Weight-bearing CT Scans in Adult-acquired Flatfoot Deformity
Adult-acquired flatfoot deformity (AAFD) encompasses a number of pathologies including medial longitudinal arch collapse, hindfoot valgus, midfoot abduction, and compensatory forefoot varus. These concomitant deformities can be difficult to evaluate individually on two-dimensional weight-bearing radiographs, and visualization of the subtalar joint and impingement between the calcaneus and fibula are better assessed on WBCT scans. Ferri et al found that CT scan measurements of forefoot arch angle, which is the angle between the horizontal and a line connecting the inferior aspect of the medial cuneiform to the inferior aspect of the fifth metatarsal, and subtalar joint subluxation were markedly changed when applying 50% weight-bearing compared with a nonweight-bearing state in a simulated weight-bearing study. Another study by Kido et al also reported the differences in the talar, navicular, and calcaneal positions on WBCT scans compared with nonweight-bearing CT scans. Consequently, WBCT scans have been used to quantify the severity of deformities in AAFD with good intraobserver and interobserver reliability. In one study comparing 10 control patients with 23 patients with stage II to IV AAFD, WBCT scans demonstrated more notable differences in radiographic parameters between control subjects and patients with AAFD than plain weight-bearing radiographs alone, although the clinical implications of these findings have not been established.
Subtalar Joint Alignment
An early study using simulated WBCT scans demonstrated frequent subluxation of the talocalcaneal joint with an average of only 68% of the posterior facet of the calcaneus in contact with the talus in eight patients with AAFD compared with 92% in control patients. In a retrospective study of 22 feet in 20 patients with symptomatic AAFD who had undergone simulated WBCT scans, the anterior aspect of the posterior facet was typically oriented in valgus in contrast to a comparison cohort of normal control patients who had a varus orientation of the anterior aspect of the posterior facet (Figure 2, A and B). This varus orientation at the anterior part of the posterior facet of the subtalar joint in normal control patients was consistent to the results reported by Colin et al, who used upright, full WBCT scans, previously discussed. Similarly, the posterior aspect of the posterior facet of the subtalar joint in patients with AAFD was found to be in statistically significantly more valgus in patients with flatfoot than in normal control subjects. More recent studies have used standing, full WBCT scans, in contrast to simulated WBCT scans, to investigate the orientation of the subtalar joint in patients with stage II AAFD compared with control patients. Cody et al, using full WBCT scans, measured the subtalar joint alignment at 50% of the anterior-to-posterior dimension of the posterior facet in 45 patients with stage II AAFD and in 17 control patients. They found that stage II patients with AAFD had an additional 10° of subtalar valgus compared with the control subjects. A similar study examining subtalar joint valgus in stage II patients with AAFD found that nonweight-bearing CTs underestimated the true subtalar joint deformity compared with WBCT scans and could not be used as a substitute for full WBCT scans.
Weight-bearing CT scan images showing the difference in the subtalar joint alignment between a patient with adult-acquired flatfoot deformity (A) and a patient with a normal hindfoot alignment (B). At 50% of the AP length of the posterior facet, the patient with adult-acquired flatfoot deformity has a notable valgus alignment of their subtalar joint, as demonstrated by the inferior facet of the talus and the horizontal line seen in white. In a patient with hallux valgus at 50% of the AP length of the posterior facet, a slight varus alignment of the subtalar joint is noted, which is demonstrated in black.
Subtalar and Subfibular Impingement
Patients with AAFD may present with pain laterally at the anterior fibula and sinus tarsi due to calcaneofibular (subfibular) and/or talocalcaneal (subtalar) impingement (Figure 3, A and B).[23,24] Malicky et al used simulated WBCT scans in 19 patients with AAFD who were indicated for surgery to study these impingement deformities. They found evidence of subtalar impingement in 92% of the WBCT scans and observed subfibular impingement in 66% of the CT scans, which differed markedly from control patients who had rates of 5% and 0%, respectively. MRI, although nonweight-bearing, may also be used to evaluate the subtalar joint for evidence of subtalar impingement and to measure the calcaneofibular distance for evidence of subfibular impingement.
Weight-bearing CT scans demonstrating subtalar (A) and subfibular (B) impingement in adult-acquired flatfoot deformity. Talocalcaneal (subtalar) impingement is seen at the angle of Gissane (dashed arrow). Calcaneofibular (subfibular) impingement is indicated by the solid black arrow.
More recently, Jeng et al used WBCT scans in 25 patients with AAFD to evaluate subtalar and subfibular impingement. Thirteen patients had stage II AAFD, 17 had stage III, one had stage IV, and three patients were not able to be staged based on chart notes. In contrast to Malicky et al, only 38% of patients had subtalar impingement and only 35% of patients had subfibular impingement on WBCT scans. Patients with subfibular impingement had a mean coronal calcaneofibular distance of 2.7 mm compared with those patients without subfibular impingement who had an average coronal calcaneofibular distance of 5.7 mm. In addition, they found that patients with AAFD with either talocalcaneal or calcaneofibular impingement had, on average, approximately 10° more of talonavicular abduction than patients with AAFD without bony impingement. These findings may have important implications for predicting which patients may fail a flatfoot reconstruction.
WBCT has also been shown to aid in the assessment of hindfoot deformity in patients with AAFD, although no direct comparisons have been made between WBCT scans and weight-bearing radiographs. In a study of 20 patients with stage II AAFD, de Cesar Netto et al reported that clinical examination of the hindfoot alignment angle underestimated the hindfoot alignment angle as measured on WBCT scans by a mean of 7.6°. They reported that the mean hindfoot moment arm measured on WBCT scans was 15.1 mm in valgus, which correlated markedly with the clinical hindfoot alignment angle. They advocated for the use of WBCT scans in patients with AAFD because they found the measurements of hindfoot alignment to be repeatable and reliable. Burssens et al were the first to describe the hindfoot alignment angle on WBCT scans, which was similar to the technique used by de Cesar Netto et al mentioned above, and found that patients with AAFD had an average hindfoot alignment angle of 20.1°. Such measurements have the potential to help surgeons titrate the amount of correction of various osteotomies.
Medial Longitudinal Arch Alignment and Forefoot Varus
To investigate the alignment of the bones along the medial longitudinal arch in AAFD, Kido et al used simulated WBCT scans to study 24 patients with flatfoot deformity and compared them with 20 healthy control subject feet. When the foot was loaded, patients with AAFD had more dorsiflexion at the first tarsometatarsal joint and more eversion at the talonavicular and talocalcaneal joints than the healthy control subjects. Their findings were supported by those of Zhang et al who compared similar WBCT measurements in patients with stage II AAFD to healthy control patients. Greisberg et al also used simulated WBCT scans in 37 patients with AAFD to study the medial longitudinal arch. Deformity along the medial longitudinal arch was found to be at either the navicular-cuneiform joint (65%) or talonavicular joint (20%) but rarely at both, with only one foot having more than 10° of collapse at both joints. They also noted severe degenerative changes in 38% of patients at the talonavicular joint, but none of the tarsometatarsal joints showed notable arthritic changes on the CT scans.
Yoshioka et al used simulated WBCT scans to study forefoot alignment in patients with stage II AAFD. They reported that the fifth metatarsal bone was more plantarflexed in patients with AAFD than in healthy control patients, and the authors described this finding as the compensatory forefoot varus deformity. However, it should be noted that the difference in plantar flexion of the fifth metatarsal bone between flatfoot patients and normal control subjects was only 1.3°. This could have important implications in the surgical management of the compensatory forefoot varus pathology and may suggest that the deformity is prone to overcorrection.
J Am Acad Orthop Surg. 2020;28(14):e595-e603. © 2020 American Academy of Orthopaedic Surgeons