Hip Dislocations

Traumatic dislocation of a native hip is a true orthopaedic emergency. The femoral head's blood supply is stretched for as long as the head sits out of the socket, the sciatic nerve lies directly behind the joint it has just left, and the energy needed to dislocate a healthy hip commonly injures the femoral head (Pipkin fracture), the acetabulum (posterior wall), the femoral neck (15%) and the knee (PCL, patella) as well. The three things to get right are the time to reduction, the sciatic nerve and the associated injuries, and each is taken in turn below.

Who. Young adults after high-energy trauma, men about twice as often as women (2:1) because of high-risk activities. A healthy native hip rarely dislocates; the far commoner hip dislocation is the prosthetic one, which is a different topic.

Mechanism. The force vector determines the direction of dislocation.

• Motor vehicle accident - the most common, the classic dashboard injury
• High-energy sport - rugby, American football, skiing
• Industrial accidents
• Fall from height

A stable joint. The hip is a ball-and-socket joint that is inherently stable, and it takes significant force to dislocate. Stability comes from bony congruity, the acetabular labrum, the strong capsular ligaments (iliofemoral, pubofemoral and ischiofemoral) and the surrounding muscles.

Blood supply of the femoral head. The retinacular vessels of the medial femoral circumflex artery carry most of it, and they are the vessels a dislocation damages.

Why dislocation causes AVN. Dislocation stretches or tears the retinacular vessels that supply the femoral head, and for as long as the head stays out the ischaemia continues. The figures that turn this into a rule about time are given with the management algorithm below.

The sciatic nerve. It leaves the pelvis through the greater sciatic foramen below piriformis, passes directly posterior to the hip joint lying on the gemelli and obturator internus, and divides into tibial and peroneal divisions. A posterior dislocation stretches it, and the nerve is injured in 10-20% of posterior dislocations. The peroneal division is injured more than the tibial: it lies more lateral, is tethered at the sciatic notch and at the fibular head, and is less mobile. The clinical picture is a foot drop from weak dorsiflexion, with numbness over the dorsum of the foot and the lateral leg. Document sensation and motor function before and after reduction.

Posterior dislocation (90%). The dashboard injury: knee flexed, hip flexed, the force transmitted along the femur and the head driven out posteriorly. The more adducted the hip, the more superior the exit. The injuries that travel with it are the posterior wall fracture and the sciatic nerve palsy.

Anterior dislocation (10%). Forced abduction with external rotation. With the hip flexed the head comes to lie inferiorly (the obturator type); with the hip extended it lies superiorly (the pubic type). The femoral vessels are at risk, rarely, and the associated bony injury is an impaction fracture of the femoral head.

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Posterior dislocations are graded by Thompson and Epstein, femoral head fractures by Pipkin and anterior dislocations by Epstein. In practice the working split is simpler: a simple dislocation is reduced urgently and a complex dislocation (one with a fracture) is reduced and then fixed definitively, with the post-reduction CT deciding whether the wall or the head needs fixation.

History. The mechanism and its force vector (motor vehicle accident, dashboard, sport), the time of injury because the 6-hour rule runs from it, associated injuries (polytrauma, knee), whether the hip is native or prosthetic, and anticoagulation for the bleeding risk.

Examination. Hip dislocation often occurs in polytrauma, where ATLS takes priority: complete the primary and secondary survey, and log-roll to assess pelvic stability. The deformity is diagnostic and the limb position names the direction, with a leg length discrepancy to match. Record the neurovascular status - the sciatic nerve (peroneal division) behind a posterior dislocation, the femoral nerve and lateral cutaneous nerve in front of an anterior one - and examine the knee for the PCL and patellar injuries of a dashboard strike. An anterior dislocation may leave the hip slightly flexed.

Reading the AP pelvis. The dislocated head is the wrong size as well as in the wrong place, because its distance from the plate has changed, and Shenton's line is disrupted in both directions.

• Posterior: head superolateral to the acetabulum; smaller apparent head (further from the plate); look for a posterior wall fragment; internal rotation of the lesser trochanter
• Anterior: head inferomedial (obturator) or anteromedial (pubic); larger apparent head (closer to the plate); external rotation makes the lesser trochanter prominent; the head may overlap the obturator foramen

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The post-reduction CT. Every reduced hip gets one, and it dictates the rest of the treatment.

Non-concentric reduction. An asymmetric joint space on the post-reduction CT means an incarcerated fragment - labrum, bone or capsule - sitting between head and socket. It needs urgent operative removal to prevent AVN and cartilage damage, and a clunk felt at reduction does not exclude it.

Algorithm

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Decision by pattern. Once the hip is in and the CT read, the pattern chooses the treatment.

The posterior wall threshold. A wall fracture over 40% of the articular surface leaves the hip unstable even after reduction and needs ORIF with buttress plating; under 40% may be stable, and stability is assessed clinically under fluoroscopy. The 40% figure is how CT triages the wall, not the last word on it. In Moed's series walls under 20% were generally stable and walls over 40-50% generally unstable, but static CT measurement was an unreliable predictor across the 20-40% band, where a dynamic fluoroscopic stress examination under general anaesthesia is the recommended test of stability. A wall proved stable on that examination can be managed non-operatively: Grimshaw and Moed's 21 such fractures all scored good to very good at a minimum of 2 years, and every hip assessed radiographically had a congruent joint without post-traumatic arthritis.

The neglected dislocation. A traumatic dislocation that presents late, weeks to months unreduced, is a recognised problem, especially where access to acute care is limited, and it behaves very differently from the acute injury. The femoral head becomes fixed in soft-tissue scar with progressive cartilage loss and capsular contracture, so simple closed reduction is almost never possible and must not be forced, because of the high risk of fracturing the femoral neck or shaft.

Management is staged and depends on age and the state of the cartilage: a period of preliminary skeletal traction to bring the head down gradually, then open reduction with capsulotomy, clearance of the scar and fibrofatty tissue from the acetabulum, and sometimes a femoral shortening osteotomy to allow tension-free relocation and protect the sciatic nerve. In a young patient with preserved cartilage the aim is to relocate and preserve the joint, accepting a high risk of AVN and arthrosis; where the head or cartilage is destroyed, arthrodesis or arthroplasty is chosen by age and demand. Outcomes are guarded, and the patient must be counselled that AVN and post-traumatic arthritis are likely.

Reducing an anterior dislocation. Closed reduction of an anterior dislocation is by external rotation, extension and traction; the injury is less common and often requires open reduction. After any reduction, posterior or anterior, confirm a concentric joint on fluoroscopy, take the hip through a full range under the image to assess stability, and send the patient for the CT.

Choosing the approach. The Kocher-Langenbeck posterior approach is the workhorse for posterior dislocations and posterior wall fractures; anterior approaches serve anterior dislocations and anterior column fractures; a surgical hip dislocation serves complex patterns and femoral head fractures. For the femoral head the choice carries a vascular cost: in Stannard's 26 head fractures the Kocher-Langenbeck approach carried 3.2 times the odds of AVN of the Smith-Petersen anterior approach, and 3-mm cannulated screws with washers gave poor results and are contraindicated for head fixation. The Ganz surgical dislocation answers the same problem from the other side: a trochanteric flip osteotomy with anterior dislocation through a posterior approach, in which the external rotators are not divided and obturator externus protects the medial femoral circumflex artery, giving full access to the head and acetabulum; in 213 hips with intra-operative confirmation of head perfusion no subsequent AVN was reported.

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Femoral head fractures by Pipkin type. Femoral head fractures have worse outcomes and higher AVN rates, and the type decides the operation.

Pipkin I (inferior to the fovea). The fragment is usually small and outside the weight-bearing surface. Excise it if it is small and not blocking reduction; fix it if it is larger or involves a weight-bearing portion.

Pipkin II (superior to the fovea). The weight-bearing portion is involved, so ORIF is preferred, with countersunk headless screws, through an anterior approach (Smith-Petersen or Watson-Jones).

Pipkin III (I or II with a femoral neck fracture). An emergency, because both blood supplies are compromised. The neck fracture takes priority: fix it in the young; the elderly often require THA.

Pipkin IV (I or II with an acetabular fracture). Both the acetabulum and the head are addressed, by a sequential or combined approach.

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Avascular necrosis. AVN may take 6-24 months to appear on the radiograph, and MRI detects the marrow changes earlier. Follow with radiographs at 6 weeks, 3 months, 6 months, 1 year and 2 years, and treat symptomatic AVN with THA.

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Sciatic nerve injury. Most are neuropraxia or axonotmesis, and the early management is to document and wait.

When the foot drop does not recover. The peroneal division recovers worst, so a proportion of patients are left with a permanent foot drop despite the favourable overall prognosis. If recovery has not occurred by around 18-24 months, with EMG showing no reinnervation, the deficit is treated as established. First-line is an ankle-foot orthosis, often all an older or low-demand patient needs to clear the foot in swing and walk safely. For a fit patient who wants to be brace-free, a tibialis posterior tendon transfer, rerouted anteriorly through the interosseous membrane to the dorsum of the foot, restores active dorsiflexion; tendo-Achilles lengthening for a fixed equinus, or ankle arthrodesis for an unstable or arthritic ankle, are added as needed. Keep the early nerve question (document, observe, EMG at 3 months, explore at 6 months) separate from the late functional one (orthosis or tendon transfer once the palsy is permanent).

Weight bearing. The stability of the hip sets the pace. A simple dislocation with a concentric reduction and no fracture may weight bear as tolerated early; after posterior wall ORIF, weight bearing is protected until the wall has healed, at 6-12 weeks; after Pipkin ORIF it depends on the size of the fragment and the fixation.

What decides the result. The favourable hip was reduced within 6 hours, had no fracture, achieved a concentric stable reduction, belongs to a young patient without comorbidities and has an intact sciatic nerve. The unfavourable one was reduced late (over 6-12 hours), carries an acetabular or femoral head fracture, was reduced non-concentrically or remains unstable, has a sciatic nerve injury (particularly of the peroneal component), or endured repeated closed reduction attempts.

In the long term. Most patients with a simple posterior dislocation reduced within 6 hours have a good functional outcome. The determinants of long-term outcome are AVN and post-traumatic osteoarthritis; younger patients may tolerate mild degenerative change better than older patients, and THA remains the salvage procedure for severe AVN or debilitating arthritis.