A fundamental problem that arises while investigating spinal fusion is the lack of definitive methods for confirming solid fusion. The fusion status can by accurately evaluated only through surgical exploration and direct inspection of the fusion mass; however, these methods are impractical for routine use. CT scanning has become the preferred diagnostic imaging modality for evaluating spinal fusion. Carreon et al. [2
] have demonstrated that the positive predictive value for solid fusion on CT scans was 89%, while that of nonunion was only 74% when PLF on both sides were not fused on fine-cut CT scans with reconstructions. Thus, CT evaluation does not seem to be very reliable for the diagnosis of nonunion. Additionally, due to the harmful effects of radiation exposure, CT is not currently used as a routine method for fusion-status evaluation in our hospital. Although plain radiography is not the best method for assessing the fusion status [11
], plain radiographs, accompanied by those in the flexion and extension bending views, are commonly used for this purpose because they are relatively inexpensive and easy to obtain [19
]. Static plain radiography is used to detect the presence of a bone bridge between the transverse processes, and functional radiography is used to detect motion at the fused segment. In this study, the overall fusion rate achieved by PLF was 74%, as evaluated by the method described by Fischgrund et al. [6
] and Kornblum et al. [13
Even though the data suggest some beneficial effects of concomitant spinal fusion in the treatment of patients with degenerative spondylolisthesis [7
], there is no agreement concerning the association between fusion status and surgical outcomes. In this study, we have retrospectively examined a minimum of 8-year surgical outcomes of decompression and PLF in the treatment of LCS with degenerative spondylolisthesis by comparing cases demonstrating union with those exhibiting nonunion. The results demonstrated that the union group achieved better clinical results than the nonunion group at the 5-year and final follow-up although no significant difference was observed at the 1 and 3-year follow-up. Additionally, the scores of LBP and leg symptoms in the union group were better than those observed in the nonunion group at the final follow-up, while these scores were not significantly different between the two groups at 1-year follow-up. In a 3-year prospective study comparing decompression alone with decompression and uninstrumented PLF in the treatment of patients with LCS and degenerative spondylolisthesis, Herkowitz and Kurz [7
] reported that patients undergoing concomitant arthrodesis demonstrated improved clinical results, regardless of the fusion status, when compared with the ‘decompression only’ group. Thereafter, Kornblum et al. [13
] described the long-term outcomes (mean, 7.7 years) of the patients treated with uninstrumented PLF in the studies by Herkowitz and Kurz [7
] and Fischgrund et al. [6
]. They demonstrated that patients exhibiting nonunion experienced significant deterioration in the surgical outcome as compared to a more stable long-term relief for patients with a solid fusion. These results together suggest that an arthrodesis attempt, regardless of the fusion status, appears to play a key role in the treatment of LCS with degenerative spondylolisthesis in the short term; however, the fusion status is a critical factor influencing the long-term operative results. The cause of deterioration in the long-term operative results in the nonunion group with time is a matter of debate. One possible explanation is that instability at the fusion segment in the nonunion group causes greater degenerative changes, such as laminar regrowth and hypertrophy of the facet joints, than that observed in the union group. These changes might lead to the recurrence of the spinal stenosis, resulting in the deterioration of surgical outcomes with time [3
Degeneration that develops at mobile segments above or below a fused spinal segment is known as adjacent segment disease (ASD) [16
]. In this study, plain radiographs obtained at the time of final follow-up revealed degenerative changes in the regions adjacent to the fused segment in the case of eight patients (herniated nucleus pulposus in one patient, disc space narrowing in 4, and instability in 3) belonging to the union group. ASD did not seem to be related to the surgical outcomes, except in the case of one patient, who required revision surgery at the adjacent level, below the fused segment, due to lumbar disc herniation.
Regarding the predictors of surgical outcomes, a multiple regression analysis revealed that the coexistence of comorbid conditions was also a key predictor of the long-term operative results, which was consistent with previous studies [10
]. This result is possibly caused by the influence of comorbid diseases on gait and ADL, which comprise 17 points in the JOA score. In contrast, a multivariate analysis was unable to identify a significant correlation between percent recovery and age, gender, preoperative %slip, preoperative JOA score, preoperative dynamic motion at the listhetic segment and postoperative %slip.
The present study has some limitations. First, it was conducted based on a set of retrospective data, and the outcomes were measured solely based on the JOA score due to the unavailability of other scales such as the Visual Analogue Scale and the Oswestry Disability Index. Second, psychosocial factors which have been reported to influence the long-term surgical outcomes of spinal fusion [22
] were not evaluated. Finally, the number of patients exhibiting nonunion was small (n
= 11). Although the small sample size in the nonunion group clearly limited the results of the statistical analysis, we believe that this fact does not invalidate the main findings of our study.