Multiplanar Semiautomatic Assessment of Foot and Ankle Offset in Adult Acquired Flatfoot Deformity
Cesar de Cesar Netto
Foot and Ankle Surgeon
Department of Orthopaedic and Rehabilitation
University of Iowa, Carver College of Medicine
Since I started working with Weightbearing Computed Tomography (WBCT) as a Research Fellow in Baltimore-Maryland back in 2014, I realized that this technology could represent a game-changer in the understanding and treatment of Adult Acquired Flatfoot Deformity (AAFD).
It is not difficult to accept the concept that two-dimensional conventional radiographic images are not enough to provide a complete assessment of this multiplanar and multifocal complex pathology.
The initial excitement of having the three-dimensional (3D) WBCT datasets available soon became a considerable challenge and sort of frustration when I started trying to apply traditional classic AAFD measurements such as talus-first metatarsal angle. It did not take a long time for me to realize that, for example, the talus and the first metatarsal were not in the same plane in most AAFD patients and that a true angulation between them could rarely be identified and calculated. The task of performing the multiple reported and validated measurements assessing hindfoot valgus, medial arch collapse, forefoot abduction, residual forefoot supination, and peritalar subluxation would also prove to be extremely complicated and time-consuming, while navigating through hundreds/thousands of axial/coronal/sagittal WBCT images.
As an attempt to solve this challenge and trying to facilitate and optimize the assessment and treatment of AAFD patients, I decided to look for measurements that in isolation could represent the multiple 3D components of this complex deformity.
The Foot and Ankle Offset (FAO) is a semi-automatic WBCT measurement where the 3D coordinates of the weightbearing points of the foot tripod are harvested (first metatarsal head, fifth metatarsal head, and calcaneal tuberosity), and is compared with the 3D position of the center of the ankle joint. Initially described by Lintz et al. in 2017 (Lintz F, Welck M, Bernasconi A, et al. 3D Biometrics for Hindfoot Alignment Using Weightbearing CT. Foot Ankle Int. 2017;38(6):684-689. doi:10.1177/1071100717690806), it provides a relative assessment of where the foot sits in relation to the ankle. In that scenario, the foot tripod can be centered underneath the ankle (neutral) or lies medially (“varus”) or laterally (“valgus”) to it. Since FAO considers the relative 3D positioning of the foot tripod and ankle joint, it would make sense that it could represent a more complete assessment of the multifocal and multiplanar AAFD as a single isolated measurement.
That was the idea of our study recently published at FAI: “Multiplanar Semiautomatic Assessment of Foot and Ankle Offset in Adult Acquired Flatfoot Deformity.” In a cohort of 113 AAFD patients, we performed multiple traditionally used AAFD manual measurements and compared them with FAO measures.
We found FAO to be more reliable than all other measurements performed, and more importantly that changes in the hindfoot moment arm (as a marker of hindfoot valgus), subtalar horizontal angle (as a marker of peritalar subluxation), talonavicular coverage angle (as a marker of forefoot abduction) and forefoot arch angle (as a marker of medial column collapse) would in combination explain almost 80% of the changes in FAO measurements. In other words, FAO measurements would strongly correlate with measurements of hindfoot valgus, medial column collapse, peritalar subluxation, and forefoot abduction, representing all those deformity components in a single and reliable measure.
Our finding is strong enough to support practitioners and medical providers to use FAO measurements as a single, reliable, and relatively quick measurement to assess AAFD baseline deformity, deformity progression, and deformity correction following surgical treatment. The widespread use of FAO by researchers would also allow standardization of study’s results, facilitating interpretation and compilation of data, potentially improving and optimizing the treatment of AAFD patients. Additional longitudinal and prospective studies further validating the use of FAO measurements are for sure needed.