Peroneal Tendon Anatomy: Retromalleolar Groove and Variants

The peroneal tendons are two rope-like bands of connective tissue running along the outer ankle and foot, connecting the peroneus longus and peroneus brevis muscles of the lower leg to bones in the foot. Together they stabilize the ankle, help you push off during walking, and resist the foot rolling inward. Their anatomy is worth understanding in detail because their winding path around bony landmarks, through tight channels, and across multiple joints makes them unusually vulnerable to tears, subluxation, and chronic irritation. Several normal anatomical variations in the surrounding bone and soft tissue make some people more injury-prone than others.

The Two Main Peroneal Tendons

The peroneus longus and peroneus brevis both live in the lateral compartment of the lower leg, the section on the outer side of the calf between the fibula bone and surrounding tissue walls. Despite sharing a compartment and traveling side by side for much of their course, they differ in where they start, where they end, and how far they travel.

The peroneus longus is the longer of the two. Its muscle belly sits higher in the leg, and its tendon begins just above the ankle at the myotendinous junction. From there the tendon makes two sharp turns: first behind and below the lateral malleolus (the bony bump on the outside of the ankle), then along the outer wall of the calcaneus (heel bone), before diving under the foot to cross the sole and insert on the medial plantar surface, attaching near the base of the first metatarsal and the medial cuneiform bone on the inner side of the foot.¹ That long, twisting path is one reason the peroneus longus tendon is so clinically interesting: each turn creates a potential friction point.

The peroneus brevis sits deeper and lower. Its muscle originates from the lower two-thirds of the fibula’s outer surface and the anterior intermuscular septum, and its tendon inserts on the tuberosity at the base of the fifth metatarsal, the bony prominence you can feel on the outer edge of your midfoot.² Because the brevis tendon ends on the lateral side of the foot rather than crossing the sole, its overall course is shorter and less convoluted.

In terms of function, the peroneus longus is the stronger evertor and also supports the arch from below, pulling the first metatarsal plantarward. The peroneus brevis everts the foot and assists with plantarflexion. Both tendons are critical for dynamic ankle stability, especially when walking on uneven ground or landing from a jump.

The Retromalleolar Groove

Behind the lateral malleolus, both peroneal tendons share a shallow bony channel called the retromalleolar (or fibular) groove. This groove acts like a pulley, redirecting the tendons’ line of pull as they curve from the leg down toward the foot. The shape of this groove varies considerably from person to person, and that variation has real consequences for injury risk.

Researchers classify the groove into several shape types. One radioanatomical study identified three main morphologies: concave, flat, and convex, finding that the concave shape was most common, appearing in about 70% of specimens.³ A more recent CT-based study added a fourth category, “irregular,” to the classification.⁴ A concave groove cradles the tendons well. A flat or convex groove offers less mechanical containment, and people with those groove shapes are at higher risk for the tendons slipping out of place.

The Retinacular Restraints and the Peroneal Tubercle

Bone shape alone does not keep the peroneal tendons where they belong. Soft tissue structures do most of the restraining work. The superior peroneal retinaculum is a band of thickened tissue that bridges across the retromalleolar groove, essentially forming a roof over the tendons as they round the ankle. It attaches to the lateral malleolus and runs backward and downward to the lateral surface of the calcaneus. If this retinaculum tears, the tendons can subluxate (partially slip) or fully dislocate out of the groove.⁵ The inferior peroneal retinaculum sits farther down and helps restrain the tendons along the lateral calcaneus.

Below the ankle, on the lateral wall of the calcaneus, a bony ridge called the peroneal tubercle (or peroneal trochlea) separates the two tendons. The peroneus brevis tendon passes above the tubercle while the peroneus longus passes below it. The size and shape of this tubercle vary. An oversized or unusually shaped peroneal tubercle can chronically irritate the peroneus longus tendon as it slides past, potentially leading to longitudinal tears over time.⁶

Blood Supply and Avascular Zones

Tendons heal slowly in general, and the peroneal tendons have an added challenge: parts of them have very little blood flow. The blood supply to both tendons arrives through small connective tissue bridges called vincula, one for each tendon, fed by branches of the posterior peroneal artery.⁷

Despite that supply, cadaver injection studies have shown areas where the intratendinous blood vessel network thins dramatically. The peroneus brevis tendon is nearly avascular in the zone where it passes through the fibular groove. The peroneus longus tendon has two such zones: one where it curves around the lateral malleolus and peroneal trochlea, and a second farther along where it wraps around the cuboid bone on the sole of the foot.⁸ These avascular zones correspond to the turning points where tendons press against bone and behave more like fibrocartilage than typical tendon tissue. Poor blood supply in these regions helps explain why peroneal tendon injuries at these locations can be stubborn to heal and prone to becoming chronic.

Anatomical Variants That Predispose to Injury

Not everyone’s peroneal anatomy follows the textbook pattern. Several common variants crowd the retromalleolar space or alter the mechanical forces on the tendons, and clinicians need to recognize them because they change both injury risk and surgical planning.

The Peroneus Quartus

An accessory muscle called the peroneus quartus is present in roughly one in ten people. An MRI study of 1,160 ankles found it in 10.6% overall, with a higher rate in males (about 13%) than females (about 9%).⁹ This extra muscle belly and tendon take up space behind the lateral malleolus. The same study found that having a peroneus quartus roughly tripled the odds of a peroneus brevis tear and about tripled the odds of peroneus brevis tendinitis. The association with peroneus longus tendinitis was also significant, with more than double the odds.¹⁰ The leading explanation is mechanical crowding: an extra tendon sharing the same confined groove compresses the primary tendons against bone.

Low-Lying Peroneus Brevis Muscle Belly

Normally, the peroneus brevis muscle transitions to tendon above the ankle. In some people, fleshy muscle tissue extends farther down into the retromalleolar groove itself. This low-lying muscle belly occupies space that would otherwise be available for the tendon to glide freely. One surgical case series reported that 90% of patients who had peroneus brevis tendon subluxation also had a low-lying muscle belly present.¹¹ While the association did not reach statistical significance in that study’s small sample, the pattern is consistent with what MRI analyses have observed: in the setting of instability, a low-lying muscle belly becomes a source of pain and pathology because of overcrowding behind the malleolus.¹²

The Os Peroneum

Embedded within the peroneus longus tendon near the cuboid bone, some people have a small sesamoid bone called the os peroneum.¹³ It varies in size and degree of ossification; in some people it remains mostly fibrocartilage and never fully ossifies. When present and fully formed, the os peroneum can fracture or displace, leading to a condition called painful os peroneum syndrome. This syndrome causes lateral midfoot pain and is underdiagnosed because the tiny bone can be overlooked on standard X-rays.¹⁴ A fractured or migrated os peroneum on imaging can also be a clue that the peroneus longus tendon itself has torn.

The Peroneus Tertius

The name “peroneus tertius” (literally “third peroneal”) is a bit misleading. Unlike the longus and brevis, which are evertors on the lateral side of the leg, the peroneus tertius sits in the anterior compartment and acts primarily as a dorsiflexor and evertor. It is often described as a separation from the extensor digitorum longus muscle rather than a true lateral compartment muscle. A meta-analysis pooling over a thousand observations found that the most common origin was the distal half of the fibula (about 70% of cases), with roughly 16% of specimens showing the muscle originating from the extensor digitorum longus itself.¹⁵

The insertion of the peroneus tertius varies widely. In about a third of cases it attaches to both the fourth and fifth metatarsals, while in other cases it inserts solely on the base, the shaft, or both areas of the fifth metatarsal.¹⁶ Some specimens even show a bifurcated insertion with two distinct tendon slips attaching at different points on the fifth metatarsal, a pattern that may contribute to stress fractures by applying torsional forces at multiple sites.¹⁷ A cadaveric study of 66 limbs documented insertion variations in 18% of specimens, including rare duplications of the entire muscle and accessory slips interacting with the extensor digitorum brevis.¹⁸ These variants matter surgically because the peroneus tertius tendon is sometimes harvested as a graft, and the surgeon needs to know what insertion pattern they are working with.

The Peroneus Longus Under the Foot

After leaving the lateral ankle, the peroneus longus tendon enters a groove on the underside of the cuboid bone called the cuboid tunnel or cuboid groove. This is the tendon’s final major turn before it crosses the sole of the foot to reach its insertion. You might expect the tendon to sit neatly inside this groove, but ultrasound studies of healthy volunteers have shown otherwise. In most foot positions, roughly 65% or more of peroneus longus tendons were partially or completely located outside the cuboid groove. The tendon’s position within the groove was linked to groove width: narrower grooves were associated with the tendon sitting higher above the groove rather than inside it.¹⁹ This finding is a good reminder that textbook diagrams showing the tendon neatly seated in a groove represent an idealized picture. In living, moving feet, the relationship between tendon and groove is more dynamic and variable.

How Anatomy Drives Peroneal Tendon Injuries

The interplay between groove shape, retinacular integrity, and space crowding explains the most common peroneal tendon injuries. Peroneus brevis splits, for example, are understood as a mechanical problem at the fibular groove. When the superior peroneal retinaculum is lax, the peroneus longus tendon presses the brevis tendon against the sharp posterior edge of the fibula during ankle motion. Over time this compression causes the brevis to splay and eventually split longitudinally.²⁰ Anatomical factors that worsen this scenario include a congenitally convex or flat fibular groove, a peroneus quartus tendon occupying additional space, and a low-lying peroneus brevis muscle belly interfering with the retinaculum’s ability to hold the tendons in place.

MRI studies of patients with confirmed peroneus brevis splits have identified statistically significant associations with several of these anatomical features. A flat peroneal groove, abnormal lateral ankle ligaments, and a chevron-shaped tendon (a sign of chronic compression) all showed strong associations with split tears.²¹ For clinicians, these associations mean that when one anatomical risk factor is seen on imaging, looking for the others is worthwhile, because they tend to cluster together.

Imaging the Peroneal Tendons

Both MRI and ultrasound are used to evaluate peroneal tendon anatomy and pathology. MRI at 1.5 or 3 Tesla with a dedicated extremity coil provides excellent static images of the tendons, the retinaculum, and the bony groove.²² It is the better choice for detecting subtle partial tears, bone marrow changes, and the os peroneum. Ultrasound has a distinct advantage, though: it can be performed dynamically. The examiner can dorsiflex and evert the ankle while watching in real time whether the tendons subluxate out of the groove, something an MRI snapshot cannot capture.²³ In practice, the two modalities complement each other, and the choice often depends on what the clinician suspects.

Recognizing the anatomical variants described earlier on imaging is important. A flat or convex groove, an enlarged peroneal tubercle, a peroneus quartus muscle, a low-lying brevis muscle belly, and an os peroneum can all be identified on MRI or ultrasound, and each one signals increased risk for peroneal tendon trouble.

Surgical Anatomy and Groove Deepening

When peroneal tendon subluxation does not respond to conservative treatment, surgery often targets the anatomy directly. One common approach involves deepening the retromalleolar groove. The surgeon lifts a thin bone flap from the back of the distal fibula, carves out additional depth in the groove, and then replaces the flap so the tendons now sit in a deeper, more secure channel. The superior peroneal retinaculum is repaired or reconstructed at the same time.²⁴ A modified version of this procedure in 18 patients with subluxation reported good clinical results with minimal surgical trauma and relatively quick recovery.²⁵

The anatomy also guides decisions about what else to address during surgery. If a peroneus quartus muscle is found crowding the groove, it may be excised. If the peroneal tubercle is hypertrophied, it can be reduced. If the peroneus brevis has a longitudinal split, the surgeon decides based on the amount of viable tendon remaining whether to repair it, debride the damaged portion, or, in severe cases, tenodese the remnant to the peroneus longus. Each of these decisions maps directly to the anatomical structures described above, which is why understanding peroneal tendon anatomy in detail is not just an academic exercise but a practical prerequisite for managing lateral ankle problems.

Naming Conventions

You will see the peroneal tendons referred to by two different naming systems. The older clinical convention uses “peroneus” (from the Greek for “pin” or “fibula”), and this remains the dominant term in orthopedic and sports medicine literature. The international anatomical terminology standard, Terminologia Anatomica, prefers “fibularis” (from the Latin for fibula). The two names are interchangeable: peroneus longus is fibularis longus, peroneus brevis is fibularis brevis, and so on. Research papers may use either term depending on regional tradition and journal preference. If you are reading an anatomy textbook that says “fibularis” and a radiology report that says “peroneus,” they are describing the same structures.