Outcomes of Sciatic Nerve Injury Repairs: A Systematic Review

The sciatic nerve is the largest nerve in the human body (approximately 2 cm in diameter)¹  formed by the amalgamation of the lumbar roots L4 and L5, sacral roots S1 to S3, and spinal nerve roots.² Originating in the buttock region, it descends posteriorly along the thigh, providing motor innervation to the hamstring muscles and adductor magnus.³ At the apex of the popliteal fossa, it bifurcates medially and laterally to form the tibial and common peroneal nerve, respectively.⁴ Damage to the nerve leads to various degrees of sensorimotor deficit defined by the anatomical level of injury. Sensory symptoms include numbness, paraesthesia, and allodynia of the lower extremities.⁵ Motor symptoms include weakness and paralysis in the associated muscles.^4,6 

The tibial nerve and its branches provide sensory and motor innervation to the posterior compartment of the leg, foot, and sole muscles⁷; damage to this nerve can result in loss or weakening of plantar flexion, toe flexion, and ankle inversion.^7,8 The common peroneal nerve innervates the lateral compartment of the leg and foot; as this nerve wraps around the neck of the fibula, its injury can result in loss of dorsiflexion and foot drop.⁸ 

Sciatic nerve injuries are most commonly seen in war/combat-related injuries, with the literature describing peripheral nerve injuries associated with as many as 30% of combat-related injuries.^9,10 These include penetrating mechanisms, such as blast shrapnel, gunshot wounds, or bone fragments/fractures as the secondary injury mechanism after explosions.¹¹ In addition, due to the diameter, length, and regeneration distance of the nerve, repairs lead to significant challenges and dilemmas. Ultimately, for these reasons, many studies have illustrated poor functional outcomes and high complication rates following sciatic nerve repair in both civilian and military practices.

Axonal regeneration occurs exclusively in peripheral nerves. Wallerian degeneration occurs at the distal axonal stump, and growth cone formation occurs at the proximal axonal stump.^12,13 In complete nerve transections, this recovery process can be hindered by neuroma formation. Incomplete regeneration requires appropriate treatment to avoid permanent muscle atrophy and functional loss.¹⁴

Surgical repair aims to reestablish continuity between the proximal and distal nerve stumps to restore innervation of end target receptors.¹⁵ Sciatic nerve injuries are commonly repaired directly or reconstructed with autologous nerve grafts along with neurolysis. Direct (end-to-end) repair is exclusively performed when the gap is short and tension-free repair is possible.^15,16 This is accomplished by approximating the ends with tissue-fibrin glue or by coapting the epineurium with sutures.¹⁷ On the other hand, nerve grafts are required when the gap is too extensive to allow tension-free repair. The sural nerve is the most common donor nerve used.¹⁸ Neurolysis aims to removes scar tissue between the fascicles (internal neurolysis) or from around the nerve (external neurolysis). Surgical neurolysis involves dissection and exploration of a damaged nerve with the goal of freeing the nerve from local tissue restrictions or adhesions. Additionally, multilevel segmental injuries may require a combination of techniques.

Evaluating patient outcomes following nerve repair using valid outcome measure tools is essential as it provides important feedback on results of sensorimotor recovery and function. A scale proposed by the Medical Research Council (MRC) is used to measure the strength of the individual muscle groups.¹⁹ The MRC scale (Table 2) grades muscle power on a scale from 0 to 5.²⁰ There are also modified versions of the MRC scale, such as the Louisiana State University Health Sciences Centre (LSUMC) muscle grading system.²¹

Injury to the sciatic nerve is a challenging and devastating complication of lower extremity trauma. Due to the large regeneration distances, many patients are often left with incomplete functional recovery even after successful surgical repair.²² Indeed, outcomes have improved significantly over the last few decades due to advances in microsurgery; however, the current literature on sciatic nerve repairs, results, and follow-up assessment is scarce. This review aims to summarize the current literature on surgical repair techniques and the motor and sensory outcomes following sciatic nerve injuries.

To assess the outcomes of various surgical repair techniques involving sciatic nerve injuries, a systematic review was designed conforming to the guidelines set by PRISMA.²³ The review was prospectively registered with PROSPERO under the identification CRD42021246717. Institutional ethics approval was not required.

Search Criteria

A thorough search of the literature was then conducted in March 2021 by identifying articles containing certain keywords in the MEDLINE, PubMed, Embase, and Cochrane databases. The keywords comprised sciatic AND nerve AND repair AND technique OR conduit OR graft, OR reconstruct* AND outcome OR rehab* OR recovery OR function AND surgery AND NOT anaesthesia. Further to the results generated by this automated search, a manual search was conducted by screening the references of the returned articles to identify any additional articles of interest.

Selection Criteria

After eliminating 440 duplicate studies, the search returned a total of 715 articles that were of potential relevance. The titles and abstracts of these articles were analyzed by two authors. There were no restrictions on language, race, sex, year, and age. The inclusion criteria used were English-language publications, case series studies with more than 2 participants, studies that focused on adult populations exclusively or included subgroup analysis of adult patients, and studies that analyzed the outcomes of multiple surgical repair techniques of the sciatic nerve. The exclusion criteria were studies without adult populations, studies on animal models, case reports of fewer than 2 participants, studies examining surgical repair of nonsciatic nerves or noncomplete branches of the sciatic nerve, and surgical repairs of iatrogenic sciatic nerve injuries. The total number of papers included within this review is 13 (Figure 1).^24-36

The various levels of lesions of the sciatic nerve were defined as “high” for the hip or buttock, “intermediate” as the thigh, and “low” as distal to the popliteal fossa. For Aydin et al,²⁸ “above knee” was reported as “intermediate,” and for Roganović 2005 et al,³¹ “above middle thigh” was reported as “high.” 

Study Quality and Risk of Bias

A risk of bias analysis using the National Institutes of Health (NIH) Quality Assessment Tool for Case Series Studies³⁷ was completed for each study included in the review (Table 1). The overall quality of the studies was then rated on a 3-part Likert scale of good, fair, and poor with poor studies being excluded from the systematic review.

Data Collection and Analysis

Full text was analyzed for each included study to extract demographic data including number of patients, mean age of patients, country of origin, and mean follow-up period. Data on repairs included location of nerve lesion (ie, sciatic and/or tibial division and/or peroneal division) and repair technique used (ie, nerve grafting, suture repair, and/or neurolysis). 

Outcomes of sciatic repairs were measured using a variety of different classification systems, including ≥3 on the LSUMC scale, >2 S-score, ≥3 M-score, and ≥3 Patient (P)-score. These criteria were selected on the basis that they were deemed functional.^24-36 Both LSUMC and M-scores of ≥3 indicate “active movement against gravity,” as shown by the MRC scale (Table 2). In addition, S-score of >2 demonstrates “Recovery of some degree of superficial pain and tactile sensibility within the autonomous zone.”²⁴ (Table 3) Motor and sensory outcomes were summarized, and weighted means were calculated for both outcomes. However, not all studies were used to calculate weighted means due to some studies having no outcome results.

A total of 13 relevant studies were included in the review. The number of patients, male-to-female ratio, age (mean or range), nerves studied, repair technique, preoperative and postoperative LSUMC/M-scores, S-scores, extent of injury, time of data collection, country where the study took place, and type of study are shown in Tables 4 and 6. Levels of evidence were allocated to each study according to the Oxford levels of evidence.³⁸  The studies ranged from as far back as 1940 to 2015 (Table 4). The peroneal nerve was the most common nerve affected, accounting for 38.6% (n = 1057) of injuries (Table 5). The thigh (intermediate) was the most common level of injury (n = 1651), and the distal to popliteal crease (low level) was the least (n = 491; Table 7). Neurolysis alone was the most common surgical technique used (n = 1192), with suture repair being the least common (n = 482; Table 8). Among the various types of injury mechanisms, gunshot wounds accounted for 22.6% of all injuries for which the cause was specified (368 of 1627 wounds; Table 9). The weighted means were calculated for the motor and sensory outcomes. The M-score and S-score weighted means were 50.11% and 66.94%, respectively (Table 6). This means that the articles that had a greater number of findings that were ≥M3 and >S2 contributed more to the weighted mean.

Sciatic nerve injuries have relatively poor but variable outcomes following surgical repair. This review identifies and analyzes 3 main techniques used in the published literature: nerve grafts, direct suture repair, and neurolysis. 

Neurolysis was the most common technique used (45.4%) and yielded more superior outcomes compared with other techniques. Neurolysis is used to “free-up” nerves that have become entrapped and/or compressed by fibrotic tissue from the resulting injury. It is mostly used to treat lesions in continuity that fall under the classification of neuropraxia or axonotmesis as opposed to neurotmesis. In more severe neurotmesis injuries, where axonal continuity is lost, neurolysis can still be used as a means of resecting fibrous scar tissue through circumferential release, allowing tension-free repairs. Interestingly, Roganović et al^30,31 carried out neurolysis on all of their patients in combination with other techniques that may have augmented the repair to produce greater functional recovery. This was also the case in the study by Aydin et al.²⁸ 

Nerve grafts were the second-most common technique used (n = 953). All the studies included in this review utilized nerve grafts. The sural nerve was the most popular donor nerve. Nerve grafts are affected by a multitude of factors that can impact the outcome of nerve repairs. For example, Roganović³⁰ identified that the length of graft significantly influenced the outcome. Successful outcomes of functional recovery were obtained in 57.4% (27 out of 47) of patients with a nerve allograft (sural nerve) shorter than 4 cm. On the other hand, a graft longer than 8 cm yielded a functional outcome in only 22.4% (19 out of 85) of patients. The differences may be attributed to the time taken for regenerating axons to cross the length of the nerve graft and each anastomosis site. Consequently, longer nerve grafts have a higher risk of fibrous ingrowth and stalling. In addition, this means that axonal diameter is reduced due to excess tension, which results in ischemic stress on the nerve and produces a worse outcome.³⁹   

Dunn 2021²⁷ was the only study within this review to measure preoperative M-scores. Similarly, no studies reported preoperative S-scores. Somasundaram et al used the MRC scale to measure the preoperative and postoperative scores of common peroneal nerves following nerve decompression in complex knee injuries. This was a retrospective study of 6 consecutive patients with foot drop due to sport-related injuries. All 6 patients had a preoperative electromyographic evaluation and clinical examination. Following this, the common peroneal nerve was decompressed surgically. As a result, a statistically significant functional and clinical recovery from preoperative MRC grade 2.5 ± 0.8 to postoperative MRC grade 4.0 ± 0.9 was identified (P = .007). Measuring motor function before surgery gives a baseline value to compare and assess the efficacy of surgical intervention and recovery. As a result, future studies should include these measurements along with standardized outcome measures to better evaluate and compare results.

The studies included in this review were limited most notably due to the heterogenous and nonstandardized methods in measuring sciatic nerve repair outcomes. Some studies analyzed outcomes using the MRC scale, whereas Kline et al,³² Kim et al (2003),³⁵ and Kim et al (2004)^33,34 all used the LSUMC scale. Overall, there was no standardized approach to measuring motor outcomes. These disparities made it difficult to directly compare different studies. The MRC scale is more accurate and reliable for the clinical assessment of weaker motor function (grades 0-3); for stronger motor function (grade 4-5), however, assessment using an analog scale is more accurate and reliable. As a result, improvements to the MRC scale have been made. This includes a further breakdown of the scale as previously there were substantial variations of the extent of muscle strength assessed by each of the 6 grades.¹⁹ Furthermore, the modified MRC scale has been recognized as having high interrater reliability.⁴⁰ Therefore, a homogenous measurement tool must be agreed upon when assessing nerve function and recovery in future studies to facilitate meaningful comparisons. 

Clawson et al²⁴ and Roganović et al^30,31 both used the MRC scale to evaluate sensory outcome. However, the assessment of sensory deficit was not reported by the other reviews. Clawson et al²⁴ and Roganović et al^30,31 stated that sensory outcomes greater than S2 were significant. S2 determines the return of superficial cutaneous pain and some degree of tactile sensibility.⁴¹ Mros et al⁴² used the Semmes Weinstein monofilament test to evaluate the threshold stimulus required for perception of light touch to deep pressure, thus estimating sensory recovery. The monofilament test encompasses quantitative data that can be used serially to assess nerve regeneration. On the other hand, the MRC has been scrutinized due to its tendency to yield inadequate, nonstandardized data and subjective findings. Hence, an accurate and valid tool needs to be agreed upon for sensory function. The Semmes Weinstein monofilament test is a good initial tool to gather important information on sensory recovery in peripheral nerve repairs. 

Roganović et al^30,31 included patient-reported outcomes in which patients were asked questions about their overall satisfaction regarding the outcome. Patients compared their outcome at the last check-up with their preoperative state. During their follow-up, they assessed the quality of recovery and explained their reasons for their selection. It was found that the patients reported overall dissatisfaction with their final outcome, but interestingly, their feedback did not correlate with the measured motor and sensory recovery. Although not exclusively stated within the methods, subjective assessment of improvement and recovery satisfaction were used in the assessment of patient-reported outcomes. The other articles included within this review did not include any patient-reported outcomes. 

Since the research by Roganović et al was published, many studies have evaluated the use of newer techniques, such as nerve transfers. Nerve transfers are used to augment motor and sensory recovery after injury.⁴³ It involves using an expendable donor nerve that serves the equivalent function as another nerve in the body and affixing it to a denervated (injured) nerve in an end-to-end or end-to-side fashion. Hence, the rerouted nerve can aid motor or sensory recovery distal to the injury site.²⁹ Moore et al evaluated the use of nerve transfers to provide better outcomes than those seen with alternative techniques.³⁰ The transfer technique was carried out on 2 male patients with sciatic nerve injuries resulting in complete palsy. Both patients underwent a transfer of distal motor branches of the femoral nerve to the gastrocnemius nerve, along with transfer of saphenous nerve to sural nerve for sensation. The researchers found that both patients achieved motor recovery of MRC grade 3 and 3+ in plantar flexion by 18 months postoperatively. The patients also recovered sensation, which was signified by the Tinel sign. Interestingly, it was found that direct end-to-end repair of nerves yielded better functional outcome than the use of a graft in nerve transfers. This study shows that the femoral nerve has promising use as a nerve transfer donor with sufficient reinnervation to distal nerves. These further supports the potential use nerve transfers use in sciatic nerve repairs.

The study by El-Taher et al utilized a dual nerve transfer approach in 31 patients with foot drop. The operative technique involved motor transfer of tibial nerve branches to the deep peroneal nerve to reinnervate muscles of dorsiflexion. They reported that 48.4% of patients achieved motor recovery of MRC grade 3 or 4 after 1 year. These findings further support the use of proximal nerve branches to reinnervate distal nerve branches.

The benefits of nerve transfers appear to be due to the shorter distance over which the regenerating axons have to travel, only having to traverse 1 neurorrhaphy site compared with 2 sites in nerve grafts.⁴⁵ These benefits can also be attributed to the specificity of transfer; that is, motor donors are joined to motor nerves and sensory donors to sensory nerves, thus optimizing nerve regeneration. 

To the authors’ knowledge, there are currently no studies that directly compare nerve transfers and nerve grafting in sciatic nerve palsy. However, Garg et al made this comparison in upper brachial plexus injuries. This systematic review found that 96% of patients achieved an MRC grade of 3 or above following nerve transfer, compared with 83% of patients that achieved an MRC grade of 3 or above following nerve grafts.⁴⁶ Therefore, nerve transfer was preferred over traditional nerve grafting. As a result, nerve transfers have the potential to be preferable to nerve grafts in the future repair of peripheral nerves. 

Moreover, an alternative approach for treatment of nerve injury is the use of an autogenous muscle-in-vein conduit (MVC), pioneered by Brunelli et al.⁴⁷ This technique involves laying muscle fibers within an autologous vein, which is then used to bridge the gap between the injured nerve stumps to act as guidance conduit for nerve regeneration. The vein acts as a wall with anti-inflammatory properties while the muscle tissue adds strength to the structure, preventing collapse and acting as a guidance network for regenerating axons.⁴⁷  Further to this, Brunelli and colleagues bridged sciatic nerve defects in animals, and they found a higher number of axons in the MVC group than in the nerve grafting group.⁴⁸ However, Ulkur et al found higher number of myelinated axons in rats that underwent autologous nerve grafting compared with those with MVCs.⁴⁹ Another studied concluded similar findings of significantly lower number of fibers in the MVC group.⁵⁰  

Furthermore, there are various other nerve repair procedures, including end-to-side repairs, vascularized nerve grafts, and cadaver transplants of the sciatic nerve. However, these studies were not included in the review as they did not meet the standard required because the studies were conducted with animal models,^51,52 which was addressed within our exclusion criteria.

Moreover, 2 other elements were not included within the current review’s data: age and timing of surgery. First, knowing the age of the patients at the time of injury in the 13 studies would be essential to identifying whether there was any significant difference in outcomes between young adults and older adults. Second, the timing of the repair—whether it was immediate or delayed—could have influenced the results. However, analyzing these cofounding variables is beyond the scope of this review and requires complex statistical analysis to draw conclusions. 

The review was also limited by the lack of recent studies being included, with most coming from the 1990s. A potential reason for this could be due to a higher number of war-related injuries during a time when more service personnel were exposed to ballistic combat than have been reported in recent years. This, however, may not be the case and would legitimize the need for more innovation in sciatic nerve repair. The lack of prospective studies is also an area for which improvement could be made. Using only retrospective studies diminishes the scientific quality of the data as it relies on recollection of events in the past and introduces a degree of bias.

Restoration of distal limb sensorimotor function was achieved in approximately half of the cases with sensation being more commonly restored than motor function. Successful outcomes as measured by the LSUMC/MRC grading scale was defined as contraction against gravity and additional mild resistance (≥3 points) in most studies. These studies found that superior success rates were achieved when neurolysis was performed compared with suture repair, even better than was seen with nerve grafts. This is possibly because neurolysis was performed on lesions in continuity (ie, nerves that are not severely severed), whereas nerve grafts were performed on nerves in discontinuity. 

Overall, the outcomes of sciatic nerve repair are relatively poor and debilitating, even with lesions in continuity requiring only neurolysis. The reasons for this could be attributed to the highly traumatic nature of these injuries, which were frequently segmental, multilevel injuries involving long regeneration distances. Further prospective trials using validated patient reporting outcome measures are needed to evaluate the outcomes of repairs and more innovative techniques, such as nerve transfers.

Affiliations: ¹Imperial College London, Faculty of Medicine, London, United Kingdom; ²Glasgow Royal Infirmary, Glasgow, United Kingdom

Correspondence: Vyshnavi Thanaraaj, MBBS, BSc; vyshnavi.thanaraaj16@imperial.ac.uk

Ethics: The review was prospectively registered with PROSPERO under the identification CRD42021246717. Institutional ethics approval was not required.

Disclosures: The authors disclose no relevant financial or nonfinancial interests.