RESUMEN
In a prospective study on the biochemical behaviour of the triceps surae in idiopathic congenital talipes equinovarus during treatment with plaster of Paris, three observations were made: [1] The muscle, put at maximum physiological length in plaster, will elongate during the time it is in the plaster. [2] The elongation of the gastrocnemius may lag behind the soleus unless the position of the knee is taken into consideration. [3] Rocker-bottom deformity may develop even if the forefoot is not actively dorsiflexed and the plaster is moulded to maintain the arch. The implications of these observations are discussed: [1] It is proposed that the methods of 'fixation' - cast, splint or strapping - are actually methods of correction acting under the biochemical law that soft tissues maintained uninterruptedly at their maximum physiological length will gradually elongate. The role of the manipulations is to reset the muscle to its new maximum physiological length each time the plaster is changed so as to maintain the stretch stimulus. [2] When applying a below-knee plaster the knee must be fully flexed. When an above-knee plaster is used the angle used while applying the below-knee part of it should be maintained when completing the plaster to above the knee. The lag in gastrocnemius response should be looked for with each change of plaster. [3] It is also proposed that the triceps surae, by levering down the distal end of the os calcis within the plaster, may actively produce a rocker-bottom deformity if the forefoot is not allowed to follow that are of movement. It is essential to arch the cast on the lateral side under the anterior part of the calcaneus to prevent it from levering downwards and to maintain the relationship of the forefoot to the hindfoot. [4] The causes of failure of the methods are either biological or mechanical and form indications for surgery