When Biomechanical Theory Clashes With Surgical EBM: A Closer Look At Flexible Flatfoot Deformity With Equinus

Questioning established medical therapies leads to advancement and innovation when it is based on scientific methodology. Biomechanical theories are often just that: theories. Often, a disconnect between biomechanical theories and surgical approaches confounds an optimal treatment plan.

One example that comes to mind is the incorrect application of kinematics and kinetics biomechanical theory to surgery in the flexible flatfoot with an associated equinus deformity. Lengthening of the gastrocsoleus complex via gastrocnemius recession or Achilles tendon lengthening (TAL) is not recommended when performing a medial calcaneal displacement osteotomy (MCDO), according to the kinematics and kinetics biomechanical approach. The premise of a medial calcaneal displacement osteotomy is that medialization of the posterior calcaneus allows the Achilles tendon to exert a supinatory moment on the hindfoot. According to this biomechanical approach, lengthening of the gastrocsoleus complex then reduces the potential corrective force. 

This specific recommendation based on the kinematics and kinetics biomechanical theory is wrong. Understanding why requires a review of the concepts of equinus biomechanics and surgical evidence-based medicine (EBM). 

Thordarson and colleagues, along with Johnson and Christensen, demonstrated the dominant planar compensation for limited ankle joint dorsiflexion in late midstance, as the body is moving from behind to over the planted foot, is in the sagittal plane.^1,2 The dominant sagittal plane deformation occurring at the naviculocuneiform joint supersedes the coronal plane supinatory moment created by the medial calcaneal displacement osteotomy without gastrocsoleus lengthening. 

The ankle joint requires at least 10 degrees of dorsiflexion in late midstance to move the body from behind to over the planted foot during late midstance.³ If less than 10 degrees of ankle joint dorsiflexion exists, sagittal plane compensation at the naviculocuneiform joint occurs as the body moves over the foot due to momentum.^1-4  The medial calcaneal displacement osteotomy produces little, if any, corrective change at the midtarsal area. Therefore, the hindfoot position improves.⁵ I prefer osseous realignment via an Evans osteotomy to an inferior osseous realignment procedure supplemented by dynamic realignment. However, the naviculocuneiform joint is still in a collapsed position. 

The other consideration relates to the subtalar joint (STJ) axis. With the realignment of the STJ axis via surgical osseous correction, in this example of a flexible flatfoot deformity, the STJ axis resumes its normal position from the posterior lateral calcaneus through approximately the first metatarsal head. In the presence of an equinus deformity, the center of pressure (CoP) moves distally and laterally from the second metatarsocuneiform joint to between the second and third metatarsal metaphyseal-diaphyseal junctions. The distance between the center of pressure and the STJ axis increases, creating an increased pronatory moment due to ground reactive forces (GRFs). The Achilles tendon insertion becomes medial to the STJ axis after a medial calcaneal displacement osteotomy, creating a supinatory moment. 

The ideal rearfoot position for correction of a flexible flatfoot deformity described by Conti and colleagues is in a slight varus hindfoot alignment.⁶ The pronatory moment created by the ground reactive forces due to the center of pressure relationship to the STJ axis is greater than the supinatory moment created by the Achilles tendon relationship to the STJ axis. The net result of not lengthening the gastrocsoleus complex despite hindfoot alignment is a pronatory moment. Additionally, if the STJ is still medially deviated due to incomplete osseous realignment associated with medial calcaneal displacement osteotomies comparative to the Evans procedure, the net pronatory moment is even greater.⁷

I find it interesting and frustrating when lower extremity biomechanical theory is put forth by non-surgical researchers. The research on the surgical side of the equation far outweighs the non-surgical side. When biomechanical theory contradicts established surgical EBM, I follow the surgical EBM every time. The EBM on the lengthening of the gastrocsoleus complex in the surgical correction of a flexible flatfoot with an equinus deformity is robust. If you are surgically correcting a flexible flatfoot deformity with an associated equinus, lengthen the gastrocsoleus complex with your procedure of choice. I prefer the Baumann procedure.

Dr. DeHeer is the Residency Director of the St. Vincent Hospital Podiatry Program in Indianapolis. He is a Fellow of the American College of Foot and Ankle Surgeons, a Fellow of the American Society of Podiatric Surgeons, a Fellow of the American College of Foot and Ankle Pediatrics, a Fellow of the Royal College of Physicians and Surgeons of Glasgow, and a Diplomate of the American Board of Podiatric Surgery.

References

1. Thordarson DB, Schmotzer H, Chon J, Peters J. Dynamic support of the human longitudinal arch. A biomechanical evaluation. Clin Orthop Relat Res. 1995;316:165-172.
2. Johnson CH, Christensen JC. Biomechanics of the first ray part V: the effect of equinus deformity: a 3-dimensional kinematic study on a cadaver model. J Foot Ankle Surg. 2005;44(2):114-120.
3. Gatt A, De Georgio S, Chockalingam N, Formosa C. A pilot investigation into the relationship between static diagnosis of ankle equinus and dynamic ankle and foot dorsiflexion during stance phase of gait: Time to revisit theory? Foot (Edinb). 2017;30:47-52.
4. Amis J. The split second effect: the mechanism of how equinus can damage the human foot and ankle. Front Surg. 2016;3:38.
5. Iossi M, Johnson JE, McCormick JJ, Klein SE. Short-term radiographic analysis of operative correction of adult acquired flatfoot deformity. Foot Ankle Int. 2013;34(6):781-791.
6. Conti MS, Ellis SJ, Chan JY, Do HT, Deland J. Optimal position of the heel following reconstruction of the stage II adult-acquired flatfoot deformity. Foot Ankle Int. 2015;36(8):919-927.
7. Zanolli DH, Glisson RR, Nunley 2^nd JA, Easley ME. Biomechanical assessment of flexible flatfoot correction: comparison of techniques in a cadaver model. J Bone Joint Surg Am. 2014;96(6):e45.