Pharmacological Mechanisms and Sequencing Rationale

The included literature consistently demonstrated that the superiority of the anabolic-to-antiresorptive sequence is rooted in divergent mechanisms of bone remodeling and modeling. Osteoporosis is characterized by an imbalance where osteoclast resorptive activity exceeds osteoblast formative capacity, leading to architectural decay [10]. Antiresorptive agents function primarily by reducing the rate of bone remodeling. Bisphosphonates, including alendronate, risedronate, and zoledronic acid, bind to hydroxyapatite and inhibit farnesyl pyrophosphate synthase within osteoclasts, inducing apoptosis and effectively closing the remodeling space [11]. Denosumab, a humanized monoclonal antibody, targets receptor activator of nuclear factor-kappa B ligand (RANKL), preventing osteoclast maturation and survival [12].

In contrast, anabolic agents stimulate modeling-based bone formation. Teriparatide (PTH 1–34) and abaloparatide (PTHrP analog) transiently activate the parathyroid hormone receptor type 1 (PTH1R), preferentially stimulating osteoblastic activity and enhancing trabecular and cortical bone formation [6, 13]. Romosozumab offers a distinct dual mechanism; by inhibiting sclerostin, it transiently boosts bone formation while simultaneously providing sustained decreases in bone resorption [6]. Critically, the bone-forming response to these agents is self-limiting and plateaus after 12 to 24 months, and cessation leads to rapid reversal of BMD gains unless followed by antiresorptive consolidation [6, 14].

Multiple studies documented the blunting effect observed when anabolic agents are administered after long-term antiresorptive therapy. The STRUCTURE trial demonstrated that patients transitioning to teriparatide after prior alendronate experienced transient hip BMD loss requiring several months for recovery [8]. This phenomenon is attributed to antiresorptive-induced suppression of bone turnover, which limits the available surface area and release of skeletal growth factors necessary for anabolic agents to exert their full effect [7]. Table 1 summarizes the comparative mechanisms and sequencing impact of major osteoporosis agents.

Table 1 Comparative mechanisms and sequencing impactBone Mineral Density Outcomes

The included studies consistently demonstrated superior BMD outcomes with sequential therapy compared to monotherapy or step-therapy approaches. In the FRAME study, postmenopausal women receiving 12 months of romosozumab followed by 12 months of denosumab achieved substantially higher BMD increases at 24 months compared to denosumab monotherapy [15]. At the lumbar spine, the sequential group achieved a 16.8% increase compared to approximately 7.5% in the monotherapy arm. Similarly striking results were observed at the hip, with the sequential cohort achieving 4.4% greater total hip BMD increase than the denosumab-only group [15].

The probability of achieving a treat-to-target goal (T-score better than − 2.5) was significantly enhanced by sequential therapy. For patients starting with a T-score of -3.0 at the lumbar spine, the romosozumab/denosumab sequence offered 92% probability of reaching target within 24 months, versus only 47% for denosumab monotherapy [15]. At the total hip, this discrepancy was even more pronounced: 50% probability for sequential therapy versus 5% for monotherapy [15]. Table 2 presents the 24-month BMD outcomes from the FRAME study and extension analyses.

Table 2 24-Month BMD Outcomes: Sequential vs. Monotherapy

The ACTIVE and ACTIVExtend trials evaluated abaloparatide followed by alendronate. After 18 months of abaloparatide, lumbar spine BMD increased by 11.2% compared to 10.5% for teriparatide [16]. Following transition to alendronate for an additional two years, the sequential group achieved total lumbar spine gains of 14.4% and total hip gains of 6.4% [16]. Notably, abaloparatide demonstrated faster and more pronounced effects on hip and femoral neck compared to teriparatide in early treatment months, potentially due to more selective PTH receptor binding [13].

The DATA-Switch study investigated teriparatide-to-denosumab sequencing, demonstrating continued substantial BMD increases across all skeletal sites. Over 48 months, this protocol resulted in 18.3% lumbar spine increase and 6.6% at total hip and 8.3% at femoral neck [17], representing one of the most potent sequential regimens documented, particularly for improving trabecular bone score reflecting microarchitectural quality [17].

Fracture Risk Reduction

The ultimate metric of effectiveness in osteoporosis treatment is anti-fracture efficacy. A comprehensive meta-analysis of 69 trials involving over 80,000 patients confirmed that bone anabolic treatments are significantly more effective than bisphosphonates in preventing both vertebral and clinical fractures [18]. In active treatment comparisons, denosumab, PTH analogs, and romosozumab were all superior to oral bisphosphonates for vertebral protection. Specifically, parathyroid hormone receptor agonists were nearly 50% more effective in reducing clinical fractures compared to bisphosphonates (OR 1.49; 95% CI 1.12-2.00) [18].

The ARCH trial, following a sequential protocol of 12 months romosozumab followed by alendronate, demonstrated 48% reduction in new vertebral fracture risk compared to alendronate monotherapy [19]. Critically for orthopedic populations, hip fracture risk was reduced by 38% in the sequential group [19]. Table 3 summarizes the meta-analytic findings for vertebral fracture risk reduction.

Table 3 Meta-Analysis of relative risk for subsequent vertebral fracture

The concept of imminent fracture risk is particularly relevant to orthopedic practice. Following a fragility fracture, the risk of a second fracture is highest in the subsequent two years [20]. Anabolic-first sequential therapy is uniquely suited for this crisis period because it induces rapid skeletal strengthening that antiresorptives cannot match [21]. In patients who have experienced osteoporotic vertebral fractures, anabolic agents provide superior prevention of subsequent fractures compared to bisphosphonates, although current evidence does not yet prove accelerated healing of the initial fracture [22].

Orthopedic Subspecialty ApplicationsHip Fracture Management

Hip fractures represent arguably the most devastating complication of osteoporosis, associated with approximately 20% first-year mortality [20]. For survivors, morbidity is severe: fewer than half regain pre-fracture functional independence, and 45% have a history of prior fragility fracture indicating earlier intervention failure [23]. Recent studies have addressed the feasibility of sequential therapy in elderly hip fracture populations where long-term adherence to daily injections is challenging. An abbreviated sequential approach employing 3 to 6 months of anabolic agent (teriparatide or romosozumab) followed by denosumab demonstrated significant one-year postoperative BMD gains at lumbar spine (3.6%), femoral neck (4.4%), and total hip (1.9%) [24]. In contrast, non-sequential groups receiving only short-course anabolics without antiresorptive follow-up showed no significant changes at any site, underscoring the necessity of the consolidation step even when the anabolic phase is abbreviated for clinical practicality [24].

Spinal Reconstructive Surgery

Adult spinal reconstructive surgery often involves multilevel instrumentation and fusion, procedures highly sensitive to surrounding bone quality [25]. The prevalence of unrecognized osteoporosis in this population is high, with up to 64% of patients unaware of their diagnosis prior to surgery [25]. Poor bone quality leads to specific mechanical failures including pedicle screw loosening, pseudarthrosis, and proximal or distal junctional kyphosis [25]. Teriparatide has emerged as the standard for pharmacologic optimization in spine surgery, with randomized studies demonstrating significantly higher fusion rates at 3D CT imaging as early as four months postoperatively, increased pedicle screw insertional torque, and promotion of active bone formation around hardware [25].

Total Joint Arthroplasty

Total hip arthroplasty, while highly successful, faces long-term durability compromise from periprosthetic bone loss due to stress shielding, potentially causing 40% decline in proximal femoral BMD in the first postoperative year [26]. A network meta-analysis of 33 RCTs clarified comparative effectiveness in THA patients: for immediate postoperative preservation (6 months), denosumab was most efficient due to rapid RANKL pathway blockade; for intermediate-term preservation (12–24 months), combination approaches including teriparatide with alendronate outperformed monotherapies [26]. For patients with systemic osteoporosis undergoing THA, sequential therapy (anabolic followed by bisphosphonate) is considered optimal strategy to maximize both systemic and periprosthetic bone health [11].

Special Populations and Health Economics

Osteoporosis in men is often underdiagnosed and undertreated despite higher hip fracture mortality rates compared to women (37.5% vs. 28.2% one-year mortality) [27]. The 2024 ESCEO guidelines specifically recommend sequential approach starting with bone-forming agents for men at very high risk, with abaloparatide now considered appropriate first-line treatment (weak recommendation) [27]. For glucocorticoid-induced osteoporosis, teriparatide has demonstrated superiority over alendronate in BMD outcomes and vertebral fracture reduction, with sequential teriparatide-to-denosumab showing significant femoral neck BMD increases not achieved with monotherapy [28].

The higher cost of anabolic agents drives step-therapy formulary preferences. However, lifetime cost-effectiveness analyses using microsimulation models demonstrate that sequential therapy often represents better long-term investment for high-risk patients [29]. Sequential abaloparatide followed by alendronate was found dominant compared to sequential teriparatide (more QALYs for lower cost) and cost-effective compared to alendronate monotherapy for men aged 50 + with prior hip or vertebral fracture [30]. These economic data suggest that step-therapy, while saving money short-term, may actually increase lifetime healthcare costs by failing to prevent expensive secondary fractures and nursing home admissions [29, 30].

Transition and Maintenance Strategies

The literature consistently emphasized that osteoporosis requires lifelong management [4]. Bisphosphonates are typically used for 3 to 5 years, after which drug holidays may be considered for patients no longer at high risk [11]. However, drug holidays are strictly contraindicated for denosumab due to rebound phenomena [31]. Denosumab discontinuation triggers massive surge in bone turnover markers that can precipitate multiple spontaneous vertebral fractures [32]. The transition from denosumab to bisphosphonate must be timed precisely, with most experts recommending initiation approximately 6 to 7 months after the last denosumab dose [33]. Selective estrogen receptor modulators have been found ineffective in preventing bone loss after denosumab discontinuation, making bisphosphonates the only reliable bridging therapy currently available [33].