A Common Error in Negative Casting
The neutral position negative casting technique relies on total patient relaxation for optimum results. Any muscular contraction which occurs during the time the plaster is drying on the foot (or the scan is being taken) can change the external morphology of the foot. In some instances, if the patient does contract one of the muscles of the leg during the casting procedure, the cast needs to be redone.
In the ideal situation, negative casting should occur when the patient is totally relaxed so the risk of muscular contraction is minimizes. However, some patients, no matter how relaxed they are, tend to tense the muscles of the leg when the foot is touched by the practitioner.
If there is only one muscle which is going to be contracted during the negative casting procedure, it will most likely be the anterior tibial muscle. The reason that the anterior tibial muscle is often involuntarily contracted by the patient is because it is the muscle which is most frequently called upon during the withdraw reflex to move the foot away from an irritating stimulus. In this case, the irritating stimulus, whether the doctor likes it or not, is the doctor’s hand.
When the foot is totally relaxed in the neutral casting position, the medial column of the foot is relatively plantarflexed in relation to the lateral column of the foot. This creates the desired situation where there is a minimal inverted forefoot deformity or a maximal everted forefoot deformity in the cast (Figure 1).
However, when the anterior tibial muscle is contracted during casting, both supination of the longitudinal axis of the midtarsal joint and dorsiflexion of the first ray occur (Figure 1). Longitudinal midtarsal joint supination introduces too much forefoot varus into the cast and flattens the contour of the medial longitudinal arch of the foot.
Dorsiflexion of the first ray elevates the plane of the first metatarsal head into a more dorsal position in relation to the second through fifth metatarsal heads. Both longitudinal midtarsal joint supination and first ray dorsiflexion within the cast will tend to cause the resulting orthotics to create irritation at the anterior aspect of the medial arch of the foot and/or dorsal jamming of the first metatarsal phalangeal joint during gait.
Many simple remedies to prevent anterior tibial muscle contraction during casting can be used:
- Make sure that the patient is thoroughly relaxed, lying or sitting comfortable with their head leaned back resting on the chair or table.
- No speaking between the patient and any office staff (including the practitioner) during the casting procedure.
- Ask the patient to concentrate on relaxing their hips and knees instead of asking them to relax their feet. Once the heel and knees are relaxed, the feet seem to relax also.
Many times, patients just can’t relax their anterior tibial muscle at all. One of the physiological findings in animals is that antagonistic muscles are reciprocally innervated. This means that when a flexor is activated, the extensor is inhibited. And when the extensor is activated, the flexor is inhibited by the nervous system.
Knowing that the gastrocnemius and soleus are antagonistic to the anterior tibial muscle across the ankle joint can help you avoid casting errors. The next time your patient fires their anterior tibial muscle during casting, don’t tell them to relax, tell them to contract! In other words, if the anterior tibial muscle becomes active during casting, just tell the patient to push slightly harder against your thumb with their fourth and fifth metatarsals. Only a little bit of contractile force from the posterior musculature is needed to totally shut down the anterior tibial muscle. This approach can allow you to get better casts even from your least relaxed patients.