Cruciate ligament degeneration and stifle joint synovitis in 56 dogs with intact cranial cruciate ligaments: Correlation of histological findings and numbers and phenotypes of inflammatory cells with age, body weight and breed.

The majority of dogs with ruptured cranial cruciate ligaments (crCLs) have inflammatory changes of the stifle joint suggesting that synovitis is an important factor which is involved in the development of degenerative ligament changes. Detailed information is not available concerning the possible occurrence of inflammatory changes in the stifle joint synovium of dogs with macroscopically intact crCLs and its correlation with presence and severity of degenerative changes of the crCLs. Therefore, the purpose of this study was to examine post mortem tissue samples of 56 dogs with non-ruptured crCLs histologically for the presence of inflammatory and degenerative changes of the stifle joint synovium and cruciate ligaments, respectively. In 25/56 (44.6%) dogs, histology showed that both lymphoplasmacytic synovitis and degenerative alterations of the crCLs were present. In these dogs, there was a significant positive statistical correlation between the severity of synovitis and degenerative crCL lesions. The results suggest that synovitis in the stifle joints of dogs with non-ruptured crCLs is a frequent event and probably is involved in the development of degenerative lesions occurring in canine crCLs. Also, the severity of crCL degeneration in these 25 dogs was significantly correlated with their age and body weight. In 2/56 (3.6%) cases, only lymphoplasmacytic synovitis was found in the absence of degenerative crCL lesions. In 15/56 (26.8%) dogs, only degenerative lesions of the crCLs without synovitis were present. Statistically, a significant correlation was found between the severity of degenerative alterations and age and body weight of these dogs. Phenotyping of inflammatory cells by immunohistochemistry showed that the synovium of dogs which histologically had lymphoplasmacytic synovitis was infiltrated with CD3+ T lymphocytes, CD79+ B lymphocytes, major histocompatibility class II antigen (MHC class II)+ cells and macrophages expressing CD163 or S100/A8/S100A9 (calprotectin), while tartrate-resistant acid phosphatase (TRAP)+ cells were absent. Quantification and statistical evaluation of inflammatory cell types in the inflamed synovium revealed that the numbers of lymphocytic cells and macrophages were significantly correlated with the severity of synovitis. These findings indicate that, besides T and B lymphocytes, both pro- and anti-inflammatory macrophages play a role in the development of degenerative crCL alterations.