Stress Distribution at the Bone-cement Interface Changes during Kyphoplasty Rehabilitation

Balloon Kyphoplasty uses an inflatable bone tamp and cement augmentation to repair vertebral compression fractures. A recent clinical study observed a 78% re-collapse rate in patients showing a radiolucent phenomenon at the bone-cement interface following Kyphoplasty. Two experimental studies showed significant height loss following Balloon Kyphoplasty under cyclical loads. The present study investigates the alteration in load angle corresponding to this height loss and its effect on load transfer to the bone-cement interface. A validated finite element model of a human thoracolumbar spine was segmented into a single L1 vertebral body and modified to replicate bilateral Balloon Kyphoplasty. Cement was modeled using prolate spheroids surrounded by an interface region divided into anterior, middle and posterior sections. Interface thickness was calculated using a mathematical model with a bone volume fraction of 0.3 and 50% bone compaction. An 800N load was applied at angles of 0o and 20o from the vertebral axis. Results indicate that a change in the applied load angle significantly alters the principal stress components and directions across the interface region. This alteration in loading must be considered in the context of the highly compliant interface region and therefore is hypothesized to be a contributory factor to vertebral re-collapse.