Iatrogenic Nerve Injury & Positioning Injuries in Orthopaedic Surgery

Iatrogenic nerve injury follows one, or several, of a small set of mechanisms, and each procedure has nerves at risk that the surgeon must know. Recognising the mechanism guides both prevention and treatment.

The six mechanisms.

• Direct laceration - scalpel, drill, saw, K-wire, screw or retractor
• Traction or stretch - limb lengthening, fracture or joint reduction, vigorous retraction
• Compression - a retractor, haematoma, tight cast or dressing, or positioning
• Thermal - bone cement, diathermy
• Ischaemia - tourniquet, vascular injury
• Patient positioning - stretch or sustained compression on an anaesthetised, unprotected limb

The hip. The sciatic nerve is at risk, especially its peroneal division, from the posterior approach, lengthening, retractors and dislocation. The femoral nerve is threatened by anterior retractors, the superior gluteal nerve by abductor-splitting approaches, and the lateral femoral cutaneous nerve (LFCN) by the anterior approach and positioning; the obturator nerve completes the list.

The order in total hip arthroplasty. The nerves most frequently involved are, in order, the sciatic, femoral, obturator, superior gluteal and lateral femoral cutaneous nerves. That ranking comes from a review pooling series of different eras and approaches, so it is an order of involvement in the literature rather than a ranked risk for an individual patient.

Other regions. The classic nerves elsewhere are set out below. The spinal accessory nerve is at risk in posterior-triangle procedures, and the brachial plexus and pudendal nerve belong to positioning, covered in the next section.

An anaesthetised, paralysed patient cannot protect their nerves from stretch or sustained compression, so correct positioning and padding of bony prominences are essential. The brachial plexus is stretched by arm abduction and extension, and each position has its own pattern of risk. "Position and pad" is the instruction everywhere; the rules an examiner expects mostly carry a number or a named landmark.

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Re-check, then record. Re-check the position after every change - draping, table tilt, repositioning for imaging - because a limb that was safe at the start may not be at hour three. Record what was padded: documentation is the only evidence that the standard was met.

Who needs the most care. The patient factors that raise risk are diabetes and pre-existing subclinical neuropathy (a "double crush" susceptibility), extremes of body habitus at either end, male sex for the ulnar nerve, prolonged operative time, and intra-operative hypotension and hypothermia.

Examine early. Map the motor and sensory deficit, distinguish a complete from an incomplete lesion, and note whether the onset was immediate or delayed.

Reading the onset

Whether the deficit was immediate or delayed is not book-keeping: it changes the differential and the urgency.

Investigation

Imaging. Ultrasound or MRI is used where structural injury is suspected.

Why the study waits three weeks. After axonal injury, Wallerian degeneration takes days to develop, and fibrillation potentials do not appear on EMG until roughly two to three weeks. A study in the first few days therefore cannot distinguish a neurapraxia from an axonotmesis, because neither yet shows denervation. At around three weeks the baseline study can grade the lesion; a repeat at around three months looks for reinnervation potentials, which appear before clinical recovery is visible.

Why an early study can matter more. Because acute denervation takes weeks to appear, abnormalities found in the first few days must pre-date the operation. Chronic neurogenic change, or an abnormality in the contralateral limb, documents a pre-existing, often subclinical, neuropathy rather than a surgical injury. That is frequently the single most useful piece of evidence in a claim, and it is unobtainable once three weeks have passed.

Where a deficit is significant and pre-existing disease is a live question, request the study early and bilaterally, and accept that a second study will be needed later to grade the lesion.

Observe or explore

Management depends on the likely mechanism and the lesion. Most positioning and retraction injuries are neurapraxias: a neurapraxia or incomplete lesion expected to recover is observed with rehabilitation (splinting, physiotherapy), and serial NCS/EMG document recovery.

Exploration and repair are indicated for:

• A suspected transection, for example after a sharp instrument near a known nerve
• A complete lesion that shows no recovery
• A deteriorating deficit
• A clearly correctable compressive cause

Communication. Communicate honestly with the patient and document. Honest communication, careful documentation of the precautions taken and appropriate management are the basis of good practice.

The decisions between neurapraxia and transection, and between observing and exploring, rest on the Seddon and Sunderland classifications. The cellular pathophysiology - Wallerian degeneration, regeneration - is developed in Nerve Injury & Regeneration.

Seddon.

• Neurapraxia - a conduction block with intact axons and no Wallerian degeneration. The typical positioning or retraction injury, recovering fully in weeks to a few months.
• Axonotmesis - axonal disruption with intact connective-tissue sheaths. The axons regrow distally at roughly 1 mm per day, with variable recovery.
• Neurotmesis - complete division of axons and sheaths. There is no recovery without surgical repair.

Sunderland refines Seddon, splitting axonotmesis into grades II-IV; grade V is neurotmesis.

• I - neurapraxia
• II - axon divided, endoneurium intact; good recovery
• III - endoneurium also divided; incomplete recovery, often needs help
• IV - only epineurium intact; a neuroma-in-continuity that usually needs resection or grafting
• V - complete transection; needs repair
• A mixed (VI) pattern can coexist

How the grade drives the decision. A likely Seddon neurapraxia or Sunderland I-II lesion is observed with rehabilitation and serial studies, expecting recovery. A Sunderland IV-V lesion, neurotmesis or a known sharp transection needs exploration and repair. NCS/EMG help to grade it: absent denervation with preserved distal conduction points to neurapraxia, while fibrillations and denervation indicate axonal loss.

Prevention is the priority. Beyond positioning, it rests on four habits in the operative field, and documenting the precautions is itself important.

Anatomy. Know the at-risk nerve for the approach and protect it during dissection, drilling and implant placement.

Retractor discipline. Place retractors correctly - in hip surgery, off the sciatic nerve posteriorly and the femoral nerve anteriorly - with minimal tension and intermittent release.

Limb length. Excessive lengthening in arthroplasty or deformity correction stretches the sciatic nerve, so avoid it.

Tourniquet. Use it judiciously, with appropriate pressure and time.

Limb-lengthening and deformity correction carry a traction risk. In these high-risk cases intra-operative neuromonitoring (IONM) detects an impending nerve injury in real time, so that the stretch can be caught before it becomes a deficit.

The modalities.

• Somatosensory evoked potentials (SSEPs) monitor the dorsal-column sensory pathway
• Transcranial motor evoked potentials (MEPs) monitor the motor tracts and muscles
• Free-running and triggered EMG detect nerve irritation and, for pedicle screws, breach
• The historical Stagnara wake-up test intentionally lightens anaesthesia to confirm voluntary movement

The alarm. A significant change - classically a 50% fall in SSEP amplitude or a 10% increase in latency, or a marked loss of MEP response - prompts immediate action.

The response. Relax the deforming force by reducing the distraction or lengthening, or backing off a correction. Check limb positioning and retractors, restore blood pressure and limb perfusion (raise the mean arterial pressure, release the tourniquet), warm the patient and re-check. The aim is to reverse the insult before it becomes a fixed deficit.

Where it is used. Spinal deformity correction is the classic indication; complex acetabular, peri-acetabular and pelvic surgery, limb-lengthening and deformity correction, and some revision or high-risk arthroplasty also qualify. IONM adds a real-time safety net to good anatomy, positioning and retractor discipline, and replaces none of them. Nerve repair for an established transection is covered in Nerve Repairs and Nerve Transfers.

Not automatically negligence. A nerve injury after a properly performed and documented operation, with standard preventive measures taken, is not automatically negligence. Many such injuries are unavoidable adverse events arising from the interaction of the patient's anatomy and the procedure. The evidence behind the principle is stronger, and stranger, than the assertion alone.

The closed claims. In the definitive closed-claims analysis, 670 of 4,183 anaesthesia claims (16%) were for nerve injury: ulnar nerve 28%, brachial plexus 20%, lumbosacral nerve root 16% and spinal cord 13%. The ulnar nerve is therefore the single commonest perioperative nerve injury, and it behaves unlike the others:

• It occurred predominantly in men (75%)
• Its onset was delayed in 62%, with symptoms appearing after the immediate postoperative period rather than on waking
• It arose overwhelmingly with general anaesthesia (85%), whereas lumbosacral root and spinal cord injuries clustered with regional techniques

The conclusion that is routinely omitted. The authors could not recommend new prevention strategies, because the mechanism of most injuries - the ulnar nerve especially - is not apparent. That finding is the substance behind the medico-legal framing. Ulnar neuropathies occur despite documented padding and correct positioning, the delayed onset is hard to reconcile with simple intra-operative compression, and the male predominance suggests an anatomical predisposition (a thinner or absent protective fat pad and a larger coronoid tubercle) rather than a lapse in care.

What that means for practice. Padding remains right and must be documented, but it is not a guarantee. The honest position is that some of these injuries are currently unpreventable.