A Musculoskeletal Model of the Lower Limbs and its Application to Clinical Paediatric Orthopaedics
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Articular cartilage accrual occurs predominantly during childhood and adolescence, with the magnitude, direction, and pattern of internal joint loads directing the cartilage growth. If any of these factors of the joint loading are abnormal, it can predispose these children to degenerative knee joint disease as an adult. To provide an estimate of the internal joint loads, a paediatric-focused, static optimization-based lower limb model was developed, compared to recorded sEMG, and analyzed for sensitivity to changes in ground reaction force and muscle attachment site. The model was found to provide consistent predictions of joint contact force predictions ranging from 0.01 to 0.35xBW with standard error of 8% to 17%, with the exception of left knee medial-lateral shear at 108%. Muscle force predictions related well to sEMG, with the standard error ranging from 14% to 36%, except for gastrocnemius lateral at 104%. The model was sensitive to variations in the ground reaction force vector, with a maximum deviation of 0.11 xBW determined as a result of a ±5% variation in GRF. The model was found to be sensitive to clinically relevant deviations in muscle attachment site. Maximum knee anterior shear was significantly changed (p < 0.05) with a 1cm posterior quadriceps insertion deviation, maximum lateral shear with a posterior semimembranosus deviation, and maximum medial shear with a posterior or medial quadriceps deviation. No deviations caused statistically significant changes in compression. Statistically significant change in joint contact force could not be predicted based on changes in muscle moment arm, but could be indirectly predicted by the predicted muscle forces. The model’s uniform convergence and sensitivity to variations in input indicate that the model is sufficiently reliable and robust. This sensitivity suggests that the model is capable of adapting to altered loading conditions and musculoskeletal geometry, either due to deformity or corrective procedure. The model was therefore deemed to be a strong platform for developing clinically specific models for analyzing internal knee loads in a diverse paediatric population.