Understanding The Biomechanical Effects Of The Evans Calcaneal Osteotomy
Reviewing the evolution of the Evans calcaneal osteotomy, this author discusses the impact on biomechanics, shares insights from the literature on osteotomy placement and bone graft size, and offers pearls on minimizing the risk of common complications.
Some of the most useful inventions in the last century were nothing more than happy accidents. Someone was testing something out and quite inadvertently made an ancillary discovery, which ultimately made more of an impact than what he or she was initially examining. From Ben Franklin’s invention of the bifocals to the apple falling on Sir Isaac Newton’s head, our history shows that when looking for something new, we generally should look to the old first.
This was the case when Evans discovered that he could realign overcorrected clubfeet with a procedure to the lateral column.1 His initial problem was that when correcting these complex clubfeet, sometimes he overshot his original goal of correction. As a result, his young patients would eventually develop extraordinarily flat feet with calcaneal valgus positions. He also had a thought about why a valgus foot is that way and concluded that it had to do with the length of the lateral column.
Evans also concluded that the lateral column of the foot was the foundation of the foot (although we now know that is not quite the case) and that the length of the medial column had a large influence on the shape of the foot. That is how his namesake procedure was born. He solved his problem by developing one of the most commonly used lateral column procedures in both the pediatric and the adult population.
At the time, the general indications that Evans came up with for the use of this new procedure included a flexible pes planovalgus foot type with an abducted forefoot, midtarsal instability with transverse plane hypermobility, an abducted lateral column and a medially deviated leg angle axis.1
The most commonly evaluated radiographic angle to determine the need for an Evans procedure is the calcaneocuboid joint, which, in most cases, should be no larger than 8 degrees. One would measure this by striking two lines, one parallel to the lateral border of the distal calcaneus and the other parallel to the lateral aspect of the cuboid. Then measure the angle between those two lines to calculate the calcaneocuboid joint angle. As I noted, an angle greater than 8 degrees is considered abnormal and is the basis behind the decision to perform the Evans procedure.
Just as a point of reference, the aforementioned information is readily available in many texts and journal articles.
Pertinent Pointers On The Biomechanics Of The Procedure
When Evans developed the procedure, the basis of the mechanics behind the procedure was to address a deformity in the transverse plane and have a uniplanar effect on function. That being said, since the foot and the mechanics of the foot are theoretically triplanar, it is difficult to imagine that any procedure, regardless of the actual plane of influence, would cause a uniplanar change in function.
This was best exemplified by Sangoerozan and colleagues, who evaluated the structures affected by this procedure as well as the radiographic changes associated with certain radiographic angles post-procedure.2 They surmised that the plantar fascia was affected as was the peroneus longus. The authors also theorized that the procedure itself aided in improving the functional locking mechanism of the midtarsal joint. They also measured radiographic angles to show functional improvement via the Evans procedure. It is important to understand that any of these changes are theoretical, based on the Root theories of the functioning of the foot.