For more than 20 years TDRs have been implanted by spine surgeons. Short- and mid-term results after TDR of different study groups have shown promising results [3
]. However, long-term results with a follow-up of up to 13 years published by van Ooij as well as a review of literature were less favorable [25
]. Therefore, the purpose of this retrospective study was to analyze the clinical and radiological long-term results of the Charité TDR in the surgical treatment of patients with DDD of the lumbar spine. Due to the fact, that the first TDRs ever implanted are part of this study, the presented data represent the longest currently possible follow-up of the Charité TDR.
In summary, this long-term follow-up study demonstrates dissatisfying results after artificial disc replacement in the majority of the evaluated cases concerning clinical as well as radiological outcome. The large percentage (83%) of radiologically confirmed fusions of the treated segments shows that the Charité artificial disc replacement cannot guarantee long-term near to normal function of the spinal motion segment in patients with moderate to severe DDD.
Multiple causes for the high rate of spontaneous fusions have to be taken into consideration. Whereas the anterior approach is carried out in the same way nowadays as during the time of these reported operations, the preservation of the anterior longitudinal ligament performed during that time, nowadays is known to be a trigger for unwanted ossifications [36
The reparative processes, which are induced by the incision of the anterior longitudinal ligament, have not been examined in detail so far, but might also have contributed to the high rate of unintended segmental fusions [31
Additionally, for a correct implantation of artificial disc prostheses a nearly complete removal of the remaining disc tissue is mandatory [5
]. This process implies the decortication of the vertebral endplates. Consecutively, osteoinductive substances are regularly released into the disc space due to the interruption of the integrity of the bone–cartilage border [1
]. In operative spondylodesis, this effect is intended to enhance and promote the osseous consolidation; however, it is unintended in TDR. This undesired process might also have contributed to the high rate of spondylodesis. Finally, since all of the reported patients suffered preoperatively from moderate to severe DDD, a progression of these processes after surgery can be assumed. This may be another cause of ongoing ossification as a result of degenerative changes of segment-related tissue such as bone and ligaments.
This study included patients who were treated with three different types of the Charité artificial disc prostheses. The history of the development of this first TDR device was previously described in detail by Link [30
]. The major changes from the first prototype to the currently commercially available Charité artificial disc replacement are summarized in the material section of this study. While the first implant was given up due to an insufficient bone implant contact area, the second was replaced because of a high rate of fractures in the wing area of the endplate. However, the third implant—the Charité III TDR—is still in use today with only minor changes. Although the basic principle based on the “low-friction” concept known from endoprosthetic joint replacements remained the same throughout the years, these three different TDR types demonstrated considerable differences regarding design, material combination, endplate geometry, endplate thickness, and size. However, interestingly, there was no significant difference in the clinical or radiographical long-term outcome between these three different TDR types. This is especially surprising, because the first two implant types were given up due to obvious design problems, which leads to the conclusion that even considerable design modifications of this TDR do not seem to have a significant input on the overall clinical and radiological long-term results.
All TDRs are promoted with the theoretical advantage to prevent an adjacent level degeneration. In fact, none of the few (n
=9; 17%) functional TDRs evaluated in this study, showed significant adjacent level degeneration. However, the overall percentage of adjacent segment degeneration (17%) after implantation of a TDR was comparable to results of follow-up studies concerning adjacent segment degeneration after fusion surgery [2
]. For example, the studies of Etebar and Kumar demonstrated 4 and 30 years after surgery a rate of adjacent segment degeneration of 14.4 and 14.2%, respectively. Due to the high rate of spontaneously ankylosed segments in this study, the high percentage of adjacent segment degenerations shown is not very astonishing.
Further, it is demonstrated in this study that the patients with preserved segmental mobility after an average follow-up of 17 years, were significantly less satisfied with the long-term outcome of the surgery than patients with spontaneous ankylosis or fused motion segments after implant failure. Due to the fact that in these cases no adjacent segment degeneration was evident, the clinical problem must be located in the operated motion segment itself. For the successful treatment of DDD with a TDR implant, anatomical reposition of the complete motion segment is as important as in fusion surgery [8
]. However, even with an ideal implantation technique the Charité TDR can only achieve the repositioning of the anterior column, while the degenerative disorders of the posterior elements are not addressed. Furthermore, it can be assumed that the occurrence of anterior and posterior degenerative changes of the lumbar motion segments is closely related to each other [9
]. It has been postulated by Dunlop that in cases of moderate to severe DDD facet joint arthritis always can be found as accompanying degenerative alteration of the posterior spine elements [15
]. Studies have further demonstrated that after implantation of a TDR, loads in the facet joints might increase 2.5-fold in comparison to the normal loading conditions [14
]. Therefore, preexisting changes in the posterior column might persist or even progress after TDR implantation resulting in deterioration of patients’ low back pain (LBP) symptoms over the years. For example, an unintended slight hyperlordosis of the motion segment after TDR implantation might lead in the long run to LBP due to progressive facet joint degeneration, and leg pain due to narrowing of the intervertebral foramen with consecutive nerve root compression. This might explain why the spontaneously ankylosed motion segments did better at follow-up than the functional and mobile segments. To prevent these problems, an optimal implantation of the TDR is mandatory. Additionally, degenerative changes of the facet joints should be evaluated very carefully prior to the implantation of a disc prosthesis [3