The skeleton, bone by bone
An anterior view of the adult skeleton (anatomical position: palms forward). Click any bone, zoom into the skull, chest, spine, hand, pelvis, knee or foot, and switch on the muscle layer to see major muscles on the body's right side.
Six kinds of synovial joint
The shape of the articular surfaces decides how a joint can move. Each model moves on its own; drag a slider to drive it by hand.
Muscles as levers
Bones are levers, joints are fulcrums and muscles supply the effort. Hold a load in your hand and find out how hard your biceps must pull, then compare the three classes of lever in the body.
Inside a sarcomere
Zoom from a muscle down to one sarcomere, the contractile unit. Thin actin filaments slide over thick myosin filaments as cross-bridges cycle; the filaments themselves never shorten.
Key facts to remember
- Describe a fracture in order: open or closed → bone and side → site (proximal, middle or distal third; intra-articular?) → pattern (transverse, oblique, spiral, comminuted, segmental, greenstick) → displacement (translation, angulation, rotation, shortening).
- Rule of twos for X-rays: two views at right angles (AP and lateral) including the joints above and below the injury.
- Open fractures (Gustilo–Anderson): I wound < 1 cm, clean; II 1–10 cm, moderate soft-tissue damage; III high-energy or > 10 cm: IIIA bone can be covered, IIIB needs a flap, IIIC has an arterial injury needing repair. Give IV antibiotics as soon as possible (within 1 hour), tetanus cover, a saline-soaked dressing and a splint.
- Compartment syndrome: pain out of proportion and pain on passively stretching the muscles are the earliest signs; pulselessness is late. ΔP = diastolic BP − compartment pressure; ΔP < 30 mmHg means urgent fasciotomy.
- Approximate closed blood loss: femur 1–2 L, tibia 0.5–1 L, humerus 0.5–0.75 L, pelvis 1.5 L or more, each rib about 125 mL. Adult blood volume ≈ 70 mL/kg.
- Colles' fracture: distal radius within 2.5 cm of the wrist, fragment displaced and angled dorsally ("dinner fork"), after a fall on the outstretched hand, often with osteoporosis. Smith's fracture is displaced towards the palm.
- Sickle cell disease: bone infarcts mimic infection; osteomyelitis is commonly due to Salmonella (also Staphylococcus aureus). Dactylitis (painful swollen hands and feet) is often the first sign in infants.
- Rickets (vitamin D or calcium deficiency): bow legs or knock knees, wide wrists, rachitic rosary, frontal bossing; X-ray shows cupped, frayed, widened metaphyses.
Sources: ATLS haemorrhage classes; Gustilo–Anderson classification; BOAST open fracture guidance; McQueen compartment pressure threshold (WikEM summary); paediatric infectious disease notes on osteomyelitis in sickle cell disease.
The skeleton
The adult skeleton has about 206 bones: 80 in the axial skeleton (skull, hyoid, ear ossicles, vertebral column, ribs and sternum) and 126 in the appendicular skeleton (limbs and the pectoral and pelvic girdles). It supports and protects the body, provides levers for movement, stores calcium and phosphate, and makes blood cells in red marrow. A newborn has around 270 bony elements, many of which fuse during growth (for example the three parts of each hip bone and the five sacral vertebrae).
Bones are classified by shape: long (femur, humerus, phalanges), short (carpals, tarsals), flat (skull vault, sternum, ribs, scapula), irregular (vertebrae) and sesamoid (patella, inside a tendon). A long bone has a shaft (diaphysis) of dense compact bone arranged in osteons around a marrow cavity, and expanded ends (epiphyses) of spongy (trabecular) bone covered by articular cartilage. Children grow in length at the cartilaginous growth plates.
The vertebral column
7 cervical, 12 thoracic, 5 lumbar, the sacrum (5 fused) and the coccyx (3–5 fused). Vertebral bodies get larger towards the base because they carry more weight. Seen from the side the column has four curves (cervical and lumbar lordosis, thoracic and sacral kyphosis) that act like a spring.
Joints
Joints are fibrous (sutures of the skull, almost immovable), cartilaginous (intervertebral discs, pubic symphysis, slightly movable) or synovial (freely movable). Synovial joints have articular cartilage, a fibrous capsule lined by synovial membrane, and lubricating synovial fluid. Their type depends on the shape of the surfaces:
| Type | Axes | Movements | Examples |
|---|---|---|---|
| Hinge | 1 | Flexion / extension | Elbow (humero-ulnar), knee (modified), interphalangeal |
| Pivot | 1 | Rotation | Atlanto-axial (shaking the head), proximal radio-ulnar (turning a key) |
| Condyloid (ellipsoid) | 2 | Flexion/extension, abduction/adduction, circumduction (no axial rotation) | Wrist (radiocarpal), knuckles (MCP) |
| Saddle | 2 | As condyloid, with more range; opposition | Thumb carpometacarpal joint |
| Ball-and-socket | 3 | All of the above plus rotation | Shoulder (glenohumeral), hip |
| Plane (gliding) | Non-axial | Small sliding movements | Intercarpal, intertarsal, vertebral facet joints |
Clinical relevance: bones and trauma
Motorcycle injuries. Motorcycle-taxi (boda boda) crashes fill Kenyan casualty departments and orthopaedic wards. Riders and pillion passengers most often break the tibia (frequently open, because the bone lies just under the skin of the shin), the femur, the forearm and the clavicle, often with head injury. Wear a helmet, and remember the hidden bleeding: a closed femoral shaft fracture can lose 1–2 litres into the thigh, enough for haemorrhagic shock. A traction splint (e.g. Thomas splint) reduces pain and bleeding.
Open fractures are graded by Gustilo–Anderson. The bone is contaminated, so give IV antibiotics as soon as possible (within an hour), check tetanus status, photograph the wound, cover it with a saline-soaked dressing and splint the limb; debridement is done in theatre, not by closing the wound in casualty. Check and record the pulses, capillary refill and sensation below the injury before and after splinting.
Compartment syndrome: bleeding and swelling inside a closed fascial compartment (most often the leg after a tibial fracture, or the forearm) raise the pressure until it exceeds capillary perfusion, so muscle and nerve die within hours. Suspect it when pain is out of proportion to the injury, needs more and more analgesia, and is worse on passively stretching the toes. Split tight casts and bandages down to skin; if the perfusion gap $\Delta P = \text{DBP} - P_\text{compartment}$ is below 30 mmHg, urgent fasciotomy is needed.
Osteomyelitis and sickle cell disease. Sickle cell disease is common in western Kenya (around Lake Victoria) and on the coast. Sickled cells block small vessels in bone, causing painful crises and bone infarcts; dead bone and a poorly working spleen invite infection, classically with Salmonella. Fever, persistent localised swelling and inability to bear weight need investigation (blood cultures, X-ray, sometimes ultrasound or aspiration), not just analgesia.
Rickets means poorly mineralised growing bone (vitamin D deficiency from little sunlight exposure, or low dietary calcium). Soft bones bend under body weight: bow legs when walking starts, widened wrists and costochondral junctions (rachitic rosary). It is prevented by sunlight, dietary calcium and vitamin D, and treated with vitamin D and calcium.
Levers in the body
A lever is in equilibrium when the moments about its fulcrum balance. With effort $F_E$ at perpendicular distance $d_E$ and load $F_L$ at $d_L$:
- First class (fulcrum between effort and load): the head balanced on the atlas, with neck extensors behind and the head's centre of mass in front.
- Second class (load between fulcrum and effort): rising on tiptoe, if the ball of the foot is taken as the fulcrum. MA > 1.
- Third class (effort between fulcrum and load): the biceps inserted 4 cm from the elbow holding a weight 35 cm away. MA < 1: the muscle must pull many times the load, but the hand moves fast and far. Most limb muscles work this way.
For the biceps model the moments about the elbow are
where $\alpha$ is the angle of the forearm from the vertical, $d$ the insertion distance and $\gamma$ the angle between the forearm and the line of the biceps. The joint reaction force is whatever balances the force vectors: $\vec R = -(\vec F_B + m\vec g + m_f\vec g)$. The model is static and assumes the biceps acts alone (in reality brachialis and brachioradialis share the load).
Skeletal muscle structure
Muscle → fascicles → muscle fibres (cells, up to several cm long, multinucleate) → myofibrils → sarcomeres in series. A sarcomere runs from one Z disc to the next. Thin actin filaments (with tropomyosin and troponin) are anchored at the Z discs; thick myosin filaments sit in the centre, held at the M line. Under the microscope this gives dark A bands (the length of the thick filaments, constant), light I bands (thin filaments only, which shorten) and a paler H zone in the middle (thick filaments only, which shortens and can vanish).
The sliding-filament mechanism and cross-bridge cycle
- A nerve impulse releases Ca²⁺ from the sarcoplasmic reticulum; Ca²⁺ binds troponin, which moves tropomyosin off the myosin-binding sites on actin.
- Energised myosin heads (holding ADP + Pi) bind actin, forming cross-bridges.
- Release of Pi triggers the power stroke: the head pivots and pulls the thin filament ≈ 10 nm towards the M line.
- A new ATP binds and the head detaches; hydrolysis of ATP re-cocks the head.
Without ATP the heads cannot detach: muscles lock in rigor (as in rigor mortis). Force depends on how many heads overlap actin, giving the length–tension curve: maximal at 2.0–2.2 µm (frog values, Gordon, Huxley & Julian 1966), falling to zero at 3.6 µm where filaments no longer overlap, and falling at short lengths as thin filaments collide and thick filaments hit the Z discs.
Compartment pressure calculator
Measured compartment pressure is judged against the diastolic blood pressure (the perfusion pressure).
Blood loss from fractures
Count the closed fractures in a trauma patient to estimate hidden blood loss and the class of haemorrhagic shock to expect (approximate teaching values).
Practice drill
Randomised questions with instant feedback. Get five right in a row to complete the challenge.
Worked examples
1. Biceps force at 90°
- Load 5 kg in the hand, 35 cm from the elbow; forearm and hand 2 kg with centre of mass 15 cm from the elbow; biceps inserts 4 cm from the elbow and pulls nearly vertically.
- Load moment: $5 \times 9.81 \times 0.35 = 17.2$ N·m. Forearm moment: $2 \times 9.81 \times 0.15 = 2.9$ N·m. Total 20.1 N·m.
- With the line of pull almost perpendicular to the forearm ($r_\perp \approx 4.0$ cm): $F_B \approx 20.1 / 0.040 \approx 500$ N, about 10 times the 49 N load. Lab 3 gives about 505 N because the muscle line is slightly oblique.
2. Standing on tiptoe on one foot
- Body mass 65 kg: ground reaction under the ball of the foot $N = 65\times9.81 = 638$ N.
- Horizontal distances: ankle joint to ball of foot $a = 12$ cm, ankle to Achilles line of pull $c = 5$ cm.
- Moments about the ankle: $F_A c = N a \Rightarrow F_A = 638 \times 12/5 = 1530$ N (about 2.4 × body weight).
- The tibia must push down on the talus with $J = N + F_A \approx 2170$ N. Treating the ball of the foot as the fulcrum, the foot is a second-class lever for the joint force $J$: $MA = (a+c)/a = 1.42$.
3. Looking down at a phone
- Head mass 5 kg (49 N); centre of mass 12 cm from the atlanto-occipital joint, 12° in front of the vertical when upright; neck extensors act 5 cm behind the joint.
- Upright: moment arm $= 0.12 \sin 12° = 2.5$ cm, so $F_m = 49 \times 2.5/5 = 25$ N.
- Flexed 45°: moment arm $= 0.12 \sin 57° = 10.1$ cm, so $F_m = 99$ N, four times as much. The joint compression rises from about 74 N to about 148 N.
4. Bands of a sarcomere
- Thick filaments 1.6 µm, thin filaments 1.0 µm each side. At SL = 2.6 µm: A band = 1.6 µm, I band = 2.6 − 1.6 = 1.0 µm, H zone = 2.6 − 2 × 1.0 = 0.6 µm.
- Contract to 2.2 µm: A band still 1.6 µm, I band 0.6 µm, H zone 0.2 µm. Only I and H shrink, the key evidence for sliding filaments.
- At 3.6 µm thin and thick filaments no longer overlap, so active force is zero.
Name the fracture
A new schematic radiograph each time. Read it like a radiographer or clinician: choose the pattern, the site and the displacement, then check your description.
Clinical scenarios
Road traffic, falls and bone disease you will meet in casualty, the wards and the orthopaedic clinic.
OSCE station: describe and manage a fracture
A timed station on imaging and managing an open fracture. The mark sheet appears at the end.
Challenges check themselves as you use the labs.