123: Comparison of Pain in Rat Femoral Nonunion Models with Different Fixation Stability

Topic: Basic Science
Subtopic: Pain category (ICD-11), Acute musculoskeletal pain, , , , , , , , , , Assessment and Measurement of Pain/Nociception, Animal imaging (includes Ca2+ imaging, fluorescence microscopy)

Background and Aims: Bone union is the primary goal of fracture treatment, yet nonunion can occur despite appropriate therapy, often resulting in persistent pain and reduced activities of daily living. However, some nonunion are associated with only mild pain, and the mechanisms underlying this variability remain unclear. Clarifying the sources of pain in nonunion may enable the development of new therapeutic strategies and less invasive treatments. This study aims to investigate how mechanical stability influences pain in a rat femoral nonunion model by comparing bone union and nonunion under different fixation conditions and characterizing their pain-related features.

Methods: Twelve-week-old male Sprague–Dawley rats underwent a femoral shaft fracture stabilized with plates and screws. Nonunion was induced by inserting a rubber sheet into the fracture gap. Fixation stability was varied by screw configuration to create small- and high-instability models. Five groups were studied: sham, small instability–union, small instability–nonunion, high instability–union, and high instability–nonunion. Fixation strength was assessed by three-point bending tests. Bone union was evaluated by X-ray at 8 weeks, and pain behavior was assessed by bilateral hindlimb weight-bearing measurements every 2 weeks. CGRP and TrkA expressions in L3 dorsal root ganglion neurons were evaluated at 4 and 8 weeks.

Results: Three-point bending tests showed significantly higher stiffness in the small-instability group than in the high-instability group. Radiographs confirmed bone union in union groups, while nonunion groups developed pseudoarthrosis. Hindlimb weight-bearing decreased in all groups at 2 weeks and recovered from 4 weeks, except in the high-instability nonunion group, which showed no improvement through 8 weeks and remained significantly lower than other groups. At 8 weeks, the proportions of CGRP- and TrkA-positive cells were significantly higher in the high-instability nonunion group, indicating enhanced nociceptive signaling under unstable nonunion conditions.

Conclusions: This study developed a novel fracture model with varying mechanical stability and bone union status. In the small-instability nonunion group, pain-related behavior improved by 4 weeks postoperatively. In contrast, the high-instability nonunion group showed persistently poor weight bearing and significantly increased proportions of CGRP- and TrkA-positive cells, indicating enhanced nociceptive signaling. These findings suggest that insufficient mechanical stability at the fracture site leads to prolonged pain in nonunion, highlighting the importance of fixation strength in regulating pain outcomes.

References: 1. Kasai Y, Aso K, Izumi M, et al. Increased calcitonin gene-related peptide expression in DRG and nerve fibers proliferation caused by nonunion fracture in rats. J Pain Res. 2021:14 3565-3571
2. Hashidate H, Kamimura M, Nakagawa H, et al. Pseudoarthrosis of vertebral fracture: radiographic and characteristic clinical features and natural history. J Orthop Sci 2006;11: 28-33
3. Mictchell SAT, Majuta LA, Mantyh PW. New insights in understanding and treating bone fracture pain. Curr Osteoporos Rep. 2018;16:325-332

Presenting Author: Shuhei Mizobuchi
Submitting Author: Shuhei Mizobuchi

Senior Author: Masahiko Ikeuchi

IASP 2026 Global Year theme of Neuropathic Pain: Yes

• Koji Aso: MD, PhD - Kochi University
• Masashi Izumi: MD, PhD - Kochi University
• Masahiko Ikeuchi: MD, PhD, Professor - Kochi University