Weight-bearing CT Scans in Hallux Valgus
Pronation of the First Metatarsal
Although hallux valgus (HV) has been understood to be a triplanar deformity of the first metatarsal, WBCT has recently helped to quantify the pathology at the first tarsometatarsal joint as well as the rotational deformity of the first ray. Collan et al used WBCT scans to compare 10 patients with HV with five asymptomatic normal control patients. They found no difference in the sagittal first metatarsal-ground angle between the HV and normal control subject groups. To study rotational deformities, they used a two-dimensional coronal view and measured the angle between the ground and either the sesamoid articulation of the first metatarsal or base of the proximal phalanx. The authors reported no notable difference in pronation of the first metatarsal in patients with HV (mean, 8°) and normal (mean, 2°) patients; however, they did note a statistically significant difference in the pronation of the proximal phalanx between the HV (mean, 33°) and control subject (mean, 4°) groups. The lack of statistical significance in pronation of the first metatarsal between HV and control patients may be due to the small number of patients included in the study and the authors' technique used to measure pronation based on the plantar aspect of the first metatarsal head, which may be deformed due to subluxation of the sesamoids and erosion of the crista.
In contrast, more recent studies have found increased pronation of the first metatarsal in patients with HV compared with normal control subjects.[30,31] Kim et al demonstrated an increase of 8.1° in pronation of the first ray in patients with HV compared with control patients. Campbell et al used a computer-aided design to create a three-dimensional model of the first metatarsal and then measured the pronation of the first ray with the second metatarsal as a reference because the second ray forms a central, structural element of the foot (Figure 4). Using simulated WBCT scans, the authors reported that patients with HV had 9.9° of increased pronation of the first metatarsal compared with normal control subjects. In addition, they did not find any correlation between pronation of the first ray and IMA or HVA in HV. A subsequent study using the technique developed by Campbell et al found that the average preoperative pronation of the first metatarsal in patients with HV was 29.0° and decreased to 20.2° postoperatively, which was statistically significant. Pronation of the first ray was not associated with sesamoid position, suggesting that the position of the sesamoids is likely not a good proxy for first metatarsal pronation. Consequently, the rotational deformity is likely independent of the other deformities in patients with HV. Proper recognition and correction of the pronation deformity of the first ray may be important in the surgical management of patients HV.
Image showing a three-dimensional computer-aided design model illustrating the pronation of the first metatarsal in hallux valgus. This figure is a reconstruction of a foot using the weight-bearing CT scan of a patient with hallux valgus to demonstrate how the deformity can be quantified in three dimensions. Courtesy of Dr. Mark Carl Miller, PhD.
Two studies using simulated WBCT scans have demonstrated increased mobility at the midfoot in patients with HV when compared with normal patients.[33,34] Kimura et al performed simulated WBCT scans in 10 patients with HV and 10 patients without HV to investigate motion at the midfoot. In the HV group, they demonstrated statistically significantly greater dorsiflexion, inversion (pronation), and adduction of the first metatarsal in relation to the medial cuneiform compared with the control patients, which suggested increased motion of the first ray in patients with HV. In a separate study using simulated WBCT scans, the same group found greater mobility of the first-second intercuneiform joint in patients with HV. Another study compared simulated weight-bearing and nonweight-bearing CT scans in 10 healthy control patients and 10 patients with HV. A three-dimensional model of each bone was created so that widening and translation of the first metatarsal-cuneiform joint could be measured. In patients with HV, the first metatarsal-cuneiform joint significantly widened and translated more in the dorsal-plantar direction when compared with the normal control patients, which supports the finding of hypermobility of the first tarsometatarsal joint in patients with HV.
Position of the sesamoids may also be more accurately measured on WBCT scans. Kim et al used simulated WBCT scans to demonstrate that no correlation was found between sesamoid position and pronation of the first metatarsal, which suggests that the degree of subluxation of the sesamoids is independent of first ray pronation. In approximately 25% of cases, they found that patients had "pseudosesamoid subluxation" on simulated WBCT scans in which there was first metatarsal pronation without true sesamoid subluxation; however, when viewed on an AP weight-bearing radiograph, the sesamoids in these cases would appear subluxated. Another simulated WBCT study demonstrated that tibial sesamoid position was correlated with HV severity as measured by the HV and intermetatarsal angles. They also found that the degree of degenerative change in the sesamoid metatarsal joint was associated with increasing lateral shift of the tibial sesamoid (Figure 5).
J Am Acad Orthop Surg. 2020;28(14):e595-e603. © 2020 American Academy of Orthopaedic Surgeons