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.

   All in all, the authors measured a total of six radiographic angles of the foot, all of which showed an improvement in position.2 That would go against the single plane of correction idea as the changes in the radiographs were multiplanar. The final conclusion was that the changes were angular, not planar, thereby showing that we cannot consider the procedure itself to be a correction in only one plane.

   This would make intuitive sense in a three-dimensional model of any kind. In a mathematical model, if you change one number (for the most part), this modifies the ultimate outcome. The foot is much like that as even a small change in motion theoretically causes a three-dimensional change in function.

Debating Osteotomy Placement And Bone Graft Size

Another point of contention is whether the placement of the osteotomy obviates a change in outcome, based on the intrusion of the articular facets within the subtalar joint. Two studies in particular evaluated the safety of osteotomy placement to avoid embarrassing the articular facets within the subtalar joint.

   Hyer and co-workers found that the optimum placement of the osteotomy was between 11.5 mm and 15 mm proximal to the calcaneocuboid joint.3 The authors showed the optimum position in order to avoid damaging either the anterior or middle facets of the subtalar joint. The authors also noted that this was only for patients with an obvious separation of these two facets. Additionally, Hyer and colleagues found that only 41.06 percent of the calcanei measured had two distinctly separate articular facets. The rest of the patients’ articular facets were conjoined, thereby eliminating the need to accurately measure the position of the osteotomy.

   Raines and Brage found a similar result and theorized that making the osteotomy at 10 mm proximal to the calcaneocuboid joint showed the least potential for intruding into the articular facets of the subtalar joint.4 They also noted that this measurement was only necessary when patients had distinctly separate articular facets.

   One other conclusion of note from Raines and Brage is that this distance for placement of the osteotomy site and its incision put some of the lateral vital structures in the least amount of danger of transection.4 The structures in jeopardy included the peroneal tendons and the sural nerve if one places the incision correctly. The authors also noted that regardless of incision and osteotomy placement, the medial structures were always at risk. These structures include the medial plantar nerve, the flexor hallucis longus, the flexor digitorum longus and the posterior tibial tendon.

   Another aspect of the procedure that Sangeorzan and colleagues discovered was that the size of the bone graft surgeons used had an impact on how much correction they achieved.2 The authors noted that the largest size bone graft tolerated by a patient due to pain was between 8 and 12 mm. The most common size surgeons use is 10 mm. However, with a graft of that size, once the bone resorbs during healing, the total amount of length one would achieve is only 4 mm.

   This is of great consequence when considering how much correction actually occurs with the various methods of lengthening available and, considering the biomechanical impact, the ultimate length of the lateral column. This also leads to how the bone grafting performs in this application. According to Mahan and Hillstrom, a bone graft works better in the calcaneus than elsewhere in the foot with respect to non-union rates.5

Avoiding The Potential Complications Of Migration And Arthrosis

Another issue in regard to the use of bone grafts is the potential for migration of the distal calcaneal fragment once the graft is in place. This is specifically the case when one uses no fixation, which is less than ideal in the pediatric population.

   Dunn and Meyer studied this recently and found a statistically significant dorsal displacement of 1.21 mm of the anterior process of the calcaneus.6 The authors did note that this did diminish to a non-statistically significant displacement of only 0.57 mm once the patients underwent evaluation at their final postoperative visit. Again, this may be of concern in the pediatric population in particular as there are no studies that suggest whether this slight shift causes any long-term issues with motion or pain.

   Davitt and Morgan have documented one long-term potential complication associated with bone grafting in the pediatric population and for this procedure.7 These authors reported that fifth metatarsal stress fractures could occur after lateral column lengthening. However, if a fifth metatarsal fracture did occur, the study authors note that one can treat it successfully with immobilization.

   Cooper and colleagues reported another potential complication with bone graft use in this procedure.8 The authors noted potential calcaneocuboid joint arthrosis in the long term. They suggested that surgeons should mostly reserve this procedure for the pediatric population as adult patients may not be able to accommodate for the increase in stress at this joint, causing pain and dysfunction. Interestingly, Cooper and co-workers also suggested the use of calcaneocuboid joint distraction arthrodesis to eliminate this potential in the adult population.

How Ilizarov Distraction Can Obviate Complications

One severely underutilized method of lengthening the lateral column is that of the Ilizarov method of callus distraction. The learning curve can be rather steep but proper use of this procedure can eliminate many of the potential aforementioned complications. Martin and co-workers first described this method.9 I learned this technique in residency and it is my preferred method of performing this procedure.

   Many who now do this procedure in the pediatric population use internal fixation to prevent any migration of the anterior fragment of the calcaneus postoperatively. There is no risk of this via callus distraction and no need for permanent internal fixation. Using an external mini-rail also allows virtually immediate weightbearing and, with proper patient education, one can tailor the amount of lengthening. There is no graft resorption so you know exactly how much correction you get based on the amount of lengthening tolerated. In addition, there is no chance of nonunion with the grafting material. Once the procedure is complete and the patient achieves full healing, the surgeon can remove the fixator.

In Conclusion

To revisit an earlier point, when describing the Evans procedure, most will automatically point to the uniplanar correction discussed due to the lengthening of one particular aspect of the foot. Sangeorzan and colleagues first challenged this.2 Twelve years later, he and his colleagues then confirmed this to be inaccurate in another journal article focusing on cadaveric measurements.10

   In a study published in 2005 by Dumontier and co-workers (of which Sangeorzan was one of the investigators), this group showed significant strides in understanding that the Evans procedure did indeed affect the foot in three dimensions.10 The authors showed that several points of influence occurred when placing a graft in the lateral calcaneus in their cadaveric specimens.

   They noted that “… relative to the talus, the navicular moved 18.6 degrees of rotation in adduction, 2.6 degrees of rotation in pronation and 3.4 degrees in plantarflexion. The average translation was 5.6 mm medial, 0.4 mm posterior and 1.8 mm plantar. The cuboid moved an average of 24.2 degrees of rotation in adduction and lengthening, 13.9 degrees in pronation and 1.9 degrees in plantarflexion. The average translation was 9.4 mm medial, 2.6 mm distal and 1.5 mm plantar. The calcaneus moved an average of 4.4 degrees of rotation in adduction, 0.1 degrees of rotation in eversion and 1.3 degrees of plantarflexion. The average translation was 3 mm medial and 0.7 mm posterior with no plantar translation.”10

   As is evident by these findings, there is truly a three-dimensional aspect to the correction surgeons achieve with this procedure. Regardless of the actual technique one uses to perform the procedure and achieve the desired results, it is clear that this procedure is anything but uniplanar. One thing that is always questionable is whether any procedure one performs on such a complex structure as the foot can ever be considered unidirectional.

   For every action, there is an equal and opposite reaction. The fault lies in considering this reaction as occurring in only one direction, plane or dimension. When dealing with a three-dimensional structure, every change affects every plane. The foot and ankle certainly fall in this realm. One should view every change in all directions.

   Dr. Raducanu is a Fellow and President of the American College of Foot and Ankle Pediatrics. He is also a Fellow of the American College of Foot and Ankle Surgeons. Dr. Raducanu is in private practice in Philadelphia. For questions on the callus distraction procedure or other questions on the article, readers can reach Dr. Raducanu at kidsfeet@gmail.com .

References

1. Evans D. Calcaneo-valgus deformity. J Bone Joint Surg Br. 1975; 57(3):270-78.
2. Sangeorzan BJ, Mosca V, Hansen ST Jr. Effect of calcaneal lengthening on relationships among the hindfoot, midfoot, and rearfoot. Foot Ankle. 1993; 14(3):136-41.
3. Hyer CF, Lee T, Block AJ, VanCourt R. Evaluation of the anterior and middle talocalcaneal articular facets and the Evans ostoetomy. J Foot Ankle Surg. 2002; 41(6):389-93.
4. Raines RA Jr, Brage ME. Evans osteotomy in the adult foot: an anatomic study of structures at risk. Foot Ankle Int. 1998; 19(11):743-7.
5. Mahan KT, Hillstrom HJ. Bone grafting in foot and ankle surgery. A review of 300 cases. J Am Podiatr Med Assoc. 1998; 88(3):109-18.
6. Dunn, SP, Meyer J. Displacement of the anterior process of the calcaneus after Evans calcaneal osteotomy. J Foot Ankle Surg. 2011; 50(4):402-6.
7. Davitt JS, Morgan JM. Stress fracture of the fifth metatarsal after Evans’ calcaneal osteotomy: a report of two cases. Foot Ankle Int. 1998; 19(10): 710-2.
8. Cooper PS, Nowak MD, Shaer J. Calcaneocuboid joint pressures with lateral column lengthening (Evans) procedure. Foot Ankle Int. 1997; 18(4):199-205.
9. Martin DE, Stran DC, Southerland JT, Wesselowski BE. Callus distraction in reconstructive foot surgery. J Foot Ankle Surg. 1996; 35(5):489-506.
10. Dumontier TA, Falicov A, Mosca V, Sangeorzan B. Calcaneal lengthening: investigation of deformity correction in a cadaver flatfoot model. Foot Ankle Int. 2005; 26(2):166-70.

   For further reading, see “Essential Insights On The Evans Calcaneal Osteotomy” in the June 2009 issue of Podiatry Today, “Key Pearls Of Calcaneal Osteotomies” in the May 2003 issue and “How To Perform The Double Calcaneal Osteotomy” in the December 2004 issue.