CASTING TECHNIQUES: DIFFERENCES AND BENEFITS
A question that I am often asked is which is the best casting method and which one do I use, so I thought I would take the opportunity to do a bit of a blog about it.
Plaster of Paris Suspension Cast
My preferred method is still a Plaster of Paris (POP) suspension cast… for no other reason than the fact that that’s what I’ve always done and I get good results (highly scientific as you can see). But why do we do it and what are the drawbacks?
Well, the purpose of doing a neutral suspension cast is to create a mould of the foot from which a functional foot orthosis can be fabricated. The reason for having the foot non-weightbearing is to try and capture the foot in an “uncompensated position”. Theoretically, the orthoses which are created to fit this uncompensated position would then counteract the resultant “compensatory” changes which occur as the foot goes into weightbearing. So the aim with this method is to prevent excessive motion of the foot during stance and gait.
The other thing that I (and many others) do with this method of casting is “put the foot into neutral” by palpating the talo-navicular congruency… but what does that mean? Well if you look at the evidence… not a lot! Although “neutral position” has been widely attributed to Merton Root, his books and papers do not say that this is the gold standard position or that talo-navicular congruency can be reliably found. However, hundreds of clinicians have used this method for decades and boasted very good results with their orthoses, so I think the lesson here is that palpating the talo-navicular joint is a handy guide but not a clinical test.
Aside from the rationale behind taking a POP, a lot of clinicians feel that it captures everything that you need from the plantar aspect of the foot right up to the malleoli. It’s also great for marking lesions on and adding any additional plaster moulds. It’s not without fault though …. the very fact that it is plaster means that it is quite fragile and it does start to crack and dry out over time. Also, unless you’re particularly well-rehearsed at taking them, it can be a time consuming messy job! I recall working with one Podiatrist some years ago and if you used the plaster room after him, it looked like a plaster bomb had gone off in there! I’ve also seen finished casts that simply look as if the clinician has stood at the back of the room and used a catapult to fire a bundle of wet bandage at the patient’s suspended foot!! ….. and don’t get me started on trying to cast hyperactive young children! So as you can see….. this method certainly has its drawbacks and some patients will think it’s a bit archaic!
Foam Impression Box
Next up is the foam impression box. Now, these are quick and simple and some will argue that by doing a semi weightbearing cast in a foam impression box, you compress the soft tissue and capture this in the cast rather than having to make assumptions with plaster additions.
However, I’ve also heard counter-arguments that the more flexible, or less stiff certain parts of the foot are, the harder it is to capture in a foam box. For example, a flexible plantarflexed 1st ray may dorsiflex as you push the foot into the foam, whereas a suspension POP cast would capture this.
Also, as quick and easy as a foam impression box is, you still need to take a good cast otherwise your orthoses won’t turn out quite as expected. For example, if the patient pushes their heel down first and then the forefoot, you get an abnormally high arch, or if the foot is allowed to excessively pronate, you may end up with a slightly longer device than you wanted.
Similarly, your patient needs to be sitting with their knee at 90 degrees so that a vertical downward force can be applied otherwise the foot impression will be altered because of the angle of the foot going into the box.
Then there are 3D scanners. If scanning is the way you want to go, do make sure that it is a 3D scan that’s being taken… a pressure plate won’t give you a true 3D image because pressure plates only capture 2-dimensional images. 3D scanners have really come along in the last few years and they are a fantastic addition to a busy practice because they are quick to do and you don’t have to mess around posting off or storing casts or foam boxes.
They are also a lot easier if you want to order repeat prescriptions of custom sandals for your patient, but there are similar issues to the foam impression casts in that you still need to make sure the foot is in the position that you need for an accurate device to be made. Again, if the patient is sat at an angle or the foot is allowed to excessively invert or evert, your orthoses may not come back as you imagined. I think because this method is so quick and simple (and looks good to the patient) there is the risk that not enough care and attention is paid to capturing the best impression… it is not simply a case of getting the foot in view and then “click”….. you still need to spend as much care and attention when getting the foot in the required position as you would doing a POP cast.
There are also slipper socks, and a new gadget which you can buy off Amazon that will take 3D impressions but I think I’ve covered the most popular 3.
At the end of the day, there is no right or wrong way and as far as I know, one method hasn’t been proven to be any better than another. As long as you understand the principle behind what you’re trying to do and how the orthosis is created from that cast then it doesn’t really matter how you capture a 3D impression.
That said… please don’t send any weird and wacky impression methods into the lab…we won’t accept, concrete, lard, playdough or any other easily accessible household or DIY methods of capturing a foot impression ;-)