Semitendinosus muscle morphology and gait kinematics after medial hamstring lengthening in children with spastic diplegia

Introduction: To improve gait in children with spastic diplegia, the semitendinosus muscle (ST) is frequently lengthened amongst other medial hamstring muscles by surgery. Side effects on gait due to weakening of the hamstring muscles and overcorrections have been reported [1]. How these side effects relate to ST morphology is unknown. Research question: Howdo ST morphology and gait kinematics change after medial hamstring lengthening in children with spastic diplegia? Methods: ST morphology and gait kinematics were determined in five children with spastic diplegia (2 boys; age at surgery: 14.4 2.4 years; GMFCS II&III) selected for medial hamstring lengthening to counteract limited knee range of motion and improve gait, before and 8–20 months after surgery. Muscle belly length, tendon length, muscle-tendon unit (MTU) length as well as muscle volume of ST at the knee angle corresponding to 4 N m knee moment in an imposed hip position of 708 flexion were assessed by freehand 3D ultrasound [2]. 3Dgait analysis was performed using an optoelectronic marker tracking system. For each participant, pelvic, hip and knee angles of the right leg were determined in the sagittal plane at midstance (MSt (30% gait cycle (GC))) and late swing (LSw (99% GC)) over three successful strides. Results: The knee angle corresponding to 4 N m knee moment was 57.2 14.98 (pre) and 35.2 12.28 (post) (p = 0.068). After surgery, muscle belly length normalized for femur length was 34 5% lower (p = 0.001), tendon length was 83 34% higher (p = 0.003) and MTU length was 10 7% higher (p = 0.044) (Fig. 1). Muscle volume was 39.6 17.6 cm2 before surgery and 21.3 17.2 cm2 after (p = 0.054). Kinematic results showed significantly improved knee extension but also increased pelvic anterior tilt, and no differences in hip angle (Table 1). Discussion: Our results show that medial hamstring lengthening leads to a longer ST MTU with a longer tendon, but a reduced muscle belly length. Differences in gait kinematics indicate that the longer ST MTU after surgery mainly leads to more knee extension during gait, while we did not find an increase in hip flexion. The increased pelvic tilt seems to be an effect of the changed knee kinematics through the femur kinematics during gait. Preoperative pelvic tilt needs to be considered when medial hamstring lengthening is indicated. References [1] Dreher J. Bone Joint Surg. Am. 2012;94:121–30. [2] Haberfehlner H. J. Anat. 2016 (in press).