The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 94, Issue 9

Scientific Articles | May 02, 2012

Asymmetric Motion Distribution Between Components of a Mobile-Core Lumbar Disc Prosthesis: An Explanation of Unequal Wear Distribution in Explanted CHARITÉ Polyethylene Cores

Avinash G. Patwardhan, PhD¹; Robert M. Havey, BS¹; Nicholas D. Wharton, MS²; Parmenion P. Tsitsopoulos, MD, PhD¹; Patrick Newman, MS²; Gerard Carandang, MS³; Leonard I. Voronov, MD, PhD¹

¹ Department of Orthopaedic Surgery and Rehabilitation, Loyola University Medical Center, 2160 South First Avenue, Maywood, IL 60153. Email address for A.G. Patwardhan: apatwar@lumc.edu
² Medical Metrics, Inc., 2121 Sage Road, Houston, TX 77056
³ Edward Hines Jr. VA Hospital, 5000 South Fifth Avenue, Hines, IL 60141

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Investigation performed at the Musculoskeletal Biomechanics Laboratory, Edward Hines Jr. VA Hospital, Hines, Illinois

J Bone Joint Surg Am, 2012 May 02;94(9):846-854. doi: 10.2106/JBJS.J.00638

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Abstract

Background:

The biconvex mobile core of the CHARITÉ lumbar disc prosthesis forms two joints (spherical bearings) with the metal end plates. We quantified the intra-prosthesis motion to test the hypothesis that the total prosthesis motion would not be equally distributed between the two bearings of implanted CHARITÉ discs, which might explain the unequal wear distribution reported in explanted cores.

Methods:

The hypothesis was tested by studying the flexion-extension motion responses of (1) twenty-six monosegmental CHARITÉ III discs implanted in nineteen human cadaveric lumbar spines, and (2) twenty-one CHARITÉ III discs (fifteen monosegmental, six bisegmental) implanted in eighteen patients in other published clinical studies. Intra-prosthesis motions were quantified with use of a radiographic image analysis technique.

Results:

Eighty-eight percent of the CHARITÉ discs implanted in cadaveric specimens exhibited larger motion at the superior bearing, with 54% demonstrating more than twice as much motion at the superior bearing as at the inferior bearing. The ratio of motion at the superior bearing to motion at the inferior bearing averaged 2.68 ± 1.84, which was significantly larger than 1.0 (p < 0.001). Ninety percent of prostheses implanted in patients showed larger motion at the superior bearing. The motion ratio averaged 2.39 ± 2.47 for monosegmental cases and 2.55 ± 2.66 for all cases; both ratios were significantly larger than 1.0 (p < 0.05).

Conclusions:

We found preferentially larger motion at the superior bearing of the CHARITÉ discs implanted in human cadaveric lumbar spines and in patients, regardless of the implanted level.

Clinical Relevance:

The findings regarding unequal motion distribution between the dual bearings of mobile-core disc prostheses are relevant to improving in vitro wear testing protocols to better replicate in vivo wear-producing conditions.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | Appendix

Total disc replacement has emerged as a treatment alternative for managing pain of discogenic origin^1,2. The CHARITÉ lumbar disc (DePuy AcroMed, Raynham, Massachusetts) has been the most extensively used artificial disc over the past twenty years^2-4. It consists of two cobalt-chromium-alloy end plates and a mobile polyethylene core. The implant geometry is fully symmetric about the midplane (Fig. 1)^5,6. The CHARITÉ disc allows angular motion to occur at the two spherical bearings (joints), one formed between the superior prosthesis end plate and the core and the other between the inferior prothesis end plate and the core. As is the case with hip and knee arthroplasty, the polyethylene core has been associated with a risk of developing clinical symptoms secondary to wear^7-11.

Previous studies have recognized the importance of incorporating clinically relevant kinematic input in the wear testing protocols to improve their ability to predict in vivo wear behavior^12-15. The current standards for wear testing of intervertebral disc prostheses (ISO-18192-1¹⁶ and ASTM-F2423-05¹⁷) specify the cyclic input profiles of total angular motions between the superior and inferior end plates of the prosthesis. However, when conducting wear tests on prostheses with two spherical bearings, specifying how to distribute the total end-plate-to-end-plate motion between the two bearings becomes problematic because of a lack of quantitative data on the intra-prosthesis motion distribution.

Kurtz et al. performed wear analysis of thirty-eight retrieved CHARITÉ artificial lumbar discs¹⁸. They measured the height from the midline to the dome on both sides of the CHARITÉ core, and the ratio of the two heights was defined as the symmetry ratio. A symmetry ratio ranging from 0.83 to 0.95 was observed in 43% of explanted cores, suggesting an asymmetric distribution of motion between the two bearings of the CHARITÉ disc implanted in patients.

The purpose of the present study was to quantify the motion distribution within CHARITÉ discs implanted in human lumbar spinal segments. The primary hypothesis was that the total prosthesis motion would not be equally distributed between the two bearings of a CHARITÉ disc, which could explain the unequal wear distribution observed in explanted polyethylene cores.

Fig. 1

The CHARITÉ lumbar disc prosthesis. The prosthesis allows motion at two joints (bearings) formed between the mobile polyethylene core and the two end plates of the prosthesis.

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Materials and Methods

Introduction | Materials and Methods | Results | Discussion | Appendix

The hypothesis was tested by examining the flexion-extension motion responses of (1) twenty-six CHARITÉ III discs implanted in human cadaveric lumbar spines, and (2) twenty-one CHARITÉ III discs implanted in patients in previously published clinical studies.

Experimental Study Using Human Cadaveric Lumbar Spines

Specimens

Nineteen human cadaveric lumbar spines (L1 to sacrum) were used in the experimental study. The mean donor age was 49.3 ± 10.4 years; eleven donors were male and eight were female. None of the specimens had radiographic signs of advanced degenerative changes. The paravertebral muscles were dissected and the discs, ligaments, and posterior osseous structures were kept intact. The specimens were wrapped in saline solution-soaked towels to prevent tissue dehydration.

Experimental Setup

The sacrum was fixed to the apparatus, whereas the L1 vertebra was free to move in each of the six degrees of freedom (Fig. 2). A moment was applied by controlling the flow of water into bags attached to loading arms fixed to the L1 vertebra. Because of the long moment arms, the variation in applied moment across the L3-L4, L4-L5, and L5-S1 segments was <5%, thereby closely approximating a pure-moment loading condition advocated for flexibility tests^19,20. The use of a multi-segment test specimen with moments applied to the L1 vertebra without constraining its motion allowed unconstrained, six-degree-of-freedom motion at each segment¹⁹.

Fig. 2

The experimental setup showing the cables for applying a compressive preload and the optoelectronic sensors for measuring intervertebral motion.

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Intervertebral motions were measured with use of an optoelectronic motion measurement system (Optotrak; Northern Digital, Waterloo, Ontario, Canada)^21,22. A six-component load cell (MC3A-6-1000; AMTI, Newton, Massachusetts) was placed under the specimen to measure the applied forces and moments. Motions between the prosthesis end plates and the core during flexion-extension were assessed with use of digital fluoroscopy (OEC 9800 Plus; GE Healthcare, Salt Lake City, Utah).

Experimental Protocol

The specimen’s motion response was quantified with use of a moment-control protocol. Specimens were initially tested in the intact state as flexion moments ranging from 0 to 8 N-m and extension moments ranging from 0 to 6 N-m were applied. These moments are within the range used in previous studies of human lumbar spines^{18,19,21,23,24}. A compressive follower preload of 400 N was applied according to a previously described technique^21,25 to simulate physiologic preloading due to gravity and muscle activity²².

All disc replacement surgical procedures were performed according to clinical guidelines²⁶ and using the operative technique described in detail in our previous cadaveric study²³. The posterior longitudinal ligament was transected to obtain proper implant positioning²³. Trial implants were used to determine appropriate prosthesis size. The prosthesis midline was centered in the frontal plane and was positioned 1 to 2 mm posterior to the midline of the disc space in the sagittal plane. Proper placement was confirmed with fluoroscopy. The specimens were then tested in flexion-extension.

Twenty-six CHARITÉ total disc replacements were implanted in nineteen lumbar spines (six at L3-L4, seven at L4-L5, and thirteen at L5-S1) (Table I). A monosegmental implantation was performed at L3-L4 in six specimens and at L5-S1 in an additional six specimens. In the remaining seven specimens, a CHARITÉ disc was initially implanted at L5-S1 and motion testing was performed. Next, a fusion at L5-S1 was simulated with use of an interbody spacer and pedicle screw instrumentation, and a CHARITÉ disc was implanted at L4-L5. Motion testing was then repeated as previously described. This allowed assessment of monosegmental total disc replacements at two spinal levels, in a cost-effective way, without affecting the motion response of the disc prosthesis. A number of studies^19,20,27 have demonstrated that under pure-moment loading conditions, each motion segment of the specimen is subjected to the same moment; therefore, motion of the L4-L5 total disc replacement will have been unaffected by the adjacent-level fusion.

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TABLE I Distribution of Twenty-six CHARITÉ Disc Prostheses Implanted in Nineteen Human Cadaveric Specimens

Specimens	L3-L4 (N = 6)	L4-L5 (N = 7)	L5-S1 (N = 13)
1 through 6	CHARITÉ	Intact	Intact
7 through 12	Intact	Intact	CHARITÉ
13 through 19, initial	Intact	Intact	CHARITÉ
13 through 19, final	Intact	CHARITÉ	Fusion

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TABLE I Distribution of Twenty-six CHARITÉ Disc Prostheses Implanted in Nineteen Human Cadaveric Specimens

Specimens	L3-L4 (N = 6)	L4-L5 (N = 7)	L5-S1 (N = 13)
1 through 6	CHARITÉ	Intact	Intact
7 through 12	Intact	Intact	CHARITÉ
13 through 19, initial	Intact	Intact	CHARITÉ
13 through 19, final	Intact	CHARITÉ	Fusion

Data Analysis

Fluoroscopic images acquired during flexion-extension were corrected for geometric distortion caused by the image intensifier and then analyzed with use of a digital image analysis technique (QMA; Medical Metrics, Houston, Texas). The ability of the QMA (quantitative motion analysis) technique to produce accurate measurements of intervertebral motion from flexion-extension radiographs has been previously validated^28,29. The images were analyzed to calculate (1) motions occurring between vertebrae before and after prosthesis implantation, (2) total motion between the superior and inferior prosthesis end plates, and (3) motion distribution between the two bearings (Fig. 3).

Fig. 3

Radiographic measurement of total prosthesis motion and its distribution between the two bearings. The QMA measurement technique was based on a digital image superimposition method that tracked the motion of each device component as an independent object. Total prosthesis motion = (C1 – C2), motion at the superior bearing = (A1 + A2), and motion at the inferior bearing = (B1 – B2). In this example of unequal distribution of intra-prosthesis motion in flexion-extension, total prosthesis motion = 12.3°, motion at the superior bearing = 10.7°, and motion at the inferior bearing = 1.6°, resulting in a motion ratio of 6.7.

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Statistical Analysis

Statistical analyses were performed with use of SYSTAT software (version 10.2; Chicago, Illinois). Normality of the data for the intervertebral range of motion and for the intra-prosthesis motion distribution was checked with use of the Kolmogorov-Smirnov test. All data sets were found to be normally distributed (p > 0.30).

The motions occurring between two vertebrae in cadaveric specimens were measured by the optoelectronic system and also calculated by QMA radiographic analysis of digital fluoroscopic images. The agreement between the radiographic and optoelectronic methods was assessed with use of a Bland-Altman analysis³⁰. The limits of agreement for the two methods of motion measurement were ±1.0°.

Intra-prosthesis motion distribution was expressed as the motion ratio, which was defined as the superior-bearing motion divided by the inferior-bearing motion. A one-sample t test was performed to determine whether the motion ratio was significantly different from 1.0, thereby assessing whether there was unequal distribution of the prosthesis motion between the two bearings.

An analysis of variance was used to determine whether the preoperative and postoperative intervertebral motions and the intra-prosthesis motion distribution were influenced by the level of implantation (segmental level L3-L4, L4-L5, or L5-S1). An a posteriori power analysis was performed to determine the statistical power of the findings in the cadaveric study.

Analysis of Intra-Prosthesis Motion in Clinical Cases

The hypothesis that motion within the mobile-core disc prosthesis is unequally distributed between the superior and inferior bearings was also tested with use of the motion response of CHARITÉ discs implanted in patients. A review of the scientific literature identified thirty-three clinical studies of CHARITÉ discs, which were published between 1994 and 2009. Thirteen of the publications featured flexion-extension radiographs^31-43; however, one of these involved an earlier prosthesis design (CHARITÉ II)⁴⁰ and was therefore excluded from further analysis. Thus, the present study included the clinical cases reported in the remaining twelve publications (see Appendix). The images were obtained from the original publications in digital (PDF) format at the highest resolution possible and stored in an uncompressed file format. Image contrast was enhanced with use of special filters to optimize implant visualization. One set of radiographs was eliminated because of inadequate image quality.

Thus, the radiographic analysis was performed on twenty-one implanted segments in eighteen patients. All of the implants were the CHARITÉ III disc prosthesis; ten were implanted at L4-L5 and eleven at L5-S1 (see Appendix)^31-39,41-43. There were fifteen monosegmental disc replacements (seven at L4-L5 and eight at L5-S1) and six bisegmental disc replacements (three L4-L5 and L5-S1 pairs).

To test the reliability of the radiographic measurements, 50% of the images were randomly selected for reanalysis after an intervening period of at least eight weeks. The reliability of the measurements was assessed with use of a Bland-Altman analysis³⁰, and the limits of agreement for the radiographic analyses were found to be ±0.5° for angular motion at each bearing of the CHARITÉ disc. These excellent reliability results are likely due to the ease with which the metallic implant components can be tracked with use of the QMA software.

Normality of the motion data was confirmed with use of the Kolmogorov-Smirnov test. The hypothesis that the prosthesis motion was unequally distributed between the superior and inferior joints of the CHARITÉ disc implanted in patients was tested separately for monosegmental total disc replacements and for all (monosegmental and bisegmental) total disc replacements. An a posteriori power analysis was performed to determine the statistical power of the findings.

Source of Funding

Research funding for this study was received from the U.S. Department of Veterans Affairs.

Results

Introduction | Materials and Methods | Results | Discussion | Appendix

Analysis of Prosthesis Motion in Cadaveric Specimens

Preoperatively, the flexion-extension range of motion between the vertebrae averaged 8.4° ± 3.2°. After disc replacement, the average range of motion for the twenty-six implanted segments increased to 11.1° ± 2.7° (p < 0.01).

The total prosthesis motion (between the superior and inferior prosthesis end plates) averaged 10.4° ± 2.4° (Table II). The small discrepancy between the total prosthesis motion and the intervertebral motion can be attributed to a combination of radiographic measurement accuracy limitations and interface motion between the prosthesis and the vertebral end plates. The angular motion at the superior bearing was 7.0° ± 1.9°, representing 67% ± 13% of the total prosthesis motion. Conversely, the motion at the inferior bearing was 3.4° ± 1.6°, representing 33% ± 13% of the total prosthesis motion.

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TABLE II Intra-Prosthesis Motion in CHARITÉ III Prostheses Implanted in Human Cadaveric Specimens

Index Level	No.	Total Prosthesis Motion*(deg)	Superior Bearing Motion*(deg)	Inferior Bearing Motion*(deg)	Superior:Inferior Motion Ratio*
L3-L4	6	10.9 ± 3.7	7.3 ± 2.5	3.6 ± 1.7	2.36 ± 0.99†
L4-L5	7	10.3 ± 2.3	6.9 ± 1.4	3.5 ± 2.2	3.21 ± 2.50†
L5-S1	13	10.2 ± 1.9	6.8 ± 2.1	3.4 ± 1.3	2.55 ± 1.81†
All	26	10.4 ± 2.4	7.0 ± 1.9	3.4 ± 1.6	2.68 ± 1.84†

*Values are given as the mean and the standard deviation.
†Significantly larger than 1.0 (one-sample t test; one-tailed p < 0.05).

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TABLE II Intra-Prosthesis Motion in CHARITÉ III Prostheses Implanted in Human Cadaveric Specimens

Index Level	No.	Total Prosthesis Motion*(deg)	Superior Bearing Motion*(deg)	Inferior Bearing Motion*(deg)	Superior:Inferior Motion Ratio*
L3-L4	6	10.9 ± 3.7	7.3 ± 2.5	3.6 ± 1.7	2.36 ± 0.99†
L4-L5	7	10.3 ± 2.3	6.9 ± 1.4	3.5 ± 2.2	3.21 ± 2.50†
L5-S1	13	10.2 ± 1.9	6.8 ± 2.1	3.4 ± 1.3	2.55 ± 1.81†
All	26	10.4 ± 2.4	7.0 ± 1.9	3.4 ± 1.6	2.68 ± 1.84†

*Values are given as the mean and the standard deviation.
†Significantly larger than 1.0 (one-sample t test; one-tailed p < 0.05).

The ratio of motion at the superior bearing to motion at the inferior bearing was 2.68 ± 1.84 (Table II, Fig. 3), which was significantly larger than 1.0 (p < 0.001). Twenty-three (88%) of the twenty-six implants demonstrated larger motion at the superior than at the inferior bearing (Figs. 4-A and 4-B). In fourteen (54%) of the twenty-six implants, motion at the superior bearing was more than twice that at the inferior bearing (Fig. 4-B). The motion ratio was significantly larger than 1.0 at each implanted level (p < 0.05), signifying preferentially larger motion at the superior bearing of the CHARITÉ discs implanted in cadaveric lumbar spines regardless of the implanted level (p < 0.05).

Fig. 4-A

Scatter plot of motion distribution in twenty-six CHARITÉ prostheses implanted in human cadaveric spines. The points lying above the diagonal line correspond to implants in which the motion at the superior bearing was larger than the motion at the inferior bearing (motion ratio >1).

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Fig. 4-B

Frequency distribution of the motion ratio (superior bearing motion divided by inferior bearing motion) of twenty-six CHARITÉ prostheses implanted in human cadaveric spines.

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Analysis of Prosthesis Motion in Clinical Cases

In one clinical case, in which the prosthesis was implanted at L5-S1, the total prosthesis motion was 7.2° but the motion at the inferior bearing was measured to be 0.1°, yielding a motion ratio of seventy-one, which was an outlier in this small sample of cases (see Appendix). Therefore, this motion ratio value was excluded from further statistical analysis.

The total prosthesis motion (between the superior and inferior prosthesis end plates) averaged 13.7° ± 4.3° for the monosegmental total disc replacements and 12.5° ± 4.5° for all total disc replacements (Table III). The motion ratio was 2.39 ± 2.47 for monosegmental cases and 2.55 ± 2.66 for all total disc replacements; both ratios were significantly larger than 1.0 (p < 0.05). Motion at the superior bearing was larger than that at the inferior bearing in 90% of the prostheses implanted in patients, and it was at least twice as large in 30%. We found preferentially larger motion at the superior bearing of the CHARITÉ discs implanted in patients regardless of the implanted level (p < 0.05).

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TABLE III Intra-Prosthesis Motion in CHARITÉ III Prostheses Implanted in Patients in Previous Clinical Studies*

Implantation	Index Level	No.	Total Prosthesis Motion†(deg)	Superior Bearing Motion†(deg)	Inferior Bearing Motion†(deg)	Superior:Inferior Motion Ratio†
Monosegmental
	L4-L5	7	14.2 ± 4.2	9.7 ± 2.8	4.5 ± 2.4	3.28 ± 3.36‡
	L5-S1	7	13.2 ± 4.7	7.9 ± 3.1	5.3 ± 1.9	1.51 ± 0.33‡
	All	14	13.7 ± 4.3	8.8 ± 3.0	4.9 ± 2.1	2.39 ± 2.47‡
Bisegmental
	L4-L5	3	10.7 ± 1.5	7.8 ± 1.8	3.0 ± 1.9	4.37 ± 4.53
	L5-S1	3	8.5 ± 5.3	5.0 ± 3.5	3.5 ± 2.1	1.45 ± 0.47
	All	6	9.6 ± 3.7	6.4 ± 2.9	3.2 ± 1.8	2.91 ± 3.29‡
All
	L4-L5	10	13.2 ± 3.9	9.1 ± 2.6	4.0 ± 2.2	3.61 ± 3.52‡
	L5-S1	10	11.8 ± 5.1	7.0 ± 3.3	4.7 ± 2.0	1.49 ± 0.35‡
	All	20	12.5 ± 4.5	8.1 ± 3.1	4.4 ± 2.1	2.55 ± 2.66‡

*L4-L5 data are from references 31-38 and 43. L5-S1 data are from references 32-34, 36, 37, 39, 41, and 42.
†Values are given as the mean and the standard deviation.
‡Significantly larger than 1.0 (one-sample t test; one-tailed p < 0.05).

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TABLE III Intra-Prosthesis Motion in CHARITÉ III Prostheses Implanted in Patients in Previous Clinical Studies*

Implantation	Index Level	No.	Total Prosthesis Motion†(deg)	Superior Bearing Motion†(deg)	Inferior Bearing Motion†(deg)	Superior:Inferior Motion Ratio†
Monosegmental
	L4-L5	7	14.2 ± 4.2	9.7 ± 2.8	4.5 ± 2.4	3.28 ± 3.36‡
	L5-S1	7	13.2 ± 4.7	7.9 ± 3.1	5.3 ± 1.9	1.51 ± 0.33‡
	All	14	13.7 ± 4.3	8.8 ± 3.0	4.9 ± 2.1	2.39 ± 2.47‡
Bisegmental
	L4-L5	3	10.7 ± 1.5	7.8 ± 1.8	3.0 ± 1.9	4.37 ± 4.53
	L5-S1	3	8.5 ± 5.3	5.0 ± 3.5	3.5 ± 2.1	1.45 ± 0.47
	All	6	9.6 ± 3.7	6.4 ± 2.9	3.2 ± 1.8	2.91 ± 3.29‡
All
	L4-L5	10	13.2 ± 3.9	9.1 ± 2.6	4.0 ± 2.2	3.61 ± 3.52‡
	L5-S1	10	11.8 ± 5.1	7.0 ± 3.3	4.7 ± 2.0	1.49 ± 0.35‡
	All	20	12.5 ± 4.5	8.1 ± 3.1	4.4 ± 2.1	2.55 ± 2.66‡

Discussion

Introduction | Materials and Methods | Results | Discussion | Appendix

Wear of the polyethylene core has been noted in retrieved CHARITÉ disc prostheses^5,9,18. Kurtz et al. evaluated thirty-eight CHARITÉ discs explanted from thirty-two patients at an average of 7.3 years after implantation¹⁸. All implants were removed because of intractable back pain and/or facet degeneration. The wear patterns of the retrieved cores were analyzed at the rim and the dome. Thirty-five cores were further processed with microcomputed tomography to determine penetration symmetry. The authors noted one-sided wear in 43% of the retrieved cores, suggesting an unequal distribution of motion between the two bearings of the CHARITÉ prosthesis. In the present study, we quantified the unequal distribution of prosthesis motion with use of flexion-extension motion responses of discs implanted in human cadaveric lumbar spines and in patients in previously published clinical studies.

The present ex vivo study of twenty-six implanted CHARITÉ discs achieved a power of >90% in testing the hypothesis of unequal motion distribution between the two bearings of the prosthesis. The motion at the joint formed between the polyethylene core and the superior prosthesis end plate was significantly larger than that between the core and the inferior prosthesis end plate. Eighty-eight percent of the CHARITÉ discs implanted in cadaveric specimens exhibited larger motion at the superior bearing, with 54% demonstrating more than twice as much motion at the superior bearing as at the inferior bearing.

The findings of the ex vivo study must be interpreted in the context of some limitations. Biomechanical testing at best mimics the immediate postoperative condition. Therefore, possible postoperative changes in the soft tissues and osseous remodeling are not incorporated, although anular relaxation may be accounted for⁴⁴. A second limitation is the inability to fully replicate physiologic loads. Although application of the follower preload provides a key element of the in vivo loading environment^19,25, the complex musculature of the lumbar region renders muscle loading nearly impossible to reproduce completely in the laboratory. More complicated testing setups have been suggested⁴⁵, but current recommendations for biomechanical testing protocols include the use of pure-moment control with follower-preload application, reflecting a balance between physiologic behavior and experimental limitations¹⁹. The experimental model used in the biomechanical study has been well documented^21-23,46 and well accepted as a testing standard in the scientific community^{19,24,45,47,48}.

The ultimate validity of a biomechanical model lies in its ability to produce results that are corroborated by clinical findings. Therefore, the hypothesis of unequal motion distribution was also tested with use of motion data from CHARITÉ discs implanted in patients. The final cohort included fifteen single-level implantations and six two-level implantations, all utilizing the CHARITÉ III disc. In one case with a CHARITÉ disc implanted at L5-S1, the total prosthesis motion was 7.2° but the motion at the inferior bearing was 0.1°, resulting in a motion ratio of seventy-one, which was an outlier in this small sample of cases. Excluding this case from statistical analysis does not bias our conclusions since the large motion ratio in this case was consistent with the existence of unequal motion distribution in the remaining cases.

We recognize that the “study population” used for the radiographic analysis of in vivo prosthesis motion has limitations. First, the sample size for the in vivo study was small. Second, we did not have firsthand knowledge of the clinical outcomes for individual patients; this potentially raises a concern that the in vivo motion patterns in some of the patients may have been associated with prosthesis-related complications or poor outcomes. In this regard, it should be noted that the implanted prostheses had good postoperative motion and, to the best of our knowledge, good clinical outcomes and no prosthesis-related complications according to the available information in these published reports (see Appendix). In light of the limitations mentioned above, no conclusions were drawn regarding the effect of the in vivo prosthesis motion patterns on clinical outcomes. The dual methodology of our study has allowed us to demonstrate that the finding of preferentially larger motion at the superior bearing of the CHARITÉ discs implanted in human cadaveric lumbar spines represents a real phenomenon that is present in vivo and is not an experimental artifact of the cadaveric study.

The motion ratio varied over a large range in the cadaveric specimens and in the patients. In the cadaveric portion of the study, an identical operative technique of disc-space preparation and implant positioning, which followed clinical guidelines, was used in all specimens^26,49. Therefore, the variability in the motion ratio may be attributed to variability in specimen anatomy. Such variability influences the way in which the mobile core sits in relation to the prosthesis end plates in the reconstructed segment, which may in turn influence the free movement of the core during segmental flexion-extension. In the clinical portion of the study, some variability in the surgical technique and implant position may be expected since the implantations were performed by different surgeons over a period of more than fifteen years. However, in spite of the variability in both portions of the study, we showed that there was preferentially larger motion at the superior bearing of the CHARITÉ prosthesis implanted in human cadaveric spines and in patients.

There is no clear consensus in the literature regarding the effect of implantation level on clinical outcome. Regan found no statistical difference in the improvements in the visual analog scale (VAS) pain score and Oswestry Disability Index (ODI) score of patients with CHARITÉ prostheses implanted at L4-L5 compared with L5-S1 over twenty-four months⁵⁰. The authors of other studies involving the ProDisc II prosthesis (Synthes Spine, Paoli, Pennsylvania) reported less postoperative motion at L5-S1 compared with L4-L5^51-54, and they suggested that this may be attributed to the discrepancy between the center of rotation of the native L5-S1 segment and that of a prosthesis such as the ProDisc that has a fixed center of rotation. The present study did not have sufficient statistical power to assess the effect of implantation level on the motion ratio. However, the motion ratio was significantly larger than 1.0 at each implanted level (p < 0.05), signifying preferentially larger motion at the superior bearing of the CHARITÉ disc in human cadaveric lumbar spines and in patients, regardless of the implanted level.

If all prosthesis motion in the CHARITÉ disc occurred at the superior spherical bearing, the center of rotation would be located at the center of curvature of the superior spherical bearing, which is caudal to the superior end plate of the inferior vertebra (Fig. 5-A). If the prosthesis motion was equally distributed between the two spherical bearings (i.e., motion ratio = 1.0), the center of rotation would be located at the center of the mobile core (Fig. 5-B). Normative reference data suggest that, within a lumbar level, the center of rotation may be found within a range of up to 8 to 10 mm in the cranial-caudal direction and that this cranial-caudal location also varies depending on the segment level⁵⁵. Hipp and Wharton measured the locations of the center of rotation for flexion-extension motion in the lumbar spine of sixty-three asymptomatic individuals with use of the computerized QMA technique⁵⁵. Using 30 mm as the typical end-plate width, the mean center of rotation of the L3-L4 and L4-L5 segments would be located caudal to the superior end plate of the inferior vertebra by 2.3 ± 1.8 and 3.6 ± 2.1 mm, respectively. This center of rotation location is clearly caudal to the midplane of this disc space. In the L5-S1 segment, the center of rotation would be cranial to the S1 superior end plate by 3.3 ± 3.1 mm. Therefore, at L3-L4 and L4-L5, unequal distribution of motion between the superior and inferior bearings of the dual-bearing disc prosthesis is needed to achieve a normal physiologic center of rotation that is asymmetrically located in relation to the prosthesis midplane (Fig. 5-C). Although the location of the L5-S1 center of rotation can coincide with the midplane of the disc, the mean location is slightly below the midplane, and the large standard deviation (3.1 mm) suggests that an asymmetric distribution of motion will be needed in a substantial proportion of cases to achieve a normal center of rotation that is below the midplane of the disc.

Fig. 5

Figs. 5-A, 5-B, and 5-C Illustration showing the effect of intra-prosthesis motion distribution on the location of the center of rotation (COR) in flexion-extension. Fig. 5-A All of the motion is occurring at the superior bearing. Fig. 5-B Fifty percent of the motion is occurring at the superior bearing and 50% at the inferior bearing, representing a motion ratio of 1. Fig. 5-C Seventy percent is occurring at the superior bearing and 30% at the inferior bearing, representing a motion ratio of 2.3.

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It is unclear how closely a prosthesis with a single spherical bearing can approximate the normal center of rotation locations at various lumbar levels. Inability to accommodate intra-level and inter-level variations in the center of rotation may lead to incongruity of the bearing surfaces, as suggested in a previous study of L5-S1 segments that had been reconstructed with use of the ProDisc-L disc prosthesis⁵⁶. The impact of such incongruity on the long-term wear or damage to the bearing surfaces remains unknown.

In contrast, in a dual-bearing prosthesis, the ability of the superior and inferior bearings to allow different amounts of motion at each interface could, in theory, accommodate variations in the center of rotation between levels and among patients. However, the theoretical existence of this ability of the CHARITÉ disc is not sufficient to conclude that the motion response of the reconstructed spinal segment resembles normal or near-normal motion characteristics. That would first require determination of the motion ratios needed to support the normal variation in the center of rotation observed in human lumbar segments. We found substantial variability in the motion ratios of the CHARITÉ discs implanted in human cadaveric specimens and in patients. A motion ratio that is well outside the normal range would indicate that motion is occurring predominantly at only one of the two joints; this suggests the presence of a locked-core phenomenon, which may produce abnormal motions and stresses at the facet joints of the implanted segment.

We often observed the core to be locked in extension relative to the inferior end plate, allowing motion predominantly at the superior bearing for much of the arc of motion from extension to flexion. The core was released only in the final segment of the arc of flexion, yielding some motion at the inferior bearing. This is consistent with the findings of O’Leary et al.²³, who illustrated this “locking phenomenon” with use of sequential fluoroscopic images and load-displacement curves that corresponded to “stick-slip” friction between the polyethylene core and the inferior prosthesis end plate. A subsequent study of CHARITÉ discs implanted in cadaveric specimens also indicated motion response that exhibited a stick-slip phenomenon in the implanted prostheses²⁷. These phenomena contribute to the degradation of other quality-of-motion characteristics of the reconstructed segment such as the neutral zone, stiffness, and hysteresis, all of which should be considered in assessing the performance of the disc prosthesis. Observations from the present study and from the study by O’Leary et al. showed impingement, which refers to a pinching of the polyethylene core between the two metal end plates of the CHARITÉ prosthesis, in extension. During impingement there is no motion at either bearing of the prosthesis. The phenomonon of asymmetric motion described in the present study refers to prosthesis motion in the nonimpingement scenario. Although there is ample evidence from retrievals that CHARITÉ implants may impinge, it is difficult to say whether there was impingement in the clinical cases that we analyzed in the present study since only the flexion and extension radiographs (and not intermediate radiographs) were available.

The findings regarding the distribution of intra-prosthesis motion are relevant to wear testing protocols. The dual-bearing design of the CHARITÉ disc raises an interesting question about how to distribute the total end-plate-to-end-plate motion between the two bearings during a wear test so that the results are clinically meaningful and predictive of the long-term wear and damage of the prosthesis. Previous studies have addressed this in differing ways. Serhan et al. positioned the CHARITÉ disc in the testing machine such that the center of the mobile core was at the testing machines’s center of rotation, thereby forcing both end plates to move relative to the polyethylene core¹⁴. A recent finite-element study simulating a wear-test setup of an isolated CHARITÉ prosthesis indicated that prosthesis articulation and wear occurred predominantly at the superior bearing surface regardless of the imposed center of rotation of the implant or the friction coefficient at the inferior bearing⁵⁷. This is consistent with the results of the present study, which found preferential motion at the superior bearing of the CHARITÉ disc prosthesis implanted in human cadaveric spines and in patients.

A number of factors may influence the in situ motion response of the prosthesis implanted in a spine segment. These include implant position within the disc space, implant height, radii of curvature of the bearing surfaces, disc-space geometry, facet orientation, and stiffness of the soft-tissue envelope. The intra-prosthesis motion data in the study describe the response of a dual-bearing prosthesis when implanted in a human lumbar segment, thereby taking into account the fact that the disc prosthesis is only one component of the three-joint complex and the host anatomy will likely influence the distribution of prosthesis motion between the two bearings.

Appendix

Introduction | Materials and Methods | Results | Discussion | Appendix

A table summarizing the analyzed clinical cases is available with the online version of this article as a data supplement at jbjs.org.

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Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Also, one or more of the authors has had another relationship, or has engaged in another activity, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

Topics

polyethylene ; intervertebral disc of lumbar vertebra ; prostheses

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Avinash G. Patwardhan, Robert M. Havey, Nicholas D. Wharton, Parmenion P. Tsitsopoulos, Patrick Newman, Gerard Carandang, Leonard I. Voronov; Asymmetric Motion Distribution Between Components of a Mobile-Core Lumbar Disc ProsthesisAn Explanation of Unequal Wear Distribution in Explanted CHARITÉ Polyethylene Cores. The Journal of Bone & Joint Surgery. 2012 May;94(9):846-854.

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