# Staging classification of atypical ulnar fractures

## TL;DR

This study classifies atypical ulnar fractures into three stages based on imaging, showing a progression from bone thickening to complete fractures with sclerosis.

## Contribution

A novel staging classification system for atypical ulnar fractures based on imaging characteristics is proposed.

## Key findings

- AUFs progress from cortical thickening to complete fractures with osteosclerosis.
- Two fractures progressed from Stage I to Stage II during follow-up.
- The classification system may help guide treatment decisions for AUFs.

## Abstract

Atypical ulnar fractures (AUFs) are rare fractures associated with long-term bisphosphonate use; their progression pattern is not understood. This study classified AUFs based on imaging characteristics, revealing a progression from cortical bone thickening to complete fractures with osteosclerosis over time. This classification may assist in determining appropriate treatments for AUFs.

The progression and characteristics of AUFs remain unclear, and no definitive treatment method has been established. This study aimed to classify AUFs based on imaging findings to elucidate their characteristic progression patterns.

We retrospectively reviewed 12 AUFs in 11 patients who had used bisphosphonates in the long term. Based on imaging characteristics, we classified the fractures into 3 stages: Stage I (incomplete fractures with cortical thickening or fracture lines only in the dorsal cortex), Stage II (complete fractures with fracture lines extending to the ventral cortex), and Stage III (complete fractures with osteosclerosis).

At the initial examination, 2 AUFs were classified as Stage I, 8 as Stage II, and 2 as Stage III. Two fractures progressed from Stage I to Stage II during follow-up. Our imaging analysis showed a consistent pattern, suggesting that AUFs begin with cortical thickening in the dorsal cortex, progress to fractures extending to the ventral cortex, and develop sclerosis resembling pseudoarthrosis.

AUFs begin with cortical bone thickening and progress to complete fractures. Over time, these complete fractures can become sclerotic, resembling pseudoarthrosis.