How Does 2D4D Predict Knee Osteoarthritis?

2D4D is not the name of Han Solo’s (Star Wars) robot sidekick but represents the length ratio of the index to the ring finger. Interestingly, this ratio predicts the occurrence of knee osteoarthritis (OA) in women. A study of >2,000 participants examined the association between the 2D:4D length ratio and OA and found a positive association with both knee OA and hip OA.  This association was stronger for tibiofemoral knee OA and for the combination of knee and/or hip OA plus IP nodes (i.e., “generalized nodal OA”). The association was dose-dependent and independent of other well-established risk factors for OA, such as age, sex, BMI, joint injury, and physical activity. Thus the 2D:4D length ratio appears to be a newly identified risk factor for the development of OA.  The mechanism that accounts for this association is unknown. Previous studies have shown the 2D:4D length ratio to associate with high standards of athletic achievement and with high levels of testosterone and “male” characteristics. Increased activity and performance in physically demanding sports could contribute to the development of OA through repetitive joint trauma and injury, and higher testosterone levels could cause hormonal modulation of joint connective tissue metabolism, including chondrocyte, osteoblast, and osteoclast activity, and thus contribute to the development and/or progression of OA.