The third objection is that examination of individual cases not infrequently shows displacements in directions opposed to that which should be determined by muscular antagonism. Volkmann has especially illustrated the latter assertion by the mechanism of genu-recurvation. In paralysis of the quadriceps extensor of the tibia the weight of the body is exercised, not merely from above downward, but from without inward, falling, therefore, on the inner malleolus. It thus tends to press the anterior part of the foot outward,66 and a valgus finally complicates the calcaneus. Seeligmüller, however, quotes two cases of pes calcaneus developed in children who had never walked: in one paralysis of the sural muscles had occurred at the age of four weeks, and the other case was observed at the age of fourteen weeks. Seeligmüller remarks that only early examination of the faradic contractility of a group of paralyzed muscles can decide whether any among them preponderate during a time sufficient to fix the limb in a vicious position. Thus in one case of pes calcaneus he found two years after the occurrence of the paralysis that some contractility still persisted in the dorsal flexors, but six months later this had quite disappeared. Had the examination then been made for the first time when all the muscles were equally paralyzed, it would have seemed impossible to explain the deformity by muscular antagonism.
66 Sayre asserts that lateral rotation cannot take place at the ankle-joint, but at the medio-tarsal articulation. Hueter also refers pes varus and pes valgus to the talo-tarsal articulation.
RELATION OF WEIGHT AND MUSCULAR FORCES.—The influences of weight and of muscular action sometimes concur, sometimes are opposed to each other. Thus, the weight of the foot alone always tends to produce equino-varus; it acts therefore to intensify the action of the sural muscles when the anterior tibial are paralyzed, but to diminish the influence of paralysis of the gastrocnemius when the foot is being drawn into dorsal flexion. Hence one reason for the comparative rarity of pes calcaneus.
Paralytic deformities at the hip and knee are much rarer than those of the foot. At the hip this immunity is partly due to the relative rarity of paralysis in the muscles surrounding the joint—still more to the fact that the weight of the limb tends to correct excessive flexions. These are therefore more likely to occur in children allowed to remain in bed than in those who are encouraged to walk by means of suitable apparatus. The use of crutches, however, favors the development of deformity, because, since with paralysis of the thigh- or leg-muscles pes equinus nearly always exists, the thigh is unduly lengthened. To palliate this inconvenience the patient instinctively flexes the knee or hip, or both, and the position tends to become permanent.
When the flexion is rigid and extension becomes impossible, the gastrocnemii are relaxed until they lose their power of tension, and thence of fixing or raising the heel. Further, as by the flexion the limb is moved in front of the body, it is necessary to project the body forward again over the support. Hence a lordosis is developed, to be distinguished from that caused by paralysis of the vertebral extensors (a) by the rigid flexion of the thigh; (b) by the facility with which the patient can extend the back as soon as he is placed in a sitting position.
Although the quadriceps extensor is so frequently paralyzed, rigid flexion at the knee—such as on the theory of muscular antagonism might be expected from the action of the hamstring muscles—is very rare. As already observed, it occurs, if at all, in neglected children allowed to lie or sit with the leg partly flexed. In those who attempt to walk the leg is not flexed, but forced into hyper-extension by the following mechanism: The muscles inserted on the upper part of the thigh swing the leg forward like a passive support. Afterward the body bends forward over the support, and its weight, pressing from above downward and from before backward, and pressing the articular surfaces of the joint together, forces the head of the tibia backward until the movement is checked by the posterior ligaments. The deformity is the same whether the quadriceps or one or all the muscles surrounding a joint are paralyzed (Volkmann).
DISLOCATIONS.—Dangling limbs are, however, much more frequently the consequence of total paralysis, with extraordinary relaxation of the ligaments of the joint permitting dislocation. Reclus67 has published several such cases. Verneuil has even suggested that congenital luxation of the hip-joint always depends on an intra-uterine spinal paralysis.68<
67 Revue mensuelle de Méd. et de Chir., Mars, 1878.
68 Quoted from Seeligmüller.