Abstract
To increase knee range of motion and improve gait in children with spastic paresis (SP), the semitendinosus muscle (ST) amongst other hamstring muscles is frequently lengthened by surgery, but with variable success. Little is known about how the pre-surgical mechanical and morphological characteristics of ST muscle differ between children with SP and typically developing children (TD). The aims of this study were to assess (1) how knee moment-angle characteristics and ST morphology in children with SP selected for medial hamstring lengthening differ from TD children, as well as (2) how knee moment-angle characteristics and ST morphology are related. In nine SP and nine TD children, passive knee moment-angle characteristics and morphology of ST (i.e. fascicle length, muscle belly length, tendon length, physiological cross-sectional area, and volume) were assessed by hand-held dynamometry and freehand 3D ultrasound, respectively. At net knee flexion moments above 0.5 Nm, more flexed knee angles were found for SP compared to TD children. The measured knee angle range between 0 and 4 Nm was 30% smaller in children with SP. Muscle volume, physiological cross-sectional area, and fascicle length normalized to femur length were smaller in SP compared to TD children (62%, 48%, and 18%, respectively). Sixty percent of the variation in knee angles at 4 Nm net knee moment was explained by ST fascicle length. Altered knee moment-angle characteristics indicate an increased ST stiffness in SP children. Morphological observations indicate that in SP children planned for medial hamstring lengthening, the longitudinal and cross-sectional growth of ST muscle fibers is reduced. The reduced fascicle length can partly explain the increased ST stiffness and, hence, a more flexed knee joint in these SP children.
Highlights
Children with spastic paresis (SP) who are walking with a flexed knee gait pattern are frequently treated by single-event multilevel surgery (SEMLS) [1, 2]
SP and typically developing children (TD) children did not differ in body mass, body mass index (BMI) or femur length (Table 1)
In TD children, knee angles measured at 0 Nm and 4 Nm were 78.6±5.6 ̊ and 37.6±7.7 ̊, respectively
Summary
Children with spastic paresis (SP) who are walking with a flexed knee gait pattern are frequently treated by single-event multilevel surgery (SEMLS) [1, 2]. With musculoskeletal modeling generally only origin-insertion length of a muscle (i.e. length of the muscle-tendon unit (MTU)) is estimated based on joint angles and moment arms This does not provide full insight in morphological alterations that may potentially underlie the reduced range of motion (ROM) around the knee in children with SP. Assessment of tendon length, fascicle length, and physiological cross-sectional area (PCSA) by ultrasound allows to relate alterations in muscle morphology to limitations in knee ROM and to increased joint stiffness (e.g. a smaller PCSA will decrease while shorter fascicles will increase passive MTU stiffness) Such information may provide indications for the magnitude of effects to be achieved by hamstring lengthening. Insight in morphological variables that affect knee-joint mechanics in children with SP prior to such surgery is the first step to identify factors that explain the side effects
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