Weight-Bearing CT Scans in Foot and Ankle Surgery

Matthew S. Conti, MD; Scott J. Ellis, MD

Disclosures

J Am Acad Orthop Surg. 2020;28(14):e595-e603. 

In This Article

Normal Anatomy on Weight-bearing CT Scans

WBCT scans have been used to investigate normal foot and ankle anatomy in patients without notable pathology.[7–11] In healthy control patients, WBCT has been most frequently used to study hindfoot alignment, which has important implications for load-bearing through the lower extremity.[8,11,12] A recent study that reviewed the WBCT scans of 48 patients without hindfoot pathology described a neutral hindfoot alignment in this control subject cohort rather than the presence of innate valgus of the hindfoot, which had been previously suggested based on weight-bearing radiographs.[9] Others have taken advantage of a newer measurement called the foot and ankle offset (FAO), which is a three-dimensional measurement that defines a relationship between three points on the sole of the foot and one in the center of the ankle joint, to describe the overall hindfoot alignment.[8,11] In these studies, they found that the hindfoot alignment in normal patients lies slightly valgus to the midline.[8,11] These studies describing hindfoot alignment in normal control patients may provide surgeons with a reference when planning for deformity correction.

Another study used a WBCT scanner with a custom pedography sensor to compare the morphologic center of the foot with the center of force during weight-bearing.[10] They found that the morphologic center of the foot was distal to the center of gravity or force of the foot in 97% of feet at a mean distance of 27.5 mm distal.[10] In 62% of feet, the foot center was lateral to the center of gravity but only by a mean of 2.0 mm.[10]

In addition to investigating alignment and force distribution through the foot and ankle, WBCT scans have been used in normal patients to describe subtalar joint position and rotational dynamics of the talus because these are difficult to visualize well on plain radiographs.[13] A study by Colin et al[13] in 59 patients without hindfoot pathology demonstrated that the most posterior aspect of the posterior facet of the subtalar joint is consistently oriented in valgus, whereas the most anterior aspect of the posterior facet of the subtalar joint is typically oriented in varus. This study suggests that the measurements of the varus-valgus orientation of the posterior facet of the subtalar joint are dependent on where the CT image is taken in the AP direction. Lepojärvi et al,[14] using 32 healthy control subjects, showed that the talus rotates a total of 10° internally-externally about the tibia when the ankle is taken from maximal internal to maximal external rotation without substantial widening of the medial clear space.

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