Fig. 1 Measurements of the sural nerve (SN) relative to points of interest on the ankle. A features measurements from the sural nerve to different landmarks on the ankle. B shows an example measurement of the vertical distance from the SN to the distal tip of the lateral malleolus. C shows traditional incision lines for the extensile lateral approach to the calcaneus, direct lateral approach to the lateral malleolus, sinus tarsi approach, and posterolateral approach to the ankle. This figure indicates potential injury risk posted to the SN with these techniques. D shows the proposed “safe zones” to reduce SN injury risk.
Source: Kent W, et al., “Creating Safe Zones at the Lateral Ankle to Avoid Iatrogenic Injury to the Sural Nerve: An Anatomical Imaging Study Using 3T MRI.”

AAOS Now

Published 3/25/2022
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Brandon May

Identifying 'Safe Zones' to Reduce Iatrogenic Injury Risk to the Sural Nerve in Common Surgical Approaches

A team of researchers has identified certain “safe zones” of the ankle which surgeons can use as guides during incision and dissection during common orthopaedic foot and ankle procedures to reduce the risk of iatrogenic injury to the sural nerve. The findings were presented as a poster at the AAOS 2022 Annual Meeting by William Kent, MD, an assistant professor of orthopaedic surgery at UC San Diego Health.

The sural nerve, a distal cutaneous nerve which results in sensation to the posterolateral ankle and foot, is at risk of iatrogenic injury during a variety of common surgical approaches (i.e., foot and ankle surgery) given the nerve’s location and variation. Injury to the cutaneous nerves in the ankle may consequently result in symptomatic neuroma formation and chronic pain, paresthesias, or sensory deficits. According to Dr. Kent and colleagues, there have been a limited number of anatomic studies of the sural nerve, with available studies focused mainly on cadavers with small sample sizes.

“By nature, in vivo measurements may also provide a more accurate representation of the anatomy, as these methods are not subject to the effects of embalming and dissection required of cadaveric studies,” the researchers wrote in their poster presentation.

In the study, Dr. Kent and colleagues analyzed a cohort of 204 high-field 3 Tesla (3T) MRIs of the ankle. The goal was to establish a generalizable in vivo sample of the distal course of the sural nerve. Consequently, the investigators sought to propose safe zones, defined as “regions for incision and dissection that surgeons can utilize to decrease the risk of iatrogenic injury to the sural nerve during common surgical approaches.” These so-called safe zones included the extensile lateral approach to the calcaneus, the sinus tarsi approach, the direct lateral approach to the lateral malleolus, and the posterolateral approach to the ankle.

Three blinded reviewers analyzed the high-field 3T MRI studies of the ankle. They measured the distance from the sural nerve to different landmarks of interest (Fig. 1A-D). The researchers wrote that Fig. 1C indicates the potential injury risk posed to the sural nerve. The proposed safe zones for incisions (Fig. 1D) were based on the mean ± three standard deviations for a given measurement, which the researchers said avoids the sural nerve in more than 99 percent of their analyzed MRI images.

The mean vertical distance from the sural nerve to the distal tip of the lateral malleolus (DTLM) (Fig. 1A) was 2.2 ± 0.4 cm and ranged between 0.9 cm and 3.6 cm. According to the researchers, the value was higher than that observed in previous cadaveric studies (1.3 to 1.5 cm).

Additionally, the mean horizontal distance from the sural nerve to the DTLM was 1.7 ± 0.3 cm (range = 0.8–3.0 cm), which was reportedly similar to previous cadaveric studies (1.4 to 1.5 cm), whereas the mean horizontal distance from the sural nerve to the lateral border of the Achilles tendon (LBA) at the level of the DTLM was 1.9 ± 0.3 cm (range, 1.0–2.9 cm), which the researchers said was smaller than prior studies (2.6 cm). The mean horizontal distances from the sural nerve to the LBA at the level of the super Achilles tendon insertion onto the calcaneus (SAI) and at 5 cm above the SAI were 2.6 ± 0.4 cm (range, 1.4–3.7 cm) and 0.9 ± 0.2 cm (range, 0.4–1.8 cm), respectively.

Limitations included the relatively small number of cases. Additionally, because this study is the first of its type in living participants, more studies may be needed to determine the applicability of the findings across a broader population.

Dr. Kent’s coauthors of “Creating Safe Zones at the Lateral Ankle to Avoid Iatrogenic Injury to the Sural Nerve: An Anatomical Imaging Study Using 3T MRI” are Claudio Ghetti, BS; Brendon Mitchell, MD; Vrajesh Shah, BS; Wilbur Wang, MD; Brady Huang, MD; and Ian Foran, MD.

Brandon May is a freelance writer for AAOS Now.