Key Insights On Digital Casting Techniques

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Author(s): 
Bruce Williams, DPM, FACFAS

Key Biomechanical Considerations In Addressing The Forefoot Component

   As stated in the Root text, “With the subtalar joint in its neutral position, and with the forefoot pronated to its maximum about both axes of the midtarsal joint, the plantar plane of the forefoot parallels the plantar plane of the rearfoot. In this position, the forefoot is locked upon the rearfoot so as to resist any further motion of the forefoot in response to any dorsiflexion or eversion forces directed against the planter surface of the forefoot by ground reaction.”9

   Plantarly loading the lateral column of the foot under or near the fourth and fifth metatarsal heads while dorsiflexing the forefoot is the accepted technique for “locking” the midtarsal joint while the subtalar joint is in the neutral position. The importance of this positioning is that the forefoot is dorsiflexed while it is pronated. It is important when utilizing this technique not to overload the lateral column, which can cause the subtalar joint to pronate and then allow the midtarsal joint to dorsiflex and abduct in compensation.

   In a study comparing practitioner variability when utilizing subtalar neutral position casting of the foot, the researchers took an interesting step. Researchers looked at casts taken in a neutral subtalar joint position with both the midtarsal joint pronated and supinated about the long axis to decide if this made a difference in the variability of the forefoot to rearfoot relationship.10

   The same practitioner did all the casting for this portion of the study. The practitioner did five casts with the midtarsal joint pronated and utilized an adductory force to the lateral forefoot. The practitioner also did five casts with the midtarsal joint supinated and utilized an abductory force to the lateral forefoot. According to their cast assessment, the researchers found this did not make any significant difference in the repeatability of the casts or in the comparison of the forefoot to rearfoot relationship.10

   The study of the midtarsal joint, midfoot and forefoot function has been a focus of much research over the past few years. Forefoot positioning is very important when it comes to non-weightbearing neutral position casting techniques, especially in reference to the “maximally” pronated midtarsal joint of the foot, the so-called “locking” of the midtarsal joint and the reduction of forefoot supinatus via plantarflexion and adduction of the medial column of the foot.11

   Some of the studies evaluating the forefoot and midfoot joint ranges of motion have found a significant amount of range of motion available within the sagittal plane between the midtarsal bones of the cuboid and the fourth and fifth metatarsal bases, and the navicular, cuneiforms and the metatarsal bases one through three.12-14 Many practitioners regularly cast out the supinatus found within the medial column of the foot.11 This technique consists of plantarflexing and adducting or pronating the medial column or first ray of the foot to reduce the soft tissue supination component, which often mimics a forefoot varus.

A Guide To Casting Technique

   For the last five years, I have used one technique for partial weightbearing scanning or casting. This consists of having the patient place the foot on the scanner with the hip, knee and ankle joint all at 90 degrees. One then adducts or abducts the knee, which pronates and supinates the foot respectively while the foot remains fixed on the flat surface of a digital scanner.

   The practitioner then seeks to find the subtalar neutral position by either palpating the talar head or assessing the equalization of pressures between loading the dorsal aspect of the first and fifth metatarsal heads against the scanner. The foot at this point is being loaded both from a plantar direction via the scanner surface and via gravity and the weight of the patient’s leg and foot against the scanner. During the actual scan of the foot, the metatarsal heads of the first and fifth metatarsals are lightly to moderately loaded at their dorsal surfaces. Due to this positioning, one can consider the forefoot to be dorsiflexed while the ankle is at 90 degrees.

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