Praising the move into digital technology, this author notes scanner casting will save money and time, and the research shows little difference between outcomes of devices made with digital scans versus plaster.
Plaster casting has traditionally been the number one preference for most practitioners who regularly make custom foot orthotics for their patients. Since most practitioners will stick to what they learned in school or residency, they will often say “This is what works for me” when asked to cite a reason for not upgrading their casting methods. That said, many practitioners may have moved beyond plaster to foam box or digital scanner casting to save time and mess, but when people ask them what they think gives the overall best result, most will tell you that they feel plaster does it best.
Few labs make their devices from a vacuum mold of the positive cast anymore. The majority of labs scan the casts that physicians send to them, whether they are plaster or foam box impressions. Isn’t it time that we cut out the middleman, the expense of materials and shipping, and the mess and time involved with plaster, and just scan the foot directly with our choice of scanning device?
Why Time Is Money When It Comes To Scanning
A 2007 cost benefit analysis study compared plaster impression casting with digital impression scanning.1 According to this study, plaster casting costs anywhere from $28 to $50 per casting event versus $3 to $11 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. This is a significant time savings for busy practitioners and also allows more opportunity cost whereby to make up the initial investment in the acquisition of a digital scanner.
Digital scanning also saves on shipping costs to the lab for manufacturing and eliminates the risks of damage occurring to casts during shipping. The acquisition of a digital scanner can cost anywhere between $1,200 and $2,500 these days. Therefore, there is likely even more overall savings than the study showed from eight years ago as material costs and shipping have only gone up while most scanners likely cost much less.
Plaster Versus Digital Scanning: What Does The Research Say?
There are several studies that review different types of foot orthotic casting techniques, ranging from plaster to foam to digital scanning.2-5 One of these initial studies focused specifically on different types of plaster casting using some of the more common positioning techniques such as subtalar joint (STJ) neutral and partial or full weightbearing casting.5 The authors compared the orthotic devices made from these various plaster techniques with pressure mapping. They found that in general, functional types of cast-made orthotics function similarly as do accommodative types of devices made from accommodative casting techniques.
When comparing plaster impression casting, digital impression scanning and foam box impression casting, most studies have attempted to determine the reproducibility and repeatability of the specific technique used to make the impression or image of the foot.2-9 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 non-weightbearing neutral position.
A general conclusion from these studies is that non-weightbearing casting, whether one used digital scans or plaster, was more consistent in comparison to practitioners’ overall evaluation of the non-weightbearing foot. This indicates that the technique clinicians use when casting, digitally or not, really matters more than when it comes to reproducing the foot’s actual architecture when casting in non-weightbearing STJ neutral position.
A study using in-shoe pressure to examine the devices made from differing casting techniques indicated similar pressure/force changes in comparing digital scans versus foam box casts.8 This study noted that there were significantly higher medial arch pressures in the computer-aided design/computer-aided manufacturing (CAD/CAM) devices versus the foam box casted devices. However, the authors concluded that this may have been due to a difference in the techniques used for casting as they did not plantarflex the medial and lateral columns equally when partial weightbearing casting in foam in comparison to performing the partial weightbearing digital scan of the foot. This is a common technique error for any partial weightbearing casting techniques as I have learned the hard way over many years testing all types of casting techniques in my private practice.
Two more recent studies specifically comparing digital and plaster scanning found very few differences in the overall regional comparisons.4,5 One study utilizing an off-the-shelf scanning tool indicated minimal differences in arch height of the feet scanned, regardless of the use of plaster or the scanning tool.4 This study is limited in that it only utilized one participant. Another study comparing the two casting techniques concluded, “The findings of this study indicate that digital scanning is a reliable technique with reduced measurement variability irrespective of clinical experience when compared to neutral suspension casting.”5 The study authors also found that when using plaster casts, there was a noted error in the forefoot-to-rearfoot measurement when authors compared both clinicians’ measurements when they used the neutral suspension plaster technique.
Ultimately, according to the studies, there is little to no difference in the outcome of a device made from a plaster cast versus a device made from a digital scan when examining specific regional areas or comparing pressure distribution. As you can see from the research, digital scanning can allow for a true 3D representation of the plantar foot and, to some extent, the sides of the foot, equaling that of a plaster cast in most necessary ways.
Why Technique Matters
Most scanners today have the practitioner utilize either a non- or partial weightbearing position when scanning the foot. Though positioning yourself for use of the digitizer has some learning curve challenges, in general, most practitioners can adapt relatively easily to the change.
That said, there are subtle differences of technique that practitioners need to appreciate prior to working with a digital scanner. These issues of technique can have repercussions on the initial orthotic outcomes for new users.
One study touched on the fact that when researchers took a neutral position plaster cast using the STJ neutral technique, but with the knee in full extension, the cast came out with subtle differences from the comparative cast in partial weightbearing.5 This position obviously will fully load the gastrocnemius and soleus at the same time, and any effects of ankle joint equinus will be in full effect on the plantar foot. Equinus issues, or a tight gastroc-soleus complex, or just the soleus on its own can have a huge effect on the cast one takes in STJ neutral position. Root himself stated that equinus issues are the greatest threat to normal foot function.10
While I think most practitioners will appreciate this, I think it is important to mention that you can regularly cast patients in a wrong position regardless of the type of cast material or device you choose to use. For instance, if you cast partial weightbearing, as referenced in the aforementioned study, it is important to plantarflex the first and fifth metatarsals against the scanner surface, and then move the knee medially and laterally until you find the STJ neutral position.5 This is to keep the five metatarsals in the same plane and then one can work backward to position the STJ in neutral. This will not always “lock” the midtarsal joint in patients as some forefoot valgus wedging may be needed in patients with exceptionally dorsally mobile lateral columns.
While this is an excellent technique for symmetry, the practitioner still must know the resting calcaneal stance position of the patient being cast so he or she can make varus/valgus posting to correct for the heel position as desired. This is a problem in partial weightbearing casting in that you really can’t just post the heel to perpendicular and expect things to work the way you want them to for the patient.
Ultimately, practitioners will choose to change or get left behind. As our entire world, including medical practice, continues to move to a completely digital format, isn’t it time you consider keeping pace? Save some time, money and mess, and learn scanning technique. Give digital scanning for your orthotics a try.
Dr. Williams is the Director of Gait Analysis Studies at the Weil Foot & Ankle Institute and the Weil Foot-Ankle & Orthopedic Institute. He is a Past President and Fellow of the American Academy of Podiatric Sports Medicine. Dr. Williams is a sports medicine professional specializing in the treatment of foot, ankle and movement disorders. He is the Director of Breakthrough Sports Performance, LLC in Ipswich, Mass.
- 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.
- McPoil TG, Schuit D, Knecht HG. Comparison of three methods used to obtain a neutral plaster foot impression. Phys Ther. 1989; 69(6):448-52.
- Laughton C, McClay Davis I, Williams DS. A comparison of four methods of obtaining a negative impression of the foot. J Am Podiatr Med Assoc. 2002; 92(5):261-8.
- 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.
- Carrol M, Annabell ME, Rome K. Reliability of capturing foot parameters using digital scanning and the neutral suspension casting technique. J Foot Ankle Res. 2011; 4(1):9.
- 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):1088-93.
- 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):62-6.
- 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.
- Root ML, Weed JH, Orien WP. Normal and Abnormal Function of the Foot, Volume 2. Clinical Biomechanics, Los Angeles, 1977.
Citing potential cost and compatibility concerns with digital scanners, and emphasizing the ability to ensure the optimal shape of the cast, this author advocates for the continued relevance of plaster casting.
In this debate, the first thing to do is define what type of scanning we are talking about. I am talking about comparing a plaster cast to a digital three-dimensional representation of the foot. There are some scanners that only can measure a two-dimensional footprint. When people step on these scanners, they measure contact points and record a “footprint.”
There is a correlation between the footprint and arch height. However, using a correlation to guess at the height of the medial longitudinal arch of the foot is just guessing and one should not consider a device made from that scan to be custom. So we are talking about scanners that create a three-dimensional image of the cast.
We also need to define this discussion on the fabrication end. Some orthotic laboratories scan plaster negative casts and then fabricate a device using the digital model from that three-dimensional scan so the difference between plaster casting and digital “casting” is only what happens in the office. The main reason that I can say this is the “input” into fabrication process is the same with both plaster casting and digital casting. For both methods, once one uses a digital cast in fabrication, the product at the other end will have modification and milling in the same manner.
Can the software, within the limitations of the milling machine, produce the same orthotic that one makes with a plaster positive cast? That question is one of engineering and software. Whether this is as good as traditional plaster cast fabrication and pressing is a different question than the one before us.
Assessing Practical Considerations For Plaster Casting Versus Digital Scans
Obvious areas that need analysis are cost, ease of use and durability. For plaster casting, all you need is plaster, water, a container to hold the water and a room that has to be accommodated for easy cleanup. For a digital scanner, you will also need a dedicated room with electrical power and storage space for the scanner. For digital scanning, the setup of the room will have to be such that one can easily position the scanner relative to the patient and easily move it out of the way to where it is stored when the room is in use for other purposes. The scanner will most likely require other support features like a computer and wires for communication between the scanner and the computer, and the computer and the orthotic lab. There is some cost, in terms of the amount of space required, to enable the luxury of pushing a button to send your “cast” to the lab.
Depending on technology to complete your daily tasks can be risky. You wouldn’t commit to electronic medical records (EMR) without having made provisions for hardware backup. What if a part of your system breaks? Another risk of technology is obsolescence. You cannot run today’s software on a 10-year-old computer. When you figure in the cost of the scanner, you have to consider how long before you have to buy a new one. There is also the cost of software upgrades.
Another issue with technology is compatibility. When a scanner “casts” the foot, it saves the image of the foot as a digital file. Currently, there is no standard for these digital files. What this means is that all scanners cannot “talk to” all software used to create the final milled orthosis. The lab you use will have to be able to read the data from your scanner. So when you buy a scanner, you will be limited to the orthotic labs that are able to read the files you send them.
Comparing The Cast To The Foot: Where Plaster Casting Is Superior
Beyond the hardware, this discussion boils down to whether the digital cast in the office is the same as the plaster cast in the office. Theoretically, you should be able to get the foot into the exact same position with each method of casting. The hard part is knowing whether your cast represents the position that you wanted to get. This is where I feel plaster casting is superior. When you take the cast, you want to know if the sagittal plane contour of the cast is the same shape as the foot when the foot is in the position that you intended.
To assess if your cast is the shape that you want, you need to compare the cast to the foot. You have to place the foot in the proper casting position and compare the sagittal view of the cast to the sagittal view of the foot. If the forefoot is allowed to plantarflex during casting, the cast will have a higher lateral arch than the foot when the foot is in the proper casting position. To see this with a digital scanner, you would have to get an exact life-size visual representation on your monitor. This would have to be calibrated for individual monitors because the picture created by the computer will be different sizes on different monitors. Your monitor would also have to be as big as the biggest foot that you would cast.
It is important to talk briefly about the importance of getting the correct sagittal plane shape of the cast. An orthotic made from a cast in which the forefoot was plantarflexed on the rearfoot will have a higher lateral arch than intended. This orthotic will push upward in the midfoot region more than intended and could cause arch irritation.
The midfoot region of the cast is also important for getting the proper forefoot to rearfoot posting in the finished orthotic. When one adds an intrinsic forefoot valgus post, it will change the shape of the lateral arch of the orthosis as well. The intrinsic forefoot valgus post will also raise the lateral arch height in the cast but with a different shape than if the forefoot to rearfoot was plantarflexed while casting.
This is why it is important to know that the information that you are sending to the lab, whether it is a plaster negative cast or a digital representation of the foot, is the exact shape that you want. This will help ensure that the lab sends you an orthotic that is the shape you want. It will be a long time before the use of plaster becomes obsolete.
Dr. Fuller taught biomechanics at the California College of Podiatric Medicine for 14 years. He practices in Berkeley, Calif.