Posterior Hip Dislocation

Posterior hip dislocation is displacement of the femoral head behind the acetabulum. It accounts for about 90% of traumatic hip dislocations. It is an orthopaedic emergency because delay beyond 6 hours increases the risk of avascular necrosis of the femoral head.

It follows a high energy force through the flexed, adducted knee and hip, such as a dashboard injury in a head on road traffic collision, a fall from height or a sporting collision. It is most frequent in young men. Hip dislocation after a low energy fall in a prosthetic hip is common in older adults.

The Thompson-Epstein classification describes the pattern. Type I is a dislocation with no or a minor fragment. Type II has a large posterior wall fragment. Type III has a comminuted rim. Type IV has an acetabular floor fracture. Type V has a femoral head fracture (Pipkin).

Associated injuries are acetabular posterior wall fracture, femoral head and neck fractures, ipsilateral femoral shaft and knee ligament injury (posterior cruciate ligament, patellar), sciatic nerve injury in 10 to 20%, and spinal and chest injuries. Complications are avascular necrosis (10 to 25%, rising to 50% if reduction is delayed more than 6 hours), post-traumatic arthritis (20 to 50%), sciatic nerve palsy, recurrent instability, heterotopic ossification, myositis ossificans and deep vein thrombosis.

Pathophysiology.

Force through the femur drives the head out through the posterior capsule. The retinacular vessels that feed the femoral head are stretched or torn, and the sciatic nerve lies directly behind the hip, where the head or a bony fragment can tether it. Time in the dislocated position increases ischaemia.

Severe pain, inability to move the leg. The classic posture is a shortened leg held in flexion, adduction and internal rotation, with the prominent femoral head palpable in the buttock. An isolated dislocation without fracture may have no obvious shortening if the patient is obese.

Test sciatic nerve function before and after reduction: dorsiflexion and plantar flexion of the ankle and toes, and sensation over the foot and lateral leg. Check the femoral pulses and distal perfusion. Examine the knee for ligament injury, tenderness over the femoral shaft, and the spine.

• Anteroposterior pelvis radiograph confirms the dislocation: the femoral head is smaller than on the other side and lies superior to the acetabulum (the Shenton line is broken). Look for a femoral neck fracture before any attempt at reduction.
• Do not delay reduction for CT. After reduction, CT of the pelvis with thin cuts defines the congruity, intra-articular fragments (loose bodies), posterior wall fracture, and femoral head or neck fracture.
• MRI later for chondral injury, labral tear and early avascular necrosis.
• Knee and femur radiographs; trauma series when injury is high energy.

Differential diagnosis.

• Anterior hip dislocation and femoral neck fracture.
• Acetabular fracture with central dislocation.
• Femoral shaft fracture and dislocated hip prosthesis.
• Traumatic hip dislocation with an associated pelvic ring injury.

Reduction within 6 hours of injury, ideally sooner, under sedation with muscle relaxation or general anaesthesia.

• Allis manoeuvre: the patient is supine, an assistant stabilises the pelvis; flex the hip and knee to 90 degrees, apply steady upward traction along the femur, with slight adduction and internal rotation, then abduct and externally rotate to reduce the head.
• Captain Morgan manoeuvre: flex the hip and knee to 90 degrees with the knee over the clinician's knee, press down on the lower leg while lifting the thigh.
• Stimson gravity method: prone with the leg over the edge of the bed and the knee flexed to 90 degrees, with downward pressure on the calf.
• Reduction is confirmed by a smooth clunk, equal leg lengths and radiographs. Gently test stability in flexion. If two attempts fail, move to theatre; repeated forceful attempts risk femoral neck fracture and cartilage injury.
• Irreducible hip (interposed capsule, piriformis tendon, labrum, bone fragment or a buttonholed head): open reduction by a posterior approach.

Reduction under sedation. Ketamine 1 to 2 mg/kg intravenously over 1 to 2 minutes as a single dose (a further 0.5 to 1 mg/kg after 10 minutes if needed; 4 to 5 mg/kg intramuscularly without access).

Give it by a clinician skilled in airway care with continuous oximetry and capnography, after fasting where time allows. Avoid with raised intracranial pressure, psychosis or acute upper airway infection. General anaesthesia with a muscle relaxant gives the best conditions for a difficult reduction.

After reduction.

• CT to confirm a congruent joint and to show fragments. Surgery is needed for intra-articular fragments, an incongruent reduction, a posterior wall fracture (large fragment, unstable hip), femoral head fracture, or recurrent instability.
• Simple dislocations: skin traction or a knee immobiliser for pain, early mobilisation with protected weight bearing (touch to partial) for 4 to 6 weeks, then full weight bearing. Avoid flexion beyond 90 degrees, adduction and internal rotation for 6 weeks.
• Sciatic nerve palsy: if it appears after reduction, explore the hip for an entrapped fragment. Use an ankle foot orthosis for foot drop. Recovery occurs in about 50 to 60%.
• Avascular necrosis: monitor with MRI at 3 to 6 months and 1 year, treat early disease with protected weight bearing; collapse needs hip replacement.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL).

With severe penicillin allergy, use vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Thromboprophylaxis. Enoxaparin 40 mg subcutaneously once daily, started within 12 hours after reduction or surgery once bleeding is controlled, and continued for 28 to 35 days or until the patient is mobile.

Dose adjustments: 20 mg once daily at creatinine clearance under 30 mL/min; adjust by anti-Xa level in obesity and renal impairment. Check the platelet count on days 5 to 14 for heparin induced thrombocytopenia. Use mechanical prophylaxis until then.

Analgesia.

• Paracetamol 1 g orally or intravenously every 6 hours (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Morphine 2.5 to 5 mg intravenously every 5 to 10 minutes as needed before reduction, titrated to pain (halve the dose with age over 75 or eGFR under 30 mL/min/1.73 m2; reassess after 10 mg). Monitor respiratory rate, oxygen saturation and blood pressure continuously.
• Afterwards oral morphine 5 to 10 mg every 4 hours as needed for up to 3 days with a laxative (avoid exceeding 60 mg in 24 hours without senior review).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use. Short term use does not harm healing.

Monitoring. Sciatic nerve function and distal perfusion before and after reduction and every 4 hours in the first 24 hours. Radiographs at 2 weeks, 6 weeks, 3 months, 6 months and 12 months. MRI at 3 to 6 months and 12 months for avascular necrosis. Hip function and pain at each visit. Heterotopic ossification on radiographs at 6 to 12 weeks.

Referral and escalation. Emergency: refer to orthopaedics on diagnosis and arrange reduction within 6 hours. Immediate escalation for failed reduction, associated femoral neck fracture, open injury, vascular compromise, or new nerve deficit after reduction.

Special groups.

• Children: dislocation follows minor trauma and the avascular necrosis risk is lower, but reduction should still be urgent.
• Older adults: low energy injury suggests prosthetic hip dislocation, bone loss or neurological disease; reduction is often closed with revision if recurrent.
• Pregnancy: left lateral tilt of 15 degrees and obstetric input.
• Kidney impairment: avoid ibuprofen and reduce morphine doses.