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Location & Overview
The fibularis tertius, which was previously named the peroneus tertius, is a muscle located on the lateral aspect of the lower leg. This muscle is one of the three fibularis muscles, alongside the fibularis longus and fibularis brevis. This muscle is absent in some individuals. Its tendon courses deep beneath the superior extensor retinaculum and within the inferior extensor retinaculum. It also traverses anterior to the anterior talofibular ligament of the ankle joint. Notably, it often appears to be part of the extensor digitorum longus muscle, running directly alongside it until its tendon deviates toward the fifth metatarsal, indicating the close anatomical relationship between these two muscles.[1,2,3]
Origin & Insertion
The fibularis tertius primarily originates from the distal and anterior portion of the fibula, specifically the lower third or quarter, with individual variation. Additional origin points include the anterior intermuscular septum and the interosseous membrane. The intermuscular septa are fibrous partitions that separate adjacent muscle compartments, providing structural support and helping to organize muscles, nerves, and blood vessels. The interosseous membrane is a fibrous, sheet-like structure that connects the tibia and fibula bones. The origin of the fibularis tertius on the fibula sometimes appears contiguous with the extensor digitorum longus muscle, emphasizing their intertwined nature. From its origin, the fibularis tertius crosses the ankle joint and inserts onto the fifth metatarsal, specifically the dorsal aspect of its base.[1,2,3]
Action & Function
The fibularis tertius is primarily involved in dorsiflexion of the foot, elevating the foot toward the shin. It also plays a supporting role in eversion of the foot, aiding in its outward movement. These movements are demonstrated in the images below.[2,3,4]
Innervation
The fibularis tertius is innervated by the deep fibular nerve, also known as the deep peroneal nerve, with roots from spinal segments L5 and S1. The deep fibular nerve is a terminal division of the common fibular nerve, which is a branch of the sciatic nerve.[1,2,3]
Blood Supply
The fibularis tertius receives its main blood supply from branches of the anterior tibial artery. This fits with its position in the anterior compartment of the leg, where the anterior tibial artery travels with the deep fibular nerve and supplies the anterior compartment muscles.[1,5]
Because the fibularis tertius is closely associated with the extensor digitorum longus, its vascular supply is described as following the same pattern. In the leg, this includes branches of the anterior tibial artery, with additional distal contribution from the perforating branch of the fibular artery.[5]
As the tendon continues toward the dorsum of the foot, the distal part of the muscle-tendon unit also receives additional blood supply from vessels in the dorsal foot region, including branches from the termination of the arcuate artery and the fourth dorsal metatarsal artery.[5]
References
- [1] Dalley AF II, Agur AMR. Moore’s Clinically Oriented Anatomy. 9th ed. Wolters Kluwer Health; 2022.
- [2] Standring S. (2015). Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 41st Edn. Amsterdam: Elsevier
- [3] Palastanga N, Soames RW. Anatomy and Human Movement: Structure and Function (Physiotherapy Essentials), 6th ed. Elsevier; 2011. ISBN-13: 978-0702035531.
- [4] Gupton M, Munjal A, Kang M. Anatomy, Bony Pelvis and Lower Limb: Fibula. (Updated 2023 May 23). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470591/
- [5] Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Amsterdam: Elsevier; 2021.
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