A Study of Weight-Based Compensation for Asymmetric Upper Limb Growth Secondary to Pre-Pubertal Operative Humeral Epiphyseal Plate Damage

Injuries to the growth plate of the humerus can occur in children during motor vehicle accidents. These injuries can then lead to growth abnormalities and musculoskeletal issues as the child develops. This research was conducted to analyze and develop a solution to musculoskeletal strain caused by uneven weight distribution inherent in a case of upper limb length discrepancy. The issue is an imbalance due to the growth of a shorter humerus in the individual’s right upper limb (RUL) as the result of a prior surgery on the individual’s right humeral growth plate. This shortened RUL weighs less than the left upper limb (LUL). This effectively lowers the mass moment of inertia of the RUL, thus lowering the balancing moment on the torso. When the individual sprints during physical exercise, there is an imbalance in rotational momentum that is created between the two arms. This imbalance in momentum requires that the opposing lower limb of the shorter RUL, the individual’s left lower limb, drives harder, leading to eventual failure in the hip flexor. In order to solve this biomechanical problem, kinematic equations were developed to model the motion of a sprinter. These equations model the motions of the hands, torso, and legs. In particular, the model defines the influence of the imbalance of the upper limbs’ motion on the lower limbs’ motion, which results in a forward rotation of the torso while sprinting. To balance the rotational momentum of the upper limbs, a counter-acting weight was attached to the wrist of the RUL, minimizing the effects on the lower limb musculature. Hence, the left lower limb would not have to overcompensate for the shorter RUL’s lack of momentum. The equations were then reconfigured to account for the counterweight, and the effect was observed and analyzed. A simulation predicted an angle of tilt of up to 5.7° in the sagittal plane from the vertical. The force required to rotate the body to the normal position was 18N. This force was determined to cause a twist of 10.0° in the transverse plane from the frontal plane. While this study was conducted on an individual with a shortened right upper limb secondary to a surgical procedure, study results can readily be generalized to individuals with shortening of either upper limb secondary to other traumatic events, such as motor vehicle accidents.