The Foot
The calcaneus, or heel bone, is characterized by several critical articular surfaces and anatomical processes that play a vital role in foot mechanics and weight-bearing. Superiorly, it features three distinct talar articular surfaces designated for the talus: the posterior talar articular surface, the middle talar articular surface, and the anterior talar articular surface. Between the larger posterior and middle articular surfaces lies the calcaneal sulcus, which is important for creating the sinus tarsi, an area involved in the stability of the foot. A prominent medial projection known as the sustentaculum tali, often referred to as the talar shelf, provides crucial support for the talus and houses a groove on its inferior surface dedicated to the tendon of the flexor hallucis longus, which is significant for the push-off mechanism during walking and running. On the lateral side of the calcaneus, the fibular (peroneal) trochlea (or tubercle) is a critical site for the passage of the fibularis longus tendon, which plays a key role in eversion of the foot.
The functional relations of the calcaneus are illustrated through specific angles that reflect its anatomical relationships. Bohler's angle, also known as the tuber angle, normally ranges between , a measurement that is calculated between the highest point of the posterior articular facet and both the anterior process and posterior tuberosity, providing insights into potential fractures or deformities. The tuberosity of the calcaneus also features a lateral process and a more prominent medial process, both of which can be points of anatomical variation or injury.
In common outdoor terminology, the term "talus" has geological implications, describing a slope composed of large rocks and boulders. This geological formation is distinct from the medical term for the second largest bone of the foot. The geological talus is characterized by stacked rocks that can provide natural stability but also pose a high risk of ankle sprains if foot placement is not accurate.
Intrinsic Muscle Layers of the Plantar Surface
The intrinsic muscles of the plantar surface of the foot are meticulously organized into four distinct layers, each separated from the superficial plantar aponeurosis, which offers additional structural support.
Layer #1, termed the "two abductors and a flexor" layer, consists of three key muscles: the Abductor hallucis, the Flexor digitorum brevis, and the Abductor digiti minimi. The Abductor hallucis and Flexor digitorum brevis are primarily innervated by the medial plantar nerve (MPN), enabling actions such as the abduction of the big toe and flexion of the middle phalanges, respectively. Conversely, the Abductor digiti minimi is innervated by the lateral plantar nerve (LPN), facilitating the abduction of the little toe.
Layer #2 houses muscles that integrate into tendons and is positioned deep to the neurovascular plane. This layer includes the Quadratus plantae (innervated by the LPN) and four Lumbricals. The Lumbricals arise from the tendons of the flexor digitorum longus and their function is to flex the metatarsophalangeal joints while extending the interphalangeal joints. Interestingly, the medial first lumbrical is innervated by the MPN, while the remaining three lumbricals receive innervation from the LPN. The tendon of the flexor hallucis longus, which is crucial for hallux flexion, also traverses this layer, reinforcing the interconnectedness of muscle actions. The midline axis of the foot for muscle action, such as the interossei, is defined by the second toe, while in the hand, the reference point is the middle finger.
Layer #3 is recognized as the "two flexors and an adductor" layer. This layer includes the Flexor hallucis brevis (innervated by the MPN), the Adductor hallucis (innervated by the LPN), and the Flexor digiti minimi brevis (also innervated by the LPN). The Adductor hallucis is anatomically complex, comprising two heads: the oblique head and the transverse head, both crucial for stabilizing the first metatarsal and allowing effective push-off during ambulation.
Layer #4 encompasses the interossei muscles, which articulate between the metatarsals and facilitate intricate movements of the toes. There are three plantar interossei responsible for adduction of the toes and four dorsal interossei that enable abduction, with all interossei receiving innervation from the lateral plantar nerve as a significant contributor to toe stability and dexterity.
Dorsal Intrinsic Foot Muscles
The dorsal aspect of the foot, in contrast, contains fewer intrinsic muscles but still has vital functions. The Extensor hallucis brevis and the Extensor digitorum brevis are the principal muscles located here, both innervated by the deep fibular (peroneal) nerve, essential for extending the toes and aiding in the foot's dynamic functionalities during movement.
Neurovascular Summary of the Foot
Innervation of the foot primarily falls under the jurisdiction of the terminal branches of the tibial nerve. An anatomical analogy can be made where the median nerve in the hand corresponds to the medial plantar nerve and the ulnar nerve corresponds to the lateral plantar nerve, highlighting how these nerves serve equivalent roles in their respective extremities.
The Medial Plantar Nerve (MPN) supplies the key intrinsic muscles including the Abductor hallucis, Flexor digitorum brevis, the 1st Lumbrical, and the Flexor hallucis brevis. In contrast, the Lateral Plantar Nerve (LPN) innervates all other plantar intrinsic muscles including the Abductor digiti minimi, Quadratus plantae, the lateral three Lumbricals, the Adductor hallucis, the Flexor digiti minimi brevis, and all interossei, thus playing a comprehensive role in foot function.
Cutaneous innervation of the foot disperses across various nerves:
The Sural nerve provides sensation to the lateral aspect of the foot and the fifth digit.
The Saphenous nerve provides sensation to the medial aspect of the foot extending to the base of the first metatarsal.
The Superficial fibular nerve supplies the majority of the dorsum of the foot, while the Deep fibular nerve supplies the small triangular web space between the first (hallux) and second toes, indicating the complex interplay of sensory distribution in the foot.
The arterial supply follows a dedicated pathway commencing from the anterior tibial artery which continues as the dorsalis pedis artery, subsequently branching into the arcuate artery and the deep plantar artery, effectively supplying the dorsal metatarsal and dorsal digital arteries. The posterior tibial artery bifurcates into the medial and lateral plantar arteries, with the lateral plantar artery forming the deep plantar arch, which gives rise to the plantar metatarsal and plantar digital arteries, marking the intricate arterial networks that service foot structure.
Venous drainage of the foot incorporates the plantar venous arch and network, alongside the dorsal venous arch and the dorsal venous network. These venous systems facilitate drainage into the great saphenous vein (medially) and the small saphenous vein (posteriorly/laterally), ensuring efficient venous return from the foot to the systemic circulation.
Structural Arches of the Foot
The human foot is ingeniously structured into three primary arches, optimized for weight-bearing and shock absorption.
The Medial Longitudinal Arch stands as the highest and is paramount for gait efficiency. It comprises the calcaneus, talus, navicular, three cuneiform bones, and the three medial metatarsals, thereby contributing to the foot's adaptability.
The Lateral Longitudinal Arch, being flatter, involves the calcaneus, cuboid, and the lateral two metatarsals, playing a crucial role in maintaining balance during ambulation.
The Transverse Arch traverses the width of the foot, formed by the cuboid and cuneiforms, as well as the bases of the metatarsals, aiding in shock absorption.
Maintenance of these arches involves an interplay of passive and dynamic support structures. Passive support derives from the intrinsic shape of the bones, the sturdy plantar aponeurosis, and key ligaments: the long plantar ligament, short plantar ligament (plantar calcaneocuboid ligament), and the "spring ligament" (plantar calcaneonavicular ligament). Meanwhile, dynamic support stems from both intrinsic plantar muscles and tendons of extrinsic muscles, specifically the Flexor hallucis longus, Flexor digitorum longus, Fibularis longus, Tibialis posterior, and Tibialis anterior, ensuring that the foot remains functional and resilient during movement.
Ligaments of the Ankle and Foot
Ligaments of the foot and ankle are categorized by their anatomical locations as well as the joints they stabilize. The Talocrural (ankle) joint demonstrates medial support through the Medial (Deltoid) ligament, a strong and fan-shaped structure that reinforces joint stability during load. On the lateral side, the ankle is bolstered by three primary ligaments: the Anterior talofibular ligament (ATFL), the Posterior talofibular ligament (PTFL), and the Calcaneofibular ligament (CFL), with the ATFL being the most commonly injured ligament in ankle sprains, thus emphasizing the importance of its structural integrity.
Connecting the tibia and fibula are the Anterior tibiofibular ligament, the Posterior tibiofibular ligament, and the interosseous membrane, all vital for the overall stability of the ankle joint during movement and weight-bearing activities.
Additional clinical landmarks and structures pertinent to the foot and ankle include the Sustentaculum tali, the Biceps femoris tendon, Fibular (lateral) collateral ligament, Patellar ligament, Anterior and Posterior cruciate ligaments, as well as muscular tendons such as Tibialis anterior and posterior, Fibularis longus and brevis, along with long and short plantar ligaments, and the critical plantar calcaneonavicular (spring) ligament, which collectively support foot function and integrity.