Effects of bone cement volume and distribution on vertebral stiffness after vertebroplasty

Study Design. The biomechanical behavior of a single lumbar vertebral body after various surgical treatments with acrylic vertebroplasty was parametri cally studied using finite-element analysis. Objectives. To provide a theoretical framework for understanding and optimi zing the biomechanics of vertebroplasty, Specifically, to investigate the e ffects of volume and distribution of bone cement on stiffness recovery of t he vertebral body. Summary of Background Data. Vertebroplasty is a treatment that stabilizes a fractured vertebra by addition of bone cement. However, there is currently no information available on the optimal volume and distribution of the fil ler material in terms of stiffness recovery of the damaged vertebral body. Methods. An experimentally calibrated, anatomically accurate finite-element model of an elderly L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements in response to a uniform co mpressive load. After virtual vertebroplasty (bone cement filling range of 1-7 cm(3)) on the damaged model, the resulting compressive stiff ness of th e vertebral body was computed for various spatial distributions of the fill ing material and different loading conditions. Results, Vertebral stiffness recovery after vertebroplasty was strongly inf luenced by the volume fraction of the implanted cement. Only a small amount of bone cement (14% fill or 3.5 cm(3)) was necessary to restore stiffness of the damaged vertebral body to the predamaged value. Use of a 30% fill in creased stiffness by more than 50% compared with the predamaged value. Wher eas the unipedicular distributions exhibited a comparative stiffness to the bipedicular or posterolateral cases, it showed a medial-lateral bending mo tion ("toggle") toward the untreated side when a uniform compressive pressu re load was applied. Conclusion. Only a small amount of bone cement (similar to 15% volume fract ion) is needed to restore stiffness to predamage levels, and greater fillin g can result in substantial increase in stiffness well beyond the intact le vel. Such overfilling also renders the system more sensitive to the placeme nt of the cement because asymmetric distributions with large fills can prom ote single-sided load transfer and thus toggle. These results suggest that large fill volumes may not be the most biomechanically optimal configuratio n, and an improvement might be achieved by use of lower cement volume with symmetric placement.