Kinematics of cervical total disc replacement adjacent to a two-level, straight versus lordotic fusion.

By London Spine
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Kinematics of cervical total disc replacement adjacent to a two-level, straight versus lordotic fusion.

Spine (Phila Pa 1976). 2011 Aug 1;36(17):1359-66

Authors: Martin S, Ghanayem AJ, Tzermiadianos MN, Voronov LI, Havey RM, Renner SM, Carandang G, Abjornson C, Patwardhan AG

Abstract
STUDY DESIGN: In vitro biomechanical study.
OBJECTIVE: To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response.
SUMMARY OF BACKGROUND DATA: 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.
METHODS: 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.
RESULTS: 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).
CONCLUSION: 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.

PMID: 21629170 [PubMed – indexed for MEDLINE]

The effect of remodeling on the kinematics of the malpositioned disc allograft transplantation.

By London Spine
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The effect of remodeling on the kinematics of the malpositioned disc allograft transplantation.

Spine (Phila Pa 1976). 2012 Mar 15;37(6):E357-66

Authors: Lam SK, Xiao J, Ruan D, Ding Y, Lu WW, Luk KD

Abstract
STUDY DESIGN: A postoperative biomechanical study.
OBJECTIVE: This study aimed to assess whether the mal-alignment of the intervertebral disc (IVD) allograft during transplantation would negatively affect the biomechanics of the spinal segment.
SUMMARY OF BACKGROUND DATA: Studies of human IVD allograft transplantation have observed remodeling of the allograft implant, suggesting that the remodeling of the allograft may be able to restore the natural mechanics of the IVD.
METHODS: Eighteen male goats (age: 6-12 months; weight: 25-30 kg) were randomly assigned into control (n = 5), aligned (n = 5), or malpositioned (n = 5) groups. Transplantation of a size-matched cryopreserved IVD allograft was performed in the lumbar region (L4-L5) after disc excision. In the aligned group, the IVD allografts were placed aligned and flush with the anterior vertebral margin. In the malpositioned group, the allografts were placed proud anteriorly by 25% of the anterior-posterior diameter of the allograft. The lumbar spines were harvested at 6 months after transplantation. Three-dimensional kinematic assessment of the lumbar spines was performed using an MTS testing machine and an optoelectronic camera system. The range of motion, neutral zone, and instantaneous axis of rotation were calculated.
RESULTS: No significant difference in range of motion was noted between the groups in flexion, axial rotation, and lateral bending. Significance was noted with extension range of motion as detected in both the aligned (17.51 ± 1.97 degrees; P = 0.019) and malpositioned groups (16.61 ± 2.35 degrees; P = 0.027) compared with the control (10.11 ± 1.03 degrees). No significant difference was detected in the neutral zone between the groups. Significant difference in the instantaneous axis of rotation orientation between the malpositioned and control groups was detected in the sagittal plane during lateral bending motion (P = 0.036).
CONCLUSION: Kinematic parameters in both the aligned and malpositioned allograft were similar to those of the intact spine. This suggests that precise positioning of the IVD allograft may not be an essential factor affecting the biomechanics of the spinal segment after transplantation.

PMID: 21912315 [PubMed – indexed for MEDLINE]

Sagittal alignment and kinematics at instrumented and adjacent levels after total disc replacement in the cervical spine.

By London Spine
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Sagittal alignment and kinematics at instrumented and adjacent levels after total disc replacement in the cervical spine.

Eur Spine J. 2012 Aug;21(8):1648-59

Authors: Barrey C, Champain S, Campana S, Ramadan A, Perrin G, Skalli W

Abstract
BACKGROUND: The purpose of the study was to report radiological outcomes after total disc replacement (TDR) in the cervical spine through a 24 months follow-up (FU) prospective study with a special focus on sagittal alignment and kinematics at instrumented and adjacent levels.
MATERIALS AND METHODS: Thirty-two patients, who sustained one-level TDR with a ball-and-socket arthroplasty (Discocerv(™) implant, Scient’x/Alphatec Spine, USA) were consecutively included in the study. Clinical (visual analogical scale and neck disability index) and radiological parameters were measured preoperatively and postoperatively at 3/6 months, 1-year and 2-year FU. Sagittal alignment, ranges of motion (ROM) and center of rotations (CORs) were analyzed using specific motion analysis software (Spineview(™), Paris, France). Patients CORs were compared with those of a control group of 39 normal and asymptomatic subjects.
RESULTS: Both local and C3-C7 lordosis significantly increased postoperatively (+8° and +13° at 2 years, respectively). At instrumented level ROM in flexion-extension (FE) was measured to 10.2° preoperatively versus 7.5° at 1 year and 6.1° at 2 years. There were no differences in ROM at adjacent levels between pre and postoperative assessments. When compared with control group and preoperative measurements, we noted postoperative cranial shift of the COR at instrumented level for patients group. In contrast, there was no difference in CORs location at adjacent levels.
CONCLUSION: Through this prospective study, we observed that cervical lordosis consistently increased after TDR. In addition, although ball-and-socket arthroplasty did not fully restore native segmental kinematics with significant reduction of motion in FE and consistent cranial shift of the COR, no significant changes in terms of ROM and CORs were observed at adjacent levels.

PMID: 22331142 [PubMed – indexed for MEDLINE]