Abstract
The microstructural skeletal phenotype of Hypoparathyroidism (HypoPT), a disorder of inadequate parathyroid hormone secretion, is altered trabecular microarchitecture with increased trabecular bone volume and thickness. Using 2-D histomorphometric analysis, we previously found that 2 years of PTH(1–84) in HypoPT is associated with reduced trabecular thickness (Tb.Th) and an increase in trabecular number (Tb.N). We have now utilized direct 3-D microstructural analysis to determine the extent to which these changes may be related to bone strength. Iliac crest bone biopsies from HypoPT subjects (n=58) were analyzed by microcomputed tomography (µCT) and by microfinite element (µFE) analysis. Biopsies were performed at baseline and at 1 or at 2 years of recombinant human PTH(1–84) [rhPTH(1–84)]. In a subset of subjects (n=13) at 3 months, we demonstrated: a reduction in trabecular separation (Tb.Sp, 0.64 ± 0.1 to 0.56 ± 0.1 mm; p=0.005) and in the variance of trabecular separation (Tb.SD, 0.19 ± 0.1 to 0.17 ± 0.1 mm; p=0.01), along with an increase in bone volume/total volume (BV/TV, 26.76±10.1 to 32.83±13.5%; p=0.02), bone surface/total volume (BS/TV, 3.85 ± 0.7 to 4.49 ±1.0 mm2/mm3; p=0.005), Tb.N (1.84 ± 0.5 vs 2.36 ± 1.3 mm−1; p=0.02) and Young’s modulus (649.38±460.7 to 1044.81±1090.5 N/mm2; p=0.049). After 1 year of rhPTH(1–84), Force increased (144.08±102.4 to 241.13±189.1 N; p=0.04) and Young’s modulus tended to increase (662.15±478.2 to 1050.80 ±824.1 N/m2; p=0.06). The 1 year change in cancellous mineralizing surface (MS/BS) predicted 1 year changes in µCT variables. The biopsies obtained after 2 years of rhPTH(1–84) showed no change from baseline. These data suggest that administration of rhPTH(1–84) in HypoPT is associated with transient changes in key parameters associated with bone strength. The results indicate that rhPTH(1–84) improves skeletal quality in HypoPT early in treatment.
Keywords: Hypoparathyroidism, PTH(1–84), microcomputed tomography, finite element analysis, bone microarchitecture
Introduction
Hypoparathyroidism, a disorder characterized by deficient parathyroid hormone (PTH) and hypocalcemia, is associated with abnormal structural and dynamic skeletal parameters1,2. Areal bone mineral density (BMD) is often above average3–5. Structural abnormalities by histomorphometric analysis of iliac crest bone biopsies show increased cortical and trabecular width and cancellous bone volume2 as well as markedly reduced bone turnover1,2.
Recombinant human PTH(1–84) [rhPTH(1–84)] has recently been approved in the United States as a therapy for hypoparathyroidism6, but its three dimensional microstructure and bone strength effects have not been fully characterized. Using conventional 2-D histomorphometric analysis of transiliac bone biopsies, we previously found that rhPTH(1–84) in hypoparathyroidism is associated with a decrease in trabecular thickness (Tb.Th) at 1 year and an increase in trabecular number (Tb.N) at 1 and 2 years and in cortical porosity at 2 years1. However, conventional 2-dimensional histomorphometry is inherently limited when it is used to assess skeletal microarchitecture7. Specifically, the primary morphometric data are obtained on a relatively small number of sections, which represent only a fraction of the biopsy volume. Moreover, 3-dimensional indices have to be extrapolated from primary measurements made in 2 dimensions, introducing an element of uncertainty.
Microcomputed tomography (microCT) of bone biopsy samples overcomes these limitations by allowing direct 3-dimensional analysis of the entire biopsy specimen. In addition, microCT permits the application of microfinite element (µFE) analysis, a computational modeling technique which is generally accepted as a surrogate marker of bone strength8–10. Calculation from bone biopsy samples of Young’s modulus, an estimate of trabecular bone strength, can provide biomechanical information11,12 about rhPTH(1–84) in this disease. This information is valuable because prospective data on fracture incidence with rhPTH(1–84) in hypoparathyroidism are not available.
The purpose of this study was to apply 3-D morphometric and µFE analyses to our longitudinal rhPTH(1–84)-treated hypoparathyroid bone biopsy samples that were previously analyzed by conventional histomorphometry1 so as to provide more direct data on structural and biomechanical measures than are achievable by a 2-dimensional approach alone.
Materials and Methods
Subjects
The diagnosis of hypoparathyroidism in women and men was established by undetectable or insufficient PTH concentration in association with hypocalcemia and an absolute need for supplemental calcium and/or active vitamin D. Hypoparathyroidism was reported to have been present for at least 1 year. Subjects were excluded if they had ever been treated with PTH(1–34) or PTH(1–84). Patients were recruited from the Metabolic Bone Diseases Unit of Columbia University Medical Center (CUMC) and from the Hypoparathyroidism Association. The study was approved by the Institutional Review Board of CUMC. All subjects gave written informed consent.
Protocol
Hypoparathyroid subjects self-administered rhPTH(1–84), provided by NPS Pharmaceuticals (Bedminster, New Jersey), for 24 months at a subcutaneous dose of 100 µg every other day. This dose was selected because we showed previously that this regimen restores suppressed bone turnover markers in hypoparathyroidism to levels that are in the normal range13. Subjects were randomly assigned to 1 of 3 bone biopsy schedules: 1) paired biopsies with tetracycline double-labeling obtained at baseline and after 12 months of rhPTH(1–84) treatment; 2) paired biopsies with tetracycline double-labeling obtained at baseline and after 24 months of rhPTH(1–84) treatment; 3) one biopsy with a quadruple-label protocol after 3 months of rhPTH(1–84). In the quadruple-label protocol, 2 sets of tetracycline labels were sequentially administered, before rhPTH(1–84) initiation and 2 weeks prior to the biopsy which was obtained after 3 months exposure to rhPTH(1–84). This method permits a comparison of dynamic formation indices at baseline and after PTH(1–84) by measuring characteristics of each set of double labels separately and then comparing them with each other14.
Microcomputed tomography
Prior to embedding for histomorphometric analysis, which was performed as previously described1, samples were analyzed by high-resolution microCT. The microtomographic imaging system (µCT 40, Scanco Medical AG, Brüttisellen, Switzerland) was equipped with a 5 µm focal spot X-ray tube as a source. A two-dimensional charge-coupled device (CCD), coupled to a thin scintillator as a detector permitted acquisition of 210 tomographic images in parallel. The long axis of the intact biopsy was oriented along the rotation axis of the scanner. The X-ray tube was operated at 50 kVp and 160 µA with an integration time set to 200 ms and all projection frames were recorded 6 times and then averaged. Scans were performed at an isotropic, nominal resolution of 8 µm (high resolution mode).
The whole intact biopsy was scanned, which resulted in an average scan height of 10 mm and a measurement time of approximately 10 hours. A cylindrical volume of interest was then placed in the digital image data to select the trabecular bone compartment. The mineralized tissue was segmented from soft tissue by a global thresholding procedure15, with a threshold value set to 34% of the maximum grayscale value. 3-D morphometric analysis was as previously described16.
We used µFE of the microCT images to calculate the apparent Young’s modulus, an index of resistance to compressive forces (strength or mechanical competence), and Force, the maximum load sustained, in the long axis (transverse direction) of the biopsies. A rectangular volume of interest of 640×640×300 voxels, corresponding to 5.12×5.12×2.40 mm3, was isolated in the center of the biopsy image. Images were then converted to finite-element models. The images of VOIs were processed to construct micro-finite-element models by converting the bone voxels to eight-node brick elements. µFE was restricted to geometric and material linearity. A single load case was applied, representing uniaxial compression along the long axis of the specimen; for the µFE models, this is the direction of highest stiffness. For the solution of the resulting systems, a custom in-house parallel conjugate gradient finite-element solver with multilevel preconditioning17 was used.
Statistical analysis
Data are expressed as mean ± SD. Because the measures were not all normally distributed, Wilcoxon test for two independent samples was used to compare all of the baseline values (n=45) to the 3 months values (n=13). Signed rank test was used for the paired 3 month biopsy comparison for MS/BS, because baseline and 3-month data on the same patients were available for this parameter. Wilcoxon test for two independent samples was used to compare the 1 year measures (n=14) with baseline values of those not in the 1 year group (n=31), and to compare the 2 year measures (n=16) with baseline values of those not in the 2 year group (n=29). Correlation analyses were performed using Spearman rank order correlation. A p-value <0.05 was considered significant. All statistical analyses were performed using SAS for Windows (version 9.4; SAS, Cary, NC).
Results
Subjects
The characteristics of the 58 hypoparathyroid subjects are presented in Table 1. All subjects were receiving calcium and vitamin D, and on this replacement therapy most had normal calcium values; a few subjects had serum calcium levels below the lower limits of normal but within a generally acceptable range for this population. The 22 idiopathic patients had a diagnosis of hypoparathyroidism for 20 ± 13 yrs. Of the idiopathic patients, four (18%) had a prior kidney stone; six (27%) had a prior fracture (2 hand, 2 metatarsal, 1 heel, 1 tibia), and 2 (9%) had prior basal ganglia calcifications. There were no differences in the baseline characteristics of the three cohorts who had sampling at different times after rhPTH(1–84) administration.
Table 1.
Baseline Characteristics of Hypoparathyroid Population
| N=58 a | Normal Range | |
|---|---|---|
| Age (yrs) | 46 (38; 59) | |
| Sex | Male: 16 Female: 42 (Premenopausal: 26 Postmenopausal: 16) |
|
| Etiology | Postoperative: 34 Idiopathic: 22 DiGeorge: 2 |
|
| Duration of hypoparathyroidism (yrs) | 10 (5; 22) | |
| Number of patients with prior fractures (%) | 13 (22%)b | |
| Calcium supplement dose (g/d) | 2400 (1500; 3300) | |
| Calcitriol supplement dose (mcg/d) | 0.50 (0.375; 1.00) | |
| Daily vitamin D dose (IU) n=23 | 1000 (400; 14286) | |
| Thiazide dose (mg) n=17 | 25 (12.5; 25) | |
| Serum calcium (mg/dL) | 8.6 (8.0; 9.2)c | 8.5–10.4 |
| PTH (pg/ml) | 2.0 (2.0; 2.0) | 10–65 |
| Phosphate (mg/dL) | 4.4 (3.8; 4.9) | 2.5–4.5 |
| Total alkaline phosphatase activity (IU/L) | 62 (52; 75) | 33–96 |
| Bone specific alkaline phosphatase (µg/L) | 23.4 (18.6; 31.3) | ≤ 22 |
| Urinary calcium excretion (mg/d) | 217 (141; 301) | |
| 25-hydroxyvitamin D (ng/ml) | 32.5 (21.3; 46.6) | 9–52 |
| 1,25-dihydroxyvitamin D (pg/ml) | 30.6 (22.4; 42.3) | 15–60 |
| TSH (mIU/L) | 1.38 (0.04; 2.17) | 0.40–4.50 |
Data are median (25th; 75th percentiles)
Types of fractures: tibia, wrist, ankle, digits, hand, metatarsal, heel, collar bone
The range of serum calcium is as expected for a hypoparathyroid cohort being treated with calcium and active vitamin D supplementation
Bone remodeling
Cancellous histomorphometric mineralizing surface (MS/BS) increased significantly at 3 months and 1 year, but was not different from baseline at 2 years (Table 2).
Table 2.
Changes in Biopsies with 3 Months, 1 Year or 2 Years of PTH(1–84)
| Baseline to 3 Months | Baseline to 1 Year | Baseline to 2 Years | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Baseline (n= 45) |
3 months (n=13) |
p | Baseline (n=31) |
1 Year (n=14) | p | Baseline (n=29) |
2 Years (n=16) |
p | |
| MS/BS (%) | 0.86±1.7 | 6.58±6.5 | 0.0002* | 1.93±3.3 | 6.73 ±6.5 | 0.001 | 3.80±5.5 | 2.59±3.7 | 0.30 |
| BV/TV (%) | 26.76±10.1 | 32.83±13.5 | 0.02 | 26.17±10.1 | 30.63±13.7 | 0.13 | 27.54±12.7 | 27.58±9.0 | 0.38 |
|
BS/TV (mm2/mm3) |
3.85±0.7 | 4.49±1.0 | 0.005 | 3.94±0.8 | 4.67±2.0 | 0.09 | 4.19±1.5 | 4.11±1.0 | 0.42 |
|
BS/BV (mm2/mm3) |
15.69±4.2 | 14.28±2.1 | 0.06 | 16.25±3.9 | 15.90±3.3 | 0.29 | 16.52±4.2 | 15.46±2.5 | 0.32 |
| Tb.Th (mm) | 0.19±0.1 | 0.20±0.1 | 0.06 | 0.18±0.1 | 0.18±0.1 | 0.33 | 0.18±0.1 | 0.18±0.1 | 0.32 |
| Tb.Sp (mm) | 0.64±0.1 | 0.56±0.1 | 0.005 | 0.65±0.1 | 0.62±0.2 | 0.18 | 0.64±0.1 | 0.65±0.1 | 0.34 |
|
Tb. Sp.SD (mm) |
0.19±0.1 | 0.17 ±0.1 | 0.01 | 0.20±0.1 | 0.21±0.1 | 0.15 | 0.20±0.1 | 0.22 ±0.1 | 0.21 |
| Tb.N (mm−1) | 1.84±0.5 | 2.36±1.3 | 0.02 | 1.82±0.5 | 2.22±1.1 | 0.17 | 1.99±0.8 | 1.87±0.4 | 0.39 |
| SMI (1) | −0.64±2.2 | −1.22±3.6 | 0.11 | −0.50±1.7 | −1.51±2.7 | 0.10 | −0.85±2.2 | −0.74±1.7 | 0.30 |
|
Conn.D (/mm3) |
17.75±26.2 | 103.83±318.2 | 0.06 | 20.27±29.2 | 164.36±501.6 | 0.17 | 86.55±350.4 | 26.21±37.1 | 0.33 |
| Force (N) | 153.27±107.7 | 227.78±254.2 | 0.11 | 144.08±102.4 | 241.13±189.1 | 0.04 | 175.03±156.1 | 172.65±115.0 | 0.35 |
|
Young’s modulus (N/mm2) |
649.38±460.7 | 1044.81±1090.5 | 0.049 | 662.15±478.2 | 1050.80 ±824.1 | 0.06 | 788.68±692.6 | 772.31±510.8 | 0.34 |
Mean±SD
MS/BS is mineralizing surface; BV/TV is bone volume fraction; BS/TV is bone surface density; BS/BV is bone surface to volume ratio; Tb.Th is trabecular thickness; Tb.Sp is trabecular separation; Tb.SD is trabecular heterogeneity; Tb.N is trabecular number; SMI is Structure Model Index; ConnD is connectivity density.
Signed rank test for paired 3 month biopsy comparison only
Primary indices
Bone volume fraction (BV/TV) and bone surface density (BS/TV) increased at 3 monthsand BS/TV tended to remain increased at 1 year (Table 2). There were no changes at 2 years. When analyzed by etiology of hypoparathyroidism, postoperative subjects at 3 months had a decrease in BS/BV, while at 1 year the idiopathic subjects had an increase in BS/TV.
Directly assessed indices
At 3 months there was a decrease in trabecular separation (Tb.Sp) and Tb.Sp heterogeneity (Tb.Sp.SD), with an increase in trabecular number (Tb.N; Table 2). There were no changes at 1 and 2 years.
Directly assessed nonmetric indices
ConnD tended to increase at 3 months (Table 2), while the Structure Model Index (SMI), a parameter that assesses the plate-rod characteristic in trabecular bone, remained abnormally low. This parameter typically ranges from 0 (most plate-like) to 3 (most rod-like). When analyzed by etiology of hypoparathyroidism, in the postoperative patients alone there was an increase in SMI from baseline to 3 months (p=0.03). There were no changes at 2 years.
µFE analysis
Force increased at 1 year and Young’s modulus tended to increase at 3 months and 1 year (Table 2). Representative microCT images are shown in a subject who had the mean change of the cohort in Young’s modulus with one year of rhPTH(1–84) treatment (Figure 1). There were no changes at 2 years.
Figure 1.
3-D µCT images of iliac crest biopsies in a representative hypoparathyroid subject (53 year old male) before (left) and after 1 year of rhPTH(1–84) treatment (right). The patient shown had the mean change in Young’s modulus of the cohort with one year of rhPTH(1–84) treatment.
Relationship of microcomputed tomography and histomorphometric bone formation
Correlation analyses between the changes (differences from baseline) of the paired microCT outcomes versus the changes (differences from baseline) of paired cancellous histomorphometric mineralizing surface (MS/BS) revealed significant relationships at 1 year. The difference in MS/BS was directly associated with the differences in BV/TV, BS/TV, Tb.N and ConnD and inversely associated with the differences in Tb.Sp, with a trend toward an inverse association with SMI (Table 3). There was no relationship between changes in MS/BS and changes in microCT variables at 2 years.
Table 3.
Relationship Between Changes in microCT Variables and Histomorphometric Cancellous Bone Formation (MS/BS)
| Change in microCT variable |
Predicted by change in MS/BS (%) |
p | |
|---|---|---|---|
|
1yr PTH(1–84) treatment |
BV/TV (%) | 0.62 | 0.03 |
| BS/TV (mm2/mm3) | 0.70 | 0.01 | |
| BS/BV (mm2/mm3) | −0.43 | 0.17 | |
| Tb.Th (mm) | 0.23 | 0.47 | |
| Tb.Sp (mm) | −0.68 | 0.01 | |
| Tb. SD (1) | 0.31 | 0.36 | |
| Tb.N (mm−1) | 0.69 | 0.01 | |
| SMI (1) | −0.57 | 0.05 | |
| Conn.D /mm3 | 0.68 | 0.02 | |
| Force (N) | 0.37 | 0.29 | |
| Young’s modulus (N/mm2) |
0.39 | 0.26 | |
|
2yr PTH(1–84) treatment |
BV/TV (%) | 0.23 | 0.42 |
| BS/TV (mm2/mm3) | 0.24 | 0.40 | |
| BS/BV (mm2/mm3) | −0.28 | 0.31 | |
| Tb.Th (mm) | 0.12 | 0.66 | |
| Tb.Sp (mm) | −0.29 | 0.30 | |
| Tb. SD (1) | −0.02 | 0.94 | |
| Tb.N (mm−1) | 0.28 | 0.30 | |
| SMI (1) | −0.36 | 0.18 | |
| Conn.D (/mm3) | 0.22 | 0.43 | |
| Force (N) | 0.39 | 0.19 | |
| Young’s modulus (N/mm2) |
0.40 | 0.18 |
Relationship of microcomputed tomography and histomorphometric variables of bone structure
Variables of cancellous bone structure in the hypoparathyroid subjects assessed by microCT were significantly correlated with those assessed by conventional histomorphometry at 1 year and at 2 years of rhPTH(1–84) treatment (Figure 2).
Figure 2.
Correlation between 2-dimensional and 3-dimensional variables of cancellous bone structure in subjects with hypoparathyroidism at 1 year of rhPTH(1–84) treatment (n=14) and at 2 years of rhPTH(1–84) treatment (n=16).
Discussion
These data show that PTH exposure to the hypoparathyroid skeleton has effects on 3-D cancellous bone microarchitecture and µFE early in treatment. Our prior 2-D quantitative histomorphometry data of rhPTH(1–84) administration in hypoparathyroidism provided detailed information about bone microarchitecture1, but certain indices of trabecular microarchitecture were indirectly derived, with the assumption of a fixed structural model18. Because trabecular bone might have a variable structure, extrapolation of 2-D analyses to 3-D quantities is fraught with uncertainties of representativeness7. The application of microCT overcame this limitation because the analysis is direct, volumetric, encompasses the entire specimen and does not rest upon assumptions of the underlying trabecular plate- or rod-like nature. Moreover, µFE analysis provides a measure of how much force is needed to compress the bone19. Using direct 3-D microcomputed analysis with µFE, we now extend our previous 2-D findings, demonstrating that administration of rhPTH(1–84) in hypoparathyroidism is associated with early but transient increases in trabecular bone strength.
We previously performed microCT analysis of our baseline hypoparathyroid samples prior to rhPTH(1–84) treatment16. Results from that study showed increased cancellous bone volume, trabecular thickness, number and connectivity in comparison with matched control subjects. In addition, the structural model index was lower in hypoparathyroidism, indicating that the trabecular structure was more plate-like than rod-like16. In the current study, the early changes with rhPTH(1–84) administration, namely increases in bone volume, bone surface and trabecular number with a decrease in trabecular separation, are consistent with the occurrence of trabecular tunnelling resorption. This occurs when the trabecula is split longitudinally into two thinner trabeculae by basic multicellular unit (BMU)-based bone remodeling20. Tunneling gives rise to smaller spaces between trabeculae and thus the increased the heterogeneity we observed at 3 months. Trabecular tunnelling would also explain the increase in spine BMD that we previously reported13, with more numerous trabeculae being detected as greater BMD by densitometry21. Interestingly, there were no changes in the estimation of the plate-rod characteristic (SMI). The absence of difference in the structure model index with rhPTH (1–84) treatment indicates that the plate-like structure characteristics of the disease were maintained during rhPTH(1–84) exposure. Postoperative subjects in particular might have been expected to respond differently to rhPTH(1–84) than the others since their growth and development would have been unaffected by the hypoparathyroidism. Although we saw some differences in the postoperative subjects, including perhaps more changes at 3 months, the small numbers limit conclusions regarding differing effects of etiology.
With regard to bone strength, the µFE measurements show an early but transient improvement in biomechanical indices. This suggests that with rhPTH(1–84) treatment there is an early improvement in the ability of the bone to withstand changes in length when under lengthwise tension or compression. This might be partially due to early changes that we observed in mineralization density. Using quantitative backscattered electron imaging (qBEI), we found a decrease in the weighted mean Ca concentration of the bone area (CaMean) in cancellous and cortical bone after 1year of rhPTH(1–84)22. This transient decrease in mineralization density perhaps allows the bone to deform and thus absorb and dissipate energy23, making it more structurally sound19. Apart from changes in mineralization, bone strength could potentially also be affected by altered cross linking of bone matrix. Collagen crosslinks increase collagen fibril stiffness and contribute to increased tissue strength24. Future studies will need to take this potential confounder into account.
We found that certain microCT changes in the paired biopsies at 1year correlated with changes in cancellous histomorphometric bone formation (MS/BS) during treatment, including a positive correlation with bone volume, bone surface density trabecular number and connectivity density and a negative correlation with trabecular separation. This is consistent with the time course of the known dynamic changes with rhPTH(1–84) administration to hypoparathyroid patients. With rhPTH(1–84) treatment, we previously found an initially dramatic change in remodeling1, perhaps reflecting a heightened state of PTH responsiveness, which became tempered during the second year. Similarly, with microCT analysis, at 2 years there was no difference from baseline in any of the parameters and there was no relationship between the change in MS/BS and in the microCT parameters. In the initial period, bone turnover rate was increased for approximately 9–12 months, but then fell during the subsequent second year1. It is thus possible that the microCT parameters at 2 years represent an overall composite of changes throughout the two years of treatment.
The clinical consequences of our findings may relate to bone fragility with rhPTH(1–84) treatment of hypoparathyroidism. Hypoparathyroidism is known to be protective against age-related bone loss25 but whether fracture risk is altered is unclear. It should be emphasized that very little is known about the risk of fracture in hypoparathyroid patients. In recent registry studies in Denmark, fracture risk was not increased in patients with post-surgical hypoparathyroidism26, although fractures at the upper extremity were increased in non-surgical hypoparathyroid patients27. With such a rare disorder and limiting numbers of subjects, it is not possible to address the question of fracture risk after rhPTH(1–84) treatment. Nevertheless, the improvements in Young’s modulus after one year with rhPTH(1–84) would be consistent with improved structural integrity. Recent data suggest that continuous administration of PTH in hypoparathyroidism via pump delivery may have enhanced physiological benefits, with normalization of bone turnover markers28. It is possible, although as yet unproven, that early increases in bone strength with PTH would persist for longer with continuous PTH exposure.
A major strength of this study is that it is an unusually large histomorphometric study of rhPTH therapy in hypoparathyroidism. A related limitation is that, by necessity, the 3 month quadruple-label biopsies and the 1 year paired and 2 year paired biopsies were each performed in three different groups of subjects, since each subject can only undergo a maximum of two biopsies. The random assignment to the different biopsy groups was designed to address this point, and, in fact, there were no apparent differences in the three cohorts who had sampling at different times after PTH administration. Although we did not have a placebo group who underwent biopsies over time, the ethics of which could be questioned, it nevertheless appears all but certain that the observed changes are attributable to rhPTH(1–84) administration.
An important limitation is the fact that cortical bone was left out of the analysis. PTH induced increases in cortical porosity and subendosteal resorption might potentially offset some of the early improvements in bone strength. Future studies of µFE with PTH use in hypoparathyroidism will need to include cortical regions. An additional limitation is that assumptions of uniform bone mineralization are incorporated into µFE analyses, which could affect results in the different time points of rhPTH(1–84) treatment that were studied. This concern is underscored by our recent report that bone mineralization density distribution varied in these samples at different time points of rhPTH(1–84) treatment22. However, agreement between identical variables measured by 2-D histomorphometry or 3-D microCT was excellent. The strong correlations we observed between these methodologies are consistent with previous studies in our laboratories and those of other investigators conducted in populations across a wide range of ages and diverse clinical diagnoses7,29–31.
We conclude that PTH exposure to the hypoparathyroid skeleton has effects on 3-D cancellous bone microarchitecture and µFE early in treatment. Further studies are necessary to explore to what extent these microCT changes affect skeletal strength and fracture incidence with rhPTH(1–84) treatment of hypoparathyroidism.
Acknowledgments
Dr. Bilezikian is a consultant for Amgen, Radius, and Merck. Dr. Bilezikian and Dr. Rubin receive research support from NPS/Shire Pharma.
Funding source: NIH DK069350, NPS/Shire Pharma
Footnotes
Disclosures: No conflicts of interest reported for the other authors.
Authors’ roles: Study design: MRR, DWD and JPB. Study conduct: MRR, NEC and JPB. Data collection: AZ, DWD, HZ and RM. Data analysis: CZ and MRR. Data interpretation: MRR, DWD, RM and JPB. Drafting manuscript: MRR. Revising manuscript content: MRR. Approving final version of manuscript: MRR, AZ, DWD, HZ, NEC, CZ, RM and JPB. MRR takes responsibility for the integrity of the data analysis.
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