Genetic variation in mice affects closed femoral fracture pattern outcomes

Intramedullary implant stability affects patterns of fracture healing in mice with morphologically different bone phenotypes

Almost all prior mouse fracture healing models have used needles or K-wires for fixation, unwittingly providing inadequate mechanical stability during the healing process. Our contention is that the reported outcomes have predominantly reflected this instability, rather than the impact of diverse biological conditions, pharmacologic interventions, exogenous growth factors, or genetic considerations. This important issue becomes obvious upon a critical review of the literature. Therefore, the primary aim of this study was to demonstrate the significance of mouse-specific implants designed to provide both axial and torsional stability (Screw and IM Nail) compared to conventional pins (Needle and K-wires), even when used in mice with differently sized marrow canals and diverse genetic backgrounds. B6 (large medullary canal), DBA, and C3H (smaller medullary canals) mice were employed, all of which have different bone morphologies. Closed femoral fractures were created and stabilized with intramedullary implants that provide different mechanical conditions during the healing process. The most important finding of this study was that appropriately designed mouse-specific implants, providing both axial and torsional stability, had the greatest influence on bone healing outcomes regardless of the different bone morphologies encountered. For instance, unstable implants in the B6 strain (largest medullary canal) resulted in significantly greater callus, with a fracture region mainly comprising trabecular bone along with the presence of cartilage 28 days after surgery. The DBA and C3H strains (with smaller medullary canals) instead formed significantly less callus, and only had a small amount of intracortical trabeculation remaining. Moreover, with more stable fracture fixation a higher BV/TV was observed and cortices were largely restored to their original dimensions and structure, indicating an accelerated healing and remodeling process. These observations reveal that the diaphyseal cortical thickness, influenced by the genetic background of each strain, played a pivotal role in determining the amount of bone formation in response to the fracture. These findings are highly important, indicating the rate and type of tissue formed is a direct result of mechanical instability, and this most likely would mask the true contribution of the tested genes, genetic backgrounds, or various therapeutic agents administered during the bone healing process.

A Murine Delayed-Healing Model Associates Immune Response with Functional Bone Regeneration after Trauma

Nonunion and delayed-union fractures pose a significant clinical challenge, often leading to prolonged morbidity and impaired quality of life. Fracture-induced hematoma and acute inflammation are crucial for establishing the healing cascade. However, aberrant inflammatory phenotypes can suppress healing and cause bone resorption. Elucidating these mechanisms is necessary to develop potent immunomodulatory therapies and prevent nonunion. Here, we report a delayed fracture healing model enabling the modulation of interfragmentary strain that mimics the etiology of hypertrophic nonunions to elucidate the role of dysregulated immune response in poor healing outcomes. High interfragmentary strain (>15%) was associated with larger callus volumes but delayed bone healing, increased inflammation, and inferior healing outcomes, while lower strain levels (<5%) corresponded to normal bone healing. In addition, we found distinct differences in the ossification, chondrification, and fibrosis patterns between high and low-strain groups, underscoring the significant impact of strain on the healing process. A comprehensive analysis of the systemic immune response revealed dynamic alterations in immune cell populations and factors, particularly within the early hours and days post-fracture. Several immune factors exhibited significant correlations with various functional healing outcomes, indicating their potential as predictive markers for assessing fracture healing progression. Our results also highlighted the significance of timely resolution of proinflammatory signals and the elevation of pro-regenerative immune cell phenotypes in promoting bone regeneration. Multivariate analysis revealed that CD25+ T-regulatory cells were influential in predicting proper bone healing, followed by CD206+ macrophages, underscoring the pivotal role of immune cell populations in the bone healing process. In conclusion, our study provides valuable insights into the intricate interplay between interfragmentary strain, immune response, and the ultimate outcomes of fracture healing. By shedding light on the underlying mechanisms that drive hypertrophic nonunion pathogenesis, our research lays the foundation for enhanced surgical management of nonunions and offers a promising avenue for developing targeted therapeutic interventions and personalized treatment strategies for individuals suffering from fracture nonunion.

Treatment of Femoral Shaft Pseudarthrosis, Case Series and Medico-Legal Implications

Pseudarthrosis (PSA) is a possible complication of femoral shaft fracture treatment. It is often associated with reduced bone quality and can, therefore, adversely affect quality of life. Its treatment poses a major challenge for orthopaedic surgeons. Several authors have set forth different surgical approaches for the treatment of pseudarthrosis, such as internal fixation with plate and screws, replacement of an intramedullary nail or prosthetic replacement. In cases associated with bone loss, osteopenia, or comminution of fracture fragments, autologous or homologous bone grafts may also be used. The chronic outcomes of the surgical treatment of femoral shaft pseudarthrosis, even when consolidation is achieved, are linked to disabling sequelae of clinical-functional relevance, deserving an adequate medico-legal evaluation. The purpose of this retrospective study is to analyse a clinical case series of patients treated for atrophic femoral shaft pseudarthrosis at the IRCCS Orthopaedic Institute Galeazzi, Milan, Italy, from 2014 to 2020 and their orthopaedic-traumatological and medico-legal implications.

What Did We Learn About Fracture Pain from Animal Models?

Progress in bone fracture repair research has been made possible due to the development of reproducible models of fracture in rodents with more clinically relevant fracture fixation, where there is considerably better assessment of the factors that affect fracture healing and/or novel therapeutics. However, chronic or persistent pain is one of the worst, longest-lasting and most difficult symptoms to manage after fracture repair, and an ongoing challenge remains for animal welfare as limited information exists regarding pain scoring and management in these rodent fracture models. This failure of adequate pre-clinical pain assessment following osteotomy in the rodent population may not only subject the animal to severe pain states but may also affect the outcome of the bone healing study. Animal models to study pain were also mainly developed in rodents, and there is increasing validation of fracture and pain models to quantitatively evaluate fracture pain and to study the factors that generate and maintain fracture pain and develop new therapies for treating fracture pain. This review aims to discuss the different animal models for fracture pain research and characterize what can be learned from using animal models of fracture regarding behavioral pain states and new molecular targets for future management of these behaviors.

Different Responses to Identical Trauma Between BALB/C and C57BL/6 Mice

Background

Different responses to identical trauma may be related to the genetic background of individuals, but the molecular mechanism is unclear. In this study we investigated the heterogeneity of trauma in mice and the potential biological explanations for the differences.

Material/Methods

Compared with other organs, the pathological response of the lung after injury is the earliest and most serious. We used C57BL/6 and BALB/C mice to explore the genetic background of different responses to trauma in the lung. We measured mortality rate, pulmonary microvascular permeability, and Cxcl15 gene expression in BALB/C and C57BL/6 mice before and after blast-wave injury. Microvascular permeability was measured using a fluorescent tracer, and Cxcl15 gene expression level and expression distribution were measured using fluorogenic probe quantitative polymerase chain reaction and northern blot.

Results

C57BL/6 mice showed lower mortality rates and pulmonary microvascular permeability than BALB/C mice after blast-wave injury; there was no significant difference in the permeability before blast-wave injury. The Cxcl15 gene was expressed specifically in the lung tissue of mice. The level of Cxcl15 expression in BALB/C mice was higher than in C57BL/6 mice before and after injury, and the variation trend of Cxcl15 expression level after injury was significantly different between BALB/C and C57BL/6 mice.

Conclusions

Our results indicated that BALB/C and C57BL/6 mice had significant heterogeneity in posttraumatic response in terms of mortality and degree of lung damage. The differences in genetic factors such as Cxcl15 may have played a role in this heterogeneity.