**4. Discussion**

From 2002 to 2008, a significant volume of data was made available on the clinical impact of arthroplasty. Sixty studies related to the clinical use of arthroplasty were published in peerreviewed papers, of which 35 described data collected prospectively and 18 represented data from multi-center studies. The total number of patients described in the literature for spinal arthroplasty is difficult to evaluate, since many studies are early data releases or subanalyses of the main randomized controlled trials performed for each new device. Thus, a given patient population might have been discussed in multiple papers. Overall, however, it was estimated that approximately 1,600 patients were included in the current literature.

The RCTs for the ProDisc-L and the CHARITÉ Artificial Disc both demonstrated the noninferiority of the arthroplasty procedure, compared to their respective controls (ALIF for CHARITÉ Artificial Disc, 360 degs fusion for ProDisc-L. The complete RCT data for Maverick Total Disc Arthroplasty System and FlexiCore Intervertebral Disc are not yet published.) For some specific clinical outcomes such as pain, disability, and hospital stay, arthroplasty patients experienced greater clinical outcomes at some of the follow-up time points. For the CHARITÉ Artificial Disc RCT, arthroplasty patients fared statistically better than fusion on pain and disability for all but the 24-month follow-up time point. Hospital stay was also significantly shorter in the CHARITÉ Artificial Disc group as compared to the fusion group. As for ProDisc-L, pain scores were statistically better in the investigational cohort as compared to control. In addition, most other case series, for any of the given arthroplasty products, including those with short- and long-term follow-up data, presented favorable overall outcomes for spinal lumbar arthroplasty.

The issues of range of motion, heterotopic ossification, and sagittal balance have also drawn significant attention. While accurate and reproducible measurement of the lumbar ROM may be a significant limitation to collect meaningful data, average ROM at the 2-year time point throughout the studies evaluated herein were at ~ 7-10 degs. In addition, restoration of sagittal balance was observed for each device, the CHARITÉ Artificial Disc, the ProDisc-L as well as the Maverick Total Disc Arthroplasty System.

This review contained however somewhat contradictory data on the issue of facet and adjacent-level degeneration. While two of the 10year studies showed very small instances of facet and/or adjacent-level degeneration, others reported nearly a 1/3 of all cases with either CHARITÉ Artificial Disc or ProDisc-L developing changes in facet morphology. The inconsistency in these results points at potential surgeon-specific techniques and approaches that may impact the long-term benefits of both procedures.

Surgeon-specific variability in technique and proficiency was also cited in cases of revision. In fact, a study by Regan et al evaluating the occurrence of revision in low-volume vs. highvolume center, confirmed that surgeon in low-volume centers may incur greater perioperative complications, however none that affected long-term outcomes59. Nevertheless,

A similar analysis by Levin et al evaluated the costs of 1- and 2-level arthroplasty vs. 360 degs fusion. This study did not include possible needs for revisions. Nevertheless, one-level arthroplasty cases were found to be less costly than one-level fusions (\$35,592 for arthroplasty vs. \$46,280 for 360 deg fusion) while two-level cases were similar for both

From 2002 to 2008, a significant volume of data was made available on the clinical impact of arthroplasty. Sixty studies related to the clinical use of arthroplasty were published in peerreviewed papers, of which 35 described data collected prospectively and 18 represented data from multi-center studies. The total number of patients described in the literature for spinal arthroplasty is difficult to evaluate, since many studies are early data releases or subanalyses of the main randomized controlled trials performed for each new device. Thus, a given patient population might have been discussed in multiple papers. Overall, however, it was estimated that approximately 1,600 patients were included in the current literature. The RCTs for the ProDisc-L and the CHARITÉ Artificial Disc both demonstrated the noninferiority of the arthroplasty procedure, compared to their respective controls (ALIF for CHARITÉ Artificial Disc, 360 degs fusion for ProDisc-L. The complete RCT data for Maverick Total Disc Arthroplasty System and FlexiCore Intervertebral Disc are not yet published.) For some specific clinical outcomes such as pain, disability, and hospital stay, arthroplasty patients experienced greater clinical outcomes at some of the follow-up time points. For the CHARITÉ Artificial Disc RCT, arthroplasty patients fared statistically better than fusion on pain and disability for all but the 24-month follow-up time point. Hospital stay was also significantly shorter in the CHARITÉ Artificial Disc group as compared to the fusion group. As for ProDisc-L, pain scores were statistically better in the investigational cohort as compared to control. In addition, most other case series, for any of the given arthroplasty products, including those with short- and long-term follow-up data, presented

The issues of range of motion, heterotopic ossification, and sagittal balance have also drawn significant attention. While accurate and reproducible measurement of the lumbar ROM may be a significant limitation to collect meaningful data, average ROM at the 2-year time point throughout the studies evaluated herein were at ~ 7-10 degs. In addition, restoration of sagittal balance was observed for each device, the CHARITÉ Artificial Disc, the ProDisc-L

This review contained however somewhat contradictory data on the issue of facet and adjacent-level degeneration. While two of the 10year studies showed very small instances of facet and/or adjacent-level degeneration, others reported nearly a 1/3 of all cases with either CHARITÉ Artificial Disc or ProDisc-L developing changes in facet morphology. The inconsistency in these results points at potential surgeon-specific techniques and approaches

Surgeon-specific variability in technique and proficiency was also cited in cases of revision. In fact, a study by Regan et al evaluating the occurrence of revision in low-volume vs. highvolume center, confirmed that surgeon in low-volume centers may incur greater perioperative complications, however none that affected long-term outcomes59. Nevertheless,

groups (\$55,524 for arthroplasty vs. \$56,823 for fusion)58.

favorable overall outcomes for spinal lumbar arthroplasty.

as well as the Maverick Total Disc Arthroplasty System.

that may impact the long-term benefits of both procedures.

**4. Discussion** 

revisions were often found to be associated with technical errors, such as errors in positioning or sizing of the implant.

This critical importance of proper technique was in fact described in the 2 technique papers discussed herein. Both these publications stressed the importance of proper patient selection, a recurrent theme in almost all arthroplasty discussions.

Three types of major complications were described or foreseen in the current literature: 1) vertebral body split due to the ProDisc-L keel design; 2) oxidation of the core polymer nucleus, a problem specific to first generation CHARITÉ I prosthesis; and 3) metallosis and long-term impact of metal ions in the body. While the issue of metallosis and metal ions still needs to be thoroughly investigated, the problems related to vertebral body split only occurred once in the published literature, and as such, may represent a rare occurrence, and that of core oxidation, has been since resolved with new sterilization techniques following which core oxidation of the polymer nucleus is not observed. Thus, possible complications related to devices with metal-on-poly designs seem to be fairly limited.

Finally, low rates of complication and adverse event were observed for most of all analyzed patients. For the ProDisc-L, smokers as well as patients 60 years of age seemed to experience similar benefit from the procedure. In all these studies, however, authors reiterated the importance of proper patient selection, thus concluding that, while all analyzed patient types experienced clinical benefit from the procedure, specific care must be given to only operate on appropriate patients.

Finally, no technology is sustainable in today's market place if its cost is prohibitive. Thus, the impact of arthroplasty on health economics was also investigated. Both, the CHARITÉ Artificial Disc and the ProDisc-L study came up at a lower cost than their fusion controls, whether potential revision costs were included or not. No data on cost exists yet for the Maverick Total Disc Arthroplasty System and the FlexiCore Intervertebral Disc.
