Journal of Foot and Ankle Surgery (Asia-Pacific)
Volume 10 | Issue 3 | Year 2023

Identification of the Superficial Peroneal Nerve: Is the Ankle Plantar Flexion Fourth Toe Flexion Sign Useful in a South-East Asian Population?

Yuet Peng Khor1, Manu J Abraham2, Audrey WC Tan3, Christopher J Pearce4

1–4Department of Orthopaedic Surgery, Ng Teng Fong General Hospital, Singapore

Corresponding Author: Yuet P Khor, Department of Orthopaedic Surgery, Ng Teng Fong General Hospital¸ Singapore, Phone: +6569082222, e-mail: yuet_peng_khor@nuhs.edu.sg.

Received on: 09 February 2023; Accepted on: 17 March 2023; Published on: 07 July 2023


Aim and background: Iatrogenic injury to the superficial peroneal nerve (SPN) can occur during surgery on the foot or ankle. The purpose of the study was to evaluate the prevalence of the SPN clinically when using the ankle plantar flexion fourth toe flexion sign in our local population.

Materials and methods: Between August and November 2020, all new patients between 21 and 70 years of age who attended our foot and ankle specialist clinic were invited to participate in the study. Participation was voluntary, and individuals with a history of surgery to the foot were excluded from the study. Volunteers were provided with a patient information leaflet and asked to complete an anonymous questionnaire with details of their age, sex, weight, height, race, shoe size, and weekly sports participation. Their feet were then examined by a consultant foot and ankle surgeon to record the presence of the SPN. The SPN can be identified by flexing the fourth toe with the ankle in plantar flexion and inversion (Fig. 1). This was recorded as either visible, not visible but palpable, or absent.

Results: A total of 154 volunteers were recruited for the study (male = 93, female = 61). The SPN was visible in 35% (n = 54) of patients. In those patients where the nerve was not visible, another 14 patients had the nerve identified by means of palpation. In total, the nerve was identifiable 44% of the time on clinical examination.

Body mass index (BMI) (p = 0.01) and sports participation (p = 0.011) were significant determinants of whether the nerve was clinically identifiable.

Age, sex, shoe size, and race did not differ between the groups where the SPN was identifiable clinically.

Conclusion and clinical significance: The ankle plantar flexion fourth toe flexion sign is a useful clinical sign to identify the SPN when planning incisions over the foot or ankle during surgery to reduce the chance of iatrogenic injury. This sign can aid in identifying the nerve in 44% of our local population. A lower BMI and the population who participated in sports were factors associated with identifying the nerve.

How to cite this article: Khor YP, Abraham MJ, Tan AWC, et al. Identification of the Superficial Peroneal Nerve: Is the Ankle Plantar Flexion Fourth Toe Flexion Sign Useful in a South-East Asian Population? J Foot Ankle Surg (Asia-Pacific) 2023;10(3):119–122.

Source of support: Nil

Conflict of interest: None

Keywords: Fourth toe flexion sign, Observational study, Superficial peroneal nerve


The superficial peroneal nerve (SPN) is a branch of the common peroneal nerve that has combined motor and sensory innervations. It provides motor supply to the lateral compartment muscles of the leg and sensory supply to most of the skin at the dorsum of the foot. At the level of the ankle joint, its terminal sensory branches are the medial and intermediate dorsal cutaneous branches.1 The intermediate dorsal cutaneous branch can be potentially injured when placing the anterolateral portal for anterior ankle arthroscopy, whilst the medial dorsal cutaneous branch may be injured during anterior surgical approaches to the ankle.2,5 Whilst the SPN is largely a sensory nerve at the level of the ankle with no motor loss in case of an iatrogenic injury, this can lead to longstanding numbness over the dorsum of the foot and even painful neuromas. Neuromas are often difficult to treat, resulting in chronic pain, impairment, psychological distress, and possibly even additional surgery.6 The adoption of arthroscopic techniques in the ankle for procedures such as ligament repairs or reconstruction, cartilage resurfacing, and joint washout or arthrodesis has helped offer advantages such as shorter hospital stay, reduced pain, faster rehabilitation, and decreased wound-related morbidity.7-9 The prevalence of iatrogenic injury to the SPN has been reported between 3 and 14% of complications in anterior ankle arthroscopy.10-12 As such, being able to locate the SPN with clinical tests may help prevent or reduce the incidence of iatrogenic nerve injury.

The identification of the SPN clinically is made possible with ankle plantar flexion in combination with inversion of the ankle and fourth toe flexion.13-17

Our study aimed to assess if the ankle plantar flexion fourth toe flexion sign improved the clinical identification of the SPN in a South-East Asian population.


We obtained local institutional review board approval before the commencement of this study. All patients who presented to our Foot and Ankle Orthopaedic Specialist Clinic for the first time between August and November 2020 were invited to participate in the study. The patients were given a patient information leaflet in English detailing the purpose of the study to read at the clinic reception, and subsequent participation in the study was voluntary. The volunteers were then asked to fill up a questionnaire recording their age, gender, height, weight, race, shoe sizes, and their participation in sports with no personal identifiers. Patients who were below 21 years which is the legal age of consent in Singapore, and above 70 years were excluded, along with those who had previous surgery to the foot and ankle. The questionnaire was then handed over to a consultant foot and ankle surgeon, who then examined their feet. The examination technique was standardized by placing the ankle in maximum plantar flexion and inversion and flexion of the fourth toe (Fig. 1). The identification of the SPN was recorded as either visible, palpable, or absent.

Fig. 1: The course of the superficial perineal nerve can be clearly seen here (red arrows) with the ankle placed in plantar flexion and inversion and passive flexion of the fourth toe

The data was then collated and analyzed on Microsoft Excel 2016. Patients where the SPN was identified clinically (either visible or palpable only) were compared with the group where it was absent. Where the SPN was only identified in one foot, the patient was analyzed in the group where it was identified. The unpaired student t-test was used to compare the continuous variables and the Chi-squared test for categorical data. A p-value of <0.05 was deemed clinically significant. A binary logistic regression was performed, and their odds ratio (OR) and confidence intervals (CIs) were presented.


There were 154 participants who volunteered in our study, of whom 93 were men, and 61 were women. The SPN was identified on visual inspection in 54 patients, of which 35 were men (Table 1). In another 14 patients, the SPN was identified by palpation only (male = 11). In total, the SPN could be identified in 68 patients. Among the male volunteers, the SPN could be identified in 49.4% of patients, whereas it was 36% among the women, but gender was not a statistically significant variable in identifying the nerve (p-value = 0.10). The other variables, such as age, race, and shoe size, were not found to be statistically significant variables.

Table 1: Univariate binary logistic regression analysis of factors for identification of the SPN with the ankle in plantar flexion and fourth toe in flexion
Volunteers (n = 154) SPN identified (n = 68) SPN not identified (n = 86) p-value Coefficient SE OR 95% CI
Sex 93 (60.4%) 46 (67.8%) 47 (54.6%) 0.10 0.55 0.337 1.73 0.89, 3.36
 Male 61 (39.6%) 22 (32.3%) 39 (45.3%)
Age (mean) 38.4 39.8 0.84 −0.0022 0.011 0.99 0.97, 1.02
Race 53 57 0.11 0.5865 0.3709 1.79 0.86, 3.71
 Chinese 15 29
Shoe size [median standard deviation (SD)] 8
SD 2.12
SD 2.16
0.38 0.0668 0.0763 1.06 0.92, 1.24
Sports participation 40 83 0.011 0.399 0.169 1.49 1.06, 2.08
None 21 16
At least once a week
BMI (mean) 17.6 27.7 0.01 −0.1045 0.04 0.90 0.83, 0.97

SPN, superficial peroneal nerve; SE, standard error; OR, odds ratio; CI, confidence interval

Mean BMI was found to be 27.7 in the group where the nerve could not be identified compared to the mean BMI of 17.6 in the group where the nerve was identified, and this was found to be statistically significant (p-value = 0.01). In addition, individuals who did not participate in sports were found to be associated with a lower probability of identifying the SPN, as evidenced by the observation that 83 of the 86 did not participate in any sports (p-value = 0.01).

On univariate binary logistic regression, a lower BMI had an OR of 0.9 with a 95% CI of (0.832–0.975) and sports participation had an OR of 1.49 with a 95% CI of (1.069–2.08) for identifying the SPN. Figure 2 shows the binary logistic regression chart for these two factors.

Fig. 2: Binary logistic regression chart showing probability of identifying the superficial peroneal nerve vs body mass index (kg/m2) and sports participation (number of times/week)


Iatrogenic injury to the SPN can result in morbidity, and having useful clinical tests is helpful to all surgeons who operate on the ankle and foot. Several clinical tests have been described, such as ankle plantar flexion with inversion, fourth toe passive plantar flexion, and transillumination test, to aid in the identification of the SPN preoperatively.11,13,14,17,18 However these tests are not entirely reproducible in all individuals.

Intraoperative identification of the SPN has been attempted with transillumination through the anteromedial portal with the scope.11,18 Hanroongroj and Chuckpaiwong studied this however found that due to the similar water content between the nerve and surrounding tissues, the nerve did not refract light, and none of their patients had a positive transillumination test making this an ineffective test for identification of the nerve.18

Stephens and Kelly,17 first described the fourth toe flexion sign as a clinical test to identify the SPN. The nerve was identified with passive flexion of the fourth toe in 26 of the 30 feet. The authors confirmed the identification of the nerve by performing subcutaneous infiltration of 1 mL of 0.5% lignocaine that corresponded to the hypoesthesia noted over the area of innervation of the intermediate dorsal cutaneous branch of the SPN. The authors identified the nerve in 86% of their patients; however, one major confounder would be the ability of the local anesthetic to diffuse through a larger plane which may have accounted for their high identification rate.

De Leeuw et al.,14 in their study, which involved 99 volunteers, found that keeping the ankle in maximum plantar flexion and inversion helped in the identification of the SPN in 57% of their patients. Fourth toe flexion alone identified the nerve by 38% only. BMI was a statistically significant factor in identifying the nerve in their population of patients, whereby the racial distribution was equal between Asian, Negroid, and Caucasian races. Our study was in agreement with de Leeuw et sl.14 with regards to lower BMI being a positive factor in identifying the nerve. They found that skin colour and race did not influence their results. Our rate of identification is 44%, and this is much lower than that of both Stephens and de Leeuw et al’s. studies. Although we did not find race to be a significant factor in our study population, this may still be a significant factor when comparing these different study populations.

One of the limitations on the usefulness of this clinical test is the change in the position of the SPN with respect to the position of the ankle. In the technique for our study, the ankle was kept in maximum plantar flexion and inversion for the clinical detection of the SPN. However, the position of the foot and ankle varies during placement of the anterolateral portal during arthroscopic ankle surgery or other surgeries for the foot and ankle. In their study, de Leeuw et al. reported that the SPN was found to excurse about 2.4 and 3.6 mm laterally when the ankle ranged from 10° of plantar flexion and inversion to the neutral position and 5° of dorsiflexion, respectively.15 There are anatomical variations of the SPN. Darland et al. described the presence of at least three branches of the SPN in about 29% of the cadaveric specimens.16 There is also a difference in the branching pattern of the SPN between two ankles of the same individual in most cadavers. Surgeons should adopt caution given these limitations. Nonetheless, it is still important to actively identify the nerve preoperatively. Suzangar and Rosenfeld reported in their study a 1.04% incidence of SPN injury when a preoperative identification and marking of the nerve was performed.13 This is one of the lowest incidences of the SPN in the literature.

One other limitation of our study and a potential area for further clinical studies is to utilize ultrasound to correlate our clinical examination findings with ultrasound localization of the nerve and also the correlation between ankle position and location of the nerve. A cadaveric study by Poggio et al.19 investigated the correlation between visual inspection and palpation, and ultrasound in the identification of the SPN. In their study, foot and ankle surgeons performed the examination and were able to locate the nerve clinically. The surgeons used ankle inversion and supination as maneuvers to identify the nerve. In this study, an ultrasound found a 92% correlation with cadaveric dissection of the nerve, whilst clinical examination by experienced surgeons only found a 58% (at the level of the ankle) and a 33% (at the subcutaneous course and division of the nerve) correlation with cadaveric dissection of the nerve. Whether the results of such a high level of identification of the nerve in cadavers can be transferred to living humans is unknown as the tissue elasticity is different. We found no clinical studies looking at the utility of ultrasound in pre or intraoperative marking of the nerve and reduction of risks of nerve injury. From a practical point of view, whilst the utility of ultrasound may provide higher accuracy for nerve identification, this mode of assessment requires equipment and training for it to be useful in a wider area of practice. Surgeons may consider employing the expertise of anesthesia colleagues who may be more proficient in nerve mapping, given the wide use of ultrasound-guided regional anesthesia in the preoperative area to mark the nerve.

The ankle plantar flexion fourth toe plantar flexion can be easily applied by all surgeons operating in the area of the foot and ankle, and we believe that having a knowledge of the limitations of this clinical test and local prevalence is helpful.


This study was conducted to assess the prevalence of the SPN using the ankle plantar flexion, inversion, and fourth toe flexion sign in our local population. We were able to identify the SPN in 44% of our patients as compared to the 57–86% reported in the literature.14,17

Individuals with a higher BMI had a statistically lower probability of identifying the nerve and thus may pose a higher risk of an iatrogenic injury. In our study, we found that individuals who took part in sports at least once a week had a positive influence on identifying the nerve. However, factors such as age, gender, shoe size, and race were not significant factors for the identification of the nerve.

Clinical Significance

Whilst we believe this clinical test is useful, the prevalence in our population is not as high as described by other authors and surgeons should take into account the limitations of the test and supplement the use with cadaveric training20 or the use of ultrasound19 to further reduce their risk of iatrogenic injury to the SPN when operating in the foot and ankle. We hope to create awareness that the SPN can be identified with the simple clinical examination maneuver of ankle plantar flexion and fourth toe flexion.


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