SP1: THE IMPACT OF CEMENT STIFFNESS, BONE DENSITY AND FILLING VOLUME AFTER CEMENTOPLASTY ON THE RISK OF ADJACENT VERTEBRAL FRACTURES

Abstract

A recurrent fracture rate after vertebroplasty and balloon kyphoplasty is as high as 20%. Biomechanically, it has not been proven that refracture rate is due to the cement stiffness alone. This finite-element study investigated effects of cement-stiffness, bone-quality, cement-volume and height-restoration in treatment of vertebral compression fractures using balloon kyphoplasty.

A finite-element model of the lumbar spine was generated from CT-scans. The model comprised of two functional spinal-units, consisting of L2-L4 vertebral bodies, intervertebral-discs, and spinal ligaments. Cement volumes modelled were in the order of 15% and 30% of total vertebral body (VB) volume. Spinal fracture was modelled as being reduced and height of VB was restored. Kyphoplasty was performed. Three different bone qualities were modelled: healthy, osteopenic, osteoporotic. A compressive load was applied to the proximal endplate of L2. An anterior shift of the centre-of-gravity of upper body was simulated by increasing the moment arm of the applied load.

All results of the analysis were compared back to an intact spinal model of the same region under the same loading regime. All parameters affected the mechanical behaviour of the spine model, although changing the bone quality from normal to osteoporotic resulted in the least change. The cement stiffness was initially modelled with an elastic modulus between 0.5GPa and 2GPa. The results showed small differences relative to intact case in the lower modulus cement. A much higher cement stiffness of 8GPa resulted in larger changes in the stresses. The most significant parameter in this study was found to be the changed load path as a result of partial height restoration. This induced a moment in the construct and increased the stresses and strains in the anterior compartments of each vertebra as well as marked in the adjacent (upper and lower) vertebrae. The factor of safety calculation showed the centre of the L3 vertebra to be the most failure prone in all cases, with the osteoporotic bone models showing higher fracture tendencies.

This study indicates that healthier bone has a better chance of survival. Cement properties with lower cement elastic moduli induce stresses/strains which are more similar to the intact model. The best way to reduce the likelihood of failure is to restore the vertebral height.