Phillip M Stevens, MEd, CPO
Specialized Prosthetic and Orthotics Technologies
Salt Lake City, Utah
In considering Duchenne muscular dystrophy, the characteristic gradual loss of functional muscle and the concurrent developments of progressive contracture often invite orthotic interventions. Progressive patterns of weakness, accommodation and contracture development characterize the early stages of the disease. However, the underlying pathomechanics of the disease should be understood during considerations of lower extremity orthotic intervention.
The unique compensations adopted by children with muscular dystrophy are secondary to the weakness of the hip and knee extensors. Regarding standing balance, hip extensor weakness leads to hip flexion contracture, anterior pelvic tilt, compensatory lordosis and shortening of the tensor fascia lata. Knee extensor weakness leads to compensatory plantar flexion. These compensations position the weight line posterior to the hip and anterior to the knee, assisting the weakened extensors at the knees and hips.
Regarding gait, as the quadriceps become progressively weaker, the combination of dorisflexor weakness and active "equinus posturing" cause the center of pressure to be born further and further distally on the forefoot. This brings the ground reaction force relatively anterior and reduces the demands placed on the knee extensors during loading response (1-2). Therefore, a degree of plantarflexor tightness is beneficial.
These compensatory strategies reduce proximal eccentric contractions of the knee and hip extensors. Recent studies on animal models have confirmed the hypothesis that dystrophic muscle is particularly susceptible to injury during eccentric contractions (3-4). This largely explains the targeted weakness that occurs at the hip and knee extensors.
In light of this understanding of the disease and its pathomechanics, day time AFO's which prevent plantarflexion should generally be avoided. By bringing the ground reaction force relatively posteriorly, they serve to increase the demands upon the weakened knee and hip extensors (2,5). As these induced contractions are eccentric in nature, they are especially damaging to these at-risk muscle groups, and might serve to decrease a child's duration of ambulation before wheelchair confinement. Thus, attempts to delay the progression of plantarflexion contractures should be confined to nocturnal bracing, where they will not have adverse affects on more proximal muscle groups.
Sutherland DH, Olshen R, Cooper L, et al. The pathomechanics of gait in Duchenne muscular dystrophy. Dev Med Child Neurol, 1981, 23(10):3-22.
Sussman M. Duchenne muscular dystrophy. J Am Acad Orthop Surg, 2002, 10(2):138-51.
Childers MK, Okamura CS, Bogan DJ et al. Eccentric contraction injury in dystrophic canine muscle. Arch Phys Med Rehabil, 2002, 83(11):1572-8.
Allen DG. Eccentric muscle damage: mechanism of early reduction of force. Acta Physiol Scand 2001;171(3):311-319
Thompson N, Robb SA. The orthotic management of gait pathology in Duchenne muscular dystrophy. Physiotherapy, 1995, 81:459-460.