In the clinical arena, composite scalp-cranial defects arising from penetrating trauma, burns, tumor resections, radiation, or infections significantly compromise patient function and aesthetics, while producing a huge societal burden 59. The heterogeneity of these conditions demands considerable resources and versatility from reconstructive surgical teams to manage such complex injuries 5. Currently, the gold standard for reconstruction involves the use of autologous materials such as nonvascularized bone grafts, free soft tissue transfers, or combinations of vascularized bone and soft tissue (free composite or chimeric flaps) 60.
However, many of these cases cannot undergo one-stage repair due to factors such as concurrent infections, polytrauma, ongoing radiation therapy, or prior cranioplasty failures, thereby leading to “chronic defects” 3,60. Furthermore, the concept of replacing “like-with-like” in these defects may demand a large donor-site burden. Biomaterial-based reconstruction with cellular-based tissue engineering strategies represents a viable alternative in such circumstances, but it requires extensive validation before clinical adoption 61. Despite the clinical impact, research on composite scalp-cranial defects is sparse, often focusing solely on bone or skin. Thus, there is a clear need for a simple, clinically relevant composite defect model to serve as a testbed for regenerative approaches.
We established an autologous reconstruction model as the positive control, closely aligned with clinical practice. After replanting the cranial bone and utilizing a rotational flap for wound closure, rapid multilayer healing was observed within 3 weeks postoperatively. PPCNg 37, a thermoresponsive biomaterial that transitions reversibly from liquid to solid at 37°C, was applied to provide semi-rigid fixation of the bone graft and potentially accelerate the healing process. The PPCNg has been shown to promote wound closure without inducing a significant inflammatory response 62. However, it is important to note that although composite wound healing is achieved under these conditions, variations in autologous bone graft “take” and osseointegration in our model, which mimics clinical conditions, demand regenerative strategies to improve composite tissue healing. Additionally, in the clinical realm, often autologous options are not available or feasible due to patient factors. Nevertheless, the creation of a rodent positive control that mimics the “gold standard” is a novel feature of our project and establishes a benchmark to which to compare all other therapeutic approaches.
The composite scalp-cranial defect model is a key focus of our study. The primary challenge is maintaining the scalp wound in an unhealed state for a longer duration. Previously, several approaches have been developed to establish chronic skin conditions in rodents. Chen et al. 62 created ischemic wounds within a bi-pedicled dorsal flap using six uniformly placed incisions, demonstrating that wounds in non-necrotic ischemic zones heal more slowly than those on normally perfused skin; however, these effects were transient, delaying only 4 days more than control group, and the technique is unsuitable for used on head. Peirce et al. 13 surgically implanted a metal plate beneath the skin and applied periodic compressions using an external magnet. This method allows control over the size and severity of the injury by varying the number and duration of compressions, replicating features of human chronic wounds such as reduced blood flow, hypoxia, and immune cell influx. However, this technique is unsuitable for cranial defect studies due to interference with micro-CT scans from the metal. While diabetes models are beneficial for studying diabetes-related wounds, they may not be appropriate for trauma studies 14,63,64. Infection models, although relevant, pose challenges in controlling outcomes and may negatively impact further regeneration studies 15.
Our approach employs mechanical resistance to counteract wound centripetal contraction. Over time, chronic wounds exhibit significantly elevated levels of proinflammatory cytokines and matrix metalloproteinases, whereas activities of matrix metalloproteinase inhibitors and growth factors are reduced, thereby decelerating the healing process 65–67. Compared to previously described methods, our method is simpler to implement, more closely aligns with trauma-related composite defects, and effectively extends the duration of a non-healing wound.
In addition to mirroring clinical practice, ease of operation is crucial for animal models; rodents provide this advantage, both in terms of husbandry and surgical procedures. Rodents also provide the advantage of being genetically modifiable and thus the opportunity to evaluate knockout or knock-in effects of essential signaling pathways on composite tissue healing and bone regeneration 68–77.
To inhibit the proliferation of adherent granulation tissue during the splinting process, we employed POC as a barrier and a bio-release medium, combined with Vincristine (cell proliferation decelerator) and Kanamycin (antibiotics). POC has been demonstrated to have minimal cytotoxicity and immune response but increased cell compatibility. In our study, POC did not elicit any related adverse reactions. Instead, it significantly reduced dural adhesions, thereby simplifying the surgical environment for subsequent research.
The longest remaining duration of WO tested was 5 weeks, which showed no difference in wound healing speed between the obturator removed at 5 weeks and those removed at 3 weeks. Thus, we surmise that 3 weeks is sufficient for the obturator to bypass the peak healing period of the skin. This model, capable of maintaining a non-healing state for extended periods, is suitable not only for the development of regenerative strategies but also for investigating inflammatory pathways, and repair mechanisms of chronic composite defects.