A literature review of different pressure ulcer models from 1942-2005 and the development of an ideal animal model

Challenges faced in developing an ideal chronic wound model

INTRODUCTION

Chronic wounds are a major drain on healthcare resources and can lead to substantial reductions in quality of life for those affected. Moreover, they often precede serious events such as limb amputations and premature death. In the long run, this burden is likely to escalate with an ageing population and lifestyle diseases such as obesity. Thus far, the identification of beneficial therapeutics against chronic wounds have been hindered by the lack of an ideal chronic wound animal model. Although animal models of delayed healing have been developed, none of these models fully recapitulate the complexity of the human chronic wound condition. Furthermore, most animals do not develop chronic wounds. Only the thoroughbred racehorse develops chronic ulcers.

AREAS COVERED

In this review, the different characteristics of chronic wounds that highlight its complexity are described. In addition, currently available models reflecting different aspects of chronic wound pathology and their relevance to human chronic wounds are discussed. This article concludes by listing relevant features representative of an ideal chronic wound model. Additionally, alternative approaches for the development of chronic wound models are discussed.

EXPERT OPINION

Delayed models of healing, including the streptozotocin diabetic model, skin flap model and magnet-induced IR models have emerged. While these models have been widely adopted for preclinical therapeutic testing, their relevance towards human chronic wounds remains debatable. In particular, current delayed healing models often fail to fully incorporate the key characteristics of chronic ulcers. Ultimately, more representative models are required to expedite the advancement of novel therapeutics to the clinic.

Three-Dimensional-Printed External Scaffolds Mitigate Loss of Volume and Topography in Engineered Elastic Cartilage Constructs

OBJECTIVE

A major obstacle in the clinical translation of engineered auricular scaffolds is the significant contraction and loss of topography that occur during maturation of the soft collagen-chondrocyte matrix into elastic cartilage. We hypothesized that 3-dimensional-printed, biocompatible scaffolds would "protect" maturing hydrogel constructs from contraction and loss of topography.

DESIGN

External disc-shaped and "ridged" scaffolds were designed and 3D-printed using polylactic acid (PLA). Acellular type I collagen constructs were cultured in vitro for up to 3 months. Collagen constructs seeded with bovine auricular chondrocytes (BAuCs) were prepared in 3 groups and implanted subcutaneously in vivo for 3 months: preformed discs with ("Scaffolded/S") or without ("Naked/N") an external scaffold and discs that were formed within an external scaffold via injection molding ("Injection Molded/SInj").

RESULTS

The presence of an external scaffold or use of injection molding methodology did not affect the acellular construct volume or base area loss. In vivo, the presence of an external scaffold significantly improved preservation of volume and base area at 3 months compared to the naked group (P < 0.05). Construct contraction was mitigated even further in the injection molded group, and topography of the ridged constructs was maintained with greater fidelity (P < 0.05). Histology verified the development of mature auricular cartilage in the constructs within external scaffolds after 3 months.

CONCLUSION

Custom-designed, 3D-printed, biocompatible external scaffolds significantly mitigate BAuC-seeded construct contraction and maintain complex topography. Further refinement and scaling of this approach in conjunction with construct fabrication utilizing injection molding may aid in the development of full-scale auricular scaffolds.

Pressure ulcer healing promoted by adequate protein intake in rats

The effect of protein intake on rat pressure ulcer healing was evaluated. One hundred rats were numbered according to body weight and then they were randomly divided into 4 groups (n=25) using the random number table. After rat models of stage II pressure ulcer were established, they were fed with feed containing different protein levels (10, 15, 20 and 25%). Healing time, pressure ulcer area, body weight, albumin (ALB) and hemoglobin (Hb) levels among groups were compared. Hematoxylin and eosin (H&E) staining was also performed to observe pressure ulcer tissue structure. In the healing process of pressure ulcer, rats with 20% protein intake had the shortest healing time and the smallest pressure ulcer area. Body weight, ALB and Hb levels were much closer to the normal level. H&E staining result also suggested that the pressure ulcer healing degree of rats with 20% protein intake was much better than the others. Adequate protein intake is therefore conducive to pressure ulcer healing, while excessive or insufficient protein intake has negative impact on healing.

Adipose stromal cells repair pressure ulcers in both young and elderly mice: potential role of adipogenesis in skin repair

More than 2.5 million patients in the U.S. require treatment for pressure ulcers annually, and the elderly are at particularly high risk for pressure ulcer development. Current therapy for pressure ulcers consists of conservative medical management for shallow lesions and aggressive debridement and surgery for deeper lesions. The current study uses a murine model to address the hypothesis that adipose-derived stromal/stem cell (ASC) treatment would accelerate and enhance pressure ulcer repair. The dorsal skin of both young (2 months old [mo]) and old (20 mo) C57BL/6J female mice was sandwiched between external magnets for 12 hours over 2 consecutive days to initiate a pressure ulcer. One day following the induction, mice were injected with ASCs isolated from congenic mice transgenic for the green fluorescent protein under a ubiquitous promoter. Relative to phosphate-buffered saline-treated controls, ASC-treated mice displayed a cell concentration-dependent acceleration of wound closure, improved epidermal/dermal architecture, increased adipogenesis, and reduced inflammatory cell infiltration. The ASC-induced improvements occurred in both young and elderly recipients, although the expression profile of angiogenic, immunomodulatory, and reparative mRNAs differed as a function of age. The results are consistent with clinical reports that fat grafting improved skin architecture in thermal injuries; the authors of this published study have invoked ASC-based mechanisms to account for their clinical outcomes. Thus, the current proof-of-principle study sets the stage for clinical translation of autologous and/or allogeneic ASC treatment of pressure ulcers.

SIGNIFICANCE

Adipose-derived stromal/stem cells (ASCs) promote the healing of pressure ulcer wounds in both young and old mice. ASCs enhance wound healing rates through adipogenic differentiation and regeneration of the underlying architecture of the skin.

Husbandry risk factors associated with hock pododermatitis in UK pet rabbits (Oryctolagus cuniculus)

Pododermatitis, often called 'sore hocks', is a chronic, granulomatous, ulcerative dermatitis which most commonly affects the plantar aspect of the caudal metatarsal and tarsal areas. Pododermatitis is a common clinical finding in the pet rabbit population, but no data is available regarding the actual prevalence of this condition in the UK pet rabbit population or possible husbandry-related factors which may predispose pet rabbits to development of this condition. It was the aim of this study to determine the prevalence of pododermatitis within a sample pet rabbit population, and study possible correlations with husbandry, sex, breed and origin of the rabbits. Findings suggested that young rabbits are at a lower risk of pododermatitis compared with older rabbits; female domestic rabbits are more predisposed to pododermatitis than males; and 100 per cent of the neutered females examined showed clinical evidence of pododermatitis. The effect that different types of bedding may have on the prevalence of pododermatitis was also investigated. This study also produced a scoring system which can be used to score clinical cases. Our study is of clinical importance because it helps to recognise many of the factors which predispose pet rabbits to pododermatitis, representing the first step towards increased awareness of this extremely common problem.

The angiogenic peptide vascular endothelial growth factor-basic fibroblast growth factor signaling is up-regulated in a rat pressure ulcer model

The purpose of this study is to investigate the mRNA and protein expression levels of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) in pressure ulcers, and to elucidate the molecular mechanism by which VEGF and bFGF are involved in pressure ulcer formation. A rat model of ischemia-reperfusion pressure ulcer was established by magnetic disk circulating compression method. Real-time fluorescence quantitative PCR and Western blot assays were conducted to detect the mRNA and protein expression of VEGF and bFGF in the tissues of rat I-, II-, and III-degree pressure ulcers, the surrounding tissues, and normal skin. Our study confirmed that the mRNA and protein expression levels of VEGF and bFGF in the tissues of rat I-degree pressure ulcer were significantly higher than that in the II- and III-degree pressure ulcer tissues (P < 0.05). The expression of VEGF and bFGF in the tissues surrounding I- and II-degree pressure ulcers were higher than the rats with normal skin. The expression of VEGF and bFGF in the tissues of rat III-degree pressure ulcer was lower than that in the surrounding tissues and normal skin (P < 0.05). There was a significant positive correlation between change in the VEGF and bFGF. The results showed that with an increase in the degree of pressure ulcers, the expression of VEGF and bFGF in pressure ulcers tissue are decreased. This leads to a reduction in angiogenesis and may be a crucial factor in the formation of pressure ulcers.

Deep tissue injury rat model for pressure ulcer research on spinal cord injury

Many rat/mouse pressure ulcer (PU) models have been developed to test different hypotheses to gain deeper understanding of various causative risk factors, the progress of PUs, and assessing effectiveness of potential treatment modalities. The recently emphasized deep tissue injury (DTI) mechanism for PU formation has received increased attention and several studies reported findings on newly developed DTI animal models. However, concerns exist for the clinical relevance and validity of these models, especially when the majority of the reported rat PU/DTI models were not built upon SCI animals and many of the DTI research did not simulate well the clinical observation. In this study, we propose a rat PU and DTI model which is more clinically relevant by including chronic SCI condition into the rat PU model and to simulate the role of bony prominence in DTI formation by using an implant on the bone-tissue interface. Histological data and imaging findings confirmed that the condition of chronic SCI had significant effect on pressure induced tissue injury in a rat PU model and the including a simulated bony prominence in rat DTI model resulted in significantly greater injury in deep muscle tissue. Further integration of the SCI condition and the simulated bony prominence would result a rat PU/DTI model which can simulate even more accurately the clinical phenomenon and yield more clinically relevant findings.