Understanding the Relationship between Torque, Rotational Speed (RPM), and Tissue Characteristics in Follicular Unit Excision Harvest

ABSTRACT

Introduction: The aim of follicular unit excision (FUE) is to harvest grafts without transection. To date, the effect of motor dynamics such as torque and revolutions per minute (RPM) on harvest success has not been studied; therefore, we chose to study the effect of torque and RPM variation on FUE graft harvest.

Materials & Methods: We analysed hair diameter and skin laxity on 40 patients prior to surgery. Patients were randomized into two groups. In the first group of 20, RPM was fixed and torque varied; in the second group of 20, it was the opposite and RPM was varied and torque fixed. Grafts were harvested at each setting and analysed for transection, and the time of harvesting was noted. This was correlated with hair diameter and skin laxity.

Results: At high laxity, low torque was required. As the laxity reduced, higher torque was needed to overcome the resistance. Torque had a minimum value required based on the tissue resistance for the harvest to be viable. It was found that increasing RPM increased the speed of harvest. The maximum RPM tolerated before transection rate was unacceptable was proportional to the hair diameter.

Discussion: Energy spent is a product of torque and RPM. Torque is the minimum force needed to drill through the surface. RPM is the speed at which the drill rotates. Our energy expended should be lower than the energy needed to damage the graft. Our recommendation is to keep the torque as low as needed: the greater the skin laxity, lower the torque needed. Additionally, we recommend keeping the RPM as high as possible to increase the speed of the harvest without leading to unacceptable levels of transection: the higher the hair diameter, the higher the RPM tolerated. With these manipulations, a successful harvest would be possible in nearly all cases.