Abstract
Introduction
The ideal patient for a disc replacement with a mechanical prosthesis, such as Discocerv of Scient̀X or a M6 of spinal kinetics with a soft core, is under 60 years of age with monosegmental, symptomatic degenerative disc disease without degeneration in the facet joints. Following implantation of prosthesis any changes in load through the facet joints appear as changes in bone mineral density at the operative and adjacent levels. A comparing study was designed to investigate whether facet joint sclerosis and degeneration occurs following the implantation of the two different disc types.
Materials and Methods
In this prospective study, suitability for disc replacement was determined by CT scan and clinical assessments. Patients were followed up 6 and 12 months postoperatively. X-ray and CT-controlled osteoabsorptiometry were used to assess any changes in bone mineral density following implantation of the two different disc models. A 3D reconstruction enabled visualization of the bone density distribution in the facet joints. Pain and function was assessed using standard outcome scores (visual analog scale [VAS] back pain score, Oswestry disability index [ODI], and short form-36 [SF-36]). Mobility was determined by flexion/extension X-rays.
Results
In total, 20 patients received disc replacement, 10 patients in each group. The postoperative location of the areas of maximum bone mineral density did not change compared with the preoperative condition. There was a reduction in subchondral mineralization density in the facet joints of the operated segments and, to a lesser degree, in the neighboring segments. The mean improvement in the VAS back pain score 6-months postoperatively was 60%. Function, as measured by ODI and SF-36 scores, had improved by up to 70% at the same postoperative time point. Improvements in pain and function scores were maintained 12 month postoperatively. On average, normal movement and weight bearing was observed 8 weeks postoperatively. Mobility increased by a mean of 8 degrees in the operated segment. There was no big difference between the two disc designs.
Conclusion
Facet joint sclerosis and degeneration did not occur following cervical disc replacement. There was no big change in the location of the areas of maximum density, indicating that mobility approached normal physiological levels. The reductions in subchondral mineralization density suggest normalization away from degenerative sclerosis in both disc types. This did not appear to be due to adoption of postoperative postures as normal movement and weight bearing was achieved. Further study is needed to assess these two different disc designs and the degree to which restoration of segmental mobility leads to the normalization of degenerative sclerosis.