This study designed a reliable and economical animal model for persistent gout arthritis in relative short time, manifested by significantly prolonged gross and behavioral abnormalities, as well as intra-articular MSU deposition and the tophi formation. This research may attract broad interest for researchers focusing on a various of crystalline arthritis.
Animal models of gout are necessary basis for researchers to understand the disease progress and to determine potential treatments and prevention measures. A literature search based on PubMed database was conducted among studies of gouty arthritis over the past decade, more than 120 articles applied model mentioned above to study the pathophysiological mechanism and therapies efficacy. However, due to the lack of appropriate animal models, limited studies are available regarding the formation, growth, attachment and deposition of MSU in vivo, which leaves a research gap in the study of the continuous interaction of crystals within the tissues in vivo.
Intra-articular injection of MSU is an optimal model for acute inflammation, while no report of chronic pathological changes is reported in injection models.[12, 20] Active uric acid oxidase is lost in human but presenting in most mammals (such as rats, rabbits, etc.), which causes that serum uric acid levels of other mammals are only one-tenth of human. [21, 22] Hence, limited dose of MSU injection often cause acute joint inflammation, but the threshold of tophi formation and crystal deposition are rarely reached. Wu et.al introduced a method of homologous recombination in mice embryonic stem cells to establish a mice model lacking urate oxidase. Congenital MSU deposition in kidney were observed in mutated mice, but the mortality rate exceeded 50 percent in the first four weeks after birth. Due to the low survival rate, the difficulty of gene editing and cost of experiment, it is hard to be comprehensively applied for further studies. Other researchers performed intraperitoneal or subcutaneous injection of MSU to simulate gout synovitis and have achieved good results, but this air-pouched method is not suitable for the study of chronic gouty arthritis focusing the changes of synovial membrane, cartilage and bone[24–26]
The present study puts accurate repeatable implantation of larger doses of MSU into the articular cavity and maintains a continuous high concentration of MSU in the articular cavity to resist the effect of uric acid oxidase. Pain and joint swelling are typical symptoms of gout arthritis. In injection model, local symptoms are proved to present as self-limiting and relive within 72 hours. Consistently, The reduction of the PWT in MSU injection models was reported maintained within 48 to 72 hours in previous researches. In contrast, this study significantly prolonged the persistence of local symptoms, with significant synovial swelling and mechanical hyperalgesia more than five days after interventions. This outcome is caused by MSU embedding rather than surgical incision due to the presence of a control group. In terms of pathology, MSU crystal deposition was observed surrounded by IL -1β, TNF -α positive inflammatory cell. This is the first report of exogenous MSU deposition in synovium in animal models into, providing potential tools for clinical identification and debridement. The model also provides an ideal approach for further studying the pathophysiological changes of MSU deposition in vivo and accurate removal of MSU deposition in tissue.
Compared with the injection modeling method, more persistent joint inflammation and crystal deposition are observed in this study for three main reasons, including higher dose, more accurate implantation, and combination with artificial defect. 1) The implantation dose was significantly more sufficient than the traditional injection method. Injection dose distributed between 0.5 mg to 5.0 mg in previous research,[12, 15, 16, 27] while the total embedding dose of MSU in this experiment reached 60 to 100 mg. High-dose MSU intervention, though larger than other injection models, was shown to be safe and no lesions or symptoms outside the joint was found. The results proved that MSU crystals were deposited in the synovium by inflammatory cells before being decomposed by uricase. 2) The space of rats’ knee articular cavity is limited with multiple contents including ligaments, cartilage, and meniscus. In addition, saturated MSU crystals tend to precipitate at room temperature and then block injection needles. Swell and effusion of tissues induced by MSU could further aggravate the stenosis of cavity and increase the failure rate of repeated injection. Minimally invasive approach solved this problem by exposing the joint cavity and locating position of MSU crystals accurately. 3) Previous studies suggested that physic impact would result in traumatic arthritis in rabbits.[28, 29] A radiographic study also found a strong relationship between bone erosion and tophi formation. Therefore, impaired cartilage accelerate the establishment of gouty arthritis model and then shorten the trial period. 
There are some limitations in the present study as the following: 1) The procedures of this minimally invasive operation required practice and repetition in order to get a satisfied result of MSU embedding. Basic surgical training is required to meet the operational requirements. 2) The duration of this model was still much less than the natural course of chronic gouty arthritis in human which lasting for a few years. Further studies have to include a more convenient way to embed MSU and increase the sessions of embedment to extend the affect time. It is not verified how long the model will last without MSU embedding, which will be one of the research directions in the future. 3) In this study, synovial crystal deposition was observed, but tendon, ligament, cartilage, or bone was not involved. Due to the process of sections decalcification, some technical difficulties of preserving crystals from strong acid need to be overcome. Sections of osteochondral specimens should also be analyzed in further researches.