Predicting the loss of knee flexion during limb lengthening using inherent muscle length

Limb lengthening procedures can be associated with loss of range of knee movement, although the amount of knee flexion lost varies markedly between patients. The exact aetiology of this problem is not fully understood. This study investigates whether the amount of joint range that is lost during limb lengthening might be affected by the inherent passive compliance and length of the patients' soft tissues determined preoperatively. A simple mathematical model has been developed to calculate the inherent length and 'spare' length of the hamstring and quadriceps muscle groups in 28 patients undergoing limb lengthening procedures. The range of knee motion was recorded preoperatively and loss of movement recorded during follow-up. The results show a strong correlation between predicted 'spare' length of quadriceps and loss of knee flexion. An association was noted between loss of knee extension and the straight leg raise and correlation of knee extension and the spare length of the hamstrings. The straight leg raise test, and calculation of spare quadriceps length using our formula, as part of the preoperative assessment, can help predict which patient is at risk of a reduced range of motion of the knee.