A Preliminary Study of Anatomical Factors Determining Independent Extension of the Ring Finger

A clinical test was designed to study factors determining independent exten-sion of the ring finger. 202 randomly selected cases (404 hands) were asked to extend their ring fin-gers while the middle finger was passively flexed and pressed firmly on the table. The following items were recorded : (1) the ability to extend the ring fin-ger in this position, (2) the angle of extension, and (3) the position of extensor digitorum communis tendon of the middle finger ( EDC-long ) related to the metacarpal head before and after trying to extend the ring finger. The basic data of the patients included : age, gender, occupation, hand dominance, general hyperlaxity of joint , and whether one had hobbies that made independent extension of finger as a desirable benefit. All factors are ana-lyzed statistically to correlate with the independent extension of the ring finger. The results were as followed : There were 82 males & 120 females. The mean age was 38.6 (5~69) years old. 196 were right hand dominant & 6 were left. Eighty hands (20 percent) were able to extend the ring finger independent-ly & 324 hands (80 percent) were not. Ulnar subluxation of the EDC-long before trying to extend the ring finger was significantly related to the ability of inde-pendent extension of the ring finger (p=0.046). There was no relationship between the ulnar subluxation of the EDC-log after trying to extend the ring finger and the ability of independent extension of the ring finger (p=0.17). Genders, hand dominance, & hobbies which prefer independent finger extension as a desirable benefit were not related (p=0.37, 0.65 & 0.82). General hyperlaxi-ty of joint may play an important role in the independent extension of the ring finger(P＜0.05). The dragging effect of the juncturae tendinum (JT) between the EDC-long and EDC-ring is considered to be the most important factor that limits the inde-pendent extension of the ring finger. The preliminary results of our study sug-gested that the ulnar subluxatin of the EDC-long may decrease the dragging effect & contribute to the independent extension of the ring finger. General hyperlaxity of joints may enable the ring finger to extend independently because the laxity of web structures allows ulnar subluxation of EDC-long, which in turn loosen the dragging effect of JT on EDC-ring. The clinical value of the results from this study remains to be decided by further studies.