Posterior Dynamic Stabilization System Research Articles

Assessment of outcome of posterior dynamic stabilization system in degenerative lumbar disease

Background: Degenerative disc disease is a leading cause of chronic back pain in the aging population in the world. The present study was conducted to assess outcome of posterior dynamic stabilization system in degenerative lumbar diseases. Materials & Methods: 78 patients of degenerative lumbar diseases of both genders were divided into 2 groups of 39 each. Group I patients underwent decompression and implantation of Dynesys and group II patients underwent PLIF. ODI, VAS scores for back pain and leg pain, operation time, blood loss, and complications were compared in both groups. Results: There were 25 males and 14 females in group I and 20 males and 19 females in group II. The operative time (min) was 136.2 in group I and 172.4 in group II. Blood loss (ml) was 362.4 in group I and 438.2 in group II. Complications were dural tear seen 2 in group II, wound infection was seen in 1 in group II, screw loosening was 1 in group I and 2 in group II and back & leg pain seen 3 in group I and 4 in group II. The mean ODI was 32.5 and 38.5, VAS back pain was 2.6 and 3.12.

Fatigue wear test comparing vitamin-E-blended crosslinked polyethylene and conventional polyethylene in a Posterior Dynamic Stabilization System of the spine in the laboratory

BackgroundAlthough artificial joints using polyethylene have been developed for various joints, the development of Posterior Dynamic Stabilization system of the spine using polyethylene has proceeded at a much slower pace. There are no studies which compare the abrasion resistance of vitamin-E-blended crosslinked polyethylene (VE) and conventional polyethylene (Virgin) in the spinal region. The purpose of this study was to compare the wear resistance of VE and Virgin in a Posterior Dynamic Stabilization System of the spine. MethodsPosterior Dynamic Stabilization System of the spine uses a polyethylene ball as a sliding surface. A fatigue wear test was repeated up to 1 million cycles at a speed of ±5°, 1 Hz while the rod was being pulled at a load of 50 N. Balls were compared using VE and Virgin in 6 samples each. Ti-6AL-4 V (Ti 64) and Co–Cr–Mo (CoCr) rods were used. Abrasion loss and shape change of the polyethylene balls were compared. ResultsWhen Ti 64 was used as the rod, the average wear amount was −0.01 mg (0.02 mg, 0.01 mg, −0.06 mg) for VE, and 0.23 mg (0.18 mg, 0.13 mg, 0.38 mg) for Virgin. When CoCr was used as the rod, the average wear amount was 0.42 mg (0.71 mg, −0.06 mg, 0.61 mg) for VE, and 0.73 mg (0.72 mg, 0.70 mg, 0.76 mg) for Virgin. Most polyethylene samples showed indentations of 0.1 m or less at the contact point with the set screw. In the combination of Virgin and CoCr, a white patch was observed on the inner side of the polyethylene samples, with a maximum depression of 0.1 mm. ConclusionsA fatigue wear test showed VE to be more efficient in abrasion resistance than Virgin in a Posterior Dynamic Stabilization System of the spine in the laboratory.

A Prospective Long-term Follow-up Study of the Posterior Dynamic Stabilizing System to Treat Back Pain Associated With Degenerative Disc Disease.

Study Design:Prospective cohort study.Objective:To evaluate the long-term effectiveness of a posterior dynamic spine stabilization (PDS) System called PercuDyn system as a treatment for low back pain (LBP) in patients with degenerative disc disease who have failed conservative treatment.Methods:Thirty-five patients (21 males, 14 females, mean age 36 years) with chronic LBP due to degenerative disc disease underwent percutaneous facet augmentation with the PercuDyn system. Indications for implanting the PercuDyn device were chronic LBP without sciatica, significant disc degeneration, and who failed conservative treatment. Follow-up after surgery ranged from 4 to 8 years. Patients were clinically evaluated using visual analogue scale (VAS), Oswestry Disability Index (ODI) questionnaire, and the EQ-5D3L.Results:The VAS scale and ODI scores improved at 6 months, 1 year, 2 years, and 4 to 8 years in comparison with the baseline values (P < .001 and P < .01, respectively). Equally, data from the EQ-5D-3L questionnaire suggested improvements in all domains, including pain and discomfort, over the follow-up period. However, during follow-up, 16 patients (46%) have had unsatisfactory outcome including failure of implant to resolve symptoms (n = 8), developed symptomatic disc protrusion (n = 4), and loosening of the screws (n = 4).Conclusion:Our clinical experience highlights the rather high overall failure rate of this system. Although, some patients show significant improvement in their symptoms, we believe that patients with LBP related to degenerative disc disease are a poor indication for this PDS device.However, large scale studies, with optimised selection criteria are needed to demonstrate the exact role of this device in spinal patients.

Durability and Biological Response of a New Posterior Dynamic Stabilization System Using Polyethylene with Vitamin E.

Objective The purpose of this study was to evaluate the durability and biological response of a new Posterior Dynamic Stabilization system using polyethylene with vitamin E on the sliding surface. Summary of Background Data The use of polyethylene with vitamin E on the sliding surface in Posterior Dynamic Stabilization has not been reported previously. Methods A developed pedicle screw-based Posterior Dynamic system consists of four parts: a set screw, a rod, a ball, and a pedicle screw. The rod is inserted into the through hole of the ball, and the ball is sandwiched by the set screw. (1) Fatigue Wear Test. Testing was conducted under a dynamic compressive load of 50N at a speed of 1 Hz for 1 million cycles. We examined the loss of polyethylene due to abrasion in 3 units. (2) Biological Response in Pigs. In two pigs, a new pedicle screw and a conventional pedicle screw were inserted in L2 and L3/4, and L4 and L2/3, respectively. After breeding for 6 months, autopsies were performed. CT imaging was used to evaluate bone union of the facet joint. Abrasive specimens were prepared, and abrasion powder and inflammatory cell infiltration were evaluated microscopically. Results The average loss of polyethylene due to abrasion was -0.01 mg. In all units, polyethylene showed a decrease of 0.1 mm or less at the contact point with the set screw. The facet joints between the conventional screws exhibited bone fusion, but the facet joint between the conventional and the new screw retained mobility with no bony fusion. No abrasion powder was found and inflammatory cell infiltration was only minimally observed. Conclusion The new Posterior Dynamic Stabilization system exhibited a high level of durability and biological safety.

Clinical Study of a New Transpedicular Nonfusion Posterior Dynamic Stabilization System for Treating Herniated Lumbar Intervertebral Disks

Nonfusion posterior dynamic stabilization systems, in which semirigid rods are used to restore spinal stability and to provide flexibility, aim to reduce or eliminate drawbacks associated with rigid fusion. In this study, we evaluated the 12-month clinical outcomes of patients treated with the K-Rod, a pedicle-based dynamic stabilization system. Patient participation was based on the presence of a herniated lumbar intervertebral disk complicated with endplate osteochondritis at L4-L5 or L5-S1. The clinical outcome at each assessment was assessed by Visual Analogue Scale to measure back pain, and the Oswestry Disability Index and range of motion to measure functional status preoperatively and postoperatively. Thirty-three patients (20 men and 13 women) with a mean age of 45.5 years were included. The mean Visual Analogue Scale score improved by 77% and the mean Oswestry Disability Index score improved by 77% from preoperative to postoperative. The range of motion results were statistically significant in the disease segment, but were not significant in adjacent segments. Nonfusion posterior dynamic stabilization using the K-Rod system is effective in improving clinical outcome, with sustained medium-term clinical improvements. Further observation is needed to evaluate its long-term effects.

In Vitro Comparison of Dynesys, PEEK, and Titanium Constructs in the Lumbar Spine.

Introduction. Pedicle based posterior dynamic stabilization systems aim to stabilize the pathologic spine while also allowing sufficient motion to mitigate adjacent level effects. Two flexible constructs that have been proposed to act in such a manner, the Dynesys Dynamic Stabilization System and PEEK rod, have yet to be directly compared in vitro to a rigid Titanium rod. Methods. Human lumbar specimens were tested in flexion extension, lateral bending, and axial torsion to evaluate the following conditions at L4-L5: Intact, Dynesys, PEEK rod, Titanium rod, and Destabilized. Intervertebral range of motion, interpedicular travel, and interpedicular displacement metrics were evaluated from 3rd-cycle data using an optoelectric tracking system. Results. Statistically significant decreases in ROM compared to Intact and Destabilized conditions were detected for the instrumented conditions during flexion extension and lateral bending. AT ROM was significantly less than Destabilized but not the Intact condition. Similar trends were found for interpedicular displacement in all modes of loading; however, interpedicular travel trends were less consistent. More importantly, no metrics under any mode of loading revealed significant differences between Dynesys, PEEK, and Titanium. Conclusion. The results of this study support previous findings that Dynesys and PEEK constructs behave similarly to a Titanium rod in vitro.

Short term outcome of posterior dynamic stabilization system in degenerative lumbar diseases.

Background:Decompression and fusion is considered as the ‘gold standard’ for the treatment of degenerative lumbar diseases, however, many disadvantages have been reported in several studies, recently like donor site pain, pseudoarthrosis, nonunion, screw loosening, instrumentation failure, infection, adjacent segment disease (ASDis) and degeneration. Dynamic neutralization system (Dynesys) avoids many of these disadvantages. This system is made up of pedicle screws, polyethylene terephthalate cords, and polycarbonate urethane spacers to stabilize the functional spinal unit and preserve the adjacent motion after surgeries. This was a retrospective cohort study to compare the effect of Dynesys for treating degenerative lumbar diseases with posterior lumbar interbody fusion (PLIF) based on short term followup.Materials and Methods:Seventy five consecutive patients of lumbar degenerative disease operated between October 2010 and November 2012 were studied with a minimum followup of 2 years. Patients were divided into two groups according to the different surgeries. 30 patients underwent decompression and implantation of Dynesys in two levels (n = 29) or three levels (n = 1) and 45 patients underwent PLIF in two levels (n = 39) or three levels (n = 6). Clinical and radiographic outcomes between two groups were reviewed.Results:Thirty patients (male:17, female:13) with a mean age of 55.96 ± 7.68 years were included in Dynesys group and the PLIF group included 45 patients (male:21, female:24) with a mean age of 54.69 ± 3.26 years. The average followup in Dynesys group and PLIF group was 2.22 ± 0.43 year (range 2-3.5 year) and 2.17 ± 0.76 year (range 2-3 year), respectively. Dynesys group showed a shorter operation time (141.06 ± 11.36 min vs. 176.98 ± 6.72 min, P < 0.001) and less intraoperative blood loss (386.76 ± 19.44 ml vs. 430.11 ± 24.72 ml, P < 0.001). For Dynesys group, visual analogue scale (VAS) for back and leg pain improved from 6.87 ± 0.80 to 2.92 ± 0.18 and 6.99 ± 0.81 to 3.25 ± 0.37, (both P < 0.001) and for PLIF, VAS for back and leg pain also improved significantly (6.97 ± 0.84–3.19 ± 0.19 and 7.26 ± 0.76–3.56 ± 0.38, both P < 0.001). Significant improvement was found at final followup in both groups in Oswestry disability index (ODI) score (both P < 0.001). Besides, Dynesys group showed a greater improvement in ODI and VAS back and leg pain scores compared with the PLIF group (P < 0.001, P = 0.009 and P = 0.031, respectively). For radiological, height of the operated level was found increased in both groups (both P < 0.001), but there was no difference between two groups (P = 0.93). For range of motion (ROM) of operated level, significant decrease was found in both groups (P < 0.001), but Dynesys showed a higher preservation of motion at the operative levels (P < 0.001). However, no significant difference was found in the percentage change of ROM of adjacent levels between Dynesys and PLIF (0.74 ± 8.92% vs. 0.92 ± 4.52%, P = 0.91). Some patients suffered from degeneration of adjacent intervertebral disc at final followup, but there was no significant difference in adjacent intervertebral disc degeneration between two groups (P = 0.71). Moreover, there were no differences in complications between Dynesys and PLIF (P = 0.90), although the incidence of complication in Dynesys was lower than PLIF (16.67% vs. 17.78%).Conclusion:Dynamic stabilization system treating lumbar degenerative disease showed clinical benefits with motion preservation of the operated segments, but does not have the significant advantage on motion preservation at adjacent segments, to avoid the degeneration of adjacent intervertebral disk.

Adjacent Intervertebral Disc Changes in Patients with Isobar Semirigid Dynamic Stabilization System

Introduction The reported incidence of adjacent segment degeneration (ASD) after lumbar fusion ranges from 8 to &gt; 100%, average 27%. Another literature review reported that 5.2 to 18.5% of patients required surgery for adjacent segment disease after lumbar fusion surgery. Posterior dynamic stabilization systems may prevent the development and progression of ASD after surgery. Hybrid dynamic stabilization with posterior spinal fusion, or “Topping-off surgery,” is applied in patients with preoperative adjacent segment degeneration. The objective of this approach is to protect the degenerative adjacent segment from progressing. Efficacy of this procedure is debated. The objective of this study was to evaluate intervertebral disk changes of the dynamically stabilized spine segment in patients receiving single-level intervertebral fusion and adjacent segment dynamic stabilization using the Isobar (Alphatec Spine, Inc.) semirigid dynamic stabilization system. Materials and Methods Twenty-seven patients with confirmed degenerative disk disease were enrolled in a retrospective study. Patients received single level intervertebral fusion and dynamic stabilization of the adjacent level using the Isobar semi-rigid dynamic stabilization system. Patient outcomes were evaluated by visual analogue scale for pain (VAS) and Chinese version of Oswestry Disability Index (ODI). Intervertebral space ratio (IVS) and range of motion (ROM) were measured on plain radiographs. MRI relative gray-scale values of the central bright area of discs were analyzed by Image J software. Results Of the 27 patients, 16 were male and 11 were female. Ages ranged from 31 to 65 years (average 47 years). Average duration of surgery was 213 (SD = 52) minutes and average blood loss was 225 mL (SD = 105). The average follow-up time was 27 months, ranging from 24 to 40 months. There was no significant change in relative gray-scale value of adjacent disc between preoperative and two year follow-up (23.98 ± 8.86 and 22.22 ± 6.25, respectively, p = 0.46). Relative gray-scale values of first cephalic adjacent disc were reduced at 2 years follow-up compared with baseline (28.63 ± 8.35 and 32.54 ± 10.75, respectively, p = 0.06). Significant difference was found in relative gray-scale value of second cephalic adjacent disc between baseline and 2-year follow-up (35.51 ± 8.15 and 29.29 ± 7.56, respectively, p &lt; 0.01). The inter-vertebral space (IVS) of index level was maintained after the surgery (Table 1). There was no statistical difference between the IVS before the surgery and final follow-up ( p = 0.33). The range of motion (ROM) of the adjacent level was reduced from 6.91 ± 3.14 preoperatively to 2.46 ± 1.03 degrees at the final follow-up ( p &lt; 0.05). There were no significant differences in the index level ROM among any of the follow-up times. VAS and ODI scores significantly improved preoperatively to final follow-up (VAS: 6.12 ± 0.80 and 1.65 ± 0.69, respectively, p = &lt; 0.01; ODI: 54.91 ± 10.96 and 14.61 ± 3.88, respectively, p = &lt;0.01). One patient received a debridement due to wound infection after operation, but the fixation system and interbody cage were not removed. This patient recovered well after the debridement. At the 2-year follow-up, one patient had two screw fractures at S1 level and another patient had two screws loosen. Conclusion Dynamic stabilization of the level adjacent to the fused level using the Isobar TTL Semi-rigid Spinal System seems to prevent progression of degenerative changes in the stabilized spine segment and is associated with favorable patient outcomes. Disclosure of Interest Q. Zhou: Conflict with Third Military Medical University, AO International R. Shi: None declared B. Kopjar: Conflict with AO Foundation, Smith &amp; Nephew, Cerapedics H. Wang: None declared D. Chen: None declared H. Li: None declared N. Di: None declared F. Luo: None declared F. Dai: None declared J. Xu: None declared

A Computational Biomechanical Investigation of Posterior Dynamic Instrumentation: Combination of Dynamic Rod and Hinged (Dynamic) Screw

Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.

Influence of Screw Augmentation in Posterior Dynamic and Rigid Stabilization Systems in Osteoporotic Lumbar Vertebrae

Biomechanical cadaveric study. To determine whether augmentation positively influence screw stability or not. Implantation of pedicle screws is a common procedure in spine surgery to provide an anchorage of posterior internal fixation into vertebrae. Screw performance is highly correlated to bone quality. Therefore, polymeric cement is often injected through specifically designed perforated pedicle screws into osteoporotic bone to potentially enhance screw stability. Caudocephalic dynamic loading was applied as quasi-physiological alternative to classical pull-out tests on 16 screws implanted in osteoporotic lumbar vertebrae and 20 screws in nonosteoporotic specimen. Load was applied using 2 different configurations simulating standard and dynamic posterior stabilization devices. Screw performance was quantified by measurement of screwhead displacement during the loading cycles. To reduce the impact of bone quality and morphology, screw performance was compared for each vertebra and averaged afterward. All screws (with or without cement) implanted in osteoporotic vertebrae showed lower performances than the ones implanted into nonosteoporotic specimen. Augmentation was negligible for screws implanted into nonosteoporotic specimen, whereas in osteoporotic vertebrae pedicle screw stability was significantly increased. For dynamic posterior stabilization system an increase of screwhead displacement was observed in comparison with standard fixation devices in both setups. Augmentation enhances screw performance in patients with poor bone stock, whereas no difference is observed for patients without osteoporosis. Furthermore, dynamic stabilization systems have the possibility to fail when implanted in osteoporotic bone.

Posterior dynamic stabilization system (Dynesys) with interbody fusion for treating two-segment lumbar degenerative disc disease

Summary Purpose To assess clinical outcomes and kinematic changes in bridged and adjacent segments after hybrid stabilization by interbody fusion. Background Conventional management for multilevel lumbar degenerative disc disease is limited to either fusion or nonfusion surgery alone. The severity of degeneration and instability in multilevel lumbar degenerative disc disease vary widely. Therefore, surgical treatment for multilevel lumbar degenerative disc disease is usually performed in a single stage by either fusion, nonfusion, or a hybrid technique. To the best of our knowledge, no kinematic study of a Dynesys fusion–nonfusion technique has yet been reported. Methods This study analyzed 28 patients who had undergone posterior lumbar interbody fusion with posterior dynamic stabilization fixation for spondylolisthesis with adjacent level stenosis between May 2007 and June 2008. All patients were followed up for at least 2 years. Clinical characteristics were then retrospectively reviewed. The visual analog scale (VAS) was used to score both lower limb pain and back pain. Patient functioning was evaluated using the Oswestry Disability Index (ODI). Results Both VAS and ODI significantly improved during the follow-up period ( p p = 0.027). Analyses of adjacent segment motion after surgery showed significant increases in neutral lordotic curves ( p p Conclusion Dynesys stabilization with interbody fusion improves clinical outcomes, enables successful interbody fusion, and maintains the ROM in the bridged segment. However, the procedure may decrease ROM in the global segment and cause hypermobility in the adjacent segment. Further in vivo or in vitro experiments are needed to optimize spacer length and cord tension when performing this procedure.

Posterior Transpedicular Dynamic Systems in the Treatment of Chronic Lumbar Instability

Dynamic stabilization is a new concept and a new technology in spinal stabilization. There are a lot of debates on this topic and most of the criticisms may be true; however, it should be kept in mind that every technology develops from necessity. If fusion could solve all the problems, scientists would not try to develop other solutions. Posterior dynamic stabilization is a hopeful technology and we believe that tremendous innovative surgical techniques and surgical instruments will develop in near future. We expect that we will open new horizons in your minds when you are considering treatment plans about your patients which need lumbar stabilization procedures. In this special issue, you will find several examples of dynamic stabilization surgeries for different spinal diseases. Clinical results were also given with expanded review of the relevant literature. The papers in this journal also open a new window about treatment of lumbar disc herniations. Type 2 and type 4 disc herniations, according to the Carragee classification, treated with dynamic stabilization instead of subtotal discectomy were discussed by A. F. Ozer et al. You will find manuscripts in this special issue regarding recurrent disc herniations, painful black discs, degenerative spondylolisthesis, and painful Modic degenerations with degenerative disc disease treated with posterior dynamic stabilization surgery. We believe that dynamic systems will be discussed more intensely in the treatment of degenerative disc and bony pathologies in the lumbar spine. There are a lot of biomechanical and clinical studies about dynamic stabilization systems of the lumbar spine. Recently, finite element studies also have been published frequently concerning these systems. D. U. Erbulut et al. have performed an excellent review on biomechanics of posterior dynamic stabilization systems, and readers will learn too much from this paper in this special issue. We know that load sharing of the spinal column is an important biomechanical factor which may directly affect a patient's pain level, fusion success, and future disease progression, following spine surgery. In this journal, there is also a well-written research about load-sharing and regional load distribution of the interbody area investigating comparisons with between rigid rods, semirigid PEEK rods, and semirigid dynamic posterior instrumentation with flexion-extension dampening materials. There is a very important paper about adult's degenerative scoliosis which concluded that pedicle screw-based dynamic stabilization can be used in elderly patients with mild degenerative lumbar scoliosis because this is a less invasive surgery with short operative duration, moderate blood loss, and low adverse event rates. Moreover, they also reported scoliosis curve stabilization, at an average followup of more than 5 years. It was believed that fusion is a gold standard for low back pain treatment so far. However, there have been several complications reported clinically. These complications mainly are related to pseudoarthrosis and adjacent segment degeneration due to high stiffness at the stabilized segment. As an alternative treatment, nonfusion stabilization systems became more and more popular in order to preserve mobility of a motion segment and eliminate adjacent segment phenomena. You will see research studies emphasizing the benefits of dynamic stabilization system to prevent pseudoarthrosis and adjacent segment disease in this issue. The posterior transpedicular dynamic stabilization method is a very good surgical procedure in the patients with segmental instability. When we consider the biomechanical problems including widening of neutral zone and weakness of parts which stabilize the spine, one can easily understand the effectiveness of dynamic systems in treating patients who have lumbar segmental instability. There are also reports about the benefits of interspinous spacers and the results in terms of pain control, motion preservation, and prevention of adjacent segment degeneration. The authors in this journal recommend its use in treatment as well as in prevention of adjacent segment disease specifically in young patients where spinal fusion for early degenerative disease is needed. We know that dynamic stabilization of the lumbar spine not only stops the degeneration process but also starts the regeneration. L.-Y. Fay et al. reported the rehydration of intervertebral disc after dynamic system surgery using MRI evaluation. These papers prove radiological improvement of the disc tissue after this surgery. Ali Fahir Ozer Vijay K. Goel Ahmet Alanay Mehdi Sasani Tunc Oktenoglu Deniz Erbulut

Effect of Graded Facetectomy on Biomechanics of Dynesys Dynamic Stabilization System

Finite element (FE) method was used to compare the biomechanics of L3-S1 lumbar spine with graded facetectomy before and after placement of Dynesys. To evaluate the biomechanics of Dynesys as a function of graded bilateral facetectomies. Spinal fusion or posterior dynamic stabilization systems are used to restore stability after facetectomies. The intact FE spine was modified to simulate decompression at L4-L5 with 50% and 75% and total facetectomy with/without dynamic stabilization with Dynesys. Biomechanics of the implanted level was investigated under different physiological loadings. Total facetectomy increased the motion in extension (8.7° vs. 2.7° for intact) and axial rotation (8.4° vs. 2.4° for intact). However the decrease in motion in the Dynesys model ranged from 65% in axial rotation to 80% in flexion for all facetectomies, except in the total facetectomy axial rotation case (motion higher than intact). The center of rotation of dynamic stabilized segment moved inferior/posterior in partial facetectomy and superior/posterior in total facetectomy with respect to the intact and destabilized cases. The Dynesys screws observed peak stresses up to 28% higher than those of a rigid fixation system in certain loadings, such as lateral bending and extension. The critical loosening torque applied to the screws in total facetectomy case was 6 times the partial facetectomy case in axial rotation. Partial facetectomy had a minimal effect on range of motion on the Dynesys-implanted segment. However, in the case of total facetectomy the motion increased by almost 40% in flexion and by 200% in axial rotation. The higher stresses applied to the screws in Dynesys in specific loadings may lead to higher risk of screw failure in Dynesys than in a generic rigid fixation construct.

Rationale of a Test Setup with a Defined COR for Extra-Discal Motion-Preserving Implants with a Low Implant Stiffness

Abstract In the current version of ASTM F2624, the center of rotation (COR) is not specified. Potentially, each device can be tested using a different COR, which subsequently makes a direct design comparison of results difficult. Four posterior dynamic stabilization (PDS) devices (Dynesys, DYN, Zimmer; DSS, Paradigm Spine; and two Aesculap implant concepts) were tested in comparison to a rigid-fixation device and to the native situation of the lumbar spine on fresh-frozen human lumbar spines (L3–L5). The instrumented level was L4–L5. The PDS systems have axial compressive stiffness values ranging from 10 N/mm to 230 N/mm and were all made compatible to connect with the pedicle-screw system. The specimens were loaded in a spinal simulator, applying pure moments for flexion/extension, lateral bending and axial rotation (+/−7.5 Nm) with a defined velocity. The COR was analyzed based on the data measured with a 3-dimensional (3D) motion-analysis system. The effect of the PDS on the location of the COR is most pronounced in the sagittal plane. In general, the higher the implant stiffness, the more the COR shifted in a posterior direction. The DYN had a similar COR to the rigid fixator. However, the PDS systems with low axial compressive stiffness values (range: 10–70 N/mm) showed very similar results on CORs, which are located in the region of the posterior border of the intervertebral disc. In the frontal and transversal plane, the COR was found to be close to the native situation for each system. Therefore, for PDS devices with low implant stiffness, the location of the COR varies only marginally and can be specified for a test setup. An initial proposal that will allow side-by-side comparison for these kinds of PDS systems is given and the feasibility of the new test setup could be proven for all three loading conditions.

Could Junctional Problems at the End of a Long Construct be Addressed by Providing a Graduated Reduction in Stiffness?

The effect of long, rigid fixation on adjacent level hypermobility was investigated in a human cadaver model with and without a transitional posterior dynamic stabilization (PDS) device placed at the last caudal level. To evaluate if PDS devices are useful in the setting of spinal deformities to restore increased adjacent level motions, which occur in long constructs. The hypothesis is that load-sharing benefits of these devices will be most suitable in long constructs and may reduce thoracolumbar junctional effects. The PDS device evaluated has a compressive spacer and flexion-dampening bumper. Mechanical factors such as excessive mobility, increased disc height due to instrumentation, and abnormal loading are thought to accentuate distal level problems, which occur in extended instrumentation. Specifically adjacent level degeneration and distal junctional kyphosis are known to occur in these cases. Seven cadaver spines were tested from T7 to L3. Long instrumentation was applied in 2 rigid groups, R1: Rigid (T8-L2) and R2: Rigid (T8-L1), and PDS to the last caudal level of each, RP1: Rigid (T8-L1) + PDS (L1-L2), and RP2: Rigid (T8-T12) + PDS (T12-L1). Range of motion was evaluated at surgical and distal adjacent levels after displacement controlled loading in a spine tester. Distal adjacent level motion was increased after 5- and 6-level rigid fixation in flexion-extension, lateral bending, and axial rotation. Most of the increases were seen in axial rotation and lateral bending. Replacing the last caudal instrumented level with the PDS test device was able to alleviate hypermobile conditions of the adjacent noninstrumented level, closer to intact (24%, 12% reduction in RP2, RP1, respectively). Reduction of hypermobility caused by extended arthrodesis may represent a new and ideally suited function for PDS devices. Mechanically, the devices were seen to kinematically restore abnormal distal motion, especially with placement of the PDS at the thoracolumbar junction.

Biomechanics of Disc Degeneration

Disc degeneration and associated disorders are among the most debated topics in the orthopedic literature over the past few decades. These may be attributed to interrelated mechanical, biochemical, and environmental factors. The treatment options vary from conservative approaches to surgery, depending on the severity of degeneration and response to conservative therapies. Spinal fusion is considered to be the “gold standard” in surgical methods till date. However, the association of adjacent level degeneration has led to the evolution of motion preservation technologies like spinal arthroplasty and posterior dynamic stabilization systems. These new technologies are aimed to address pain and preserve motion while maintaining a proper load sharing among various spinal elements. This paper provides an elaborative biomechanical review of the technologies aimed to address the disc degeneration and reiterates the point that biomechanical efficacy followed by long-term clinical success will allow these nonfusion technologies as alternatives to fusion, at least in certain patient population.

Novel indication for posterior dynamic stabilization: Correction of disc tilt after lumbar total disc replacement

Background The increase in total disc replacement procedures performed over the last 5 years has increased the occurrence of patients presenting with postoperative iatrogenic deformity requiring revision surgery. Proposed salvage treatments include device retrieval followed by anterior lumbar interbody fusion or posterior fusion. We propose a novel approach for the correction of disc tilt after total disc replacement using a posterior dynamic stabilization system. Methods Pedicle screws can be inserted either in an open manner or percutaneously by standard techniques under fluoroscopy. The collapsed side is expanded, and the convex side is compressed. Universal spacers are placed bilaterally, with the spacer on the collapsed side being taller by 6 mm. Cords are threaded through the spacers and pulled into place with the tensioning instrument. Extra tension is applied to the convex side, and the wound is closed by standard techniques. Results Three patients presenting with tilted total disc replacement devices underwent corrective surgery with posterior dynamic stabilization. Radiographs confirmed correction of deformity in all cases. Conclusions/Level of Evidence This technical note presents a novel indication for posterior dynamic stabilization and describes its surgical application to the correction of disc tilt after total disc replacement. This is level V evidence.

A minimally invasive technique for percutaneous lumbar facet augmentation: Technical description of a novel device

Background:We describe a new posterior dynamic stabilizing system that can be used to augment the mechanics of the degenerating lumbar segment. The mechanism of this system differs from other previously described surgical techniques that have been designed to augment lumbar biomechanics. The implant and technique we describe is an extension-limiting one, and it is designed to support and cushion the facet complex. Furthermore, it is inserted through an entirely percutaneous technique. The purpose of this technical note is to demonstrate a novel posterior surgical approach for the treatment of lumbar degenerative.Methods:This report describes a novel, percutaneously placed, posterior dynamic stabilization system as an alternative option to treat lumbar degenerative disk disease with and without lumbar spinal stenosis. The system does not require a midline soft-tissue dissection, nor subperiosteal dissection, and is a truly minimally invasive means for posterior augmentation of the functional facet complex. This system can be implanted as a stand-alone procedure or in conjunction with decompression procedures.Results:One-year clinical results in nine individual patients, all treated for degenerative disease of the lower lumbar spine, are presented.Conclusions:This novel technique allows for percutaneous posterior dynamic stabilization of the lumbar facet complex. The use of this procedure may allow a less invasive alternative to traditional approaches to the lumbar spine as well as an alternative to other newly developed posterior dynamic stabilization systems.

Radiostereometric Analysis of Postoperative Motion After Application of Dynesys Dynamic Posterior Stabilization System for Treatment of Degenerative Spondylolisthesis

Prospective case series This was designed to precisely measure motion after posterior dynamic stabilization using Dynesys instrumentation. The Dynesys posterior dynamic stabilization system, which stabilizes the spinal segment while potentially decreasing the risk of adjacent segment disease, is undergoing evaluation by the US Food and Drug Administration for treatment of degenerative spondylolisthesis without fusion. Evaluation of adjacent segment disease requires precise characterization of motion on the surgical level. Unfortunately, routine clinical radiographic techniques are imprecise and unreliable for full characterization of spinal segment motion. Radiostereometric analysis, which is very precise and reliable for in vivo measurement of motion, was used to examine spinal segment motion after dynamic stabilization with Dynesys. Six patients (age 59+/-7 y) underwent posterior decompression followed by posterior stabilization using Dynesys instrumentation (4 one-level, 2 two-levels). Three to 5 tantalum beads were placed in each vertebral body. Postoperative biplanar radiographs were obtained in flexion, extension, right, and left lateral bending, and 3-dimensional reconstruction was performed using radiostereometric analysis at 3, 6, 12, and 24 months postoperatively. The translations and rotations of the superior vertebral body were measured relative to the inferior vertebral body. Over the 24-month follow-up period, mean flexion, extension, left, and right lateral bending of the motion segments were noted to be 1.0 degrees, 2.4 degrees, 0.6 degrees, and 0.6 degrees or less, respectively. There were no statistically significant changes in the degree of motion. During follow-up, no significant changes in neutral position of the device were noted in any of the 3 planes, and minimal translation was noted in the postoperative period. The Dynesys dynamic instrumentation system seems to stabilize degenerative spondylolisthesis. As expected in the degenerative lumbar spine, the segmental motion of the implanted level in this study was limited and considerably less than normal spinal motion.

Preclinical evaluation of the Dynesys posterior spinal stabilization system: a nonhuman primate model

Background context Posterior dynamic spinal stabilization systems are intended to restore near-normal biomechanical function of the spine without inducing unnatural stresses to the spinal elements or eliciting a histopathological response. These devices must resist loosening within the challenging biomechanical environment of the lumbar spine. Purpose To determine the biomechanical effects of the Dynesys dynamic stabilization system (Zimmer, Inc., Warsaw, IN, USA) in the acute postoperative period and after 6 and 12 months in vivo; to examine the facet joints at the same postoperative intervals for signs of degeneration; and to measure the incidence of screw loosening after in vivo loading. Study design/setting This was an in vitro and in vivo animal survival study. Methods Fourteen baboons were used. Eight animals underwent survival surgery to implant a posterior dynamic stabilization system spanning two lumbar levels. Six animals were sacrificed acutely, and their spines were biomechanically tested in the intact condition and with instrumentation implanted as described above. Six animals in the survival group were sacrificed at 6 months postoperatively and two animals at 12 months postoperatively. Their spines were biomechanically tested with instrumentation in situ and explanted. The facets were then processed using undecalcified technique. Microradiographs of the facets were examined for signs of arthrosis, inflammation, and degenerative changes. Results The range of flexion-extension motion for the acute group of instrumented spines was 27% of the intact condition. After 6 months with instrumentation in situ, flexion-extension was 56% of the intact condition. After 12 months with instrumentation in situ, flexion-extension was 70% of the intact condition. With instrumentation explanted, flexion-extension at 6 and 12 months was not different from the intact condition (p>.05). Similar results were observed for lateral bending. There were no significant differences in axial rotation between any groups at any time point (p>.05). The facet joints at the operative and adjacent levels exhibited normal articular cartilage at both the 6- and 12-month postoperative time points. There was no evidence of facet arthrosis in any animal. At 6 months postoperatively, 0 of 36 screws exhibited radiolucency at the bone-metal interface. At 12 months postoperatively, 3 of 12 screws exhibited radiolucency. Conclusions After 12 months in vivo, spinal motions were stabilized by the dynamic instrumentation system. No facet arthrosis was observed at 6 and 12 months postoperatively. Explantation of the instrumentation restored motion to intact levels. A 25% rate of screw loosening (3 of 12 screws) was observed at the 12-month postoperative time point.