The Spinal Kinetics M6 Cervical Disc Studies
The Spinal Kinetics M6 artificial cervical disc is designed to replicate the structure and performance of a healthy disc. Its innovative design incorporates an artificial nucleus to allow shock absorption and a woven fiber annulus for controlled motion in all directions.
These characteristics accurately replicate the healthy disc, allowing the implant to work in concert with the remaining human discs.
Unlike earlier implant designs with M6 the resulting natural functionality of the entire spinal curve should provide the best chance for a full recovery, and prevent additional adjacent level degeneration.
This "Quality of Motion" is a major benefit not available in any other implant we have seen!
From the Studies below:
"The kinematic signatures of the intact human disc and the M6 artificial disc are nearly identical"
"There have been no device-related adverse events, re-operations, revisions, removals supplemental fixation or evidence of device migration, expulsion or subsidence in this patient group.
"
Testing and Studies of the Spinal Kinetics M6 Disc
Clinical Study Results
Sustained Clinical Improvement following Cervical Disc Replacement with Spinal Kinetics M6-C; 12 Month Pilot Results
As presented at the Global Symposium on Motion Preservation Technology | 7th Annual Meeting | May 1-4 2007 Berlin Germany
Purpose
This single-arm, prospective feasibility trial evaluated the preliminary safety and effectiveness of a next generation artificial cervical disc replacement in the treatment of patients with symptomatic cervical radiculopathy. This novel disc system replicates the anatomic , physiologic and biomechanical characteristics of the native disc by incorporating a compressible nucleus within a woven fiber annulus. These unique properties allow for natural kinematics including axial compression, translation independent of rotation, and progressive resistance to motion resulting from a physiologically restrained construct. Thus, the quality of motion closely mimics that of the native intervertebral cervical disc.
Methods
Thirty two patients (18 1-level, 14 2-level) have undergone standard decompressive discectomy and artificial disc implantation for persistent neurological symptoms associated with cervical radiculopathy, nonresponsive to at least 6 weeks of conservative management. Patient reported outcomes were measured prior to surgery as well as at 6 weeks, 3, 6 and 12 months. Condition-specific functional impairment was evaluated with the Neck Disability index (NDI). Arm and neck pain severity was evaluated with a standard 10-point visual analog scale (VAS). Health-related quality of life was evaluated with the SF-36 Health Survey. Intervertebral range of motion (ROM) and disc height were determined independently from anteroposterior and literal radiographs using proprietary, quantitative imaging software. Outcomes for the initial 15 patients with 12 months follow-up are provided.
Results
Thirteen (13) females and two (2) males with a mean age of 41.2 years are included in this analysis. All patients presented with cervical radioculopathy and were un-responsive to at least 6 weeks of non-operative treatment. Significant improvement was seen at 12 months for all clinical outcomes measured. The NDI score improved from 48.1% to 20.4% (p<0.0001) at 12 months. The arm pain VAS score decreased significantly with a mean change from 6.8 to 3.3 at 12 months (p<0.0001) and there was also a significant improvement in neck pain at 12 months with a change from 7.2 to 3.5 (p=0.0014). There have also been significant improvements in both the Physical Component Summary (PCS) and Mental Component Summary (MCS) scores of the SF-36.
Post-operative mean disc height has remained constant throughout follow-up and is 5.8mm at 12 months compared to 3.4mm at baseline. Global ROM has steadily improved to the baseline level at 12 months (47.3' to 47.7') and the index level ROM has returned to near pre-operative motion levels. Device position has been maintained for all patients.
There have been no device-related adverse events, re-operations, revisions, removals supplemental fixation or evidence of device migration, expulsion or subsidence in this patient group.
Conclusions
These 12 month pilot findings suggest robust and consistent clinical improvement for all function, pain and quality of life outcomes. There has been sustained and durable effectiveness after implantation of this artificial disc with clinically relevant gains realized early postoperatively and maintained through 12 months of follow-up. This novel disc system has an excellent safety profile.
Migration and Expulsion Testing
Following implantation of the device in cadaver cervical spine specimens, the M6 Artificial Disc construct was subjected to natural range of motion under normal and excessive loads. The device remained acutely fixed without notable migration in any direction.
Biocompatibility
The M6 Artificial Disc is composed of well-accepted and characterized biomateriels with a long history of use in medical device applications. Standard biocompatibility testing showed no evidence of tissue sensitization, irritation, or toxicity, either locally or systematically, and no indication of pyrogenicity or genotoxicity.
Animal Implantation Studies
Two animal studies were undertaken to evaluate the vivo characteristics of the device after long implantation. First, using a caprine model, the biological response of the implanted device after both one and two level spinal procedures was determined at 3, 6, and 12 months. Histopathologic assessment was conducted for the functional spine units, including the spinal cord, periprosthetic tissues, and distant organs. There was no evidence of acute or systemic toxicity, no significant microscopic lesions, and minimal particulate materiel.
Second, using a rabbit model, the biologic response with respect to wear debris affecting the spinal cord was evaluated. Histopathologic assessment of the spinal cord and adjacent tissues 3 and 6 months after device implantation found acceptable cellular reaction with no evidence of systemic or neurotocicity.
Biomechanical Evaluation
Three-dimensional kinematic assessment of cervical spines implanted with the M6 Artificial Disc was performed using fresh human cadaveric cervical spine specimens. In addition to quantify the ROM, the ability of the implanted segments to replicate the kinematic signature of a healthy cervical spine segments over the entire available ROM was also assessed.
Six human cervical spine specimens (C3-C7, 51.5+-5.5yr) were tested in flexion-extension, lateral bending, and axial rotation. Flexion-extension was tested under physiologic preload. Disc prostheses were implanted at C5-6 with prosthesis midline slightly posterior to midpoint of the superior end plate of C6 vertebrae.
The M6 artificial disc restored ROM in flexion-extension and axial rotation to previously reported physiologic norms. There was loss of ROM in lateral bending, similar findings have been reported in literature for other disc designs, and this observation is likely related to the surgical implantation technique used in the two vitro study wherein anterolateral annulus was retained to serve as an anterior tension band and unicinate processes were left intact.
The Kinomatic signature of implanted segments approximated intact controls, both qualitative and in its quantitative measures. The center of rotation for flexcion-extension motion of the implanted segments was located posterior of the midpoint of C6 vertebra, similar to intact controls, and agreed well with the values reported in the literature for healthy subjects.
Overall, the results of this kinematic assessment suggest that the M6 Disc allows restoration of physiologic quality as well as quality of motion like no other implant studied to date. C5-C6 flexion extension load displacement curves show biomechanical results that the M6 artificial cervical disc maintained total range of motion with excellent quality of motion.
The kinematic signatures of the intact human disc and the M6 artificial disc are nearly identical.
Quality and Quantity of Motion of Cervical Spine after M6 Disc Replacement
Purpose
A novel compressible six-degree-of-freedom cervical disc prothesis
(Spinal Kinetics, Sunnyvale, CA) composed of fiber matrix and
polymer core between two metal endplates, is designed to
replicate the response of the native annulus and nucleus. We
compared the load-displacement response of segments
implanted with this prosthesis to that of intact controls.
Methods
Six human cervical spines (C3-C7, 51.5±5.5 years) were tested in
flexion-extension, lateral bending and axial rotation (±1.5 Nm).
Flexion-extension was tested under 150N follower preload. Disc
prostheses were implanted at C5-C6 with the prosthesis midline
0.9±0.6 mm posterior to the segment midline. Range of motion
(ROM) was calculated in all tested directions. The quality of motion
was assessed in flexion-extension by calculating: (1) stiffness (slope
of load-displacement curve) in the high flexibility zone in flexion
and extension; and (2) center of rotation (COR) assessed using
digital fluoroscopic images. Data after TDR were compared to (i)
intact controls of the specimens, and (ii) “population” intact
controls from our database of 36 cervical spines tested using
an identical flexibility protocol.
Results
After prothesis implantation, C5-C6 flexion-extension ROM
increased from 13.2 +- 3.1 degrees to 15.1 +- 2.5 degrees (p=0.11).
Total axial rotation decreased from 9.9 +- 2.2 degrees to 8.0 +- 1.9
degrees (p=0.01), and total lateral bending decreased substantially
from 9.0 +- 1.6 degrees to 4.4 +- 1.0 degrees (p<0.01).
The load-displacement curve pattern in flexion-extension after
TDR was sigmoidal, and closely approximated intact controls.
The flexion-extension stiffness in the high flexibility zone was not
different between implanted and specimens’ intact segments or
population controls (p>0.30). The COR for total motion from
extension to flexion was 2.7 +- 0.7 mm posterior to the midpoint
of C5 superior endplate in the implanted segment, similar to intact
controls (p=0.74); but was 3.4 +- 0.8 mm more cephalad than the
intact location which was just below the endplate within C6
vertebral body (p<0.01).
Conclusions
The prosthesis restored quantity of motion (ROM) in flexion-extension
and axial rotation to previously reported physiologic
norms. The decrease in lateral bending motion after implantation
may be a multi-factorial phenomenon. The antero-lateral annulus
was preserved during implantation to minimize the loss of
anterior tension band. Increased pre-tensioning of annulus fibers
after prothesis insertion might have increased stiffness in lateral
bending and axial rotation. Further, the uncinate processes
were untouched. Previous studies suggest that uncinate process
resection, apart from allowing neural decompression, may
also restore lateral bending to normal values.
The location of the composite COR calculated in the intact
controls agreed well with the in vivo data reported in healthy
subjects, validating the in vitro method used for TDR kinematics
assessment. The pattern of load-displacement curves of
implanted segments approximated intact controls. COR for
total extension-to-flexion motion of implanted segments was
posterior to the midpoint of C5 vertebra, similar to intact
controls; but was about 3 mm more cephalad. Further studies
are needed to assess the long-term clinical implication of COR
location on the fate of facets joints. Overall, the data suggest
that this TDR provides similar kinematics to the lower cervical
spine as compared to the intact spine.
Patwardhan A.G.1, Tzermiadianos M.N.1, Voronov L.I.1,
Renner S.M.2, Carandang G.2, Havey R.M.2
1 Loyola University Chicago, Orthopaedic Surgery and Rehabilitation,
Maywood, USA
2 Edward Hines, Jr. VA Hospital, Hines, USA
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