Fractures and dislocations
Fractures, dislocations, subluxations and nerve entrapment
Fracture questions test how bone heals, why some sites fail to heal or die, how children's bones differ, and which nerve or vessel lies at risk. Imaging questions add the signs that reveal subtle fractures and dislocations on plain films, CT and MRI.
How bone heals
- Secondary (indirect) healing: haematoma, inflammation, soft callus (fibrocartilage), hard callus (woven bone), remodelling to lamellar bone. Needs some movement (casts, nails).
- Primary (direct) healing: with absolute stability (compression plate), osteons cross the gap directly; no external callus forms. Callus around a rigid plate suggests instability.
- Hypertrophic non-union: abundant flared callus around a gap from too much movement; needs rigid fixation, not graft. Atrophic non-union: poor biology (blood supply, infection, smoking).
- Malunion: children remodel well in the plane of joint movement but not angulation in the coronal plane or rotation, so a supracondylar fracture can leave a cubitus varus ('gunstock') elbow.
Sites at risk of avascular necrosis or non-union
| Bone | Reason | Imaging sign |
|---|---|---|
| Scaphoid | Supply enters distally (retrograde); proximal pole dies | Sclerotic proximal pole |
| Femoral neck | Retinacular vessels torn in intracapsular fracture | — |
| Talar neck | Tenuous supply to the body | Hawkins sign: subchondral lucency at 6–8 weeks means the body kept its blood supply |
| Tarsal navicular | Central third poorly vascular | Sagittal line on CT |
- Steroids and alcohol cause femoral head necrosis: the MRI double-line sign (inner bright, outer dark band) is the reactive interface around dead bone. A medullary bone infarct has a serpiginous rim around fatty marrow.
- Freiberg infraction: flattening of the second metatarsal head (adolescent girls).
Stress and fragility fractures
- Fatigue fracture: abnormal load on normal bone (recruits, runners); insufficiency fracture: normal load on weak bone.
- Radiographs are often normal early; MRI STIR shows periosteal and marrow oedema first. Calcaneal stress fracture shows a vertical sclerotic band.
- Femoral neck: the superolateral (tension) side tends to open and displace, so it is treated urgently; the inferomedial (compression) side is safer.
- Female athlete triad: low energy availability suppresses GnRH, giving low FSH, LH and oestradiol, amenorrhoea and low bone density.
- Postmenopausal bone loss hits trabecular bone fastest, so vertebral bodies collapse first (height loss).
Children
- The physis is weaker than ligaments: a twisting knee avulses the tibial spine rather than tearing the ACL.
| Salter–Harris | Line |
|---|---|
| I | Through the physis |
| II (commonest) | Physis and metaphysis |
| III | Physis and epiphysis (intra-articular) |
| IV | Across metaphysis, physis and epiphysis |
| V | Crush of the physis (may look normal) |
- Juvenile Tillaux: the medial distal tibial physis closes first, so the anterior inferior tibiofibular ligament avulses the anterolateral epiphysis.
- A bony bar across the physis after a crush injury tethers growth (shortening and angulation).
- Toddler fracture: undisplaced spiral distal tibial fracture; may show only on later films.
- Neck: pseudosubluxation of C2 on C3 is normal if the posterior cervical (spinolaminar) line is respected; the dentocentral synchondrosis is a normal smooth lucency; children's large heads and flat condyles predispose to atlanto-occipital dissociation; atlantoaxial rotatory subluxation follows throat infection.
- Birth clavicle fracture causes pseudoparalysis; check the brachial plexus too.
Spine
| Injury | Mechanism and point |
|---|---|
| Flexion teardrop | Hyperflexion with axial load; anterior cord syndrome (motor and pain loss, dorsal columns spared) |
| Bilateral facet dislocation | About 50% forward slip; unilateral gives about 25% |
| Chance fracture | Lap belt flexion-distraction; look for bowel or mesenteric tears |
| Ankylosing spondylitis | Rigid, brittle spine fractures through disc and posterior elements after minor falls |
Dislocations
- Anterior shoulder (commonest): axillary nerve; Hill–Sachs posterolateral head impaction, Bankart anterior glenoid lesion.
- Posterior shoulder: seizures and electric shock; arm held internally rotated; reverse Hill–Sachs on the anteromedial head; easily missed on an AP view, so obtain an axillary or Y view.
- Inferior (luxatio erecta): arm fixed overhead; high neurovascular risk.
- Perilunate dislocation: lunate stays in the radius, capitate lies behind it; in lunate dislocation the lunate tips forward and can compress the median nerve.
- Essex–Lopresti injury: radial head fracture plus distal radioulnar disruption through the interosseous membrane.
- Posterior hip dislocation: sciatic nerve.
- Zygomatic (tripod) fracture: numb cheek from the infraorbital nerve. Sternal fracture: exclude blunt cardiac injury.
Nerves at risk and entrapment
| Injury | Nerve |
|---|---|
| Surgical neck of humerus | Axillary (with posterior circumflex humeral artery) |
| Humeral shaft (spiral groove) | Radial: wrist drop |
| Supracondylar humerus | Median (anterior interosseous), brachial artery |
| Medial epicondyle | Ulnar |
| Fibular neck | Common fibular (peroneal): foot drop |
- Carpal tunnel syndrome: median nerve compression; causes include pregnancy, hypothyroidism, acromegaly (soft tissue swelling), rheumatoid arthritis and dialysis amyloid. Ultrasound shows a swollen median nerve at the tunnel inlet (cross-sectional area above about 10 mm²).
- Other entrapments: ulnar at the cubital tunnel or Guyon's canal, lateral cutaneous nerve of thigh (meralgia paraesthetica), tibial nerve in the tarsal tunnel.
- Seddon grades: neurapraxia (conduction block, demyelination, full recovery), axonotmesis (axon divided, sheath intact, regrows about 1 mm/day), neurotmesis (nerve divided).
Remember Absolute stability heals without callus; relative stability heals with it.
Exam trap A lucent band under the talar dome after a talar neck fracture is good news: the bone is alive.
Exam trap A normal radiograph does not exclude a stress fracture or a posterior shoulder dislocation.
Test yourself on this topic
5 of the 33 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 10-year-old boy falls from his bicycle and twists his knee, which swells within an hour. A bony fragment has been pulled off the intercondylar eminence of the tibia, where the anterior cruciate ligament attaches. Why does a child usually suffer this injury instead of a tear in the ligament itself?
Answer: Epiphyseal bone is weaker than the ligament
In skeletally immature children, the ligaments are strong and elastic relative to the incompletely ossified epiphyseal bone and cartilage at their attachments. A force that would tear the anterior cruciate ligament in an adult instead pulls off its bony insertion on the tibial spine (intercondylar eminence). The fragment may be undisplaced, hinged or completely displaced, and displaced fragments need reduction to restore stability and avoid a block to extension. Haemarthrosis is common because the fracture enters the joint.
Tibial spine avulsion
- Child's ACL equivalent
- Epiphyseal bone weaker than ligament
- Haemarthrosis
- Displaced fragment: reduce
Remember In children, bone gives way before ligament.
Exam trap The fragment is often much larger than it first appears, because much of it is unossified cartilage.
Why the other options are wrong
- Children's ligaments are stiffer than adult ligaments Children's ligaments are more elastic, not stiffer.
- The anterior cruciate ligament is absent before puberty The ligament is present from fetal life.
- Children's menisci absorb all rotational force The menisci do not absorb all the force; ligament attachments still take the load.
- The growth plate is stronger than the ligament The growth plate is weaker, not stronger, than ligaments in children.
A 40-year-old man falls from a ladder onto his outstretched hand. Radiographs show a comminuted radial head fracture. He also has pain and swelling at the wrist, and a wrist radiograph shows the ulna projecting beyond the radius. Tearing of which structure allows the radius to migrate upwards?
Answer: Interosseous membrane of the forearm
In an Essex-Lopresti injury, a high-energy axial load fractures the radial head and continues down the forearm, tearing the interosseous membrane and disrupting the distal radioulnar joint. The radius then loses both its proximal buttress (the radial head) and its longitudinal tie to the ulna (the central band of the interosseous membrane), so it migrates proximally and the ulna becomes relatively long at the wrist. Excising the radial head without replacement in this situation worsens the migration, so wrist pain must always be looked for with radial head fractures.
Essex-Lopresti injury
- Radial head fracture
- Interosseous membrane torn
- DRUJ disrupted, ulna relatively long
- Do not excise the radial head alone
Remember Radial head fracture with wrist pain: check the interosseous membrane.
Exam trap Wrist pain after a radial head fracture is easily ignored, but missing the longitudinal instability leads to chronic disability.
Why the other options are wrong
- Annular ligament of the radius Holds the radial head against the ulna at the elbow but does not prevent proximal migration of the whole radius.
- Ulnar collateral ligament of the elbow Stabilises the elbow against valgus stress; it does not link radius and ulna along the forearm.
- Extensor retinaculum of the wrist Holds down the extensor tendons; it does not stabilise the radius against the ulna.
- Pronator quadratus muscle A muscle at the distal forearm; it is not the main longitudinal stabiliser.
After a car crash, a 35-year-old man has quadriplegia. Lateral cervical CT shows C5 displaced forwards on C6 by about half the depth of the vertebral body, with both inferior articular processes of C5 lying in front of the superior articular processes of C6. What is the injury?
Answer: Bilateral facet dislocation
In a bilateral facet dislocation, severe flexion and distraction tear the posterior ligaments, joint capsules and disc, and both inferior facets of the upper vertebra jump over and lock in front of the facets below. This shifts the upper body forwards by about 50% of its depth and narrows the canal, so cord injury is common and the injury is highly unstable. In a unilateral facet dislocation, flexion with rotation dislocates only one side, and the forward shift is about 25% of the body depth, often with a nerve root injury rather than a complete cord injury.
Facet dislocations
- Unilateral: about 25% shift, rotation
- Bilateral: about 50% shift
- Both facets locked in front
- Bilateral: cord injury, unstable
Remember Half a body forward means both facets are locked.
Exam trap A disc herniation often accompanies facet dislocation, so MRI is needed before closed reduction in an awake patient.
Why the other options are wrong
- Unilateral facet dislocation Gives a forward shift of about a quarter of the body depth, with rotation.
- Hangman fracture A fracture of both pars of C2, not a dislocation at C5–C6.
- Clay-shoveler fracture An avulsion of a lower cervical spinous process, which is stable.
- Anterior wedge compression fracture Loss of anterior body height without forward translation of the vertebra.
A 20-year-old basketball player has vague dorsal midfoot pain on jumping and is found to have a stress fracture running sagittally through the central third of the tarsal navicular. Why does this fracture often fail to heal?
Answer: Its central third is poorly vascular
The navicular receives its blood supply from the dorsalis pedis and medial plantar arteries entering at its medial and lateral ends, leaving the central third relatively avascular. This central zone is also where compressive forces from the talus and cuneiforms concentrate during push-off. Stress fractures therefore occur there, are easily missed, and often need prolonged non-weight-bearing or fixation to heal.
Navicular stress fracture
- Jumping athletes
- Central third is poorly vascular
- Load concentrates centrally
- Often missed on radiographs: use CT or MRI
Remember Midfoot pain in a jumper: think central navicular.
Exam trap Vague midfoot pain with normal radiographs in an athlete is often labelled a sprain; a delayed diagnosis increases non-union.
Why the other options are wrong
- It is a site of tumour in young adults The fracture is from repetitive loading, not tumour.
- Its cartilage cover blocks callus formation Callus forms from the bone's blood supply; the cartilage surfaces do not prevent it.
- It carries no weight during walking The navicular is a key weight-bearing bone in the medial arch.
- It has no attachment for tendons Tibialis posterior inserts on the navicular tuberosity.
A 40-year-old man who has taken high-dose steroids for a year has right groin pain. Radiographs are normal. T2-weighted MRI shows a serpiginous line in the femoral head made of an inner bright band and an outer dark band, enclosing a segment of normal-looking fatty marrow. What does this double line represent?
Answer: Reactive interface around necrotic bone
In avascular necrosis, the dead segment of marrow often keeps its fatty signal at first. At its border, the living bone mounts a repair response: an inner zone of hyperaemic granulation tissue, which is bright on T2, and an outer rim of sclerotic, reactive bone, which is dark on all sequences. Together they form the double-line sign, which is highly specific for osteonecrosis. MRI shows this before radiographs change, and the extent of the segment predicts later collapse.
Avascular necrosis on MRI
- Dead segment may keep fat signal
- Inner bright line: granulation tissue
- Outer dark line: sclerotic bone
- Seen before radiographs change
Remember A double line around fat in the femoral head means osteonecrosis.
Exam trap The necrotic segment can look normal on T1 early on; it is the rim around it that gives the diagnosis.
Why the other options are wrong
- A subchondral fracture through the femoral head A subchondral fracture is a later stage and gives a thin line just under the cartilage, often with a crescent on radiographs.
- Synovial fluid entering the femoral head Synovial fluid does not form a serpiginous rim enclosing marrow.
- Chemical shift at a fat–water border only Chemical shift can add a dark edge, but the bright inner band reflects granulation tissue.
- Tumour spreading through the marrow Tumour replaces fat with abnormal signal rather than leaving normal fat within a rim.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Musculoskeletal and endocrine disorders, Fractures, dislocations, subluxations and nerve entrapment