Conventional posterior dynamic stabilization devices demonstrated a tendency towards highly rigid stabilization approximating that of titanium rods in flexion. In extension, they excessively offload the index segment, making the device as the sole loadbearing structure, with concerns of device failure. The goal of this study was to compare the kinematics and intradiscal pressure of monosegmental stabilization utilizing a new device that incorporates both a flexion and extension dampening spacer to that of rigid internal fixation and a conventional posterior dynamic stabilization device. The hypothesis was the new device would minimize the overloading of adjacent levels compared to rigid and conventional devices which can only bend but not stretch. The biomechanics were compared following injury in a human cadaveric lumbosacral spine under simulated physiological loading conditions. The stabilization with the new posterior dynamic stabilization device significantly reduced motion uniformly in all loading directions, but less so than rigid fixation. The evaluation of adjacent level motion and pressure showed some benefit of the new device when compared to rigid fixation. Posterior dynamic stabilization designs which both bend and stretch showed improved kinematic and loadsharing properties when compared to rigid fixation and when indirectly compared to existing conventional devices without a bumper.
Fusion using rigid pedicle screwrod instrumentation is a conventional surgical treatment for mechanical back pain due to disc degeneration when nonoperative treatment has failed. In spite of this standard, it is associated with implantrelated failures such as screw breakage or loosening. Screw breakage or loosening have been reported in the literature to range from 1% to 11.2% of the screws inserted [
The challenge for surgeons, biomechanists, and engineers has been to determine and develop an optimally stiff device that will provide enough rigidity across a destabilized spinal segment while simultaneously sharing load with the fusion mass. Posterior fixation devices have evolved from larger diameters and stiffer materials (6.5 mm cobalt chromium/stainless steel) to smaller diameters and less stiff or semirigid materials (5.5 mm poly ether ether ketone (PEEK)), respectively. Semirigid fixation or dynamic stabilization devices such as PEEK rods, titanium rods with helical grooves, and polymeric spacers with an interwoven cord tethered between pedicle screws have been designed to increase loadsharing in an attempt to induce compression on the bone graft and accentuate the concept of bone remodeling as first credited by Wolff [
In this particular study, the TRANSITION Stabilization System (Globus Medical, Inc., Audubon, PA) was utilized as the method of semirigid stabilization. The device was designed to bend and stretch by incorporating two polymeric spacers: one strategically placed above the cranial pedicle screw and the other between the pedicle screws, to allow a resistance to flexion, and a natural compression across the joint, respectively. We hypothesize that the compressibility across the surgical level may have implications on both the index and adjacent levels, but to what degree remains unknown.
The aim of this study was to evaluate the implanted and adjacent level kinematics and loadsharing effects of the human lumbosacral spine implanted with a semirigid fixation device, TRANSITION, compared to rigid fixation, and the historical performance of conventional semirigid devices. In this study, the injury model of the motion segment was created by a decompression involving facetectomy.
All spines were radiographed to ensure the absence of fractures, deformities, and any metastatic disease. The spines were stripped of paravertebral musculature while preserving the spinal ligaments, joints, and disk spaces. Subsequently, they were mounted at L1 rostrally and S1 caudally in a threetoone mixture of Bond Auto Body Filler and fiberglass resin (Bondo MarHyde Corp., Atlanta, GA). The spine was then affixed to a six degreeoffreedom (6DOF) testing apparatus, and pure unconstrained bending moments were applied in the physiological planes of the spine at room temperature using a multidirectional hybrid flexibility protocol [
Six degreeoffreedom testing apparatus, allowing unconstrained motion and rotations. Three motors, each placed in a physiological rotation direction providing pure rotations, while translational guide rails allow the forces to redistribute according to the kinematic properties of the spine. A_{AP}: guide rail with air bearings (anteriorposterior), A_{ML}: guide rail with air bearings (mediallateral), A_{CC}: guide rail with air bearings (cephaladcaudal), B: flexionextension motor, C: lateral bending motor, and D: axial rotation motor.
The semirigid device which can both bend and stretch (TRANSITION) is composed of titanium, polycarbonate urethane (PCU), and polyethylene terephthalate (PET) (Figure
The TRANSITION Stabilization System. The cephalad bumper shown in neutral and flexed position.
Nine intact fresh human cadaver lumbosacral spines (L1S1) were tested by applying a pure moment of ±8 Nm, according to the test standards for lumbar spine [
Surgical testing sequence. (1) Intact; (2) unilateral facetectomy (UF); (3) UF and unilateral TRANSITION device (UF + UT); (4) UF and bilateral TRANSITION device (UF + BT); (5) bilateral facetectomy (BF); (6) BF and bilateral TRANSITION device (BF + BT); (7) BF and bilateral rigid fixation with interbody spacer (BF + S + R).
Several comparisons were made to evaluate any statistical differences between constructs 1 and 7. The unilateral model (constructs 1, 2, 3, and 4) was evaluated separately from the bilateral model (constructs 1, 5, 6, and 7). Statistical comparisons were completed using a single factor, repeated measures analysis of variance (ANOVA). In all cases to alleviate inhomogeneity of variance, log transforms in the form of log_{10} (rawdata + 1) were applied to the raw data. Comparisons were made with a probability of type I error,
The range of motion (ROM) was determined for each surgical construct of the unilateral injury model (Figure
Index surgical level results of multidirectional flexibility testing for constructs 1, 2, 3, and 4 (unilateral model).
Increased motion due to the UF injury was expected to lead to reduced motions at the immediate adjacent levels in a displacement control protocol (Table
Unilateral model (construct 1, 2, 3, and 4) adjacent level ROM and pressure. Brackets show which construct groups are significant.
Intact  UF  UF + UT  UF + BT  






ROM (% of intact)  
Flexion  
L3L4  Mean 100 (SD 23) 
Mean 101 (SD 20)  Mean 109 (SD 24)  Mean 111 (SD 27) [1] 
L5S1  Mean 100 (SD 32)  Mean 98 (SD 26)  Mean 107 (SD 41)  Mean 108 (SD 35) 
Extension  
L3L4  Mean 100 (SD 11)  Mean 91 (SD 12)  Mean 101 (SD 16)  Mean 98 (SD 9) 
L5S1  Mean 100 (SD 26)  Mean 118 (SD 28)  Mean 133 (SD 43)  Mean 126 (SD 35) 
Lateral bending  
L3L4  Mean 101 (SD 16)  Mean 98 (SD16) [3, 4]  Mean 105 (SD 17) [2]  Mean 106 (SD 20) [2] 
L5S1  Mean 100 (SD 28) 
Mean 104 (SD 29) [3, 4]  Mean 114 (SD 31) [1, 2]  Mean 116 (SD 31) [1, 2] 
Axial rotation  
L3L4  Mean 100 (SD 31) 
Mean 89 (SD 29) [1, 3, 4]  Mean 93 (SD 28) [1, 2]  Mean 94 (SD 28) [1, 2] 
L5S1  Mean 100 (SD 22)  Mean 99 (SD 25)  Mean 110 (SD 27)  Mean 109 (SD 26) 
 
Pressure 

Flexion  
L3L4  Mean 100 (SD 42) [4]  Mean 144 (SD 33)  Mean 166 (SD 38)  Mean 220 (SD 76) [1] 
L5S1  Mean 100 (SD 58) [3, 4]  Mean 141 (SD 62)  Mean 161 (SD 53) [1]  Mean 207 (SD 82) [1] 
Extension  
L3L4  Mean 100 (SD 21)  Mean 74 (SD 26)  Mean 78 (SD 31)  Mean 99 (SD 24) 
L5S1  Mean 100 (SD 84)  Mean 103 (SD 78)  Mean 120 (SD 89)  Mean 113 (SD 96) 
Lateral bending  
L3L4  Mean 100 (SD 54) [4]  Mean 97 (SD 59) [4]  Mean 109 (SD 66) [4]  Mean 127 (SD 76) [1, 2, 3] 
L5S1  Mean 100 (SD 78)  Mean 90 (SD 70)  Mean 90 (SD 65)  Mean 94 (SD 70) 
Axial rotation  
L3L4  Mean 100 (SD 44)  Mean 92 (SD 33)  Mean 110 (SD 36)  Mean 81 (SD 21) 
L5S1  Mean 100 (SD 33)  Mean 85 (SD 35)  Mean 100 (SD 45)  Mean 87 (SD 33) 
With respect to intact, adjacent level motion was significantly increased in lateral bending at L5S1 by both PDS constructs (UF + UT: 114% of intact,
Intradiscal pressure measurements of adjacent levels (Table
The range of motion (ROM) was determined for each surgical construct of the bilateral injury model (Figure
Index surgical level results of multidirectional flexibility testing for constructs 1, 4, 5, and 6 (bilateral model).
The trend of index level motion follows the model BF + S + R < BF + BT < BF, where all constructs were statistically different than one another. The stabilization with TRANSITION PDS device reduced the ROM values, which were, in terms of intact, 44% (
Increased motion due to the BF injury at the index level is expected to lead to reduced motions at the immediate adjacent levels in a displacement control protocol (Figures
Cranial adjacent level results of multidirectional flexibility testing for constructs 1, 5, 6, and 7.
Caudal adjacent level results of multidirectional flexibility testing for constructs 1, 5, 6, and 7.
The loadbearing effect at the adjacent levels, as measured by intradiscal pressure, (Figures
Cranial adjacent level intradiscal pressures of multidirectional flexibility testing for constructs 1, 5, 6, and 7.
Caudal adjacent level intradiscal pressures of multidirectional flexibility testing for constructs 1, 5, 6, and 7.
Conventional rigid fusion in the surgical treatment for chronic low back pain has some negative side effects such as the potential for adjacent segment degeneration and screw loosening. The concept of semirigid or dynamic stabilization has evolved to possibly prevent such degeneration, if it is not a function of natural disease progression, mainly through the reduction of stress at the adjacent segments. Softstabilization devices were developed to permit loadsharing with the anterior column to accomplish solid fusion and, at the same time, provide a softer posterior implant stiffness. Consequently, semirigid instrumentation is expected to lower screw breakage associated with transmission of forces through posterior instrumentation as opposed to through the anterior column. While there is some disparity between the potential uses of PDS systems (whether they are for reducing adjacent level degeneration or for promoting fusion through loadsharing), the ubiquitousness of such systems cannot be ignored. Their prevalence currently has more to do with dissatisfaction with conventional fusion than a proven efficacy. This study attempts to characterize the biomechanical efficacy of a select system. The clinical efficacy has yet to be determined. It remains to be seen if “soft fusion” can be achieved and if, in the presence of boney ingrowth with weaker mechanical properties, adjacent level effects can be ameliorated.
The purpose of this study was to evaluate the stability of using a posterior dynamic stabilization (PDS) device which differs from conventional PDS devices in two ways: (1) by the addition of both flexion and extension dampening materials; and (2) by the addition of titanium spools (attached to the screw heads) which slide along the PET cord. The primary aim was to compare this device to rigid fixation with pedicle screws and rods. The hypothesis is that the new PDS design will loadshare with the surgical level more effectively, therefore minimizing the overload effect of the adjacent levels compared to the conventional rigid and PDS devices.
Both the PDS and rigid devices produced significant stabilization, but a consistent and significant trend of increased flexibility was observed in all loading modes for BF + BT (TRANSITION) when compared to BF + S + R (rigid). TRANSITION led to ROM values which were, in terms of intact, 44%, 62%, 58%, and 125% in
The PDS device used in this study resulted in kinematic and loadsharing trends which appear different when compared to trends observed in conventional PDS designs within the literature [
In a finite element study by Schmidt et al., the authors predicted the performance of PDS devices in different loading modes, as a function of polymer properties [
The PDS test device reduced adjacent level hypermobility caused by rigid fixation. The trend of adjacent level motions followed the model BF + S + R ≥ BF + BT ≥ BF for all loading modes at both L3L4 and L5S1, indicating the utility of semirigid stabilization to offset adjacent level effects. While this trend is encouraging to alleviate adjacent level stresses, its clinical relevance needs to be proven. The question “How much is offloading ideal?” remains to be answered. Nevertheless, the new PDS device produced significantly smaller motions than rigid fixation at the adjacent levels, in flexion (only at L5S1), extension (only at L3L4), and lateral bending (only at L3L4).
Intradiscal pressure measurements at the adjacent level reflected the same trends as the ROM, but, in flexion, the relationship between ROM and IDP was nonlinear. For example, a 22% increase in L3L4 level motion caused by L4L5 rigid fixation, resulted in 105% increase in the IDP value. Moreover, the stabilization with PDS device (BF + BT) was not able to restore these large pressure that increases to near the intact value. If adjacent level disease is indeed related to a physiological imbalance in loadsharing and kinematics of segments juxtaposed to the fusion site, then the role of motion versus pressure on the rate of disease progression needs to be determined. Since these factors are nonlinearly related, restricting the motion may not be sufficient at buffering the loadsharing effects on the adjacent level.
There were certain limitations in this study. One objective was to relate the biomechanical differences observed between this study and those found on the widely studied conventional device, Dynesys. The ideal way to evaluate the difference was to compare TRANSITION versus Dynesys directly. In the current study, this comparison was indirect from the literature data. The reason behind this was that testing TRANSITION and Dynesys on the same specimen was not possible because the pedicle screws are different in the two systems, and the reinsertion of the pedicle screws in the same specimens introduces unacceptable errors because of loosening at the screwbone interface. Removing the bumper alone from the TRANSITION does not make it comparable to Dynesys. The second limitation of this study was the bilateral facetectomy injury model, which may not be the most common scenario of a decompression clinically. However, facetectomy produced considerable instability, possibly more than what can be achieved by nucleotomy alone. The injury model was chosen because of the benefit of having a greater degree of instability (or worstcase scenario). Thirdly, testing pedicle screws and rods without an interbody device would have provided some information in the comparison of rigid rods and TRANSITION. Nevertheless, the authors were predominately interested in seeing the maximum change in the rigidity between interbody fusion with internal fixation and semirigid posterolateral fusion. Lastly, there is a certain amount of error introduced via suboptimal device placement which can occur via difficulty in the anatomy, irregular curvatures, or even screw placement. The PDS device considered made use of individually sized PCU spacers which were trialed to appropriate length. The implants are also preassembled with a constant tension of 220 N, so there should never be a case where one side of the disc space is artificially tensioned more than the other. Therefore, device placement was not separately considered in the analysis of variance.
The semirigid fixation/dynamic stabilization device investigated in this study, which utilized posteriorly placed flexion and extension dampening materials, was able to reduce the motion (
The authors would like to thank Kurt Faulhaber for his contribution to the artwork in Figure