Radiographic Factors Associated with Tibial Pain After Expandable Distal Femoral Endoprosthesis in Skeletally Immature Patients: A Retrospective Cohort Study

Background: Limb-salvage surgery using extendable distal femoral endoprostheses has become the standard reconstruction following tumor resection in skeletally immature patients, allowing continued growth and improved function. However, mechanical complications, particularly tibial pain, remain challenging and poorly understood. This study aimed to identify radiographic predictors of tibial pain and evaluate their potential utility in early risk detection. Methods: A retrospective cohort study was conducted of 29 skeletally immature patients (mean age 10.4 years) who underwent expandable distal femoral endoprosthetic replacement between 2008 and 2018 at a tertiary orthopedic oncology center. Standardized radiographs were analyzed at 6 months and final follow-up (mean 75 months) to assess cortical thickness, stem-to-cortex distances, stem migration, stress shielding, pedestal formation, and periosteal reaction. Associations between radiographic parameters and tibial pain were assessed using multivariable logistic regression, t-tests, and chi-square analyses. Results: Seventeen patients (58.6%) developed activity-limiting tibial pain requiring analgesics, as documented during follow-up. Mean medial and lateral cortical thickness increased from 3.0 mm and 3.4 mm to 4.1 mm and 5.1 mm, respectively. The logistic regression model demonstrated strong explanatory power (Pseudo R2 = 0.57, p = 0.004). Medial cortical thickness at last follow-up was the only significant independent predictor of tibial pain (p = 0.042), and was significantly associated with tibial pain. Patients with tibial pain exhibited greater medial cortical thickening (p < 0.001). Stem migration (φ = 0.421, p = 0.065), stress shielding (φ = 0.476, p = 0.044), pedestal formation (φ = 0.608, p = 0.004), and periosteal reaction (φ = 0.569, p = 0.008) were also associated with pain. Conclusions: Medial cortical hypertrophy emerged as a potential radiographic biomarker for tibial pain. after expandable distal femoral endoprosthesis in growing patients. The findings suggest that cortical remodeling, stress shielding, and pedestal formation collectively reflect stem micromotion and bone adaptation. Early radiographic surveillance of these parameters warrants further investigation in prospective studies to determine their clinical utility. Larger multicenter studies are warranted to validate these predictors and refine postoperative monitoring protocols.