Kinematics of total facet replacement (TFAS-TL) with total disc replacement.

BACKGROUND: Total disc replacement (TDR) and total facet replacement (TFR) have been the focus of recent kinematics evaluations. Yet their concurrent function as a total joint replacement of the lumbar spine's 3-joint complex has not been comprehensively reported. This study evaluated the effect of a TFR specifically designed to replace the natural facets and supplement the function with the natural disc and with TDR. The ability to replace degenerated facets to complement a pre-existing or simultaneously implanted TDR may allow surgeons to completely address degenerative pathologies of the 3-joint complex of the lumbar spine. We hypothesized that TFR would reproduce the biomechanical function of the natural facets when implanted in conjunction with TDR.
METHODS: Lumbar spines (L1-5, 51.3 ± 14.2 years, N = 6) were tested sequentially as follows: (1) intact, (2) after TDR implantation, and (3) after TFR implantation in conjunction with TDR, all at L3-4. Specimens were tested in flexion-extension (+ 8 Nm to - 6 Nm), lateral bending (± 6 Nm), and axial rotation (± 5 Nm). A 400 N compressive follower preload was applied during flexion-extension tests. Three-dimensional segmental motion was recorded and analyzed using analysis of variance in Systat (Systat Software Inc., Chicago, Illinois) and multiple comparisons with Bonferroni correction.
RESULTS: The TDR implantation (TDR + natural facets) allowed similar lateral bending (P = .66), but it generally increased flexion-extension (P = .06) and axial rotation (P < .05) range of motion (ROM) at the implanted level compared to intact. The TFR + TDR (following replacement of the natural facets with TFR) decreased ROM to levels similar to intact in lateral bending (P = .70) and axial rotation (P = .23). The TFR + TDR flexion-extension ROM was reduced in comparison to intact and TDR + natural facets (P < .05).
CONCLUSIONS: The TFR with TDR was able to restore stability to the lumbar segment after bilateral facetectomy, while allowing near-normal motions in all planes.

The clinical presence of motion preserving devices in the lumbar spine has been advanced by the approval of total disc replacements (TDR) such as the CHARITÉ Artificial Disc (DePuy Spine, Raynham, Massachusetts) and the ProDisc-L (Synthes Spine, West Chester, Pennsylvania). Extending the notion of functional replacement of the spinal anatomy, facet replacement devices have recently seen investigation in clinical studies and kinematic studies.

As total facet replacement (TFR) becomes a more accepted interventional treatment in the lumbar spine, it likely will be combined clinically with anterior column restoration, such as TDR. Recent literature has reported that TDR implantation can alter the loading on the facets[1-3] and in some clinical cases degenerative changes in the facets have been observed.[4, 5] Additionally, facet arthrosis is a contraindication for treatment with TDR, limiting the patient population.[6] As such, replacement of both the natural disc and the natural facets is probable, and it is critical to understand the functional biomechanics of associated TDR and TFR prior to clinical use.

The function of each TDR design may alter the native biomechanics of the spine, and care should be taken in the design of a TFR to be used as an adjunct device. The TFR may require specific features so as not to alter the function of the TDR while protecting the unaltered anatomy. A functional TFR must account for the mechanical interaction of the entire joint. The natural facets provide graded resistance to motion through complementary mechanics of the facet capsule and opposing articulating surfaces. The TFAS-TL (Archus Orthopedics, Inc., Redmond Washington) couples a spherical cephalad bearing that is mated with a cuplike caudal bearing. The profile of the caudal bearing provides graded resistance to angular motion of the associated functional spinal unit in the 3 major planes of motion similar to the natural anatomy. Specifically, it was designed to complement an unconstrained TDR while allowing the native elements of the spine to continue their function in stabilizing and controlling the lumbar spine.

This study evaluated the functional performance of the TFAS-TL (TFR, Fig. 1) when coupled with an unconstrained CHARITÉ Artificial Disc (TDR). We hypothesized that a TFR will reproduce the biomechanical function of the natural facets when implanted in conjunction with a TDR.

Fig. 1

Representative illustration (A) and photo (B) of an implanted TFAS-TL.

Materials and methods

Specimens and experimental setup

Six fresh-frozen human cadaveric spines (L1-5, mean age: 51.3 ± 14.2 years) with no previous spinal surgery and without bridging osteophytes and osteoporosis were tested. Prior to testing, the specimens were thawed for 24 hours at room temperature (approximately 20°C) and the paravertebral muscles were carefully resected to prevent iatrogenic damage to the discs, ligaments, and posterior elements. Hydration of the discs during testing (at room temperature) was maintained by wrapping the discs in saline-soaked towels.

The terminal vertebrae were potted using bone cement and pins, and the L5 vertebral body was fixed to a 6-axis load cell (Model MC3A-6-250; AMTI Inc., Watertown, Massachusetts) and the L1 vertebral body was unconstrained.

Specimens were tested by applying a moment to the L1 vertebra in flexion-extension (+8 Nm to −6 Nm), lateral bending (±6 Nm), and axial rotation (±5 Nm). The moment was applied by controlling the flow of water into bags attached to loading arms fixed to the L1 vertebra. This technique allows for minimal application of shear forces to the specimen.

A 400 N compressive follower preload was applied during flexion-extension tests to stabilize the spine under physiologic loads.[7]

Biaxial angle sensors (Applied Geomechanics Inc., San Francisco, California) were mounted on each vertebra to provide real-time feedback for the optimization of the follower load path (via a cable and guide mounts connected to each vertebral body) so that changes in lumbar lordosis were minimized. Follower load was not applied during lateral bending and axial rotation due to the potential for erroneous results with the current test setup.

Each specimen was tested in the following order: (1) intact; (2) after TDR implantation, (3) after TFR implantation in conjunction with TDR (Fig. 2), all at L3-4. The independent motion of each vertebral body relative to the potted caudal segment (L5) was measured using optoelectronic components (Optotrak 3020, Northern Digital Inc., Waterloo, Ontario, Canada). During implantation and flexion-extension testing, implant and vertebral positioning were monitored using fluoroscopic imaging (OEC 9800 Plus digital fluoroscopy machine; GE Healthcare, United Kingdom).

Fig. 2

Representative illustration (A) and X-ray (B) of an implanted TFAS-TL construct with a CHARITÉ TDR.

Statistical methods

Load-displacement curves were analyzed to determine L3-4 angular range of motion (ROM) in flexion-extension, lateral bending, and axial rotation. Additionally, L3-4 segmental stiffness values (Nm/degree) were calculated using previously described techniques.[8]

The statistical analysis was performed using repeated-measures analysis of variance in Systat (Systat Software Inc., Chicago, Illinois) and post hoc tests using Bonferroni correction for multiple comparisons. The following pair-wise comparisons were made: (1) intact versus TDR, (2) intact versus TDR + TFR, and (3) TDR versus TDR + TFR.

Results

The TDR implantation (with natural facets) allowed similar lateral bending (P = .66), but it generally increased flexion-extension (P = .06) and axial rotation (P < .05) ROM at the implanted level compared to intact. The TFR implantation with complementary TDR (TDR + TFR) decreased ROM to levels similar to intact in lateral bending (P = .70) and axial rotation (P = .23) (Fig. 3). The TDR + TFR flexion-extension ROM was reduced in comparison to intact and TDR with natural facets (P < .05). Figure 3 details the average ROM levels for each condition in each motion. Figure 4 demonstrates a typical flexion-extension moment curve for one specimen.

Fig. 3

Average L3-L4 range of motion.

Fig. 4

Typical flexion-extension moment versus angle curve for operated level.

Discussion

The TFR (Fig. 1) is designed to replace the facets in the lumbar spine after total facetectomy.[8] It's design provides for articulation of spherical bearings, rigidly connected to the lamina of the superior vertebral body, over two socket type bearings connected to the inferior vertebral body via pedicle screws. The caudal bearing's design promotes replication of the natural anatomy's function via surface profiles that, in flexion-extension, lateral bending, and axial rotation, provide both graduated resistance to angular motion and limits to prevent excessive motion and maintain stability. The natural facets act to guide proper motion in both flexion and lateral bending, but in extension and axial rotation they also function as a limiter at the extremes of motion to prevent instability. As such, when coupled with a TDR, the requirement of a TFR to prevent instability is magnified, particularly in axial rotation and extension, to ensure stability of the restored segment.

The TFR with TDR was able to restore stability to the lumbar segment after bilateral facetectomy, while allowing near normal motions in all planes. The ROM of the segment in lateral bending and axial rotation with TDR and TFR was generally similar to intact and within physiological norms, maintaining proper stability. While the flexion-extension ROM of TDR + TFR was statistically significantly smaller than intact and also with TDR with natural facets (P = 0.044), the resultant motion averaged 7.6 ± 2.2°, and was only 1.4 ± 1.3° smaller than the motion of the intact specimens in our sample. It should be noted that this was an in vitro study using human cadaveric specimens and the sample size was relatively small (N = 6).

The kinematic analysis did not include calculations of the center of rotation of the lumbar segments before and after implantations of the artificial disc as well as the artificial facets. When an artificial facet replacement is used in conjunction with an artificial disc, the centers of rotation of the two devices must be complementary in order for the two devices to function together. A disc prosthesis with a mobile core, such as the TDR discussed in this study, could potentially adapt to the kinematics of the facet replacement prosthesis. Further studies are needed to investigate the behavior of the center of rotation of the implanted segment in these scenarios using disc prostheses of different designs.

These results suggest that together, TDR and TFR function synergistically without one device compromising the performance of the other. Clinically, the ability to replace degenerated facets to complement a preexisting or simultaneously implanted TDR may allow surgeons to address degenerative pathologies of the 3-joint complex of the lumbar spine.