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Casting Techniques: a Review of the Literature

The casting process is the first step in the fabrication of any custom made foot orthoses. The main concern to a patient is the end product.  Is it comfortable? Is my pain going away?  From a pedorthists point of view, the concern lies a little deeper.  The movement of the patient’s foot needs to be properly controlled.  The orthoses needs to have complete contact with the plantar surface of the foot. Comfort is only achieved when the orthoses accurately contours the patient’s foot. Root’s subtalar neutral theory has been the standard casting method in the past. Today, numerous devices and methods have been developed to duplicate the foot in this particular position. The discussion in today’s literature surrounds the question of which technique leads to the “best orthoses” for a particular pathomechanical deformity. This paper highlights some of the research comparing different casting methods. Forefoot to hindfoot angles, rearfoot to forefoot relationships and plantar pressures are all discussed.

Over forty years ago, Merton Root developed the theory of casting patients in the subtalar neutral position. Root’s knowledge in podiatric medicine led to the concept that “central to appropriate foot function was the necessity of the subtalar joint coming into neutral just after heel strike, and again at the end of midstance” (UWO Pedorthic Diploma Program). Root proposed that in order for orthoses to be effective, the subtalar joint must function around its desired neutral position and the orthoses must cause a locking of the midtarsal joints. Root conceptualized that deviation from the subtalar neutral position resulted in the pathologies pedorthists’ see in clinic everyday. Emerging theories are slowly gaining respect over Root’s past beliefs. Importantly however, is that today’s “research supports that orthotic treatment from a subtalar neutral cast does improve symptoms” (UWO Pedorthic Diploma Program). Many different casting methods have emerged based on Root’s theory of casting in subtalar neutral.

In 1989, Thomas G. McPoil et al. and Laughton et al. in 2002 both compared the forefoot to rearfoot relationship found between different casting methods. McPoil compares three different casting methods; a supine and prone non-weight bearing plaster cast and a sitting semi-weight bearing foam box cast. Each subject’s foot was considered individually, as a previous study demonstrated that “in a sample of 58 women, 69% exhibited the same type of forefoot-and-rearfoot deformity bilaterally, indicating the importance of considering each foot individually for both assessment and plaster impression procedures” (McPoil, 1989). Results indicated that there were no differences in the forefoot to rearfoot relationship between both plaster casting methods. Although a neutral subtalar joint was consistent among all three casting methods, McPoil notes that professionals can expect to see differences between forefoot and rearfoot alignment when using semi-weight bearing casting versus non-weight bearing casting.

The forefoot to rearfoot relationship was only one of the examining variables in Laughton and colleagues research. This study further examined forefoot and rearfoot width, as well as arch height obtained by four casting methods. A non-weight bearing plaster cast, a semi-weight bearing foam cast, and a partial and non-weight bearing laser scan were all tested. Contrasting to McPoil and colleagues, the Laughton et al. research found no differences in the forefoot to rearfoot relationship between each casting method. There were significant differences in arch height between casting methods, the highest arch height found in the non-weight bearing laser scan, and the lower arch height found in the partial-weight bearing laser scan. Plaster casting on the other hand, produced the second lowest values on arch height. This is unexpected, as the patients were casting in a non-weight bearing position. Authors attributed this unusual finding to the subject’s casting position, where there was possibly an increased amount of tension in the plantar fascia. The least amount of reliability was reported in arch height measures. In terms of forefoot and rearfoot width, the smallest measures were found in plaster casting, whereas foam casting produced the largest measures. One of the benefits of foam box casting versus plaster casting is its ability to capture soft tissue expansion. These differences in rearfoot and forefoot width are attributed to this soft tissue expansion.

These two studies demonstrate that variation is indeed found between casting methods, regardless if the subtalar joint is held in a ‘neutral’ position. The “subtalar neutral is a position that allowed the subtalar joint to be most efficient. The purpose of this casting position is to capture the relationship of the forefoot to the rearfoot, plantar contours of the foot, and the calcaneal inclination angle” (UWO Pedorthic Diploma Program). All this might be true, however variability between casting methods devalues the importance of such subtalar neutral. Patients are casted by various methods and the finished orthoses still provides them with positive results. From this perspective, it’s understandable why new theories have emerged, and are slowly gaining precedence over subtalar neutral. The soft tissue stress theory emerged six years following the results of his previously explained study. As Thomas G. McPoil notes, it is important as clinicians to see these holes in the big picture.

Guldemond et al. studied contact area, plantar pressures, and gait lines in both accommodative and functional orthoses. Two foam box methods of casting were used; an accommodative full weight bearing method (labeled method A) and a functional semi-weight bearing subtalar neutral method (labeled method B). Two plaster methods were also used: an accommodative suspension method (labeled method C) and functional suspension subtalar joint neutral position method (labeled method D). The gait line data was described as “the path of the center of pressure from frame to frame”, therefore “movements in the sagittal plane determine the location of gait line along the longitudinal axis” (Guldemond, 2006). Results indicated that heel cups were deeper and arches were most posteriorly located on orthoses C and D compared to orthoses A and B. The total contact area of the orthoses within footwear increased by a factor of 17.4% compared to shoes without orthoses. The largest pressure increase was seen in the medial midfoot region. Furthermore, peak pressure for the total plantar surface of the foot was lower with orthoses than without. Lastly, gait lines were reduced when patients wore the orthoses in their shoes.

These important results imply to pedorthists that all orthoses do in fact increase the total contact area of the plantar surface of the foot. The largest pressure reduction is seen in the forefoot region among all casting methods. This result suggests that pressure from the casting pedorthist may be greater in the forefoot area. Guldemond et al. further noted a few interesting differences between the studies orthoses casting methods. Foam box casting was seen to create a larger surface contact area than the two plaster casting methods. This is attributed to the soft tissue expansion captured in foam box methods, and not when casting with plaster. Peak pressures remained the same within all casting methods regardless of the differences in orthotic contours and total contact area. Lastly, the gait line of functional orthoses provided promising results compared to the center of pressure paths of accommodative orthoses. In functional orthoses, the gait lines were more laterally located, “which is in accordance with the objective of functional orthoses: the change in the center of pressure reflects the redirection of forces” (Guldemond, 2008).

All these results suggest to pedorthists that specific casting methods may be better suited for some patients, as  “different methods of obtaining a representation of the foot results in different measures of foot morphology” (Laughton, 2002). When looking at the big picture, consistency on behalf of the caster is a question worth addressing. Chuter and associates noted that the experience of the caster had no correlation to the outcome of negative plaster casts. The range of variability between forefoot to rearfoot relationship was 10 degrees everted to 6.5 degrees inverted. Such results raise the question of the necessity of accurate casts. Regardless if the patient’s foot is accurately controlled and there is complete plantar surface of the patient’s foot on the orthoses, a larger question may still remain unanswered. Perhaps more research is needed to clarify inter-rater versus intra-rater reliability of all these casting studies. Additional research is obviously needed. Until answers are found, clinicians are responsible to research the pros and cons of different casting methods. Consequently, the best method can be selected for the benefit of the patient.

Kelly Levac
CK, C Ped C
KAL Orthoses