# Radiographic Classification of Hallux Valgus Interphalangeus Based on the Center of Rotation of Angulation (CORA): Prevalence, Reliability, and Surgical Implications

## TL;DR

This study introduces a new classification for a toe deformity called hallux valgus interphalangeus based on the center of rotation of angulation, which could improve surgical planning.

## Contribution

A novel radiographic classification for hallux valgus interphalangeus based on the center of rotation of angulation is proposed.

## Key findings

- 72.3% of the studied population had hallux valgus interphalangeus.
- The most common deformity type had the center of rotation at the distal end of the proximal phalanx.
- The classification may refine surgical planning for this deformity.

## Abstract

Introduction: Hallux interphalangeus (also called hallux valgus interphalangeus (HVI)) is a deformity characterized by lateral deviation of the distal phalanx of the great toe relative to the proximal phalanx. It is often seen in conjunction with hallux valgus but can occur in isolation. It is classically measured by the hallux interphalangeal angle (HIA), which is formed from the longitudinal axis bisections of the proximal and distal phalanges.

Aim: To recognize, discuss, and propose a classification for HVI considering the concept of center of rotation of angulation (CORA).

Methodology: We performed a retrospective analysis of pedobarographs performed at a single institution from December 2019 to December 2021. Inclusion criteria consisted of anterior-posterior view of foot X-rays of patients aged 18 years or older (skeletally mature). All radiographs had been performed with weight bearing. Excluded were those radiographs where a sagittal plane (flexion) deformity was seen on the lateral radiograph to reduce confounding factors. A total of 80 radiographs were collected from the patients included in this study.
Results: The radiographical evidence showed that the prevalence of “normal” hallux in the patient cohort was 27.69% (n=18). The percentage of feet with HVI was 72.30% (n=47). Within the population who presented with HVI, it was noted that the commonest deformity, with 30.76% (n=20), presented with the CORA at the distal end of the proximal phalanx (P3). The second most prevalent type of HVI is with the CORA at the midpoint of the proximal phalanx (P2) (15.3%) (n=10). The remaining classes had a CORA based at the proximal end of the proximal phalanx (P1) at 10.7% (n=7), a CORA at the base of the distal phalanx (P4) (9.23%) (n=6), or a CORA within the distal interphalangeal joint (P5) (6.15%) (n=4).

Conclusions: Hallux interphalangeus is a clinically significant deformity that often coexists with or mimics hallux valgus. This study introduces a novel classification for HVI based on the CORA. We identified five deformity types and their distribution. This CORA-based classification provides a new anatomical framework for HVI, which may refine surgical planning beyond standard osteotomy techniques. Thus, a thorough understanding of the deformity’s etiology, radiographic assessment, and individualized treatment planning plays an essential role in achieving favorable outcomes.