graft size and angle
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Graft size and angle

During a hair transplant, a hair surgeon always studies the orientation of the hair follicles so that he can graft them in the recipient site as necessary so that the natural flow of hair is retained like the patient originally had. This success of this approach was mainly due to the artistic abilities of the surgeon.

Using mathematical modeling, Alhaddab and his colleagues have arrived at a workable proportion in which they have used the size, angle of the graft and the distance between one graft from the other or the intergraft distance as the parameters with 1sq cm area as the working area.

In an adult male, the number of hairs per sq cm ranges between 130-280 with an average of 200 hairs / sq cm. If the man is diagnosed to have alopecia and is visibly seen to have thin hair, then he must have lost at least 50% of his original hair density which means he will have only 65-140 hairs/ sq cm. In order to restore normalcy the patient has to be transplanted with as many hairs lost. But in most of the hair transplants carried out so far, this has not been possible and if at all possible, the final result is not as pleasing. In terms of number of follicular units, 40 follicular units / sq cm with 2-3 hairs each give a hair density of 80-120 hairs.

The following assumptions have been made before embarking upon the calculation-
1. The angle of hair transplant – 0 deg means vertical to the skin or at 90 deg to the skin surface; 75 deg is horizontal or 15 deg from the skin surface. [Imagine a protractor with 90 deg as 0 and 0 as 90 !]
2. 1 mm is the standard hair graft diameter.
3. 5 mm is the length of the hair graft along with the shaft
4. The depth at which the graft is to be inserted is equal to the length of the graft multiplied by cosine alpha. This makes the length and width of the recipient site 10 mm each and the depth of the site 5 mm.
5. The interslit distance – maintained at 1 mm in the anterior and posterior sides of the slit. 1.5 mm is maintained at the lateral distance so that when the graft is inserted, the wound will heal and the distance would close in by 0.5 mm during the healing bringing the grafts closer. All the mentioned distances will be halved to calculate for either side of the graft.
6. Volume wasted is not taken into consideration but is presented as the areas in continuation with the adjacent grafts.

Keeping these parameters as standard a mathematical expression has been formulated that will help calculate the number of hair follicles necessary for a sq cm of hair transplant.

By this mathematical expression what is very clear is that depending on the angle of the hair follicle the number of hairs that go into a recipient site is delineated. Another significant advantage is in determining the use of nerve blocks to anesthetize patients depending on the surface area of the skin available for the procedure.

Size and angle of the grafts play very important roles in achieving higher hair density. At an angle of 90 deg, maximum number of hairs (33) can be inserted while at an angle of 15 deg only 8 can be inserted. The actual density of the hair depends on the inter distance variability and this depends on the thickness of the shaft too. When the shaft is less dense, closer packing of the grafts would result in better density, but the availability of donor hair for closer packing needs to be decided prior to the procedure. In order to minimize surgical wound, the grafts have to be dissected to near perfection.

In 1998 Dr. Meyer reported the following results he achieved in terms of density in all the hair transplant procedures he carried out. A 10 follicular unit/sq. cm had a survival rate of 97.5%, 20 with 92.5%, 30 with 72.5% and 40 with 78.1% which shows that achieving higher density results in decrease in the survival rate.

Use of this mathematical formula might help in evaluating patients to help them achieve greater hair density even if it is filling of thinned down mane and not necessarily high degree alopecia.


Graft size and angle references

  • Alhaddab M, Kohn T, Sidloi M. Effect of graft size, angle, and intergraft distance on dense packing in hair transplant. Dermatol Surg. 2005 Jun;31(6):650-3; discussion 654. PMID: 15996414
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