Objective
This study aims to clarify how the stimulation of acupuncture points is achieved by needles with different surface texture during acupuncture; it also seeks to lessen injury at the insertion site and increase the therapeutic efficacy of acupuncture, by simulating the mechanical effects of various needle surface patterns on Zusanli (ST36) without changing the radius of acupuncture needles.

Methods
Five acupuncture needle models with different surface patterns, including the smooth needle, the lined needle, the ringed needle, the left-hand threaded needle and the right-hand threaded needle, and a layered model of the Zusanli acupoint were used to investigate how to reduce tissue damage and increase stimulation during acupuncture treatment. Puncturing of the skin as well as lifting-inserting and twisting needle manipulations were simulated using these models, and the degree of damage and force of stimulation caused by the acupuncture needles with different surface patterns during acupuncture were compared.

Results
The smooth needle and the lined needle caused the least tissue damage during insertion, while the left-hand threaded and the right-hand threaded needles caused the most damage. The ringed needle, the left-hand threaded needle and the right-hand threaded needle stimulated the acupoint tissue more during lifting-inserting manipulations, while the lined needle and the smooth needle produced less stimulation. The stimulation of the lined needle on the acupoint tissue was the largest during twisting manipulation, whereas the left-hand threaded needle and the right-hand threaded needle had smaller effects. In lifting-inserting and twisting manipulations, both the left-hand threaded needle and right-hand threaded needle provided more stimulation, but the torsion direction in which they produced better stimulation was the opposite.

Conclusion
According to the simulation results, the ringed pattern enhances stimulation best in the lifting-inserting manipulation, whereas the lined pattern enhances stimulation best in the twisting manipulation. Both the right-hand and left-hand thread patterns have certain enhancing effects in these two operations. Taking the geometric properties of the pattern into account, the left-hand thread pattern and the right-hand thread pattern have the geometric characteristics of both the lined pattern and the ringed pattern. To conclude, a pattern perpendicular to the movement direction during the acupuncture manipulation creates more stimulation. These results have significance for future needle design.

Key words: Biomechanics, Finite element method, Acupuncture, Needle roughness, Mechanical effects