The effect of anti-osteoporosis medications on BMD has been well discussed in different literature. However, despite being the single most important factor in guiding treatment, BMD is not the sole factor clinicians should consider. Moreover, the improvement in BMD is often subclinical and may not drive patient-perceived outcomes. Thus, patients are seldom strongly motivated and adhere to their treatments. On the contrary, QOL measures, such as pain assessment and functional status, may be more relevant to the patients and allow patients to perceive the treatment effects more effectively.
Study results suggest that continuous treatment with parathyroid hormone analogues demonstrated significant improvements in multiple QOL parameters over one year, encompassing aspects like back pain, general health status and physical function. All the improvements were greater or equal to the MCID and were considered clinically significant. Similar trends were not observed in the other two treatment groups. In RANKL inhibitor and bisphosphonate groups, only the OPAQ scores improved but the back pain VAS worsened. The results of our study are different from those by Sambrook et al(16) and Hagino et al(19), in which bisphosphate treatment was shown to be associated with significant improvement in specific domains of QOL. One of the reasons to explain the difference is that the mean age of study subjects in our study is about 5 years higher than the mean age of the two studies (around 73 years), and older patients may be more prone to additional back pain. This hypothesis was justified by the subgroup analysis in the study by Hagino et al(19), in which older subgroup (age ≥ 75 years) failed to show significant improvement in QOL with once-weekly bisphosphonates. Thus, we should be aware that older patients are generally less likely to benefit from anti-osteoporosis medications in terms of QOL outcomes.
Back pain in osteoporosis is a complicated process with elements of fractures, postural alterations and muscle atrophy.(29) Vertebral compression fracture is a well-recognized cause of back pain in osteoporotic patients. Its pain severity can vary from mild chronic pain to incapacitating acute back pain.(30) However, the incidence of back pain correlates well with the vertebral fracture severity.(31) In osteoporotic patients without vertebral fractures, their back pain mainly originates from muscle weakness and postural alterations. In elderly patients, osteoporosis and sarcopenia often coexist, and muscle weakness plays an important role in inducing kyphosis and chronic back pain.(29) Our study results are consistent with previous studies which showed teriparatide can reduce back pain to a greater extent as compared to alendronate.(31, 32) In addition to comparison with alendronate, superiority in back pain reduction in teriparatide over denosumab was observed in our study as teriparatide is the only agent showing statistically and clinically significant back pain reduction. The reason for that is largely unknown. It is believed that back pain reduction in teriparatide is mainly mediated by the reduction in number and severity of vertebral fractures(33), but this mechanism also applies to most anti-osteoporosis medications. Moreover, unlike alendronate and denosumab which may improve muscle strength and back pain in sarcopenia(34, 35), teriparatide is not known to exert any effect on muscle strength. However, a recent study showed that teriparatide can significantly reduce mechanical hyperalgesia and the upregulation of transient receptor potential channel vanilloid 1 (TRPV1) and calcitonin gene-related peptide (CGRP) expression in mice.(36) This may offer new possible explanations for the greater back pain reduction in teriparatide.
Another interesting phenomenon was observed regarding the predictive factors for progression of back pain in osteoporotic patients. To our knowledge, there is no existing study that particularly investigates the factors that predict the progression of back pain in osteoporotic patients. However, back pain is one of the more commonly encountered symptoms by postmenopausal women with osteoporosis with reported prevalence from 85.1–91.4%.(29, 37, 38) 78.3% of patients in our study also complained of variable degree of back pain at baseline. Thus, reduction in back pain is of important concern in osteoporotic patients. In our study, a general linear model was constructed to identify the predictive factors that account for MCID in average VAS back pain. Initially, without PS weighting, both baseline pain level and treatment group were significant predictive factors, with parathyroid hormone analogues outperforming the other two. Despite this, the intergroup differences (teriparatide versus denosumab: -5.1; teriparatide versus alendronate: -4.6) were quite remarkable. Our study results raised the hypothesis that teriparatide treatment may predict better back pain improvement when compared to denosumab and alendronate, but larger scale studies are needed to prove whether the choice of anti-osteoporosis medications can influence back pain progression significantly. Nevertheless, baseline pain level remains the most important factor in predicting future back pain progression in our study, and we should not expect certain anti-osteoporosis medications to significantly improve back pain in patients with poor baseline pain level.
Poor adherence to bisphosphonates was observed in our study and several other studies.(39, 40) Patients in the alendronate group particularly performed poorly in the questions concerning forgetting their medications and inconvenient drug intake instruction. Thus, we propose that the once-weekly regimen of bisphosphonate may be difficult for elderly patients to remember when they should take drugs. The requirement of patients to maintain an upright posture for 30 to 60 minutes after ingesting bisphosphonates to avoid erosive esophagitis may also cause inconvenience to the patients and account for the lower treatment adherence score. On the other hand, the uses of parathyroid hormone analogues and RANKL inhibitors are more convenient with less restriction on drug intake. However, it is important to note that 6 patients (33.3%) discontinued teriparatide before the end of the study. It means that the most noncompliant patients have left the study and are not included in the analysis, so the ADEOS-12 score of the teriparatide group may not reflect the true extent of treatment adherence.
The first limitation of our study is the unequal numbers of patients in the three study groups, with significant differences between teriparatide group and the other two groups. Comparison of QOL outcomes across treatment groups was difficult as the unequal numbers limited the power of the statistical tests. Nevertheless, our study design should have reduced the effect of these various confounders.