Segmental degeneration and instability are the main causes for low back pain in elderly patients. After failed conservative treatment, surgery is known to be the last way to dispose of this problem.1 The gold standard for surgical treatment is the circumferential spinal fusion based on rigid pedicle screw instrumentation, it is sometimes accompanied with neural decompression.1–3 With the increase of spinal fusion operations, a documented rise of progressive adjacent segment disease (ASD) has been observed.4
ASD has been defined as “any abnormal process that develops next to a spinal fusion.”4 The most commonly reported abnormality of ASD is disk degeneration, defined by a loss of disk height or intervertebral space narrowing.4 Other complications have been reported as instability, ventrolisthesis, disk prolapsed, stenosis, or hypertrophic facet joints.4
The causes for ASDs are not yet fully resolved. For some authors, a genetic predisposition is the primary mechanism.5,6 Others believe that the confirmed presence of an increased range of motion (ROM) (hypermobility) in the segment above the fusion might contribute to overstress of the segment and hasten the development of ASD.5–8 Furthermore, the postoperative sagittal imbalance seems to be another risk factor.9,10
Several clinical studies have confirmed the presence of hypermobility and the accelerated progression of degeneration in the segment above a fusion leading to poorer outcomes with increasing back pain, a reduction of quality of life, and the necessity of reoperation.11–15
Literature suggests that the loss of the ROM within the fused segments leads to an increased motion in the adjacent segments.16,17 Another major problem of long rigid fixation is that it increases the mechanical load and the intradiscal pressure on the adjacent segment, which is prone to accelerate degeneration of that segment.4,18 Stiffness at the cranial end of the construct and the length of the fusion are also shown to have influence on adjacent segment degeneration.4,7,8,15,19,20
Dynamic implants have been developed to establish stabilization and load-bearing capacity while simultaneously allowing moderate motion of the implanted segments. Recently, hybrid systems have been proposed which place a dynamic system at the end of a rigid instrumentation (topping off). It is suggested that the hybrid stabilization might reduce adjacent segment hypermobility and, therefore, would avoid the overstress of the disk in the adjacent segment.21 Published data on hybrid systems are limited.
To the authors knowledge there are only few biomechanical studies that investigated the problem of ASD above or below a topping off system.2,15,21 Strube et al15 compared the effects of rigid and hybrid stabilization to the corresponding adjacent segments. They demonstrated that the compensatory hypermobility in the adjacent segments increased with a higher number of instrumented levels and with an increase in the stiffness of the implant. In the biomechanical study of Mageswaran et al,2 similar results were shown. Both rigid and hybrid instrumentation caused significant increase of motion in the cranial and caudal adjacent segments. Durrani et al21 investigated the ROM of the instrumented and distal adjacent segments of a 5-level and 6-level rigid instrumentations with and without a dynamic stabilization device at the caudal end. The rigid instrumentations demonstrated an increased motion in the caudal adjacent level; whereas, the construct with distal dynamic instrumentation showed an adjacent level motion near to that of the intact spine.
The aim of this ex vivo, cadaveric study is to investigate lumbar spinal motion within the instrumented levels and at the adjacent segments of a hybrid construct (2-segment rigid instrumentation plus single-level dynamic stabilization) to compare it with a 2-segment rigid instrumentation and long rigid 3-segment instrumentation. The hypothesis is that the hybrid construct limits the ROM in the dynamic instrumented level but allows more motion than a rigid instrumentation. This could possibly reduce motion in the adjacent segment and thereby prevent hypermobility and progression of ASD.
MATERIALS AND METHODS
Human spinal specimens with 6 vertebrae each (Th12–L5) were divided into 2 groups to compare the segmental motion between rigid and hybrid instrumentation.
Specimen Preparation and Instrumentation
Eight intact human spines from 4 female and 4 male donors with a mean age of 50.1 (SD, 5.2 y; range, 43–59 y) were harvested, double-sealed, and stored at −22°C until the testing day. Before testing, a quantitative computed tomographic scan (slice thickness of 0.75 mm; Siemens Somatom, Definition Flash, Siemens AG, Erlangen, Germany) was performed to determine the apparent volumetric bone mineral density (BMD) and to screen for any structural or posttraumatic deformities. Specimens were excluded from the study if they met an exclusion criterion: previous spinal surgery, trauma or metastatic disease of the spine, scoliosis deformity (>15 degrees), and an age below 40 years or above 60 years. Apparent BMD for each specimen was calculated by cropping a 25×25×25 voxel cube (Avizio 5.1, Mercury Computer Systems, San Diego, CA) from the center of the L1, L2, and L3 vertebral bodies. The average Hounsfield unit value was scaled linearly to the reference densities of a phantom (QRM-BDC, QRM, Möhrendorf, DE) containing 3 different concentrations of calcium hydroxyapitate solution (0, 100, 500 mg/mL).
The specimens were divided into 2 experimental groups—rigid and hybrid (N=4 each), based on sex, body height, body weight, spine height, ratio of disk height to total height, body mass index, and BMD. There were no significant differences between the 2 groups (Table 1).
The night before testing the specimen was thawed at 8°C. On the morning of testing the spine was prepared by removal of all soft tissue; taking care to preserve the facet joint capsules, spinal ligaments, and osseous structures. To reduce tissue degradation the specimens were kept moist during the entire preparation and testing period by spraying with Ringer solution.
All instrumentations were performed by an experienced spine surgeon under fluoroscopic guidance to ensure the proper anterior-posterior and lateral position of the pedicle screws. The rigid group (Fig. 1C1) consisted of a 3-segment (L2–L5) instrumentation (OCPS, OrthoCube AG, Baar, Switzerland). The OCPS system, consists of self-tapping, conical, titanium pedicle screws (Ø5.5–7.2 mm; length, 40–55 mm) connected by a titanium alloy rod (Ø5.5 mm) (Fig. 2A). The hybrid group (Fig. 1C2) included a 2-segment (L3–L5) rigid instrumentation with the same OCPS system connected to a 1-level (L2–L3) dynamic system (Elaspine Flextension, Spinelab AG, Winterthur, Switzerland) to form a “hybrid” construct. The dynamic system consists of self-tapping, titanium pedicle screws (Ø6.5 mm; length, 45 mm) connected by a semiflexible, polycarbonate urethane rod (Ø10 mm). The polycarbonate urethane rod was directly linked to the conventional titanium rod by a rod adapter (Fig. 2B).
The cranial half of Th12 and the caudal half of L5 of the prepared and implanted specimen were then embedded in 2-component polymer resin (Ureol, Rencast FC 53, Huntsman Advanced Materials GmbH, Basel, Switzerland). The specimen was positioned such that a neutral posture was maintained and the L2–L3 lumbar disk was aligned horizontal to the embedding plates.
Each specimen was mounted in a servohydraulic material testing machine (MTS Bionix 858.2, Eden Prairie, MN) and tested at room temperature. A cranially located, driven rotational axis was used to impart 5 degrees of extension and 5 degrees of flexion at a frequency of 0.1 Hz in angle control (Fig. 3). A caudal axis was held stable, and the vertical axis was set to maintain 0 N during the testing period. Shear load was minimized by a cranially placed x-y table that was free to move in the transverse plane. A caudally located 6-degree-of-freedom load cell recorded all forces and moments. Each specimen underwent 5 consecutive ROM cycles in 3 different configurations (Fig. 1):
- Native, pedicle screws implanted, no rods (N=8).
- Two-segment instrumentation, rigid instrumentation from L3 to L5 (N=8).
- Three-segment instrumentation subdivided into the 2 groups: rigid (N=4) and hybrid (N=4).
A motion capture system (frame rate of 102.4 Hz; Vicon-460, Oxford Metrics Ltd., Oxford, UK) was used to measure the ROM of the whole spine and of each segment. L-shaped sets of 3 reflective markers were attached to the head of each screw on the left side of the instrumentation (L2–L5), to the top embedding plate (Th12), and to the bottom embedding plate (L5, Fig. 3). In addition, a pedicle screw with a L-shaped set of 3 reflective markers was implanted in the vertebra L1. This screw was not included in the instrumentation but was necessary to measure segmental motion. For the L2–L3 segment, which underwent different instrumentations, an additional marker set was attached to the spinous process of both L2 and L3 (Fig. 3).
Data and Statistical Analysis
Data for the intersegmental ROM were evaluated for the third cycle. The motion data were used to determine the angular ROM (flexion-extension) for each spinal segment (Th12–L1, L1–L2, L2–L3, L3–L4, L4–L5). From the L-shaped 3-marker sets mounted on each pedicle screw (L1–L5), a vector was defined from the central marker to the most posterior marker and was then projected to the sagittal plane. For T12, the normal to the plane defined by the triangular 3 marker set was calculated then projected to the sagittal plane. The segmental ROM was then defined as the angle measured between the sagittal projections of 2 adjacent levels.
The segmental motions between the native, 2-segment and 3-segment instrumentations (rigid and hybrid) were assessed using 2-way analysis of variance with the Fisher Least Significant Difference used for post hoc tests. All statistical analyses were performed using SPSS Statistics 20 (IBM Corp., Armonk, NY) with a type I error probability set to 5%.
In all tested configurations the instrumented segments experienced a reduced ROM from the native spine (P<0.05, η2=0.76). All adjacent segments in both the rigid and hybrid groups underwent a significant increase in ROM from the native spine (P<0.05, η2=0.79). The mean intervertebral ROMs for the tested configurations are summarized in Table 2 and Figure 4.
Two-segment rigid instrumentation caused a significant reduction of ROM compared with the native spine in the instrumented sections L4–L5 (P<0.001, η2=0.86) and L3–L4 (P<0.001, η2=0.87). In comparison with native, the levels above the 2-segment rigid fixation revealed an increase in motion by +189% at L2–L3 (P<0.001, η2=0.72), +158% at L1–L2 (P<0.001, η2=0.80), and +153% at Th12–L1 (P=0.004, η2=0.46).
There was a significant decrease in the motion of the instrumented segments in the 3-segment rigid fixation compared with the native spine by −95% at L4–L5 (P=0.001, η2=0.86), −90% at L3–L4 (P=0.001, η2=0.86), and −95% at L2–L3 (P<0.001, η2=0.89). There was an increase of motion at the adjacent segment L1–L2 (+235%, P<0.001, η2=0.94) and in Th12–L1 (+195%, P=0.007, η2=0.73).
The 3-segment hybrid fixation caused a significant reduction in ROM for the instrumented segments, L2–L5, by −91% at L4–L5 (P=0.003, η2=0.77), −90% at L3–L4 (P<0.001, η2=0.90), −85% at L2–L3 (P<0.001, η2=0.97). The ROM increased in motion at the adjacent segment L1–L2 (+228%, P=0.002, η2=0.82) and at Th12–L1 (+220%, P<0.001, η2=0.90).
There was an increasing ROM in the last instrumented segment (L2–L3) for the hybrid construct (mean, 0.26 degrees; SD, 0.12) compared with the 3-segment rigid construct (mean, 0.10 degrees; SD, 0.03), however, the difference was not significant (P=0.06, η2=0.47). There were no significant differences in ROM from the hybrid construct compared with the 3-segment rigid fixation at the levels above the instrumentation: Th12–L1 (P=0.39, η2=0.12) and L1–L2 (P=0.79, η2=0.12).
The primary conceptual idea for a hybrid construct is to limit the ROM and the stress within the adjacent level at the end of a rigid spinal fusion by adding an additional dynamic implant to provide a softer transition of the motion distribution to further levels. This is hypothesized to reduce the hypermobility within the adjacent segment and to protect it from the development or progression of degeneration.22,23 The aim of this study was to investigate the effect of a hybrid construct (dynamic fixation at the cranial end of a 2-segment rigid fixation) on the instrumented and adjacent segments. The hypothesis that the hybrid construct limits the ROM in the dynamic instrumented level but allows more motion than the rigid instrumentation could not be proven. This study found that the use of the dynamic device at the proximal end of a 2-segment rigid instrumentation reduces the ROM in the instrumented level close to the rigid fixation, while resulting in similar increasing mobility in the segments adjacent to the instrumentation. The results presented here show that this hybrid construct cannot be recommended to prevent adjacent level hypermobility and therefore reduce the possibility of ASD.
A possible explanation for the results might be the rod design. The fundamental difference between the 2 experimental groups tested is that the uppermost segment of the hybrid group utilized a rod comprised of a more compliant material [polycarbonate urethane (PCU)] than in the rigid group (titanium). This rod of reduced modulus was used to lessen the stiffness gradient at the construct ends. However, the PCU rod design also included 2 design alterations that increased the rod stiffness: an increased radius and a reduced effective length of the rod. The increase in rod radius has a substantial effect on the stiffness, because any incremental increase in radius exponentially raises the bending stiffness to the power of 4 and the axial stiffness to the power of 2. The reduction in the effective length of the rod was due to the necessity to attach the PCU rod by a metal adapter (Fig. 2B). A reduction in the rod length causes an approximate exponential increase in bending stiffness and a linear increase in axial stiffness. The combination of a rod comprised of a more compliant material but also with increased diameter and reduced length, did not allow for a sufficient level of bending and axial compliance to permit clinically relevant observable changes to the system motion. For clinically observable changes, a substantially less flexurally and axially rigid topping off segment would be preferable.
The biomechanical impact of posterior dynamic stabilization devices on the adjacent segment is not yet resolved, especially with regard to hybrid constructs. There are some authors, which have reported that the adjacent segment is not affected by either a dynamic or rigid fixation3,24 In an in vitro experiment Schmoelz et al24 showed that the dynamic nonfusion system DYNESYS was more flexible than a rigid fixation for the bridged segment. In extension the DYNESYS device was able to restore the ROM of the destabilized segment back to the intact spine; furthermore, the adjacent segment was not shown to be influenced by any stabilization method. Schilling et al3 investigated different posterior dynamic stabilization devices and compared them with a rigid fixation. They found that the adjacent segment was not affected by the stiffness of the fixation device. The results from the current study showed that the dynamic instrumented bridged segment in fact allowed more motion (L2–L3, 0.26 degrees) than the rigid instrumented segment (L2–L3, 0.10 degrees), but the difference was not significant. In contrast to these 2 investigations a significantly higher ROM was observed in the adjacent levels (Th12–L1 and L1–L2) above the dynamic and the rigid instrumented segment.
The influence of hybrid constructs on the adjacent segment was shown in different studies,2,15 which confirm the results presented here. Under the idea to stabilize the segment adjacent to a fusion and prevent it from hypermobility, Strube et al15 performed an in vitro biomechanical study to investigate the effect of a hybrid construct (single-level rigid fixation plus a dynamic stabilization) on instrumented, adjacent, and supra-adjacent segments. They concluded that their hybrid construct limited the ROM in the instrumented levels close to that of a 2-segment rigid fixation and that the produced hypermobility of the adjacent segments increased with the number of instrumented levels. Mageswaran et al2 compared 2 constructs: a single-level rigid fusion versus single-level rigid fixation with a dynamic device at the cranial end. They demonstrated that the hybrid construct could significantly limit the motion in both instrumented levels, while still significantly increasing of ROM in all adjacent levels cranial and caudal of the instrumentation. A similar conclusion can be made in the investigation presented here. The 3-segment rigid fixation decreased the ROM in the instrumented levels (L2–L5) compared with the intact spine while the levels adjacent to (L1–L2) and above (Th12–L1) increased compared with both the intact spine and the 2-level fixation indicating a shift of the hypermobility to these 2 levels. The hybrid construct reduced the ROM in the dynamic instrumented level (L2–L3) slightly less than the 3-segment rigid construct. However, a similar significant hypermobility was exhibited in the levels above the instrumentation (Th12–L1 and L1–L2). Therefore, the hybrid, “topping off” construct shows biomechanically similar behavior to a 3-segment rigid fixation, especially for the adjacent segments (Th12–L1 and L1–L2) during extension/flexion. In contrast to the studies of Strube et al15 and Mageswaran et al2 a longer hybrid construct with a 2-segment rigid fixation and a 1-segment dynamic was used in the current study. Also the testing method used here, with an angle-controlled movement, was different to the 2 aforementioned investigations. Nonetheless, the results are comparable and come to the same conclusion.
In contrast to the presented results and the latter 2 in vitro studies, other authors found no hypermobility at the adjacent levels when using a hybrid construct.21,22 Durrani et al21 investigated the effect of long rigid fixation on adjacent levels with and without a transitional posterior dynamic stabilization at the caudal end. A 5-level and 6-level rigid fixation showed an increased motion in the distal adjacent levels in all loading modes (extension-flexion, lateral bending, axial rotation). The added dynamic stabilization device immediately distal to the long construct reduced the ROM in the dynamically instrumented segment. At the adjacent levels it led to normalized motions that were near to that of the intact spine. Cheng et al22 compared rigid, dynamic, and hybrid constructs in 6 different configurations and investigated their effects on the stabilized and adjacent levels. For the hybrid construct Cheng et al22 reported that the dynamic instrumented segment allowed significant motion in all bending modes than the 2-segment rigid stabilization. However, nothing was reported about the effects for the levels above the hybrid construct. These 2 studies are in contrast to the results presented here because with the current system there was no significant increase in motion at the dynamically instrumented segment and a hypermobility was seen in the adjacent segments.
The different results of the studies demonstrate that, from the biomechanical point of view, there is no consensus on the benefit for hybrid constructs in ASD. One problem for the biomechanical testing of these dynamic instrumentations is that there exists no standard in vitro testing method. Different protocols were used for each study presented. Some studies3,24 followed the recommendations of Wilke et al25 for in vitro stability testing of spinal implants. The result of the studies showed that the adjacent level is not effected by a fixation. The authors believe that for the testing of spinal implants, applying pure moments under load control is more adequate.24 Other researchers2,15 used the hybrid multidirectional test method of Panjabi26 to evaluate adjacent level effects. The results of these 2 investigations showed an increasing ROM in the adjacent levels of a rigid and hybrid construct. But in contrast to this Durrani et al21 also used the hybrid multidirectional test method of Panjabi and found no hypermobility at the caudal end of the long hybrid instrumentation. In the current study an angle-controlled movement (5 degrees extension, 5 degrees flexion) with a free-bending load was applied at the cranial end of the specimen, which is different to the method of Wilke et al25 where a load-controlled movement is used. Under the clinically derived assumption that a treated patient would try to preserve the preoperative spinal motion, this method was chosen to detect the hypermobility of the adjacent levels.
As in other in vitro studies, there were some limitations in this investigation. The availability of specimens was limited leading to a low sample size (N=8). The SD between the samples was relatively small and the magnitude of the motion for the instrumented segments (including the hybrid instrumentation) was low. Even with the small sample size, these values clearly show that magnitude of the extra ROM allowed by the hybrid instrumentation is extremely low (approximately 0.16 degrees) and clinically insignificant. Further, it was not possible to test all the 3 different configurations on the same specimen. The third configuration (3-segment instrumentation) had to be divided into 2 groups: hybrid or rigid because the structural design prevented the interchanging of the different rods with the screw heads. Only 1 movement direction (extension/flexion) was investigated. This loading was chosen because in daily life extension and flexion play the most important role in the bending of the spine. It is not possible to transfer the results of extension/flexion to other directions, like lateral bending or axial rotation as these may show positive effects of the hybrid construct for the limitation of the movement in the adjacent level. Like all in vitro studies, the data may only represent directly postoperative conditions. This study was not able to describe the long-term effects like pain and life quality, which are dependent on several biological and physiological factors like fusion rate, bony ingrowth of the pedicle screws, muscle regeneration, or wound healing. Also it is unclear how the role of muscle forces and everyday loading patterns would influence the results, because these are difficult to mimic in vitro. The addition of a topping off instrumentation necessitates the preparation of an additional spinal level and, therefore, it inflicts extra soft tissue damage, which has an unknown influence on the progression of adjacent level disease both in vivo or in vitro.
In conclusion, when compared with rigid fixation, the hybrid construct showed similar biomechanical behavior for extension and flexion without significant differences in the instrumented or adjacent segments. This implies that the hybrid construct will not cause a different loading scenario in the adjacent disk. Therefore, the rather short and stiff elastic construct does not seem to prevent hypermobility in the adjacent levels and poses a similar risk of transferring the hypermobility to the levels above as it is assumed for the rigid rod fixation. This compensatory hypermobility could possibly result in progressive segmental degeneration in the levels adjacent to the hybrid construct.
The authors thank the Department of Radiology, Asklepios Klinik St. Georg, Hamburg, (Dietmar Kivelitz, MD, PhD) for performing the quantitative computed tomographic scans.
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Keywords:© 2017 by Lippincott Williams & Wilkins, Inc.
Key Words: lumbar spine; adjacent segment degeneration; dynamic stabilization; hybrid construct