REIDER PART 2 17
_Chapter 5 Felvis, Hip, and Thigh 177
perpendicular to the length of the table (Fig. 5-21A). The examiner looks down on the patienfs feet from above, estimating the angle that the medial border of the foot makes with the longitudinal axis of the thighs (Fig. 5-21#). Normally, the medial border of the foot should be externally rotated about 20° from the longitudinal axis of the table. Variations from this amount are described as increased or decreased external tibial tor-sion. As in the femur, rotational malalignment of the tibia may be a developmental anomaly or the sequela of a malunited fracture.
Foot Abnormalities. The Finał potential cause of in-toeing or out-toeing is an abnormality of the foot. When this is the case, a clear-cut foot deformity is usually pres-ent. The most common example of this is the in-toeing associated with the forefoot adductus deformity of an incompletely corrected clubfoot.
■ GAIT
Anterior and Postf.rior Perspectwes Abductor Muscle Function. Observation of the pelvis during gait reveals abnormalities that may not be appreci-ated during a static standing examination. Owing to the symmetric design of human anatomy, the bodys center of gravity passes midway between the hips through the pubie symphysis. When one foot is lifted from the ground, the center of gravity generates a downward force that tends to drop the pelvis toward the side where the foot has been lifted (Fig. 5-22). This effect is similar to what would happen if a leg were removcd from a stool, causing the stool to fali toward the side of the removed leg.
In humans, the gluteus medius and minimus mus-cles counteract the tendency of the pelvis to fali toward the opposite side by pulling it toward the greater trochanter of the weightbearing limb. These abductor muscles must be very strong because they are working at a mechanical disadvantage. When one lower limb is lifted off the ground to take a step, the entire upper body must be balanced on the femoral head of the weightbearing limb. The weight of the upper body can be assumed to act through its center of gravity, which passes through the pubie symphysis. The distance from the abductor muscles to the femoral head is only about half the distance from the center of gravity to the femoral head; consequently, the abductor muscles must puli with a force approxi-mately twice the patients upper body weight to counter-balance it. This system seems to work quite well in normal individuals, although it generates tremendous forces across the weightbearing hip joint. With the weight of the patients upper body bearing down on one side of the femoral head and the abductor muscles pulling at a force twice the patients upper body weight on the other side, a compressive force of three times the weight of the patients upper body is transmitted across the weightbearing femoral head with every step.
Trendelenburg's Gait. This precariously balanced system can break down in several ways. If the gluteus medius and minimus are not quite strong enough to counterbalance the patients upper body weight, the pelvis tends to droop toward the floor when the patient
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