Morphological Characteristics of Bone Callus During Acceleration of Bone Healing in TBI-related Polytrauma

Background. The phenomenon of accelerated fracture healing in patients with cranio-skeletal polytrauma was first described qualitatively in 2005 by Morley. Already existing morphological studies allowed to form a clear hypothesis about the mechanism of formation of heterotopic ossifications after traumatic brain and spinal cord injuries. Morphological analysis of fracture healing is one of the leading methods for identifying the underlying mechanism. Materials and methods. Adult white rats (n=90) were divided into two same size groups: TBI-related polytrauma group and sham group. Harvesting was performed on day 7, 14, and 21. In both groups closed femur fracture was modelled. Modeling of polytrauma in animals of group 1 was achieved by modelling closed TBI followed by fracture of the femur, contralateral to the side of the brain lesion, by method described above. Animals were euthanized on days 7, 14, and 21 after surgery collecting of bone callus in the fracture area and the brain tissue which were examined by both light and electron microscopy. Results. The study of histological sections of the callus in animals with an isolated fracture on the seventh day showed its connective tissue organization. Ultrastructural analysis reveals activated fibroblasts with well-developed membrane organelles in the cytoplasm. Examination of section series from polytrauma group on day 7 demonstrates the presence of skeletal tissue components, such as cartilage and bone, in addition to connective tissue elements. The light-microscopy of the connective tissue components of the callus reveals its vascularization and grouping of collagen fibers into bundles. In animals with polytrauma on the day 14, the callus has cartilaginous and bone structures, while cytoplasm is characterized by pronounced granularity. On the day 21, the callus of animals with an isolated fracture contains islands of cartilage tissue, which is represented by a few isogenic groups of chondrocytes surrounded by a connective tissue framework. The callus of animals with polytrauma on day 21 has elements of calcified bone matrix, which makes it difficult to make sections. Further examination reveals numerous trabeculae with osteocytes in the unligated bone matrix with islands of multinucleated osteoclasts. Conclusions. In animals with polytrauma, the dynamics of transformation is accelerated in time, when cartilage tissue elements and activated osteoblasts are observed on the seventh day, and loci of calcified bone matrix are already detected on the twenty-first day. This dynamics confirms the existing theory of acceleration of fusion in the cranial component of the injury. Further studies of the intermediate bone repair terms are needed.