Hip dislocation and avascular-necrosis risk

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Hip dislocation is a high-energy time-critical injury. Begin with trauma physiology, not the deformity. A posterior dashboard mechanism can coexist with patellar, femoral, acetabular, pelvic, abdominal and spinal injury. Support the limb where found, provide analgesia and examine skin and distal circulation. Record individual sciatic-nerve functions before sedation: ankle and great-toe dorsiflexion, plantarflexion and sensation over the dorsum and sole.

Direction predicts position but radiographs confirm anatomy. Posterior dislocation produces flexion, adduction and internal rotation; anterior dislocation produces abduction and external rotation, with flexion or extension depending on obturator or pubic position. Obtain AP pelvis and cross-table or appropriate lateral imaging without forcing movement. Look for femoral-neck and head fractures, acetabular wall fragments and pelvic disruption before selecting the reduction environment.

Reduction is urgent because prolonged displacement worsens cartilage pressure and femoral-head vascular risk. Use an experienced operator, anaesthetic assessment, adequate sedation or general anaesthesia, muscle relaxation, monitoring and a clear rescue plan. The chosen manoeuvre applies in-line traction and reverses the mechanism without levering on the femoral neck. A fracture-dislocation, prosthesis, delayed case, open injury or failed controlled attempt usually belongs in theatre.

After reduction, the work is not finished. Re-examine sciatic, femoral and distal vascular function before residual sedation obscures change. Obtain AP pelvis and lateral imaging and assess the joint line and Shenton line. A non-concentric joint, widened space or mechanical block suggests incarcerated labrum, osteochondral or acetabular fragment. CT defines femoral-head impaction, occult neck fracture, acetabular morphology and loose bodies and guides open reduction or fixation.

Associated fractures determine stability and loading. A small stable posterior-wall injury differs from a large weight-bearing dome fragment, femoral-head shear or neck fracture. Do not test stability aggressively outside the specialist plan. Open fracture-dislocation receives immediate intravenous antibiotics and orthoplastic principles. Vascular injury requires emergency repair, and a new or worsening sciatic deficit after reduction raises concern for entrapment requiring urgent review.

Avascular necrosis may not appear on early radiographs. Explain that prompt reduction reduces but does not abolish risk. Follow pain, movement and radiographic change over months and longer, using MRI selectively for symptoms or specialist surveillance. Weight-bearing status follows joint stability, associated fracture and fixation, not the dislocation label alone. Rehabilitation restores protected motion and abductor strength without positions that threaten instability.

Special situations require a separate lens. A prosthetic dislocation can damage components, dissociate a modular head or indicate infection and should not be managed as a native joint without checking implant films. In children and adolescents, physeal injury and slipped epiphysis change manoeuvre risk. In older adults, low-energy hip deformity is more likely fracture; forceful reduction before adequate imaging can convert an occult neck fracture into a displaced one.

Key points

Posterior hip dislocation usually presents with a shortened flexed, adducted, internally rotated limb; anterior dislocation is typically abducted and externally rotated.
Run ABCDE after high-energy trauma and inspect the knee, femur, pelvis, abdomen and spine because dashboard force rarely injures the hip in isolation.
Before and after every attempt, document dorsiflexion, great-toe extension, plantarflexion, sole and dorsum sensation, femoral function, pulses and capillary refill.
Obtain immediate AP pelvis and targeted hip radiographs if this does not delay lifesaving care; identify femoral-neck, head and acetabular fractures before choosing reduction setting.
Reduce a native dislocated hip urgently with senior orthopaedic and anaesthetic support; use adequate relaxation, one controlled technique and a theatre route when fracture or failed attempt makes closed reduction unsafe.
Do not perform repeated forceful reductions, especially with a femoral-neck fracture, large acetabular fragment, prosthesis or non-concentric first reduction.
Confirm clinically and radiographically, repeat nerve and vascular examination, then obtain CT to assess congruity and femoral-head, neck or acetabular fragments where indicated.
Arrange fracture-specific weight bearing and long-term follow-up for avascular necrosis, post-traumatic arthritis, nerve recovery, heterotopic ossification and instability.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Dashboard posterior injury

A flexed hip struck through the knee drives the femoral head posteriorly, often with acetabular-wall fracture and sciatic nerve stretch.

Abduction external-rotation injury

Force through an abducted externally rotated hip produces anterior dislocation, with the femoral head displaced into obturator or pubic regions.

High-energy sport or fall

Collision, skiing, contact sport and falls can dislocate a native hip, particularly when force acts at the end of motion.

Prosthetic instability

Component position, soft-tissue weakness, impingement, infection and extreme movement can dislocate arthroplasty, requiring a separate implant-specific pathway.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Posterior displacement
The head leaves the acetabulum posteriorly, typically leaving the hip flexed, adducted and internally rotated and threatening the sciatic nerve.
2
Anterior displacement
The head passes anteriorly with an abducted externally rotated limb and can injure femoral nerve, vessels and overlying skin.
3
Vascular interruption
Capsular and retinacular vessels are stretched or torn during displacement, and longer dislocation duration increases avascular-necrosis risk.
4
Chondral and bony injury
Shear can fracture the femoral head or acetabular rim and leave loose fragments, cartilage damage or an incongruent joint after reduction.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Posterior dislocation

A shortened flexed, adducted and internally rotated limb after dashboard force is characteristic and requires sciatic-nerve documentation.

Anterior dislocation

An abducted externally rotated limb with a palpable groin or obturator head suggests anterior displacement and potential femoral neurovascular pressure.

Fracture-dislocation

Crepitus, high-energy impact and radiographic head, neck or acetabular fragments make routine repeated closed reduction unsafe.

Sciatic deficit

Foot dorsiflexion, great-toe extension, plantarflexion or dorsal and plantar sensation loss may predate reduction or emerge from fragment entrapment.

Non-concentric reduction

Persistent pain, joint-space widening, loss of smooth alignment or mechanical block after apparent reduction suggests an incarcerated fragment.

Vascular threat

Coolness, weak or absent pulses, delayed refill or expanding groin and buttock swelling demands immediate vascular and orthopaedic escalation.

Red flags requiring action

A cool pulseless limb, expanding groin or buttock swelling, open injury or progressive neurological loss requires immediate trauma, orthopaedic and vascular intervention.
Associated femoral-neck fracture, acetabular fracture, prosthetic hip, delayed presentation or failed gentle reduction makes repeated closed attempts hazardous and requires consultant-led theatre management.
Persistent joint widening, incongruity, mechanical block or new nerve deficit after reduction suggests an incarcerated fragment or iatrogenic injury and requires urgent CT and specialist review.

05InvestigationsWhat to request, why it matters and how to interpret it.

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
First-line AP pelvis radiographFirst stepFirst line
Why
Confirm direction and identify associated proximal femoral, acetabular and pelvic fracture.
Interpretation and limitations
Assess femoral-head position, neck continuity and acetabular wall; do not manipulate an apparent fracture-dislocation repeatedly before senior review.
02
Lateral hip radiograph
Why
Define anterior or posterior displacement and confirm post-reduction position.
Interpretation and limitations
Select a view that avoids dangerous limb movement; inadequate or painful positioning should not delay controlled reduction in the appropriate setting.
03
Post-reduction CT
Why
Assess congruity, loose fragments, impaction and occult head, neck or acetabular injury.
Interpretation and limitations
CT is particularly important after fracture-dislocation or any non-concentric result and directs fixation, open reduction and weight bearing.
04
Serial neurovascular examination
Why
Distinguish injury present before reduction from iatrogenic or entrapped nerve and vessel damage.
Interpretation and limitations
Record named sciatic divisions, femoral function, pulses and refill with times before sedation, after reduction and after splintage.
05
Whole-body trauma CT
Why
Detect multisystem injury when mechanism and physiology justify it.
Interpretation and limitations
Use in a suitable stable or responding major-trauma patient; a persistent non-responder needs haemorrhage control rather than scanner delay.
06
MRI during follow-up
Why
Assess symptomatic avascular necrosis, labral or chondral injury when it will change care.
Interpretation and limitations
MRI can detect marrow perfusion change earlier than radiographs, but routine immediate MRI does not replace urgent reduction and CT.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Proximal femoral fracture

Shortening and rotation with severe pain can reflect neck or trochanteric fracture; radiographs must be checked before forceful reduction.

Acetabular fracture without dislocation

Hip pain after dashboard force may arise from acetabular column or wall injury with a located but unstable joint.

Pelvic ring injury

Shock, perineal bruising and pelvic mechanism can coexist and require binder and haemorrhage care rather than isolated hip manipulation.

Slipped upper femoral epiphysis

An adolescent with atraumatic or low-energy hip pain and obligatory external rotation needs paediatric non-weight-bearing assessment, not adult dislocation reduction.

Septic arthritis

Atraumatic painful immobile hip with fever and inflammatory features requires urgent aspiration and washout pathway rather than closed reduction.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ImmediateAssess before reductionFirst stepA painful fixed hip deformity follows trauma.

1Run ABCDE, support the limb, give analgesia and examine the pelvis, femur, knee, abdomen and spine for associated injury.
2Document sciatic and femoral motor and sensory function, distal pulses, refill and skin before sedation or manipulation.
3Obtain urgent AP pelvis and a safe lateral view and identify femoral-neck, head and acetabular fracture or prosthetic components.
4Call orthopaedics and anaesthesia and choose resuscitation room or theatre according to physiology, fracture, implant and operator experience.

02ReductionRestore the joint urgentlyA native dislocation is confirmed without a fracture pattern requiring primary open management.

1Provide full monitoring, adequate analgesia, sedation or general anaesthesia and muscle relaxation with an airway and failed-attempt plan.
2Use one controlled in-line technique appropriate to direction and stop if unexpected resistance, crepitus or instability occurs.
3Confirm reduction clinically, repeat named nerve and vascular findings immediately and obtain post-reduction radiographs.
4EscalationEscalate a failed, unstable or non-concentric reduction to urgent CT and consultant-led open or operative management.

03AftercareProtect the femoral head and functionThe joint is concentrically reduced and associated injuries are defined.

1Use CT findings and stability to set fixation, range and weight-bearing restrictions and provide VTE prevention where indicated.
2Begin protected rehabilitation with explicit avoidance positions tailored to direction, repair and prosthetic status.
3Follow sciatic and femoral nerve recovery with motor, sensory and functional assessment and investigate deterioration promptly.
4Arrange long-term review for avascular necrosis, post-traumatic arthritis, heterotopic ossification and recurrent instability.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

Provides titratable analgesia before imaging, transfer and definitive anaesthesia while allowing response and adverse effects to be assessed.

First-line intravenous morphine for severe major-trauma pain

Give 2 mg intravenously, reassess after 5 minutes and repeat small 1–2 mg increments to effect under monitored local trauma guidance rather than giving an unobserved large bolus.

Use lower increments in frailty or cardiorespiratory disease, monitor respiratory rate, sedation, pressure and oxygen saturation and ensure naloxone and airway support are available.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Avascular necrosis

Interrupted femoral-head perfusion can produce collapse and secondary arthritis months or years later despite an apparently successful reduction.

Sciatic nerve injury

Posterior displacement or a fragment can injure peroneal and tibial divisions, causing foot-drop, sensory loss and neuropathic pain.

Post-traumatic arthritis

Cartilage impact, femoral-head or acetabular fracture and residual incongruity accelerate progressive pain, stiffness, functional loss and later arthroplasty.

Heterotopic ossification

Trauma and operative exposure can form ectopic bone around the hip, limiting motion and causing pain or nerve compression.

Recurrent instability

Labral, capsular, bony and muscle damage may allow redislocation, particularly after prosthetic dislocation or inadequate rehabilitation.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat sciatic and femoral motor and sensory findings and distal perfusion immediately after reduction and at every transfer.
Review post-reduction radiographs and CT for congruity, loose fragments, fracture and femoral-neck integrity before mobilisation.
Monitor analgesia and sedation recovery, respiratory function and pressure before leaving a staffed observation environment.
Track pain, range, stability, nerve recovery and weight-bearing compliance through fracture or dislocation follow-up.
Investigate new deep groin pain, stiffness or collapse symptoms for avascular necrosis and post-traumatic arthritis even months later.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

The knee transmits the force

Dashboard injury can damage patella and femoral shaft while the same axial load dislocates the hip and fractures acetabulum.

Nerve timing matters

Pre- and post-reduction documentation separates the original sciatic injury from entrapment or iatrogenic change.

A clunk is not confirmation

Only clinical reassessment plus imaging establishes a concentric reduction and identifies an incarcerated fragment.

Reduction reduces but cannot erase risk

Prompt restoration improves femoral-head prospects, yet vascular disruption at impact can still lead to later avascular necrosis.

A prosthesis is a different problem

Component position, dissociation, infection and periprosthetic fracture require implant-specific films and senior reduction planning.

11Common pitfallsFrequent interpretation and management errors.

01
Reducing a fixed hip without documenting sciatic, femoral and vascular status first.
02
Applying repeated force after resistance without reconsidering an associated neck or acetabular fracture.
03
Calling a palpable clunk a successful reduction before radiographic confirmation.
04
Omitting post-reduction CT when the joint is incongruent or fracture fragments are present.
05
Allowing early pain relief to obscure knee, femoral, pelvic, abdominal or spinal associated injuries.
06
Discharging from short follow-up without explaining delayed avascular-necrosis and arthritis symptoms.