# Evaluating Osteoarthritis Severity in Mice Using μCT-Derived Geometric Indices

## TL;DR

This study introduces new μCT-based geometric indices to accurately assess osteoarthritis severity in mice, offering reliable and quantifiable metrics for disease progression.

## Contribution

The study introduces novel μCT-derived geometric indices for evaluating osteoarthritis severity in mice with high reproducibility and diagnostic precision.

## Key findings

- Distal femoral width-to-length ratio increased significantly in OA mice, indicating osteophyte formation.
- Proximal tibial height-to-width ratio decreased, reflecting cartilage degeneration and bone collapse.
- Indices showed strong reproducibility and were validated histologically and in aged mice with age-related OA.

## Abstract

Murine models of osteoarthritis (OA) are essential for investigating disease mechanisms and for the preclinical development of disease-modifying therapies. However, reliable, easily quantifiable, and objective metrics for assessing OA disease severity in murine models are lacking. This study addresses this gap by developing μCT-derived geometric indices in mice with knee-joint OA. We found that the distal femoral width-to-length ratio increased significantly at 4 and 8 weeks following medial meniscectomy (MMS) in 10–12-month-old middle-aged mice, reflecting osteophyte formation at the distal femur. In contrast, the proximal tibial height-to-width ratio decreased significantly, reflecting articular cartilage degeneration, subchondral bone collapse, and osteophyte formation at the proximal tibia. Similar changes in these geometric indices were also observed in 28-month-old aged mice with knee OA. Importantly, these indices demonstrated strong inter-rater reproducibility, and MMS-induced changes were attenuated in mice with genetic deletion of GM-CSF, a key mediator of OA. Notably, both indices remained unchanged in normal knee joints from young-to-middle-aged mice, enabling robust comparisons across experiments. Collectively, these μCT-based geometric indices provide reproducible and easily quantifiable metrics that overcome challenges in directly quantifying osteophytes and reconcile contradictory μCT-based subchondral bone mass measurements, thereby improving the precision of OA diagnosis and disease severity assessment.

Imaging is the gold standard for diagnosing osteoarthritis (OA). However, it remains challenging to precisely assess OA in murine models. The aim of this study is to establish μCT-based geometric indices for assessing the disease severity of post-traumatic OA (PTOA) and age-related OA (AROA). Following medial meniscectomy (MMS) in adult C57BL/6 mice, distal femoral length remained unchanged, whereas its width increased significantly. As a result, distal femoral width-to-length ratio was markedly elevated at 4 and 8 weeks after MMS (1.33 ± 0.05 and 1.47 ± 0.1, n = 7 and 5 joints, respectively) compared to contralateral normal joints (1.19 ± 0.04, n = 9), yielding an area under the curve (AUC) of 1.0 (95% CI, 1.0–1.0). Similarly, the tibial secondary ossification center (IIOC) height decreased, whereas its width increased after MMS. Consequently, the tibial IIOC height-to-width ratio was significantly reduced at 4 and 8 weeks post-MMS (0.25 ± 0.02 and 0.24 ± 0.02, respectively) compared with a normal joint (0.304 ± 0.011), with an AUC of 1.0 (95% CI, 1.0–1.0). Bland–Altman analysis demonstrated strong interrater reproducibility, with intraclass correlation coefficients of 0.853 (95% CI, 0.681–0.936) for the femoral ratio and 0.887 (95% CI, 0.748–0.952) for the tibial ratio. Notably, these MMS-induced changes in femoral and tibial geometric indices were attenuated in mice with genetic deletion of GM-CSF, a key mediator of OA. Importantly, μCT-derived geometric indices were validated by histological analysis. Furthermore, increased distal femoral width-to-length ratio and reduced tibial IIOC height-to-width ratio were also observed in the knee joints of 28-month-old mice with AROA. In addition, an enlarged patella and a calcified synovium–capsule serve as a reference for AROA and PTOA. Collectively, μCT-based geometric indices are useful and easily quantifiable metrics for assessing disease severity and therapeutic response of PTOA and AROA models in murine. Of note is that the distal femoral and proximal tibial geometric indices were developed based on severe OA induced by MMS, and their applicability to mild OA requires further investigation.