Kinematic assessment of an elastic‐core cervical disc prosthesis in one and two‐level constructs

Introduction Anterior cervical discectomy and fusion has been associated with the development of adjacent segment degeneration (ASD), with clinical incidence of approximately 3% per year. Cervical total disc arthroplasty (TDA) has been proposed as an alternative to prevent ASD. Hypotheses TDA in optimal placement using an elastic‐core cervical disc (RHINE, K2M Inc., Leesburg, Virginia) will replicate natural kinematics and will improve with optimal vs anterior placement. Methods Seven C3‐T1 cervical cadaver spines were tested intact first, then after one‐level TDA at C5‐C6 anterior placement, after TDA at C5‐C6 optimal placement, after two‐level TDA at C5‐C6 and C6‐C7 optimal placement, and finally after two‐level TDA at C5‐C6 lateral placement and C6‐C7 optimal placement. The specimens were subjected to: Flexion‐Extension moments (+1.5 Nm) with compressive preloads of 0 N and 150 N, lateral bending (LB) and axial rotation (AR) (+1.5 Nm) without preload. Results C5‐C6 TDA in optimal placement resulted in a non‐significant increase in flexion‐extension ROM compared to intact under 0 N and 150 N preload (P > 0.05). Both LB and AR ROM decreased with arthroplasty (P < 0.01). Optimal placement of C6‐C7 TDA resulted in an increase in flexion‐extension ROM with preload compared to intact (P < 0.05) while LB and AR ROM decreased with arthroplasty (P < 0.01). Conclusion This six degree of freedom elastic‐core disc arthroplasty effectively restored flexion‐extension motion to intact levels. In LB the TDA maintained 42% ROM at C5‐C6 and 60% at C6‐C7. In AR 57% of the ROM was maintained at C5‐C6 and 70% at C6‐C7. These findings are supported by literature which shows cervical TDA results in restoration of approximately 50% ROM in LB and AR, which is a multifactorial phenomenon encompassing TDA design parameters and anatomical constraints. Anterior placement of this viscoelastic TDA device shows motion restoration similar to optimal placement suggesting its design may be less sensitive to suboptimal placement.

is estimated to be about 2.9% per year for the first 10 years after fusion. Approximately two-thirds of these patients require re-operation. 1 Cervical total disc arthroplasty (TDA) has been proposed as an alternative to fusion to prevent adjacent segment degeneration and is now challenging ACDF as the "gold-standard".
Biomechanical studies have demonstrated that TDA can replicate physiologic motion at the index level and allow normal kinematics at adjacent levels. [2][3][4] Recent prospective, randomized studies using validated outcome measures including neurologic success, pain, function, and return to work have shown that treatment of single-and twolevel radiculopathy or myelopathy with cervical TDA results in outcomes superior to ACDF. [5][6][7][8][9][10] This biomechanical study sought to characterize the kinematics of human cervical spine specimens implanted with TDA at the C5-C6 and C6-C7 levels. We tested the hypotheses that (a) cervical disc replacement using an elastic-core cervical disc prosthesis (RHINE, K2M Inc., Leesburg, Virginia) will replicate the normal intact kinematics of the cervical spine, and (b) range of motion of the implanted segment will be maximized with optimal implant placement in the sagittal and coronal planes.

| Specimens and experimental set-up
Seven human cervical (C3-T1) cadaveric spine specimens (age: 41.0 AE 10.2 years) were tested ( Table 1). The specimens had no radiographic signs of metastatic disease or bridging osteophytes. The paravertebral muscles were dissected and intervertebral discs, ligaments and bony structures were left intact. The specimens were wrapped in saline soaked towels to prevent dehydration of the soft tissues and testing was performed at room temperature.
The C3 and T1 vertebrae were anchored in cups using bone cement and pins. Specimens were fixed to the test apparatus at the caudal end (T1) and were free to move in any plane at the proximal end (C3). Moment loading was achieved by controlling the flow of water into and out of bags attached to loading arms fixed to the C3 vertebra. 11 Flexion-extension and lateral bending utilized a variable force at a distance to apply the moments necessary to produce motion. This technique assures that the testing apparatus does not constrain the motion of the specimen or contribute motion artifact.
Axial rotation was performed using a pure axial rotation moment.
Due to the low mass of the apparatus attached to the specimen (0.12 kg), no counter balance was necessary. The apparatus allowed continuous cycling of the specimen between specified maximum moment endpoints in flexion-extension, lateral bending, and axial rotation.
The motion of the C3 to T1 vertebrae was measured using an optoelectronic motion measurement system (Optotrak Certus, Northern Digital, Waterloo, Ontario). In addition, bi-axial angle sensors (Model 902-45, Applied Geomechanics, Santa Cruz, CA) were mounted on each vertebra to allow real-time feedback for the optimization of the preload path. A six-component load cell (Model MC3A-6-250, AMTI Inc., Newton, Massachusetts) was placed under the specimen to measure the applied compressive preload and moments ( Figure 1A). Fluoroscopic imaging (GE OEC 9800 Plus digital fluoroscopy machine) was used to measure intact disc heights prior to implantation and in flexion and extension to monitor vertebra and implant motion.
The follower load technique was used to apply compressive preload to the cervical spine during the range of motion experiments in flexion and extension. [11][12][13][14][15][16] The compressive preload was applied along the path that follows the lordotic curve of the cervical spine.
This allowed the cervical spine to support physiologic compressive preloads without damage or instability.
The follower load was applied using bilateral loading cables that were attached to the cup holding the C3 vertebra. The cables passed freely through guides anchored to each vertebra and were connected to a loading apparatus under the specimen. The cable guides allowed anterior-posterior adjustments of the follower load path within a range of approximately 10 mm. The preload path was optimized by adjusting the cable guides to minimize changes in cervical lordosis when a compressive load of 150 N was applied to the specimen in a moderately flexed posture. The preload path was considered optimized when the preload application from 0 N to 150 N produced no more than 0.3 of segmental motion and 0.3 of motion form C3 to T1. 11,13,14

| Experimental protocol
The specimens were tested under the following conditions ( Figure 2); i) intact, ii) anterior placement of the TDA at C5-C6, iii) optimal placement of the TDA at C5-C6, iv) optimal placement of TDA at C6-C7, v) lateral placement of TDA at C5-C6.
The TDA tested in this work was an elastic-core cervical disc prosthesis (RHINE, K2M Inc.). This TDA is a one-piece non-articulating prosthesis with titanium alloy endplates which have a plasma sprayed titanium coating for integration with the bony endplates. The core is an elastomeric polyurethane sized and intended to mimic the mechanical properties (stiffness and ultimate strength) of an intact cervical disc.
Anterior placement was achieved by placing the prosthesis midline 2 mm anterior to the midline of the intervertebral disc space as determined radiographically on lateral fluoroscopy. Optimal placement was achieved by tapping the TDA into the disc space until the center  Step 1
First, the baseline range of motion of the intact specimen was determined in FE, LR, and AR under 0 N external preload. The loaddisplacement data was collected until two reproducible loaddisplacement loops were obtained. This required a maximum of three loading cycles. Following optimization of the preload path, a range of motion test on the intact spine was repeated in FE for 150 N compressive preload.
After testing the intact spine, a discectomy was performed using standard instruments. The endplates was preserved but cleaned of all cartilage. The TDA was implanted at the C5-C6 level using instrumentation and technique provided by the manufacturer. Trial sizes were used to estimate the size of the disc space for correct TDA height selection. Proper placement of the device in each protocol step was confirmed by fluoroscopy. The specimens were tested in FE, LB, and AR as described above after each protocol step.

| Data analysis
The motion data was analyzed in terms of range of motion (ROM) at

Representative applied moment vs angular displacement graphs (Figures 3 and 4) depict the classic sigmoidal behavior of the C5-C6
and C6-C7 motion segments in the intact condition, and after anterior and optimal placements of the TDA.
In the absence of a compressive preload (0 N), the C5-C6 angular range of motion of the intact spine was 11.0 AE 3.5 in FE and increased slightly to 11.4 AE 1.9 after anterior TDA placement. Following optimal placement of the TDA, C5-C6 ROM further increased In the absence of a compressive preload, the C6-C7 angular motion of the intact spine was 10.9 AE 3.9 in FE and increased to 11.9 AE 3.3 after optimal placement of TDA C6-C7 (P > 0.05) ( Figure 6A; Tables 3 and 4 Under moments of AE1.5 Nm in LB, the angular motion at C5-C6 was significantly reduced from 8.8 AE 1.5 in intact to 3.7 AE 2.1 after optimal placement of the TDA (P < 0.01). Anterior and lateral placement of the TDA did not significantly change the C5-C6 lateral bending ROM compared to optimal placement ( Figure 5B; Tables 2 and 4). C6-C7 lateral bending ROM was also significantly reduced after TDA from 8.1 AE 3.2 intact to 4.7 AE 3.0 after optimal placement of the TDA (P < 0.01) ( Figure 6B; Tables 3 and 4). C5-C6 AR, ROM was 10.0 AE 4.3 in the intact spine, which decreased to 5.9 AE 1.7 after optimal placement of the TDA (P < 0.01) ( Figure 5B; Tables 2 and 4). Anterior and lateral placement were not significantly different than optimal placement (P > 0.05).

| Flexion-extension neutral zone
Regardless of the preload applied, C5-C6 FE neutral zone was not affected by TDA placement in the anterior, optimal or lateral locations (P > 0.05) (Tables 2 and 4). Under 0 N preload, the mean change in NZ was less than 0.5 while under 150 N preload the mean change in NZ was less than 1.5 .

| Stiffness
Placement (anterior, optimal, lateral) of the TDA at C5-C6 did not significantly affect the flexion or extension stiffness either with or without preload (P > 0.05) compared to the intact condition. At C6-C7 under 0 N compressive load, flexion and extension stiffness both increased significantly with optimal placement of the cervical disc (P < 0.05). However, under 150 N of compressive preload the flexion and extension stiffness both showed a decrease in segmental stiffness reaching significance in flexion (P < 0.05) (Tables 2 and 3).

| Axis of rotation
Axis of Rotation was measured in two specimens (Specimen #5 and Specimen #6) using the specimen specific CT-based kinematic analysis. 18 The local anatomic coordinate system was located at the center of the superior endplate of the inferior vertebra of each motion segment (Anterior = +X, Cranial = +Z). The flexion-extension axes of rotation of the two specimens are shown in Figure 7 and Table 5.

| DISCUSSION
The current study confirms that implantation of a cervical TDA prosthesis can provide near physiologic mobility at the implanted levels in flexion-extension when compared to the native motion segments.
Both quantity and quality of motion of the implanted motion  This study evaluated the kinematic response of the elastic-core cervical disc arthroplasty using load control (flexibility) protocol. 20 Moments of AE1.5 Nm were utilized based on laboratory experience and published literature. 11,[13][14][15][16] Kinematic response was evaluated with and without a compressive follower preload of 150 N. The follower preload is representative of the compressive forces induced by the stabilizing musculature and the mass of the head. A total of 150 N is within the range of forces that the cervical spine experiences in vivo. 21 The follower preload was applied using optimized bilateral cables as described by Patwardhan et al. 11,12 While kinematic testing without preload represents a non-physiologic condition, this study included testing without preload in order to compare to previous studies in which TDA testing was performed without compressive preload. In addition, testing without preload was performed to determine the sensitivity of this TDA to compressive preloads due to its non-articulating design.
Normative in vivo and kinematic data from literature shows average C5-C6 flexion-extension ROM is between 12.5 AE 4.8 and 16.5 AE 5.0 and C6-C7 ROM is between 12.5 AE 4.8 to 13.7 AE 5.1 . 19,22,23 In lateral bending one sided in vivo motion is 4.3 AE 1.4 and 5.7 AE 1.9 for C5-C6 and C6-C7 24 and average one sided motion in axial rotation is 5.4 AE 4.3 and 6.4 AE 2.5 for C5-C6 and C6-C7. 25 While the ROM values in this ex vivo data set tend to be smaller than the above mentioned in vivo norms, caution must be used when making direct comparisons between these data sets. ex vivo data sets are collected in such a way that each data set (intact, anterior TDA placement, optimal TDA placement, etc.) is comparable to the previous and subsequent sets using the same loading rates and magnitudes.   Step 1 intact Step 2 anterior placement Step 3 optimal placement Step 5 lateral placement †Significantly different than optimal TDA placement (P < 0.05).

| Limitations
Interpretation of these results requires some consideration of the study limitations. Biomechanical testing at best mimics the immediate postoperative condition, and therefore changes in the soft tissues, such as annular scar tissue formation, and bony remodeling, are not incorporated, although anular relaxation may be largely accounted for. 29 A second noteworthy limitation of this study is the inability to entirely replicate in-vivo physiologic loading. Although application of  Finally, this research study was performed on relatively healthy specimens with no significant disc degeneration in order to eliminate confounding factors such as bridging osteophytes, facet and advanced disc degeneration, etc. A major advantage of testing on relatively healthy specimens is the ability to compare postoperative motions to the native disc. This provides a specimen-specific and motion segment specific control for data comparison.

| CONCLUSIONS
This six degree of freedom elastic-core disc arthroplasty device effectively restored flexion-extension motion to intact levels at both C5-C6 and C6-C7. In LB the TDA maintained 42% ROM at C5-C6 and 60% at C6-C7. In AR 57% of the ROM was maintained at C5-C6 and 70% at C6-C7. These findings are supported by literature which shows cervical TDA results in restoration of approximately 50% ROM in LB and AR, which is a multifactorial phenomenon encompassing TDA design parameters and anatomical constraints. Anterior placement of this elastic-core TDA device shows motion restoration similar to optimal placement suggesting its design may be less sensitive to sub-optimal placement than mechanical TDAs with moving parts. In this two level study, the data suggests that this device restores ROM to preoperative levels in flexion-extension. This new generation of TDA offers an alternative to fixed axis of rotation and articulating devices.  (Paid directly to Institution).