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Biomechanical evaluation of vertebroplasty and kyphoplasty with polymethyl methacrylate or calcium phosphate cement under cyclic loading.

Wilke HJ, Mehnert U, Claes LE, Bierschneider MM, Jaksche H, Boszczyk BM

Institute of Orthopaedic Research and Biomechanics, University of Ulm, Ulm, Germany. hans-joachim.wilke@uni-ulm.de

STUDY DESIGN: We developed a new method to simulating in vivo dynamic loading as closely as possible, which allows comparison of kyphoplasty and vertebroplasty, as well as augmentation materials. OBJECTIVE: Special interest was given to calcium phosphate cement, which might fail due to its brittleness. SUMMARY OF BACKGROUND DATA: Vertebroplasty and kyphoplasty are, with limitations, 2 promising alternative techniques to augment osteoporotic vertebrae with polymethyl methacrylate or calcium phosphate cements. However, little is known about the fatigue characteristics of the treated vertebrae under cyclic loading. METHODS: Twenty-four intact, osteoporotic bi-segmental human specimens were divided into 4 groups: (1) vertebroplasty with polymethyl methacrylate, (2) kyphoplasty with polymethyl methacrylate, (3) kyphoplasty with calcium phosphate cement, and (4) untreated control group. After augmentation of the middle vertebrae, all specimens underwent 100,000 cycles of eccentric loading during which the specimen revolved around its longitudinal axis. Pre-loading and post-loading radiographs, and subsidence measurements at different sites of the vertebrae were taken. The overall height was additionally determined every 20,000 cycles in the material testing machine. Finally, the specimens were cryosectioned to examine the cements. RESULTS: Loss of height progressed with strong individual differences in all groups, with an increasing number of load cycles up to median values of 2.8 mm for both augmented groups and 4.2 mm for the nonaugmented group. At the center of the upper endplate, subsidence in kyphoplasty was greater than in vertebroplasty, with little differences with respect to the kind of cement. The cryosections did not show any signs of fatigue in the polymethyl methacrylate, but small cracks were in the calcium phosphate. CONCLUSIONS: Vertebroplasty and kyphoplasty seem to be equivalent methods in strengthening osteoporotic vertebrae. However, these results cannot be transferred to the treatment of fractures with these methods. A "physiologic" loading situation was achieved by complex motion, including all combinations of flexion/extension with lateral bending during eccentric cyclic loading.

Published 1 December 2006 in Spine, 31(25): 2934-41.
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