The Role of Weightbearing Computed Tomography Scan in Hallux Valgus

Abstract

Background: Hyperpronation of the first metatarsal in hallux valgus (HV) is poorly understood by conventional weightbearing radiography. We aimed to evaluate this parameter using weightbearing computed tomography (WBCT) and to understand its association with other standard measurements. Methods: Retrospective evaluation of WBCT and weightbearing radiographs (WBXRs) was performed for 20 patients with HV feet and 20 controls with no such deformity. Axial computed tomography images of both groups were compared for the first metatarsal pronation angle (alpha angle) and tibial sesamoid subluxation (TSS) grades. The HV angle (HVA), first-second intermetatarsal angle (IMA), first metatarsal-medial cuneiform angle (MMCA), Meary’s angle, and calcaneal pitch (CP) angle of the study and control groups were compared on both WBXR and the corresponding 2-dimensional images of WBCT. All measurements were independently performed by 1 musculoskeletal radiology fellow and 1 foot and ankle surgical fellow. Measurements were averaged and interobserver reliability was calculated. Results: The HV group demonstrated significantly higher values for TSS grade ( P < .001) but not for alpha angle ( P = .121) compared with controls. Likewise, significantly elevated HVA and IMA were noted in the HV group on both imaging modalities, while no such differences were observed for the CP angle. Higher MMCA and Meary’s angle in the HV group were evident only on WBXR (MMCA, P = .039; Meary’s, P = .009) but not on WBCT (MMCA, P = .183; Meary’s, P = .171). Among all, the receiver operating characteristic (ROC) curves demonstrated the greatest area under the curve (AUC) for HVA, followed by IMA. The alpha angle performed only just outside the range of chance (AUC, 0.65; 95% CI, 0.52-0.69). The Pearson’s correlations of the alpha angle, in the HV group, revealed a significant linear relationship with TSS grade and with HVA on WBXR, and only trended toward a weak linear relationship with IMA and with HVA on WBCT. Conclusion: The alpha angle, a measure of abnormal hyperpronation of the first metatarsal, was an independent factor that may coexist with other parameters in HV, but in isolation had limited diagnostic utility. “Abnormal” alpha angles were even observed in individuals without HV. Increases in IMA and MMCA were not necessarily associated with similar increases in alpha angle, despite moderate correlations with TSS grade and HVA on WBXR. Nevertheless, the WBCT was a useful method for assessing hyperpronation and guiding surgical management in individual cases. Level of Evidence: Level III, retrospective comparative study.

Normative Distribution of First Metatarsal Axial Rotation

“…The 95% CIs demonstrate that mean normative M1 rotation lies slightly pronated, rather than parallel to a ground reference (Figure 4). However, the relatively wide ranges reported with both methods suggest significant natural variability in M1 rotation, 19 even among feet without clinical/radiographic deformity. Additionally, this study showed that measuring the M1 head lateral edge shape can also give clinicians an estimate of M1 rotation using anteroposterior (AP) radiographs.…”

“…In the last 10 years, axial rotation of the first metatarsal (M1) in the coronal plane has gained increasing attention as having a clinically meaningful impact in HV deformity. 15,19,43 This rotational deformity is referred to as “pronation” when the plantar aspect of the M1 faces inward, toward the second metatarsal, and “supination” when the plantar aspect of the M1 faces outward, away from the second metatarsal. 10,29,43…”

“…5,35,40 Next, the alpha angle (or Kim et al 15 method), developed using simulated WBCT, has also been verified by later articles. 19,30 Appearing concurrently in literature with these measurement methods are derotational operative techniques such as the modified Lapidus procedure, 6 proximal supination osteotomy, 47 and proximal rotational metatarsal osteotomy, 44 among others.…”

The Assessment of First Metatarsal Rotation in the Normal Adult Population Using Weightbearing Computed Tomography

“…35 Based on values that are 2 standard deviations higher than the mean, the upper normal limit of MPA can be as high as 30 degrees in hallux valgus, 5,28 and the upper normal limit of the α angle has been reported as high as 33 degrees in hallux valgus. 15,20 Failure to correct coronal rotation may be a cause of recurrent deformity and persistent sesamoid subluxation. 14 Many procedures have been described to supinate the metatarsal in hallux valgus, ranging from proximal osteotomies to modified first tarsometatarsal joint arthrodesis, with promising results.…”

Investigation on the Site of Coronal Deformities in Hallux Valgus

“…Recently, Mahmoud et al [33] found that almost half of the subjects in the control group had an abnormal alpha angle greater than 16° (the value below the cut point should be regarded as a normal alpha angle according to Kim's method [34]), indicating that the pronation may also exist in feet without HV. It should be noted that their coronal rotation may be affected by the pronation of the midfoot and hindfoot bones, as it used the ground level and the distal 1 st MT as the measurements reference points.…”