Rib fractures (RF) represent the most common bone fracture after blunt trauma, occurring in 10–20% of all trauma patients and leading to concomitant injuries of the inner organs in severe cases. However, a standardized classification system for serial rib fractures (SRF) does still not exist. Basic knowledge about the facture pattern of SRF would help to predict organ damage, support forensic medical examinations, and provide data for in vitro and in silico studies regarding the thoracic stability. The purpose of our study was therefore to identify specific SRF patterns after blunt chest trauma. All SRF cases (≥3 subsequent RF) between mid-2008 and end of 2015 were extracted from the CT database of our University Hospital (n=383). Fractures were assigned to anterior, antero-lateral, lateral, postero-lateral, and posterior location within the transverse plane (36° each) using an angular measuring technique (reliability ±2°). Rib level, fracture type (transverse, oblique, multifragment, infracted), as well as degree of dislocation (none, </≥ rib width) were recorded and each related to the cause of accident. In total, 3747 RF were identified (9.7 per patient, ranging from 3 (n=25) to 33 (n=1)). On average, most RF occurred in crush/burying injuries (15.9, n=13) and pedestrian accidents (12.2, n=14), least in car/truck accidents (8.8, n=76). Altogether, RF gradually increased from rib 1 (n=140) towards rib 5 (n=517) and then decreased towards rib 12 (n=49), showing a bell-shaped distribution. More RF were detected on the left thorax (n=2027) than on the right (n=1720). Overall, most RF were found in the lateral (33%) and postero-lateral (29%) segment. Posterior RF mostly occurred in the lower thorax (63%), whereas anterior (100%), antero-lateral (87%), and lateral (63%) RF mostly appeared in the upper thorax. RF were distributed symmetrically to the sagittal plane, showing a hotspot (up to 98 RF) at rib levels 4 to 7 in the lateral segment and rib level 5 in the antero-lateral segment. In the car/truck accident group, 47% of all RF were in the lateral segment, in case of frontal collision (n=24) even 60%. Fall injuries (n=141) entailed mostly postero-lateral RF (35%). In case of falls >3 m (n=45), 48% more RF were detected on the left thorax compared to the right. CPR related SRF (n=33) showed a distinct fracture pattern, since 70% of all RF were located antero-laterally. Infractions were the most observed fracture type (44%), followed by oblique (25%) and transverse (18%) fractures, while 46% of all RF were dislocated (15% ≥ rib width). SRF show distinct fracture patterns depending on the cause of accident. Additional data should be collected to confirm our results and to establish a SRF classification system

Summary Statement. MCP-1/ CCR2 axis at the early phase plays a pivotal role in the fracture healing. Inflammation plays a pivotal role in fracture healing. Among them, chemokines play key roles in inflammation. Monocyte chemotactic protein-1 (MCP-1), via its receptor C-C chemokine receptor 2 (CCR2), acts as a potent chemoattractant for various cells to promote migration from circulation to inflammation site. Thus, the importance of MCP-1/CCR2 axis in fracture healing has been suggested. However, the involvement of MCP-1/CCR2 axis tofracture site is not fully elucidated. Results. PCR Array: The expression of MCP-1 and MCP-3 had increased on day 2 than 0 or 7 in the rib fracture healing. Immunohistochemistry Staining: To verify the localization of MCP-1 expression, we examined the Wild type (WT)-mouse rib fracture healing. We observed high expression of MCP-1 and MCP-3 at the periosteum and the endosteum on post-fracture day 3. In vivo Antagonist Study: To elucidate whether MCP-1/CCR2 axis is involved during the early phase of fracture healing, we continuously administered RS102895, CCR2 antagonist, before or after rib fracture. Micro-CT analysis showed delayed fracture healing in the before-group compared with both the control and after-group. On day 21, the hard callus volume in the before-group was significantly smaller than that in the control-group. Histological analysis showed that fractures in both the control and the after-groups were healed by day 21. In contrast, less of cartilage in the callus was observed in the before-group on day 7. Gain of Function: To examine the roles of MCP-1 at the periosteum and the endosteum during the fracture healing, we created a segmental bone graft exchanging model. The bone grafts were transplanted from MCP-1. −/−. mice to another MCP-1. −/−. mice (KO-to-KO). Micro-CT analysis showed that KO-to-KO transplantation led to the delay of fracture healing on day 21. Next, we created exchanging-bone graft models between MCP-1. −/−. and WT mice, in which a segmental bone derived from a WT mouse was transplanted into a host MCP-1. −/−. mouse (WT-to-KO). In contrast to KO-to-KO bone graft transplantation, the transplantation of WT-derived graft into host KO mouse resulted in a significant increase of new bone formation on day 21. Histological analysis revealed that marked and localised induction of MCP-1 expression in the periosteum and the endosteum around the WT-derived graft was observed in the host MCP-1. −/−. mouse. Loss of Function: To validate whether MCP-1 is a crucial chemokine for fracture healing, we created WT-to-WT and KO-to-WT bone graft models. When WT-donor graft was transplanted into WT-host, abundant new bone formation was observed around a WT-derived graft on day 21. In contrast, transplantation of KO-derived graft into WT-host resulted in a marked reduction of periosteal bone formation on a donor graft. Discussion. In this study, we demonstrated that MCP-1/ CCR2 axis at the early phase modulates the fracture healing. Furthermore, we showed that MCP-1 in the periosteum and the endosteum promotes the fracture healing in vivo. Thus, these results clearly suggest that MCP-1 in the periosteum and the endosteum at the early inflammatory phase is an essential component for successful fracture healing