Yes. Dispelling myths about the differences between casting and scanning, this author also notes that scanning is a cost-effective and time-saving alternative that should inspire providers to rethink their old algorithms.
By Bruce Williams, DPM
Despite digital scanners for foot imaging being around since the 1990s and despite the fact that most medium- and large-sized orthotic labs currently use digital technology for the scanning of plaster and foam box impressions, 74 percent of foot and ankle practitioners continue to cast feet using a non-digital format. According to a 2019 survey, 41 percent of respondents use plaster casting for their foot impressions, versus 26 percent who use digital or plantar pressure scanning (21 percent digital scans, five percent plantar pressure scans).1
With the ready availability of inexpensive and portable scanning tools, such as the Structure Sensor (Occipital), a scanner you can attach to an iPad, you would think that most podiatrists would move on to quicker and less messy options versus traditional plaster molding.
In 2007, Craig Payne, PhD did a cost benefit analysis study comparing plaster impression casting with digital impression scanning.2 According to the study, plaster casting costs from 28 to 50 dollars per casting event versus three to 11 dollars per digital scanning event. It takes on average 11 minutes to make a plaster cast versus only two minutes on average for a digital scan.2 This is a significant time savings for busy practitioners and allows more opportunity for income whereby to make up the initial investment in the acquisition of a digital scanner.
What The Literature Reveals
According to the literature, there is little, if any difference between casting a foot with plaster and taking a digital scan of the foot, especially if you capture both in a neutral non-weightbearing subtalar joint position. In a 2011 study comparing neutral position suspension casting using plaster versus a digital scan, Carroll and colleagues found “ … that digital scanning is a reliable technique with reduced measurement variability irrespective of clinical experience when compared to neutral suspension casting.”3
Two other studies looked at digital scanning for overall foot measurements, like those used in shoe sizing and the gathering of foot anthropomorphic data.4,5 The authors of these studies noted that benefits of the digital scanner include precision, repeatability and accuracy.
Many practitioners complain about what they perceive to be a difference between casting a foot in a neutral non-weightbearing position as opposed to a partial- or full-weightbearing position as may occur with scanning. In a study looking at data from several published studies on foot scanning, Lee and colleagues noted that “increases in length and breadth between unloaded and half loaded are greater than those found between half and full weightbearing.”5 The takeaway here is that there is, of course, a difference in the sizing of the foot when comparing off-weight bearing measures to partial- or fully-loaded weightbearing measures. However, in general, this is not an issue so long as the lab understands what type of scan the practitioner is sending.
Many orthotic labs may prefer a partial weightbearing cast or scan as that gives them a truer representation of the foot’s dynamic or weightbearing arch height. Most of the studies comparing techniques examine arch height and forefoot to rearfoot relationships and sometimes the heel width of a patient’s foot in a non-weightbearing neutral position.3-13 A general conclusion from these studies is that non-weightbearing casting, whether one used digital scans or plaster, was more consistent with the practitioners’ overall evaluation of the non-weightbearing foot. This indicates that the impression technique, digital or not, really matters more than when it comes to reproducing the foot’s actual architecture than when casting in a non-weightbearing subtalar joint neutral position. This can be the difference between a comfortable working device for a patient and a device the practitioner returns to the lab for modification.
Other Considerations With Digital Scanners And Positional Scanning
Most digital scanners are set up with stands to enable non-weightbearing capture of the foot held in a neutral subtalar joint position. However, almost all of these digital scanners can accommodate partial weightbearing scans as well if the practitioner prefers. The differences between these types of casts should be obvious to most practitioners. With a non-weightbearing cast, regardless of whether plaster or digital, the medial arch will almost always be higher and shorter in length than a cast one takes with the patient partially or fully weightbearing.
There is also a difference in positioning of the lateral and possibly the medial columns between non-weightbearing casts versus partial- or full-weightbearing casts. But does this really make a significant difference? In one study, Lee and coworkers attempted to understand if the forefoot and rearfoot were in alignment at the time of the casting event, would that affect the outcome of the casts when capturing the subtalar joint slightly beyond a position of subtalar neutral?13 They specifically compared neutral non-weightbearing subtalar joint casts versus casts taken with the subtalar joint in 2 and 4 degrees of varus and 2 degrees of valgus. The study authors determined that “the changes in arch heights, navicular height, and protrusion were insignificant and very small.”13
Consider that when you perform a partial or full weightbearing scan, you capture the forefoot and rearfoot in the same plane because of the flat glass surface of the digital scanner. The only other aspect that is essentially different with this type of positional scanning is to place the subtalar joint in a so-called “neutral position.” One can accomplish this easily enough by using the knee as a “joystick,” moving it medially or laterally until the practitioner feels that the talus is then in a “neutral position” via the usual palpation technique.
Finally, Chuter and coworkers looked at whether the forefoot position in a neutral non-weightbearing cast made a difference.14 In this study, a single practitioner either adducted or abducted the forefoot during the neutral cast capture. According to the authors, “If the forefoot was supinated about the oblique axis of the midtarsal joint, it would be assumed that the forefoot would be inverted relative to the rearfoot compared with a cast of the foot that was pronated about the oblique axis of the midtarsal joint.”14 The study authors maintained there was no difference “in the frontal plane forefoot-to-rearfoot relationship between the two midtarsal joint positions.”14
While none of these studies are conclusive, there is definitely a lot we are assuming about which type of technique is best overall for capturing a non-weightbearing foot impression, regardless of what cast type, plaster or digital, that we choose to utilize. Ultimately, in my opinion, it is much more important to know the prescriptive elements you want to program into the prescription orthotic device than it is to split hairs about how the foot is cast.
Ultimately no amount of peer-reviewed data is going to change a practitioner’s preference for how he or she has always captured impressions for foot orthotics. Most practitioners will use techniques they learned in school or residency and whichever techniques seem to give them a “perceived” best result for their patients. Of course, the interesting thing is that often the perceived best result for patients is first based on the comfort of the device and then whether the orthotic device alleviated most or all the patient’s painful symptoms. Overall, though, scanning technology provides us a reliable, cost-effective option for the creation of custom orthotics.
Dr. Williams is a Past President and Fellow of the American Academy of Podiatric Sports Medicine. He is the Director of Breakthrough Sports Performance, LLC in Chicago. Dr. Williams has disclosed that is the Medical Director for Go 4-D and a consultant for HP FitStation.
No. Noting the high cost of scanners, variable quality and other challenges with scanning devices, this author emphasizes that casting provides a more accurate, three-dimensional representation of the contour of the foot.
By Jeff Root
Casting or scanning the foot enables practitioners to produce custom foot orthoses that have the potential to reduce or eliminate symptoms, alter function of the foot and lower extremity, improve athletic performance or simply increase comfort. The ultimate measure of the effectiveness of foot orthoses is patient outcomes. I am not aware of any evidence which suggests that patient outcomes of orthoses produced from scanning are superior to devices produced by casting the foot. As a result, I do not believe one can say that scanning is more effective than casting for creating custom foot orthoses.
Aside from patient outcomes, there are other factors to consider when deciding whether it is better to cast or scan the foot. First, let us examine the objective of casting or scanning the foot. Merton Root, DPM developed a non-weightbearing casting technique using plaster of Paris splints, casting the foot with the subtalar joint (STJ) in the neutral position and with the midtarsal joint (MTJ) in a fully pronated position.1 This casting technique creates a three-dimensional model that results in a unique contour of the foot. Any alteration in the positions of the joints of the foot will result in a different unique contour of the foot and in the resulting foot model.
Root also developed a technique using a plaster of Paris slurry to modify the positive model of the foot in order to produce a functional foot orthosis.2,3,4,5 The contour of the positive model and modification of the shape of the positive model with additive (e.g. adding plaster) or subtractive (e.g. carving or shaving) techniques has a direct influence on the shape and efficacy of the finished foot orthosis.2,3,5-7 While there are a number of different casting/scanning techniques and methods for modifying the positive model, Root’s techniques still serve as a foundation for the construction of most modern-day functional-type foot orthoses.
Many in the prescription foot orthotic manufacturing industry consider plaster of Paris slipper casts to be the gold standard by which they compare other forms of casting or scanning of the foot. Developers of foot scanners typically try to match digitally what some achieve physically using plaster of Paris casting splints. In other words, they are trying to capture the unique contour of the outer surface of the foot with specific joints held in specific positions.
Recognizing Varying Quality Among Digital Scanning Devices
There are a variety of different types and brands of commercial foot scanners available to practitioners today. The quality of the scan directly relates to the quality of the scanner. Some scanners produce better quality scans than others. Scanners that utilize inferior technology are not capable of producing scans that are as accurate as scanners that employ superior technology. Extremely accurate scanners can cost in excess of $20,000, making them economically impractical for most labs and clinicians.
The quality of the scan is important when comparing one foot scanner to the next or when comparing any scanner to a physical, negative cast of the foot. One important factor is whether the scanner is capable of accurately capturing anatomical features and landmarks. For example, if the scanner does not capture or does not capture accurately the posterior surface of the heel, the lab may not be able to bisect the heel with the same degree of accuracy as it can a physical cast of the foot. As a result, the lab may not be able to position the scan in the frontal plane with the same degree of accuracy as it can a physical cast of the foot. Differences in foot scanners and scan quality is another reason why we cannot assume that scanning is necessarily more effective than casting for custom foot orthoses.
Other Challenges And Considerations With Digital Scans
Many orthotic labs consider accurate positioning of the cast/scan in the frontal plane to be extremely important because it directly influences orthotic reaction forces acting on the foot. Changes in the position of the foot model in the frontal plane will result in different orthotic reaction forces.8-10 Accurate positioning of the heel enables us to more accurately predict orthotic reaction forces, thereby creating more predictable results. Also, it may be more difficult for a lab technician to identify the metatarsophalangeal joints on a scan as opposed to a cast, which can result in an orthosis being too long or too short. Even if the scan quality is excellent, one must appreciate the fact that the lab technician is working on a two-dimensional computer screen and not on an actual three-dimensional model. This can make it more difficult to accurately identify anatomical landmarks.
Other issues that can impact the quality of foot scans are ergonomics, environment and scanner calibration. Ergonomic factors include portability and ease of use. Some scanners are large and not easily transportable between treatment rooms, or between different office locations. Scanners that are large and bulky may make it more difficult to accurately position the foot if the scanner is between the practitioner and the foot. Some scanners require the operator to stand farther away from the foot, which can create two problems. First, if the treatment room is small, there may not be sufficient space in the room to scan the foot. Secondly, an assistant may be necessary to position the foot or to operate the scanner while the practitioner positions the foot during scanning. A plaster of Paris cast does not require an assistant or additional room space.
Environmental factors such as light pollution can also be a problem. Light pollution can adversely affect scan quality and can even make the scan unusable. As a result, scanners do not necessarily work or work as well in some treatment environments. Scanner calibration is another issue of concern. The clinician may not be able to determine if the scanner is accurately calibrated. If the scanner is not accurately calibrated, the dimensions of the resulting foot model will not be accurate, potentially resulting in poor fitting or poorly functioning foot orthoses.
What About Positioning Of The Foot?
One solution to avoid the need for an assistant when scanning is to use a foot positioning device. While a foot positioning device may be helpful, it may not always be possible to get the patient’s foot in the desired casting position at the subtalar joint and especially at the midtarsal joint (MTJ) One can use a foot positioning device to apply a dorsiflexory force to the plantar lateral surface of the forefoot.
When casting the foot using plaster of Paris, the practitioner uses his or her hands to manually position the ankle, midtarsal joint subtalar joint and the digits. Therefore, the clinician has the benefit of both visual and tactile positioning of the joints of the foot. Rather than just applying a dorsiflexory force to the plantar lateral aspect of the forefoot, the clinician may feel that it necessary to simultaneously abduct the forefoot relative to the rearfoot in order to fully pronate the midtarsal joint. A positioning device does not have the clinician’s sense of feel and may not necessarily be the best technique for positioning the foot during scanning.
Understanding The Intersection Of Casting And Scanning For Orthotic Labs
Today, many, if not most, modern orthotic labs use CAD/CAM systems in the manufacturing of custom foot orthoses. When a practitioner sends the lab a physical cast, the lab must scan the cast in order to convert it to a digital format. The orthotic lab technician has the benefit of being able to evaluate and measure the negative cast in order to compare it to the orthotic device. When a practitioner scans the foot, the scan is transmitted to the lab electronically and the lab technician cannot compare the scan to a physical model of the foot in order to verify accuracy. Therefore, the lab technician must assume that the scan is an accurate model of the foot.
Another factor to consider is the capability of the lab’s CAD/CAM system and technician. If the CAD/CAM system or the lab technician lacks capability or skill, the finished orthosis may not produce the desired clinical outcome. As a result, the CAD/CAM system and lab technician are just as important as the quality of the cast or scan. In addition, there are times when it may be better to produce an orthosis manually from a cast due to a highly unusual foot type.
Another possible disadvantage of a foot scanner is if you are only able to use it with one orthotic lab. When a foot scanner is tied to one lab, it limits the practitioner’s source of foot orthoses. A practitioner can send a physical cast to any orthotic lab. We must also recognize that the design of some foot scanners are for semi-weightbearing or full weightbearing scanning of the foot. Recognizing that a semi-weightbearing or full weightbearing scan results in a relatively flat representation of the plantar surface of the foot, these scanners may not produce the type of foot model the practitioner desires.11
Foot scanning has some noteworthy advantages. Some of these advantages are cost-related. Scanning is clean and may be less time consuming than casting, and it eliminates the cost of casting supplies. Scanning has advantages over impression foam because it is difficult to accurately position the joints of the foot when using impression foam, especially if one intends to capture the foot with the midtarsal joint in a fully pronated position.
I am not opposed to foot scanning or CAD/CAM technology. We use these techniques every day in my lab to produce excellent quality custom foot orthoses. All methods of creating models for the production of custom foot orthoses have distinct advantages and disadvantages. No single method is right or best for every practitioner. Ultimately, the practitioner must weigh the advantages and disadvantages of each option in order to determine if casting, scanning or some combination thereof is best for him or her. In either case, the practitioner should attempt to provide the lab with the highest possible quality model of the patient’s foot, regardless of the technique.
Mr. Root is the President and CEO of Root Orthotic Laboratory in California. He is a Past President of PFOLA (the Prescription Foot Orthotic Laboratory Association).
- Donoghue SK. PM’s 36th annual survey: money management 2.0. Podiatry Management. 2019. Available at: https://podiatrym.com/pdf/2019/2/Survey219web.pdf . Accessed April 15, 2020.
- Payne CB. Cost benefit comparison of plaster casts and optical scans of the foot for the manufacture of foot orthoses. Australas J Podiatr Med. 2007;41(2):29-31.
- Carroll M, Annabell M-E, Rome K. Reliability of capturing foot parameters using digital scanning and the neutral suspension casting technique. J Foot Ankle Res. 2011;4(9);1-7.
- Telfer S, Woodburn J. The use of 3D surface scanning for the measurement and assessment of the human foot. J Foot Ankle Res. 2010;3(19):1-9.
- Lee Y-C, Lin G, Wang M-JJ. Comparing 3D foot scanning with conventional measurement methods. J Foot Ankle Res. 2014;7(44):1-10.
- McPoil TG, Schuit D, Knecht HG. Comparison of three methods used to obtain a neutral plaster foot impression. Phys Ther. 1989;69(6):448-452.
- Laughton C, McClay DI, Williams DS. A comparison of four methods of obtaining a negative impression of the foot. J Am Podiatr Med Assoc. 2002;92(5):261-268.
- Dombroski CE, Balsdon ME, Froats A. The use of a low cost 3D scanning and printing tool in the manufacture of cutom-made foot orthoses: a preliminary study. BMC Research Notes. 2014;7:443.
- Chu WC, Lee SH, Chu W, et al. The use of arch index to characterize arch height: a digital image processing approach. IEEE Trans Biomed Eng. 1995;42(11):1088-1093.
- Shiang TY, Lee SH, Lee SJ, et al. Evaluating different footprint parameters as a predictor of arch height. IEEE Eng Med Biol Mag. 1998;17(6):62-66.
- Ki SW, Leung AK, Li AN. Comparison of plantar pressure distribution patterns between foot orthosis provided by the CAD-CAM and foam impression methods. Prosthet Orthot Int. 2008;32(3):356-362.
- Trotter LC, Pierrynowski MR. Ability of foot care professionals to cast feet using the nonweightbearing plaster and the gait-referenced foam casting techniques. J Am Podiatr Med Assoc. 2008;98(1):14–18.
- Lee WCC, Lee CKL, Leung AKL, Hutchins SW. Is it important to position foot in subtalar joint neutral position during non–weight-bearing molding for foot orthoses? JRRD. 2012;49(3):459–466.
- Chuter V, Payne C, Miller K. Variability of neutral-position casting of the foot. J Am Podiatr Med Assoc. 2003;93(1):1-5.
Additional Point References
- Williams B. Key insights on digital casting techniques. Podiatry Today. 2010;23(1).
- Williams B, Fuller E. Point-counterpoint: scanner casting: is it better than plaster impression casting? Podiatry Today. 2015;28(6):50-55.
- Root ML, Orien WP, Weed JH. Neutral Position Casting Techniques. Clinical Biomechanics Corporation;1971.
- Anthony RJ. The Manufacture and Use of the Functional Foot Orthosis. Switzerland:Karger Publications;1991.
- Philps JW. The Functional Orthosis. London: Churchill Livingstone;1990.
- Lee WE. Podiatric biomechanics: An historical appraisal and discussion of the Root model as a clinical system of approach in the present context of theoretical uncertainty. Clin Podiatr Med Surg. 2001;18(4):555-684.
- Root ML. Development of the functional orthosis. Clin Podiatr Med Surg. 1994;11(2):183-210.
- Colson MJ. (AKA Coulson MJ) An effective orthotic design for controlling the unstable subtalar joint. Orthot Prosthet. 1979;33(1):39-49
- Kirby KA. The medial heel skive technique: improving pronation control in foot orthoses. J Am Podiatr Med Assoc. 1992;82(4):177-188
- Blake RL. Inverted functional orthosis. J Am Podiatr Med Assoc. 1986;7(5):275-276.
- Blake RL. The Inverted Orthotic Technique: A Process of Foot Stabilization for Pronated Feet. Pennsauken, N.J.;Book Baby Publishing:2019.
- Baitch SP, Blake RL, Fineaga PL, Senatore J. Biomechanical Analysis of Running with 25 Degrees Inverted Orthotic Devices; J Am Podiatr Med Assoc. 1991;81(12):647-652.
- Richie D, Root J. A comparison of negative casting techniques used for the fabrication of custom foot orthoses. Podiatry Management. 2007;26(7):129.
From reading your current blog, it appears you are a fan of the "twisted plate" orthosis. To clarify, that concept was first described by MacConaill: MacConaill MA. The postural mechanism of the human foot. Proceedings of the Royal Irish Academy 1944-45; 50B, 265-278
If you position the foot on a flat plate scanner in a semi-weight bearing or full weight bearing position, the lamina pedis will "untwist" and will tension the plantar aponeurosis. In order to capture the foot in a "twisted" configuration, you must pronate the forefoot on the rearfoot. This is best accomplished by taking a scan or making an impression cast, off-weight bearing, holding the midtarsal joint maximally pronated.
Thank you for the reference. I was not aware someone else had written about that first.
Regarding casting on a flat plate: I understand your point, but my point is that if you know how much the lateral column can dorsiflex, then you can wedge the lateral column at approximately that amount and get the same outcome as casting in the traditional non-weight bearing manner. I’ll be writing more about that in my next blog post.