Scanning Technology And Orthotic Casting: What You Should Know
Are 3D scans or plaster casting more beneficial for custom orthoses? Offering a closer look at the technology, key findings from the literature and sharing insights from his experience, this author says 3D scans can play a viable role in a podiatrist’s armamentarium for custom orthoses prescriptions.
Since my days as a student at the New York College of Podiatric Medicine, I learned from my mentors, Richard Schuster, DPM, Justin Wernick, DPM, Joseph D’Amico, DPM, Rob Roy McGregor, DPM, et al, that plaster casting for prescription orthotics was both a science as well as an art. I spent over 35 years trying to perfect that art, molding with TLC the plaster strips around a patient’s foot and pressing up that fifth metatarsal to stabilize the forefoot and simulate semi-weightbearing. I would locate the lateral margin of the talus, find the congruency of the ankle mortise and lock the midtarsal joint, always mindful not to pronate the medial column or supinate the lateral column.
I felt plaster casting was the only way to go and I would never have considered three-dimensional scanning. Then it all changed.
At a podiatry seminar, a colleague noted at a reception: “Oh, we use the same lab. Do you cast or 3D scan?” To which I replied, “Cast, of course. Is there any other way?” His reply, “That’s the way I used to do it until I discovered 3D scanning and I have never gone back. You should try it.”
My response: “No way, I’m a ‘caster’ and will always be.” His response was: “Try it. You might be pleasantly surprised. It changed the way I prescribe orthotics.”
I procrastinated for about a year. Then I saw this colleague again. He asked if I tried the 3D scan. I told him I had not used it but I was thinking about it. I finally took the initiative and contacted my orthotics lab. Then a representative came by and gave me a complete in-service on the use of the 3D scanner. The good news was the lab was offering the 3D scanner without charge. That changed the equation. Before I gave it a try, I did my due diligence and researched the pros and cons of 3D digital scanning versus neutral suspension casting.
I continued to hold out the option for my runners and other athletes to impression cast when necessary for conditions such as high degrees of forefoot varus, hallux limitus with metatarsus primus elevatus and hallux elevatus, semi-rigid plantarflexed first rays with sesamoiditis, high degrees of medial column instability and pronation, and cavus feet with significant rearfoot varus. I also wanted to reserve the opportunity to use impression casting for my patients with diabetes who had pre-ulcerative and existing ulcerative conditions as well as those with Charcot arthropathy.
Comparing 3D Scanners With Plaster Casting In Terms Of Cost And Time
One of the first eye openers was the cost differential of 3D scanning versus traditional plaster strip casting. According to a study by Payne, the costs for the plantar cast method vary from $27.94 to $49.60.1 The costs for the optical scan vary from $3.30 to $10. These costs do not include the initial costs of purchasing an optical scanner or the costs of maintaining an Internet connection. Some 3D scanners may typically cost between $1,200 and $2,500, and some go as high as $15,000 but, in many cases, orthotic labs may provide the scanner at no cost.
The time estimates are even more impressive. Payne’s study showed that the total time involved with a plaster of Paris casting takes approximately 11 minutes while the optical scan takes two minutes.1 Time savings also involve the immediate transmission of the digital data to the orthotic lab in comparison to the time involved with packing and shipping the plaster casts, and the worry of the casts being crushed or lost in the mail. Inquiring with the lab as to the whereabouts of the casts can be worse when one faces the delays in getting the orthotics into production. In addition, the custom-fabricated orthotic is only as good as the molding process used to create it.
If one utilizes digital technology in the fabrication of a custom foot orthotic, the laser or scanning system must create a three-dimensional foot image from points directly from the foot itself or from a direct model of the foot. The scanning technology must not use computer algorithms, extrapolations or interpretations to calculate shapes and contours from two-dimensional pressure readings.2
When utilizing the 3D scanner for capturing a 3D model of the foot, there are three acceptable types of 3D digital scanners. The laser triangulation (red light) uses a laser light to measure the distance between the laser source and the foot to create an accurate model of the foot. Structured light (white or infrared light) uses the same trigonometric triangulation that laser scanning does but instead of using laser light, the scanner projects a light pattern onto an object and calculates the distance to the light source. Contact digitization uses a three-dimensional pin matrix to capture the contours of the plantar aspect of the foot.
A 3D scanning method developed through advanced optoelectronic technologies can also collect anthropometric data.3–5
A study by Lee and Wang compared the precision and accuracy of four foot dimension measurement methods.5 Looking at the precision and accuracy evaluation results, they found that 3D scanning had better performance than the digital caliper, digital footprint and ink footprint methods when it came to collecting the foot dimensions. Based on the findings, Lee and Wang supported the use of 3D scanning method for collecting foot anthropometric data.
An advantage of using the 3D scan is that it allows the quick scanning of a large number of participants and the measurement is robust and efficient.6 A previous study indicated that using the digital footprint to collect foot dimensions is reliable.7 Mall and coworkers compared optical techniques and caliper measurements in collecting foot dimensions.8 They found similar reliability with the two techniques and reported that using the optical techniques was as reliable as the caliper measurements, but the measurement time was reduced.8
In another study, DeMits and colleagues evaluated the validity of 3D scanning measurements using comparisons with X-rays and manual instruments.9 The results showed that 3D scanning provided good validity when scanning participants. In cases of abnormal feet, the 3D scan can also screen for foot deformities before the presence of foot erosions.10 One study also showed good validity and reliability for 3D scanning in comparison with clinical measurements.11
How 3D Scanning Can Accommodate Foot Deformities
The 3D scanner has a role in the prescription of foot orthotics and customized shoes intended to accommodate deformities related to the foot. One can use scanning technology for research and clinical assessments of various medical conditions.5 Patients who require custom shoes and insoles—such as those with diabetic peripheral neuropathy, rheumatoid arthritis and psoriatic arthritis—can also benefit from the 3D scanner in addition to the traditional impression casting technique.
Borchers and colleagues investigated a laser scanner’s potential for generating the data needed for the design of custom shoes intended to reduce the risk of ulceration in insensate feet.12 Their study revealed that in comparison to a standard shoe last, the hallux and the fifth metatarsal head both protruded outside the orthoses’ last shape, both areas common for irritation and development of diabetic ulcerations.
Kouchi and Mochimaru described the development of the Infoot 3D foot digitizer (I-Ware Laboratory).13 They investigated the validity and reliability of the 3D scanner in comparison to manual measurements for patients with rheumatoid arthritis. The authors determined that the device was a fast and reliable method of obtaining 3D anthropometric data of the foot.
Another use of the 3D scanner is for measuring the surface area of the foot. Traditionally, it has been an estimated percentage of total body surface area.14 The 3D scanning system provides a means to increase the accuracy of the measurement by taking into account many parts of the foot surface that may be omitted utilizing previous physical measuring techniques such as plaster wrapping.15
One may use the 3D scanner as a means for creating a baseline reproduction of the foot, particularly for the athlete or for patients with diabetic neuropathy. Repeated scans over a period of years can help determine changes in the foot and how one can modify prescription orthotics for such patients, taking into consideration those physiologic changes. These scans will be saved indefinitely in cases in which the patient has changed foot physicians or moved.
The 3D digital foot scan creates a full-color 3D digital cast or impression, which incorporates a weightbearing process in which the patient rests one foot or both feet on the sensor plate with the knee bent at 90 degrees. The assistant clicks a button to begin the process and the scanner takes a computerized image of the foot. It is important to note that some pressure mapping systems only provide a two-dimensional representation of the foot. The computer then extrapolates or “guesses” the remaining details of the foot to create the 3D cast.
As Williams has stated, the foot specialist can utilize either a non-weightbearing or partial weightbearing position when scanning the foot.16 Positioning of the foot on the digitizer can have some learning curve challenges but after a period of time, the practitioner will develop a level of comfort. The challenge is achieving a “neutral” position of the foot without pronating or supinating the rearfoot while maintaining a stable midtarsal joint and medial column.
Pertinent Insights On Plaster Casting
The traditional approach to custom functional foot orthoses is to take an initial plaster cast of the foot.17,18 Researchers have shown plaster casting is widely used in the fabrication of foot orthoses but has some reliability issues.19 Despite the issues of reliability, reviews of outcome surveys have shown plaster casts are clinically successful.20
Before fabricating a custom-made foot orthotic, there is an involved process that includes capturing a three-dimensional foot shape, an orthotic design process and finally a manufacturing process. The cast is the capture of foot structure and foot position. The fabrication of a foot orthotic first requires both an accurate evaluation of the foot structure and a precise neutral impression of the foot morphology.
This plaster neutral foot impression is to replicate the patient’s forefoot to hindfoot alignment that would occur at the midstance phase of the gait cycle.21 At the midstance instance, the subtalar joint should be in a neutral position, which is neither pronated nor supinated.17 Concomitantly at midstance, the midtarsal joint becomes fully locked, placing the plane of the metatarsal heads in a position that is perpendicular to the bisector of the calcaneus.22 The normal forefoot to hindfoot alignment at midstance phase of gait should be 90 degrees. To duplicate the midstance phase of the gait cycle, take the neutral plaster impression with the subtalar joint in neutral and the midtarsal joint fully locked. Research has shown that the principal cause of incorrect positioning of the foot when taking the neutral plaster impression is the failure to lock the midtarsal joint fully.23
There are also forefoot deformities that can alter the normal forefoot to hindfoot alignment and contribute to the abnormal movement patterns of the foot.21,24 Two of those forefoot deformities include a forefoot varus or forefoot valgus.
There are several casting methods one can employ to create an impression of the foot. These methods include plaster casting, slipper casting and impression foam in addition to the 3D digital scanner. All of these methods can be successful if one performs them properly.
The three most common methods clinicians use to obtain a neutral plaster foot impression are the supine non-weightbearing method, the prone non-weightbearing method and the sitting semi-weightbearing method. In a study by McPoil and coworkers, one can obtain the same forefoot to hindfoot alignment by using either the supine non-weightbearing or the prone non-weightbearing methods, but not with the semi-weightbearing method.25 Clinicians can also expect a difference in the degree of forefoot deformity between the left and right feet.
Valmassy described the advantages and disadvantages of all three impression techniques.26 He suggested using the variation in forefoot to hindfoot alignment in the semi-weightbearing method for obtaining a neutral foot impression in comparison with the prone non-weightbearing and supine non-weightbearing methods. Authors have shown that the difference is caused by the inability to fully locate the midtarsal joint when the foot is in a semi-weightbearing position.25,26 This can also be a problem when one is utilizing the 3D scanner.
A number of additional studies have reviewed the various methods of plaster casting utilizing some of the more common positioning techniques such as neutral, partial or full weightbearing casting, and subtalar joint neutral positioning. In addition, studies have looked at the reliability of capturing foot parameters with digital scanning versus the neutral casting technique.25–28
Positioning the foot for plaster casting is important for reproducing the sagittal contour. You should be able to assess the cast you molded and compare it to foot shape and any idiosyncrasies of the rearfoot or forefoot. During the casting process, it is important to remember not to allow the forefoot to plantarflex as this will cause the cast to have a higher lateral arch than the foot. There are some advantages to casting as it can allow the practitioner to slightly plantarflex the first metatarsal and dorsiflex the hallux for a functional or structural hallux limitus. Balancing the forefoot to correct various degrees of forefoot varus is another advantage to the casting technique. Remember, a gastroc-soleus equinus can also have an effect upon neutral plaster casting, particularly with the knee extended, in comparison with casting of the foot in partial weightbearing.
Comparing different plaster impression casting techniques to the foam box casting devices, Ki and colleagues compared the plantar pressure distribution patterns between foot orthoses by the computer assisted design-computer assisted manufacturing (CAD-CAM) system and the foam impression method.29 Their results showed that the CAD-CAM system produced a pressure distribution pattern, with the exception of the mid-forefoot region, similar to the one provided by the foam impression method. The authors noted that peak pressure and the pressure-time integral in the midfoot regions were lower with the CAD-CAM approach in comparison to the foam impression approach.
Podiatrists, sports medicine biomechanics specialists and diabetic wound care specialists now have the luxury of choosing between 3D digital scanners and traditional plaster casting methods. As technology has advanced, replicating the foot in a digital manner and transmitting the scan immediately to the orthotics laboratory is a distinct advantage. We can now eliminate the use of plaster impression casts, that easily can be crushed in boxes, lost or delayed in the mail, or thrown out inadvertently by the lab.
One can store the 3D scan data indefinitely so if patients move locations and need a new foot specialist to continue care, their biomechanical information will be stored at the lab, which offers continuity of care. Clinicians can order additional orthotics at any time in the future without having to recast the patient. The process can increase efficiency in allowing the practitioner to speed up the orthotic process, which also allows the patient to spend less time in the office and with cleaner feet. It allows the practitioner to spend more time for a gait analysis and/or a pressure mapping test.
Now this does not eliminate the need for traditional plaster casting for the fabrication of prescription orthotics. It also does not preclude the need for a biomechanical evaluation and measuring inversion/eversion of the subtalar joint, obtaining neutral measurement of the rearfoot/forefoot, assessing for equinus, first ray mobility and hallux dorsiflexion, and assessing foot type off-weightbearing and fully weightbearing.
There are many occasions when I have an athlete with certain foot types and rearfoot/forefoot imbalances. In these cases, the athlete can benefit from a foot impression that can offer exact contour and better locking of the midtarsal joint while preventing the rearfoot from supinating or pronating. I also consider plaster casting in some cases for the insensate Charcot arthropathy patient. I always have my plaster splints available when I assess the foot and decide whether to use a 3D scan or impression cast.
Dr. Ross is an Associate Professor in the Division of Vascular Surgery and Endovascular Therapy in the Michael E. DeBakey Department of Surgery at the Baylor College of Medicine in Houston. He is a Fellow of the American College of Foot and Ankle Surgeons, and a Diplomate of the American Board of Podiatric Surgery. Dr. Ross is a Past President of the American Academy of Podiatric Sports Medicine and a Fellow of the American College of Sports Medicine.
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