Essential Keys To A Pre-Op Biomechanical Evaluation

Author(s): 
Robert D. Phillips, DPM

   What podiatry has attempted to change in the last 50 years is the idea that we do not treat flat feet but we do treat abnormal pronation.2 Abnormal pronation becomes a problem when it produces stress and strain on soft tissues, and finally bone deformity and cartilage degeneration.3,4 Unfortunately, we have not yet developed the sophistication to accurately predict when those symptoms or deformities, or degenerative changes will occur.5,6

   For most cases of abnormal pronation, clinicians can adequately treat symptoms with non-surgical appliances and in-shoe custom foot orthotics are the most common. However, sometimes surgery may be the most desirable biomechanical approach.

   Over the years, authors have proposed a myriad of surgical procedures and as one reviews any foot surgery textbook, it appears that all types of tendon procedures, osteotomies and fusions are available. Whenever one sees a wide variety of optional treatments for any problem, one can assume there are either multiple causes of that problem or that there is very little known about the etiology of that problem.

   In the case of abnormal pronation, podiatrists have been teaching for many decades that there are multiple etiologies of the problem. The better the surgeon can identify the etiology, the better the chance of picking the correct surgical procedure.

Why One Should Determine The Heel To Leg Angle And Whether There Is Calcaneal Bisector Displacement

The first biomechanical measurement that should be part of the evaluation is the resting calcaneal stance position (RCSP).7,8 Is the calcaneus everted, vertical or inverted? It is important that the clinician not automatically equate the amount of calcaneal eversion relative to the vertical with whether the patient is pronated or supinated.

   The heel is most stable when it is vertical and centered under the center of the leg. However, if the patient has a fully compensated rearfoot varus, the subtalar joint will be pronated from neutral and the heel will still be vertical. If the patient has a partially compensated rearfoot varus, the heel will be inverted in stance and the subtalar joint will be maximally pronated.

   For people with both fully and partially compensated rearfoot varus, it would usually be a mistake to try to do a surgical procedure that would make the rearfoot stand more inverted.

   After measuring the RCSP, the clinician also should measure the relaxed tibial stance position (RTSP).9 By subtracting the RTSP from the RCSP, the clinician can calculate the heel to leg angle. Then compare this angle with the maximum inversion and eversion range of motion of the subtalar joint when the patient is non-weightbearing.10

   Does the heel to leg angle show the subtalar joint to be maximally pronated? Does the angle show the calcaneus to be further everted to the leg than the maximum pronated position during the range of motion exam of the subtalar joint? It is my opinion that one of the criteria for flatfoot surgery is that the patient should be standing with the subtalar joint maximally pronated or subluxed.

   Another biomechanical measurement to assess is the displacement of the calcaneal bisector relative to the lower leg bisector.11 One can do this with the patient standing or laying prone. The clinician moves the subtalar joint until the heel bisector is parallel to the lower leg bisector. Radiographic methods can also accomplish this.12

   Measure the number of millimeters that the calcaneal bisector is medial or lateral to the leg. Normal should be 5 mm or less. The more lateral the calcaneal bisector is, the more that an eversion couple is set up for the subtalar joint to pronate when the patient places an axial load on the lower limb.

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