Can repeatability be improved when taking foam impressions of the foot?

´│░an> Orthotic Biomechanical Solutions 2010

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Foam impression boxes are widely used to capture the plantar surface of the foot whilst in a sub talar joint neutral position. This is difficult to achieve, as the foot often tends to pronate or supinate as it is pushed through the foam.
Foot orthoses are produced from these impressions so it is important that they represent an accurate re-creation of the foot . Foam impression boxes sent to central fabrication sites may need to provide the bulk of the information to produce effective orthoses. Previous studies have shown that there is variation, when multiple casts of the same subject are compared.1 This study assessed the variation produced by 20 different clinicians casting the same subject. Also if a plantar flexed casting technique was more accurate, than the subject producing weight bearing casts, with no assistance, over consecutive impressions.

20 practitioners took a none non weight bearing impression of the same subject. They were free to take the cast using the method that they were most accustomed to.
To determine if a total weight bearing and modified none weight-bearing casting technique could produce a repeatable impression, 15 casts were taken using each method. This was undertaken by one practitioner and on one subject. All the foam impressions were scanned in to a CAD package and the height relationship between the first and fifth met head was compared.
The foam boxes were also filled with plaster and once set, cleaned to expose the plaster model of the foot. The calcaneus was bisected along the plantar surface and this bisected line was extended through to the intersection between the first and fifth metatarsal head. The distance between the side of the first met head and the intersection of the centre line was calculated for each model. This measurement was used to relate adduction and abduction of the forefoot in relation to the hind foot. The gradient of the arch at its highest to lowest point was also calculated for each cast.

The first part of the study showed that there was variation in all of the parameters measured in each of the casts taken by the 20 practitioners.

The distance between the projected calcaneus bisection line and the first metatarsal head showed a standard deviation (S.D) of 6.4 and a range of 16 mm. The relationship between the height of the first and fifth metatarsal head varied by a S.D of 4.1 and an arch gradient S.D of 3.5 and a mean arch gradient of 19 degrees
When comparing weight bearing casts, with plantar flexed none weight bearing casts, a difference could be seen by the two casting methods (undertaken by the same practitioner). The relationship between the first metatarsal head and the calcaneus was lower in the weight bearing trial with a S.D of 2 compared with a S.D. of 6.5 with the plantar flexed group. This was slightly higher than the results produced by 20 different clinicians. The mean arch gradient was lower in the plantar flexed group with a mean gradient of 14 degrees, the weight bearing casts being 2 degrees higher. The weight bearing casts predictably produced a more repeatable position of the first and fifth metatarsal head with a S.D of 1.8 (plantar flexed casts 2.6)

Comparing the results from the 20 casts taken by different clinicians it could be seen that there was an inter cast variation. This would make producing an accurate foot orthoses difficult to achieve by a central fabrication process. It was considered that taking an impression with the foot plantar flexed might have increased the ability of the Practitioner to produce a more repeatable impression of the foot. This was not found in this study as the relationship between the calcaneus and the first metatarsal varied greater than the trials using 20 different clinicians to cast one person. The use of a weight bearing cast technique did take the most repeatable impression but allowed the foot to be captured in an un corrected position. Further work will need to be conducted to examine if new technologies such as CAD CAM can be used coupled with more information from the foot, to produce a corrective insole from a weight bearing un-corrected foam impression of a patient’s foot.

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