We found that ibandronate increased BMDs in patients newly diagnosed with osteoporosis. In both groups, the mean BMD at the 3-year follow-up improved significantly compared to the initial value (-3.71 vs. -3.13 for group 1; -3.80 vs. -3.45 for group 2, all p < 0.001). This result was consistent with a previous report showing that BMDs at the lumbar spine and hip increased significantly (3.1% and 1.8%, respectively; p < 0.0001 vs. placebo) with daily 2.5-mg ibandronate after 24 months [19].
In patients with osteoporosis alone, the annual BMD increased significantly over the previous BMD for three years, which suggests that the therapeutic effect of ibandronate is significant every year in patients with osteoporosis alone. However, in patients with osteoporosis and LSS, the annual BMD increased significantly from the initial value only during the first year but did not show a significant increase after that, which suggests that in patients with osteoporosis and LSS, the efficacy of ibandronate was sufficient for the first year, after which the effect of the treatment decreased, perhaps because of the progression of spinal stenosis over time, worsening the symptoms of stenosis, and thus reducing the physical activities of patients, which may reduce the therapeutic effect of ibandronate.
LSS causes neurologic claudication and reduces the strength of the lower limb, which decreases physical activity [10]. Walking difficulty due to claudication or physical inactivity can be associated with decreased BMD [20]. A previous study reported a relationship between physical inactivity and decreased BMD in patients with vascular claudication originating from peripheral arterial disease [20]. Physical activities in seniors can induce the maintenance of BMD or increase BMD by means of physical load [11, 21, 22]. Lee et al. [23] have also reported that increased physical activity and regular walking exercise could prevent osteoporosis in a study of older women aged 65 years and over. In the present study, the direct comparison of annual BMDs between the two groups over three years showed that the annual BMDs in patients with osteoporosis and LSS were significantly lower than in patients with osteoporosis alone, which suggests that LSS may interfere with the improvement of BMD in the treatment of osteoporosis.
These findings were consistent with the correlations between initial BMD and total change of BMD in both groups. The correlation analysis showed that in patients with osteoporosis alone, the initial BMD and the total change of BMD had a strong negative correlation. It suggests that in patients with osteoporosis alone, the more severe the initial osteoporosis, the higher the degree of osteoporosis improvement after treatment. However, in patients with osteoporosis and LSS, there was a weak negative correlation between initial BMD and total change of BMD, which means that the relationship between the severity of initial osteoporosis and the degree of osteoporosis improvement after treatment is weaker in patients with osteoporosis and LSS than in those with osteoporosis alone.
In patients with LSS, a decrease in the therapeutic effect of ibandronate may also be associated with deterioration of bone metabolism [24]. Lee et al. [14] reported that 55.6% of patients with LSS had hypovitaminosis D, which reduced the effectiveness of osteoporosis treatment. Kim et al. [25] reported that limited physical activity in symptomatic LSS patients resulted in high bone turnover rates, including bone formation and bone resorption markers. Moreover, in a subsequent study, they reported that decompression surgery for symptomatic LSS patients had a positive effect on bone metabolism by reducing the increased bone resorption rate [26]. These reports show that improved walking ability and physical activity resulting from the active treatment of LSS can help improve the effectiveness of osteoporosis treatment. Therefore, active treatment for LSS to alleviate neurological symptoms, combined with more potent treatment for osteoporosis should be taken to increase BMD for patients with osteoporosis and LSS.
This study has some limitations. First, we did not make an objective measure that daily activity was lower in patients with osteoporosis and LSS than in patients with osteoporosis alone. Previous studies have reported that daily activity decreases in patients with LSS, so we were able to conduct this study based on the earlier results [8–10]. Further research using objective tools, such as questionnaires on daily activities, is needed. Second, we could not include bone turnover markers in this study. Because it was retrospective, test values were not present for some patients. Third, this study included only patients treated with ibandronate. Thus, the results of this study may not apply to patients treated with other types of osteoporosis drugs. Additional prospective trials are needed to validate our findings and extend these results to different kinds of osteoporosis drugs. Finally, clinical outcomes of osteoporosis treatment, such as osteoporotic fractures, were not evaluated during the follow-up, because we focused only on whether LSS affects BMD in the treatment of osteoporosis. Despite these limitations, our study has the strength that it is the first case series to evaluate the effect of LSS on BMD in the treatment of osteoporosis patients.