Lab 16 Pre-Lab Exercise:
1. The foot
There are many homologies between structures in the hand and the foot. But there are many differences also.
Understanding the foot is relatively simple if you consider the homologies to the hand. The joints of the foot are similar to those of the hand. There are two rows of tarsals:
- Talus and calcaneus, and navicular
- cuboid, and three cuneiforms
In total there are seven tarsal bones.
The largest tarsal bone is the calcaneus, or the heel bone. The calcaneal (Achilles) tendon attaches to the calcaneus. The calcaneus bears the weight of heel-strike during waling and running. It is cushioned by a heel pad.
The talus is the only tarsal bone that articulates with the leg bones (tibia and fibula). One interesting note about the talus is that it is the only bone in the body that does not have a muscle attached to it.
The navicular articulates with the talus and the other tarsals. Accordingly, it has a distinctive concave posterior surface for articulation with the head of the talus and a convex anterior surface where the other tarsals articulate.
The next row of bones includes the cuneiforms -- medial, intermediate, and lateral. These bones are wedge-shaped in cross-section and contribute to the or transverse arch of the foot. The medial cuneiform is the largest of the three and articulates with the first metatarsal.
The intermediate cuneiform (the smallest) articulates distally with the second metatarsal, while the lateral cuneiform articulates distally with the third metatarsal.
The final tarsal bone is the cuboid, which articulates distally with the fourth and fifth metatarsals. It is the centerpiece in the low lateral arch of the foot, lying between the calcaneus and the two metatarsals.
After that there are the homologus metacarpal and phalanges. Arranged this way it can be said there is a rear-foot, mid-foot, and fore-foot.
1b. Joints of the foot
The joints of the foot are as follows:
- The tibiotarsal (true ankle) where much dorsiflexion and plantarflexion occurs. This is a hinge joint in which the range of motion is limited by the shape of the talus. The joints of the ankle are protected on the medial side by the deltoid ligament which consists of an anterior tibiotalar, tibionavicular, and tibiocalcaneal part. This ligament is so strong that it rarely sprains. Instead, the medial malleolus fractures leading to a Pott’s fracture of tibia and fibula. The joints are protected on the lateral side by the anterior talofibular ligament, calcaneofibular ligament, and posterior talofibular ligament. The anterior talofibular ligament is most often sprained because the foot is often simultaneously plantarflexed and inverted.
- The subtalar joint between talus and calcaneus. Here the shape of the talus allows dorsiflexion and plantarflexion accompanied by inversion. The axis of rotation of this joint is around the big toe.
- Transverse tarsal joint allows rotation around the axis of the third digit.
1c. The arch of the foot
The mechanics of the foot relies also on the arches that give the foot its shape, store energy, and allow the foot to function as a lever.
The arch of the foot is maintained by the plantar aponeurosis, spring ligament, the long plantar ligament, the short plantar ligament and the tendon of fibualris longus. Failure of the structural support of the arch leads to “flat feet”, mainly a drop in the medial longitudinal arch. Pronation and supination involves changes in arch height and shape. In pronation the midfoot rolls medially, the arch compresses, and the medial edge of the foot approaches the substrate (foot “flattens”). Supination is opposite (the arch raises and the foot rolls laterally).
In general, the metatarsals are quite similar to the metacarpals. There are five in each foot, numbered I through V from the medial to lateral position. Metatarsals form the support for distal part of the arch of the foot. Like the metacarpals of the hand, each metatarsal consists of a proximal base, a shaft, and a distal head. However, metatarsals differ from metacarpals in having a more slender shaft and a more triangular cross-section.
The pedal phalanges are very similar to the manual phalanges. There are two phalanges for the great toe (hallux), one proximal and one distal; all other pedal digits have three phalanges, a proximal, a middle, and a distal. Thus, there are fourteen phalanges in each foot, just as there are in each hand.
It is worth noting how small the fifth phalanx is and how large the hallux is. The hallux, unlike the thumb, has been brought in line with the other phalanges and is much more robust because it has to bear the weight of a propulsive toe-off.
2. Bony landmarks of the foot
Tendons, muscles and bony landmarks in the lower limb are used to locate major arteries, veins, and nerves. Because vessels are large, they can be used as entry points to the vascular system and in addition, the vessels in the lower limb are farthest from the heart and most inferior in the body. Therefore, the nature of peripheral pulses in the lower limb can give important information about the status of the circulatory system in general.
Talus: the sustentaculum tali is a horizontal shelf that arises from the anteromedial portion of the calcaneus. The superior surface is concave and articulates with the middle calcaneal surface of the talus. The inferior surface has a groove for the tendon of the flexor hallucis longus. It can be felt just distal to the tip of the medial malleolus (approach from below and press upwards with your fingers). The head of the talus is between the sustentaculum tali and the tuberosity of the navicular bone (see below).
Navicular: palpate anterior to the sustentaculum tali to feel the tuberosity of the navicular bone.
Medial cuneiform: can be felt anterior to the tuberosity of the navicular.
Metatarsals: the tip of the base of the 5th metatarsal (tuberosity of the 5th metatarsal) is easily seen and palpated on the lateral aspect of the foot. The sesamoid bones of the head of the 1st metatarsal can be felt if the great toe is passively moved. All metatarsals and phalanges are easily palpable on the superior surface of the foot.
3. Soft tissue of the foot
Sensation and muscle action in the lower limb are tested to assess lumbar and sacral regions of the spinal cord.
Calcaneal tendon: largest tendon entering the foot and formed by the combined tendons of gastrocnemius, soleus, and plantaris. It is prominent on the posterior aspect of the foot as is descends from the leg to the heel. A tap with a tendon hammer on this tendon tests reflex activity of the spinal cord levels S1-S2.
Tendon of tibialis posterior: can be seen and felt posterior and inferior to the medial malleoulus if the foot is plantar flexed and inverted.
Tendon of tibialis anterior: is visible on the medial side of the ankle anterior to the medial malleolus.
Tendon of fibularis longus and brevis: pass around the posterior and inferior aspects of the lateral malleolus (going to the base of the 5th metatarsal) and can be seen when the foot is dorsiflexed and everted.
Extensor tendons: can be seen and felt as they cross the anterior aspect of the ankle joint when the foot is dorsiflexed against resistance. From the medial to lateral palpate the tendons of tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius.
4. Visualizing the tarsal tunnel and its contents
The tarsal tunnel is formed on the medial side of the foot in the groove between the medial malleolus and the calcaneal tuberosity and by the overlying flexor retinaculum. The posterior tibial artery and the tibial nerve enter the foot through the tarsal tunnel. The tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus also pass through the tarsal tunnel in compartments formed by the septa of the flexor retinaculum.
The order of the structures passing through the tunnel from the anteromedial to posterolateral are the tendon of the tibialis posterior, the tendon of the flexor digitorum longus, the posterior tibial artery and associated veins, the tibial nerve, and the tendon of the flexor hallucis longus. The tibial artery is palpable just posteroinferior to the medial malleolus on the anterior face of the visible groove between the heel and medial malleolus.
5. Visualizing the position of the major superficial veins
Superficial veins in the lower limb often become enlarged. Because the veins are large, they can be removed and used elsewhere in the body as vascular grafts.