Finding line of action of the force exerted on erect spine based on lateral bending test in personalization of scoliotic spine models

Abstract

In multi-body models of scoliotic spine, personalization of mechanical properties of joints significantly improves reconstruction of the spine shape. In personalization methods based on lateral bending test, simulation of bending positions is an essential step. To simulate, a force is exerted on the spine model in the erect position. The line of action of the force affects the moment of the force about the joints and thus, if not correctly identified, causes over/underestimation of mechanical properties. Therefore, we aimed to identify the line of action, which has got little attention in previous studies. An in-depth analysis was performed on the scoliotic spine movement from the erect to four spine positions in the frontal plane by using pre-operative X-rays of 18 adolescent idiopathic scoliosis (AIS) patients. To study the movement, the spine curvature was considered as a 2D chain of micro-scale motion segments (MMSs) comprising rigid links and 1-degree-of-freedom (DOF) rotary joints. It was found that two MMSs representing the inflection points of the erect spine had almost no rotation (0.0028° ± 0.0021°) in the movement. The small rotation can be justified by weak moment of the force about these MMSs due to very small moment arm. Therefore, in the frontal plane, the line of action of the force to simulate the left/right bending position was defined as the line that passes through these MMSs in the left/right bending position. Through personalization of a 3D spine model for our patients, we demonstrated that our line of action could result in good estimates of the spine shape in the bending positions and other positions not included in the personalization, supporting our proposed line of action.

Computational modelling of the scoliotic spine: A literature review

“…As they note in the context of lateral bending tests, the line of action is crucial as it determines the loads and moments. Incorrect lines of action would potentially lead to inferior characterisation of the IVJ stiffness 135 …”

“…Incorrect lines of action would potentially lead to inferior characterisation of the IVJ stiffness. 135 …”

“…Many scoliotic models are validated by simulating the surgical intervention performed and then comparing predicted curvature to the post‐operative curvature 13 , 15 , 70 , 72 , 73 , 74 , 81 , 84 , 106 , 127 , 128 , 129 , 131 , 132 , 133 , 139 or by simulating a bending test and comparing the predicted curvature to the measured curvature from radiographic images. 69 , 85 , 96 , 134 , 135 , 136 , 137 , 140 Predicted Cobb angles within 5° of the clinically measured values are often predicted and are considered acceptable as it corresponds to the clinical accuracy. 155 …”

“…Incorrect lines of action would potentially lead to inferior characterisation of the IVJ stiffness. 135 The study by Jalalian et al used X-rays of left and right bending and erect positions to personalise their model. They were able to establish a relationship between the bent and erect positions and were able to accurately predict positions which were not used in the characterisation.…”

“…Many scoliotic models are validated by simulating the surgical intervention performed and then comparing predicted curvature to the post-operative curvature 13,15,70,[72][73][74]81,84,106,[127][128][129][131][132][133]139 or by simulating a bending test and comparing the predicted curvature to the measured curvature from radiographic images. 69,85,96,[134][135][136][137]140 Predicted Cobb angles within 5 of the clinically measured values are often predicted and are considered acceptable as it corresponds to the clinical accuracy. 155 The downside of this method is that it only offers retrospective validation of the models 132 and in the case of postoperative radiographs only for the procedure performed.…”