# Effect of distraction length on the morphology of knee cartilage in a rat model of femoral distraction osteogenesis

## TL;DR

This study shows that excessive femoral distraction in rats causes knee cartilage degeneration and osteoarthritis-like changes due to inflammation and bone remodeling.

## Contribution

The study identifies a threshold for femoral distraction beyond which knee joint damage becomes irreversible, linking it to specific molecular pathways.

## Key findings

- 20 mm distraction caused severe cartilage erosion and subchondral sclerosis, unlike 10 mm distraction.
- Group B showed elevated catabolic markers (IL-1β, MMP-13, RANKL) and reduced anabolic markers (COL-II, SOX9, OPG).
- Excessive distraction triggers sterile inflammation and osteochondral uncoupling via IL-1β/MMP-13 and OPG/RANKL pathways.

## Abstract

This study investigated the dose-dependent impact of femoral distraction length on knee joint integrity and characterized the molecular mechanisms driving cartilage degeneration in a rat distraction osteogenesis (DO) model.

Thirty-six Sprague-Dawley rats underwent femoral DO and were randomly assigned to three groups: Control (5 mm), Group A (10 mm), and Group B (20 mm). Following a consolidation phase, knee joint morphology and subchondral bone microstructure were evaluated using digital radiography and micro-computed tomography. Histological assessment included H&E, Safranin O-Fast Green, and Masson’s trichrome staining. Furthermore, immunohistochemical analysis quantified the expression of catabolic (IL-1β, MMP-13, RANKL) and anabolic (COL-II, SOX9, OPG) biomarkers in the articular cartilage and subchondral bone.

Radiographic and histological findings demonstrated successful osseointegration and physiological tolerance in the Control and Group A. Conversely, Group B exhibited severe osteoarthritis-like pathology, including cartilage erosion, proteoglycan depletion and subchondral sclerosis. Quantitative analysis confirmed significantly elevated bone mineral density and bone volume fraction in the subchondral bone of Group B (P < 0.05). Molecularly, Group B showed significant upregulation of catabolic biomarkers (IL-1β, MMP-13 and RANKL) with concurrent downregulation of anabolic biomarkers (COL-II, SOX9 and OPG; P < 0.05).

Extensive distraction (20 mm, ∼50% of original length) surpasses the physiological adaptive capacity of the knee joint, triggering irreversible degeneration via IL-1β/MMP-13-mediated sterile inflammation and OPG/RANKL-driven osteochondral uncoupling. While moderate distraction (10 mm, ∼25% length) remains compensatory, this pathological transition highlights the critical necessity of joint protection strategies when limb lengthening targets exceed ∼25% of the original bone length.

Scientific diagram illustrating distraction osteogenesis in SD rat femurs at 5 millimeters, 10 millimeters, and 20 millimeters, correlating increased distraction to histomorphological knee joint changes, subchondral bone sclerosis, and a mechanistic pathway involving mechanical overload, inflammation, and altered bone nutrient support.