[Adjacent segment degeneration after lumbosacral fusion in spondylolisthesis: a retrospective radiological and clinical analysis].

PURPOSE OF THE STUDY: Whereas the posterior lumbar interbody fusion (PLIF) technique with pedicle screw fixation has shown satisfactory clinical results, solid fusion has been reported to accelerate degenerative changes at adjacent unfused levels, especially at the cranial level. The aim of this retrospective study was to evaluate a group of patients with adjacent segment disease (ASD) developed after 360-degrees lumbar fusion for spondylolisthesis performed by PLIF with transpedicular fixation and posterolateral fusion (PLF).Radiographic examinations were focused on the origin or progression of degenerative changes at the adjacent segments after the operation, with statistical evaluation of some parameters. Clinical evaluations included back pain or neurologic symptomatology which emerged later in the post-operative period in patients with adjacent segment degeneration.

MATERIAL: The authors performed a retrospective analysis on a group of 91 patients (49 females, 42 males) with isthmic, degenerative or dysplastic spondylolisthesis at the L4-L5 level who had undergone the PLIF technique on L4/L5 or L5/S1 with transpedicular fixation surgery and PLF in the period from 1990 to 2001. Isthmic spondylolisthesis was observed in 70 patients, degenerative or dysplastic forms were found in 14 and 7 patients, respectively.The patients were operated on at 40.8 years on average, and were followed-up for an average of 6.1 years. Seven patients had isthmic, two had degenerative and one had dysplastic spondylolisthesis.

METHODS: The data for the patients with ASD were obtained retrospectively, based on radiographic examinations and clinical sequential follow-up examinations. The radiographs were analysed with regard to degeneration at the adjacent levels pre- operatively, immediately after surgery and at the time of the last follow-up visit. The origin or progression of L3-L4, L4-5 or L5-S1 segment degeneration was defined, as a condition giving rise to segmental instability (defined by White and Panjabi), significant disc herniation, spinal stenosis, disc narrowing or slippage (spondylolisthesis or retrolisthesis), on the basis of a comparison with the pre-operative and post-fusion lateral radiographs, those before additional surgery and at the time of the last follow-up. The following sagittal parameters were measured and compared: lumbar lordosis (L1-S1); distal lordosis (L4-S1) segmental lordosis--the slip angle (SA) at the fused and the adjacent segment, respectively; sacral slope (SS) and slippage (SLIP). The correlation and regression analyses were used for the statistical evaluation of angular characteristics. The results were statistically analysed using MINITAB statistical software. Functional disability was measured by the Oswestry disability index (ODI) questionnaire and pain was assessed using a 100-mm VAS.

RESULTS: Of the 91 patients, symptomatic adjacent segment disease developed from a previously asymptomatic level in 10 (11%) patients. Their mean age at the time of initial surgery was 42.8 years and the mean follow-up period was 8.7 years. The mean period between the initial surgery and the onset of adjacent segment degeneration was 3.8 years. In every case fusion involved the use of autologous bone graft and, with the PLIF technique, cages were used in three, bone dowels in six and an autofibular graft in one patient The patients of this group frequently had more than one degenerative process. Four patients had signs of instability abo- ve the fusion and seven patients showed degeneration which was above the fusion in four and below it in three. The degenerative changes included spinal canal stenosis due to disc herniation and/or facet hypertrophy in four, disc narrowing in five and spondylolisthesis or retrolisthesis in five patients. Clinical deterioration was manifested as progressive back pain in three, back and leg pain in seven and lower extremity paresthesia in two patients. The mean pre- and post-operative values were 50.5% and 28.6% for ODI scores and 7.1 and 3.5 for VAS scores, respectively. At the time of ASD, the ODI value was 39% and the VAS was 5.2. The four patients with instability in the cranial adjacent segment successfully underwent additional surgery by 360-degree instrumented fusion (anterior lumbar interbody fusion--ALIF in three patients and PLIF with decompression in one patient). No statistically significant correlations were revealed by the comparison of radiological angular characteristics before surgery, after it and at the onset of ADS.

DISCUSSION: On X-ray images obtained prior to surgery, signs of hypermobility in the cranial adjacent segment were present in one patient. This hypermobility affected the rigidity of fusion in the caudal segment, which accelerated the progress of instability and required further surgery. The subsequent clinical deterioration, which usually develops due to a combination of significant disc degeneration, herniation, degenerative stenosis, segmental instability, spondylolisthesis or retrolistesis at the motion segment adjacent to fusion, is in agreement with the findings presented by the authors using the same surgical technique.

CONCLUSIONS: An increased occurrence of degenerative changes and the instability predominately at the level immediately above single-segment instrumented 360-degree fusion with clinical deterioration give support to the view that this is due to increased mechanical stress at the motion segments adjacent to fusion. However, the size of our sample was not large enough to allow us to draw generally valid conclusions from the results of radiological angular characteristics. The causes of instability in younger patients could also include spine overloading, damage to the stability of ligaments and bone structures sustained during the operation, or a combination of both. The authors recommend a permanent reduction in physical activity after lumbar or lumbosacral spinal fusion and, in cases where symptomatic instability or degeneration of the adjacent motion segment is manifested, the use of 360-degree instrumented fusion (ALIF or PLIF), dynamic or semi-rigid stabilisation or total disc replacement. A thorough examination of levels adjacent to the planned spinal fusion will prevent termination of the fusion at the potentially painful segment, with a possibility to use a fusion or combined with dynamic neutralisation at the adjacent segment.