Influence of foot position on the measurement of first metatarsal axial rotation using the first metatarsal axial radiographs

Repeatability of Weightbearing Computed Tomography Measurement of First Metatarsal Alignment and Rotation

BACKGROUND

Weightbearing computed tomography (WBCT) can be used to assess alignment and rotation of the first metatarsal. It is unknown whether these measures remain consistent on sequential WBCTs in the same patient when a patient's standing position may be different. The aim of this study was to establish the repeatability (test-retest) of measurements of first metatarsal alignment and rotation in patients without forefoot pathology on WBCT.

METHODS

We retrospectively identified 42 feet in 26 patients with sequential WBCT studies less than 12 months apart. Patients with surgery between scans, previous forefoot surgery or hallux rigidus were excluded. Hallux valgus angle (HVA) and intermetatarsal angle (IMA) were measured using digitally reconstructed radiographs. Two methods of calculating metatarsal rotation (metatarsal pronation angle [MPA] and alpha angle) were measured on standardized coronal CT slices. Interobserver agreement and test-retest repeatability were assessed using intraclass correlation coefficients (ICCs). Standard error of measurement (SEM) and minimally detectable change (MDC95) were calculated.

RESULTS

Interobserver agreement was excellent for HVA and IMA (ICC 0.96 and 0.90, respectively) and was good for MPA and alpha angle (ICC 0.81 and 0.80, respectively). There was excellent test-retest repeatability for HVA (ICC=0.90) and good test-retest repeatability for IMA (ICC=0.77). There was excellent test-retest repeatability for MPA (ICC=0.91) and good test-retest repeatability for alpha angle (ICC=0.87). The MDC95 was 4.6 degrees for MPA and 6.1 degrees for alpha angle. Five percent of patients had a difference outside of the MDC95 for the alpha angle, compared with 2% for the MPA.

CONCLUSION

Measurements of first metatarsal alignment and rotation are reliable between assessors and repeatable between sequential WBCTs in patients without forefoot pathology. Subtle differences in patient positioning during image acquisition do not significantly affect measurements, supporting the validity of this method of assessment in longitudinal patient care.

LEVEL OF EVIDENCE

Level IV, retrospective case series.

Assessing the Rotation of the First Metatarsal on Computed Tomography Scans: A Systematic Literature Review

BACKGROUND

Hallux valgus is a multiplanar deformity that is often treated on the basis of 2-dimensional (2D) parameters and radiographs. Recurrence rates after surgical correction remain high, and failure to correct pronation of the metatarsal is increasingly stipulated as being part of the problem. Multiple methods of assessing metatarsal pronation have been proposed.

METHODS

We performed a systematic literature review identifying studies that measured metatarsal pronation and torsion on computed tomography (CT) scans. Specific methodology, patient groups, results, and reliability assessments were all reported.

RESULTS

We identified 14 studies that fulfilled the inclusion criteria. Eleven studies measured 2D values on CT scan, and 3 studies used computer-based 3-dimensional (3D) modeling and artificial intelligence systems to help calculate pronation. Metatarsal pronation angle, α angle, sesamoid rotation angle, and measurements for torsion were the most commonly used methods. All angles and measurements were performed as 2D measurements, but the metatarsal pronation angle was also performed with 3D modeling. Reliability and reproducibility of the α angle and metatarsal pronation angle were excellent, despite being performed on studies with small numbers.

CONCLUSION

Multiple methods have been reported to demonstrate first metatarsal pronation on CT, of which the α angle and the metatarsal pronation angle are the most pragmatic and useful in a clinical setting. Further work is needed to further validate the reliability of these measurements in larger series and to identify normal pronation and metatarsal torsion on weightbearing imaging. Further work is required to determine whether addressing pronation reduces recurrence rates and improves outcomes in surgery for hallux valgus.

LEVEL OF EVIDENCE

Level III, retrospective cohort study.

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

BACKGROUND

The importance of the rotational profile of the first metatarsal is increasingly recognized in the surgical planning of hallux valgus. However, rotation in the normal population has only been measured in small series. We aimed to identify the normal range of first metatarsal rotation in a large series using weightbearing computed tomography (WBCT).

METHODS

WBCT scans were retrospectively analyzed for 182 normal feet (91 patients). Hallux valgus angle, intermetatarsal angle, anteroposterior/lateral talus-first metatarsal angle, calcaneal pitch, and hindfoot alignment angle were measured using digitally reconstructed radiographs. Patients with abnormal values for any of these measures and those with concomitant pathology, previous surgery, or hallux rigidus were excluded. Final assessment was performed on 126 feet. Metatarsal pronation (MPA) and α angles were measured on standardized coronal computed tomography slices. Pronation was recorded as positive. Intraobserver and interobserver reliability were assessed using intraclass correlation coefficients (ICCs).

RESULTS

Mean MPA was 5.5 ± 5.1 (range, -6 to 25) degrees, and mean α angle was 6.9 ± 5.5 (range, -5 to 22) degrees. When considering the normal range as within 2 standard deviations of the mean, the normal range identified was -5 to 16 degrees for MPA and -4 to 18 degrees for α angle. Interobserver and intraobserver reliability were excellent for both MPA (ICC = 0.80 and 0.97, respectively) and α angle (ICC = 0.83 and 0.95, respectively). There was a moderate positive correlation between MPA and α angle (Pearson coefficient 0.68, P < .001).

CONCLUSION

Metatarsal rotation is variable in normal feet. Normal MPA can be defined as less than 16 degrees, and normal α angle can be defined as less than 18 degrees. Both MPA and α angle are reproducible methods for assessing rotation. Further work is needed to evaluate these angles in patients with deformity and to determine their significance when planning surgical correction of hallux valgus.

LEVEL OF EVIDENCE

Level III, retrospective comparative study.