General Session: Biomechanics
Presented by: S. Nagaraja - View Audio/Video Presentation (Members Only)
V. Palepu(1), J. Peck(2), M. Helgeson(3), D. Simon(1), S. Nagaraja(1)
(1) US Food and Drug Administration, Divison of Applied Mechanics, Silver Spring, MD, United States
(2) US Food and Drug Administration, Office of Device Evaluation, Silver Spring, MD, United States
(3) Walter Reed National Military Medical Center, Bethesda, MD, United States
Background: In recent years lumbar integrated fixation cages (IFC) have been introduced as a new fusion technology to reduce surgical time and implant profile. IFC devices have different screw insertion locations (through the vertebral endplate) and trajectories (30°-60° from the vertebral endplate) than screws in traditional supplemental fixation devices such as anterior plating. However, it is unclear if the fixation strength and stability of IFCs are comparable to an anterior plate and traditional cage (AP+C), particularly before fusion has occurred. Therefore, the purpose of this study was to evaluate the biomechanical stability of IFC devices compared to AP+C under fatigue loading.
Methods: L2-3 and L4-5 functional spinal units (FSUs) were obtained from twelve human lumbar spines. These FSUs were subjected to pure moments (7.5 Nm) in flexion-extension (FE), lateral bending (LB), axial rotation (AR) and the resulting range of motion (ROM) was recorded. These FSUs were then reconstructed using an anterior approach by a board certified spine surgeon. Screws used for both AP+C and IFC were of the same size (5.5mm diameter and 25mm length). To minimize variability, the L2-3 and L4-5 of a given donor were randomized to receive either AP+C or IFC. Moments were applied to these reconstructed specimens to record change in ROM. Next, the reconstructed specimens were fatigued with a flexion-extension load of ±3 Nm at 1Hz for first 5,000 cycles and the load was raised to ±5 Nm until 20,000 cycles. ROM difference from post reconstruction was calculated in FE, LB, and AR, at 5,000, 12,500 and 20,000 cycles. Elastic zone stiffness and lax zone parameters were also calculated for flexion and extension in all specimens.
Results: In FE, AP+C specimens had similar (p>0.28) ROM difference compared to IFC specimens at 5,000 cycles, 12,500 cycles, and 20,000 cycles (Figure 1). In LB, AP+C specimens and IFC specimens also exhibited similar (p≥0.28) ROM changes at 5,000 cycles, 12,500 cycles, and 20,000 cycles. In AR, AP+C specimens showed increased changes in ROM compared to IFC specimens at 5,000 cycles (p=0.03), 12,500 cycles (p=0.07), and 20,000 cycles (p=0.14). There were no significant differences in elastic zone stiffness (p >0.30) and lax zone (p>0.20) between AP+C and IFC specimens in flexion and extension.
Conclusions: Overall, these results indicate that IFC performs similarly to a traditional fixation method in the short term prior to fusion. In fact, IFC appeared to outperform AP+C in AR, possibly due to the location of the IFC screws in the intervertebral space offering increased mechanical advantage during rotation. Future work should assess the performance of cervical IFCs, lateral lumbar IFCs, and IFCs compared to pedicle screw systems, the gold standard for lumbar fixation.