Staged Reconstruction of Distal Femoral Osteosarcoma in a 10-year-old Girl using a Hybrid Endoprosthesis-Motorised Lengthening Nail Construct: A Case Report.

BACKGROUND: Reconstruction of tumour defects in skeletally immature patients poses a particular challenge due to the resection of growing physes and resulting limb-length discrepancies. Expandable prostheses allow outpatient lengthening with fewer surgical revisions, but the limb is lengthened within the prosthesis rather than native bone. CASE DESCRIPTION: A 10-year-old girl with no medical history presented with distal right thigh pain and swelling. Imaging and biopsy confirmed osteosarcoma. She underwent wide local excision and initial endoprosthetic reconstruction, followed by a planned staged exchange to a hybrid endoprosthesis (BioXpand), in which the usual prosthetic stem is replaced by a motorised intramedullary nail (Fitbone), allowing limb lengthening via distraction osteogenesis. The patient began non-invasive lengthening on postoperative day five (approximately 0.96 mm/day), with the rate adjusted weekly based on clinical and radiological parameters. After 12 weeks, 6 cm of length was achieved, and consolidation took a further 6 weeks before full weight-bearing. Finally, the reconstruction was converted into a definitive static prosthesis once the final limb length was achieved. At the latest follow-up, the patient attends school and participates in sports, reports no pain, and is skeletally mature, with a 1-centimetre leg-length difference. CONCLUSION: This case demonstrates the feasibility of a staged approach to the reconstruction of paediatric osteosarcoma by combining the principles of endoprosthetic reconstruction with distraction osteogenesis. A staged strategy enabled oncologic clearance, growth accommodation, and functional preservation. CLINICAL SIGNIFICANCE: Staged BioXpand reconstruction offers a 'bioexpandable' alternative to expandable endoprostheses by restoring length through distraction osteogenesis rather than metal extension. A modular sequence (primary reconstruction, hybrid nail exchange for outpatient lengthening, then definitive static conversion) can equalise limb length while generating bone stock, supporting future revisions.