A Guide To Orthotic Prescription Writing With The Tissue Stress Theory Approach

Can the tissue stress theory approach to biomechanics help podiatrists make more specific orthotic modifications to reduce pathological forces on the foot? With this question in mind, this author offers helpful diagnostic pearls and a closer look at key variables that can influence orthotic treatment decisions.

The tissue stress approach to biomechanics tries to explain foot pathology by examining the forces acting on the anatomic structures of the foot. Damage to the foot results from high forces on particular parts of the foot. This is different than what we learned in the past, namely that just the position of joints can explain pathology of the foot.

The first step in tissue stress biomechanics is to identify the injured anatomical structure. For example, if there is pain at the first metatarsophalangeal joint (MPJ), examine the forces at that joint and do not look at the position of the subtalar joint. After identifying the anatomical structure that has been injured, one can create a mechanical model of that structure and use it to identify the forces acting on that structure. The clinician can subsequently use the mechanical model to ascertain how to use an orthotic or some other treatment modality to reduce those forces acting on the injured structure.

Then one can design a treatment to reduce those pathological forces. After reducing the damaging forces, the clinician can allow the injured part to heal. Whether the forces acting on the structure have decreased enough is apparent by the patient’s response to the mechanical intervention. If the initial response is insufficient, one can modify the orthotic further to reduce the pathologic forces even more.  

When using the tissue stress approach, it is important to understand what prescription variables alter the forces acting on various anatomical structures of the foot. The focus of this article is on the specific measurements that one uses and how those measurements determine which variables to choose when making the orthotic. I will briefly discuss the results of the modeling in terms of how a particular pathology occurs and how the modeling predicts which orthotic variables will help reduce forces in a specific pathology.

An important caveat in the tissue stress approach is that many of the things that you do will shift forces from one anatomic structure to another. It is possible to, and often the specific intention to, increase force on one structure to reduce force in another. Most of the time, these increased forces will not cause pathology. Sometimes though, the forces on a structure may be too high and pain can develop. If a clinician understands how the forces change, he or she can predict what to look for in follow-up visits and then adjust the orthotic accordingly.

Accordingly, let us discuss how to group various pathologies and how various orthotic modifications can address those pathologies.

Key Insights Into Common Gait Pathologies

Broadly, we can group pathology into three categories. When patients overpronate, their feet have pathology-related high pronation moments. For those who over-supinate, their feet have very low pronation or even supination moments. A third broad category is pathology resulting from a high load on a specific anatomic location, causing point high stress that is not necessarily related to either pronation or supination moments.   

Overpronators will tend to have posterior tibial tendon dysfunction, sinus tarsi syndrome and pathology related to the windlass mechanism. The windlass mechanism is what holds up the medial arch. Anatomically, the windlass includes the talus, calcaneus, navicular, medial cuneiform, first metatarsal, the first proximal phalanx, the sesamoid bones at the head of the first metatarsal, the plantar fascia and the intrinsic muscles that insert into the base of the proximal phalanx.

Modelling predicts that high load in the windlass mechanism is responsible for hallux valgus, hallux limitus/rigidus, plantar fasciitis and plantar fascia enthesopathy.¹ Beyond those conditions, high loads on the windlass may create pain in any of the anatomical structures that make up the windlass.

As the subtalar joint pronates, the arch gets flatter and there is increased distance from the origin of the plantar fascia at the medial tubercle of the calcaneus to the insertion of the plantar fascia at the base of the first proximal phalanx. This will tend to increase tension in the plantar fascia. Increased tension in the plantar fascia will tend to increase the compressive forces on the bones of the arch.¹

Also, as the subtalar joint pronates, there is a tendency for higher medial forefoot loads and this will also tend to increase tension forces in the plantar ligaments that connect the bones of the windlass mechanism as well as the plantar fascia. Accordingly, decreasing pronation can decrease the load in the windlass mechanism.

Over-supinators will tend to get peroneal tendonitis, lateral ankle instability and high lateral forefoot loads. The high lateral forefoot loads can cause problems at the fifth metatarsal head, the fifth metatarsal, cuboid, calcaneus and any of the plantar ligaments that limit dorsiflexion motion of those joints.  

High point loads can be the result of anatomical variation. A long second metatarsal will tend to have higher pressure than the other metatarsals, especially during the push-off phase of gait.² This high load can create a capsulitis or plantar plate injury of the second MPJ. Another cause of high loads is the partially compensated varus foot. With this foot, there will be high loads on the lateral forefoot.

Historically, there was a distinction between a forefoot varus and a rearfoot varus. However, when either of those foot types run out of pronation range of motion with the forefoot still inverted to the ground, there will be high load on the lateral forefoot and low load in the medial forefoot in static stance.

Note that the over-supinating foot can also cause high lateral forefoot loads. One should treat the partially compensated varus foot entirely differently than a patient with an over-supinating foot. A plantarflexed first metatarsal can create a tripod effect in which there are high loads on the first and fifth metatarsal.  

We should not expect the same basic orthotic protocol to work for all feet. This is one problem with doing research on the effectiveness of orthotic therapy. The orthotic that is good for one foot won’t necessarily be good for another foot. Just casting a foot and sending the cast to a lab is not utilizing the full capabilities of a custom orthotic. The real issue is figuring out the important things that make feet different.   

Understanding The Impact Of The Subtalar Joint Axis Position In The Transverse Plane

One of the most important variables that makes one foot different from another is the location of the subtalar joint axis in the transverse plane. The position of the axis varies from 4 to 47 degrees of deviation from the longitudinal axis of the foot.³ We need to treat individual feet and not the average foot.

The location of the axis determines the leverage ground reactive force has on the subtalar joint. Upward ground reaction force on the first metatarsal head will cause a supination moment (torque) in a foot with a more laterally deviated subtalar joint axis and a pronation moment in a foot with a more medially deviated subtalar joint axis.⁴ In the foot that has a medially deviated axis, the ground reaction force will pronate the subtalar joint harder than in a foot with an average or lateral axis location. It is not the amount of pronation that causes stress on anatomical structures, it is how hard or the magnitude of the pronation moment that determines the stress on anatomical structures. It is not the pronation motion that causes problems, it is the stopping of pronation that causes problems.

Conversely, in the foot with a laterally deviated subtalar joint axis, ground reaction force will tend to cause supination and the anatomical structures that resist supination will be under stress. Therefore, the position of the subtalar joint axis in the transverse plane is what determines whether a patient has an overpronating or over-supinating foot.   

There is a clinical test to find the location of the subtalar joint axis in a foot.^5,6 When a patient is seated in an exam chair, the foot is in the same subtalar joint position that it would be when the patient is standing. If the subtalar joint is at its end range of motion in stance, then the subtalar joint is at its end of range of motion on the examining table. One can ascertain the position of the subtalar joint in stance by viewing the patient from behind in stance and asking the patient to evert. (Describe the motion to the patient and the vast majority of patients can understand the instructions.) Tell the patient just to move the foot and not the leg. Observe, roughly, how many degrees of eversion of the calcaneus there are to the leg. If you see 2 degrees of eversion, then when the patient is seated on the exam chair, invert the foot 2 degrees from maximal pronation and then assess the position of the location of the subtalar joint. The reason for this is you want to know the location of the subtalar joint axis in the transverse plane in the position (e.g. 2 degrees from maximal pronation) when the foot is in stance. (As the talus and calcaneus move around the subtalar joint, the projection of the location of the subtalar joint axis onto the transverse plane will move as well.) It is important to understand the difference between the position of the subtalar joint (e.g. 2 degrees from maximal pronation) and the location of the subtalar joint axis (e.g. angled 16 degrees from the sagittal plane).

With the patient in the chair, grasp the fifth metatarsal head with one hand and place the foot in the stance position. Then use your other hand to push on several locations on the bottom of the foot. When applying pressure lateral to the subtalar joint axis, the subtalar joint will pronate. When applying pressure medial to the axis, the subtalar joint will supinate. If there is no motion, apply force directly toward the axis. Mark this point with a pen. Then repeat the palpation further distally. A line connecting the points of no motion represents the location of the subtalar joint axis under the foot when the foot is in stance. Researchers have found this test to be reproducible.⁷

There is quite a lot of variation when it comes to the subtalar joint axis location among people. Feet with a more medially deviated subtalar joint axis will tend to have a larger pronation moment from ground reaction force. This will tend to stress the structures in the foot that resist pronation. One can average force under the foot to a single point.⁸ The same location of force in a medially deviated axis foot will create a greater pronation moment than a foot with an average axis location because the distance from force to the joint axis is farther. Moment = force x distance. This is one thing that can make a foot overpronate. People with medially deviated subtalar joint axes are pronators because of the pronation moment from ground reaction force.

Conversely, those with a laterally deviated subtalar joint axis will have a higher supination moment from the ground. Sometimes, the supination moment will be so high that if there were no muscular action opposing the supination moment, the foot would supinate to its end range of motion. The people with lateral subtalar joint axes have learned not to let their feet go to the supination end range of motion by using their peroneal muscles to create a pronation moment that is larger than the supination moment from the ground. These people will often exhibit late stance phase pronation and often get peroneal tendonitis. Additionally, the pronation moments from the muscles can cause problems like excessive stress in the windlass mechanism. This muscular activity is why those who over-supinate can have pronation-related problems. One should treat the muscular pronators differently than those who pronate due to ground reaction forces.  

Treatment decisions are based on the location of the subtalar joint axis. One can alter the moment from ground reaction force about the subtalar joint axis by changing the shape of the orthotic. One technique is to shift ground reaction force medially with the medial heel skive.⁹ This will decrease the pronation moment from the ground by changing the lever arm of the ground reaction force on the subtalar joint. When there is a lower pronation moment from the ground, the anatomical structures that resist pronation will have less stress on them. Conversely, a foot with a laterally deviated axis can have an orthotic with a valgus heel cup (lateral heel skive) or a forefoot valgus post to shift the ground reaction force more laterally to increase pronation moment from the ground. For the over-supinating foot, reducing the supination moment from the ground will decrease the need for peroneal muscle activity. Many people with peroneal tendonitis feel immediate relief with a valgus forefoot wedge.  

To reiterate, not all patients with problems related to high pronation moments need to have their orthotic increase supination moments from the ground. The decision of whether to add a medial heel skive and create a varus heel cup should be based on the location of the subtalar joint axis. For the foot with a laterally deviated subtalar joint axis, one may use a forefoot valgus post and/or a rearfoot valgus heel cup.

A foot with a medially deviated subtalar joint axis should get a rearfoot varus heel cup. Sometimes a forefoot extrinsic post is indicated for a medially deviated subtalar joint axis and sometimes it is not.
When it comes to the location of most medially deviated subtalar joint axes, force on the first metatarsal head will cause a pronation moment. If there is high stress in the windlass mechanism (hallux valgus, hallux limitus, plantar fasciitis), additional force from a forefoot varus wedge attempting to supinate the subtalar joint will be ineffective because that same force will increase load on the windlass mechanism.  

Pertinent Pearls On Forefoot Valgus Posting And Maximum Eversion Height

One indication for the use of forefoot valgus posting, which I discussed in the previous section, is the over-supinating foot. Another indication is overloading of the windlass mechanism. Research has shown that a forefoot valgus wedge can reduce tension in the plantar fascia.¹⁰ There needs to be a measurement that will determine how much forefoot valgus post to use for a specific foot. Different feet have different amounts of eversion range of motion available. It is possible that a forefoot valgus wedge will attempt to evert the subtalar and midtarsal joints further than the available range of motion of both of those joints. Since both the midtarsal joint and subtalar joint contribute to the available range of motion, the measurement has to take both of those joints into account when the foot is weightbearing.  

I have developed a maximum eversion height measurement to estimate how much forefoot valgus wedge one can use without stressing the structures that limit further eversion range of motion. The problems that occur when the forefoot valgus post is too large are lateral column pain and sinus tarsi pain. The physical end of range of motion of the subtalar joint occurs when the lateral process of the talus slides all the way down the posterior facet of the subtalar joint and the lateral process abuts the floor of the sinus tarsi. Further attempted eversion just compresses the talus and calcaneus together at the floor of the sinus tarsi.⁴

Measure the maximum eversion height with the patient standing and demonstrating eversion motion. Ask the patient to attempt to evert. The vast majority of patients can do this. This can happen at the same time as the observation of the calcaneus to see how much subtalar joint eversion there is for the location of the subtalar joint axis test. Note the distance from the fifth metatarsal to the ground. The amount of forefoot valgus wedge cannot be higher than this distance. One can convert the distance to a degree measurement with simple trigonometry but conversion for orthotic manufacturing is not necessary.
The maximum eversion height test is very similar to the Coleman block test. One can use both to differentiate the over-supinator with high lateral loads from the maximally pronated varus foot with high lateral loads.

Determining The Correct Amount Of Arch Fill

Root wrote that one should add medial expansion plaster to a cast taken in subtalar joint neutral position because some people could not tolerate the orthosis with no additional arch fill.¹¹ Orthotic laboratories have made the amount of arch fill part of the orthotic prescription. Unfortunately, the labs tended to use vague terms like minimal, normal and maximal. This variation is necessary because different feet will have different distance between the neutral position arch height and the standing arch height.

A way to make the amount of arch fill more precise is to have the patient stand. Then, using your fingers with a comfortable amount of force pressing up into the arch, measure the height of the fingers off the ground. This number can give the lab a better idea of how much arch fill to add. If the patient is an over-supinator, the clinician can press less hard and create an orthosis with a lower arch, which will attempt to supinate the foot less than if the arch was higher.  

Key Considerations With Calcaneal Fat Pad Expansion And Plantar Fascia Prominence

Root also noted that heel cup irritation could occur if one did not add expansion plaster to the positive cast when the orthotic was manufactured. The amount of fat pad expansion is not consistent across feet.¹¹ Therefore, one should use calipers to measure the width of the fat pad when the patient is standing and make the orthotic heel cup as wide as that measurement to prevent heel cup irritation.

During the non-weightbearing exam, the examiner should dorsiflex the hallux and note whether the plantar fascia, when tight, significantly deforms the arch of the foot. If it does, mark the plantar fascia so its location can transfer to the negative cast and the lab knows exactly where to place the plantar fascial groove in the orthotic.  

Ascertaining The Relative Metatarsal Length And Necessary Extensions

The traditional plastic shell of an orthosis ends behind the metatarsal heads. Devices that end behind the metatarsal heads can be quite effective and all that the patient needs. However, the shell cannot alter forces after heel off. Some foot problems need to be addressed during the propulsive phase of gait.

Windlass mechanism-related problems can benefit from a reverse Morton’s extension. Specifically, one may employ a piece of cork attached to a top cover that extends beyond the shell under the second, third, fourth and fifth metatarsal heads. The clinician can use extensions for any specific location problems by just altering the configuration of the extension. Of course, before ordering an extension, you have to have the conversation with the patient about which shoes will be able to accommodate the additional volume of the extension.

Other Pertinent Benefits Of The Tissue Stress Theory Approach

One difference between the tissue stress approach and the Root approach is that the tissue stress approach may require capturing a forefoot to rearfoot measurement in the cast rather than capturing what was measured. For example, one may measure a forefoot to rearfoot relationship of 3 degrees varus, but a maximum eversion height of 3 mm is visible. Using the tissue stress approach, you would want to plantarflex the medial column to the point where you would see a forefoot valgus relationship in the cast. One could then add an intrinsic forefoot valgus post without creating an everted appearing heel cup.
Another way to deal with this situation is to take the cast that has a forefoot varus relationship and still add an intrinsic forefoot valgus post to the cast. One could make the resulting everted heel appear vertical or even inverted with the addition of a medial heel skive. Thus, the finished orthotic made from that cast could still have an intrinsic forefoot valgus post and heel cup that has a varus heel wedge.   

If the orthotic does not at first achieve the desired results, the tissue stress approach can give guidance on how to modify and improve the performance of the orthotic. Other protocols for orthotic manufacturing would expect you to repeat the same process that just failed. With the tissue stress theory, if you found that the orthosis did not reduce pronation symptoms caused by ground reaction forces sufficiently, then you could increase the varus wedge effect in the heel cup of the orthosis. The emphasis of the tissue stress approach to orthotic prescription writing is to envision the shape of the finished orthotic the patient will be standing on. It may not matter so much in which position the clinician cast the foot in as one can add intrinsic forefoot valgus posts and heel skives to almost any cast. The finished orthosis is what alters the location of forces on the foot.

In Conclusion

An orthotic shell that is off the shelf or made from a cast of the foot will alter the forces that are applied to the foot. This can explain why prefabricated devices can provide relief for some foot complaints. The tissue stress approach can help providers choose modifications beyond basic shells that can make orthotics work better than just a basic shell. Some feet get better with generic shells. Other feet may need specific orthotic modifications to get better. The tissue stress approach gives providers a rationale for choosing appropriate modifications.

Dr. Fuller taught biomechanics at the California College of Podiatric Medicine for 14 years. He is in private practice in Berkeley, Calif.

References

1.     Fuller EA. The windlass mechanism of the foot: a mechanical model to explain pathology. J Am Podiatr Med Assoc. 2000; 90(1):35-46.
2.     Morag E, Cavanagh PR. Structural and functional predictors of regional peak pressures under the foot during walking. J Biomech. 1999;32(4):359-70.
3.     Inman VT. The Joints of the Ankle. Williams and Wilkins, Baltimore, 1976.
4.     Kirby KA. Rotational equilibrium across the subtalar joint axis. J Am Podiatr Med Assoc. 1989;79(1):1-14.
5.     Kirby KA. Methods for determination of positional variations in the subtalar joint axis. J Am Podiatr Med Assoc. 1987;77(5):228-234.
6.     Van Alsenoy KK, De Schepper J, Santos D, Vereecke EE, D’Août K. The subtalar joint axis palpation technique-part 1: validating a clinical mechanical model. J Am Podiatr Med Assoc. 2014;104(3):238-46.  
7.     De Schepper J, Van Alsenoy K, Rijckaert J, et al. Intratest reliability in determining the subtalar joint axis using the palpation technique described by K. Kirby. J Am Podiatr Med Assoc. 2012;102(2):122-9.
8.     Fuller EA. Center of pressure and its theoretical relationship to foot pathology. J Am Podiatr Med Assoc. 1999;89(6):278-91.
9.     Kirby KA. The medial heel skive technique. Improving pronation control in foot orthoses. J Am Podiatr Med Assoc. 1992;82(4):177-88.
10.     Landorf KB, Keenan A. Efficacy of foot orthoses: what does the literature tell us? J Am Podiatr Med Assoc. 2000; 90(3):149–58.
11.     Root ML. Development of the functional orthosis. Clin Podiatr Med Surg. 1994;11(2):183-210.
12.     Fuller E, Hogge JD. Measurement of the expansion of the calcaneal fat pad upon weightbearing. J Am Podiatr Med Assoc. 1998;88(1):12-6.  

For further reading, see “Prescribing Orthoses: Has Tissue Stress Theory Supplanted Root Theory?” in the April 2015 issue of Podiatry Today, “What A Recent Study Gets Right And Wrong About Root Biomechanics” by Kevin Kirby, DPM at https://tinyurl.com/ycx8fe2k, or the DPM Blog “Defending Root: Is The New Study By Jarvis And Colleagues All It Is Cracked Up To Be?” by Doug Richie, Jr., DPM, FACFAS at https://tinyurl.com/yd5ynr7r .