Development of a novel external fixation device for inducing knee arthrofibrosis in rats

Arthrofibrosis (AF) is a common pathological condition characterized by joint dysfunction. However, traditional non-invasive external fixation methods are difficult to maintain on the small limbs of rats. This study aimed to develop a novel, non-invasive, and stable rat model of knee arthrofibrosis using a thermoplastic polymer resin. Sixty male Sprague-Dawley rats were randomly assigned into a Sham group and immobilization groups (1, 2, 4, and 6 weeks). Utilizing the material's property of being malleable at high temperatures and rigid at room temperature, a custom-fitted "thigh-crus-trunk" external fixation device was fabricated to immobilize the knee at 135° of flexion. Total, arthrogenic, and myogenic contractures were assessed by measuring the range of motion (ROM). Histopathological changes were evaluated using H&E and Masson's trichrome staining. The expression of fibrotic markers (α-smooth muscle actin, α-SMA and collagen type I alpha 1 chain, COL1A1) in synovial tissues was detected via immunohistochemistry, RT-qPCR, and Western blotting. Biosafety was assessed through histological and serum biochemical analyses of major organs. Prolonged immobilization resulted in a significant decrease in knee ROM, while joint capsule thickness, synovial hyperplasia, and collagen deposition increased, stabilizing after 4 weeks. Analysis revealed that myogenic contracture predominated in the first 2 weeks, whereas arthrogenic contracture became dominant in the later stage. Molecular analysis confirmed a time-dependent upregulation of α-SMA and COL1A1 in synovial tissues. Furthermore, no abnormalities were observed in major organs or serum biochemical indices, indicating favorable biosafety. A novel non-invasive rat model of knee arthrofibrosis was successfully established using thermoplastic polymer resin. This device is cost-effective, user-friendly, stable, and biocompatible. It effectively simulates immobilization-induced joint contracture without surgical trauma, serving as a valuable model for future arthrofibrosis research.