THA Neurovascular Injury

Neurovascular injury is among the most feared and devastating complications of total hip arthroplasty. It is relatively uncommon, but it can leave permanent disability, a dissatisfied patient and litigation, and that makes prevention and early recognition paramount.

Nerve injury. The sciatic nerve accounts for 80% of nerve injuries, and its peroneal division is affected more often than the tibial. The femoral nerve accounts for 10-15% and is associated with the anterior approach. The superior gluteal nerve accounts for 5%, and presents as abductor weakness.

Vascular injury. The external iliac artery is the most commonly injured vessel. The common femoral artery is at risk with the anterior approach, and the superior and inferior gluteal vessels with the posterior. Injury shows as immediate bleeding or later as a pseudoaneurysm or arteriovenous fistula, and major vascular injury carries a mortality risk of 5-10%.

The sciatic nerve. The largest nerve in the body, formed from the L4-S3 roots. It leaves the pelvis through the greater sciatic notch and, at the level of the hip joint, lies 2-3cm posterior to the posterior acetabular rim, protected by the short external rotators (quadratus femoris and the obturators). That distance is critical: excessive posterior wall reaming, retractor placement or cement extrusion can injure the nerve directly.

Its two divisions. The peroneal division lies lateral and posterior to the tibial division, has less connective tissue support, and is the more vulnerable to stretch. The tibial division is medial and better supported. It is less commonly injured, usually as part of a complete sciatic injury, and carries a better prognosis than the peroneal.

The piriformis relationship. This matters for the posterior approach, and it varies:

• 85% - the nerve exits below piriformis (the standard anatomy)
• 12% - the nerve divides at the sciatic notch and the peroneal division passes through piriformis
• 3% - the nerve exits above piriformis

The variant cannot be predicted from external landmarks, so assume the vulnerable anatomy.

The other nerves. The femoral nerve lies anterior to the hip, beneath the inguinal ligament and lateral to the femoral artery. Anterior retractor compression, haematoma and cement extravasation injure it, and the injury is often a neuropraxia. The superior gluteal nerve exits above piriformis and supplies gluteus medius and minimus. Excessive superior dissection or retractor trauma during exposure produces abductor weakness and a Trendelenburg gait, and the injury often goes unrecognised.

The arteries. The external iliac artery runs along the pelvic brim, where medial acetabular screw penetration, intrapelvic cement extravasation and overzealous anterior retractor placement put it at risk. Atherosclerosis increases the risk of vascular injury, especially to this artery. The common femoral artery is vulnerable to direct injury in the anterior approach, to the saw during the femoral neck osteotomy, and to anterior perforation of the femoral canal.

The veins. The external iliac vein has a thinner wall than the artery and is easily torn. It presents with bleeding and haematoma formation, and may cause a delayed DVT if the injury goes unrecognised. The gluteal vessels are at risk with the posterior approach; bleeding is usually controlled with packing, and pseudoaneurysm is rare but reported.

Vascular anomalies. These include a high bifurcation of the common femoral artery (5%), an accessory obturator artery (20-30%) and an aberrant external iliac course. Pre-operative vascular imaging is the mitigation in revision cases and protrusio repair. Both the nerve and the vascular variants are the argument for careful dissection and for adherence to recognised surgical landmarks rather than presumed anatomy.

Nerve injuries are graded by Seddon (neuropraxia, axonotmesis, neurotmesis), and recovery by grade is set out under Outcomes. The classification that shapes prevention is of risk, which comes from the patient and from the operation.

Patient factors. DDH and revision carry the largest multiples of risk, and each factor has its own preventive answer.

Surgical factors. Each approach puts a different nerve at risk. Lengthening threatens the peroneal division through traction, and protrusio repair threatens the external iliac vessels.

Examine before the patient leaves theatre. The single most important step in diagnosing neurovascular injury is a systematic post-anaesthesia examination before the patient leaves the operating theatre. Failure to perform it is the most common reason for delayed diagnosis of nerve injury: patients emerging from anaesthesia may not recognise or report subtle motor or sensory deficits, and a nerve injury missed in the immediate post-operative period becomes far more difficult to prove causation for and to manage appropriately.

The examination covers:

• Foot dorsiflexion (peroneal division of the sciatic nerve) and plantarflexion (tibial division)
• Toe extension and flexion
• Hip flexion and knee extension (femoral nerve)
• Dorsalis pedis and posterior tibial pulses, and capillary refill in both limbs

Record it within the hour. Motor power on the MRC 0-5 scale, sensation to light touch and pinprick, and vascular status (pulses, temperature, colour) go into the notes, compared with the contralateral limb and with any pre-operative baseline.

When a deficit is found. Inform the supervising consultant immediately. Check the leg-length differential clinically and radiographically, review the intraoperative notes for risk factors, consider urgent imaging if there is a vascular concern, and start the management algorithm.

The patterns. Each nerve leaves its own combination of motor deficit, sensory loss and special signs.

In the common peroneal-division pattern, tibialis anterior and the peroneal muscles are weak, and the numb dorsum of the foot and lateral leg is the territory of the superficial peroneal nerve. The tibial division is less commonly affected; with it spared, sole sensation is intact and plantarflexion may be preserved.

The differential. Not every post-operative motor or sensory deficit is a direct surgical nerve injury. The following must be actively excluded, because management differs substantially.

Recognising vascular injury. Acute injury shows as absent or diminished distal pulses, a cold, pale limb compared with the other side, an expanding haematoma in the wound or thigh, hypotension if blood loss is significant, and pain out of proportion as compartment syndrome develops. The delayed complications are:

• Pseudoaneurysm - a pulsatile mass with a bruit
• Arteriovenous fistula - a continuous thrill
• Compartment syndrome (see Management)
• DVT from venous injury
• Heterotopic ossification from haematoma

At the bedside. Investigation starts with the examination above: systematic motor and sensory testing graded on the MRC scale, a clinical measurement of leg lengths, and a look for signs of compartment syndrome.

Radiographs within 1-2 hours. An AP pelvis and lateral hip show:

• Component position and orientation
• Limb length discrepancy
• Femoral offset restoration
• Cement extruded medially
• Acetabular screws penetrating the medial wall

Nerve studies at 3 weeks. Baseline EMG and nerve conduction studies are done 3 weeks post-injury. Earlier studies are unhelpful because Wallerian degeneration takes 2-3 weeks to develop. At 3 weeks the study distinguishes neuropraxia from axonotmesis, establishes baseline severity for monitoring recovery, localises the level of injury and guides the prognosis discussion. Repeat studies at 3 and 6 months track the recovery trajectory.

Cross-sectional imaging. MRI of the pelvis and hip, if surgical exploration is planned, shows nerve compression from haematoma or cement, assesses nerve continuity and rules out a space-occupying lesion. CT angiography, if vascular injury is suspected, shows active extravasation, pseudoaneurysm or arteriovenous fistula.

Suspected arterial injury. Within the first hour, a bedside duplex ultrasound assesses flow, the ankle-brachial pressure index (ABPI) is compared with the contralateral side, and compartment pressures are measured if there is clinical concern. Within 6 hours, CT angiography, the gold standard, defines the level and extent of injury, with conventional angiography if intervention is planned. The findings decide the pace:

• Hard signs - absent pulses, expanding haematoma, pulsatile bleeding, limb ischaemia - mean immediate theatre for exploration
• ABPI under 0.5 means urgent vascular surgery consultation
• Soft signs mean close observation with serial ABPI and duplex

Suspected venous injury. The clinical features are a large haematoma, limb swelling disproportionate to the surgery, persistent bleeding from the drain, or hypotension without an obvious bleeding source. Duplex ultrasound assesses for DVT, CT venography is used if major vessel injury is suspected, and haemoglobin is measured serially. Small venous injuries are often self-limiting with compression; a large haematoma may require drainage to prevent compression of adjacent structures, and DVT prophylaxis is adjusted to the bleeding risk. Venous injuries are less immediately life-threatening than arterial, but haematoma mass effect and DVT can cause significant morbidity.

Algorithm

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The first hours. Within 0-2 hours, confirm the diagnosis: document the deficit completely (motor and sensory), review the intraoperative events, measure leg length radiographically, and grade the palsy as complete or partial. Between 2 and 6 hours, assess the components. The decision point comes at 6-12 hours.

When to go back urgently. Any of these is an indication for urgent revision, exploration or evacuation:

• Limb lengthening over 4cm with a complete sciatic palsy
• Cement extrusion compressing the nerve or other neurovascular structures
• Component malposition causing direct nerve compression
• A haematoma compressing the nerve - evacuate urgently, and revise if the compression does not resolve with drainage alone
• A progressive deficit: partial becoming complete, or worsening despite initial observation
• Severe pain suggesting nerve compression
• Vascular compromise accompanying the nerve injury

A complete palsy with substantial lengthening is the combination that argues for urgent revision to shorten the limb; not every post-operative palsy needs the components out. A partial deficit without lengthening, where neuropraxia is likely, is observed. The 4cm in the first criterion is the conventional figure discussed under leg length, a planning trigger rather than a validated threshold.

Observation. If observation is chosen, close neurovascular monitoring is essential:

• Daily neurovascular examination
• A foot drop splint (ankle-foot orthosis) to prevent contractures
• Physiotherapy for passive range of motion
• EMG/NCS at 3 weeks
• Re-assessment at 6 weeks, 3 months and 6 months

Revising for lengthening. Component exchange aims to bring the limb-length differential to under 4cm:

1. Remove the acetabular component and insert a smaller liner
2. Reduce femoral offset with a different stem and head combination
3. Consider a femoral shortening osteotomy if needed
4. Verify leg length with fluoroscopy before closure

Decompressing the nerve. Decompression releases external compression of the nerve:

1. Extend the posterior approach proximally
2. Identify the sciatic nerve at the sciatic notch
3. Trace it distally to the point of compression
4. Remove the cement, haematoma or scar tissue
5. Do not manipulate the nerve itself, which risks further injury

What going back achieves. 50-60% recover useful function after component revision for lengthening, and nerve decompression for compression brings 40-70% improvement. Outcomes are better if the operation is within 72 hours. A complete nerve transection has a poor prognosis regardless of intervention.

Vascular injury. The presentation sets the investigation and the clock.

Compartment syndrome. Haematoma from a vascular injury, or reperfusion after ischaemia, can lead to compartment syndrome, typically 24-48 hours post-operatively.

The decision at a glance. The management above, condensed into four scenarios:

Prevention is far superior to management. The intraoperative techniques that minimise neurovascular risk concern where the screws go, how the patient lies, how the nerve is protected during exposure, how much the leg is lengthened and how the cement is contained.

The Acetabular Quadrant System - Where Screws May and May Not Go

Why a map is needed. The structures at risk lie on the other side of the bone, and nothing of the intrapelvic anatomy is visible from inside the socket. Screw safety therefore cannot be judged by direct vision the way it can at most other sites. Wasielewski's cadaveric study answered this by fixing the danger to landmarks the surgeon can see from within the acetabulum.

Constructing the quadrants. Two lines are drawn on the acetabulum as it faces you:

1. The first runs from the anterior superior iliac spine, through the centre of the acetabulum, to the posterior fovea, dividing the socket into anterior and posterior halves
2. The second is drawn perpendicular to the first at the mid-point of the acetabulum, splitting each half into superior and inferior

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The rule. Screws go posteriorly, and preferably posterosuperiorly. The two anterior quadrants are avoided whenever possible, because a screw that exits there meets vessels which will not tolerate it.

Two cautions for the viva. The quadrant system is a guide to direction, not a licence on length: a correctly aimed posterosuperior screw can still be too long, so measure, and check the trajectory under fluoroscopy when the anatomy is distorted. And the landmarks assume recognisable acetabular anatomy. In revision, dysplasia or protrusio the fovea and rim may be absent or displaced, which is precisely when screws are most needed and the map is least reliable; in that setting pre-operative CT, not the quadrant rule, tells you where the vessels are.

Positioning

Lateral decubitus for the posterior approach. The patient lies with the affected side up, the lower limb flexed at hip and knee for stability, the operative limb free to move through its range, and pelvic posts anterior and posterior to stabilise the pelvis. To protect the sciatic nerve, avoid hip flexion over 90 degrees, which increases its tension, and extreme internal rotation, which stretches it; at rest the operative limb sits in 20-30 degrees of flexion and neutral rotation. Excessive flexion, adduction or rotation during surgery increases nerve stretch.

Supine for the anterior approach. The patient lies on a radiolucent table with the operative leg on an extension attachment and a perineal post for countertraction. Limit hip extension beyond neutral, which stretches the femoral nerve, and avoid excessive lateral translation of the femoral shaft, which compresses it.

Exposure

The posterior approach protects the sciatic nerve in four steps.

1. Superficial dissection. Identify the sciatic nerve before placing deep retractors: palpate it posterior to the short external rotators, protect it with a blunt retractor or sponge stick placed posteriorly, and mark its position with a suture if the anatomy is unclear.
2. Capsulotomy. Open the capsule along the femoral neck, staying anterior to the nerve's course, under direct vision rather than blindly with scissors. Avoid excessive posterior capsule release, because in revision cases the nerve is adherent to the posterior capsule.
3. Retractor placement. The anterior retractor sits on the anterior wall of the acetabulum, which is safe. The posterior retractor goes medially, not posterior to the acetabulum. Use blunt retractors only, since pointed ones can impale the nerve, and release them every 20 minutes to reduce ischaemia.
4. Reaming. Use hand-held retraction, have the assistant monitor the retractor position continuously, and do not ream excessively posteriorly, which risks cement extrusion toward the nerve.

The anterior approach protects the femoral nerve. It uses the interval between tensor fascia lata (superior gluteal nerve) and sartorius and rectus femoris (femoral nerve), a true internervous plane. Stay in the muscular interval and avoid excessive medial dissection, where the femoral nerve and vessels lie. The anterior retractor over the femoral neck goes against bone, not soft tissue, is released every 15-20 minutes, and is not levered against the anterior structures. The medial retractor protecting the femoral vessels is hand-held only, not self-retaining, and placed intermittently during femoral preparation. Safe exposure depends on adjusting retractor position frequently rather than on static retraction.

Leg Length

Planning. Template to plan offset and leg length restoration, measure the contralateral limb length as a baseline, and identify the anatomical landmarks (lesser trochanter to centre of rotation).

Measuring in theatre. Fluoroscopy and pin-to-pin measurement give a number; the shuck test does not.

Cement

The medial wall. Cement extruding medially through a thin wall or an acetabular defect can compress the external iliac vessels, the femoral nerve or the obturator nerve. It may present as delayed vascular compromise or a progressive nerve palsy, hours to days after the operation. Prevention:

• Bone graft medial wall defects before cementing
• Contain the cement with a restrictor or mesh
• Inject at low pressure
• Watch for medial extrusion with fluoroscopy during injection

The posterior wall. Cement extruding posteriorly can directly contact or compress the sciatic nerve. Do not over-ream posteriorly, check the integrity of the posterior wall by direct vision before cementing, place the posterior retractor to shield the nerve during injection, and remove excess cement immediately, before it polymerises.

The femur. Canal reaming can perforate anteriorly, especially in dysplastic femurs or in revision with a thin anterior cortex. Prevention is hand-feel during reaming, radiographic landmarks, and considering an uncemented stem if the canal is abnormal. Pressurise with a cement gun and canal restrictor, but avoid excessive pressurisation in thin cortices, which risks perforation and cement extrusion.

What it offers. Some high-volume centres use intraoperative SSEP or EMG neuromonitoring for Crowe III-IV cases, while routine use is not evidence-supported (see Guidelines). Its value is a real-time warning: the surgeon can release traction, reduce lengthening or reposition a retractor before the injury becomes permanent.

The risk. An acetabular cup, screw or cement mass that has migrated medial to the quadrilateral plate lies against the external iliac and common femoral vessels, and the obturator. It is the highest-stakes vascular scenario in revision THA, and that single radiographic finding changes how the revision is planned and staffed: blind extraction from the hip risks fatal haemorrhage.

Removing it safely. Imaging, vessel control, and a willingness to leave it:

1. Image first. CT, ideally CT angiography, defines the relationship of the intrapelvic hardware to the vessels. Involve vascular surgery before the day of surgery.
2. Plan vessel control. For hardware intimate with the vessels, plan a retroperitoneal (ilioinguinal or Stoppa) approach to control the iliac vessels first, or keep vascular surgery and an endovascular balloon-occlusion catheter on standby. Do not simply pull it out from the hip.
3. Know when to leave it. A well-fixed intrapelvic fragment encasing the vessels may be safer left in situ than risking vascular catastrophe or precipitating uncontrollable haemorrhage. Have cross-matched blood and cell salvage ready.

Secondary complications. Beyond the primary nerve and vascular injuries, several complications arise from the injury or its management.

The medicolegal position. Nerve injury after THA is a common source of litigation. Defensibility rests on five things: a documented pre-operative baseline neurovascular examination; informed consent that specifically discusses nerve injury risk, especially in high-risk cases; a leg length measurement documented before closing; a post-operative neurovascular examination documented in the recovery room; and timely revision or observation with a clearly documented rationale. Failure in any of these areas significantly weakens the medicolegal position. The full documentation checklist sits under Guidelines.

The first 48 hours. In recovery, alongside the examination described under Clinical Assessment, the leg lengths are checked radiographically on an AP pelvis, the distal pulses are confirmed and documented, and the pain management protocol is started.

• 2-24 hours. Neurovascular checks every 4 hours for the first 24 hours, watching for compartment syndrome (pain, paraesthesia, pressure) and for excessive drain output, which may indicate vascular injury. Weight bearing follows the protocol, protected if nerve injury is suspected.
• Days 1-2. Physiotherapy assessment of gait and transfers, an AFO fitted if foot drop is present, passive range of motion exercises to prevent contractures, and continued neurovascular observations if there is a deficit.

Neuropraxia, weeks 0-6. The goals are to prevent contractures and maintain joint mobility. The AFO is worn continuously (removable for hygiene), alongside daily passive range of motion exercises, protected weight bearing with a walking aid and weekly physiotherapy review.

Neuropraxia, weeks 6-12. The goals shift to monitoring recovery and strengthening the recovering muscles. The AFO continues while there is no dorsiflexion power, active-assisted exercises begin as motor recovery returns, and a repeat EMG at 12 weeks assesses the trajectory; prognosis is discussed on the basis of the recovery pattern. Electrical stimulation may help, though the evidence is limited.

Axonotmesis. The nerve regenerates at 1mm per day (1 inch per month), and a sciatic injury at the hip is 80-90cm from the foot. The expected time to reach the foot muscles is 24-30 months, functional recovery may take 2-3 years, and many patients recover incompletely.

Secondary procedures for persistent foot drop. Indication, expected outcome and timing for each:

Tibialis posterior transfer. The tendon is passed through the interosseous membrane to the dorsum of the foot and attached to the middle cuneiform and the bases of the 2nd-4th metatarsals. The patient must have tibialis posterior strength of MRC grade 4 or greater. The foot is immobilised for 6 weeks, followed by intensive physiotherapy, and functional dorsiflexion is achieved in 70-80% of carefully selected patients.

Recovery by Seddon grade. Timeline, expected outcome and prognostic factors for each grade:

Other prognostic factors. A young patient has better regeneration capacity, and an elderly one reduced capacity. No lengthening is favourable, the injury being from transient compression rather than traction, whereas lengthening over 4cm means a traction injury and a poor prognosis. An isolated peroneal-division injury leaves less disability than a complete sciatic nerve injury, which causes severe functional impairment. Traditional lists also put a partial deficit at presentation among the good signs and a complete palsy from the time of surgery (read as suggesting neurotmesis) among the bad; on the evidence above, that distinction is a poor guide.