A geometric method of calculating tendo Achillis lengthening.

Twenty-six open "Z-type" tendo Achillis lengthenings were performed on 20 children with equinus contractures secondary to spastic cerebral palsy. The patients' average age was 8.6 years (range 4.0-17.5 years). A preoperative calculation of the amount of lengthening needed to accomplish a satisfactory correction of equinus was used before all procedures in the hope of avoiding iatrogenic overlengthening. This calculation was done with simple clinical measurements on the foot and ankle and is based on elementary lever arm and geometric principles. After an average follow-up of 3.1 years (range 6 months to 7 years), there were no instances of overlengthening, nor was there residual equinus.