Age-Related Changes and Effects of Bisphosphonates on Bone Turnover and Disease Progression in Fibrous Dysplasia of Bone

Abstract

Fibrous dysplasia (FD) is a mosaic disease in which bone is replaced with fibro-osseous tissue. Lesions expand during childhood, reaching final burden by age 15 years. In vitro data suggest that disease activity decreases in adulthood; however, there is no clinical data to support this concept. Bone turnover markers (BTMs) have been used as markers of disease activity in FD; however, the natural history of BTM changes, the effects of antiresorptive treatment, and their association to clinical outcomes have not been described. The goals of this study are to describe 1) the natural history of FD disease activity and its association with pain; 2) the impact of bisphosphonates on the natural history of BTMs; and 3) the effect of bisphosphonates on progression of FD burden during childhood. Disease burden scores and alkaline phosphatase, osteocalcin, NTx, FGF23, and RANKL levels from 178 subjects in an FD/MAS natural history study were reviewed, including 73 subjects treated with bisphosphonates. BTMs, RANKL, and FGF23 demonstrated a sustained reduction with age. Bisphosphonate treatment did not significantly impact this age-dependent decrease in BTMs. Pain was more prevalent and severe in adults compared with children and was not associated with BTMs. In children, the progression of disease burden was not affected by bisphosphonates. In conclusion, FD is associated with an age-dependent decline in bone turnover and other markers of disease activity. Pain, in contrast, is more frequent and severe in adults with FD and is not related to bone turnover. Bisphosphonate treatment does not significantly impact the age-dependent decrease in bone turnover, nor does it prevent the progression of FD disease burden in children. These findings, in association with the established adverse effects of antiresorptives, should be considered when evaluating use and response to bisphosphonates in patients being treated for FD and in any study using BTMs as surrogate endpoints.

Introduction

Fibrous dysplasia (FD) is a mosaic disorder caused by early postzygotic mutations in GNAS, leading to constitutive activation of Gαs G-protein-coupled receptor signaling.^(1,2) In the skeleton, these mutations result in a subset of skeletal stem cells that are unable to differentiate into normal bone-forming and hematopoietic supporting mesenchymal cells,⁽³⁾ leading to replacement of bone and marrow with abnormal fibro-osseous tissue. These discrete, expansile lesions lead to fractures, deformity, functional impairment, and pain.^(2,4,5) FD can involve one bone (monostotic) or multiple (polyostotic), and can affect any part or combination of the skeleton.⁽⁶⁾ FD may occur in isolation or in association with café-au-lait skin macules and hyperfunctioning endocrinopathies, including hyperthyroidism, precocious puberty, growth hormone excess, hypercortisolism, and fibroblast growth factor 23 (FGF23)-mediated hypophos-phatemia.⁽⁷⁾The combination of FD and one or more extraskeletal features is termed McCune-Albright syndrome (MAS).⁽⁸⁾

Age-related changes in FD can be considered from two perspectives. Disease burden, defined as the extent of the skeleton involved with FD lesions, has been extensively described. Using a validated ⁹⁹Tc-methylene diphosphonate (⁹⁹Tc-MDP) bone scan instrument, the skeletal disease burden score (SDBS),⁽⁵⁾ our group has previously shown that 90% of the total skeletal disease is established by age 15 years. After this age, SDBS plateaus and remains stable throughout adulthood. The implication of this finding is that the window of time for preventative therapies is narrow and that any intervention aimed at decreasing progression of disease burden should be instituted at an early age.⁽⁹⁾ The second perspective is disease activity, reflecting the metabolic activity within specific FD lesions. In vitro evidence suggests that there is a histologic age-dependent normalization of FD lesions, in which mutated bone marrow stromal cells fail to self-renew and are consumed by apoptosis, in contrast to residual normal stem cells that survive and enable formation of histologically normal structures.⁽¹⁰⁾ No clinical studies to date have provided evidence to support this assertion. Bone turnover markers (BTMs) have been used as a surrogate marker of disease activity in FD,⁽¹¹⁾ but the natural history of changes in bone turnover have not been studied. Age-related changes in nontraditional markers of disease activity that are secreted by metabolically active FD lesions, FGF23⁽⁷⁾ and receptor activator of nuclear factor kappa-B ligand (RANKL),⁽¹²⁾ have likewise not been studied. This information is critical to provide context to the effects of any treatment in which bone turnover or other markers of disease activity are used as endpoints.

The clinical implications of elevated BTMs in patients with FD are unknown. Based on the marked increase in osteoclastogenesis and bone resorption in FD lesions,⁽¹³⁾ bisphosphonates have been commonly used in the medical treatment of this disease. Evidence to support their use originates primarily from open-label and uncontrolled studies, which have suggested potential benefits on FD-associated pain and a decrease in BTMs^(11,14–18) It has been postulated that increased bone turnover may contribute to FD-related bone pain, suggesting that elevated BTMs may be used as a marker to identify patients who may benefit from the analgesic effect of bisphosphonates.⁽¹¹⁾ However, the only placebo-controlled trial of bisphosphonates in FD did not show a significant effect on pain control or disease progression.⁽¹⁹⁾ Concerningly, reports have emerged suggesting a high prevalence of osteonecrosis of the jaw associated with bisphosphonate treatment in patients with FD.⁽²⁰⁾ Therefore, the natural history and clinical utility of BTMs in FD, as well as the role of bisphosphonate treatment, remain important unanswered questions.

The goals of this study are to describe 1) the natural history of disease activity in subjects with FD, including its association with FD-related pain; 2) the impact of bisphosphonates on the natural history of bone turnover in FD; and 3) the effect of bisphosphonates on progression of FD burden during childhood.

Materials and Methods

Subjects

Subjects were evaluated at the National Institutes of Health (NIH) Clinical Center as part of a longstanding cohort study in FD/MAS (http://clinicaltrials.gov/show/NCT00001727). The study was approved by the Institutional Review Board of the National Institute of Dental and Craniofacial Research (NIDCR), and all subjects and/or their guardians gave informed consent/assent. The diagnosis of FD/MAS was established on clinical grounds with molecular diagnosis as needed, according to previously reported guidelines.⁽²¹⁾ Associated endocrinopathies, including hypophosphatemia, were treated according to previously reported guidelines.⁽²¹⁾

Bisphosphonate treatment

Information was collected regarding bisphosphonate treatment, including formulations, doses, and dates administered. All subjects with complete data regarding bisphosphonate use were included. Subjects were treated on clinical grounds, primarily outside of NIH by local clinicians. Regimens were therefore individualized and varied for each subject. Although some subjects received fixed dosing intervals, others were infused at variable intervals, as needed to control bone pain. To evaluate overall bisphosphonate exposure, each individual’s cumulative dose was quantified according to typical treatment of osteoporosis, determined as an osteoporosis equivalent treatment year. One osteoporosis equivalent treatment year was defined as: 1) zoledronic acid (0.1 mg/ kg/yr in pediatric subjects, 4 to 5 mg/yr in adults); 2) alendronate (20 mg daily for subjects >50 kg, 10 mg for 30 to 50 kg, 5 mg for 20 to 30 kg, over a 1-year period; 3) risedronate (30 mg daily over a 1-year period); and/or 4) pamidronate (4mg/kg/yr in pediatric subjects, 60 to 90 mg/yr in adults). Clinically significant bisphosphonate exposure was defined as ≥2 osteoporosis equivalent treatment years. Subjects were excluded from the analyses if they had received bisphosphonates with an accumulated dose <2 osteoporosis equivalent treatment years, if bisphosphonate exposure could not be verified, and/or if they had received treatment with additional bone-altering therapies.

Determination of disease burden

SDBS is a quantitative measure of total skeletal FD involvement validated to predict clinical outcomes. SDBS was determined from all available ⁹⁹Tc-MDP bone scintigraphy using previously reported methodology.⁽⁵⁾ The rate of SDBS progression was determined in all subjects who had available serial bone scans performed at age ≤15 years.

Bone turnover and nontraditional markers of disease activity

All measurements of total alkaline phosphatase, osteocalcin, N-terminal telopeptide (NTx), carboxy-terminal FGF23 (C-FGF23), and RANKL in the 1998 to 2017 period were analyzed. Total alkaline phosphatase was measured by colorimetric method on a Roche Hitachi 917 or Cobas 6000 analyzer (Roche Diagnostics, Basel, Switzerland). Plasma N-MID osteocalcin was measured by electrochemiluminescence assay on a Roche Elecsys or Cobas 6000 analyzer (Roche Diagnostics). Urine NTx was measured by VITROS chemiluminescence immunoassay (Mayo Medical Laboratories, Rochester, MN, USA), and results expressed as nmol bone collagen equivalent/mmol creatinine. C-FGF23 was measured in plasma by immunometric enzyme assay (Immutopics [Quidel], San Diego, CA, USA), and RANKL was measured in plasma by MILLIPLEX MAP Human RANKL kits (EMD Millipore Corporation, Burlington, MA, USA; HBN51 K1 RANKL) following manufacturer recommendations.

Pain determination

Pain was evaluated using the NIH Functional Assessment Pain Score (NIHFA),⁽²²⁾ a four-point pain self-assessment at each visit: 4 = no pain; 3 = pain not interfering with functional activities; 2 = pain interfering with functional activities; 1 = intractable pain. When more than one pain determination was available for a subject in a specific age group, a weighted average of NIHFA scores and its concomitant BTMs values were considered for analysis.

Statistical analysis

Numerically continuous variables are reported as means (standard error [SE]), unless otherwise indicated. Analyses of BTMs in different age groups were performed based on a mixed model repeated measures analysis of covariance, in which treatment and age group are fixed effects, intercept and subject are random effects, with the SDBS as a covariate and the log of the studied BTMs as the endpoint. BTMs and other markers of disease activity were not normally distributed but were normally distributed on a logarithmic scale. For this reason, a log transformation was used for statistical analyses and summary figures. Age groups were defined to allow similar number of measurements in each group and to be clinically relevant and informative: 0 to 9 years (young children); 10 to 17 years (older children and adolescents); 18 to 30 years (young adults); >30 years (adults). SDBS was considered as a covariate given the previously reported significant positive correlation of BTMs with SDBS.⁽⁵⁾ For FGF23 and RANKL, in which only one determination was available per subject, Pearson’s correlation coefficients were calculated between age and the natural log of FGF23 and RANKL. Because SDBS has previously been shown not to be associated with pain status,⁽⁴⁾ absolute values of BTMs were used for these analyses. Unpaired t tests and Fischer’s exact tests were also performed for comparisons between treated and untreated groups. These analyses were performed using SAS software (version 9.4; SAS Institute, Inc, Cary, NC, USA). Statistical significance was a priori set at alpha = 0.05. No Bonferroni adjustments for simultaneous multiple inferences were made.

Results

Subject characteristics

Of 211 total subjects enrolled in the NIH FD/MAS cohort, 178 subjects were included in the analyses (105 subjects without any bisphosphonate use and 73 with significant bisphosphonate exposure). Thirty-three subjects were excluded from the analyses: 7 with uncertain bisphosphonate treatment records, 4 with concomitant teriparatide and/or denosumab use, and 22 with clinically insignificant bisphosphonate exposure (<2 osteoporosis equivalent treatment years) (Fig. 1). Subject characteristics, organized by groups, are included in Table 1. Subjects exposed to bisphosphonate treatment had a relatively high mean accumulated dose of 7 osteoporosis equivalent treatment years (range 2 to 27). Forty-two subjects (57%) received a single formulation (22% pamidronate, 20% zole-dronic acid, 14% alendronate, 1% risedronate), whereas 31 subjects (43%) received a combination of formulations. Compared with untreated subjects, bisphosphonate-treated subjects were older at enrollment (23 versus 17 years, p = 0.01), had a higher final SDBS (36 versus 24, p < 0.001), and had a higher prevalence of renal phosphate wasting (44 versus 16%, p < 0.001).

Fig.1.

Patient study flowchart. All subjects with complete data regarding bisphosphonate use were included. Bisphosphonate exposure was quantified as number of osteoporosis equivalents treatment years. One osteoporosis equivalent treatment year was defined as: 1) zoledronic acid (0.1 mg/kg/yr in pediatric subjects, 4 to 5 mg/yr in adults); 2) alendronate (20 mg daily for subjects >50 kg, 10 mg for 30 to 50 kg, 5 mg for 20 to 30 kg, over a 1-year period; 3) risedronate (30 mg daily over a 1-year period); and/or 4) pamidronate (4 mg/kg/yr in pediatric subjects, 60 to 90 mg/yr in adults). Significant bisphosphonate exposure was defined as ≥2 osteoporosis equivalent treatment years. SDBS = Skeletal Disease Burden Score.

Table 1.

General Subject Characteristics

General characteristics	Total	No bisphosphonates	Bisphosphonates	p Valuea
No. of subjects	178	105	73
Age (years) enrolled, mean (range)	19 (2–80)	17 (2–80)	23 (3–69)	0.01
Female, n (%)	105 (59)	66 (63)	39 (53)	NS
SDBS, mean (range)	30 (1 −75)	24 (1 −75)	36 (1 −75)	<0.001
Endocrinopathies, n (%)
Precocious puberty	80 (45)	44 (42)	36 (49)	NS
Renal phosphate wasting	49 (28)	17 (16)	32 (44)	<0.001
Hyperthyroidism	47 (26)	26 (25)	21 (29)	NS
Growth hormone excess	35 (20)	19 (18)	16 (22)	NS

SDBS = skeletal disease burden score; NS = not significant.

^ap values represent the statistical comparison between the bisphosphonate versus no bisphosphonate exposed groups.

A subgroup of 35 subjects had >1 ⁹⁹Tc-MDP bone scintigraphy study available from age ≥15 years, allowing for determination of serial SDBS during the period in which final FD burden was established. Twenty received significant exposure to bisphosphonates during this period, and 15 subjects were not treated with bisphosphonates. Subject characteristics included in this analysis are shown in Table 2.

Table 2.

General Characteristics of Subjects With Serial SDBS Before Age ≤15 Years

General characteristics	No bisphosphonates	Bisphosphonates	p Valuea
No. of subjects	15	20
Female, n (%)	10 (67)	10 (50)	NS
Age (years) at first SDBS, mean (range)	7 (3–12)	10 (4–14)	p < 0.05
Initial SDBS, mean (range)	27 (3–63)	32 (3–65)	NS
Years of follow-up mean (range) Endocrinopathies, n (%)	4.0 (1 −9)	3.6 (1 −9)	NS
Endocrinopathies, n (%)
Precocious puberty	10 (67)	11 (55)	NS
Hyperthyroidism	7 (47)	8 (40)	NS
Hypophosphatemia	4 (27)	7 (35)	NS
Growth hormone excess	4 (27)	7 (35)	NS

SDBS = skeletal disease burden score; NS = not significant.

^ap values represent the statistical comparison between the bisphosphonate versus no bisphosphonate exposed groupsTable 1: General subject characteristics

Age-related changes in markers of disease activity

Age-related changes in BTMs were evaluated in the 105 subjects who had no exposure to bisphosphonates (number of determinations: 246 for alkaline phosphatase; 108 for osteocalcin 108; 201 for NTx). Mean values of alkaline phosphatase, osteocalcin, and NTx significantly decreased with age. For alkaline phosphatase and NTx, the highest mean values occurred in the 0 to 9 years age group, with a sustained decrease in older age groups. For osteocalcin, the highest mean values occurred in the 10 to 17 years age group, with a sustained decrease in older age groups (Fig. 2).

Fig. 2.

Age-related change in bone turnover markers in subjects with fibrous dysplasia. Mean values of alkaline phosphatase (A), osteocalcin (B),and NTx (C) for each defined age group in subjects with fibrous dysplasia without exposure to bisphosphonates. Results are expressed as mean and standard error for natural logarithmic units of bone turnover markers. *p < 0.05, statistical differences referred to comparison between contiguous age groups.

C-FGF23 levels were negatively correlated with age (Pearson correlation coefficient = −0.46, p < 0.001, n = 57) and plasma levels of RANKL were negatively correlated with age in untreated adult subjects (Pearson correlation coefficient −0.50, p = 0.02, n = 20) (Fig. 3).

Fig. 3.

Age-related changes in FGF23 and RANKL in subjects with fibrous dysplasia. (A) C-terminal FGF23 levels (n = 57) were negatively correlated with age in subjects with fibrous dysplasia. (B) RANKL levels (n = 20) were negatively correlated with age in untreated adult subjects with fibrous dysplasia. Results are expressed in natural logarithmic units. Dashed lines represent upper limit of normal (ULN) in the general population expressed in natural logarithmic units. ULN for C-FGF23 as defined by manufacturer. ULN for RANKL defined in 24 healthy volunteers. r = Pearson correlation coefficient.

Despite this progressive decline in BTMs, most subjects persisted with values that were higher than their age-specific normal range. For example, alkaline phosphatase remained elevated in 79% of FD subjects aged 18 to 29 years, and 60% remained above the upper limit of normal for subjects 30 years or older.

Effect of bisphosphonate use on age-related changes in BTMs

All BTMs measured in the 73 subjects with clinically significant bisphosphonate exposure were evaluated (number of determinations: 340 for alkaline phosphatase; 73 for osteocalcin; 171 for NTx). Alkaline phosphatase, osteocalcin, and NTx in bisphosph-onate-exposed subjects decreased with age in a similar pattern to untreated subjects. When comparing BTMs between treated and untreated groups, we observed similar mean values of alkaline phosphatase and osteocalcin in each age group, except for alkaline phosphatase in the >30 years age group, where treated subjects had higher values (p < 0.05). NTx levels were lower in bisphosphonate-exposed subjects in the 0 to 9 and 10 to 17 years age groups (p < 0.05), whereas mean values in subjects >18 years were not significantly different (Fig. 4). In addition, we analyzed 32 subjects for whom we had alkaline phosphatase values just before and 1 and 2 years after starting bisphosphonate treatment. We observed an 18% mean reduction in the first year of treatment, but only 8% of them had normalized alkaline phosphatase values for their age-specific normal range. After 2 years of treatment, 83% of subjects persisted with elevated values of alkaline phosphatase.

Fig. 4.

Age-related changes in bone turnover markers in subjects treated and untreated with bisphosphonates. Mean values of alkaline phosphatase (A), osteocalcin (B), and NTx (C) for each defined age group in subjects with fibrous dysplasia with and without exposure to bisphosphonates. Results are expressed as mean and standard error for natural logarithmic units of bone turnover markers. *p < 0.05, statistical differences referred to the comparison of bisphosphonate and no-bisphosphonate exposed subjects of the same age group.

Effect of bisphosphonate use on FD disease burden progression

SDBS increased over time in both the untreated and treated subjects ≤15 years old, and the increase in the bisphosphonate-treated group was not different from the untreated group. The absolute yearly mean increase in SDBS was 3.1 for untreated and 2.4 for treated subjects (p = 0.37), with an 18%-year SDBS increase for treated subjects versus 14% for untreated subjects (p = 0.58) (Fig. 5).

Fig. 5.

Rate of fibrous dysplasia disease burden progression according to bisphosphonate exposure. (A) Absolute rate of Skeletal Disease Burden Score (SDBS) progression in treated versus untreated subjects with fibrous dysplasia age ≤15 years. (B) Relative rate of SDBS progression in treated versus untreated subjects with fibrous dysplasia age ≤15 years. Results expressed as mean and standard error.

Age-related changes in pain and relation with bone turnover

Sixty-one pediatric subjects (< 18 years old) and 64 adults had at least one pain determination. The presence of pain, defined as a NIHFA score <4, was significantly more frequent in adult subjects compared with pediatric subjects (68.7% versus 27.8%, p < 0.001). Additionally, pain with an intensity that interfered with functional activities (NIHFA score 2) was also more frequent in adult subjects compared with pediatric subjects (25% versus 3.2%, p < 0.001). Of those subjects who had >1 pain determination (n = 61), 56% reported stable pain scores, while 38% reported an increase in pain intensity. Mean values of alkaline phosphatase, osteocalcin, and NTx were similar in subjects (both pediatric and adult) that had pain (NIHFA score <4) compared with those who did not have pain (NIHFA score 4) (Fig. 6).

Fig. 6.

Bone turnover markers in pediatric and adult fibrous dysplasia subjects according to pain status. Values of alkaline phosphatase (A), osteocalcin (B), and NTx (C) in pediatric and adults subjects without pain (NIHFA 4) compared with subjects who presented with any intensity of pain at time of evaluation (NIHFA <4). Bars show median values, boxes show interquartile range, and whiskers show range.

Discussion

This large longitudinal series provides the first clinical evidence to support an age-dependent decline in bone turnover and other markers of disease activity in FD. Pain, in contrast, is more frequent and severe in adults with FD and is not related to changes in bone turnover. Bisphosphonate treatment does not significantly impact this age-dependent decrease in bone turnover, nor does it prevent the progression of FD disease burden in children.

This age-related decrease in BTMs in patients with FD is distinct from the natural history of BTM changes in healthy individuals. In the general population, BTMs peak at ages 11 to 13 years, and then decline toward adult levels in late puberty. Levels remain stable throughout adulthood, with a mild increase in postmenopausal women and older men.^(23–25) In contrast, patients with FD have a persistent and progressive decline in BTMs with age, without the described postpubertal stabilization or late-life increase in turnover. This difference is likely related to the abnormal bone formation and resorption that takes place within FD lesions. Dysplastic woven bone is formed and resorbed, elevating BTMs, FGF23, and RANKL, resulting in marrow fibrosis and mechanically unsound bone.^(7,12,13) This age-related decline in BTMs, FGF23, and RANKL is consistent with previous in vitro findings that describe an age-dependent demise of GNAS mutation-bearing bone marrow stromal cells and histological “normalization” of FD lesions.⁽¹⁰⁾

Delineating the age-related decline in bone turnover in FD has several important clinical implications. Together with the previously reported improvement in histologic changes,⁽¹⁰⁾ these findings support a model in which the metabolic activity of FD lesions decreases over time, potentially due to a shortened life span of mutation-bearing bone marrow stromal cells. A potential therapeutic strategy, for example, may be to harness this by promoting the apoptosis of mutated stromal cells, in order to “burn out” FD lesions and decrease the period of time during which they expand. It also establishes a critically important potential pitfall in evaluating clinical studies that use BTMs as surrogate endpoints. Because BTMs decline naturally over time, in the absence of a control group, longitudinal studies are likely to attribute or overestimate the effects of interventions on bone turnover. These age-related changes must therefore be taken into account in any study utilizing BTMs as surrogate outcome measures. This is particularly important when evaluating the effects of bisphosphonates. BTMs have been frequently used as surrogate endpoints in studies of bisphosphonate efficacy in FD,^{(11,15,18,26)} the majority of which were uncontrolled. The only placebo-controlled trial of bisphosphonates in the treatment of FD showed a decline in NTx in the treated group but no difference in osteocalcin between groups.⁽¹⁹⁾ These findings are consistent with the current retrospective analysis, where we observed lower mean values of NTx in the treated subjects, specifically in the age <18 years subgroup. This is consistent with the more potent effects of bisphosphonates in children, who have higher levels of osteoclast activity related to skeletal growth.

Bisphosphonate treatment does not significantly impact the age-dependent decrease in bone turnover, nor does it prevent the progression of FD disease burden in children. These findings likely reflect the limited activity of bisphosphonates on FD tissue. Plotkin and colleagues reported that histomorphometric findings in FD tissue of patients receiving pamidronate were similar to untreated patients.⁽¹⁷⁾ Corsi and colleagues recently reported the case of a child with FD treated with pamidronate, in whom radiographic and histologic evidence of bisphosphonate exposure was only present in normal bone and was absent within FD lesions, which continued to expand despite treatment.⁽²⁷⁾ It is possible that the decrease in BTMs observed with bisphosphonate treatment in subjects with FD is explained by a combination of the effect of the drug on healthy bone and the age-related decrease in FD activity. These findings, in addition to the fact that 5 of the 73 subjects (6.8%) with significant exposure developed osteonecrosis of the jaw as a complication of bisphosphonate treatment,⁽²⁰⁾ highlights the need for caution in prescribing bisphosphonates in patients with fibrous dysplasia.

Consistent with previous studies,^(4,28) we observed that pain is more frequent and severe in adults compared with pediatric subjects with FD. A novel finding from this study was that BTM levels did not vary according to the presence or absence of pain. We observed a wide range in BTM values in subjects who endorsed pain, from extremely elevated to well within normal range (for example, alkaline phosphatase in adults with pain ranged from 57 to 1107 U/L). Likewise, subjects who lacked pain also presented with a wide range of BTMs (alkaline phosphatase in adults without pain ranged from 42 to 1493 U/L). These findings, in addition to the inconsistent response of pain to bisphosphonates, support the concept that the pathophysiology of pain in FD is complex⁽²⁹⁾ and does not appear to be entirely related to bone turnover or disease activity.

Strengths of this study include the long-term follow-up and detailed clinical description of both treated and untreated subjects. This is the largest and longest-term study of FD/MAS to date. Limitations include the retrospective nature of the study design, which resulted in significant variability in bisphosphonate dosing. Although a variety of regimens were used, most subjects received a relatively high cumulative exposure. A definitive answer to the question of the utility of bisphosph-onates in FD would require a large, long-term, prospective, controlled study, which may not be feasible given the rarity of the disease. Finally, subjects were evaluated at a tertiary referral center, which likely resulted in selection bias toward more severely affected patients. This is evidenced by the fact that only 9.5% of our cohort presented with a monostotic form of the disease. This is relevant when considering the applicability of these results to less severely affected patients. Additional studies are needed to describe age-related changes of FD in mildly affected subjects, as well as to define potential differences according to regional complications of FD. Longitudinal studies of functional status and quality of life are also needed to understand age-related changes in the clinical features of FD.

In summary, FD is associated with an age-dependent decline in bone turnover and other markers of disease activity. Pain, in contrast, is more frequent and severe in adults with FD and is not related to significant changes in bone turnover. Bisphosphonate treatment does not significantly impact the age-dependent decrease in bone turnover, nor does it prevent the progression of FD disease burden in children. These findings, in association with established side effects of antiresorptive treatment, should be considered when evaluating use and response of bisphosphonates and other clinical interventions in FD.