The Coverage Value Calculation: An Objective Method of Evaluating Donor Supply and Coverage

55B The Coverage Value Calculation: An Objective Method of Evaluating Donor Supply and Coverage

Koray Erdogan

Summary

The concept of coverage value (CV) calculation was developed as an objective method to standardize scientific criteria for calculating coverage through two components: hair density and hair shaft thickness. These criteria not only address overall coverage but also create a standard for avoiding donor depletion, an issue that has been of concern to hair transplant surgeons for some time and especially where higher graft counts are concerned. In addition to the calculation, a Graft Calculator app was created to assist surgeons evaluate the estimated hairs per graft being harvested during surgery in real time. This enables physicians to make adjustments during surgery (if needed) in the hairs per graft being produced in order to meet the target CV.

Keywords: coverage value calculated density hair caliber density graft calculator graft coefficient

Key Points

•Coverage value provides scientific criteria for determining minimal coverage.

•Coverage value is determined by using a formula that considers the number of hairs by their caliber or thickness.

•The minimum coverage value of 5.4 determines target graft excision numbers that avoid depletion.

•The Graft Calculator is a downloadable application that assists surgeons in evaluating the actual average hairs/graft being harvested during surgery in real time. This enables the physicians to make adjustments in punch size (if needed) in order to obtain the desired hair/graft and meet the target coverage value.

55B.1 Introduction

In searching for a scientific solution to the question of hair coverage using objective criteria, nature provided an excellent model. Grass grows much in the same pattern hair does, with its appearance being determined by both blade density and thickness (Fig. 55B.1). This model provides the understanding needed to approach the question of hair coverage. The hair coverage of an area is determined by the number of hairs in a given area multiplied by the hair shaft thickness. This is similar to the conclusion arrived at by James Harris in 2003 when he stated, “Visual density does not correlate to the calculated volume of hair, but to the hair shaft diameter and the number of hairs.” See Video 55B.1.

Fig. 55B.1 Model of grass coverage determined by number of blades and their thickness.

The two greatest concerns to both the patient and the surgeon are the following: (1) How many grafts can we safely take from the donor area (donor capacity)? (2) How many grafts does it take to create good coverage in the recipient area?¹^,² Determining these is the main objective of the coverage value (CV) calculation.³

With the assistance of several instruments, the CV calculation becomes a very powerful tool, allowing the surgeon to calculate the starting CV in each scalp area and then determining the total number of grafts that can safely be excised from each area. This is an extremely important concept when considering the trends in follicular unit excision (FUE) surgery where multiple surgeries and larger sessions are becoming more common. See Video 55B.2, 55B.3, and 55B.4 for more details on using coverage value.

55B.2 Calculation Tools

There are several tools necessary to obtain the necessary data for correct CV calculations (Fig. 55B.2 and Fig. 55B.3). They are (1) a micrometerused for determining the hair caliber from different zones of the scalp (temporal, parietal, and occipital); (2) a dermatoscope to obtain detailed photographs of each of these scalp areas to determine the follicular density (FU/cm2), hair density(hair/cm²), and calculated hairs per follicular unit (hairs/FU); and (3) the Graft Calculator downloadable application used to determine the average hair/graft actually produced during surgery, enabling adjustments in punch size to be made to modify (if needed) the hairs/graft in order to meet the target CV. The calculated hairs per FU (or hairs per graft) is also called “ calculated density,” “ FU coefficient,” or “ graft coefficient” (Video 55B.5)

Fig. 55B.2 Calculation tool used to determine coverage value including (1) micrometer, (2) dermatoscope and software for counting hairs, and (3) Graft Calculator app to determine real-time hairs per graft.