Little and Ring Fingers May Do Better with Silicone Arthroplasty Than Index and Middle FIngers
Arthroplasty (joint replacement) can be done on many joints now, including the fingers. Metacarpophalangeal joints, or the knuckles at the base of the fingers, that have been damaged by rheumatoid arthritis can be replaced using silicone arthroplasty, but some studies have shown that this may not be as effective in some fingers than in others. The authors of this study wanted to see if different fingers reacted differently to the silicone arthroplasty in how they were able to move, extend, and move in an arc motion.

To do the study, researchers studied 68 patients who were between 18 and 65 years old; all had rheumatoid arthritis and had hand deformities due to arthritis. The researches were looking for deviations and lags in movement in the index, middle, ring and little fingers. All the patients received the same implant, using the same technique.

To measure the function of the fingers, hand therapists measured the arc of motion of the fingers, and ulnar drift, which was how the finger moved while in a resting position.

The drift angle before surgery for the index finger was 26 degrees before the surgery and 13 degrees one year after surgery. For the middle finger, it was 33 degrees before surgery, 12 degrees one year later. The ring finger had an angle of 40 degrees before surgery and 23 degrees after, and the little finger had 48 degrees before surgery and 14 degrees after. This means that the biggest improvement was with the little finger (30 degrees improvement) and the least amount of improvement was the index finger with only 12 degrees improvement.

The same type of finding was noted with the extension lag. The index finger had 53 degrees before surgery, 32 degrees after; middle finger had 65 degrees before surgery, 35 degrees after; ring finger had 68 degrees before surgery, 27 degrees after, and the little finger had 72 degrees before surgery and 47 degrees after. Again, the better improvement was seen with the little finger at 47 degrees and the least amount of improvement in the index finger at 21 degrees.

When measuring the arc of motion, there was a slight change in which finger did best. The least amount of improvement was still with the index finger (8 degrees), but the best was the ring finger (15 degrees) followed by the little finger (13 degrees).

In conclusion, the researchers found that the ulnar fingers, the ring and little fingers had the best improvement after the surgery, despite having the most damage, while the radial fingers, the index and middle fingers, didn't do quite as well. The differences could well have to do with the mechanics of the different knuckles. The authors had thought that the ulnar fingers would not do as well with the silicone arthroplasty because the knuckles don't have the same stability as the knuckles of the radial fingers, but they were proved wrong in this case.
References
Kevin C. Chung, MD, et al. Outcomes of Silicone Arthroplasty for Rheumatoid Metacarpophalangeal Joints Stratified by Fingers. In Journal of Hand Surgery. Nov. 2009. Vol. 34A. Pp. 1647 to 1652.