Essential Keys To A Pre-Op Biomechanical Evaluation

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Robert D. Phillips, DPM

   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.

   When it comes to the design of orthotics and shoes, it is desirable for the center of the shoe to be under the center of the leg. Once the heel gets more than 10 mm lateral to the leg, it is almost impossible to utilize an in-shoe orthotic to equalize the inversion-eversion moments.

   For these cases, either a shoe has to have modifications to bring the center of the sole under the center of the leg or surgery has to bring the center of the calcaneus under the leg.13

What You Should Know About Examining The Subtalar Joint Axis

The plotting of the subtalar joint axis on the plantar foot is a very important part of the preoperative exam. I am fond of the following quote by Close:

   “The position of this (subtalar joint) axis in the foot is thus of great importance in the study of the action of muscles whose tendons pass in relation to it. The problem of exactly where to place the insertion of a tendon to be transferred is solved by knowledge of the position of the axis and the state of other muscles passing it.”14

   The plotting of the subtalar joint axis should occur with the subtalar in the “forefoot flat position,” which is defined as that subtalar joint position in which the forefoot is parallel to the ground when the midtarsal joint is fully pronated. The normal subtalar joint axis should lie lateral to the first metatarsal head.15 If it is medial, then any surgery should try to bring it more lateral or else the foot may still pronate to the end of its range of motion after surgery. One of the most effective procedures that moves the subtalar joint axis more laterally is the Evans osteotomy to lengthen the lateral side of the os calcis.

Evaluating The Forefoot To Rearfoot Relationship

Assessing the forefoot to rearfoot relationship also needs to be an important part of the pre-op evaluation.16 If the patient has a forefoot varus, one must address this in any surgery.17 Evaluate the forefoot with the subtalar joint in a position in which the calcaneus is perpendicular to the ground. While the traditional method of measuring has been with the patient supine and non-weightbearing, this may also occur with the patient standing.18

   With the heel in the desired post-op position, if the first metatarsal is off the ground, the clinician should gently push down on the first metatarsal head to see if it can make contact with the ground. If it cannot, then the clinician must determine whether the patient has a true forefoot varus or a metatarsus primus elevatus. The recording of the forefoot to rearfoot relationship needs to be in the evaluation as well as the range of motion of the first metatarsal.

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