The design and biomechanical analysis of a dynamic positioning frame for scoliosis surgery

Scoliosis is a three dimensional deformity of the spine and trunk. In severe cases of scoliosis, surgery is required. Positioning of patients is an important step in the surgical procedure and has been shown to correct the scoliotic curve by 37% prior to the instrumentation which then fixes the final correction at 57%. Biomechanical models have been used to help study scoliosis but these models have not been used to study patient positioning.

The general objective of this work is to develop and validate a dynamic positioning frame (DPF) that will be used in the operating room to improve the surgical correction of spinal deformities. It is hypothesised that the utilization of the DPF can improve the correction of the spinal deformity and the overall trunk geometry compared to the Relton-Hall frame used in the conventional approach. In parallel, it is hypothesised that the osseo-ligamentous structures of the patient, the effect of gravity, anaesthesia and the surgical position can be modeled, and such computer models can be used to recommend placement of the DPF components on a patient for correction of the spinal deformities.

In order to maximise the amount of correction observed in the prone position a DPF was designed. The DPF is composed of a base of four cushions that contour to the sides of the patient that can be easily displaced on rails. There are also external correction cushions that can be used to apply force to the trunk deformities. A clinical evaluation was performed of 11 unanesthetised scoliotic patients; standing, lying prone on the Relton-Hall frame, prone on the DPF and prone on the DPF with external manually applied forces. Trunk geometry and pressure measurements were recorded. The results showed a statistically significant lengthening of the spine, improvement in the torsional deformity (delta hump) and less of a reduction in kyphosis when the patient was lying prone on the DPF compared to the Relton-Hall frame. Higher pressures were recorded on the DPF when compared to the Relton-Hall frame necessitating modifications to the DPF cushions.

Modifications were made to the cushions and the improved DPF was tested on three patients in the operating room. As a safety precaution, pre-operative pressures were measured the day before with the patient lying on both the Relton-Hall frame and the DPF. The pre-operative pressures on the DPF were equal too or less than the pressure on the Relton-Hall frame. This study showed the feasibility of using the DPF during scoliosis surgeries but a larger clinical trial is underway to determine if there is a significant improvement in correction achieved with this system. (Abstract shortened by UMI.)