Hip joint dislocation sits at the intersection of high-energy trauma, time-critical intervention, and procedural skill, making it a reliable target for MRCEM SBA practice questions and FRCEM SBA clinical scenarios alike. The ability to recognise the injury rapidly, select the correct reduction manoeuvre, and anticipate neurovascular complications is precisely what the RCEM curriculum tests under its Major Trauma and Musculoskeletal domains. This article walks through the core science and clinical decision-making, then maps each element directly to the format and pitfalls of the SBA exam.
MRCEM SBA practice questions: Key Points
- Posterior dislocation accounts for 85 to 90% of traumatic hip dislocations; the classic mechanism is an axial load transmitted through a flexed, adducted hip (the dashboard injury).
- Closed reduction should be achieved within six hours of injury to minimise the risk of avascular necrosis of the femoral head.
- Sciatic nerve injury complicates up to 20% of posterior dislocations; a full neurological assessment of the lower limb is mandatory before and after reduction.
- Adequate analgesia and procedural sedation with monitoring equivalent to that used for general anaesthesia is required before any reduction attempt in the ED.
- Post-reduction CT of the hip is now standard practice to exclude incarcerated fragments and occult acetabular fractures before weight-bearing is permitted.
- MRCEM SBA practice questions on this topic frequently hinge on the correct sequence of assessment, the named reduction technique, and recognition of the vascular anatomy at risk.
Clinical Context and Epidemiology
Hip dislocation is uncommon in absolute terms, representing approximately 5% of all dislocations presenting to emergency departments, but its consequences are disproportionately severe. The dominant mechanism in the UK is road traffic collisions, particularly the dashboard mechanism in which an axial load is transmitted through the flexed knee along the femoral shaft to a flexed and adducted hip. Sporting injuries (particularly contact sports and equestrian falls) and falls from height account for the remainder in adults. In patients with total hip replacements, dislocation can occur with surprisingly minor force and is an increasingly common presentation as the prosthetic hip population grows.
Posterior dislocations constitute 85 to 90% of all traumatic cases. Anterior dislocations (10 to 15%) result from forced abduction and external rotation or direct anterior trauma, and are subdivided into superior (pubic and iliac) and inferior (obturator and perineal) subtypes. The distinction matters clinically because the neurovascular structures at risk differ: the sciatic nerve is at risk posteriorly, whereas the femoral nerve and femoral vessels are threatened by anterior dislocation. The Royal College of Emergency Medicine curriculum explicitly requires trainees to recognise both patterns and their associated complications.
Pathophysiology: Why Time Is Tissue
The hip joint is one of the most inherently stable articulations in the body. The femoral head is deeply seated within the acetabulum, reinforced by the iliofemoral, pubofemoral, and ischiofemoral ligaments, a fibrocartilaginous labrum, and a thick fibrous capsule. Dislocation therefore demands substantial energy, invariably disrupts the capsule, and in most cases ruptures the ligamentum teres.
The critical vulnerability is vascular. The blood supply to the femoral head is derived predominantly from the medial circumflex femoral artery, a branch of the profunda femoris, which ascends in the retinacular vessels along the posterosuperior femoral neck. Dislocation stretches, kinks, or ruptures these retinacular vessels. Ischaemia is progressive, and the probability of avascular necrosis (AVN) of the femoral head rises steeply with time. Cohort data consistently support a threshold of six hours from injury to reduction as the point beyond which AVN rates increase substantially, though some series suggest harm accrues even earlier. This six-hour window is the single most important time-critical concept tested in MRCEM SBA trauma questions on this topic.
The Thompson-Epstein classification grades posterior dislocations from I (pure dislocation without significant fracture) through to V (dislocation with comminuted acetabular fracture), informing both reduction strategy and prognosis. Grade I injuries are amenable to closed reduction in the ED; higher grades increasingly require orthopaedic involvement and may necessitate operative management. Understanding this classification framework helps candidates interpret question stems that describe radiological findings alongside the dislocation.
ED Assessment: History, Examination, and Investigations
History and Mechanism
Establish the mechanism carefully. A high-energy mechanism in a young adult implies a native joint injury; lower-energy mechanisms in older patients raise the probability of a prosthetic dislocation or an underlying pathological process. Document the precise time of injury: this drives the urgency of the reduction attempt. Establish pre-injury mobility and any previous hip surgery, as the presence of a prosthesis changes both the reduction technique and the threshold for immediate orthopaedic referral.
Clinical Examination
Posterior dislocation presents with the leg held in flexion, adduction, and internal rotation. The limb appears shortened. Anterior dislocation presents with the leg held in extension, abduction, and external rotation, which may superficially resemble a femoral neck fracture. In both patterns, any attempted passive movement is exquisitely painful and should not be forced in the resuscitation bay.
A structured neurovascular examination is mandatory before any intervention. For posterior dislocation, assess sciatic nerve function: dorsiflexion and plantar flexion of the foot, sensation over the dorsum and sole, and the distribution of any paraesthesiae. Document this formally in the notes, as post-reduction comparison is both clinically and medicolegally essential. Assess the dorsalis pedis and posterior tibial pulses. For anterior dislocation, pay particular attention to femoral pulse quality and compare with the contralateral limb.
Imaging
Plain anteroposterior pelvis and lateral hip radiographs confirm the diagnosis and classify the dislocation. Scrutinise for associated fractures of the acetabular wall, femoral head, and femoral neck. A femoral neck fracture is a contraindication to closed reduction by standard manipulation techniques, as rotational forces applied during reduction can displace a minimally displaced neck fracture, converting it to a displaced injury with catastrophic vascular consequences. CT scanning post-reduction (and pre-reduction if the plain films suggest complex fracture-dislocation) is now considered standard practice, consistent with guidance from the National Institute for Health and Care Excellence on major trauma imaging pathways.
Reduction Techniques and Procedural Sedation
Procedural Sedation
Closed reduction requires adequate muscle relaxation and analgesia. This mandates procedural sedation titrated to a level that allows full muscular relaxation, monitoring to a standard comparable to that used for general anaesthesia, and a trained assistant dedicated to monitoring. Ketamine (with or without propofol) and fentanyl-based sedation regimens are commonly employed. The Resuscitation Council UK provides guidance on standards for procedural sedation that underpin safe practice.
Principal Reduction Manoeuvres for Posterior Dislocation
Allis technique: The patient is supine. The assistant stabilises the pelvis by pressing firmly on the anterior superior iliac spines. The operator stands over the patient on the trolley, flexes the hip and knee to 90 degrees, and applies steady inline traction along the axis of the femur, with gentle internal and external rotation to disengage the femoral head. A palpable and audible clunk indicates reduction. This is the most widely taught technique for the ED setting.
Stimson (gravity) technique: The patient is positioned prone with the hip and knee flexed to 90 degrees over the edge of the trolley. The operator applies steady downward traction on the lower leg while stabilising the pelvis. The advantage is that gravity assists reduction and muscular relaxation is facilitated by the prone position, but the technique requires a cooperative patient and sufficient staffing to safely position and monitor a sedated trauma patient prone.
Captain Morgan technique: The patient is supine. The operator’s knee is placed beneath the patient’s flexed knee, acting as a fulcrum. Upward traction is applied by the operator elevating their own knee, converting the force to a lever rather than a direct pull. This technique is ergonomically advantageous and reduces physical strain on the operator.
All three techniques are tested in MRCEM SBA practice questions. Question stems typically describe the patient position and operator action and ask candidates to identify the named manoeuvre, or conversely describe the named manoeuvre and ask for the optimal patient position.
Post-Reduction Management and Disposition
Immediately after reduction, repeat the full neurovascular examination and document findings explicitly. Obtain post-reduction plain radiographs to confirm concentric reduction, then arrange CT of the hip to identify incarcerated bony fragments, chondral injury, or occult acetabular fractures that may not be visible on plain film. Even a Thompson-Epstein grade I dislocation with successful closed reduction requires orthopaedic follow-up within 24 to 48 hours. Non-concentric or unstable reduction, any associated fracture requiring fixation, neurovascular deterioration post-reduction, and prosthetic dislocation that cannot be managed with standard closed techniques all constitute indications for urgent inpatient orthopaedic referral.
Sciatic nerve injury detected pre-reduction, in the absence of worsening, is not a contraindication to closed reduction: indeed, prompt reduction is the intervention most likely to relieve nerve traction. New or worsening neurological deficit identified after reduction should prompt urgent senior orthopaedic review and cross-sectional imaging to exclude an incarcerated fragment impinging on the nerve.
How the MRCEM SBA Exam Tests This Topic
The MRCEM SBA exam draws on the RCEM curriculum’s Major Trauma and Musculoskeletal competencies. Questions on hip dislocation cluster around several high-yield themes. First, candidates are expected to identify the dislocation type from the clinical description (leg position, mechanism) and map it to the correct anatomical structure at risk. A question describing a leg held in flexion, adduction, and internal rotation after a dashboard injury, with weakness of foot dorsiflexion, is testing knowledge of posterior dislocation and sciatic nerve injury simultaneously. Second, the six-hour reduction window is a recurring SBA pivot: questions may present a delayed presentation and ask the candidate to prioritise intervention or identify the complication that is now most likely. Third, contraindications to closed reduction (principally the presence of an ipsilateral femoral neck fracture on plain film) are tested because this requires a change in management pathway. Fourth, post-reduction CT is increasingly the correct answer in SBA stems asking about the most appropriate next investigation after successful closed reduction.
Mapping your revision to the RCEM curriculum domains is essential. The MRCEM SBA syllabus is broad, and high-yield trauma topics such as this one reward candidates who understand the underlying mechanism well enough to reason through novel question stems rather than relying on rote memorisation.
Revision Pearls: High-Yield Facts for the MRCEM SBA
- Posterior dislocation: leg in flexion, adduction, internal rotation. Anterior dislocation: leg in extension, abduction, external rotation.
- The six-hour window from injury to reduction is the critical time threshold for minimising avascular necrosis risk.
- Sciatic nerve injury occurs in up to 20% of posterior dislocations; the common peroneal division is more vulnerable than the tibial division.
- Always exclude an ipsilateral femoral neck fracture on plain radiograph before attempting closed reduction; rotational force during reduction can displace a previously undisplaced neck fracture.
- The medial circumflex femoral artery, via its retinacular branches, provides the dominant blood supply to the femoral head; disruption causes progressive ischaemia.
- Post-reduction CT is standard practice to detect incarcerated fragments and guide further management even after an apparently successful closed reduction.
- Thompson-Epstein grade I posterior dislocation (no significant associated fracture) is the pattern most amenable to closed reduction in the ED; higher grades require orthopaedic input.
- Prosthetic hip dislocation may occur with minimal force; always consider this in older patients presenting after a seemingly trivial fall.
Common Pitfalls: Where Candidates Lose Marks
- Confusing the leg position of anterior and posterior dislocation, particularly when internal versus external rotation is the pivot of the question stem.
- Failing to identify the femoral neck fracture on the radiograph description before selecting closed reduction as the correct answer.
- Selecting plain radiograph rather than CT as the appropriate post-reduction investigation when the question stem specifies that reduction has been confirmed clinically and on initial X-ray.
- Neglecting to document or reassess neurological function after reduction, which is both a clinical and a curriculum competency.
- Underestimating the urgency of reduction in a question that presents a patient who is haemodynamically stable but several hours post-injury: time to reduction is the key variable.
- Assuming that sciatic nerve injury is a contraindication to closed reduction rather than recognising that prompt reduction is the primary treatment.
How EM Learning Centre Supports Your MRCEM Single Best Answer Revision
Preparing for the MRCEM SBA exam requires more than reading around individual topics. The exam rewards the ability to apply clinical reasoning under time pressure across a broad curriculum, from trauma and orthopaedics to toxicology, critical care, and paediatric emergencies. The MRCEM Single Best Answer revision course at EM Learning Centre is structured around the current RCEM syllabus, with topic-by-topic lessons, integrated MRCEM SBA practice questions, and worked explanations written at the level of detail illustrated in this article.
Whether you are working through high-yield trauma scenarios such as hip dislocation, refining your approach to anaphylaxis and angioedema, or building confidence across the full breadth of the SBA syllabus, the platform provides structured, exam-mapped content designed specifically for UK trainees. Visit the EM Learning Centre homepage to explore the full course library and access MRCEM SBA question bank resources built around the way the exam actually tests clinical knowledge.
The BMJ and RCEM Learning both emphasise that deliberate, case-based practice with immediate feedback is the most effective revision strategy for high-stakes single best answer examinations. Every question you work through on a topic like hip dislocation builds the pattern-recognition that distinguishes a candidate who has revised from one who has trained.
References
- Royal College of Emergency Medicine. RCEM Curriculum and Competency Framework. Available at: rcem.ac.uk
- National Institute for Health and Care Excellence. Major Trauma: Assessment and Initial Management (NG39). Available at: nice.org.uk
- Resuscitation Council UK. Safe Sedation Practice for Healthcare Procedures. Available at: resus.org.uk
- The BMJ. Clinical evidence and education resources for emergency medicine. Available at: bmj.com
