Metaphyseal Fracture Healing follows Similar Biomechanical Rules as Diaphyseal Healing

It is generally supposed that the pattern of fracture healing in trabecular metaphyseal bone differs from that of diaphyseal fractures. However, few experimental studies to date have been performed, even though clinically many fractures occur in metaphyseal bone. Particularly, the influence of biomechanical factors has not yet been investigated under standardized conditions. Our aim was to correlate the interfragmentary strain (IFS) with the bone-healing outcome in a controlled metaphyseal fracture model in sheep. Twelve sheep received a partial osteotomy in the distal femoral condyle close to the trochlea. The determination of the IFS by in vivo X-ray analyses and a finite element model revealed that the deflection of the osteotomy gap by the patello-femoral force during walking provoked increasing strains of up to 40%. Bone healing was evaluated after 8 weeks by the assessment of the bone mineral density and by histomorphometry in regions of interest that displayed differing magnitudes of IFS. In areas with strains below 5% significantly less hone formation occurred compared to areas with higher strains (6-20%). For strains larger than 20% fibrocartilage layers were observed. Low IFS (<5%) led to intramembranous bone formation, whereas higher strains additionally provoked endochondral ossification or fibrocartilage formation. It is therefore proposed that metaphyseal bone healing follows similar biomechanical principles as diaphyseal healing. (C) 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:425-432,2011