Discussion

As a common emergency in the field of trauma, ACS has a high incidence of fractures and crush injuries [3]. Its pathological feature is that pressure within the confined fascial chamber increases dramatically, triggering a cascade of microcirculatory disorders and tissue ischemia [22]. Due to the hidden symptoms in the early stage, the diagnosis is easily delayed. If not intervened in time, ACS will not only block the blood circulation of the limbs, leading to muscle necrosis and even amputation, but also induce rhabdomyolysis, which will lead to acute renal failure and systemic inflammatory response syndrome, seriously threatening patients' lives [1, 24, 25]. Once ACS is diagnosed, immediate action should be taken to reduce the ICP and restore blood circulation [22]. Although fasciotomy and decompression are the main treatments for ACS, it is not suitable for all patients, and risks such as infection and poor wound healing exist in the postoperative period [26, 27]. In recent years, some scholars have also believed that ACS treatments were too aggressive. Most patients do not need surgical incision and decompression, and ICP can be reduced spontaneously, especially in patients with blisters after limb injuries, but they suffer from huge surgeries, prolonged hospitalization, and high medical costs [28, 29]. Therefore, the construction of experimental models that can accurately simulate the pathogenesis of ACS is essential for early diagnosis and the development of more effective therapeutic strategies.

Current modeling and manometry methods have many limitations in ACS research. In animal models, the anatomical and physiological differences among rats, rabbits, and human fascial compartments render the simulated ACS process inconsistent with reality [27]. Especially, rats are the most frequently used experimental subjects due to their small size and low cost, suitable for large‐scale experiments [16]. However, the rat fascia has excessive elasticity and is prone to significant swelling, and the elevation of ICP and hindlimb ischemia are not obvious [13]. Our pretest found that rats without plaster after injury induction showed no significant fluctuation in ICP, slight cellular swelling, and that the injury subsided on its own after about 3 days without further injury. Therefore, in the present study, we applied plaster to limit the expansion of rat fascia, thereby more realistically simulating the role of limited fascial expansion in the progression of ACS. Currently, the most used method to construct the model is the intrafascial fluid infusion method, which centers on a continuous fluid drip through the intrafascial compartment to simulate the high ICP pathology after trauma [27]. This method has obvious limitations, including the difficulty of maintaining a stable pressure, and the uninterrupted drip and the systemic reaction triggered by fluid absorption, which can hardly be ignored in the results. Alternatively, the use of a catheterized balloon placed within the fascial compartment or an inflatable cuff around the limb to simulate the clinical state of high ICP is also a classic modeling method [19]. In practice, however, the placement of a balloon within the fascial compartment disrupts the fascial structure and may introduce additional inflammation or edema. The inflatable cuff surround makes it difficult to simulate the pressure gradient distribution of ICP due to bleeding or edema in clinical ACS, and the mechanical artificial pressure only simulates the result of elevated pressure, ignoring important pathological mechanisms such as posttraumatic systemic inflammatory response and coagulation changes. Some studies have attempted to elevate the ICP with fracture followed by crush to form the ACS [27]. However, our pre‐experiment revealed that fracture trauma tends to disrupt the integrity of the fascial compartment. Although it is clinically accepted that fascial integrity does not affect ICP, the effect was significant in the rats, leading to poor reproducibility [30]. Another study simulated ACS by blast‐induced fracture, which required surgical suture after fascial compartment disruption, and then built a high‐pressure environment with a high impact of manual intervention. Moreover, this study measured only pressure and lacked blood indices and pathology to provide strong evidence of secondary damage [13]. Compared with other studies, the present study used direct crush injury to develop a model that more closely matched the pathogenesis of clinical fractures. With the gradual increase in intrafascial pressure, the muscle tissue showed initial signs of edema and ischemia, as well as serious lesions such as swelling and fracture of muscle fibers, which were highly consistent with the biopsy results from clinical tissues [31, 32]. Meanwhile, with the self‐developed balloon catheter, high‐frequency continuous pressure measurement of intrafascial pressure can be realized after one time of zero adjustment, which avoids the interference of local pressure and inflammatory manifestations caused by the traditional methods, such as repeated injection of fluids, and is able to accurately capture the details of pressure changes [27, 33, 34]. In addition, the modeling process in this study is greatly simplified, eliminating reliance on complex surgery and specialized equipment, reducing experimental cost and difficulty, and improving reproducibility and stability. Notably, our choice of injury method and the use of plaster immobilization were not arbitrary but were informed by multiple pre‐experiments and existing research evidence. In our pre‐experiments, simple crush injury, bone injury, or ischemia‐only intervention (without plaster immobilization) induced only mild hindlimb swelling in rats, with no significant fluctuations in ICP that met the diagnostic criteria for ACS. The fundamental reason for this phenomenon is that the fascial extensibility of rat hindlimbs is far higher than that of human lower extremities, which is a well‐recognized species‐specific limitation of rat ACS models. Thus, plaster immobilization was applied in our model to simulate the low‐extensibility fascial compartment structure of humans, which is a necessary design to obtain a clinically relevant ACS model with measurable and diagnostic ICP elevation.

In our model, pressure and blood flow in the rat hindlimb fascial cavity exhibited a distinct pattern of change following simulated injury. The pressure reached its maximum value in about 80 min, and the blood flow started to decrease after returning to the level of the Sham group in about 30 min. The phenomenon suggested that the decrease in blood flow depends not only on the peak pressure but also on vascular status and local inflammatory manifestations [20, 35]. During the rise of ICP, blood vessels were gradually compressed, and blood flow was gradually reduced by the peripheral inflammatory reaction and changes in the state of the vessels themselves. Our pretest data showed that the ICP could be maintained at 60 mmHg with little change over 2–8 h after injury, and the perfusion volume did not change much, so we cut off the measurement time for ICP and blood flow at 2 h. We chose 8 h as the cutoff time for the overall observation because muscle necrosis typically appears after 6 h of ischemia. Continued observation for 6 h after ACS formation, that is, until 8 h postinjury, revealed that pathological and hematologic findings were more severe at this stage than before, confirming that elevated ICP and decreased blood flow after ACS formation can lead to further secondary damage. In contrast to other studies, the ICP trend in this model was similar to that observed in clinical ACS, with a rapid pressure‐increase phase and a plateau phase [16, 36]. Most studies have also observed a gradual decrease in blood flow as ICP increases. However, the present study clarified the three‐step changes in perfusion and the quantitative pressure‐blood flow relationship, reflecting the body's initial compensatory mechanism and subsequent compensatory failure. A significant decrease in early blood perfusion was observed in a similar trauma model, corresponding to a decrease in blood perfusion at the initial stage of injury in the present study, but which did not detail the subsequent recovery [37]. We found that compensatory recovery of blood perfusion occurred in the ACS group at approximately 27 min, likely because the body sensed ischemic and hypoxic signals and initiated vasodilation and collateral circulation opening to maintain blood supply [38]. From about 30 min onwards, limb perfusion gradually decreased as ICP rose, and perfusion entered a plateau after about 80 min. These suggested that elevated ICP had a continuous effect on limb perfusion and that raising ICP to a certain level would break the body's compensatory mechanisms. The combination of blood flow and ICP showed perfusion was negatively correlated with ICP when ICP was > 30 mmHg, consistent with previous findings and confirmed that a change in ICP was a key factor affecting hindlimb blood flow [14]. These results demonstrated that an increase in ICP beyond the threshold would decrease perfusion and trigger ischemia and hypoxia‐induced injury in muscle tissue, suggesting that clinical treatment requires accurate monitoring of ICP and providing new avenues for exploring the pathological mechanisms and therapeutic strategies of ACS.

To explore the pathology and changes of muscle necrosis, the ACS model was analyzed with H&E staining and biochemical indexes. The results showed that the histopathological changes were obvious at all time points after injury. From basically normal at 0 h after injury, to mild swelling of some muscle fibers at 1 h, increasing swelling at 2 h, increasing swelling, disordered alignment, and inflammatory cell infiltration at 3 h, and finally to severe destruction of muscle fibers and a strong inflammatory reaction at 8 h. Compared with other research models, this process is clearer and more gradual, and is closer to the pathologic changes in muscle tissue of clinical patients [27]. The proinflammatory cytokines IL‐1β and TNF‐α, muscle damage index CK, renal function index BUN, and anaerobic metabolites LAC and lipid peroxidation products MDA increased over time after injury, following a typical time‐dependent pattern, which was highly compatible with the results of continuous ICP and blood flow tracking. In the first 3 h, no significant secondary damage was observed due to the short duration of ischemia. In the 8 h, there was a significant positive result, and the index change was consistent with the degree of inflammatory reaction and muscle damage on H&E staining. Compared with other studies, the magnitude and timing of the index change in this model were more specific, and it could more accurately and reliably reflect the biochemical and pathological changes of ACS and provide a quantitative basis for assessing disease progression [14, 39].

Despite the promising features of our novel ACS model, several limitations should be acknowledged. First, due to the inherent anatomical differences between rodent and human hindlimbs—particularly the higher extensibility of rat fascia—we applied a plaster cast to simulate the low‑compliant human fascial compartment. While this manipulation was necessary to achieve clinically relevant ICP elevation, it introduces an external constraint that may not fully replicate human ACS pathophysiology. Second, our model focused on the early phase of ACS (within 8 h postinjury) to investigate the window for potential interventions; consequently, long‑term outcomes such as neuromuscular dysfunction, fibrosis, and functional recovery were not assessed. Finally, functional outcome measures (e.g., pain behavior and motor function) were not included, which limits the model's clinical relevance. Future studies using larger animals, longer observation periods, more specific biomarkers, and functional assessments are warranted to further validate and refine this model.