288


PART IV THE LOWER EXTREMITY


ray of the foot. The second ray is formed by the middle cuneiform and the second metatarsal, and the third ray is formed by the lateral cuneiform and the third metatarsal. The fourth and fi fth rays correspond to the fourth and fi fth metatarsals.


INTERMETATARSAL JOINTS


The base of each metatarsal forms a joint with the adja- cent metatarsal base and is labeled an intermetatarsal joint. These are synovial joints between the second and third bases and between the third and fourth bases of the metatarsals. Although not synovial joints, ligaments connect the fi rst and second metatarsal bases.


Kinematics


Because the second ray has the least amount of movement of the metatarsals, it is often used as a reference point for motion of the forefoot. Compared with the medial fore- foot, which is more stable, the lateral forefoot has greater range of motion. The fi rst ray has only a limited amount of movement, occurring mainly in fl exion and extension. This limited movement allows the medial aspect of the foot to move around the second ray as the foot adapts to various surfaces during weight-bearing activities. It also allows the medial border of the foot to make contact with the ground as the heel rises during the push-off phase of gait.


Rotation of the fi rst, third, fourth, and fi fth metatar- sals around the second metatarsal affects the mobility and stability of the foot. It provides a means for the forefoot to invert and evert independently of the rearfoot. Inver- sion and eversion of the metatarsals at the tarsometatarsal joints and the intermetatarsal joints is minimal and is


described relative to the second metatarsal rather than the midline of the body.


METATARSOPHALANGEAL AND INTERPHALANGEAL JOINTS


The metatarsophalangeal (MTP) joints are made up of the convex metatarsal head and the concave base of the proximal phalanx (Fig. 13.16A). The joints are biaxial— they permit movement in the sagittal and horizontal planes. The joints permit 90° of hyperextension and 30° to 45° of fl exion. The range of motion for hyperexten- sion enables standing on the toes and permits suffi cient push-off as the heel rises off the ground at the end phase of stance when walking. A much smaller degree of abduc- tion and adduction of the toes also occurs at the MTP joints (Fig. 13.16B–D).


The interphalangeal (IP) joints of the toes, similar to the IP joints of the fi ngers, are hinge joints that permit 1° of motion: fl exion and extension. Similar to the thumb, the great toe has only one joint, between the proximal and the distal phalange. The four additional toes have three phalanges with a proximal IP joint between the proximal and middle phalanx and a distal IP joint between the middle and distal phalanx. Toe fl exion in a closed chain standing position helps to balance the leg. Closed chain toe extension assists the MTP joints to enable standing on the toes and to push off during walking (Fig. 13.16D).


FOOT ARCHES


Three structural arches stabilize the foot and enable the foot to rebound and adapt to weight-bearing loads. The


Interphalangeal joint


Metatarso- phalangeal joints


Distal


interphalangeal joint Proximal


interphalangeal joint B1 A Flexion


Figure 13.16 (A) Metatarsophalangeal and interphalangeal joints. Flexion (B1), extension (B2), hyperextension (B3), adduction (C1), abduction (C2), and closed chain hyperextension (D) all are movements at the metatarsophalangeal, proximal interphalangeal, and distal interphalangeal joints.


Continued


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