Effect of needle geometry on insertion force of different hair implanters

Surgeons insert approximately 3000 hair implants in a single hair restoration surgery, and the needle shape critically affects surgical performance including surgical time, survival rate, and the surgeon’s fatigue. There are several experimental studies on the relationship between needle shape and performance; however, there are no studies on hair implant needles, which are different from other medical needles as they have a com-paratively short bevel. This study compared several hair implant needles to determine the relationship between needle shape and insertion force. Eight hair implant needles with the same diameter were collected, and their geometrical data were visually measured. The insertion force and total work of each needle were measured us-ing custom-made force measurement equipment; the results were statistically compared, and their correlation was analyzed. Moreover, a qualitative comparison was made. The needle insertion force ranged from 1.30 to 2.97 N, and the total work ranged from 8.15 to 10.97 mJ. The primary bevel and acute angles showed a moder-ate positive correlation with the total work. A longer point length, smaller primary bevel, and acute angle de-crease the total work of hair implant needles. As per the results of the qualitative comparison, we found that the surgeons can identify the differences in needle insertion force among various hair implant needles, and their grading was consistent with the grading according to the total work of needle insertion.