Suture | A thin layer of fibrous tissue between interlocking bone edges | The joints between skull vault bones | Found only in the skull. Wide at birth, leaving the fontanelles; they narrow through childhood and eventually ossify. |
| Syndesmosis | Bones held by a ligament or an interosseous membrane, further apart than in a suture | The inferior tibiofibular joint; the interosseous membranes of forearm and leg | Permits a small but genuinely useful amount of movement. The ankle syndesmosis is injured in a high ankle sprain. |
| Gomphosis | A peg-in-socket joint | A tooth in its socket, held by the periodontal ligament | The only example in the body. |
| Type | What joins the bones | Examples | Notes |
|---|---|---|---|
| Primary synchondrosis |
Hyaline cartilage | The epiphyseal growth plate; the first sternocostal joint | Usually temporary. Most ossify once growth is complete. |
| Secondary symphysis |
Fibrocartilage, with a thin layer of hyaline cartilage on each bone surface | Pubic symphysis, intervertebral discs, manubriosternal joint | Permanent, slightly movable, and always in the midline. Built to absorb shock and permit a little give. |
A pattern worth noticing
Secondary cartilaginous joints are midline joints, and they contain fibrocartilage. Both facts follow from the job: the midline of the body must transmit load between two halves while allowing a small amount of movement, and fibrocartilage is the tissue that does exactly that.
These are the joints you will treat. The defining feature is a joint cavity containing synovial fluid, which separates the bone ends and allows free movement.
Figure 2 · The structure of a synovial joint
| Feature | What it does | Clinical relevance |
|---|---|---|
| Articular cartilage | Covers the bone ends. Smooth, low friction, spreads load. | Has no nerve supply and heals poorly. |
| Joint cavity | The space between the surfaces. | Can fill with fluid, blood or pus. An effusion limits movement and inhibits the surrounding muscles. |
| Fibrous capsule | A sleeve of dense connective tissue joining the two bones. | Richly innervated. Tightens at the end of range, and shortens if a joint is held still. |
| Synovial membrane | Lines the capsule and every internal surface except the cartilage. Produces synovial fluid. | The tissue that inflames in inflammatory arthritis. |
| Synovial fluid | Lubricates, nourishes the cartilage, and absorbs shock. | Becomes thinner when a joint is warmed and moved, which is part of why warming up works. |
| Ligaments | Reinforce the joint. May thicken the capsule or lie separate from it. | Guide and check movement. Contain the receptors that tell you where the joint is. |
| Discs and menisci | Fibrocartilage pads improving fit, spreading load and absorbing shock. | Knee, temporomandibular joint, sternoclavicular joint, wrist. |
| Labrum | A fibrocartilage rim deepening a shallow socket. | Glenoid and acetabular labra. Tears reduce stability. |
| Bursae | Fluid-filled sacs reducing friction where structures rub. | Inflame readily. Subacromial bursitis is one of the commonest shoulder problems. |
| Fat pads | Fill dead space and cushion. | Can be pinched and become painful, as at the front of the knee. |
Figure 3 · The six types of synovial joint
Illustration to be added
Six panels in two rows. Each panel pairs a real anatomical example with a simple mechanical analogy beside it, and arrows showing the movement permitted. Plane: intercarpal joints, with flat sliding blocks. Hinge: elbow, with a door hinge. Pivot: atlantoaxial joint, with a peg in a ring. Condyloid: wrist, with an oval in an oval socket. Saddle: first carpometacarpal joint, with two saddle shapes interlocking. Ball and socket: hip, with a sphere in a cup. Label each type, its example, and the number of axes. Navy line work, bone warm ivory, movement arrows in gold.
Synovial joints are grouped by the shape of their surfaces, and the shape decides how many directions the joint can move in.
| Type | Shape | Axes | Movements | Examples |
|---|---|---|---|---|
| Plane | Flat or nearly flat surfaces | Non-axial | Gliding and sliding | Intercarpal, intertarsal, facet joints, acromioclavicular |
| Hinge | A convex surface in a concave trough | One (uniaxial) | Flexion and extension only | Elbow, ankle, interphalangeal joints |
| Pivot | A rounded process inside a ring | One (uniaxial) | Rotation only | Atlantoaxial, proximal and distal radioulnar |
| Condyloid ellipsoid | An oval head in an oval socket | Two (biaxial) | Flexion, extension, abduction, adduction and so circumduction. No true rotation. | Wrist, metacarpophalangeal joints |
| Saddle | Each surface concave one way and convex the other | Two (biaxial) | As condyloid, with more freedom; permits opposition at the thumb | First carpometacarpal joint of the thumb |
| Ball and socket | A spherical head in a cup | Three (multiaxial) | Movement in every direction, including rotation | Shoulder, hip |
The one comparison examiners love
The shoulder and hip are both ball and socket, and they behave completely differently. The hip has a deep socket, a strong capsule and powerful ligaments — stable, and it dislocates only under great force. The shoulder has a shallow, small socket and a lax capsule, so it is the most mobile joint in the body and the most commonly dislocated. Same design, opposite compromise.
Figure 4 · The stability and mobility trade-off
Every joint sits somewhere on this line, and what holds it together depends on where.
| What provides stability | How it works | Where it dominates |
|---|---|---|
| Bone shape | A deep socket physically prevents the head leaving it. | Hip, elbow. The strongest and most reliable form of stability. |
| Ligaments and capsule | Passive checks that tighten at the end of range. | Knee, ankle. Effective, but they do not adapt quickly and take months to heal once torn. |
| Muscle | Active control, adjusting continuously. | Shoulder, spine. The only stabiliser that can be trained — which is why so much rehabilitation targets it. |
| Atmospheric pressure and fluid | Negative pressure inside the sealed cavity holds surfaces together. | Contributes at the hip and shoulder. Small, but real. |
This table is the anatomical basis of a great deal of physiotherapy. You cannot change the shape of a socket and you cannot make a stretched ligament short again. What you can change is muscle. When passive stability is lost, active stability is the part that remains available, and training it is the treatment.
Figure 5 · What stops a joint moving further
When a joint will not move further, something is stopping it — and identifying which thing changes what you do next. A joint blocked by bone will not respond to stretching. A joint blocked by a tight capsule may. A joint blocked by pain needs the pain addressed before range is tested at all.
The two-joint muscle catch
A muscle crossing two joints limits one according to the position of the other. Straighten the knee and the hamstrings limit hip flexion sharply; bend the knee and hip flexion increases immediately. If range changes when you move a neighbouring joint, muscle length is the limiting factor — not the joint itself.
Joints receive blood from an anastomosis of vessels around them, which matters because a joint must keep its supply through the full range of movement. If a single vessel were responsible, some positions would cut it off.
The nerve supply follows a rule you should know by name.
Hilton's law
A nerve supplying the muscles that move a joint also supplies the joint itself and the skin over it. This is why hip pathology is often felt at the knee: both are supplied in part by the obturator and femoral nerves. Whenever a patient's pain does not sit over the structure you suspect, this law is worth remembering.
Joint capsules and ligaments are also densely supplied with receptors that report position and movement. This is the anatomical basis of proprioception, and it explains why a joint that has been injured often feels unreliable even after the tissue has healed — the mechanical repair does not automatically restore the information coming from it.
They are two descriptions of the same joints. Structural classification asks what lies between the bones; functional asks how much movement results.
A plane joint is synovial and moves only by gliding. The presence of a cavity tells you the joint is built for movement, not how much.
A synchondrosis is a primary cartilaginous joint. It behaves like a joint in that it is a line of relative weakness, which is exactly why it fails in children.
It is described as a hinge, but rotation occurs at it, particularly near full extension. Describing it as a pure hinge will make the cruciate ligaments impossible to explain.
A labrum is a rim deepening a socket, at the shoulder and hip. A meniscus is a crescent pad improving fit and sharing load, most importantly at the knee. Both are fibrocartilage.
Fluid in a joint does not only limit movement mechanically; it inhibits the muscles acting across it. This is why a swollen knee loses quadriceps control quickly, and why reducing swelling is part of restoring strength rather than a separate task.
Ten questions on this chapter. Tap one to see the answer and the reasoning.
Answer: (B) A syndesmosis. It is a fibrous joint, but the bones are further apart than in a suture, so genuine movement is possible.
Answer: (B) The pubic symphysis. Secondary cartilaginous joints contain fibrocartilage, are permanent, slightly movable, and lie in the midline. The growth plate is primary.
Answer: (B) It stops at the margin of the articular cartilage. Covering the cartilage would defeat its purpose as a gliding surface.
Answer: (C) Condyloid, or ellipsoid. Two axes allow four movements and therefore circumduction, but the oval shape prevents rotation about the long axis. The wrist is the example.
Answer: (B) Both are ball and socket. The shoulder trades bony stability for range, leaving it dependent on muscle, whereas the hip has a deep socket and strong ligaments.
Answer: (C) Muscle. Bone shape cannot be changed and a stretched ligament does not shorten again, so active stability is where rehabilitation does its work.
Answer: (C) Hilton's law. A nerve supplying the muscles moving a joint also supplies the joint and the skin over it, and the hip and knee share obturator and femoral supply.
Answer: (C) A two-joint muscle. The hamstrings cross both hip and knee, so slackening them at the knee immediately releases hip flexion.
Answer: (B) A labrum, found at the glenoid and acetabulum. A meniscus is a crescent-shaped pad improving fit and sharing load, as at the knee.
Answer: (B) Joint swelling reflexly inhibits the surrounding muscles. This is why reducing the effusion is part of restoring strength rather than a separate problem.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| B D Chaurasia's Handbook of General Anatomy Chaurasia and Garg |
The chapter on joints follows this classification closely and is well matched to Indian examination questions. |
| Anatomy and Human Movement: Structure and Function Palastanga, Field and Soames |
The best treatment of joint mechanics for a physiotherapist: what actually limits range, and how stability is shared between structures. |
| Clinical Anatomy by Regions Snell |
For seeing each joint in its regional context once you move on to the limbs. |
| Gray's Atlas of Anatomy Drake, Vogl and Mitchell |
Keep it open at the relevant plate while reading about any individual joint. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents