Joint dislocation imaging

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Dislocation is loss of normal joint congruence; subluxation is partial loss. Radiographs establish direction and reveal associated fractures, but the patient determines timing. A threatened limb or skin cannot wait for a complete elective-style series. Conversely, in a stable patient, pre-reduction images help define a fracture-dislocation that could make a routine manipulation dangerous.

Every joint requires projections that show the articulating surfaces in more than one plane. AP images alone miss posterior shoulder dislocation and can understate rotational malalignment. At the elbow and knee, alignment may have spontaneously reduced despite major ligament and vascular injury. The report should therefore state congruence, direction, impaction, avulsion and fracture, not merely “no dislocation”.

Post-reduction assessment is a new clinical event. Repeat exactly the pre-reduction pulse, perfusion and motor-sensory examination, then confirm concentric reduction with imaging. Persistent widening or asymmetry suggests interposed tissue or an unrecognised fragment. CT maps complex bony anatomy; MRI is generally reserved for stable patients when soft-tissue mapping will alter rehabilitation or surgery.

Key points

Before imaging or reduction, record joint position, wounds, distal pulse, refill and the named motor and sensory nerve functions at risk.
Obtain orthogonal views that demonstrate the joint above and below relevant fractures; direction cannot be trusted from one projection.
When perfusion or skin is threatened, controlled emergency reduction takes priority over routine pre-reduction films, with imaging immediately afterwards.
Describe dislocation direction, congruence, associated fractures and impaction, then look for characteristic instability patterns rather than reporting alignment alone.
After every reduction, repeat the same neurovascular examination and obtain radiographs confirming congruence and excluding an iatrogenic or previously hidden fracture.
Use CT for complex fracture-dislocation geometry and MRI later for selected labral, ligament, cartilage or tendon injuries after urgent limb threats are settled.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

Anterior shoulder dislocation

The humeral head lies anteroinferior to the glenoid, often with greater-tuberosity fracture or humeral-head impaction; axillary nerve function must be documented.

Posterior shoulder dislocationRed flag

Internal rotation after seizure with a light-bulb appearance on AP imaging is suspicious, but an axillary or scapular-Y projection confirms direction.

Elbow fracture-dislocation

Assess radiocapitellar and ulnohumeral alignment and seek coronoid and radial-head fractures that convert a simple dislocation into complex instability.

Knee dislocation patternRed flag

Gross multidirectional instability or periarticular avulsions may indicate a spontaneously reduced dislocation with popliteal artery risk despite a palpable pulse.

Hip dislocation

Femoral-head displacement after high-energy trauma is an orthopaedic emergency; associated acetabular or femoral-head fragments influence reduction and post-reduction CT.

Reduction incongruence

Residual joint-space widening, asymmetry or a nonconcentric contour after manipulation suggests trapped soft tissue, osteochondral fragment or incomplete reduction.

Red flags requiring action

A cool or pulseless limb, expanding haematoma or abnormal perfusion after knee dislocation requires urgent vascular assessment even if alignment has spontaneously returned.
Open dislocation, skin necrosis risk or an irreducible joint needs immediate orthopaedic management and should not undergo repeated forceful attempts.
Posterior shoulder dislocation is commonly missed on an AP view, especially after seizure or electric shock; obtain an orthogonal view.
Post-reduction new nerve deficit, loss of pulse, persistent incongruence or a trapped fracture fragment requires immediate reassessment and escalation.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Pre-reduction radiographsFirst step
Why
Confirm direction and identify fractures before manipulation when the limb and skin are not immediately threatened.
Interpretation and limitations
Choose orthogonal joint-specific views; a single AP image cannot safely characterise three-dimensional congruence.
02
Post-reduction radiographs
Why
Document concentric alignment and detect fracture or fragment made clearer after reduction.
Interpretation and limitations
A radiographically reduced joint does not replace repeated neurovascular examination or clinical stability assessment.
03
Computed tomography
Why
Define acetabular, glenoid, radial-head, coronoid or other complex fracture-dislocation geometry.
Interpretation and limitations
CT is especially useful when plain films show fragments or incongruence, but it should not postpone urgent reduction of a threatened limb.
04
Magnetic resonance imaging
Why
Map labral, ligament, cartilage, tendon and occult osseous injury after acute stabilisation.
Interpretation and limitations
Abnormal signal is common and must match instability pattern and symptoms; MRI is not the first response to vascular compromise.
05
CT angiography or vascular imaging
Why
Evaluate arterial injury after knee or other high-risk dislocation when perfusion examination or protocol indicates.
Interpretation and limitations
A palpable pulse can coexist with intimal injury; local vascular pathways integrate examination, pressure indices and serial review.

04Clinical next stepsHow the result changes management or prompts escalation.

01Stable jointImage before reductionFirst stepA dislocation is suspected and distal perfusion and skin remain satisfactory.

1Document wounds, pulses, refill and named peripheral nerve functions before analgesia and manipulation.
2Obtain adequate orthogonal views and identify direction, fracture, impaction and any barrier to closed reduction.
3Perform one controlled reduction using an appropriate trained technique and analgesia or sedation.
4Repeat clinical findings and radiographs, then arrange specialty follow-up according to stability and associated injury.

02Threatened jointReduce without harmful delayDislocation is causing vascular compromise, progressive nerve deficit or severe skin tenting.

1Call senior emergency and orthopaedic help while preparing immediate analgesia and safe monitoring.
2Record the rapid neurovascular examination and photograph or document skin and wounds when this does not delay care.
3Undertake prompt controlled reduction; do not wait for routine imaging if delay threatens the limb.
4EscalationRe-examine immediately, image the reduced joint and escalate persistent abnormality for operative or vascular management.

03Complex patternMap instability after reductionPost-reduction films show fracture, widening, impaction or nonconcentric alignment.

1Immobilise in the position advised for the specific joint and maintain repeated neurovascular observation.
2Obtain CT when detailed fracture geometry or an intra-articular fragment will change treatment.
3Use MRI later when soft-tissue mapping is necessary for a stable patient’s operative or rehabilitation plan.
4DefinitiveCommunicate the complex pattern directly and assign review of definitive imaging to the treating specialist team.

05Risks, monitoring and follow-upComplications, safety checks and further assessment.

Record identical pre- and post-reduction pulse, refill, motor and sensory examinations so any change is visible.
Observe for recurrent deformity, increasing pain, swelling, skin compromise and evolving compartment or vascular signs.
Confirm that post-reduction imaging demonstrates a concentric joint and that any fracture has a management plan.
For knee dislocation risk, follow the local serial vascular-observation and imaging pathway even after apparently successful reduction.
Ensure follow-up addresses instability, stiffness, avascular necrosis where relevant and return-to-activity criteria.

06Special situationsVariants, exceptions and circumstances that change the usual approach.

Spontaneous reduction misleads

A knee may appear anatomically aligned after relocation while retaining multiligament disruption and serious popliteal arterial injury.

Direction needs orthogonality

Posterior shoulder dislocation can appear deceptively subtle on AP imaging because the abnormality lies mainly in the unseen plane.

Reduction exposes fractures

Overlapping structures before manipulation can hide an osteochondral or rim fragment that becomes apparent on post-reduction films.

Pulse is incomplete evidence

Collateral circulation may preserve a distal pulse despite intimal disruption, so perfusion quality and protocol-directed assessment still matter.

Do not chase MRI first

Soft-tissue detail has value after the joint and limb are safe; it should not delay reduction or vascular investigation.

Name the nerve

Documenting a specific motor action and sensory territory is more useful than writing only that the limb is neurovascularly intact.

07Common pitfallsFrequent interpretation and management errors.

01
Waiting for routine imaging when a dislocated joint is threatening perfusion or skin.
02
Accepting one AP shoulder view after a seizure as exclusion of posterior dislocation.
03
Failing to document named nerve functions before manipulation.
04
Assuming a palpable pulse excludes arterial injury after knee dislocation.
05
Omitting post-reduction radiography and repeated examination.

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