LEP and obesity due to melanocortin 4 receptor deficiency: For example, the synovial and serum leptin levels have both been correlated with the radiologic score for OA severity [14]; the leptin concentration in synovial fluid has been correlated with body mass index (BMI) [16]; higher leptin expression has been detected in vitro in OA cartilage tissues compared with healthy control tissues [13]; and obesity due to impaired leptin signaling in mice does not lead to OA, in contrast to what is observed in obesity due to overfeeding [15].