Hybrid dynamic stabilization: a biomechanical assessment of adjacent and supraadjacent levels of the lumbar spine

Biomechanical effects of screw loosening after lumbar PEEK rod and titanium rod fixation: a finite element analysis

Objective

Screw loosening is a common complication following lumbar spine fixation surgery, yet the biomechanical outcomes after screw loosening remain rarely reported. This study aims to utilize finite element (FE) models to compare the biomechanical performance of PEEK rod dynamic fixation and titanium rod rigid fixation in the postoperative lumbar spine, exploring potential biomechanical mechanisms for re-stabilization of loosened screws.

Methods

A FE model of the lumbar spine from L3 to the sacrum was developed using CT image segmentation. Four L4-S1 fixation models were constructed: PEEK rod dynamic fixation (PEEK model), titanium rod rigid fixation (titanium model), PEEK rod with pedicle screw loosening (PEEK-PSL model), and titanium rod with pedicle screw loosening (titanium -PSL model). A preload of 300 N was applied to the superior surface of L3. Stress distributions in the intervertebral discs, facet joints, pedicle screws, and rods were calculated to evaluate the biomechanical effects of different fixation methods.

Results

Across four physiological loading conditions, the stress differences in intervertebral discs, facet joints, and nucleus pulposus between the PEEK model and titanium model were minimal. However, vertebral body stress was significantly higher in the PEEK model, whereas screw and rod stresses were greater in the titanium model. Screw loosening further increased stress in all models. The S1 screw in the PEEK-PSL model exhibited lower and more uniform stress, while stress was concentrated at the screw-rod junction in the titanium-PSL model.

Conclusion

The PEEK rod fixation system demonstrated superior stress distribution, reducing stress concentration risks and improving stability while minimizing screw loosening rates. In contrast, the titanium rod system offers advantages in scenarios requiring high rigidity, potentially making it more suitable for patients with greater stability needs.

Percutaneous Transforaminal Endoscopic Discectomy for Adjacent Segment Disease versus Lumbar Disc Herniation in Elderly Patients

Purpose

Percutaneous transforaminal endoscopic discectomy (PTED) was used as a minimally invasive treatment option for lumbar disc herniation (LDH). However, studies focusing on the clinical outcomes of PTED for elderly patients with adjacent segment disease (ASD) were limited. This study aims to compare the clinical outcomes of PTED between ASD and LDH in elderly patients.

Patients and Methods

This retrospective study enrolled 39 patients with ASD and 39 patients with LDH. Both groups had undergone PTED in Beijing Chaoyang Hospital from July 4, 2016 to July 30, 2021. Visual analog scale for back pain (VAS-BP) and leg pain (VAS-LP) and Oswestry disability index (ODI) were used to value the clinical outcomes of patients preoperatively, immediately postoperatively, 12, and 24 months postoperatively, and at final follow-up. Patients' satisfaction was evaluated based on the MacNab criteria.

Results

All operations were completed. The excellent or good clinical outcomes at final follow-up was demonstrated by 87.15% (34/39) and 89.74% (35/39) in ASD and non-ASD patients, respectively. Clinical improvement was observed immediately postoperatively in both groups and sustained stability during the postoperative follow-up. The ASD group demonstrated significantly longer hospital stays (p = 0.02) and operative time (p < 0.01) than the non-ASD group.

Conclusion

PTED is an effective and minimally invasive treatment option for revision surgery of ASD, especially for elderly patients. However, the long-term prognosis of PTED treating ASD still needs further exploration.

The effects of topping-off instrumentation on biomechanics of sacroiliac joint after lumbosacral fusion

BACKGROUND

Lumbar/lumbosacral fusion supplemented with topping-off devices has been proposed with the aim of avoiding adjacent segment degeneration proximal to the fusion construct. However, it remains unclear how the biomechanics of the sacroiliac joint (SIJ) are altered after topping-off surgery. The objective of this study was to investigate the biomechanical effects of topping-off instrumentation on SIJ after lumbosacral fusion.

METHODS

The validated finite element model of an intact lumbar spine-pelvis segment was modified to simulate L5-S1 interbody fusion fixed with a pedicle screw system. An interspinous spacer, Device for Intervertebral Assisted Motion (DIAM), was used as a topping-off device and placed between interspinous processes of the L4 and L5 segments. Range of motion (ROM), von-Mises stress distribution, and ligament strain at SIJ were compared between fusion (without DIAM) and topping-off (fusion with DIAM) models under moments of four physiological motions.

RESULTS

ROM at the left and right SIJs in the topping-off model was higher by 26.9% and 27.5% in flexion, 16.8% and 16.1% in extension, 18.8% and 15.8% in lateral bending, and 3.7% and 7.4% in axial rotation, respectively, compared to those in the fusion model. The predicted stress and strain data showed that under all physiological loads, the topping-off model exhibited higher stress and ligament strain at the SIJs than the fusion model.

CONCLUSIONS

Motion, stress, and ligament strain at SIJ increase when supplementing lumbosacral fusion with topping-off devices, suggesting that topping-off surgery may be associated with higher risks of SIJ degeneration and pain than fusion alone.

Biomechanical Effect of Hybrid Dynamic Stabilization Implant on the Segmental Motion and Intradiscal Pressure in Human Lumbar Spine

The hybrid dynamic stabilization system, Dynesys-Transition-Optima, represents a novel pedicle-based construct for the treatment of lumbar degenerative disease. The theoretical advantage of this system is to stabilize the treated segment and preserve the range of motion within the adjacent segment while potentially decreasing the risk of adjacent segment disease following lumbar arthrodesis. Satisfactory short-term outcomes were previously demonstrated in the Dynesys-Transition-Optima system. However, long-term follow-up reported accelerated degeneration of adjacent segments and segmental instability above the fusion level. This study investigated the biomechanical effects of the Dynesys-Transition-Optima system on segment motion and intradiscal pressure at adjacent and implanted levels. Segmental range of motion and intradiscal pressure were evaluated under the conditions of the intact spine, with a static fixator at L4-5, and implanted with DTO at L3-4 (Dynesys fixator) and L4-5 (static fixator) by applying the loading conditions of flexion/extension (±7.5 Nm) and lateral bending (±7.5 Nm), with/without a follower preload of 500 N. Our results showed that the hybrid Dynesys-Transition-Optima system can significantly reduce the ROM at the fusion level (L4-L5), whereas the range of motion at the adjacent level (L3-4) significantly increased. The increase in physiological loading could be an important factor in the increment of IDP at the intervertebral discs at the lumbar spine. The Dynesys-Transition-Optima system can preserve the mobility of the stabilized segments with a lesser range of motion on the transition segment; it may help to prevent the occurrence of adjacent segment degeneration. However, the current study cannot cover all the issues of adjacent segmental diseases. Future investigations of large-scale and long-term follow-ups are needed.

Biomechanical comparative analysis of effects of dynamic and rigid fusion on lumbar motion with different sagittal parameters: An in vitro study

Background: Although the management of the lumbar disease is highly dependent on the severity of the patient’s condition, optimal surgical techniques to reduce the risk of adjacent degeneration disease (ADS) remain elusive. Based on in vitro biomechanical tests of the cadaver spine, this study aimed to comparatively analyze the kinematic responses of the spine with dynamic and rigid fixations (i.e., Coflex fixation and posterolateral fusion) after single-or double-level lumbar fusion in daily activities.

Methods: Six human lumbar specimens (L1-S1) were selected for this experiment, and the sagittal parameters of each lumbar specimen were measured in the 3D model. The specimens were successively reconstructed into five groups of models: intact model, single-level L4-5 Coflex fixation model, single-level L4-5 Fusion (posterior pedicle screw fixation) model, double-level L4-5 Coflex + L5-S1 Fusion model; and double-level L4-5 Fusion + L5-S1 Fusion model. The pure moment was applied to the specimen model to simulate physiological activities in daily life through a custom-built robot testing device with an optical tracking system.

Results: For single-level lumbar fusion, compared to the traditional Fusion fixation, the Coflex dynamic fixation mainly restricted the extension of L4-L5, partially retained the range of motion (ROM) of the L4-L5 segment, and reduced the motion compensation of the upper adjacent segment. For the double-level lumbar fixation, the ROM of adjacent segments in the Coflex + Fusion was significantly decreased compared to the Fusion + Fusion fixation, but there was no significant difference. In addition, PT was the only sagittal parameter of the preoperative lumbar associated with the ROM under extension loading. The Coflex fixation had little effect on the original sagittal alignment of the lumbar spine.

Conclusion: The Coflex was an effective lumbar surgical technique with a less altering kinematic motion of the lumbar both at the index segment and adjacent segments. However, when the Coflex was combined with the fusion fixation, this ability to protect adjacent segments remained elusive in slowing the accelerated degradation of adjacent segments.

Is Pedicle-Based Hybrid Stabilization (PBHS) protecting posterior lumbar fixation from adjacent-segment failure? Finite element analysis and comparison of different systems

INTRODUCTION

Interbody fusion is a very common surgical treatment for degenerative disc diseases. It is necessary to explain the effect of Pedicle Based Hybrid Stabilization systems (PBHS) on the lumbar spine, as there is no consensus in the literature about their performance.

HYPOTHESIS

Topping off a fusion with a PBHS may provide some protection against adjacent segment failure.

MATERIAL AND METHODS

The biomechanical effect PBHS on fused and adjacent to fusion levels were investigated, including range of motion, bending stiffness, Von Mises stress A 3D Finite Element model of the L2-S1 spine was used and modified to simulate pre and postoperative changes during combined loading. Five models instrumented with different systems [Titanium and PEEK fusion; Dynesys hybrid system; NFlex hybrid stabilization and PEEK topping off fusion] were compared to those of healthy model.

RESULTS

After hybrid instrumentation, the L4-L5 level did not lose its motion completely, NFlex hybrid stabilization system maintained 82% of flexion at the adjacent to fusion level, reduced bending stiffness by 40% in axial rotation. Dynesys hybrid system represented more restricted motion than NFlex. PEEK topping off fusion system was the most rigid one among all three systems. It increased bending stiffness at the adjacent level and increased the axial motion by 25%. High risk of rod breakage was computed for PEEK topping off system as 48.8MPa in lateral bending.

CONCLUSION

Hybrid stabilization can delay adjacent segment failure and compensate lumbar spine mobility. However, it is clear that PBHS need to be further tested before being considered for clinical use.

LEVEL OF EVIDENCE

III; well-designed computational non-experimental study.

Biomechanical investigation of topping-off technique using an interspinous process device following lumbar interbody fusion under vibration loading

Topping-off technique has been proposed to prevent adjacent-segment degeneration/disease following spine fusion surgery. Nevertheless, few studies have investigated biomechanics of the fusion surgery with topping-off device under whole-body vibration (WBV). This biomechanical study aimed to investigate the vibration characteristics of human lumbar spine after topping-off surgery, and also to evaluate the effect of bony fusion on spine biomechanics. Based on a healthy finite-element model of lumbosacral spine (L1-sacrum), the models of topping-off surgery before and after bony fusion were developed. The simulated surgical procedures consisted of interbody fusion with rigid stabilizer at L4-L5 segment (rigid fusion) and dynamic stabilizer at degenerated L3-L4 segment. An interspinous implant, Device for Intervertebral Assisted Motion (DIAM, Medtronic Inc., Minnesota, USA), was used as the dynamic stabilizer. The stress responses of spine segments and implants under a vertical cyclic load were calculated and analyzed. The results showed that compared with rigid fusion alone, the topping-off technique significantly decreased disc stress at transition segment (L3-L4) as expected, and resulted in a slight increase in disc stress at its supra-adjacent segment (L2-L3). It indicated that the topping-off stabilization using DIAM might provide a good tradeoff between protection of transition segment and deterioration of its supra-adjacent segment during WBV. Also, it was found that bony fusion decreased stress in L4 inferior endplate and rigid stabilizer but had nearly no effect on stress in DIAM and L3-L4 disc, which was helpful to determine the biomechanical differences before and after bony fusion.

Finite element modelling of hybrid stabilization systems for the human lumbar spine

Intersomatic fusion is a very popular treatment for spinal diseases associated with intervertebral disc degeneration. The effects of three different hybrid stabilization systems on both range of motion and intradiscal pressure were investigated, as there is no consensus in the literature about the efficiency of these systems. Finite element simulations were designed to predict the variations of range of motion and intradiscal pressure from intact to implanted situations. After hybrid stabilization system implantation, L4-L5 level did not lose its motion completely, while L5-S1 had no mobility as a consequence of disc removal and fusion process. BalanC hybrid stabilization system represented higher mobility at the index level, reduced intradiscal pressure of adjacent level, but caused to increment in range of motion by 20% under axial rotation. Higher tendency by 93% to the failure was also detected under axial rotation. Dynesys hybrid stabilization system represented more restricted motion than BalanC, and negligible effects to the adjacent level. B-DYN hybrid stabilization system was the most rigid one among all three systems. It reduced intradiscal pressure and range of motion at the adjacent level except from motion under axial rotation being increased by 13%. Fracture risk of B-DYN and Dynesys Transition Optima components was low when compared with BalanC. Mobility of the adjacent level around axial direction should be taken into account in case of implantation with BalanC and B-DYN systems, as well as on the development of new designs. Having these findings in mind, it is clear that hybrid systems need to be further tested, both clinically and numerically, before being considered for common use.

Dynamic Stabilization Adjacent to Fusion versus Posterior Lumbar Interbody Fusion for the Treatment of Lumbar Degenerative Disease: A Meta-Analysis

This study evaluated differences in outcome variables between dynamic stabilization adjacent to fusion (DATF) and posterior lumbar interbody fusion (PLIF) for the treatment of lumbar degenerative disease. A systematic review of PubMed, EMBASE, and Cochrane was performed. The variables of interest included clinical adjacent segment pathologies (CASPs), radiological adjacent segment pathologies (RASPs), lumbar lordosis (LL), visual analogue scale (VAS) of back (VAS-B) and leg (VAS-L), Oswestry disability index (ODI), Japanese Orthopaedic Association (JOA) score, duration of surgery (DS), estimated blood loss (EBL), complications, and reoperation rate. Nine articles identified as meeting all of the inclusion criteria. DATF was better than PLIF in proximal RASP, CASP, and ODI during 3 months follow-up, VAS-L. However, no significant difference between DATF and PLIF was found in distal RASP, LL, JOA score, VAS-B, ODI after 3 months follow-up, complication rates, and reoperation rate. These further confirmed that DATF could decrease the proximal ASP both symptomatically and radiographically as compared to fusion group; however, the influence of DATF on functional outcome was similar with PLIF. The differences between hybrid surgery and topping-off technique were located in DS and EBL in comparison with PLIF. Our study confirmed that DATF could decrease the proximal ASP both symptomatically and radiographically as compared to the fusion group; however, the influence of DATF on functional outcome was similar with PLIF. The difference between hybrid surgery and topping-off technique was not significant in treatment outcomes.

Adjacent segment degeneration and topping off. Never stop at the apex!

We investigated if applying the Transition system (Globus Medical Inc., Audubon, PA, USA) as topping off can prevent Adjacent Segment Degeneration (ASD) and if rate of ASD is increased if instrumentation stopped at the apex of the Lumbar Lordosis (LL). We enrolled 99 consecutive patients in a retrospective study who have been operated by instrumented fusion of the lumbar spine. Thirty patients were treated by topping of (Group 1), 69 patients received the standard procedure (Group 2). 18 patients of group 1 (60%) and 38 patients of group 2 (55%) developed ASD. The difference was not significant (P>0.05). In 17 patients (17%) instrumentation stopped at apex of LL. 14/17 patients (82%) developed an ASD. This influence was significant (P<0.05). Instrumented fusion of the lumbar spine should not stop at the apex of the lumbar curve. Topping off by hybrid dynamic fixation does not reduce the rate of ASD.

Material failure in dynamic spine implants: are the standardized implant tests before market launch sufficient?

PURPOSE

International Standards Organization (ISO) 12189 and American Society for Testing and Materials F2624 are two standard material specification and test methods for spinal implant devices. The aim of this study was to assess whether the existing and required tests before market launch are sufficient.

METHODS

In three prospective studies, patients were treated due to degenerative disease of the lumbar spine or spondylolisthesis with lumbar interbody fusion and dynamic stabilization of the cranial adjacent level. The CD HORIZON BalanC rod and S⁴ Dynamic rod were implanted in 45 and 11 patients, respectively.

RESULTS

A fatigue fracture of the material of the topping off system has been found in five cases (11%) for the group fitted with the CD HORIZON BalanC rod. In the group using the S⁴ Dynamic rod group, a material failure of the dynamic part was demonstrated in seven patients (64%). All three studies were interrupted due to these results, and a report to the Federal Institute for Drugs and Medical Devices was generated.

CONCLUSION

Spinal implants have to be checked by a notified body before market launch. The notified body verifies whether the implants fulfil the requirements of the current standards. These declared studies suggest that the current standards for the testing of load bearing capacity and stand ability of dynamic spine implants might be insufficient. Revised standards depicting sufficient deformation and load pattern have to be developed and counted as a requirement for the market launch of an implant. These slides can be retrieved under Electronic Supplementary Material.

Clinical experiences with a PEEK-based dynamic instrumentation device in lumbar spinal surgery: 2 years and no more

Background

Dynamic spine implants were developed to prevent adjacent segment degeneration (ASD) and adjacent segment disease (ASDi). Purpose of this study was to investigate the clinical and radiological outcomes of “topping off” devices following lumbar spinal fusion procedure using a PEEK-based dynamic rod system. Moreover, this study focused on the hypothesis that “topping off” devices can prevent ASD.

Methods

This prospective nonrandomized study included patients with indication for single-level lumbar fusion and radiological signs of ASD without instability. The exclusion criteria were previous lumbar spine surgery and no sign of disc degeneration in the adjacent segment according to magnetic resonance imaging. All patients were treated with single-level lumbar interbody fusion and dynamic stabilization of the cranial adjacent segment. Patients underwent a clinical examination and radiographs preoperatively and at 1 and 2 years after surgery. Analyses were performed on clinical data collected with the German Spine Registry using the core outcome measure index (COMI) and visual analogue scale (VAS) scores for back and leg pain.

Results

A total of 22 patients (6 male and 16 female) with an average age of 57.6 years were included in the study; 20 patients completed the follow-up (FU). The average COMI score was 9.0 preoperatively, 4.2 at the 1-year FU, and 4.7 at the 2-year FU. The average preoperative VAS scores for back and leg pain were 7.7 and 7.1, respectively. At the 1-year FU, the scores were 4.25 for back pain and 2.2 for leg pain, and at the 2-year FU, the scores were 4.7 for back pain and 2.3 for leg pain. At FU, failure of the dynamic topping off implant material was verified in four cases, and ASD of the segment cranial to the topping off was confirmed in three cases.

Conclusions

These results demonstrate significant improvements in clinical outcomes and pain reduction after lumbar spinal fusion with topping off at 2 years after surgery. However, the implant failed due to the high rate of implant failure and the development of ASD in the segment cranial to the dynamic stabilized segment.

Biomechanical Effects of a Dynamic Topping off Instrumentation in a Long Rigid Pedicle Screw Construct

STUDY DESIGN

Biomechanical ex vivo study.

OBJECTIVE

To determine if topping off instrumentation can reduce the hypermobility in the adjacent segments when compared with the classic rigid spinal instrumentation.

SUMMARY OF THE BACKGROUND DATA

Long rigid instrumentation might increase the mechanical load in the adjacent segments, the resulting hypermobility, and the risk for adjacent segment disease. Topping off instrumentation intends to reduce the hypermobility at the adjacent level by more evenly distributing segmental motion and, thereby, potentially mitigating adjacent level disease.

MATERIALS AND METHODS

Eight human spines (Th12-L5) were divided into 2 groups. In the rigid group, a 3-segment metal rod instrumentation (L2-L5) was performed. The hybrid group included a 2-segment metal rod instrumentation (L3-L5) with a dynamic topping off instrumentation (L2-L3). Each specimen was tested consecutively in 3 different configurations: native (N=8), 2-segment rod instrumentation (L3-L5, N=8), 3-segment instrumentation (rigid: N=4, hybrid: N=4). For each configuration the range of motion (ROM) of the whole spine and each level was measured by a motion capture system during 5 cycles of extension-flexion (angle controlled to ±5 degrees, 0.1 Hz frequency, no preload).

RESULTS

In comparison with the intact spine, both the rigid 3-segment instrumentation and the hybrid instrumentation significantly reduced the ROM in the instrumented segments (L2-L5) while increasing the movement in the adjacent segment L1-L2 (P=0.002, η=0.82) and in Th12-L1 (P<0.001, η=0.90). There were no ROM differences between the rigid and hybrid instrumentation in all segments.

CONCLUSIONS

Introducing the dynamic topping off did not impart any significant difference in the segmental motion when compared with the rigid instrumentation. Therefore, the current biomechanical study could not show a benefit of using this specific topping off instrumentation to solve the problem of adjacent segment disease.

Hybrid Instrumentation in Lumbar Spinal Fusion: A Biomechanical Evaluation of Three Different Instrumentation Techniques

STUDY DESIGN

Ex vivo human cadaveric study.

OBJECTIVE

The development or progression of adjacent segment disease (ASD) after spine stabilization and fusion is a major problem in spine surgery. Apart from optimal balancing of the sagittal profile, dynamic instrumentation is often suggested to prevent or impede ASD. Hybrid instrumentation is used to gain stabilization while allowing motion to avoid hypermobility in the adjacent segment. In this biomechanical study, the effects of two different hybrid instrumentations on human cadaver spines were evaluated and compared with a rigid instrumentation.

METHODS

Eighteen human cadaver spines (T11-L5) were subdivided into three groups: rigid, dynamic, and hook comprising six spines each. Clinical parameters and initial mechanical characteristics were consistent among groups. All specimens received rigid fixation from L3-L5 followed by application of a free bending load of extension and flexion. The range of motion (ROM) for every segment was evaluated. For the rigid group, further rigid fixation from L1-L5 was applied. A dynamic Elaspine system (Spinelab AG, Winterthur, Switzerland) was applied from L1 to L3 for the dynamic group, and the hook group was instrumented with additional laminar hooks at L1-L3. ROM was then evaluated again.

RESULTS

There was no significant difference in ROM among the three instrumentation techniques.

CONCLUSION

Based on this data, the intended advantage of a hybrid or dynamic instrumentation might not be achieved.

The unfeasible made feasible: lumbar minimally invasive hybrid stabilization with dynamic rod and mini-open transforaminal lumbar interbody fusion

Posterior dynamic stabilization of the lumbar spine is spreading as a viable alternative to spinal fusion, aiming to achieve an equally satisfactory clinical outcome without making the spine completely rigid. We describe the feasibility of a minimally invasive surgical technique used to implant a hybrid system and perform a mini-open (m-open) transforaminal lumbar interbody fusion (TLIF) in patients suffering from degenerative spondylolistesis and adjacent level's degenerative disc disease (DDD). Three patients (2 females), suffering from degenerative spondylolistesis and adjacent level's DDD, underwent two-level hybrid stabilization combining a rigid, circumferential fusion (with m-open TLIF) at the level involved by spondylolistesis and a dynamic stabilization at the adjacent one. Screws, hybrid rods as well as interbody cages were introduced using a simple minimally invasive technique. Clinical and radiological evaluation was performed pre- and postoperatively, and at 3, 6 and 12 months, respectively, using the Visual Analogue Scale and the Oswestry Disability Index questionnaire. Mean VAS and ODI score reduced from 8.3, preoperatively, to 5 and from 72.66 to 43.98, respectively. No surgery-related complications were observed and the mean postoperative hospitalization was 2.5 days. Postoperative and follow-up flexion-extension X-rays showed persisting motion at dynamically stabilized levels. Follow-up CT imaging confirmed interbody fusion at TLIF levels in all patients. Dynamic and hybrid stabilizations of the lumbar spine are typically performed using open surgery. This study reports the feasibility of a hybrid stabilization with m-open TLIF performed using a minimally invasive technique.

Hybrid Surgery Combined with Dynamic Stabilization System and Fusion for the Multilevel Degenerative Disease of the Lumbosacral Spine

BACKGROUND

As motion-preserving technique has been developed, the concept of hybrid surgery involves simultaneous application of two different kinds of devices, dynamic stabilization system and fusion technique. In the present study, the application of hybrid surgery for lumbosacral degenerative disease involving two-segments and its long-term outcome were investigated.

METHODS

Fifteen patients with hybrid surgery (Hybrid group) and 10 patients with two-segment fusion (Fusion group) were retrospectively compared.

RESULTS

Preoperative grade for disc degeneration was not different between the two groups, and the most common operated segment had the most degenerated disc grade in both groups; L4-5 and L5-S1 in the Hybrid group, and L3-4 and L4-5 in Fusion group. Over 48 months of follow-up, lumbar lordosis and range of motion (ROM) at the T12-S1 global segment were preserved in the Hybrid group, and the segmental ROM at the dynamic stabilized segment maintained at final follow-up. The Fusion group had a significantly decreased global ROM and a decreased segmental ROM with larger angles compared to the Hybrid group. Defining a 2-mm decrease in posterior disc height (PDH) as radiologic adjacent segment pathology (ASP), these changes were observed in 6 and 7 patients in the Hybrid and Fusion group, respectively. However, the last PDH at the above adjacent segment had statistically higher value in Hybrid group. Pain score for back and legs was much reduced in both groups. Functional outcome measured by Oswestry disability index (ODI), however, had better improvement in Hybrid group.

CONCLUSION

Hybrid surgery, combined dynamic stabilization system and fusion, can be effective surgical treatment for multilevel degenerative lumbosacral spinal disease, maintaining lumbar motion and delaying disc degeneration.

Comparison of clinical and radiographic results between isobar posterior dynamic stabilization and posterior lumbar inter-body fusion for lumbar degenerative disease: A four-year retrospective study

PURPOSE

A retrospective study was conducted to compare clinical outcome with radiographic data and clinical complications between isobar posterior dynamic stabilization (IPDS, Scient'x, France) and posterior lumbar inter-body fusion (PLIF) for lumbar degenerative disease.

METHODS

113 consecutive patients (IPDS group, N=62; PLIF group, N=51) with lumbar degenerative disease were operated on between March 2009 and November 2011. Patient charts, radiographic films and medical records were reviewed. Clinical outcomes including the visual analog scale (VAS), Oswestry disability index (ODI) scores, and radiographic outcomes, including disk height index (DHI) and range of motion (ROM) were retrospectively analyzed.

RESULTS

The ODI and VAS leg and back pain scores in two groups were significantly improved at 6 and, 24 months and at the final follow-up (all, P<0.05). The degree of improvements in the ODI and VAS back pain scores, the incidence of complications and the rate of adjacent segment degeneration were similar in both groups (P>0.05). However, operation times and blood loss were significantly reduced in the IPDS group (P<0.05).

CONCLUSION

In summary, with similar symptoms improvement and complication rates, the results of this study demonstrate that IPDS is an effective and safe treatment for lumbar degenerative disease. There is currently insufficient evidence to indicate that the IPDS can avoid adjacent segment degeneration therefore, it is essential to conduct prospective, randomized, controlled multicenter studies with larger sample size and longer follow-up.

Biomechanical effect of interspinous dynamic stabilization adjacent to single-level fusion on range of motion of the transition segment and the adjacent segment

BACKGROUND

Despite numerous biomechanical studies have been carried out on dynamic stabilizers, there is very little information on their hybrid application, especially when combined interspinous dynamic stabilization with single-level fusion. The aim of this study is to assess the biomechanical effect of interspinous dynamic stabilization adjacent to single-level fusion on range of motion of the transition segment and the adjacent segment.

METHODS

Six fresh lumbosacral spines (L2-S1) were tested in the following sequence: 1) intact (Construct A); 2) fusion in L5/S1 and intact in L4/5 (Construct B); 3) fusion in L5/S1 and unstable state in L4/5 (Construct C); 4) fusion in L5/S1 and Coflex in L4/5 (Construct D). Range of motion (at L3/4 and L4/5) was recorded and calculated.

FINDINGS

Range of motion in L3/4 in the four constructs showed no difference under all motion states. Under flexion/extension, the range of motion of L4/5 in Construct B and Construct C increased, while the range of motion of L4/5 in Construct D decreased compared with Construct A. Compared with Construct D, the range of motion of L4/5 in Constructs B and C showed a significant increase. Under lateral bending and axial rotation, Construct A showed similar range of motion of L3/4 compared with other constructs.

INTERPRETATION

Fusion combined with Coflex is able to stabilize the transition segment and restrict flexion and extension in that segment, while having no significant effect on the range of motion of the adjacent segment or the range of motion of the transition segment under lateral bending and axial rotation.

The current testing protocols for biomechanical evaluation of lumbar spinal implants in laboratory setting: a review of the literature

In vitro biomechanical investigations have become a routinely employed technique to explore new lumbar instrumentation. One of the most important advantages of such investigations is the low risk present when compared to clinical trials. However, the best use of any experimental data can be made when standard testing protocols are adopted by investigators, thus allowing comparisons among studies. Experimental variables, such as the length of the specimen, operative level, type of loading (e.g., dynamic versus quasistatic), magnitude, and rate of load applied, are among the most common variables controlled during spinal biomechanical testing. Although important efforts have been made to standardize these protocols, high variability can be found in the current literature. The aim of this investigation was to conduct a systematic review of the literature to identify the current trends in the protocols reported for the evaluation of new lumbar spinal implants under laboratory setting.

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.

Biomechanical evaluation of a simulated T-9 burst fracture of the thoracic spine with an intact rib cage

Object

Classic biomechanical models have used thoracic spines disarticulated from the rib cage, but the biomechanical influence of the rib cage on fracture biomechanics has not been investigated. The well-accepted construct for stabilizing midthoracic fractures is posterior instrumentation 3 levels above and 2 levels below the injury. Short-segment fixation failure in thoracolumbar burst fractures has led to kyphosis and implant failure when anterior column support is lacking. Whether shorter constructs are viable in the midthoracic spine is a point of controversy. The objective of this study was the biomechanical evaluation of a burst fracture at T-9 with an intact rib cage using different fixation constructs for stabilizing the spine.

Methods

A total of 8 human cadaveric spines (C7–L1) with intact rib cages were used in this study. The range of motion (ROM) between T-8 and T-10 was the outcome measure. A robotic spine testing system was programmed to apply pure moment loads (± 5 Nm) in lateral bending, flexion-extension, and axial rotation to whole thoracic specimens. Intersegmental rotations were measured using an optoelectronic system. Flexibility tests were conducted on intact specimens, then sequentially after surgically induced fracture at T-9, and after each of 4 fixation construct patterns. The 4 construct patterns were sequentially tested in a nondestructive protocol, as follows: 1) 3 above/2 below (3A/2B); 2) 1 above/1 below (1A/1B); 3) 1 above/1 below with vertebral body augmentation (1A/1B w/VA); and 4) vertebral body augmentation with no posterior instrumentation (VA). A repeated-measures ANOVA was used to compare the segmental motion between T-8 and T-10 vertebrae.

Results

Mean ROM increased by 86%, 151%, and 31% after fracture in lateral bending, flexion-extension, and axial rotation, respectively. In lateral bending, there was significant reduction compared with intact controls for all 3 instrumented constructs: 3A/2B (−92%, p = 0.0004), 1A/1B (−63%, p = 0.0132), and 1A/1B w/VA (−66%, p = 0.0150). In flexion-extension, only the 3A/2B pattern showed a significant reduction (−90%, p = 0.011). In axial rotation, motion was significantly reduced for the 3 instrumented constructs: 3A/2B (−66%, p = 0.0001), 1A/1B (−53%, p = 0.0001), and 1A/1B w/VA (−51%, p = 0.0002). Between the 4 construct patterns, the 3 instrumented constructs (3A/2B, 1A/1B, and 1A/1B w/VA) showed comparable stability in all 3 motion planes.

Conclusions

This study showed no significant difference in the stability of the 3 instrumented constructs tested when the rib cage is intact. Fractures that might appear more grossly unstable when tested in the disarticulated spine may be bolstered by the ribs. This may affect the extent of segmental spinal instrumentation needed to restore stability in some spine injuries. While these initial findings suggest that shorter constructs may adequately stabilize the spine in this fracture model, further study is needed before these results can be extrapolated to clinical application.

Biomechanics of the lower thoracic spine after decompression and fusion: a cadaveric analysis

BACKGROUND CONTEXT

Few studies have evaluated the extent of biomechanical destabilization of thoracic decompression on the upper and lower thoracic spine. The present study evaluates lower thoracic spinal stability after laminectomy, unilateral facetectomy, and unilateral costotransversectomy in thoracic spines with intact sternocostovertebral articulations.

PURPOSE

To assess the biomechanical impact of decompression and fixation procedures on lower thoracic spine stability.

STUDY DESIGN

Biomechanical cadaveric study.

METHODS

Sequential surgical decompression (laminectomy, unilateral facetectomy, unilateral costotransversectomy) and dorsal fixation were performed on the lower thoracic spine (T8-T9) of human cadaveric spine specimens with intact rib cages (n=10). An industrial robot was used to apply pure moments to simulate flexion-extension (FE), lateral bending (LB), and axial rotation (AR) in the intact specimens and after decompression and fixation. Global range of motion (ROM) between T1-T12 and intrinsic ROM between T7-T11 were measured for each specimen.

RESULTS

The decompression procedures caused no statistically significant change in either global or intrinsic ROM compared with the intact state. Instrumentation, however, reduced global motion for AR (45° vs. 30°, p=.0001), FE (24° vs. 19°, p=.02), and LB (47° vs. 36°, p=.0001) and for intrinsic motion for AR (17° vs. 4°, p=.0001), FE (8° vs. 1°, p=.0001), and LB (12° vs. 1°, p=.0001). No significant differences were identified between decompression of the upper versus lower thoracic spine, with trends toward significantly greater ROM for AR and lower ROM for LB in the lower thoracic spine.

CONCLUSIONS

The lower thoracic spine was not destabilized by sequential unilateral decompression procedures. Addition of dorsal fixation increased segment rigidity at intrinsic levels and also reduced overall ROM of the lower thoracic spine to a greater extent than did fusing the upper thoracic spine (level of the true ribs). Despite the lack of true ribs, the lower thoracic spine was not significantly different compared with the upper thoracic spine in FE and LB after decompression, although there were trends toward significance for greater AR after decompression. In certain patients, instrumentation may not be needed after unilateral decompression of the lower thoracic spine; further validation and additional clinical studies are warranted.

Zero-profile hybrid fusion construct versus 2-level plate fixation to treat adjacent-level disease in the cervical spine

OBJECT

Single-level anterior cervical discectomy and fusion (ACDF) is an established surgical treatment for cervical myelopathy. Within 10 years of undergoing ACDF, 19.2% of patients develop symptomatic adjacent-level degeneration. Performing ACDF adjacent to prior fusion requires exposure and removal of previously placed hardware, which may increase the risk of adverse outcomes. Zero-profile cervical implants combine an interbody spacer with an anterior plate into a single device that does not extend beyond the intervertebral disc space, potentially obviating the need to remove prior hardware. This study compared the biomechanical stability and adjacent-level range of motion (ROM) following placement of a zero-profile device (ZPD) adjacent to a single-level ACDF against a standard 2-level ACDF.

METHODS

In this in vitro biomechanical cadaveric study, multidirectional flexibility testing was performed by a robotic spine system that simulates flexion-extension, lateral bending, and axial rotation by applying a continuous pure moment load. Testing conditions were as follows: 1) intact, 2) C5-6 ACDF, 3) C4-5 ZPD supraadjacent to simulated fusion at C5-6, and 4) 2-level ACDF (C4-6). The sequence of the latter 2 test conditions was randomized. An unconstrained pure moment of 1.5 Nm with a 40-N simulated head weight load was applied to the intact condition first in all 3 planes of motion and then using the hybrid test protocol, overall intact kinematics were replicated subsequently for each surgical test condition. Intersegmental rotations were measured optoelectronically. Mean segmental ROM for operated levels and adjacent levels was recorded and normalized to the intact condition and expressed as a percent change from intact. A repeated-measures ANOVA was used to analyze the ROM between test conditions with a 95% level of significance.

RESULTS

No statistically significant differences in immediate construct stability were found between construct Patterns 3 and 4, in all planes of motion (p > 0.05). At the operated level, C4-5, the zero-profile construct showed greater decreases in axial rotation (-45% vs -36%) and lateral bending (-55% vs -38%), whereas the 2-level ACDF showed greater decreases in flexion-extension (-40% vs -34%). These differences were marginal and not statistically significant. Adjacent-level motion was nearly equivalent, with minor differences in flexion-extension.

CONCLUSIONS

When treating degeneration adjacent to a single-level ACDF, a zero-profile implant showed stabilizing potential at the operated level statistically similar to that of the standard revision with a 2-level plate. Revision for adjacent-level disease is common, and using a ZPD in this setting should be investigated clinically because it may be a faster, safer alternative.

Retrieval analysis of PEEK rods for posterior fusion and motion preservation

INTRODUCTION

The purpose of this study was to analyze explanted PEEK rod spinal systems in the context of their clinical indications. We evaluated damage to the implant and histological changes in explanted periprosthetic tissues.

METHODS

12 patients implanted with 23 PEEK rods were revised between 2008 and 2012. PEEK rods were of the same design (CD Horizon Legacy, Medtronic, Memphis TN, USA). Retrieved components were assessed for surface damage mechanisms, including plastic deformation, scratching, burnishing, and fracture. Patient history and indications for PEEK rod implantation were obtained from analysis of the medical records.

RESULTS

11/12 PEEK rod systems were employed for fusion at one level, and motion preservation at the adjacent level. Surgical complications in the PEEK cohort included a small dural tear in one case that was immediately repaired. There were no cases of PEEK rod fracture or pedicle screw fracture. Retrieved PEEK rods exhibited scratching, as well as impressions from the set screws and pedicle screw saddles. PEEK debris was observed in two patient tissues, which were located adjacent to PEEK rods with evidence of scratching and burnishing.

CONCLUSION

This study documents the surface changes and tissue reactions for retrieved PEEK rod stabilization systems. Permanent indentations by the set screws and pedicle screws were the most prevalent observations on the surface of explanted PEEK rods.

Pedicle screw-based posterior dynamic stabilization: literature review

Posterior dynamic stabilization (PDS) indicates motion preservation devices that are aimed for surgical treatment of activity related mechanical low back pain. A large number of such devices have been introduced during the last 2 decades, without biomechanical design rationale, or clinical evidence of efficacy to address back pain. Implant failure is the commonest complication, which has resulted in withdrawal of some of the PDS devices from the market. In this paper the authors presented the current understanding of clinical instability of lumbar motions segment, proposed a classification, and described the clinical experience of the pedicle screw-based posterior dynamic stabilization devices.