When the Z-plasty is used to release a contrac-ture, the common limb, i.e. the central limb of the Z, is positioned along the line of the contracture. The size of each of the angles of the Z is 60°, a compromise figure which has been reached as a result of experience. The reasons for selecting this angle size and the effects of altering it are discussed later, but 60° will be the size used in the present discussion.
Constructed in this way the two triangles together have the shape of a parallelogram with its shorter diagonal in the line of the contracture, its longer diagonal perpendicular to it. The two diagonals can conveniently be referred to as the contractural diagonal and the transverse diagonal.
In order to understand the sequence of events when a Z-plasty is used in releasing a contrac-ture it is essential to bear in mind that the com-mon limb of the Z, being along the line of the contracture, is under tension. Its ends spring apart when the interdigitating flaps are raised and the fibrous tissue band responsible for the contracture is divided.
The springing apart of the divided contracture results in a change in the shape of the parallelogram, and the triangular flaps become transposed, the contractural diagonal lengthens and the transverse diagonal shortens. It is important to appreciate that when a Zsplasty is used properly to correct a linear contracture the surgeon does not actively transpose the Z flaps. Flap
transposition follows naturally from the change in shape of the parallelogram, as do the lengtlmling and the shortening. The changes in length are such that the length of the contractural diagonal after transposition equals that of the transverse diagonal before transposition. The contractural diagonal has lengthened at the expense of the transverse diagonal, which has shortened as much as the contractural diagonal has lengthened.
Translated into practical terms this means that skin has been brought in from the sides with a tightening effect, as shown by the shortening of the transverse diagonal, to allow the lengthening of the contractural diagonal. The difference in length of the two diagonals indicates the actual amount of lengthening and shortening.
The surgeon’s interest is in the lengthening rather than the shortening, but it is crucial to suc-cessful Z-plasty practice to realise that lengthen-ing cannot take place without the transverse shortening. Translated into practical terms, this means that unless there is transverse skin slack available, equal in quantity to the length differ-ence between the axes of the Z, the method will not work.