Yoon Hyuk Kim

Increase of load-carrying capacity under follower load generated by trunk muscles in lumbar spine:

Abstract The load-carrying capacity of the spine has been experimentally shown to increase substantially when the resultant force of all loads applied on the spine is directed in accordance to its curvature, which is called a ‘follower load (FL)’. However, there have been few studies to investigate the muscle forces producing the FL owing to the difficulty of the relevant experimental measurements. This study investigated whether trunk muscle activations could be found for transmitting an external load within a range of FL direction. A two-dimensional finite element model of a lumbar spine and 117 pairs of trunk muscles was developed in the sagittal plane. An optimization technique was utilized to estimate the muscle forces generating the FL and the corresponding responses of the lumbar spine under two loading cases: the upright neutral standing posture and the posture holding 200 N in the hands. For each loading case, the muscle forces required to generate the FL could be found and the corresponding responses of the lumbar spine validated that the FL could increase the load-carrying capacity. The results showed that the FL could be produced in vivo by trunk muscles to increase the load-carrying capacity.

Stress analysis in a pedicle screw fixation system with flexible rods in the lumbar spine

Abstract Breakage of screws has been one of the most common complications in spinal fixation systems. However, no studies have examined the breakage risk of pedicle screw fixation systems that use flexible rods, even though flexible rods are currently being used for dynamic stabilization. In this study, the risk of breakage of screws for the rods with various flexibilities in pedicle screw fixation systems is investigated by calculating the von Mises stress as a breakage risk factor using finite element analysis. Three-dimensional finite element models of the lumbar spine with posterior one-level spinal fixations at L4—L5 using four types of rod (a straight rod, a 4 mm spring rod, a 3 mm spring rod, and a 2 mm spring rod) were developed. The von Mises stresses in both the pedicle screws and the rods were analysed under flexion, extension, lateral bending, and torsion moments of 10 N m with a follower load of 400 N. The maximum von Mises stress, which was concentrated on the neck region of the pedicle screw, decreased as the flexibility of the rod increased. However, the ratio of the maximum stress in the rod to the yield stress increased substantially when a highly flexible rod was used. Thus, the level of rod flexibility should be considered carefully when using flexible rods for dynamic stabilization because the intersegmental motion facilitated by the flexible rod results in rod breakage.

Investigation of the compressive stiffness of spinal cages in various experimental conditions based on finite element analysis

Recently, novel polymers, including polyetheretherketone and carbon fibre reinforced polymer, have been used for spinal implants. Because the in vitro experimental test uses metal blocks with different material properties from those of polymer cages in standard test protocols for prediction of the mechanical performance, it is necessary to analyse the influence of various experimental conditions, such as the material of the blocks. In this study, the compressive stiffness of spinal cages was investigated for different materials (polyetheretherketone, carbon fibre reinforced polymer, and titanium) under simulations of the mechanical experimental tests and the in vivo situation based on finite element analysis. The stiffness was affected by shapes of cage as well as experimental conditions, such as the load application method or fixation block. In the open cages, the polymer cages showed a greater dependence on the experimental situation than the metal cages. Hence, it may be necessary to consider the experimental conditions during in vitro mechanical tests for the stiffness evaluation of spinal cages made of novel polymers to obtain results relevant for an in vivo situation.

Interjoint coordination of the lower extremities in short-track speed skating

In short-track speed skating, the three-dimensional kinematics of the lower extremities during the whole skating cycle have not been studied. Kinematic parameters of the lower extremities during skating are presented as joint angles versus time. However, the angle–time presentation is not sufficient to describe the relationship between multi-joint movement patterns. Thus, angle–angle presentations were developed and used to describe interjoint coordination in sport activities. In this study, 15 professional male skaters’ full body motion data were recorded using a wearable motion capture system during short-track speed skating. We investigated the three-dimensional kinematics of the lower extremities and then established the interjoint coordination between hip–knee and knee–ankle for both legs during the whole skating cycle. The results demonstrate the relationship between multi-joint movements during different phases of short-track speed skating. This study provides fundamentals of the movement mechanism of the lower extremities that can be integrated with physiotherapy to improve skating posture and prevent injuries from repetitive stress since physiological characteristics play an important role in skating performance.