Physiologic impact of inflammation in the polytrauma patient

Authors: Chukwuebuka Achebe, Arun Aneja, Melanie Haffner-Lutzner, Gareth Ryan, Prism Schneider, Michel P.J. Teuben, Hans-Cristoph Pape, Justin Haller

Abstract

Polytrauma represents one of the most challenging scenarios in modern trauma care, with fracture healing outcomes that defy conventional expectations. Although isolated fractures typically follow predictable healing patterns, the presence of multiple injuries creates complex interactions that can either accelerate or severely impair bone regeneration. Remarkably, patients with traumatic brain injury often demonstrate enhanced fracture healing with rapid callus formation and shorter time to union, whereas those with systemic inflammatory burden from thoracic or multiorgan trauma frequently experience delayed healing and complications. These paradoxical outcomes reflect distinct biological pathways that are only beginning to be understood. Malnutrition, affecting up to one-third of hospitalized orthopaedic patients, further complicates recovery by impairing both soft tissue and bone healing. Emerging research has identified key molecular mediators including complement factors, inflammatory cytokines, and potentially leptin as critical determinants of healing trajectories. However, translating these laboratory findings into clinical practice remains challenging because of the heterogeneous nature of polytrauma populations and the complexity of coordinating multicenter research. The purpose of this review is to synthesize current understanding of nutritional optimization strategies in polytrauma, delineate the molecular mechanisms underlying both accelerated and delayed fracture healing, explore the unique effects of traumatic brain injury on bone regeneration, and describe the development of collaborative research infrastructure necessary to advance the field.

1. Introduction

Polytrauma—defined as multiple traumatic injuries with at least one being life-threatening—presents unique challenges for fracture healing that extend beyond simple mechanical considerations.^1^ Perhaps most intriguing is the observation that patients with traumatic brain injury (TBI), present in up to 30% of those with long bone fractures, often demonstrate paradoxically accelerated bone healing with profound early callus formation and shorter time to union.^2^ This phenomenon stands in stark contrast to the delayed healing typically seen in polytrauma patients without TBI, where systemic inflammation and metabolic derangements impair normal regenerative processes (Fig. 1).^3^

The complex interplay between neurological injury, systemic inflammation, and tissue repair creates divergent healing trajectories that complicate clinical management. Although TBI appears to trigger beneficial humoral factors that enhance osteogenesis, the broader inflammatory burden of polytrauma—particularly with thoracic trauma—can overwhelm these mechanisms, leading to prolonged inflammation and impaired bone formation.^4^ Compounding these challenges, malnutrition affects up to one-third of hospitalized orthopaedic patients, further compromising both soft tissue and bone healing through protein depletion and metabolic dysfunction.^5^

Understanding these contrasting biological responses is critical for developing targeted therapeutic strategies. This collaborative review synthesizes current research examining how TBI paradoxically enhances fracture healing, elucidates the molecular mechanisms by which systemic inflammation delays bone regeneration, evaluates evidence-based nutritional interventions that can optimize recovery, and describes the multicenter research infrastructure necessary to translate these findings into clinical practice. By integrating these complementary perspectives, we aim to provide a comprehensive framework for understanding and managing the complex healing patterns observed in polytrauma patients.

2. Effects of Traumatic Brain Injury on Bone Healing in Polytrauma Patients

Polytrauma accounts for up to 25% of trauma unit admissions,^1^ with 60% of patients presenting with orthopaedic injuries.^6^ TBI is encountered in up to 30% of patients with upper limb injuries, 17% of patients with tibia fractures, and 28% of patients with femur fractures.^7–9^ The presence of a concomitant TBI has profound implications throughout multiple stages of caring for these patients, including determining when it is safe to operate, safe initiation of thromboprophylaxis, timing of mobilization, and quantifying fracture healing. Interestingly, patients with long bone fractures and concomitant TBI tend to produce a profound amount of early fracture callus, develop more frequent heterotopic ossification, and experience a shorter time-to-union (Fig. 2); however, the underlying mechanisms behind these phenomena are poorly understood.^2,10,11^

2.1. Experimental Evidence

Preclinical studies have explored the effects of TBI on biomechanical strength, histomorphometry, and microarchitecture during fracture healing, demonstrating that the presence of TBI results in formation of up to 2 times the volume fracture callus from 2 weeks onward, and 90% union rate at 4 weeks, compared with a 60% in those without TBI.^12,13^ This accelerated healing response appears to equalize around 8 weeks in rodents.^14^ Although most animal models concur that fracture healing is accelerated, the impact on the biomechanical properties of the healed bone remains unclear.^13,15^ The inherent challenges with studying humans with TBI and fracture has limited the volume of clinical research in this population. Despite this, several authors have confirmed the presence of earlier callus formation and shorter time to union in humans.^10,11,16–18^

2.2. Proposed Mechanisms

Numerous humoral mechanisms have been proposed to explain the observed acceleration in healing. Interestingly, serum transfer from TBI rodents to control rodents, without TBI, results in accelerated healing equivalent to the TBI rodents, suggesting at least a component of peripheral regulation.^19^ Various cytokines, growth factors, hormones, neuropeptides, and neurotrophins have been shown to play a role in the accelerated healing associated with TBI (Table 1). Recently, animal models have suggested that leptin, an adipokine that functions primarily to regulate metabolism, may be one of the central mediators of accelerated healing, as evidenced by impaired healing in leptin-deficient rodents with induced TBI.^20^ Leptin can induce osteogenesis directly by binding to leptin receptors on osteoblasts and also indirectly through upregulation of various growth factors. There is considerably less clinical research surrounding mechanisms given the variety of heterogeneity of the population. Despite this, similar alterations in inflammatory markers, cytokines, growth factors, and hormones have been observed.^2,10,21–23^ Although there is a considerable body of evidence supporting the role of leptin in accelerated fracture healing, this has yet to be confirmed in humans. Only a single study has measured leptin levels; however, between-group comparisons were not performed.^16^ Interestingly, a secondary analysis of their findings suggests an early elevation in leptin levels in patients with combined TBI and fracture compared with controls. Further research surrounding humoral mechanisms in humans is needed, specifically with respect to leptin.

2.3. Clinical Implications

Determining the mechanisms responsible for accelerated fracture healing in patients with combined TBI and fracture has the potential for profound advances in orthopaedic care. Although there are likely numerous mechanisms, isolation of specific key proteins may allow for the development of novel therapeutic targets for patients with impaired fracture healing and nonunion. This is an exciting area of research, with the potential to inform the way we treat impaired healing and to reduce the morbidity associated with delayed union and nonunion.

3. Molecular Mechanisms of Delayed Fracture Healing in Polytrauma

Delayed fracture healing is a significant challenge in polytrauma patients, where the combined effects of multiple traumatic injuries complicate recovery.^24,25^ The physiological demands on these patients—balancing survival, wound healing, inflammation, and tissue repair—are severe. Persistent systemic inflammation is a primary disruptor of normal bone regeneration, with the complex interplay of immune signaling, cytokine release, and cellular response creating a molecular environment where typical healing mechanisms are compromised. In this context, the molecular pathways involved in inflammation and bone repair are altered, slowing or even preventing proper fracture healing.^26^

3.1. Inflammatory Response in Polytrauma

Inflammation is crucial for bone repair, but in polytrauma patients, this response often becomes dysregulated. The initial inflammatory phase of bone healing involves the release of immune mediators, including interleukins (IL-1, IL-6), tumor necrosis factor-alpha (TNF-α), complement factors like C3a and C5a, and transforming growth factor-beta, all essential for initiating repair processes.^26,27^ However, in polytrauma, the inflammatory response is often hyperactivated and prolonged, leading to “systemic inflammatory response syndrome” (SIRS). This excessive inflammatory state causes widespread endothelial and tissue damage,^28^ compromising the controlled, localized inflammation and resolution of inflammation necessary for bone regeneration. A key feature of SIRS is immune cell dysregulation after trauma. Neutrophils and mast cells are recruited in excessive numbers to the injury site, where they release inflammatory cytokines, creating a hostile microenvironment that hinders osteoblast and promote osteoclast function.^3,4,29^ This dysregulated immune cell activity leads to prolonged inflammation, impeding granulation tissue formation, which is essential for proper fracture healing.

3.2. Delayed Fracture Healing in Mice After Thoracic Trauma

Recknagel et al^4^ further investigated molecular and cellular mechanisms underlying disturbed fracture healing in polytrauma patients by using a murine model for a “double hit” a femur fracture combined with a blunt chest trauma. Mice and rats subjected to concurrent thoracic trauma and bone fractures experience delayed or incomplete healing because of an amplified inflammatory state, creating an unfavorable repair environment. Thoracic trauma–induced hypoxia and subsequent hypoxemia trigger excessive complement activation, particularly of anaphylatoxins like C5a, leading to increased recruitment of neutrophils and mast cells to the fracture site. In studies with neutrophil- or mast cell–depleted mice subjected to thoracic trauma and fracture, neutrophil depletion did not resolve the delayed healing, mast cell depletion did.^30^ Moreover, blocking C5aR and IL-6 trans-signaling pathways showed promising results in alleviating delayed bone regeneration.^31–34^ These findings support the hypothesis that excessive complement activation after trauma stimulate release of mast cell–derived mediators like IL-6, which, through interaction with osteoclasts and osteoblasts at the fracture site, delays healing. This pathway may be one mechanism by which additional trauma exacerbates fracture healing complications.

3.3. Therapeutic Considerations

Current evidence shows that delayed fracture healing in polytrauma patients is driven by a sustained, dysregulated inflammatory response involving excess cytokine production and immune cell activation. Addressing these molecular mechanisms through targeted therapeutic strategies may improve healing outcomes and reduce the morbidity associated with prolonged fracture repair.

In particular, research using murine models has highlighted the potential therapeutic role of anti-inflammatory interventions and oxygenation modulators to enhance fracture healing after thoracic trauma. By targeting the dysregulated inflammatory pathways and supporting the transition from inflammation to repair, such treatments could improve outcomes in fracture healing, offering insights that may extend to human polytrauma management.

4. Polytrauma Patients: Soft Tissue Healing

Polytrauma often leads to increased metabolic rates, protein loss, and catabolism in patients.^35^ These physiological changes can contribute to increased wound and soft tissue healing complications.^35^ As a result, ensuring that polytrauma patients meet a number of nutritional requirements is of the utmost importance for soft tissue healing.

4.1. Prevalence and Impact of Malnutrition

Malnutrition has a prevalence between 22% and 33% among hospitalized orthopaedic patients and has been associated with impaired wound and bone healing, increased infection rates, sepsis, progressive weakness, lethargy, delayed union and malunion, longer hospital stays, and delayed physical rehabilitation.^5^ Malnutrition can be present regardless of body mass index, and the detrimental role it plays in the healing of soft tissue wounds is not limited to underweight patients.^36^ Typically seen in those with low skeletal muscle mass and nonvolitional weight loss, “anabolic resistance” leads to a blunted response of muscle protein synthesis to nutrients, propagating a cycle of malnutrition.^36^ Because of the urgency with which polytraumatized patients must often be brought into the operating room, presurgical nutritional optimization is rarely feasible for this population, and as such patients identified as high risk for malnourishment should be promptly identified and treated with aggressive feeding in the postoperative period.^37^

4.2. Nutritional Intervention Strategies

One of the most crucial steps in postoperative optimization for polytrauma patients is protein-calorie supplementation. A diet consisting of complex carbohydrates and 1.2–2.0 g/kg/day of protein has been promoted in previous literature, and several studies have demonstrated decreased complication rates with protein-calorie supplementation after orthopaedic surgery.^38,39^ Amino-acid supplementation also plays an important role. As demonstrated in a randomized controlled trial conducted by Hendrickson et al,^40^ supplementing orthopaedic polytrauma patients with conditionally essential amino acids can lower rates of complications, mortality, and nonunion as compared with standard nutrition alone. Immunonutrition supplementation, which combines carbohydrates and amino acids with nucleotides and fatty acids which have been implicated in immune and inflammatory responses, has been shown to decrease rates of mortality and infection, and length of intensive care unit and hospital stay.^41^ In a retrospective study of 3015 orthopaedic patients, immunonutrition supplementation significantly decreased both rate of infection and length of hospital stay in patients after total knee or hip arthroplasty when compared with controls.^42^ Shumaker et al^43^ also demonstrated decreased readmission rates and total complication rates for patients treated with perioperative immunonutrition supplementation. Between the demonstrated effectiveness of protein-calorie, amino-acid, and immunonutrition supplementation methods in improving outcomes and decreasing complication rates after orthopaedic surgery, the importance of nutritional optimization is abundantly clear. Although each of the 3 methods have proven worthwhile in isolation, combining supplementation strategies may further optimize patients' soft tissue healing.

4.3. Multidisciplinary Approach

To facilitate nutritional optimization postoperatively, a multidisciplinary team is key. Involvement of a variety of health care providers (ie, hospitalists, geriatrics, nurses, and nutritionists) provides the best opportunity for addressing and optimizing the multifaceted management of comorbidities, malnutrition, and complications in the perioperative phase.^44^ Leveraging the expertise of various types of providers has the potential to optimize the management of malnutrition, and thus decrease complication rates and length of stay, and improve overall outcomes for polytrauma patients.

4.4. Cost-Effectiveness

Appropriate nutrition is not just essential to ensure proper wound healing, it has also been shown to reduce health care costs as well. Nutritional optimization is an economical strategy, with minimal risk to patients. For every dollar spent on perioperative nutritional optimization, there is an overall reduction of approximately $52 in hospital costs.^45^ Stated simply, the cost of care for malnourished patients is minor compared with the potential costs of postoperative complications and reoperations.

Malnutrition is common in orthopaedic polytrauma patients and presents a larger barrier to soft tissue healing. Without necessary nutrients and proteins, patients face difficulties in healing soft tissue wounds, increasing the risk of an array of complications such as infection, nonunion, malunion, lethargy, weakness, sepsis, and mortality. Nutritional optimization is critical to promote healing, and through protein-calorie supplementation, amino-acid supplementation, immunonutrition supplementation, or some combination of the 3, postoperative outcomes in polytrauma patients may be significantly improved. Through the use of a multidisciplinary approach, care teams can make use of this extremely cost-effective and low-risk method to improve soft tissue healing, and overall outcomes, in polytraumatized patients.

5. The Value of a Multicenter Serum Data Bank for Trauma Patients

Understanding of the pathophysiology of life-threatening post-traumatic complications such as acute respiratory distress syndrome, sepsis, and multiple organ dysfunction syndrome has improved significantly in recent decades.^46,47^ In addition to cellular dysregulation, post-traumatic humoral factors dictate the post-traumatic immune response. Polytrauma is increasingly seen as a disease and polytrauma patients are sick and should be treated as such.^48^ However, the clinical translation of findings from experimental research to clinical application is still limited. In particular, there is a lack of reliable biomarkers to guide treatment decisions or predict outcomes, and no (immuno) modulatory therapies have found their way into the treatment of trauma patients.^49^

5.1. Challenges in Clinical Trauma Research

The complexity of clinical trauma research is because of its dynamic and multifaceted nature. Both patient and injury characteristics vary widely. Differences in treatment strategies add to the heterogeneity of the data collected. Ethical issues related to the treatment of severely injured patients complicate patient enrollment and may introduce selection bias. The quality of data collection and sampling in unstable patients may be compromised by the circumstances in trauma bay and on the intensive care. In addition, dropout rates are high and long-term follow-up is difficult in (poly) trauma study cohorts. All these factors add to the potential for serious standardization problems, which are intensified in multicenter projects.

Prospective studies or clinical trials are essential next steps to translate findings from controlled (animal) experimental studies to the care of trauma patients. To avoid underpowered studies, which are likely given the heterogeneous patient populations in (poly) trauma, clinical studies require large numbers of patients. Given the limited average volume of severely injured trauma admissions to level 1 trauma center in Western and Central Europe, multicenter studies are mandatory. Despite the introduction of trauma systems and triage guidelines, it still seems easier to centralize data about polytrauma patients than the polytrauma patients themselves.

5.2. Development of the German Multicenter Initiative

To address these challenges, the German Society of Traumatology (DGU) launched a multicenter serum database project in 2011. Several online and in-person meetings were held to define the main research objectives of the multicenter project. The interests of the various research groups involved were brought together, and it was decided to focus project on traumatic injury, inflammation, and coagulation on an organ and cellular level. The consensus primary research objective of the task force (Network Trauma Research) was to further investigate the complexity of the post-traumatic immune response and to potentially identify novel research pathways for exploration.^50,51^

It was attempted to integrate the novel biobank project into the infrastructure of the (DGU) national trauma registry, in which 675 hospitals are involved and more than 30,000 patients are enrolled each year. Herewith, clinical and experimental parameters could be fused. From 2013 on, all participating institutions developed local experimental study protocols, tailored to their local hospital and laboratory infrastructure. Thereafter, ethical approval for local biobanks was obtained, and data collection was initiated.^50–52^

5.3. Logistical Framework

The logistical framework was integrated into the existing laboratory infrastructure at the different participating institutions. The key stages of the process are (i) clinical sampling, (ii) sample storage and parallel clinical data collection, (iii) sample analysis including validation and ongoing quality control, and (iv) data processing. According to the final protocol, sampling begins immediately upon patient admission to the emergency department and includes a follow-up period of up to 10 days. In total, sampling occurs at 6 different time points. To improve standardization, it is preferable to pool material and data as early in the process as possible.^51^

5.4. Current Progress and Achievements

From the beginning on, evaluation-meetings were organized in which experiences were shared and discussed. Protocols have been shared and optimized continuously. This resulted in the successful build-up of local prospective biobanks and structural patient enrollment at the trauma bay occurred in all participating institutions. Patient recruitment increased gradually over time. Pilot projects on specific patient cohorts of trauma were performed, and (pilot-) data were analyzed and presented at structural meetings with all participating institutes. This further resulted in the first publications using local serum databanks.^53^ Additional biobanks have been formed to investigate specific topics such as extracellular vesicles.^54^ In the future, biobanks should be set up to study the most important effector cells of the immune response after a the neutrophils.^48^

In our experiences, difficulties in optimization and implementation of the multicenter sampling workflow are mainly related to legal aspects, such as alignment of standard operating procedures, ethical approval, privacy issues, and data-protection. Regarding data collection and storage, aliquot handling, validation, and standardization with pooled experimental samples are challenging.

5.5. Future Directions

Currently, not all samples and attached clinical data sets are stored centrally, but early sample sharing and centralization of clinical and experimental material should be a next step. To facilitate future data pooling and centralization of data/samples, additional ethical approval and data transfer agreements will be required. Multicenter biobanks are crucial, especially as advances in digitization, automatization, and AI offer new opportunities for managing big data. For now, analysis of data from the multicenter serum data bank may help testing specific hypotheses, extracted from interesting findings from preclinical trauma studies. In the future, the high quality data generated by this project may form the basis for powerful AI-analysis. This is crucial because AI's effectiveness depends on the quality of the data it is provided.

6. Conclusions

Polytrauma fracture healing follows divergent pathways shaped by complex biological interactions. Traumatic brain injury paradoxically accelerates bone regeneration through humoral factors, with leptin emerging as a potential key mediator that induces earlier callus formation and shorter time to union. Conversely, systemic inflammation from thoracic trauma triggers excessive complement activation and mast cell–derived IL-6, creating hostile microenvironments that impair osteogenesis and delay healing. Malnutrition compounds these challenges, affecting up to one-third of hospitalized orthopaedic patients and significantly increasing complication rates. Evidence-based nutritional interventions—including protein supplementation at 1.2–2.0 g/kg/day, amino acid supplementation, and immunonutrition—have demonstrated clear benefits in reducing complications and hospital costs, with every dollar spent on nutritional optimization yielding approximately $52 in savings. The development of multicenter biobanking infrastructure through initiatives like the German trauma registry provides essential frameworks for translating these molecular insights into clinical practice. Moving forward, trauma surgeons must recognize polytrauma as a systemic disease requiring multidisciplinary management. Early nutritional optimization, targeted anti-inflammatory strategies based on emerging biomarkers, and continued investment in collaborative research networks will be essential to improve outcomes in this complex patient population.

Data Availability Statement

There is no publicly available data for this manuscript.