Expert Insights On Performing The Akin Osteotomy
After making the incision, identify the extensor hallucis longus tendon and retract it laterally. Incise the periosteum and retract it only enough to perform the osteotomy. Surgeons should never dissect the periosteum from the lateral cortex because this will disrupt the blood supply and could result in postoperative hinge failure.
Some variation exists regarding placement of the osteotomy and its orientation. Frequently, one places the bone cut transversely in the center of the phalanx, probably as a matter of convenience for the surgeon. This is the area where exposure is good and there is adequate bone on either side of the osteotomy for fixation.
It is also possible to correct an abnormal PASA and DASA from this location. A disadvantage of this location is that the osteotomy is entirely in diaphyseal bone where healing may be prolonged. Most agree that the surgeon should place the cut in the metaphyseal bone and proximally this is usually about 5 to 7 mm distal to the articular surface of the base of the phalanx.11 A distal osteotomy can be challenging because the area of the metaphysis is small.
Use an oblique osteotomy when the goal is to achieve interfragmentary compression. Usually one orients the osteotomy from distal medial to proximal lateral, inserting the bone screw perpendicular to the osteotomy site. An advantage of the oblique osteotomy is that the distal and proximal aspect of the bone cut extends into the metaphysis for faster healing. When performing the oblique osteotomy, take care not to inadvertently cut into the joint. Without good fixation, if the lateral hinge breaks, this osteotomy is very unstable and displacement results in shortening of the toe.
Wherever one places the osteotomy, a good technique is to place a loose Kirschner wire (K-wire) over the phalanx and use fluoroscopy to confirm the position.
When making the osteotomy, it is recommended to score the bone before making the cuts. This allows for final confirmation of the osteotomy prior to making the cut. With a proximal osteotomy, make the first cut parallel to the base and about 5 to 7 mm distal to the joint. Place the second arm of the osteotomy distally with both arms intersecting just medial to the lateral cortex but leaving the lateral cortex intact. One can also use a guide wire to preserve the lateral cortex. Make the cuts from dorsal to plantar, and remove the wedge of bone.
Then position the saw from medial to lateral to feather the lateral cortex. While advancing the saw laterally, apply a gentle adductory pressure to the great toe until the lateral cortex weakens and the osteotomy is closed. Since one does not dissect the soft tissue laterally, this maneuver is essentially a closed or percutaneous procedure.
How Much Bone Should One Remove?
Preoperative planning is necessary to determine the amount of bone to remove.
Gerbert advocated using templates based on the preoperative radiographs.12 Removing just enough bone to make the medial and lateral lengths of the phalanx equal is a good guide, but this may not result in enough correction. McGarvey indicated that usually the width of bone he removed was 2 to 3 mm and this gave a good result.13 Frey had devised a complicated mathematical formula to assist in determining how much bone to remove.11 He determined that removing 1/8-inch (3 mm) of bone resulted in 8 degrees of correction. Removing 3/16-inch (5 mm) yielded 16 degrees of correction and 5/16-inch (8 mm) gave 24 degrees of correction.
Even with this data, determining the amount of bone to remove is often based on the surgeon’s judgment. If one does not remove enough bone, the great toe can remain abducted and if one removes too much of a wedge, this can result in a varus deformity. The variable is the intraoperative instability of the first metatarsophalangeal joint. This makes it difficult to determine if the amount of bone removed is enough or too much.