Intervertebral disc recovery after dynamic or static loading in vitro: is there a role for the endplate?

In vivo studies on disc mechanics show loss of fluid from the intervertebral disc (IVD) during loading and full recovery during rest. Previous work indicated that in vitro recovery is hampered after static loading. The aim of the present study was to investigate the role of the endplate after dynamic and static loading on mechanical recovery in vitro. Lumbar spines (caprine) were obtained from the local slaughterhouse and stored frozen. Twenty-four intervertebral discs were thawed and subjected to a compression test in a saline bath (37 degrees C). The discs were pre-loaded at 20 N for 15 min. Three 15-min loading cycles (static: 2.0 MPa or dynamic: average load 2.0 MPa at 0.5 Hz) were applied, each followed by a 30-min period of unloading (20 N). After this protocol, the endplates of half of the discs were blocked with silicone paste and the long-term recovery protocol was applied; the discs were subjected to a single loading cycle (15 min of static or dynamic loading) followed by 10h of unloading at 20 N. All specimens showed a net loss of height and a gain in stiffness during the first part of the test. Eventually, height and stiffness were restored during a long-term recovery test. The difference in recovery between blocked and free endplates was marginal. If fluid flow plays a role during recovery in vitro, the role of the endplate appears to be limited. Our findings show no influence of loading type on recovery in vitro.