Issue 469

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Sep 20, 2022

Practice Perfect 828
Making Practice Perfect Decisions
Part 5 - Function Follows Form

  Jarrod Shapiro, DPM, FACFAS, FACPM, FFPM, RCPS Glasg

PRACTICE PERFECT   September 20, 2022

Today’s Practice Perfect starts a slight divergence from our topic of making decisions, moving into the surgical realm. The concept that function follows form is an old one and is not specific to foot and ankle medicine and surgery. Consider the very simple example of a wheel. The form of a wheel (a simple circular form) is made for one primary function: moving forward by rolling. It performs that function very well, being round. Take that same round shape and morph it into a square, and you no longer have that rolling function.

The lower extremity has a similar general goal to a wheel-forward movement - but it accomplishes this in the human in a much different way. Instead, we have three large joints (the hips, knees, and ankles) all of which have large amounts of sagittal plane movement, and several smaller joints in the foot that create a complex machine relying on a specific form to function well. The subtalar joint is a torque converter that transforms the sagittal plane movements of the other joints into frontal plane movement. The joints of the medial longitudinal arch work together to form a dynamic but stable arch, while the lateral column joints create stability. Additionally, other machines, such as the windlass, a transverse arch, and irregular joint shapes, also participate. When all of these joints work in synchrony, they create a device that interacts with the ground to alternately provide shock absorption, spring action, and a rocker system, leading to smooth forward motion with less energy use and an efficient gait. Of course, any complex machine may fail in a number of ways, which is why foot and ankle medicine and surgery is so complex and necessary. When form breaks down, function declines. That leads us to Principle #5:

Now, before we get into the nitty gritty of this rule, especially as it pertains to surgery, I need to make one quick modification to the order of our rules in this series. Until this editorial, #5 (Function follows form) was listed as #7, and the new #7 (Know where you are) was #5. I switched these two around because knowing where you are is more pertinent to the specifics of surgical technique (don’t worry, we’ll get to that eventually), and our new #5 is most important for choosing surgical procedures. I felt this change was an improvement in organization of the materials. So, that brings in our modified graphic showing the ten principles of decision making in the foot and ankle shown in Figure 1.

Figure 1. The new and improved 10 principles for making foot and ankle treatment choices.

Ok, let’s get to it! If the foot has a specific form to allow for proper function, then for those patients with pedal pathology, it will require us to understand where the form breaks down so we can address it to improve function. Recall, principles 2 and 3 have us focus on the underlying biomechanical cause of a problem (#2) and then treat that underlying biomechanical cause by adjusting forces or correcting deformity (#3). In reality, correcting the deformity adjusts forces, so they’re actually the same thing.

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Take a look at the table below (Table 1), extracted from an article I wrote in January 20201. This is part of a larger table that lists the known kinematics (joint positions) and kinetics (pressures and forces) of surgical procedures applied to the foot and ankle as reported in the medical literature. These two characteristics are what we cause to happen when doing these procedures. As an example, the Evan’s calcaneal osteotomy corrects form by elongating the lateral column in the flatfooted patient, improving the peroneus longus pulley function as it wraps around the cuboid, locking the first ray at its insertion on the proximal 1st metatarsal, thereby reestablishing proper medial column function. The resulting improved form and function decrease 1st MTP joint load, decrease rotation about the talonavicular joint with the secondary effect to increase lateral column load. This information demonstrates that every surgical procedure potentially effects both form and function.

Table 1. Excerpt from a larger table in Reference 1 showing that all procedures have kinetic and kinematic effects.

Here’s a relatively simple example. The radiographs in Figure 2 demonstrate a patient with hallux valgus and bunion deformity for which I performed a Lapidus bunionectomy.

Figure 2. Pre- and post-operative dorsoplantar radiographs demonstrating normalization of the intermetatarsal and realignment of the metatarsal head with the sesamoid complex for improved function.

When considering what procedure to perform, I utilized Principle 5 - Function follows form. The apex of the deformity was at the 1st tarsometatarsal joint, rather than within the metatarsal shaft itself, dictating the location of the surgery. During the procedure, the goal was to realign the 1st metatarsal head with the sesamoids. The evidence for this goal comes from an article by Shibuya and colleagues2, who found that sesamoid rotation was independently associated with hallux valgus, tibial sesamoid position, and medial column collapse, demonstrating the importance of an anatomically normal 1st MTP joint.

Similarly, the use of the CORA principle (center of rotation of angulation) provides a way to determine realignment procedures based on normalization of form (and the mechanical effects of the location of osteotomies). A somewhat more complex example of function following form is shown in Figure 3. For this patient with Stage 4 progressive collapsing foot deformity (AKA posterior tibial tendon dysfunction) I performed an ankle fusion as part of the reconstruction (also included was a talonavicular fusion and Cotton procedure). The preoperative axial view shows the severity of the frontal plane deformity with the apex near the ankle joint as evidenced by the tibiocalcaneal angle (a subtalar fusion and medial displacement calcaneal osteotomy had been done in the remote past). Normalization of the form, shown in the postoperative axial view led to improved function for the entire foot.

Figure 3. Normalization of the tibiocalcaneal angle leading to normal form and improved function.

It’s appropriate that Principle 5, Function follows form, is now set squarely in the middle of our 10 principles, since this is such an important decision-making principle. I challenge each of you to try these five principles when making your next decision and see where it lands you.


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Desert Foot 2022

Best wishes.

Jarrod Shapiro, DPM
PRESENT Practice Perfect Editor
[email protected]


References
  1. Shapiro J. Surgical Biomechanics: Principles of Procedure Choice. Clin Podiatr Med Surg. 2020 Jan;37(1):101-1616
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  2. Shibuya N, Jasper J, Peterson B, Sessions J, Jupiter DC. Relationships Between First Metatarsal and Sesamoid Positions and Other Clinically Relevant Parameters for Hallux Valgus Surgery. J Foot Ankle Surg. 2019 Nov;58(6):1095-1099.
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