[ProDisc-C Total Disc Replacement. A four-year prospective monocentric study].

Purpose Of The Study: To present the results of an independent prospective monocentric study of patients with ProDisc-C Total Disc Replacement (CTDR) followed up for 4 years, and to analyse the most frequent late complications, in particular heterotopic ossification.

Material: In the period from October 2004 to May 2006, a total of 61 patients underwent ProDisc-C CTDR involving one or two segments at the Department of Spinal Surgery, University Hospital in Motol. This study included 39 patients who were followed up for at least 4 years. With the exception of one patient operated on two segments, the patients were treated by ProDisc-C CTDR at one level.

Methods: In the study, only the surgical procedure recommended by the implant manufacturer (Synthes, USA) was used and all operations were performed by a team with the same leading surgeon. Clinical assessment. The patients were examined before surgery, immediately after it and at 6 and 12 weeks and 6, 12, 24 and 48 months post-operatively. At each follow-up, responses to the questionnaire were obtained, and the patients' health status was evaluated on the basis of Neck Disability Index (NDI) and Visual Analogue Score (VAS) values for cervical spine and radicular pain, the use of analgesics and personal satisfaction Radiographic assessment. Pre- and post-operative radiographs were taken in antero-posterior and lateral projection, and flexion, extension and lateral bending films were obtained The height of the intervertebral disc space at the affected level was measured and range of motion in flexion and extension was evaluated together with the adjacent levels. In addition, subsidence, loosening, failure or displacement of the implant was assessed, as well as the presence of heterotopic ossification. The results were statistically analysed using Student's t-test.

Results: The clinical results at 1, 2 and 4 years of follow-up were as follows: NDI values, 44.9 pre-operatively, 26.1, 25.8 and 25.1 post-operatively, improvement by 44.1% after 4 years; VAS for cervical spine pain, 5.8 pre-operatively, 3.0, 2.7 and 2.7 postoperatively, improvement after 4 years by 53.7%; VAS for radicular pain, 6.3 pre-operatively, 2.9, 2.9 and 2.7 postoperatively, improvement by 57.1% after 4 years. The radiographic findings showed the average intervertebral disc space height of 3.2 mm at the affected level before and 7.4 mm after surgery, with no significant change in the following period. The average range of disc motion at the affected level was 4.2 degrees before and 11.1 degrees after surgery, with 11.4 degrees at 4 years of follow-up. During that period, heterotopic ossification was recorded in 10 (25%) treated discs, with five of them (12.5%) classified as grade III or IV. Spontaneous fusion across the disc replacement level was found in three cases (7.5%). Two patients (5%) developed kyphosis at the affected disc level. The statistical analysis showed a significant difference between the pre-operative VAS values and those at 6 post-operative weeks for both cervical spine and radicular pain (t = 4.4 and t = 5.3, respectively; p < 0.05). No significant difference in VAS values was found between 6 weeks and 3 months after surgery for either condition (t = 1.69 and t = 0.3; p > 0.05). Changes in VAS values in the following period were minimal and non-significant. The differences in NDI values before surgery and at 6 weeks after it, and between 6 weeks and 3 months post-operatively were significant (t = 11, p < 0.05 and t = 3.8, p < 0.05, respectively). In the following period, changes in the values were minimal and non-significant.

Discussion: Short-term studies on various types of cervical disc replacement have been optimistic and reported good clinical results and few complications. However, with longer follow-ups there has been an increasing incidence of heterotopic ossification as the most frequent late complication. Although the number of patients diagnosed with it is growing, heterotopic ossification influences the patient's clinical problems only little. What are its causes and how to prevent it are questions to be fully answered yet

Conclusions: Heterotopic ossification is the most frequent late complication of total disc replacement. Many factors may be responsible for its development and therefore its prevention is not clear. The correct indication and appropriate surgical technique are most often recommended, and are considered also by the authors to be most important. Restricted motion at the treated segment/s has no significant effect on the patient's clinical status.