PURPOSE:
The initial interfragmentary motion (IFM) at a fracture site determines the mode of fracture healing. Understanding the impact of orthopaedic interventions on the fracture environment is essential to advancing our knowledge of fracture healing. The purpose of this scoping review is to analyze the orthopaedic literature to assess our understanding of the effects of early axial IFM on fracture healing outcomes.

METHODS:
PubMed, OVID, and Scopus databases were queried to identify all studies from inception until June 2023 assessing axial IFM on fracture healing outcomes in animal and human subjects. We collected information regarding the amount of IFM, osteotomy/fracture location, experimental methodology, and outcomes (histologic, biomechanical, and radiographic evidence of fracture healing) for each study. Data synthesis is presented as a narrative review of our findings.

RESULTS:
In total, 4,972 studies were identified. Fifteen studies were included, totaling 605 fractures/osteotomies. Of the included studies, 423 animal and 182 human subjects were examined. Nine studies investigated IFM at the tibia, four at the metatarsus, and two at the femur. The median time to analysis was nine weeks. The fracture gap size did not exceed 6 mm in any study. The range of IFM in tibias, metatarsi, and femurs was 0.3-2.0 mm, 0.1-2.4 mm, and 0.03-1.0 mm, respectively. No experiment using a femur model identified an association between early axial IFM and healing outcomes. All studies at the level of the tibia exhibited positive effects on callus formation with small-to-moderate axial IFM (mean 0.54, SD 0.30; range 0.2-0.9 mm). Most studies (9/13, 69.2%) found that early micromovement produced superior stiffness and biomechanical rigidity at the fracture site compared to absolute stability. While larger IFMs (mean 1.28, SD 0.70; range 0.25-2.4 mm) frequently led to a larger callus area, the callus quality and biomechanical strength of the callus was compromised.

CONCLUSION:
The definitive range of axial IFM conducive to a favorable healing environment remains elusive. However, preliminary evidence suggests an association between small-to-moderate (mean 0.41, SD 0.32; range: 0.03- 1.0 mm) initial axial IFM for stimulating successful fracture healing. This review found that the cumulative evidence present in the literature is insufficient to determine a definite correlation between the early axial IFM and outcomes.