Since the skin flaps must fit together in their transposed position, the limbs of the Z are con-structed equal in length. The angles of the Z are also usually made equal in size. The factors which do vary are angle size and limb length, and the ways in which variation in these factors affects the result provide an explanation of why specific constructions are used in particular sets of circumstances.
Once the lengths of the limbs of the Z have been fixed the amount of lengthening to be expected is determined by the size of the angle, its amount increasing with increase in the size of the angle. With an angle of 30° there is theoretically a 25% increase in the original length, with 45° a 50% increase, and with an angle of 60° the increase rises to 75%.
These increases are theoretical and they cannot be applied with strict accuracy to the clinical situation, though when account is taken of variations in skin extensibility, pre-existing scar-ring, etc., it is surprising how well they do apply. The theoretical lengthening usually exceeds slightly what can be achieved in practice. In releasing a contracture the object of the Z- plasty is to maximise the amount of lengthening.
Tissue for transverse shortening is seldom available in unlimited quantity, and it is found in practice that when the angle increases beyond 60° the tension produced in the surrounding tissues tends to be so great that the flaps cannot read-ily be brought into their transposed position. For this reason 60° is the compromise figure used for angle size. Limb length.
With the use of 60° as the routine Z-plasty angle, it is length of limb which pro-vides the major variable in practice. The amount of tissue available on either side determines the practicable limb length -a large amount permits a large Z, a small amount correspondingly limits the size of the Z.