A Quick Check Ensures Quality Casts
It is imperative to evaluate negative casts for proper positioning before sending them to the lab for orthotic manufacturing. A quick evaluation of 10 features will help ensure optimum outcomes. The full evaluation should be performed regardless of whether you use plaster, the STS sock, or optical scanning. In fact, the ability to evaluate these points should be a criterion for evaluating the effectiveness of digital casting technology.
Features of a Good Negative Cast for Functional Orthoses
Hallux position is one of the most important points to evaluate on your negative casts. Dorsiflexing the hallux while casting helps to plantarflex the first metatarsal. Studies have demonstrated that plantarflexion of the first metatarsal enhances the windlass mechanism function and decreases tension on the plantar fascia. An orthosis made from a cast with a dorsiflexed hallux is more likely to provide optimum clinical outcomes for patients with hallux limitus, hallux valgus, and plantar fasciitis. If the hallux is plantarflexed while casting, the first metatarsal is dorsiflexed. An orthosis made from this cast error (as shown on the left below), will prevent hallux dorsiflexion leading to functional hallux limitus.
The right cast has a dorsiflexed hallux and is acceptable for use in manufacturing a functional foot orthosis. The cast on the left has a plantarflexed hallux and should be rejected
5th Digit Proximal Phalanx almost Parallel to 5th Metatarsal Shaft
Capture an accurate forefoot to rearfoot relationship by holding the 4th and 5th digits parallel to the metatarsal shafts. This ensures that you are not plantarflexing the 5th metatarsal and introducing a false forefoot varus into the negative cast.
The right cast shows the proximal phalanx of the 5th is almost parallel to the metatarsal shaft. The left cast shows the proximal phalanx is greatly dorsiflexed resulting in plantarflexion of the metatarsal and capturing excessive forefoot varus in the cast.
Thumb Impression in the Sulcus of the 4th and 5th Toes Only
Proper positioning of the thumb is in the sulcus of the 4th and 5th toes. The thumb should not extend any farther medial than the sulcus of the 4th digit. If your thumb is placed more medial than the 4th, you are likely to apply an inversion force on the forefoot, causing excessive varus to be captured in the forefoot to rearfoot relationship.
The cast on the right shows appropriate thumb placement with the thumb in the sulcus of the 4th and 5th toes. The cast on the left has the thumb impression too far medial.
There should be a distinct concave impression of the first metatarsal head captured in the negative cast. You should be able to visualize the 1st metatarsal head impression as concave both from medial-to-lateral and anterior-to-posterior. A flat or convex head impression represents a dorsiflexed 1st ray.
Note the concavity of the first metatarsal head in the cast on the right
On a plaster cast, skin lines should be visible in the mid-arch. This indicates that the plaster had good contact with the arch during casting. Absence of skin lines indicates that the plaster pulled away from the arch. This will result in an orthosis that does not conform adequately to the arch of the foot. This evaluation point is not available when using the STS sock.
The cast on the left has visible skin lines mid-arch indicating good contact with the arch of the foot.
The slope of the medial arch indicates if the STJ was held excessively pronated or supinated during casting. If the foot is held correctly with the subtalar joint in neutral and the midtarsal joint pronated, then the arch shape on the inside of the cast should be sloped on the medial 2/3 and flat on the lateral 1/3. If the subtalar joint is excessively pronated, then the arch will be flat across the entire inside of the cast. If the subtalar joint is excessively supinated, then there will be a steep slope extending the entire width of the arch.
The center cast cross section shows the appropriate 2/3 slope at the mid-tarsal joint indicating correct positioning during casting. The left cast has a flat arch across the entire foot, indicating excessive STJ pronation during casting. The cast on the right shows a steep slope from medial to lateral indicating excessive supination of the STJ and/or MTJ during casting.
Pronated Correct Supinated
Ensure that the cast captures the correct forefoot to rearfoot relationship. This must be noted by evaluating the foot before casting in order to compare to the casts. If the foot has a forefoot valgus, then the cast should capture that valgus. If the foot has a forefoot varus, then that should be captured. To evaluate the forefoot to rearfoot relationship, place the negative cast on a flat surface and evaluate the position of the heel relative to the surface. If a forefoot valgus is present in the cast, then the heel will invert. If a forefoot varus is present, then the heel will evert. A vast majority of feet will have forefoot valgus so, in a majority of cases, the heel should invert. Only in the case of a true forefoot varus should the heel of the cast evert. Forefoot varus is a very rare situation and usually found only on the severe pes planus foot type.
The cast on the left inverts when placed on the supporting surface. This indicates that the cast has captured a valgus forefoot to rearfoot relationship. The cast on the right has a perpendicular heel indicating a perpendicular forefoot to rearfoot relationship.
The shape of the lateral border of the negative cast can help you to determine midtarsal and subtalar position during casting. When a cast is taken with the subtalar joint held in neutral and the midtarsal joint locked in a pronated position, the lateral border is usually straight or slightly abducted – as shown below in the picture on the left. An exception to this rule is in the case of of metatarsus adductus, in which case the lateral border may be somewhat “C”-shaped. In the absence of metatarsus adductus, a “C”-shaped border indicates that the foot was held in a position with the midtarsal joint and /or the subtalar joint excessively supinated.
A C-shaped border, indicating excessive STJ supination during casting, is shown on right. Appropriate slightly abducted lateral border is shown on left cast.
The lateral arch should be visible with the apex at the calcaneocuboid joint. A flat lateral arch can indicate that the casting material gapped from the foot. An apex that is too far distal can indicate the MTJ was not fully pronated allowing the forefoot to plantarflexed on the rearfoot or that the cast was still soft when removed from the foot, causing the forefoot to plantarflex on the rearfoot.
Cast on the left shows excellent capture of lateral arch with apex at calcaneocuboid joint. On the right, the cast has an uneven lateral border and the apex is too far distal. It is likely that the plaster was still soft when this cast was removed from the foot, resulting in the forefoot plantarflexing on the rearfoot.
It is imperative that you capture the shape of the heel with deformation. A proper cast will reflect the rounded shape of the plantar heel. Applying pressure to the heel while casting, can flatten the heel of the cast. Maximally pronating the subtalar joint will also produce a non-concave and asymmetrical heel impression.
Note the rounded heel shape of the cross section cast on the right which is desired and correct. The cast on the left is excessively flat, indicating pressure on the heel during casting.
Clearly mark both the pateint's and doctor's names on the bottom of the negative casts.
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