I will start this post recognizing the wonderful discussion I had with Dr Mark Warford on biomechanics which sparked this post and a few more. Thank you Mark. I hope to encourage my colleagues to bring up topics we can discuss.
Whether you believe in one or two midtarsal joints, both concepts make sense to me in different ways, when we bear weight on the forefoot ground reactive force places upward pressure across the distal part of the joint and loading of the midtarsal joint(s) begin(s). One thing that an orthotic device should prevent is complete dorsal jamming of the foot joints due to orthotic positioning of these joints more mid-range with no ligament or muscular strain needed to maintain stability. This is not what "locking" meant to Dr Root. He meant that a joint could move from one side to the other, and be stable at the end range of either side. By stable, he meant that joints had bony end points, and as long has they stayed within a certain range, the protective muscles or ligaments of that joint would not be stressed. Dr Root argued that the midtarsal had better bony alignment when the subtalar joint was near neutral and when ground reactive force maximally pronated the midtarsal joint. He looked at joint articulations and explained that we lose that bony locking if our subtalar joint gets too pronated. Genius for the 1950s? I think so.
The word "locking" in our more modern day venacular is pathological. A joint is stiff, stuck, dysfunctional, and needs treatment. You can comment on whether we should change that word for another. The cuboid gets "locked" in a plantarflexed position after an Inversion sprain, or in a dorsiflexed position after miles of walking with pronated feet on cobblestone streets. These are painful pathological situations. Dr Root's locking meant bone on bone stability.
What I believe is that the closer to neutral subtalar joint, the better that locking or stability across that joint or joints. We all know if you pronate the subtalar joint you make the midtarsal joint floppy and unstable, and when you supinate the subtalar joint, you make the joint more rigid stable. Easy peasy!
We must remember Goldilocks and The Three Bears. The soft bed to lay on was too soft signifying a pronated unstable foot. The hard bed to lay on was too hard signifying the supinated rigid foot with poor shock absorbing qualities. The medium bed, the one in the middle of my mind in the story, was just right signifying neutral position.
Those of us trained with Root concepts, and practiced them with good intentions, have had great fortune with taking this middle ground with the bed just right. There is some disagreement in the amount of force needed when pronating the midtarsal joint, but I will err on less force for sure. This is the starting point to Root biomechanics. Whether you cast or scan, the subtalar joint should be in neutral and the midtarsal joint maximally pronated and locked with very little force applied to dorsiflex the 4th and 5th metatarsals. Do not load the foot to influence the ankle joint which severely dorsally jams the midtarsal.
Can you lock the midtarsal joint in any subtalar joint position? Of course. Locking the midtarsal joint is done by grabbing the 4th and 5th toes and lifting the foot up just off the table. Your direction of pull is a slight dorsiflexion of the lateral forefoot just to resistance. You do not dorsiflex the ankle to neutral (right angle of foot to leg). Dr Root demonstrated the slight upward pressure needed for this procedure with his rheumatoid arthritic hands to our group one day. It is light pressure to stretch out the long plantar ligament under the cuboid, place the midtarsal joints in the center of their typically gliding motion. And, although this can be done in any subtalar joint position you want, I prefer the subtalar neutral position.
We know that every measurement has an ideal and then subject variability. Most feel that 4 degrees of motion variance between us all is great. Some of us will say neutral is 2 degrees invereted to its true position and others will say neutral is 2 degrees everted to its true position. But, this is normal variance. We do not want to capture with our imaging/casts a midtarsal joint too much on the pronated subtalar joint side, or a midtarsal joint too much on the supinated subtalar joint side. One produces an orthotic device with stability issues, and one produces an orthotic device with rigidity issues.
If I ask podiatrists whether they love orthotic devices, it is typically 2 camps. Either, "Why would you ever use those?", or "I love my orthotic practice." This speaks volumes. Those practicing good orthotic techniques of casting and prescribing should love them. In private practice, I made around 500 pairs a year, and just loved this. Of course, I could devote this time because I surrounded myself with good surgeons: Dr Ronald Valmassy, Dr Bill Olson, and Dr Remy Ardizzone. But, I digress!
Now comes my discussion (in parts) with Dr Warford Part 1. If we are using neutral (within 4 degrees) for our imaging, it must translate into an orthotic device that holds the foot within 4 degrees also of neutral. I measure RCSP and NCSP for each foot, and they should be different in feet and between feet which speaks of the asymmetries we all have. Then I stand my patients on top of their orthotic devices and see what their new resting position is. I am going to call it OCSP or Orthotic Calcaneal Stance Position (which I probably stole from someone LOL). You then compare it with your NCSP, or standing on your orthotic device reset the patient in subtalar neutral. See how far different it is. Within 4 degrees of neutral, you are almost guaranteed a great orthotic device. What if not?
Let us assume that you made your Type A orthotic device with forefoot balancing of any deformity captured in your cast (Golden Standard for you). If you have forgot about Type A, please review the previous post. The patient is here for dispense. Your nurse has dispensed them and the patient loves them before you walk into the room. You are making for stability, not comfort (although always good to get good feedback), as the patient has chronic bilateral posterior tibial tendon pain. You want a neutral to slightly inverted device to stabilize the posterior tibial tendon on both sides. RCSP was 4 everted right and 7 everted left without orthotics. OCSP was 2 everted right and 5 everted left. NCSP however was 6 right inverted and 8 left inverted due to high degrees of tibial varum. We have made a comfortable orthotic device, the patient actually looked okay to good in gait walking, but our gold standard placed them 8 degrees from neutral on the right and 13 degrees from neutral on the left. Your cast/image captured the foot architecture near neutral but the orthotic device you put them in did not and they will pronate on the device with both an unstable subtalar joint and unstable midtarsal joint. I propose that this will continue to make the posterior tibial work too hard and it will fail again. Yes, they are more stable, and we have learned that one of the benefits of orthotic devices is that pronation moments (forces) are slowed down.
The big point for this post is that if you care about a cast or scan capturing neutral, make sure your orthotic device first of all makes the foot move in the direction of neutral #1, which it has, and then suggest to the patient if this is not enough to help their post tib issues that you can raise the correction more (see posts on the various Types of Orthotic Correction to choose one). If you miss this vital discussion with the patient, they will assume this is the best you can do, and move on to someone else if symptoms continue or return with activity (you have to assume this person is not as skilled as you).