Purpose: Cage fusion of the L5–S1 segment is a controversial issue due to the weak stabilisation of the spine during extension and axial rotation. Complementary fixation appears to be needed to improve stability, but the presence of the bifurcation of the great vessels is an anatomic limitation. We studied the anatomy of this area to examine the feasibility of anterior plate fixation.

Material and methods: According to the recognised anatomic references (Rouvière, Bouchet and Cuilleret, Louis), a 33-mm safety zone was described at the aortoiliac and iliocaval bifurcation. This space free from contact with the greater vessels lies in front of the L5–S1 disc. Pre-operative angio-MRI was used to assess the size of this safety zone. A triangular anterior plate was designed for arthrodesis (Pyramid Sofamor Danek, USA). An L5–S1 arthrodesis was performed in 15 consecutive patients using this plate. Follow-up was one year. A video-assisted anterior retroperitoneal approach was used in all cases. The Prolo and Oswestry scores were used for the preoperative and last follow-up evaluations.

Results: According to the anatomic study and the MRI views, 89% of the patients had a sufficient safety zone for plate fixation. The method was contraindicated in two patients who had a low bifurcation. Angio-MRI was found to be simple and reproducible. The MRI analysis was confirmed at surgery (no false negatives). The plate was successfully implanted in 15 patients with no contact with the great vessels. There were no serious complications (vascular, neurological, urological, digestive). The economic Prolo score was improved from 2.7 to 4.2 and the functional score from 2.6 to 4.3. The Oswestry score improved 33%. The rate of clinical success was 93% (14/15 patients).

Discussion and conclusion: Implantation of an anterior plate for L5-S1 fusion is feasible. The key points are: 1) rigorous preoperative evaluation of the greater vessel bifurcation; 2) anatomic plate design; 3) appropriate surgical technique. The risk of retrograde ejaculation is related to retraction of the hypogastric plexus and should be investigated with a prospective study. This osteosynthesis technique can avoid secondary operations for fixation with pedicular or transarticular screwing.