This study aimed to develop a rabbit model of knee contracture in extension and investigate the natural history of motion loss and time-dependent changes in the joint capsule after immobilization. We immobilized the unilateral knee joints of 32 rabbits by maintaining the knee joint in a plaster cast at full extension. Eight rabbits were euthanized at 2, 4, 6, and 8 weeks after casting, respectively, and the lower extremities were disarticulated at the hip joint. Eight control group rabbits that did not undergo immobilization were also examined. We assessed the progression of joint contracture by measuring the joint range of motion, evaluating the histologic alteration of the capsule, and assessing the mRNA levels of transforming growth factor β1 (TGF-β1) in the anterior and posterior joint capsules. After 2 weeks of joint immobilization, the knee joint range of motion was limited, the synovial membrane of the suprapatellar and posterior joint capsules was thickened, the collagen deposition was increased, and the mRNA levels of TGF-β1 were elevated in the anterior and posterior joint capsules. These changes progressed rapidly until 6 weeks of immobilization and may advance slowly after 6 weeks. Joint contracture developed at the early stage of immobilization and progressed over time. The changes in the anterior and posterior joint capsules after joint immobilization may contribute to the limitation in flexion. The elevated mRNA expression of TGF-β1 may be related to joint capsule fibrosis and may be one of the causes of joint contracture.

Stretching combined with ultrashort wave treatment was effective in improving joint range of motion, reducing the biomechanical, histological, and molecular manifestations of joint capsule fibrosis in a rabbit model of extending joint contracture.

It was with great interest that I read the study entitled 'Risk factors for surgical site infection and urinary tract infection after spine surgery.' published online in June 2016 in European Spine Journal [1]. In this paper, the authors report that risk factors for post-operative surgical site infection and urinary tract infection were operative time and ASA classification 3. It is an interesting study. Nevertheless, we have some queries which we would like to communicate with the authors.As is a retrospective observational study, only association and not causation could be inferred from the results of the above cohort study. It was not possible to measure and then control for the confounding via statistical analysis in the retrospective study. In this study, the authors did not exclude risk factors of UTIs preoperative such as urinary tract abnormalities, blockages in the urinary tract, a recent urinary procedure and asymptomatic UTIs, which may affect the incidence of UTIs secondary to the spine surgery. Moreover, before the surgery, the patients did not undergo urinalysis. The authors cannot differentiate that spine surgery was the primary or secondary risk factor of postoperative UTIs.