Animal models for critical-sized long-bone defects and long-bone non-union: a scoping review

Critical-sized long-bone defects and non-unions are among the most challenging complications in fracture care, as they lack the capacity for spontaneous healing and often lead to prolonged disability and repeated surgical intervention. Their complex aetiology involves mechanical, biological, and systemic factors that make consistent clinical management difficult. Preclinical animal models remain crucial for understanding the biological and mechanical factors underlying bone repair. This scoping review summaries existing animal models of long-bone defects and non-unions and identifies methodological gaps affecting reproducibility and clinical translation.

A comprehensive literature search across five databases (PubMed, Web of Science, Embase, Scopus, and BIOSIS) was performed. Preclinical in vivo animal studies with experimentally induced long-bone defects and non-unions that were published prior to October 2024 were included in the search. The following parameters were collected: animal species, number of animals, definitions of "non-union" and "long-bone defect", specific bone studied, experimental setup (e.g., unilateral or bilateral model), method of defect creation, fixation technique, defect size, defect filling, additional therapeutic interventions, infection, experiment duration, non-union rate, and the employed surgical approach (single-stage or multi-stage).

A total of 117 studies were included for analysis, whereby 54 studies addressed non-union and 63 focused on long-bone defects. Amongst the included studies, the most frequently investigated species were rats (40.2%), mice (21.4%), sheep (15.4%), and dogs (11.1%). Definitions for non-union and critical-sized defects varied widely, with the most commonly applied method determining non-union being radiographic analysis (74.4%) and time-based criteria (35%). The fixation methods applied were intramedullary nails (35%), external fixators (22.2%), and plates (28.2%), with the reported average experimental duration across studies of 116 ± 101 days.

Preclinical models for critical-sized defects and non-union remain highly heterogeneous, limiting reproducibility and translational relevance. Clear, species-appropriate definitions, improved reporting of defect and fixation parameters, and alignment of study duration with species-specific healing dynamics are essential to strengthen comparability. While model selection should remain driven by the research question, greater standardisation and the development of biologically faithful non-union models are needed to improve the clinical predictive value of preclinical research.