The ratio of the length of the index finger to the ring finger (referred to as 2D:4D) is a trait that is identified with difference between men and women. Men tend to have shorter index than ring fingers while in women they tend to be about the same length. Smaller 2D:4D ratios have interesting hormonal relationships such as higher prenatal testosterone levels, higher sperm counts and lower estrogen concentrations. Smaller ratios have also been linked to athletic and sexual prowess.

Now there has been a study to determine the relationship between the 2D:4D ratio and the risk of osteoarthritis (OA) of the knee or hip.

Researchers and the University of Nottingham performed a case-controlled study of over 2,000 subjects, with an average age of 67, which were recruited from hospital surgery patients and a rheumatology clinic in Nottingham. All of the participants had osteoarthritis of the hip or knee to the extent that they were considering joint replacement surgery.

Data from these participants were compared to over 1,000 people, with an average age of 63, who did not have any radiographic evidence of OA.

All of the study subjects had radiographs taken of their knees and hips, as well as their left and right hands. The 2D:4D ratios were determined by the measurements of the radiographs of their hands.

Three different methods were used to determine the finger length ratios: visual comparison of the 2 finger ends; the measurement of the fingers from the base to the tip of the upper finger joints; and measurement of the metacarpal bones.

The 2D:4D ratios were classified as type 1, where the index finger was longer than the ring finger; type 2, where the fingers were equal length; or type 3, where the index finger was shorter than the ring finger. As was expected, men were more likely than women to have a type 3 ratio, by 2.5 to 1.

When comparing the finger ratio types and the incidence of hip or knee OA the researchers found that type 3 was associated with an increased risk of OA. In addition, they found that participants with the type 3 ratio showed double the risk of those with type 1 or 2. Women with type 1 fingers had a greater risk of osteoarthritis of the knee than men.

Another trend that was uncovered was the smaller the upper finger joint ratio, the greater the risk of knee OA, even after adjusting for normal OA risk factors.

The team concluded that the 2D:4D ratio appears to be a new risk factor for osteoarthritis and that women with the “male” type 3 2D:4D pattern are at higher risk to develop knee osteoarthritis. The underlying mechanism for this risk is unclear and the team called for more research.