Total Disc Replacement—Charite™ Artificial DiscThe Charité Artificial Disc is a total disc replacement technology that uses two metal alloy endplates and its unique sliding core. This offers the theoretical advantage of allowing the spacer to shift dynamically within the disc space during spinal motion, moving posteriorly with flexion and anteriorly in lumbar extension. Some experts feel this may improve the segmental rotation and decrease the possibility of facet impingement at extremes of motion. This has not yet been clinically demonstrated.
Figure 14: Charité artificial disc
Figure 15a and 15b: Charité Artificial Disc design to mimic normal disc motion
The Charité Artificial Disc was designed to restore disc space height, to restore motion segment flexibility, to prevent disc degeneration at adjacent segments, to reduce or eliminate pain from motion or from nerve compression, and to improve the patient's functional activities. It was designed to be biocompatible and durable. It has a life span of 40 years (85 million cycles).
The Charité Artificial Disc has kinematics that mirror the segmental motion of a normal spine. It is designed to allow anatomic alignment in lordosis, and to allow normal facet joint loading and unloading.
Wear debris, a concern with polyethylene implants in the peripheral joints, has been studied in the Charité, given the implant's proximity to the spinal canal and nerve roots. In a long-term laboratory test of cyclical motion simulating >11 years of use, no wear debris particles were identified. There is minimal deformation of the core, with less than 8% height loss expected in 10 years of use.
Meticulous attention to implantation is required to ensure that the articulating surfaces of the endplates are parallel in order to restore normal biomechanics. Angled prosthetic endplates are available and were designed to produce parallel surfaces while accommodating lumbar lordosis. Different size endplates are available to the surgeon, so the largest size possible can be used to minimize the chance of subsidence into the bone. Care must be exercised by the surgeon to place the implant centrally in both the sagittal and antero-posterior planes.
Figure 16: Charité Artificial Disc is centralized, front view
Figure 17: Charité Artificial Disc is centralized, side view
Rotation must be controlled by the surgeon during implantation. The endplates are inserted , and the polyethylene core placed into position as the disc space is distracted. Core dislocation is a rarely reported complication.
The surgical approach is typically through an anterior retro-peritoneal route. Patient positioning is important so that radiographic confirmation of the implant position can be seen easily by the surgical team. Factors critical for a good result using the Charité are proper patient selection, selecting the correct prosthesis size, and proper prosthesis positioning with the CentreLine Instruments
Several clinical studies have been published documenting the European experience with this disc since 1987. Worldwide experience with this unconstrained anatomic disc replacement is now greater than 10,000 cases.
Cinotti reported on 46 Italian patients in 1996, with 2-5 year follow-up. He saw no implant failures, but did report reoperation in 19% for continued pain and one case of implant dislocation due to wrong size selection. Overall patient satisfaction was 63%.
LeMaire reported his French series in 1997, following 105 patients a mean of 51 months with 79% good outcomes and no device failures.
Zeegers reported 50 patients in 1999 in a Dutch series. He showed 70% good results with 2 year follow-up, but he did report 24 reoperations in 12 patients, none due to device failure.
The US FDA study was launched at the Texas Back Institute with the first US implantation in March 2000. Since that time, 294 patients have been enrolled in the FDA multicenter (14) study. The study was completed in December 2001, and the FDA approved the Charite for single-level L4-5 and L5-S1 implantation as of October 2004.
The study protocol called for a 2:1 randomization of Charite:BAK threaded fusion cages with autogenous bone graft. 196 patients received the Charité Artificial Disc as part of the randomization, with another 71 non-randomized patients receiving the disc as part of the experience portion of the protocol, so that a total of 267 discs were implanted.
Inclusion criteria were age 18-60, single-level L4-5 or L5-S1 symptomatic degenerative disc disease confirmed by discography, Oswestry score >30, VAS score >4/10, failed >6 months of nonoperative care, back and/or leg pain without nerve compression, <3 prior abdominal surgeries, and compliance with the follow-up schedule.
Exclusion criteria included previous fusion, multilevel degeneration, prior fracture in the lower lumbar spine, non-contained HNP, osteoporosis or metabolic bone disease, spondylolisthesis >3 mm, positive straight leg raise, scoliosis >11 mm, spinal tumor, infection, facet joint arthrosis, psychological disorder, and morbid obesity.
Additional exclusions were metal allergy, bone growth stimulator elsewhere in the spine, participation in another study, arachnoiditis, pregnancy, chronic steroid use, or presence of an autoimmune disease.
Figure 18: Front view of implanted Charité Artificial Disc
Figure 19: Side view of implanted Charité Artificial Disc
