How Does Casting Technique Impact Orthotic Form and Function?

At the NYSPMA Clinical Conference in New York City, Joseph C. D’Amico, DPM, DSc shares his findings from a controlled, comprehensive experiment he conducted using himself as the subject and a qualified practitioner (Samantha Landau, DPM) as the clinician who performed plaster cast impressions with 3 different techniques to fabricate custom foot orthotics (CFOs). He then went on to evaluate the differences in fit, form, function, alignment, and tolerability of the resultant devices. The same orthotic laboratory fabricated all 3 sets of devices from the original casts with no modifications or alterations to the cast or the positive models except those called for in each prescription. Podiatry Today had the chance to learn more about Dr. D’Amico’s goals for this experiment, and what he thinks DPMs can learn from his findings.

Q: What motivated or inspired you and your team to look at these specific casting technique features in your study?

Dr. D’Amico shares that he noted many CFOs patients brought to his office over the past 40 years did not have the desired result, and that there was significant room for improvement in matching foot architecture, and optimizing alignment and function. He feels this may happen for a variety of reasons including a need for improvement in assessment, prescription, fabrication, skill, or experience.

“Medicine is an art as well as a science, and there is an art component involved in performing and assessing a comprehensive biomechanical examination, observing and analyzing gait, and appropriately positioning and capturing the foot in impression casting,” he says. “The static and dynamic analysis of the foot, ankle, knee, hip and spine requires time. Time not only for the performance of the tasks required, but time to contemplate the data acquired.”

Furthermore, he shares that since no 2 patients are structurally the same, that there is no universal simplistic solution to each patient’s uniquely complex presentation. Dr. D’Amico goes on to say that these observations led him to begin his pilot study assessing 3 common casting techniques. He adds that he feels results could be applicable to scanning techniques as well, but that this topic deserves a separate study.

Q: What were the most impressive findings you observed in the orthotics made with a dorsiflexed hallux cast?

This casting technique resulted in the highest dorsiflexory position of the first MTPJ (46 degrees), and first metatarsal inclination of 30 degrees, says Dr. D’Amico. The custom foot orthotics required forefoot posts of 6 degrees of varus on the left and 4 degrees of varus on the right. The maximum medial longitudinal arch height was significantly (and expectedly) higher than with any other cast technique studied.

“While the CFO arch morphology matched proximally, the mid- and especially distal segments were markedly displaced from the first ray and proximally to the navicular segments,” he adds. “Upon weightbearing in the orthotics, the hallux was clinically unable to be dorsiflexed even when significant upward pressure was applied.”

Total varus present in the subject’s foot without orthoses was 43 degrees on the left and 45 degrees on the right, and with the dorsiflexed hallux orthoses, the measurement was approximately 50% improved with a measurement of 24 and 20 degrees on the right and left, respectively.

“The devices were initially and subsequently very uncomfortable to wear, especially in the medial longitudinal arch region,” explains Dr. D’Amico. “There was associated low back, medial knee, and patellar strain and discomfort, along with a loss of fluidity with transverse plane motion. The right navicular, first MTPJ, and medial ankle were also uncomfortable with prolonged use of the devices.”

Q: What were the most impressive findings you observed in the orthotics made from a valgus twist cast?

The valgus twist orthotic casts exhibited the second highest dorsiflexory position of the first MTPJ, with a value of 37 degrees and a first metatarsal inclination of 28 degrees. The custom foot orthotics required a 1-5 bar on the left foot of 3 degrees varus, says Dr. D’Amico, whereas the right forefoot called for a 2-5 bar of 4 degrees of valgus.

“This is notable since the subject has a confirmed forefoot varus on the right of 22 degrees,” he adds.

The maximum medial longitudinal arch height was lower than the dorsiflexed hallux cast with a value of 31.5mm on the right and 31 mm on the left. The medial longitudinal arch morphology deviated plantarly, beginning more proximally (at the talonavicular  articulation) than that observed in the dorsiflexed hallux device. The total varus measurement demonstrated the least improvement, measuring in degrees 38 on the right and 30 on the left. Upon weight-bearing in the orthotics, the hallux was clinically unable to be dorsiflexed even with significant upward pressure.

“The devices produced immediate and subsequent medial longitudinal arch, medial collateral ligament as well as lumbosacral strain, pressure and discomfort,” he shares. “The right forefoot was especially uncomfortable, which was most likely due to the conversion of a structural, 22 degree forefoot varus deformity into a “valgus” forefoot position requiring 4 degrees of valgus posting. This eventually resulted in increasing right lateral foot pressure and discomfort.”

Dr. D’Amico adds that left foot pronated throughout midstance with increasing medial longitudinal arch pain, and gluteal and piriformis stress and irritation.

Q: What were the most impressive findings you observed in the orthotics made from a Root suspension technique?

Conformity, comfort, and improvements in alignment and function of not only of the foot but the entire musculoskeletal system were overarching findings Dr. D’Amico notes from his observations related to the Root suspension cast technique. These casts exhibited the lowest hallux dorsiflexory position at 25 degrees, and a first metatarsal inclination angle of 20 degrees.

“The cast itself was the only one to produce a markedly everted calcaneus as a result of capturing the forefoot varus deformity present,” he notes. “The forefoot correction consisted of 1-5 bars with 7 degrees of varus on the right and 6 degrees of varus on the left. Maximum medial longitudinal arch height was similar to that of the valgus twist cast technique.”

Dr. D’Amico continues on to say that the orthotics fabricated with this technique conformed to the foot and exhibited 0 degrees of total varus in the devices, meaning that the foot in the device was optimally positioned. Hallux dorsiflexion was available in the devices upon weight-bearing, at least 45 degrees. This was the only technique that allowed facilitation of this critical motion, he says.

“Upon wearing these devices, it was immediately and subsequently noted an improvement in posture, stability, comfort, absence of medial longitudinal arch strain and pressure, and a reduced lumbosacral angle.”

The study also performed pressure testing barefoot, in shoes, and in shoes with each type of device. All 3 cast techniques produced an advantageous, more compact, and increased surface contact area, he shares. The center of force pathway was likewise improved, however the rearfoot force versus time curves did not improve in the dorsiflexed hallux and valgus twist casts. The forefoot somewhat improved in the dorsiflexed hallux devices but not in the valgus twist devices.

“The only waveforms that resembled a normally functioning foot was that obtained with the Root suspension cast technique (devices),” says Dr. D’Amico. “The midstance phase of gait was also lowest with the Root suspension cast technique CFOs. Cadence and propulsion were also greatest with the Root CFOs.”

To further explain, Dr. D’Amico says that the “lowest midstance phase of gait” is a temporal parameter. In essence, the lower the better. Lowering the midstance value (% of the stance phase of gait - ideally 18-22 in younger individuals and up to 30% in adults) almost always increases the propulsive phase of gait. He adds that the higher the midstance value, the more dysfunctional the foot. Waveforms are force versus time and represent pressure application in a time sequence.

He adds that the dorsiflexed hallux cast technique produced an orthotic that increased pressure on the hallux and first metatarsal head, but only on one side. Also the propulsive sequence was disrupted to a pattern of 5,2,3,H,1, instead of 5,2,3,1,H which was observed in shoes without an orthotic.

Q: What features or consequences of the Root technique do you feel contributed most significantly to the performance, alignment, and precision of its associated CMOs?

Dr. D’Amico reminds the audience that it comes down to remembering that the proper positioning of the foot for foot orthosis fabrication is an art and a science.

“The success of the Root technique when properly performed is that it captures the “normal” contours and relationships of that individual’s particular foot type without practitioner-induced distortion,” he says.

Q: What do you want podiatrists to remember about the findings of your study that they can apply in their practices right now?

Dorsiflexing the hallux during impression casting has a negative impact on CFO form and function, says Dr. D’Amico. Introducing a valgus force into a varus foot degrades CFO function, which he says he does not recommend. He shares that in his experience, accurate recognition and neutralization of forefoot varus is a requirement for optimum CFO function. Finally, and perhaps most important, he says, is that the casting clinician should take care to avoid unduly pronating the forefoot during impression casting in all foot types.