Modern medicine posits that the pathogenesis of diabetic shoulder periarthritis is complex, and the specific mechanisms of diabetic shoulder periarthritis are still unclear. It is likely related to factors such as the deposition of advanced glycation end products (AGEs) in the shoulder joint, leading to alterations in the microstructure of collagen fibers, increased production of diabetes-related inflammatory factors and proinflammatory mediators, microcirculatory disturbances associated with diabetes, and lipid metabolism disorders(Date and Rahman, n.d.). These factors can induce inflammatory responses and fibrotic changes in the joint capsule surrounding the shoulder joint, resulting in shoulder dysfunction(Struyf et al., 2022). Under hyperglycemic conditions, excessive sugar and protein combine to form AGEs, which accumulate within the body. AGEs cause the deposition of cross-linked collagen within the shoulder joint capsule, resulting in the denaturation of collagen and making tendons and ligaments harder, with reduced strength and elasticity, causing joint stiffness and chronic inflammatory processes, thereby enhancing the inflammatory response of the synovium and ultimately resulting in fibrotic changes in the shoulder joint capsule(Aljethaily et al., 2020; Tuleta and Frangogiannis, 2021). Studies have shown that the content of this abnormal collagen in diabetic patients is at least twice that in nondiabetic individuals of the same age, which is one of the pathological reasons for the progressive changes in the elasticity of the shoulder joint(Shanmugam et al., 2003).
Another potential link between diabetes and shoulder periarthritis is that hyperglycemia may induce the expression of proinflammatory cytokines, which are elevated in the joint capsule and synovium of patients with shoulder periarthritis. Metabolic dysfunction in diabetic patients often affects microvessels, resulting in poor vascular conditions, causing low perfusion in the shoulder joint and subsequently triggering tissue hypoxia, aseptic inflammatory responses, and the release of inflammatory factors and a large number of fibrotic factors, promoting tissue adhesion, exacerbating malnutrition of muscles, bones, joints, and ligaments, and causing enhanced platelet aggregation and increased blood viscosity, increasing the likelihood of shoulder joint dysfunction(Date and Rahman, n.d.; Shanmugam et al., 2003). Lipid metabolic disorders caused by diabetes, such as increased levels of non-high-density lipoproteins composed of low-density lipoproteins and very low-density lipoproteins rich in triglycerides, can cause arterial wall inflammation, inducing atherosclerosis, arterial narrowing, and reduced blood flow to bone and joints, affecting their nutritional supply and making the shoulder joint more susceptible to inflammatory responses following chronic injury(Lui, 2017; Segrest, 2002).
Poulsen RC (Poulsen et al., 2014) placed human tendon cells in low-glucose and high-glucose culture media and treated them with hydrogen peroxide. This study revealed that tendon cells survived and exhibited increased collagen synthesis in low-glucose environments. In contrast, high-glucose environments led to apoptosis and loss of biological activity. Burner T(Burner et al., 2012) used high-glucose pig patellar tendons and reported that tendon collagen is bound by proteoglycans and that high glucose levels reduce the levels of proteoglycans in tendons, causing them to harden. Long-term exposure to high-glucose environments may lead to permanent changes in the composition or structure of the extracellular matrix of tendons(Lin et al., 2017; Snedeker, 2016). Another study on rats showed that high glucose levels induce chronic inflammatory responses characterized by increased cell density in the middle section of the tendon and increased levels of tendon IL-1b (Thomas et al., 2014). Under the influence of high glucose, the collagen fibers of tendons become irregularly arranged and chaotic, with reduced diameter and density. Tendon cells undergo rupture and degeneration, and the spaces between fibers correspondingly expand(Shi et al., 2021). Pathology revealed greater levels of fibrosis in the diabetic model group than in the control group, with the tendons exhibiting irregular arrangements and wavy changes, as well as some inflammatory cell infiltration.
The diabetic shoulder periarthritis model is a composite model, and thus, it is necessary to consider animals that can adapt to both modeling methods. The shoulder periarthritis model was selected through screening by the research team, adopting a continuous strain and ice compression model similar to the pathogenesis of shoulder periarthritis, and convenient and economical New Zealand rabbits were chosen as the experimental subjects. Many animals can be used for the preparation of diabetic models, but the establishment of diabetic models in dogs, pigs, sheep, pigeons, monkeys, etc., is relatively difficult, with limited breeding conditions, and it is challenging to create shoulder periarthritis models on the basis of diabetes. Rodents are often used for the preparation of diabetic models, but due to their small shoulder joints, it is difficult to establish shoulder periarthritis models, and they are often injected with streptozotocin, which is expensive. The New Zealand white rabbit, previously chosen by the research team as an animal model for shoulder periarthritis, has a gentle temperament, is easy to control and operate, and is cost-effective, with a reliable and safe animal source and convenient breeding conditions, thus making it suitable as an experimental subject for the diabetic shoulder periarthritis model.
With respect to the preparation of drug-induced diabetic models, alloxan is currently the most common drug used in the preparation of blood sugar models. Alloxan can now be successfully prepared domestically, and the drug is relatively inexpensive, requires small dosages, has simple injection operations, is sensitive to blood sugar responses, has a high modeling rate, and has a low animal mortality rate, effectively promoting the establishment of diabetic models. The intraperitoneal injection of alloxan in rabbits requires anesthesia, which is inconvenient, so this study used alloxan via auricular vein injection for model production.
Alloxan was synthesized in 1838 and was first discovered in 1943 to cause specific pancreatic islet necrosis in rabbits, with the ability to induce diabetes mellitus. Alloxan is a β-cell toxin that destroys β-cell structures by producing superoxide free radicals, contributing to cell damage and necrosis and resulting in insulin deficiency and experimental diabetes. Sustained hyperglycemia can occur within 24 hours after injection, with irreversible necrosis of β-cells. When alloxan enters the body, pancreatic islet cells are damaged and destroyed, leading to insufficient insulin secretion and increased blood sugar. To balance this, the islets release large amounts of insulin to lower blood sugar. Over time, the demand for insulin exceeds the supply, leading to persistent hyperglycemia. Experimental studies have shown that after the injection of alloxan, there are three stages: the first stage is the hyperglycemic period, which usually lasts 2–4 hours; the second stage is the hypoglycemic period, during which blood sugar begins to decrease and lasts approximately 12–16 hours, a dangerous period during which severe cases may die from hypoglycemia; and the third stage begins the morning after the second day of medication, during which blood sugar levels gradually increase and maintain a state of sustained hyperglycemia(Ighodaro et al., 2017). Therefore, appropriate prevention is needed during the hypoglycemic period, with attention given to the mental changes and reactions of the rabbits, as well as random blood sugar measurements for observation and timely intervention during the hyperglycemic period to avoid excessively high blood sugar levels, which could lead to diabetic ketoacidosis.
After modeling, rabbits exhibit disease symptoms similar to those of humans, such as reduced food intake, weight loss, restricted joint movement, and high fasting blood sugar. In terms of pathological tissue morphology, the diabetic shoulder periarthritis model causes pathological changes in the joints and synovial tissues of rabbits, including the destruction of islets, dissolution and disappearance of islet nuclear cells, disordered arrangement of tendon fibers, dense synovial structure, inflammatory cell infiltration, severe cell proliferation, and severe fibrosis, which are characteristics of both diabetes and shoulder periarthritis, further validating the effectiveness of the model. Moreover, as time progresses, the results show that these pathological changes do not diminish, to some extent indicating that the model is effective in simulating the progression of the disease and that the increase in fibrosis may reflect the chronic development of the disease.
In terms of the degree of fibrosis, both the shoulder periarthritis model group and the diabetic shoulder periarthritis model group exhibited high levels of fibrosis, but the diabetic shoulder periarthritis model group had an even greater degree of fibrosis, indicating that the diabetic shoulder periarthritis model group had more severe fibrosis than the simple shoulder periarthritis model group. According to the semiquantitative statistics of the islet number, compared with those in the control group, both the diabetic model group and the diabetic shoulder periarthritis model group exhibited a significant reduction in islet number, with the diabetic shoulder periarthritis model group showing a greater reduction. This indicates that the model can effectively simulate the impact of diabetes on islets, and the effect on the diabetic shoulder periarthritis model is more pronounced than that on the diabetic model.
In summary, this animal model has been successfully used to simulate both shoulder periarthritis and diabetes. The diabetic shoulder periarthritis model group effectively simulates human diseases and is replicable, operable, economical, and convenient, making it an excellent animal model.
Authors’ contributions
Sichen PENG and Shihui WANG: methodology, investigation, formal analysis, and writing—original draft. Shao-dan CHENG and Cheng GE and Yinghui MA: methodology, investigation, formal analysis, and validation. Zichao XIONG and Yunwen GAO: investigation and formal analysis. funding acquisition, conceptualization, supervision, writing—reviewing & editing, project administration, and data curation.
All authors have read and approved the final manuscript and therefore have full access to all the data in the study and take responsibility for the integrity and security of the data.