[Mid-term results of 360-degree lumbar spondylodesis with the use of a tantalum implant for disc replacement].

PURPOSE OF THE STUDY

The study presents the mid-term results in patients treated with circumferential, i.e., 360-degreee, fusion of the lumbar spine carried out by posterolateral instrumented spondylodesis and anterior intersomatic fusion using a tantalum implant for replacement of an intervertebral disc. The aim of the study was to verify tantalum implant quality, to evaluate segment fusion and to assess the outcome of this method by patients' subjective findings.

MATERIAL AND METHODS

The prospective non-randomised study included the implantation of 47 tantalum cages in 40 patients by the technique of anterior lumbar interbody fusion (ALIF) and posterolateral spondylodesis. Only patients followed up longer than for one year were included in this evaluation. The patient group comprised 16 men and 24 women, with an average age of 47.9 years. The patients indicated for this procedure had mono- or bisegmental lumbar disc degeneration with advanced chan- ges of articular facets. The procedure involved posterior transpedicular screw fixation with decompression, if spinal stenosis existed, and reposition of the segment, if spondylolisthesis was present, and posterior spondylodesis by the open book method. No autogenous bone grafts were used because of the risk of donor-site pain and because one of the aims of the study was to test tantalum cage properties. Subsequently, ALIF and disc replacement with a tantalum cage were carried out after cage insertion, the anterior borders of the upper and lower adjacent vertebral bodies were adjusted so that the lamellar bone should overlap the cage and thus provide continual bridging for the segment, with an emphasis being on joining the lamellae with preserving their blood supply from the respective vertebral bodies. RESULTS Implant subsidence in a rotation/flexion fashion, i.e. ventrally into the upper endplate of the distal vertebra and dorsally into the lower endplate of the proximal vertebra, was observed in two cases; subsidence in a vertical fashion, i.e., symmetrically into the lower endplate of the upper vertebra was found in one patient. Neither vertical mode of subsidence into the upper endplate of the lower vertebra only, nor rotation/extension type of subsidence was recorded.No frontal/rotational mode of subsidence was found either. No osteolytic lesion between the implant and adjacent bone was diagnosed. No migration of an implant sideways or in a ventral/dorsal direction was detected. Anterior bridging fusion was observed in 32 cages. No fusion dorsal to or lateral to the cage was seen. Evaluation of bone ingrowth into the cage was not possible due to a high X-ray contrast effect of the material. High-quality bridging posterolateral spondylodesis was diagnosed in 33 cases. Subjective evaluation by the patients was based on a visual analogue scale (VAS) and an Oswestry Disability Index (ODI) questionnaire. For the whole group, the average VAS value for back pain dropped from 58.3 points before surgery to 18.1 points at one year post-operatively; there were no differences between the genders. The VAS value for leg pain decreased from 54.1 pre-operatively to 9.4 at one year after surgery. Also, the results of ODI assessment were similar in both genders, with the average value for the whole group having decreased from 53.5 % pre-operatively to 29.0 % at one year post-operatively.

DISCUSSION

By using the mechanical properties of a tantalum implant, i.e., its high strength and flexibility, the structure almost identical to cancellous bone and its high stability following implantation, we avoided the necessity of harvesting autogenous bone grafts from the iliac crest. Instead, we introduced the method of bridging a stable and strong implant with lamellar bone, while preserving its blood supply from the adjacent vertebral bodies. Our results showed that this approach resulted in implant subsidence in 1.8 % of cases only.We achieved good-quality fusion by bridging the whole segment in 68 %, and diagnosed good posterolateral fusion in 71 % of the cases. All patients showed good spondylodesis, which was either ante- rior, posterior or bilateral.

CONCLUSIONS

Avery stable fixation of the lumbar spinal segment can be achieved by using posterolateral fusion and ALIF.With this approach, ALIF is augmented with a porous tantalum cage, and the use of autogenous bone grafts, derived from the adjacent anterior vertebral borders and placed before the cage, results in high-quality anterior bridging spondylodesis in a lar- ge proportion of cases. The subjective evaluation by the patients is in agreement with the stability and fusion achieved.