Peroneus Tertius
| Peroneus Tertius | |
|---|---|
| Muscle Type | |
| Origin | Distal anterior fibula (often continuous with extensor digitorum longus) |
| Insertion | Dorsum of base of 5th metatarsal |
| Action | Dorsiflexes ankle; everts foot (pronation) |
| Synergists | |
| Antagonists | Tibialis Posterior, Triceps Surae |
| Spinal innervation | |
| Peripheral Innervation | Deep fibular (peroneal) nerve (L5–S1) |
| Vasculature | Anterior tibial artery |
The fibularis tertius (formerly peroneus tertius) is a small muscle of the anterior compartment of the leg. It is essentially a part of extensor digitorum longus that inserts on the 5th metatarsal. Its primary action is to dorsiflex and evert the foot (pull the foot upward and outward). Not everyone has a distinct fibularis tertius, but when present, it assists in stabilizing the lateral ankle and guiding the foot during swing phase to avoid excessive inversion (helping to prevent ankle sprains). It is considered an accessory dorsiflexor and evertor.
Structure
Fibularis tertius is a small fusiform muscle slip, effectively the lower part of extensor digitorum longus, with a separate tendon going to the fifth metatarsal. It lies in the anterior compartment (so it's a dorsiflexor like EDL/TA) but functionally assists the lateral compartment evertors.
Origin
It arises from the distal third of the anterior surface of the fibula and the interosseous membrane, often blending with the EDL muscle belly. It essentially peels off from EDL near where the EDL tendons branch off.
Insertion
It inserts on the dorsum of the base of the 5th metatarsal. Sometimes it also attaches to the 4th metatarsal or has multiple small insertions, but typically the 5th metatarsal base (just in front of the tuberosity). This is the same bone that fibularis brevis attaches to from below (brevis attaches to the tuberosity on lateral side).
Action
Fibularis tertius dorsiflexes the foot at the ankle (like EDL) and everts the foot (pronation) because it inserts laterally. Its action is weaker compared to main evertors (fibularis longus and brevis) but it helps smooth the transition between foot up and foot out motions. Functionally, when the foot is off the ground, fibularis tertius aids in keeping the foot neutral to slightly everted, which can help prevent the foot from inverting excessively (thus possibly contributing to reduced risk of rolling the ankle on heel strike).
Synergists
- For dorsiflexion: Tibialis anterior and extensor digitorum longus are main synergists.
- For eversion: Fibularis longus and fibularis brevis (the lateral compartment muscles) are the primary synergists. Tertius joins brevis and longus in turning the sole outward.
- In gait, fibularis tertius contracts with the other dorsiflexors during swing phase to ensure the foot clears. It particularly fires in late swing to prepare foot for heel strike in a slightly everted position (co-contracting with tib anterior).
- It may assist the fibularis brevis in resisting sudden inversion (like if stepping on an uneven surface, tertius along with brevis reflexively contract to counteract inversion).
Antagonists
- Tibialis posterior and triceps surae muscles are antagonistic. Tibialis posterior inverts (opposes eversion), and triceps surae plantarflexes (opposes dorsiflexion).
- Also, tibialis anterior inverts while dorsiflexing; fibularis tertius everts while dorsiflexing, so they are synergistic in DF but antagonistic in the inversion/eversion plane. Usually, they coordinate to position the foot appropriately.
- In stance, fibularis longus and tibialis anterior/posterior have a tug-of-war for foot position (balance inversion/eversion); fibularis tertius being weak, it mainly helps longus/brevis group.
Spinal Innervation
L5–S1 via deep fibular nerve (like EDL).
Peripheral Innervation
Deep fibular nerve (the nerve for anterior compartment) innervates fibularis tertius. It's basically a part of EDL, so same nerve supply. In common fibular nerve injuries, fibularis tertius is lost along with dorsiflexors, contributing to inability to evert in a dorsiflexed position (foot tends to invert when dorsiflexion is attempted, because tib anterior (if intact) inverts without tertius to counter).
Vasculature
Supplied by the anterior tibial artery, similar to EDL. The arterial supply is via branches that go to the lower anterior compartment.
Clinical Relevance
Not Everyone Has It: Fibularis tertius is absent in around 5-10% of people, and its absence is usually asymptomatic. In such cases, EDL still dorsiflexes and the main evertors (longus, brevis) handle eversion, albeit without a dorsiflexion component. Evolutionarily, it's thought to help with bipedal gait (preventing foot drop and ankle sprains). On exam or cadaver, an absence might just be noted incidentally.
Ankle Sprain Relevance: A well-developed fibularis tertius can provide some active resistance to inversion at heel strike, potentially reducing lateral ankle sprain risk. People with recurrent ankle sprains sometimes do exercises to strengthen all evertors (including tertius indirectly via dorsiflexion+eversion movements). While tertius is small, every bit helps in proprioception and quick correction.
Shin Splints (Anterior-Lateral): If fibularis tertius is overworked (like running downhill or extensive speed walking), it can cause pain over the lower lateral shin. This is less common than medial shin splints, but patients might feel pain just above the ankle on outer shin. Treatment is similar – rest, ice, correct training errors. Usually coexists with tibialis anterior issues.
Foot Drop Gait: In foot drop, absence of fibularis tertius contraction means the foot tends to invert (because tib anterior still works partially often). So in some neuropathies, patients not only have foot drop but also foot supination (inversion). Strengthening the evertors (including any function of tertius if present) is part of physical therapy to even out foot position. In nerve recovery, sometimes fibularis tertius (if it recovers earlier) can be seen to flicker with attempts to dorsiflex – giving a clue that deep fibular nerve function is returning.
EMG: If testing deep fibular nerve, fibularis tertius can be used as an EMG site (especially if tib anterior is inaccessible or already tested). It's innervated by the same nerve but a different branch near ankle. Rarely needed, but e.g., in a selective deep peroneal nerve injury (which might spare tib anterior which has some L4 innervation from common fibular directly), tertius could be out (but such selective injuries are rare).
Strengthening/Stretching: It's hard to target fibularis tertius specifically, but dorsiflexion with eversion exercises (e.g., resistance band pulling foot up and out) will engage it. Runners often do ankle exercises to strengthen the anterior-lateral compartment to improve foot control. Stretching fibularis tertius is effectively accomplished by plantarflexing and inverting the foot (the opposite of its action), which also stretches EDL and tib anterior somewhat.
Tendon Visibility: While tib anterior and EHL tendons are quite visible at the ankle, fibularis tertius tendon can sometimes be seen or felt just lateral to EDL tendons on the dorsum of foot (leading to 5th toe area). It's small and often merges with EDL slip to 5th toe. In some cases, it attaches into the 5th toe extensor expansion as well. Variation in insertion can mean some "extra" extension of 5th toe present even if EDL is typical.
Differential Insertion: Rarely, fibularis tertius can be misidentified as a split of fibularis brevis on imaging if one doesn't realize it's anterior to lateral malleolus (brevis goes posterior to lateral malleolus). Radiologists note the presence or absence of tertius on MRI, which is usually just a variant not pathology unless torn. A torn fibularis tertius would likely be in context of ankle injuries and might not be specifically repaired if others are intact, due to minor role.
Evolution Note: Fibularis tertius is considered unique to humans (absent in great apes). It’s thought to aid efficient bipedal gait (by enabling heel strike on a slightly everted foot, distributing forces). Its absence in some people doesn't drastically impair gait, showing redundancy in system (other muscles compensate).
Ankle Brace: In people with frequent ankle sprains, braces or tape often aim to mimic fibularis muscles' action (prevent inversion). Strengthening fibularis tertius along with longus and brevis is part of prophylactic exercise programs for athletes at risk of sprains.
