FROZEN STORAGE AFFECTS THE COMPRESSIVE CREEP-BEHAVIOR OF THE PORCINE INTERVERTEBRAL DISC

Study Design. A biomechanical study of the compressive creep behavior of the porcine intervertebral disc before and after frozen storage. Ob jective. To determine whether frozen storage alters the creep response , hydration, and nuclear swelling pressure of the intact intervertebra l disc. Summary of Background Data. The mechanical response of the dis c is dominated by swelling and fluid flow, whose effects are time-depe ndent. Because fluid content, which may change during storage, plays a significant role in the disc's time-dependent behavior, changes in me chanical response due to freezing may have been missed in previous stu dies that focused on time-independent behavior only. Methods. Porcine intervertebral discs were tested in repealed cycles of compressive cre ep either immediately postmortem or after 3 weeks of frozen storage. S welling pressure and nuclear hydration were also measured in fresh and frozen discs. A fluid transport model was used to analyze the creep d ata. Results. The creep behavior of the intact porcine intervertebral disc is dramatically affected by frozen storage. The apparent permeabi lity of the frozen discs was 82% higher than that of the fresh discs, and the swelling pressure of frozen discs was 25% lower in frozen disc s (P < 0.01). The behavior of fresh and frozen discs became more dissi milar with repeated cycles of creep. Conclusions. In vitro tests of fr ozen porcine intervertebral discs do not represent fresh behavior. Fro zen storage appears to permanently alter disc behavior. The precise na ture of any freezing-induced damage, and whether frozen storage simila rly affects human discs, remains to be seen.