Biomechanical evaluation of a new pedicle screw-based posterior dynamic stabilization device (Awesome Rod System) - a finite element analysis

Acting as a tension band at the posterior spinal column, the pedicle-screw-based PDS system has been proved to share loads with the anterior spinal column [ 7- 9, 21, 22]. According to Wolff’s law, the force transmitted to an interbody graft and avoidance of stress shielding could potentially increase the rate of successful arthrodesis [ 8, 23, 24], which was also proved by the biomechanical study of Scifert et al. [ 25]. Moreover, pedicle-screw-based PDS system standing alone as a non-fusion procedure have been reported to yield satisfactory clinical results, as evaluated by postoperative radiographs and functional scores [ 10- 13]. Without the requirement of bone grafting, soft tissue stripping would not occur beyond bilateral facet joints, blood loss and surgical times would be diminished, and degeneration at the instrumented levels would have the opportunity to be reversed. Reyes-Sa’nchez et al. [ 10] presented a 2-year clinical report of 20 patients with an AccuFlex system implanted as a stand-alone device for non-fusion procedures. The termination of disc degeneration was observed in 83% of the patients and rehydration was observed in 16% of the patients, despite two cases of implant failure. Regarding the use of a similar system as a preventative reinforcement and an adjunct to fusion surgery, Hudson et al. implanted a rigid fixation system for fusion at the caudal levels, and implanted dynamic instruments at the more cephalic level of the instrumented lumbar vertebrae without fusion in a 28-patient clinical report with 2-year follow-up [ 26]. No change at the adjacent level observed and disc height was preserved at all levels; furthermore, improved functional outcome was observed. The authors concluded the 2-year study of hybrid dynamic stabilization with a pedicle-screw-based PDS system, which showed satisfactory performance. However, there was no evidence to suggest pedicle-screw-based PDS superior to traditional fusion constructs [ 13, 26, 27].

Similarly to the AccuFlex system, the flexibility of the Awesome Dynamic Rod System originates from the cuts created on the surface of the rods. The curved- cut design is similar to that of the AccuFlex, which features helical cuts on the surface of regular rods with the same diameter [ 21]. The AccuFlex system is approved by the Food and Drug Administration for lumbar fusion when used in conjunction with an anterior interbody device, and the helical cuts on the rods provide flexibility that allows for motion in the flexion-extension mode. According to the biomechanical study conducted by Mandigo et al. [ 21], the intended flexion and extension movements of the AccuFlex system increase the load transmitted through an anterior interbody graft by more than 50% compared with that afforded by a rigid construct, creating a “load-sharing” capability to increase the potential for fusion through Wolff’s law [ 23]. Biomechanical tests revealed the rods could withstand the normal stresses exerted on the lumbar spine with an adequate fatigue life. Clinical results indicated that patients to which the AccuFlex system was administered for fusion surgery exhibited statistically similar fusion rates and outcomes compared with patients receiving rigid rod fixation after one year. Another clinical study conducted by Reyes-Sánchez et al. [ 10] demonstrated that 22.22% of patients receiving AccuFlex constructs for posterior dynamic stabilization required hardware removal due to fatigue, including two cases with broken rods, whereas in 83% of cases no progression of disc degeneration was observed at the instrumented level. Moreover, three patients showed disc rehydration. To prevent cuts from jeopardizing the strength of the rods [ 10], the Awesome Dynamic Rod System reinforces the segments of rods where cuts are created by enlarging the rod diameter, which are referred to as the “joint parts” of rod; those segments not reinforced are referred to as the “rigid parts” of rods. In addition, unlike the helical cuts of the AccuFlex system, the curve cuts of the Awesome Dynamic Rod System are not helical and allow for the vacant spaces of joint parts to be pressed only in the flexion-extension direction, hindering extra movement of the rods in lateral bending or torsion. However, the effect of enlarging the rod diameter may also neutralize the semi-rigid characteristics provided by the curve cuts.

In the current study, as a posterior dynamic stabilization system, AWE reduced the ROM at the instrumented levels L4-L5 the least in all moments except for extension. As an adjunct to lumbar fusion, AWEFUS reduced the ROM at the instrumented level less than fusion with the rigid rod system, FUS, at all moments except for left bending. Thus, Awesome Dynamic Rod System could preserve motion at the instrumented level better than fusion with rigid rods, not only as a posterior dynamic stabilization system but also as an adjunct to lumbar fusion, although not in all moments. This result is in accord with an in vitro study conducted by Jin et al. [ 28] comparing the semi-rigid PEEKs rods as a dynamic fixation system, as well as an adjunct to interbody cage fusion procedures, to traditional rigid rods. The authors reported that the PEEK rods sustained lower ROM decrement at the instrumented level than did the traditional rigid rods during flexion, but no significant difference was observed during extension. The curve cuts on the thicker joint parts caused the Awesome Dynamic Rod system to be semi-rigid in the flexion-extension mode but not in lateral bending or in the torsion direction. The Awesome Dynamic Rods did not enhance the dynamic design during extension perhaps due to facet joints withstanding the moment and backward shifting of the helical axis of motion of the lumbar spinal column [ 29] producing a shorter radius of curvature for a negative bending moment and making it more difficult to bend the dilated joint part of the dynamic rods. Moreover, as an adjunct to lumbar fusion with a cage installed in the disc space, the Awesome Dynamic Rod still demonstrated more flexibility than the rigid rod system did, as indicated by the results obtained for AWEFUS and FUS.

For the adjacent level, i.e., L3-4 for AWE and FUS and L2-3 for AWEFUS, AWE showed a minimal increment, but AWEFUS showed a maximal increment in ROM during flexion, left bending and left torsion. These results suggest that AWE had a weaker effect on the ROM of the adjacent level than did FUS, and potentially alleviating the rate of acceleration of degeneration at the adjacent level. However, to compensate for the maximal decrement in the ROM at the instrumented level during extension, AWE increased the ROM mostly at the adjacent level in the same direction. According to the data presented Table  1, AWEFUS shows a greater increase in ROM at the L1-L2 and L2-L3 level than did the other three models. For AWEFUS, L2-L3 was the level adjacent to the instrumented segments, but the increase in ROM at L2-L3 for AWEFUS was even greater than that at L3-L4 for FUS. However, it is too early to conclude that AWEFUS is almost as stiff as two-level fusion. Using the hybrid method [ 20] to evaluate the adjacent level effect, the total ROM of the entire spine would be shared by the non-operated levels of the spine models (3 levels for FUS and AWE, and 2 levels for AWEFUS). Hence, observing a much higher ROM for L1-L2 and L2-L3 of AWEFUS would not be surprising. If we summed up the ROM of L1-L2, L2-L3 and L3-L4 of FUS and subtracted less than one degree (the supposed ROM of fused L3-L4) and then divided the result by 2 to simulate the ROM of L1-L2 and L2-L3 at two-level fusion, we would find that the result would still be greater than that for AWEFUS. Thus, when used as a preventative reinforcement, AWEFUS could certainly alleviate the adjacent level effect to a certain extent but not by much. Gillet et al. advocated the application of a type of preventative reinforcement for transitional segments to delay transitional segment alteration after lumbar fusion [ 14]. Awasthi et al. [ 16] the presented clinical outcomes of 13 patients treated with the Scient’X Isobar TTL system topping off lumbar fusion. Owing to the distinct improvement in functional scores, including the Oswestry Disability Index and Prolo scale, the authors concluded that the posterior dynamic stabilization device does provide effective stabilization for the transitional segment above a fusion and postpones the rate of degeneration at these cephalic adjacent levels. Hudson et al. [ 26] prospectively implanted dynamic rod systems at the most cephalic level of lumbar degeneration as a preventative reinforcement on the transitional zone in a clinical study, with fusion performed at the causal levels a of degenerative lumbar spine. The authors did not observe any change in the ROM at the levels above the transitional zone 2 years after operation in compared with that observed upon preoperative measurement. However, the disc height ratio at the level above the index level increased significantly by 14.6%. Nevertheless, the author concluded that the preliminary results at 2 years were satisfactory. As a preventative reinforcement at a level adjacent to that of fusion surgery, AWEFUS did not demonstrate a suitable effect in diminishing the hypermobility of the cephalic adjacent segment, but conversely aggravated its effect on the ROM of the adjacent level, despite providing a sufficient motion constraint at the transitional segment L3-L4.

The same trend was reflected in the disc stress at the adjacent level. That disc stress increases more extensively in flexion than in other moments suggests that discs sustain more stress during flexion. The maximum annulus stress occurring at the anterior edge of the annulus fibrosus during flexion, as shown in Figure  2, corresponds to that reported in a previous study [ 18], which also demonstrated that the greatest annulus stress occurred at the adjacent cranial level located at the anterior edge of the annulus fibrosus. The disc stress distribution shown in Figure  2 reveals that AWE showed a smaller increase in stress at the adjacent level than did FUS. This finding corresponds well with the results obtained for ROM at the adjacent level and suggests that AWE showed less stress concentration than did FUS at the adjacent level. Disc stress at L3-4 of AWEFUS was shielded by the Awesome Dynamic Rod System. As was observe for ROM, however, the disc stress at the L2-L3 level of AWEFUS was much greater than that observed for the other three models. AWEFUS did not eliminate stress effectively at the adjacent disc when acting as a preventative reinforcement in the transitional zone.

The maximal facet contact forces of INT at the L4-L5 level were 135 N in torsion, 54 N in extension, and 21 N in lateral bending. There was no contact force in flexion. This result is in agreement with the FE study conducted by Wilke et al., who loaded a pure 7.5 Nm stress on a L4-L5 FE spinal model. The authors predicted maximum facet joint forces of 105 N under torsion, 50 N under extension and 36 N under lateral bending, and the facet joints remained unloaded during flexion [ 30]. At the adjacent level, AWE showed a lower contact force than did FUS in left bending and left torsion but a greater contact force in extension. As demonstrated in a previous study [ 17, 18], the incremental increases in the facet joint contact forces at the cranial adjacent level were proportional to the segment stiffness at the instrumented level.

The limitations of the FE analysis conducted in this study are as follows. The characteristics of disc degeneration, such as dehydration and reduced disc height, were not taken into account. The thread on the pedicle screw was ignored. Because this study aimed to examine how the new Awesome Rod System affects spinal biomechanics rather than the mechanical interaction between screw threads and bone, this study assumed complete osteointegration in between bone and screw. The conclusions drawn in this study are based on these limitations. Additionally, the FE study could only demonstrate the biomechanical effects of the implants when moments were loaded on the models. Further clinical studies should be conducted to monitor long-term implant fatigue and graft fusion.