Pathophysiology

Cerebral palsy (CP) is, by definition, a static encephalopathy with onset before maturation of the central nervous system. Although most cases are present at birth, most experts include injury to the brain before age 3 in the diagnostic grouping of CP. In general, anoxic events lead to quadriplegia. Prematurity resulting in periventricular leukomalacia results in diplegic CP. Vascular events such as intrauterine strokes cause hemiplegia.

Regardless of the type of CP, it is the central control system that is damaged. The neurological lesion may produce different tone abnormalities. In a patient with pure spasticity, only the pyramidal system is damaged. In a patient with athetoid CP, only the extrapyramidal system is involved. Both systems are injured when a mixed pattern is seen. The central nervous system lesion affects the musculoskeletal system. Primary abnormalities include the following⁸:

These primary abnormalities cause secondary growth disorders of the musculoskeletal system as the neurological impairments occur early during growth and development. Normal bone growth occurs only if the bones are subjected to typical development stresses. Children who are unable to walk, run, and play at typical ages with typical movement patterns are likely to develop bone and joint deformities. Infantile bony alignment is markedly different than typical adult alignment. Normal developmental remodeling of fetal femoral anteversion and internal tibial torsion is unlikely in the absence of typical growth stresses. Development of normal foot alignment and function is in jeopardy if muscle function is abnormal or if the stresses on the foot are excessive due to spasticity or abnormal weight-bearing positions. Longitudinal bone growth occurs at the physes present at both ends of long bones. Much of that growth probably occurs while the child is resting. Muscle growth, on the other hand, is driven by stretch. Normally that stretch occurs when a child, whose bones have grown during sleep, gets up and starts to run and play. Ziv et al.³¹ showed that in order for normal muscle growth to occur, 2 to 4 hours of stretch per day is necessary. In addition, they showed that the muscles of spastic mice do not grow in response to stretch at the typical rate. The combination of this lack of response and the lack of “normal play” leads to secondary musculotendinous contractures.

Gait dysfunction in individuals with CP occurs as a result of these primary and secondary abnormalities, which rarely occur in isolation. Rather, they are multiple and consist of primary effects (due to the damage to the central nervous system), secondary deformities (from abnormal bone/muscle growth), and tertiary compensations (individual coping responses to minimize the gait inefficiency resulting from the primary and secondary abnormalities). Tertiary abnormalities are coping responses to restore lost attributes of normal gait¹⁹ and include the following:

One example of a tertiary gait compensation is circumduction to compensate for clearance problems caused by cospasticity of the rectus femoris and hamstrings. This produces the stiff knee gait so common in patients with CP. Another example is premature plantarflexion in stance phase (typically referred to as vaulting), which is present in a child with hemiplegic CP to compensate for a lack of clearance on the hemiplegic side caused by either a drop foot in swing (tibialis anterior dysfunction) or rectus femoris spasticity. Too often, the “difficult to identify” vaulting is misinterpreted as a part of the pathology and is not recognized as compensation on the nonaffected side. As such, orthotic or surgical management may be prescribed, resulting in a detrimental effect on the child’s function. Because primary, secondary, and tertiary abnormalities occur at all levels in the lower extremity, the management of gait dysfunction in children with CP truly is complex and was (and continues to be) the driving force for the development of computerized gait analysis laboratories.

Neurological conditions create greater distal than proximal dysfunction. The weakness and loss of motor control that can occur in individuals with CP typically are worse at the foot and ankle than at the hip. The unstable foot induces additional alignment abnormalities at the hip and knee. This explains why orthotic management is primarily directed toward compensating for foot and ankle dysfunction to improve walking function. These issues are addressed in the treatment sections of this chapter.

It is primarily the two-joint muscles (e.g., psoas, rectus femoris, hamstrings, and gastrocnemius) that are affected by excessive tone caused by CP. With time and growth, these muscles become contracted. It is interesting that the one-joint muscles (e.g., gluteus maximus, vasti, and soleus) typically are too long due to the chronic effects of walking in crouch. The crouch position of excessive hip and knee flexion in conjunction with either excessive ankle dorsiflexion or midfoot breakdown puts these muscles in a position of excessive elongation as they cross the joints. These are also the muscles that are primarily responsible for our ability to maintain upright posture.²⁹ Upright posture requires good antigravity function of the one-joint muscles responsible for supporting body weight. Weakness and excessive elongation of these muscles in conjunction with spasticity of the two-joint muscles is primarily responsible for crouch gait. The management of ambulatory dysfunction in CP in the vast majority of cases is geared toward the treatment of crouch. The long-term consequences of crouch are excessive joint stress and gait inefficiency. This combination leads to decreasing ambulatory function in adolescence and adulthood due to joint pain and excessive energy costs.^13,27 The frequency of these problems in adulthood is disturbingly high¹⁶ and ultimately is the reason why treatment of these problems in children and adults is so important.

To understand and treat physical function deficits requires a basic understanding of mechanics. A lever arm or moment arm is defined as the distance from a point to a force that is perpendicular to the line of action of that force. The force (measured in Newtons) times the length of the lever arm (in meters) is equal to the moment that acts around the center of rotation (in Newton-meters). In general, the length of the bone serves as the lever, and the joint at the end of that bone serves as the center of rotation or fulcrum. The magnitude and direction of the moment depends upon the point of action of the applied force (Fig. 38-1).

Fig. 38-1 Moments and lever arms. The magnitude of a moment (M) is the product of force (F) times the length of the lever arm (d). A lever arm is defined as the perpendicular distance between the force and the center of rotation. A change in either the position or the orientation of the applied force will cause a change in the magnitude of the moment. To create the largest moment, the force must be perpendicular to the lever.

Moments are perhaps understood most easily if one thinks of a see-saw in which the mass of the larger individual times his or her distance from the fulcrum is equal to that of the smaller individual times her or his distance from the fulcrum (Fig. 38-2).

Fig. 38-2 Easily understood example of moments, shown as the relationship between an adult and a child on a teeter-totter. The larger mass of the adult sitting closer to the pivot point (fulcrum) balances the smaller mass of the child sitting further away.

The principle is the same in walking. External moments produced by the ground reaction and inertial forces plus weights of the lower extremity segments are resisted by internal moments produced by the action of muscles, tendons, and/or ligaments (Fig. 38-3).

Fig. 38-3 Relationship between the external moment produced by the ground reaction force (GRF) and the internal moment produced by the muscles. Both forces act on a skeletal lever around the fulcrum (joint center). In this illustration, the lever arm of the GRF is twice as long as that of the ankle plantarflexors. As a result, the magnitude of the muscle force would be twice that of the GRF.

Lever-arm dysfunction is a term originally coined to describe the particular orthopedic deformities that arise in an ambulatory child with CP. However, the condition is common to any traumatic or neuromuscular problem that produces alteration of the bony skeleton. Lever-arm dysfunction, then, describes a general class of bone modeling, remodeling, and/or traumatic deformities that includes hip subluxation, torsional and angular deformities of long bones, and/or foot deformities. Because the muscles and/or ground reaction forces must act on skeletal levers to produce locomotion, abnormalities of these lever-arm systems greatly interfere with the child’s ability to walk.^8,9

In a condition such as CP, the muscle and/or ground reaction forces are neither appropriate nor adequate because of muscle contractures, poor selective motor control, and/or abnormality of the bony lever arms. The five distinct types of lever arm deformity are (1) short lever arm, (2) flexible lever arm, (3) malrotated lever arm, (4) abnormal pivot or action point, and/or (5) positional lever-arm dysfunction (Table 38-1). A comprehensive discussion of lever-arm dysfunction is beyond the scope of this chapter, but a common example of lever-arm dysfunction that is seen in spastic diplegia will serve to illustrate the problem.

Table 38-1 Examples of lever arm dysfunction

In normal gait during the second half of the stance phase, stability of the knee is maintained without quadriceps action by a mechanism termed the plantarflexion/knee-extension couple (Box 38-1). That is, the action of the soleus at the ankle restrains forward motion of the tibia over the foot and in so doing maintains the ground reaction force in front of the knee. The result is that the ground reaction force acting on the lever arm of the forefoot produces an extension moment at the knee, which in turn maintains the joint in extension without the aid of the quadriceps (Fig. 38-4). However, the typical child with spastic diplegia frequently has femoral anteversion in conjunction with pes valgus and/or external tibial torsion. The plane of the foot often is as much as 40 degrees external to the plane of the knee. In addition, a valgus foot is an ineffective lever because it is supple rather than rigid. As a result, even if the magnitude of the ground reaction force were normal, because the lever arm is supple and maldirected, the magnitude of the extension moment can be greatly reduced (Fig. 38-5). Fortunately, lever-arm dysfunction usually is correctable with appropriate orthopedic surgery and/or bracing.

Fig. 38-4 Plantarflexion/knee-extension couple. During midstance in typical gait, the soleus resists ankle dorsiflexion and slows the forward movement of the lower leg (tibia). As a result, the GRF acting on the lever arm of the foot generates an extension moment on the knee that stabilizes the knee in an extended position without muscular activation of the larger muscle mass of the quadriceps (which would consume more energy to accomplish the same task). This extension moment is referred to as the plantarflexion/knee-extension couple.

Fig. 38-5 Flexible lever arm dysfunction. A, During terminal stance, the arch typically stiffens due to tension created in the plantar fascia as it winches around the metatarsal heads during metatarsophalangeal joint extension. The heel moves into relative varus and the arch lifts. The rigidity of the foot renders it an effective lever for the ankle plantarflexors to generate push-off power. The midfoot instability associated with pes valgus allows the hindfoot to remain in valgus while the forefoot persists in a position of abduction and varus throughout stance. As a result, the foot is externally rotated to the knee axis. The lever arm is both maldirected and an ineffective lever arm, like a crowbar made of rubber. B, Typical pes valgus secondary to spastic diplegia. The normal plantarflexion/knee-extension couple is disrupted because the GRF is behind the knee axis in midstance. The entire task of sustaining an upright position with lower extremity extension falls to the hip and knee extensors. Unfortunately, as body size increases with age, the strength and power in these two muscle groups are not enough to assume this burden, and crouch gait develops.

Many children with CP have weak ankle plantarflexors (gastrocnemius and soleus). Typically the ankle plantarflexors restrict dorsiflexion and tibial advancement in second rocker (midstance phase of the gait cycle) and act like a spring, providing power for push-off in third rocker (just before toe-off).

When the plantarflexors fail to function adequately in midstance, excessive dorsiflexion results and is accompanied by excessive knee flexion, or crouch gait. When the activity of the gastrocnemius is inadequate, insufficient power for push-off results in decreased clearance in swing, reduced step length, and decreased walking speed. Therefore, plantarflexor dysfunction leads to both stance phase (supportive) and swing phase (propulsive) deficiencies.

Evaluation of foot deformity is challenging (see Box 38-2). Two common foot deformity types exist in children with CP. In patients with hemiplegia, the equinovarus foot deformity is most common and may be associated with pes cavus. In individuals with diplegic and quadriplegic CP, the dominant foot deformity is equinovalgus. Although the etiology of this foot deformity is not completely certain, it most likely is related to equinus of the hindfoot (primarily gastrocnemius contracture and spasticity) leading to excessive forefoot weight-bearing early in a child’s development when the structure of the foot’s longitudinal arch is incompletely developed. Up until about age 6 years, pes planus is typical. With typical development, the arch forms and develops. In the case of neuromuscular pathology, this process may not occur, and equinovalgus foot deformity with midfoot instability can develop. With time, a forefoot varus deformity is not uncommon (Fig. 38-6). Treatment of this complex foot deformity type may require management of not only hindfoot equinus but also midfoot instability and forefoot varus.