An intraoperative neurophysiological monitoring method for testing functional integrity of the low extremity peripheral nerves during hip surgery

31.1

Introduction

Peripheral nerve injury is a serious potential complication in hip procedures, which may result in postoperative permanent sensory and/or motor deficits. The incidence of sciatic nerve palsy after hip surgery ranges from 0.3% to 4% in primary total hip arthroplasty (THA) up to 7.6% in revision surgery or developmental dysplasia of the hip (DDH) However, Weber in 1976 found evidence of subclinical nerve injury in 21 out of 30 patients (70%) using preoperative and postoperative electromyographic evaluation .

More than half of the sciatic nerve palsies after hip surgery have no explanation for the mechanism that caused the injury. Nevertheless, several etiologic factors have been associated with an increased risk for iatrogenic sciatic nerve damage, including gender (female>male), previous neuropathy (e.g., diabetic), posttraumatic arthritis and preoperative diagnosis of hip dysplasia . Other proposed risk factors are revision surgery, retractors’ placement and its duration, posterolateral approach, leg lengthening (more than 4 cm), direct trauma to the nerve either by retraction, laceration, thermal injury, position of the limb, impaired vascularity, postoperative intraneural hematoma and the use of cemented femoral stems by hitting or trapping the sciatic nerve .

Many studies have suggested that sciatic nerve lesions are often incomplete in most patients as isolated peroneal nerve palsy. In 1945 Sunderland proposed two factors to explain this higher peroneal susceptibility to injury. For starters, its tethering at the sciatic notch and at the neck of the fibula makes the nerve more sensitive to stretching. Furthermore, it has been shown that resistance to stretching or mechanical injury increases as the amount of connective tissue increases. The peroneal nerve is composed of tightly packaged funiculus, while the tibial nerve has a large amount of connective tissue with scattered funiculus . On the other hand, delayed sciatic nerve palsy has been attributed to entrapment within hematomas or fibrous adhesions that compress the nerve leading to axonal degeneration.

In more than 80% of these cases, the lesions may result in some permanent sequelae .

Experiments in animal models have demonstrated that extensive dissection, stretching or compression may significantly reduce peripheral nerve blood flow . Furthermore, when compression is administered sequentially, its effects appear to be potentiated by previous ischemia, even if the ischemic period was short and, by itself, did not affect nerve conduction. Kennedy et al. placed recording electrodes directly on the sciatic nerve and noted decreased amplitudes and increased latencies of compound nerve action potential in response to nerve retraction. Upon removal of the retractors these changes resolved .

Previous studies have been performed in order to assess impending injuries of sciatic nerve during specific maneuvers. In hip surgery, using noninvasive neurophysiological methods, Stone et al. in 1985 were the first to report somatosensory-evoked potentials (SEPs) changes caused by retractors’ position or its direct pressure on the sciatic nerve.

Another potential cause is represented by the extreme internal rotation of the leg or by traction forces during reduction maneuvers during THA . Nercessian et al. in 1989 reported that SEP monitoring was also a useful method to minimize risk of nerve injury during revision surgery or reoperation . Nevertheless, Black et al. in 1991 found no difference in the incidence of intraoperative sciatic nerve palsy during THA between the monitored group with SEP and the unmonitored group . Later on, another group concluded that SEP monitoring was a safe and useful method in assessing sciatic nerve compromise within a group of patients with higher risk, such as in revision surgery or DDH surgery .

Sutherland et al. in 1996 found that spontaneous electromyography (EMG) might provide real-time monitoring of nerves’ functional integrity that allows immediate warning signals for the surgeons to take corrective actions in order to avoid nerve injury . Satcher et al. in 2003 described the use of motor-evoked potentials (MEPs) combined with free-running EMG to monitor sciatic nerve status and reduce risk of sciatic nerve injury during THA . In 2017 Novais et al. used transcranial electrical stimulation (TES) for eliciting MEPs, SEPs, and spontaneous EMG in a series of 34 patients undergoing periacetabular osteotomy (PAO) and succeeded in detecting temporary sciatic nerve warnings . However, due to their small sample size, they do not specify if their multimodal method helps in preventing impending injury. Other groups have also monitored femoral nerve, although it is less frequently damaged during hip procedures and is the second most frequent neurological complication .

So far, multimodal intraoperative neurophysiological monitoring (ION) using multimodal evoked potentials and specific, more sensitive methods to assess the functional integrity of the peripheral nervous system to evaluate sciatic and femoral nerves in hip surgeries has not been described.

31.2

Material and methods

31.2.1

Subjects

A series of 100 patients operated by two surgical teams, working at two separate institutions, underwent ION of the sciatic and femoral nerve by the same neurophysiologist during hip surgery between June 2008 and December 2013 are included in this study.

The patients included 67 females and 33 males, ages ranging from 28 to 81 years (average age 44.96±18.7 SD). Subjects were admitted for dysplasia of the hip in 45 patients, failed THA (cup aseptic loosening, stem subsidence, and periprosthetic fractures) in 21 patients, primary coxarthrosis in 19 patients, femoral-acetabular impingement in four patients, avascular necrosis in three patients, and a miscellaneous group of eight patients. Patients with neurological deficit prior to the surgery were not included. In 24 patients a THA had been performed, in 46 patients a PAO for a DDH (Ganz osteotomy), in 22 patients a revision of THA surgery, in four patients femoral-acetabular osteoplasty, and in four patients other procedures have been reported ( Fig. 31.1 ). All patients from the two institutions signed a consent form for the surgery procedure with ION.