P19. Vertebra/implant response to cyclic loading is influenced by central strut support in intervertebral devices

BACKGROUND CONTEXT The presence of a central strut may serve to strengthen a spinal spacer. However, it is unknown how a strut located in the central endplate region will influence vertebral body response under cyclic loading. PURPOSE The authors hypothesized that devices with an open design will display comparable stiffness to devices with a central strut. Further, open devices would not restrict movement of the vertebral endplate and therefore display improved viscoelastic properties of the construct as compared to implants with a central strut. STUDY DESIGN/SETTING Implants were fabricated from Ti-6Al-4V alloy represented an Open (contact area 427.5mm2) and Strutted device (contact area 407.9mm2 to 504.5mm2 depending on strut contact). PATIENT SAMPLE Fourteen L4 or L5 porcine vertebra were randomly assigned to the implant groups. OUTCOME MEASURES The ratio of the unloading/loading (loss/storage) stiffness values are used to compute Tan (δ). Traditionally Tan (δ) is associated with the loss and storage moduli. Such a calculation is not possible due to the complexity of contact area for a vertebral body. However, within any one specimen, the contact area is uniform and results in a scalable factor of the actual Tan (δ). METHODS Loading from 50N to 500N was applied at 1Hz for 500 cycles. Load and deformation data were acquired at 60Hz at 20 cycle intervals. The loading (storage) stiffness, unloading (loss) stiffness and mean Tan (δ) were computed at each cycle interval and subjected to non-linear regression. The respective values for K (Rate) and Span (Change from initial value) for each implant type were compared using an unpaired t-test. RESULTS For the loading stiffness, the regression analysis resulted in two rate parameters: KFast and KSLow. In the case of unloading stiffness, single rate functions were preferred. No statistical differences for loading stiffness were found between open and strutted devices for KFast (P>0.12) and KSlow (P>0.18). For the unloading stiffness, the K value for open devices was significantly decreased as compared to the strutted devices (P CONCLUSIONS While metal devices encompassing an open concept are comparable in stiffness to devices containing a central strut, open designs can permit the unimpeded, viscoelastic response of the vertebral body which may be beneficial in the remodeling process. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.