[Show abstract][Hide abstract] ABSTRACT: Study Design. In vitro Biomechanical Study.Objective. To test the hypotheses: (1) an anchored-spacer device would decrease motion similarly to a plate-spacer construct, and (2) the anchored-spacer would achieve a similar reduction in motion when placed adjacent to a previously fused segment.Summary of Background Data. An anchored-spacer device has been shown to perform similar to the plate-spacer construct in previous biomechanical evaluation. The prevalence of adjacent segment disease after fusion is well established in the literature. There is currently no evidence supporting the use of an anchored interbody spacer device adjacent to a previous fusion.Methods. Eight human cervical spines (age: 45.1±13.1) were tested in moment-control (±1.5Nm) in flexion-extension, lateral bending, and axial rotation without preload. Flexion-extension was then retested under 150N preload. Spines were tested intact and after ACDF at C4-C5 and C6-C7 with either a plate-spacer or anchored-spacer construct (randomized). The specimens were tested finally with an ACDF at the floating C5-C6 segment using the anchored-spacer device adjacent to the previous fusions.Results. Both the plate-spacer and anchored-spacer significantly reduced motion from the intact spine in flexion-extension, lateral bending and axial rotation (p<0.005). There was no statistically significant difference between the two fusion constructs in their abilities to reduce motions (p = 1.0). ACDF using the anchored-spacer at the floating C5-C6 level (in between the plate-spacer and anchored-spacer constructs) resulted in significant motion reductions in all modes of testing (p<0.05). These motion reductions did not significantly differ from those of a single-level anchored-spacer construct or a single level plated ACDF.Conclusion. The anchored-spacer provided significant motion reductions, similar to a plated ACDF, when used as a single-level fusion construct or placed adjacent to a previously plated segment.
[Show abstract][Hide abstract] ABSTRACT: Study Design. Analysis of data collected prospectively from the US FDA IDE clinical trial for the 1 and 2 Level ProDisc-L arthroplasty patients performed at two study centers.Objective. To determine if there is any clinical or radiographic differences between 1-level ProDisc-L patients and 2-level ProDisc-L patients with a minimum follow-up of 5 years.Summary of Background Data. Recent prospective US FDA clinical trial results have been published showing efficacy of the ProDisc prosthesis in both single and two level surgical procedures. Results of these prospective, randomized multicenter FDA IDE clinical trials showed safety, efficacy and superiority over circumferential fusion.Methods. Patients were part of the FDA clinical trial for the ProDisc-L versus circumferential fusion study at two institutions. We identified 20 patients who received ProDisc-L at 1 level and 21 who received it at 2 levels for a total of 41 patients. Pre- and post-operative pain, clinical function and range of motion (ROM) were measured. An unpaired t-test was done to compare the 2-verus 1-level disc replacement patients.Results. All patients had more than 60 months of clinical follow-up. The mean scores on the VAS were 37.1 and 28.7 respectively (P = 0.33) for one and two level patients. ODI scores were 33 and 29.1 respectively for one and two level patients (P = 0.60); and SF36 physical scores were 43 and 43.9 respectively for one and two level patients (P = 0.81); SF-36 mental scores were 46.2 and 46.6 respectively for one and two level patients (p = 0.923). The average final follow-up post-op ROM for one and two level TDR were 6.0º and 5.4º respectively (p = 0.632).Conclusion. This study was unable to identify statistical differences in all clinical and radiographic outcomes between 1- and 2-level ProDisc arthroplasty patients in a cohort from two centers. Future studies with longer-term follow-up are needed to confirm these results.
[Show abstract][Hide abstract] ABSTRACT: Study Design. Prospective retrieval analysis of failed Prodisc-C CTDRs from 24 explanting surgeons over a 6-year period.Objective. To determine the in vivo mechanical performance and fixation to bone of explanted Prodisc-C CTDRs.Summary of Background Data. The nature and quantity of damage sustained by an implanted device has proven to be important in the prediction of clinical longevity. We hypothesized that retrieval analysis of the Prodisc-C will display characteristic modes of wear consistent with increased posterior angulation and translation of the functional spinal unit following resection of the discoligamentous anatomy.Methods. Thirty CTDRs from 29 patients (mean age 45.1±1.9, range 31-57yrs) after a mean length of implantation of 1.0±0.2 years (range 2 days-3.5 yrs) were studied. Operative level was C4-C5 in 20%(6/30), C5-C6 in 47%(14/30), C6-7 in 20%(6/30), and unknown in 13%(4/30). Polyethylene(PE) and metallic(CoCrMo) components were examined using light stereo-microscopy(6X-31X), scanning electron microscopy(SEM), and energy dispersive x-ray analysis (EDAX).Results. CTDRs were explanted for indications of axial pain(n = 9), radicular symptoms(n = 6), atraumatic loosening(n = 6), trauma(n = 5), metal allergy(n = 1), myelopathy(n = 1), hypermobility(n = 1), and unknown(n = 1). Surface area of ongrowth (mean = 7.2±1.4%) was not associated with operative level(p = .37), surgeon reported axial pain(p = .56) or atraumatic loosening(p = .93). Burnishing consistent with metallic endplate impingement was present in 80%(24/30) of retrieved CTDRs; most commonly in the posterior quadrant(p<.001). There was no association between implant height(p = .19) or depth(p = .17) and posterior impingement. Backside wear was not observed on any of the disassembled implants(0/16). Third-body wear occurred in 23%(7/30) and the donor site was confirmed by SEM/EDAX to be the porous-coated surface of the CTDR.Conclusion. Early clinical failures of Prodisc-C CTDRs display surface damage evidence of metal endplate-endplate impingement, most commonly posteriorly. Backside wear was not evident, however, third-body wear was found. Future studies will determine the clinical impact of these predominant modes of wear on long-term metal-on-PE semi-constrained CTDR performance.
[Show abstract][Hide abstract] ABSTRACT: A retrieval analysis of wear modes and fixation of lumbar total disc replacements (TDRs). Explanted Prodisc-L TDRs were prospectively collected during a 7-year period (2005-2011) and analyzed.
To assess the in vivo modes of wear and fixation of lumbar TDR with the Prodisc-L device.
Inferior clinical outcomes and failure of lumbar TDR may occur because of suboptimal component fixation, wear properties, and impingement in a subset of patients. Posterior component TDR impingement has been demonstrated radiographically; however, despite its widespread use, the in vivo mechanical performance and fixation of the Prodisc-L device remain unknown.
Explanted polyethylene and metallic (CoCrMo) components of Prodisc-L devices were examined by light stereo-microscopy (6X-31X), scanning electron microscopy, and energy-dispersive x-ray analysis from an international retrieval registry, with 13 participating surgeons.
Nineteen ProDisc-L devices from 18 patients (age, 44.7 ± 2.9 yr) following an index TDR at L4-L5 (n = 6), L5-S1 (n = 11), and unknown level (n = 2) were explanted for pain (n = 8), prosthesis subluxation/migration (n = 4), end plate collapse/subsidence (n = 3), polyethylene dislodgement (n = 3), and unknown (n = 2) after a mean length of implantation of 13.0 ± 3.9 months. Surface area of bony ongrowth was 9.6 ± 2.9% (range, 0%-52.5%). TDR burnishing was observed posteriorly consistent with component impingement in extension in 53% (8/15) (P < 0.02), more commonly than anterior 20% (3/15) lateral 20% (n = 3) (3/15) patterns. Circumferential burnishing was not observed. Posterior impingement was associated with 6° lordotic implants (P < 0.05) and 10-mm polyethylene size (P < 0.05). Backside wear occurred in 75% (9 of 12) of the disassembled implants and third-body wear was observed in 33% (5 of 15).
Metallic end plate burnishing was evident in a large percentage of clinically failed Prodisc-L TDR devices, most commonly posteriorly, consistent with impingement in extension. Long-term follow-up studies will evaluate the effects of the observed backside wear, third-body wear, and end plate impingement on clinical outcomes.
[Show abstract][Hide abstract] ABSTRACT: We hypothesized that L5-S1 kinematics will not be affected by the lordosis distribution between the prosthesis endplates.
Twelve cadaveric lumbosacral spines (51.3 ± 9.8 years) were implanted with 6° or 11° prostheses (ProDisc-L) with four combinations of superior/inferior lordosis (6°/0°, 3°/3°, 11°/0°, 3°/8°). Specimens were tested intact and after prostheses implantation with different lordosis distributions. Center of rotation (COR) and range of motion (ROM) were quantified.
Six-degree lordosis prostheses (n = 7) showed no difference in flexion-extension ROM, regardless of design (6°/0° or 3°/3°) (p > 0.05). In lateral bending (LB), both designs reduced ROM (p < 0.05). In axial rotation, only the 3°/3° design reduced ROM (p < 0.05). Eleven-degree lordosis prostheses (n = 5) showed no difference in flexion-extension ROM for either design (p > 0.05). LB ROM decreased with distributed lordosis prostheses (3°/8°) (p < 0.05). Overall, L5-S1 range of motion was not markedly influenced by lordosis distribution among the two prosthesis endplates. The ProDisc-L prosthesis design where all lordosis is concentrated in the superior endplate yielded COR locations that were anterior and caudal to intact controls. The prosthesis with lordosis distributed between the two endplates yielded a COR that tended to be closer to intact.
Further clinical and biomechanical studies are needed to assess the long-term impact of lordosis angle distribution on the fate of the facet joints.
[Show abstract][Hide abstract] ABSTRACT: In vitro biomechanical study.
To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response.
Cervical TDR may be a promising alternative for a symptomatic adjacent level after prior multilevel cervical fusion. However, little is known about the TDR kinematics in this setting.
Eight human cadaveric cervical spines (C2-T1, age: 59 ± 8.6 years) were tested intact, after simulated two-level fusion (C4-C6) in lordotic alignment and then in straight alignment, and after C3-C4 TDR above the C4-C6 fusion in lordotic and straight alignments. Fusion was simulated using an external fixator apparatus, allowing easy adjustment of C4-C6 fusion alignment, and restoration to intact state upon disassembly. Specimens were tested in flexion-extension using hybrid testing protocols.
The external fixator device significantly reduced range of motion (ROM) at C4-C6 to 2.0 ± 0.6°, a reduction of 89 ± 3.0% (P < 0.05). Removal of the fusion construct restored the motion response of the spinal segments to their intact state. The C3-C4 TDR resulted in less motion as compared to the intact segment when the disc prosthesis was implanted either as a stand-alone procedure or above a two-level fusion. The decrease in motion of C3-C4 TDR was significant for both lordotic and straight fusions across C4-C6 (P < 0.05). Flexion and extension moments needed to bring the cervical spine to similar C2 motion endpoints significantly increased for the TDR above a two-level fusion compared to TDR alone (P < 0.05). Lordotic fusion required significantly greater flexion moment, whereas straight fusion required significantly greater extension moment (P < 0.05).
TDR placed adjacent to a two-level fusion is subjected to a more challenging biomechanical environment as compared to a stand-alone TDR. An artificial disc used in such a clinical scenario should be able to accommodate the increased moment loads without causing impingement of its endplates or undue wear during the expected life of the prosthesis.
[Show abstract][Hide abstract] ABSTRACT: A biomechanical study using human lumbar spines.
To test the hypotheses that with increasing implant height (1) the range of motion (ROM) of the implanted segment will decrease, (2) the segmental lordosis will increase, and (3) the size of the neural foramens will increase.
Little is known about the effects of the implant height on the segmental motion and foraminal size at the implanted level.
Seven human lumbar spines (age, 54.4+/-11.4 years; L1-sacrum) were tested intact, and after discectomy at L4-L5 and sequential insertion of ProDisc-L implants (Synthes Spine, Paoli, PA) of increasing heights (10, 12, and 14 mm). The specimens were tested in flexion (8 Nm) and extension (-6 Nm) with a 400 N follower preload as well as in lateral bending (+/-6 Nm) and axial rotation (+/-5 Nm) without preload. Three-dimensional motions were measured at L4-L5. Foraminal sizes at L4-L5 were measured in the specimen's neutral posture under a 400 N preload for the intact spine and after each implantation using finely graded cylindrical probes. Segmental lordosis was measured in the specimen's neutral posture under a 400 N preload by analyzing digital fluoroscopic images. Effects of implant height on the kinematics, foraminal size, and segmental lordosis were assessed using paired comparisons with Bonferroni correction.
Increasing implant height from 10 mm to 14 mm caused a significant decrease (P<0.05) in segmental ROM by up to 37%+/-21% in flexion/extension, 33%+/-18% in lateral bending, and 29%+/-28% in axial rotation. Increasing implant height also produced a significant increase in segmental lordosis (P<0.05): from 9.7 degrees+/-2.9 degrees at 10 mm, to 16.1 degrees+/-5.1 degrees at 14 mm. The increase in foraminal size, while significant, was only 4.6%+/-3.2% when comparing 10 mm to 14 mm implants.
These results suggest that a smaller implant height should be selected to optimize the ROM of the implanted segment and maintain sagittal balance.
[Show abstract][Hide abstract] ABSTRACT: Biomechanical study of the ProDisc-L in a cadaveric model under pure moment loading. OBJECTIVE.: To determine the kinematic properties of a lumbar spine motion segment and the adjacent level following ProDisc-L disc replacement in the cadaveric spine.
Total disc replacement is intended to preserve native motion, in an attempt to prevent accelerated adjacent segment degeneration. The quality and quantity of the motion following TDR may have important consequences on the facet joints of the same motion segment, as well as the motion at the prosthetic component interface.
Ten cadaveric lumbar spines were radiographed (L3-L5) and tested under pure moments (+10 Nm to -10 Nm) with an applied follower load (200 N). Load-deformation was tested in flexion/extension, lateral bending (LB), and axial rotation (AR). Range of Motion (ROM) data were recorded. Superior adjacent disc pressure (L3-L4) was measured using subminiature pressure transducers. The L4-L5 disc was subsequently instrumented with a ProDisc-L. Radiographs and biomechanical tests were repeated.
Disc replacement significantly reduced extension (ROM 2.2 degrees +/- 0.5 degrees before and 1.2 degrees +/- 0.7 degrees after instrumentation) (P = 0.001), but not flexion (ROM 5.6 degrees +/- 3.1 degrees before and 6.2 degrees +/- 1.2 degrees after) (P = 0.34). Combined flexion/extension motion was marginally reduced (P = 0.517). LB ROM (7.4 degrees +/- 2.0 degrees ) was marginally reduced (P = 0.072) following instrumentation (6.2 degrees +/- 2.5 degrees ), while ROM in AR (3.4 degrees +/- 1.1 degrees ) was significantly increased (4.4 degrees +/- 1.2 degrees ) (P = 0.001). Superior adjacent segment ROM was preserved.No significant differences in disc pressure were observed at the adjacent motion segment before (199 kPa at maximum flexion and 171 kPa at maximum extension) or after disc replacement (252 kPa and 208 kPa, respectively).
In cadaveric spines, ROM of operated and adjacent motion segments was preserved following ProDisc-L insertion. Excision of the anterior anulus may increase laxity, which is taken up by the restoration of disc height and lordosis, at the cost of a moderate loss of flexion/extension motion. Adjacent segment kinematics were unaffected following TDR.
[Show abstract][Hide abstract] ABSTRACT: Biomechanical study using human cadaver spines.
To assess the stabilizing effect of a supplemental anterior tension band (ATB, Synthes) plate on L5-S1 anterior lumbar interbody fusion (ALIF) using a femoral ring allograft (FRA) under physiologic compressive preloads, and to compare the results with the stability achieved using FRA with supplemental transpedicular instrumentation.
Posterior instrumentation can improve the stability of ALIF cages. Anterior plates have been proposed as an alternative to avoid the additional posterior approach.
Eight human specimens (L3 to sacrum) were tested in the following sequence: (i) intact, (ii) after anterior insertion of an FRA at L5-S1, (iii) after instrumentation with the ATB plate, and (iv) after removal of the plate and adding transpedicular instrumentation at the same level. Specimens were tested in flexion-extension, lateral bending, and axial rotation. Flexion-extension was tested under 0 N, 400 N, and 800 N compressive follower preload to simulate physiologic compressive preloads on the lumbar spine.
Stand-alone FRAs significantly decreased the range of motion (ROM) in all tested directions (P < 0.05); however, the resultant ROM was large in flexion-extension ranging between 6.1 +/- 3.1 degrees and 5.1 +/- 2.2 degrees under 0 N to 800 N preloads. The ATB plate resulted in a significant additional decrease in flexion-extension ROM under 400 N and 800 N preloads (P < 0.05). The flexion-extension ROM with the ATB plate was 4.1 +/- 2.3 under 0 N preload and ranged from 3.1 +/- 1.8 to 2.4 +/- 1.3 under 400 N to 800 N preloads. The plate did not significantly decrease lateral bending or axial rotation ROM compared with stand-alone FRA (P > 0.05), but the resultant ROM was 2.7 +/-1.9 degrees and 0.9 +/- 0.6 degrees , respectively. Compared with the ATB plate, the transpedicular instrumentation resulted in significantly less ROM in flexion-extension and lateral bending (P < 0.05), but not in axial rotation (P > 0.05).
The ATB plate can significantly increase the stability of the anterior FRA at L5-S1 level. Although supplemental transpedicular instrumentation results in a more stable biomechanical environment, the resultant ROM with the addition of a plate is small, especially under physiologic preload, suggesting that the plate can sufficiently resist motion. Therefore, clinical assessment of the ATB plate as an alternative to transpedicular instrumentation to enhance ALIF cage stability is considered reasonable.
[Show abstract][Hide abstract] ABSTRACT: Background
Many clinical studies have focused on clinical pain scores and less on kinematics following intervertebral disc replacement. Although flexion and extension of the motion segment can be measured on lateral X-rays, measuring lateral bending and axial rotation of the device is extremely difficult on plain radiography. This study was designed to measure, using radiostereometric analysis (RSA), the postoperative range of motion of the spinal segment following placement of ProDisc-L interbody device (Synthes Spine, West Chester, Pennsylvania).
[Show abstract][Hide abstract] ABSTRACT: An in vitro human cadaveric biomechanical study.
To evaluate intervertebral rotation changes due to lumbar ProDisc-L compared with simulated fusion, using follower load and multidirectional testing.
Artificial discs, as opposed to the fusions, are thought to decrease the long-term accelerated degeneration at adjacent levels. A biomechanical assessment can be helpful, as the long-term clinical evaluation is impractical.
Six fresh human cadaveric lumbar specimens (T12-S1) underwent multidirectional testing in flexion-extension, bilateral lateral bending, and bilateral torsion using the Hybrid test method. First, intact specimen total range of rotation (T12-S1) was determined. Second, using pure moments again, this range of rotation was achieved in each of the 5 constructs: A) ProDisc-L at L5-S1; B) fusion at L5-S1; C) ProDisc-L at L4-L5 and fusion at L5-S1; D) ProDisc-L at L4-L5 and L5-S1; and E) 2-level fusion at L4-L5 to L5-S1. Significant changes in the intervertebral rotations due to each construct were determined at the operated and nonoperated levels using repeated measures single factor ANOVA and Bonferroni statistical tests (P < 0.05). Adjacent-level effects (ALEs) were defined as the percentage changes in intervertebral rotations at the nonoperated levels due to the constructs.
One- and 2-level ProDisc-L constructs showed only small ALE in any of the 3 rotations. In contrast, 1- and 2-level fusions showed increased ALE in all 3 directions (average, 7.8% and 35.3%, respectively, for 1 and 2 levels). In the disc plus fusion combination (construct C), the ALEs were similar to the 1-level fusion alone.
In general, ProDisc-L preserved physiologic motions at all spinal levels, while the fusion simulations resulted in significant ALE.