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A 3D Computer Simulation Reveals Amount of Shortening Before Transfer Symptoms Occur in 1st Metatarsal Osteotomies

I recently came across an interesting research article that I’d like to share with the community. The reason this article is noteworthy is that it lends evidence to an important topic – hallux valgus surgery – while demonstrating the power of a relatively new research method, finite element analysis. I’ve been seeing an ever-increasing presence of this type of research, especially in the biomechanical literature. Let’s take a look at the study and the research method they employed.

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What is Finite Element Analysis (FEA)?

FEA uses mathematical computing power to create models of a particular system and then run simulations on that system to see what would happen. The Finite Element Method (FEM) takes a complex structure and breaks it up into much smaller parts, each of which can be represented by a mathematical equation, called a finite element equation (hence the name). Putting these together allows us to mathematically describe various characteristics of the original object. Complex objects can then be described by breaking them into discrete subparts that can be recombined to describe the whole. Using a computer in this process, then, allows simulations of highly complex domains such as weather patterns, car crashes, and, of course, biomechanics of the human foot. The study under discussion today used this technology to examine function of the foot.

What About this Research Study?

One of the challenges of performing surgery for hallux valgus and bunion deformities is shortening of the first metatarsal. This complication may occur as a result of bone resorption from excessive heat during the osteotomy, angling the bone cut in such a way as to shorten the bone, and unintended elevation of the bone. It has also been proposed that purposefully plantarflexing the bone during bunion procedures will offset shortening and reduce the chances of patients having problems, the primary one being transfer pressures to the lesser metatarsal heads.

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Geng and colleagues looked to better understand the biomechanical effects of this situation using a finite element model.1 They created a computer model using a single patient with a nonpathological foot who underwent a gait analysis to determine the position of the hallux and ankle joint during the push off phase of gait. A nonweightbearing CT scan was then taken of the patient’s foot and a computer model was created with the foot in the position it would normally take during the propulsive phase of gait (as determined by the previously described gait analysis). Various physical and mechanical properties of the tissues were determined to create as accurate a model as possible.

The researchers then created a simulated osteotomy at the neck of the first metatarsal perpendicular to the weightbearing surface and to the second metatarsal shaft bisection. They removed 2, 4, 6, and 8 mm lengths of bone to simulate a shortened metatarsal and then, using their model, calculated the change in plantar forefoot pressures as a result. They also analyzed what would happen if one were to transpose the metatarsal head in a plantar direction during the shortening, what they termed a “pushing down motion.”

Before I summarize the results, it is helpful to discuss one assumption the authors used that derived from a prior research study, also by Geng’s research group. In a case control study, they found that in postoperative patients with transfer metatarsal pain after hallux valgus surgery, it was not the shortening of the first metatarsal but rather the abnormal loading pattern during gait that actually caused pain.2 The amount of loading on each metatarsal head at the moment of maximal pressure (termed the instant load percent) was the most important parameter in patients with transfer metatarsalgia.2 As a result of this study, Geng found the risk of metatarsalgia to increase substantially when the loading ratio was greater than 55% during push off (greater than 55% of the plantar force was taken by the lesser metatarsal heads). They used this number as the threshold at which a simulated patient would experience metatarsalgia.

Now for the results. First, Geng found their model was a close approximation to the original patient’s foot, so we have a valid model. They also found as the first metatarsal shortened, its load decreased while the lesser metatarsal head load increased. Only when first metatarsal shortening exceeded 6 mm did the loading ratio of the central rays increase to 55%, that threshold for metatarsalgia. In the simulated feet with 8 mm of shortening, this same threshold was decreased when the metatarsal was lowered by 3 mm.

They concluded that 6 mm of shortening after a first metatarsal osteotomy is safe to prevent postoperative transfer metatarsalgia, while 3 mm of metatarsal head plantar translation can compensate when greater shortening occurs.

Geng et al concluded that 6 mm of shortening after a first metatarsal osteotomy is safe to prevent postoperative transfer metatarsalgia, while 3 mm of metatarsal head plantar translation can compensate when greater shortening occurs.

Comments About the Geng Study

This paper is important because it provides a theoretical answer to the question about shortening after a first metatarsal osteotomy and the potential success of plantarflexion as a method of compensation for shortening. This also applies to proximal bunionectomies like the Lapidus procedure.

Another important aspect of this study is the power of finite element analysis to explore biomechanical research questions. It would be very difficult to answer how much shortening leads to transfer metatarsalgia using live humans. Taking a large number of patients and deliberately shortening their metatarsals a specific amount to see what would happen is clearly unethical. This leaves us the option of a retrospective study looking at large numbers of postoperative patients to see what happened (which has been done). There are so many methodological problems with this design that it wouldn’t lead us any closer to a solid answer. FEA, on the other hand, allows us the ability to play with various parameters and see how changes affects the model.

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The last significant aspect of this study is the focus on kinetics (pressures and forces) to answer a research question dealing with a kinematic (positional) situation. It’s clearly not just shortening of the first metatarsal that is important but rather the transfer of pressures to the lesser metatarsals during the push off phase of gait that causes symptoms. That understanding is highly important when figuring out how to treat patients with this problem. For example, if I see a patient with postoperative transfer metatarsalgia, and I thought only the length of the metatarsal mattered, then I’m obliged to lengthen the bone as treatment. Alternatively, understanding the pressure redistribution allows me to focus on various types of treatments that aim to decrease lesser metatarsal head pressures (there are lots of ways to do this).

Present Treasure Hunt 2020

This study exemplifies the trend in research toward a more accurate understanding of biomechanics of the foot rather than the limited positional view of the past. Just as we now know foot orthotics work via pressure effects on the foot (kinetically) rather than “controlling” foot position (kinematically), we now have mounting evidence that positional changes during surgery create kinetic changes, and it’s these pressure and force changes that affect outcomes for our patients. Treatment as a result of these force changes should then be focused on modifying these abnormalities rather than worrying about positions. This is where research on the lower extremity should continue to go into the future.

Best wishes.

Jarrod Shapiro, DPM
PRESENT Practice Perfect Editor
jarrod@podiatry.com
References
  1. Geng X, Shi J, Chen W, et al. Impact of first metatarsal shortening on forefoot loading pattern: a finite element model study. BMC Musculoskelet Disord. 2019 Dec 27;20(1):625.
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  2. Geng X, Huang D, Wang X et al. Loading pattern of postoperative hallux valgus feet with and without transfer metatarsalgia: a case control study. J Orthop Surg Res. 2017 Jul 25;12(1):120.
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