Key Pearls For Performing Bunion Surgery

Author(s): 
John Mozena, DPM, FACFAS, and David Arndt, DPM

Pertinent Insights On Metatarsal Elevation

An osteotomy can be an integral part of a bunion procedure to realign the first metatarsal. Every time the surgeon performs an osteotomy, it shortens the bone by at least the width of the blade. The first metatarsal has a declination angle that is not parallel to the weightbearing surface. Any bone cut action will shorten the first metatarsal longitudinally and thereby elevate it by the amount of the blade width.

   Therefore, one can determine elevation of the first metatarsal not only by the width of the blade but also by the declination of the first metatarsal in relationship to the ground. The greater the declination, the greater the elevation as the metatarsal shortens along its longitudinal axis. Elevation can lead to many complications in bunion surgery. These complications include: second ray overload resulting in painful callus; capsulitis; plantar plate tears leading to predisloaction syndrome; neuromas; and stress fractures.

   Another way elevation can occur is by wedge resection of the bone. Consider again the proximal closing base wedge osteotomy. Determine the amount of elevation by the angle of the hinge of the apex or axis. In performing this osteotomy with the axis perpendicular to the bone, the bone will elevate along the arc of a circle. The higher the pitch of the bone, the more significant this elevation becomes. If the osteotomy has an axis that is 90 degrees to the ground, the bone will move along the transverse plane and not elevate. Of course, one must be conscious of the orientation of the axis in the frontal plane as well. If the axis has any valgus angulation, there can be elevation and shorting in this plane.

What About The Relationship Between A High IM Angle And A High Hallux Abductus Angle?

There is a directly proportional relationship between a high intermetatarsal angle and high hallux abductus angle. As the hallux deviates laterally and the metatarsal deviates medially, the extensor hallucis longus tendon deviates laterally and becomes more bowstrung. The higher the intermetatarsal angle, the shorter the effective length of the extensor hallucis longus tendon becomes. This lateral deviation of the extensor hallucis longus tendon accelerates bunion formation and valgus rotation of the hallux.

   When one performs bunion surgery to realign the first ray, one may need to lengthen the shortened extensor hallucis longus tendon or hallux extensus could occur. Using the Pythagorean theorem can prove this.4 The theorem can be written as an equation relating to lengths of the sides of a right triangle: A2 + B2 = C2.

   The hypotenuse or C, which represents the extensor hallucis longus tendon, is actually shorter when the intermetatarsal angle is higher. As one places the first ray into its proper alignment, the extensor hallucis longus tendon may not have enough length to balance the hallux in its neutral position between flexion and extension.

Comments

Dear Drs. Mozena and Arndt,

Thank you for a very well written article.

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