Essential Keys To A Pre-Op Biomechanical Evaluation

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Robert D. Phillips, DPM

Fundamental Biomechanical Insights For Hallux Valgus Cases

Another common surgical procedure is the correction of hallux valgus. The literature is replete with the concept that most people with hallux valgus have some type of pronation problem during the gait cycle.20,21

   Again, the clinician should identify whether pronation is the etiology of the deformity as well as the etiology of any pronation that is occurring. Most of the patients I evaluate for hallux valgus get one of two options: orthotic therapy or surgery with post-op orthotic therapy.

   It is important to identify the stage of the deformity and convey to the patient what an orthotic can and cannot do.22 Good orthotic therapy can theoretically reverse Stage I bunions. One can theoretically stop Stage II bunions from progressing but they will not reverse. Theoretically, clinicians can slow down but not stop the progression of Stage III bunions. One cannot change Stage IV bunions in their progression with orthotic therapy but orthotic therapy may alleviate some of the secondary symptoms.

   Before performing bunion surgery, assessing first ray range of motion is of prime importance in the biomechanical examination.23,24 Unfortunately, there are no instruments that can measure this.25,26 There is also a tremendous amount of research into how this range of motion changes in reaction to other joint motions. I have noted clinically that when the long axis of the midtarsal joint is supinated, the first ray seems to have more range of motion than when the long axis is pronated. No controlled study has confirmed this observation.

   I recommend that physicians observe the total first ray range of motion with the midtarsal joint fully pronated when the subtalar joint is in its neutral position. It has been traditional to record this total range of motion in the number of millimeters that the first metatarsal head moves above and then below the second metatarsal head. We assume that the neutral position is halfway between the maximal dorsiflexed position and the maximal plantarflexed position.

   When doing hallux valgus surgery, it is important that the surgeon not create an inverted forefoot to ground position. If the patient has a rearfoot varus deformity — which may be due to a subtalar varus or a lower leg varus — the surgeon will want to plantarflex the first ray during surgery.27 Likewise, if the patient has a forefoot varus deformity, the surgeon will also want to plantarflex the first ray when performing surgery.

In Summary

I want to impress on all the importance of analyzing the mechanical function of the foot before and after any musculoskeletal surgery on the foot and the ankle. Surgeons sometimes get caught up in the aesthetics of the foot after their surgery, believing that if it looks good, it must function well.

   However, it is well known that changes in the structure of the foot can have marked effects not only on those structures in close proximity to the surgical site but on structures that are far distant to the surgical site. For example, there is good evidence as to the effects of limited motion of the first MPJ having a negative impact on the function of the spine. Changes in subtalar joint position may produce changes in hip or patellar function.28,29

   Unfortunately, there is much research and development yet to occur in analyzing the structure and function of the foot. There is much to do in understanding how the ligamentous structures interact to create the ranges of motion of the midtarsal joint and the midfoot joints.

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