The hind limb is directly attached to the axial skeleton through the os coxae or hip bone ("innominate bone"). The two innominate bones join with the sacrum to form the pelvis. The os coxae are formed ontogenetically by the fusion of the ilium, ischium, and the pubis.
The ilium (pl. ilia) is the largest bone of the innominate, and serves as the origin for the gluteal muscles. It is the thin, blade-like section superior to the hip socket. In the lateral aspect, the most obvious landmark of the innominate is the acetabulum, which provides the articulation for the femoral head. Other landmarks include the iliac crest, which defines the superior border of the ilium, and the sciatic notch, an indentation on the postero-inferior surface of the ilium, just superior and dorsal to the acetabulum.
The ischium (pl. ischia) is the most inferior bone of the innominate. The body of the ischium defines the border of the obturator foramen. The most caudal portion of the ischium is marked by the ischial tuberosity (what you sit on- it’s the bone that carries the weight of the trunk in a sitting person).
The pubis is the most anterior bone of the innominate. Left and right pubic bones articulate at the pubic symphysis, which aligns with the sagittal midline of the trunk. The pubis has superior (iliopubic) and inferior (ischiopubic) rami (sing. ramus) that surround the obturator foramen.
1. Examine an innominate and identify the following:
- The points of attachment of the sacrospinous, sacrotuberous, and posterior sacroiliac ligaments. These ligaments resist sacroiliac dislocation and rotation of the sacrum.
- The ischial tuberosity, bears the weight of the trunk when we sit, and serves as the site of attachment of the hamstring muscles
- The C-shaped articular surface of the acetabulum
- The anterior superior iliac spine, where the inguinal ligament and sartorius muscle attach
- The anterior inferior iliac spine, where the rectus femoris muscle and the iliofemoral ligament attach. Note also the osseous parts of the boundaries of the greater and lesser sciatic foramina, separated by the sacrospinous ligament, and completed medially by the sacrotuberous ligament.
The femur is the largest long bone in the body and the only bone in the thigh (as the humerus is the only bone in the arm). Proximally, it is marked by the large, globular femoral head, the femoral neck, and the greater trochanter. The femoral head articulates with the acetabulum of the innominate (note the smooth surface of the femoral head at this articulation).
The femoral shaft (diaphysis) is generally very robust with a slight anterior curvature. Dorsomedialy, the shaft is marked near its proximal end by the lesser trochanter. At its distal end, the femoral shaft expands into the lateral and medial femoral condyles. These condyles provide articulation with the proximal tibia, forming the knee joint. However the femur is not in direct line with the tibia; it angles medially from the acetabulum toward the knee (valgus knee). The articular surface of the condyes is expanded anteriorly to provide a groove for the patella (knee cap).
1. Examine a disarticulated femur. Locate the most important sites of muscle attachment to this bone:
- The greater trochanter (insertions of the deeper glutei and lateral rotator muscles)
- The lesser trochanter (insertion of the iliopsoas muscle)
- The linea aspera along the whole length of the posterior surface of the shaft (origin of the medial and lateral vasti muscles and the short head of the biceps femoris; insertion of the adductors muscles, pectineus, and gluteus maximus)
- Medial and lateral epicondyles (origin of the plantaris and gastrocnemius muscles)
- The adductor tubercle (insertion of the adductor magnus muscle - hamstring portion).
The patella (knee cap) is the largest sesamoid bone in the body. Sesamoid bones develop within tendons and often function to increase the leverage of the tendons in which they are imbedded. The patella resides within the quadriceps tendon and also helps stabilize the tendon during knee flexion. The patella does not develop until between two to six years of age.
1. Examine a disarticulated patella and identify the following:
- The flat and proximal base.
- The distally pointing apex.
- Two articular facets on the dorsal side of the patella: the smaller medial facet and the larger lateral facet that articulate with the medial and lateral femoral condyles, respectively.
4. The leg
The leg contains two bones. The tibia is the larger of the two and is in the medial part of the leg. The fibula is in the lateral part of the leg and is much thinner.
The tibia is generally triangular in cross-section, with one of the angles of the triangle forming the sharp surface of the shin. Proximally, the lateral and medial condyles articulate with their femoral counterparts. Distally, the tibia articulates with the talus at the ankle joint. The distal medial malleolus lies just medial to the tibio-talar articulation.
1. Examine a disarticulated tibia and identify the following:
- The proximal medial and lateral condyles of the tibial plateau.
- The proximal and anteior tibial tuberosity, the attachment site for the quadriceps tendon.
- The sharp and palpable anterior border.
- The lateral interoseous border.
- The distal medial malleolus.
- The distal articular surface for the talar trochlea.
The fibula is the smaller bone of the leg. It lies lateral to the tibia. Proximally, it articulates with the tibia at the superior tibiofibular articular surface. Distally, it articulates with the tibia and talus, and expands laterally to form the lateral malleolus.
1. Examine a disarticulated fibula and identify the following:
- The proximal fibular head.
- The medial interosseus border.
- The distal lateral malleolus.
5. 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 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 and thus 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 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.
5b. 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.
5c. 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 hwo small the fifth phalanx is and how large the hallus. 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.