Essential Keys To A Pre-Op Biomechanical Evaluation

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Robert D. Phillips, DPM

Pertinent Pearls On Evaluating The Midtarsal Joint

The clinician should also assess the range of motion of the midtarsal joint, both the oblique axis as well as the longitudinal axis. While there are no published methodologies for goniometric recording of these measurements, the clinician can at least make some qualitative observations.

   One can grade the longitudinal axis as having a low amount of motion, which I consider to be less than 5 degrees; a moderate amount of motion, which I consider to be between 5 and 20 degrees; or a high amount of motion, which I consider to be more than 20 degrees of motion. The reason for this is that during the contact period of gait, the anterior tibialis fully supinates the long axis of the midtarsal joint.

   Then as the forefoot makes contact with the ground from lateral to medial, the anterior tibialis relaxes and the forefoot starts its eversion motion relative to the rearfoot. If the forefoot excessively inverts to the rearfoot during contact, then the subtalar joint may function as if the patient has a forefoot varus. Most of the time, a functional orthotic can be effective for these patients.

   The oblique axis direction of motion is also important to observe. Does the patient have a transverse plane dominance of motion or sagittal plane dominance? Again, no quantitative methods exist for this observation yet there are differences in the type of pronated feet that will result.19

   Transverse plane dominance will produce a foot that exhibits the “too many toes” sign whereas the foot with sagittal plane dominance will exhibit more of an effect of fallen arches. The foot with transverse plane dominance will show a high degree of cuboid abduction on a dorsal-plantar view of the foot whereas the foot with sagittal plane dominance will show a much higher degree of talar declination and loss of calcaneal inclination angle.

   The foot with the transverse plane dominance will function much more like a foot that has a medially displaced subtalar joint axis. The foot with the high degree of sagittal plane mobility may exhibit an “equinus-like” function of the foot, in which it takes a lot less tension in the Achilles tendon to produce a pronated foot. For these types of patients, 10 degrees of ankle joint dorsiflexion may not be enough and it may be desirable for the patient to exhibit at least 15 degrees of ankle joint dorsiflexion.

   Assess the degree of forefoot adductus before surgery. Theoretically, one do this by taking a dorsal-plantar X-ray with the patient in standing position and the subtalar joint in neutral position. The angle is the comparison of the longitudinal bisector of the second metatarsal and the long axis of the rearfoot. If this angle is greater than about 15 degrees, the practitioner should be very careful about trying to do flatfoot surgery as the patient may have a difficult time postoperatively fitting shoes that have enough of a curved last.

   When there is a high degree of forefoot adductus, the clinician needs to determine why the patient is still having such a high degree of pronation that he or she is considering doing surgery. One may also need to address the forefoot adductus as part of the surgical procedure.

   When considering surgery for abnormal pronation, the clinician needs to ensure that he or she has identified all of the pronatory etiologies. For example, examination for internal femoral torsion or internal tibial torsion should be part of the evaluation. If the patient has either of these abnormalities, correcting the pronation may actually cause the patient to walk with a pigeon-toed gait. This may exacerbate clumsiness.

   The clinician should also ensure that the patient does not have extremely short hamstring muscles or contraction of the iliopsoas muscles. Both of these conditions create strong pronation forces on the foot that will continue even after any surgery.

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