JBMR
ORIGINAL ARTICLE
In Vivo Assessment of Bone Quality in Postmenopausal Women With Type 2 Diabetes Joshua N Farr,1 Matthew T Drake,1 Shreyasee Amin,2 L Joseph Melton III,3 Louise K McCready,1 and Sundeep Khosla1 1
Department of Medicine, Division of Endocrinology, Mayo Clinic College of Medicine, Rochester, MN, USA Department of Medicine, Division of Rheumatology, Mayo Clinic College of Medicine, Rochester, MN, USA 3 Department of Health Sciences Research, Division of Epidemiology, Mayo Clinic College of Medicine, Rochester, MN, USA 2
ABSTRACT Although patients with type 2 diabetes (T2D) are at significant risk for well‐recognized diabetic complications, including macrovascular disease, retinopathy, nephropathy, and neuropathy, it is also clear that T2D patients are at increased risk for fragility fractures. Furthermore, fragility fractures in patients with T2D occur at higher bone mineral density (BMD) values compared to nondiabetic controls, suggesting abnormalities in bone material strength (BMS) and/or bone microarchitecture (bone “quality”). Thus, we performed in vivo microindentation testing of the tibia to directly measure BMS in 60 postmenopausal women (age range, 50–80 years) including 30 patients diagnosed with T2D for >10 years and 30 age‐matched, nondiabetic controls. Regional BMD was measured by dual‐energy X‐ray absorptiometry (DXA); cortical and trabecular bone microarchitecture was assessed from high‐ resolution peripheral quantitative computed tomography (HRpQCT) images of the distal radius and tibia. Compared to controls, T2D patients had significantly lower BMS: unadjusted (11.7%; p < 0.001); following adjustment for body mass index (BMI) (10.5%; p < 0.001); and following additional adjustment for age, hypertension, nephropathy, neuropathy, retinopathy, and vascular disease (9.2%; p ¼ 0.022). By contrast, after adjustment for confounding by BMI, T2D patients had bone microarchitecture and BMD that were not significantly different than controls; however, radial cortical porosity tended to be higher in the T2D patients. In addition, patients with T2D had significantly reduced serum markers of bone turnover (all p < 0.001) compared to controls. Of note, in patients with T2D, the average glycated hemoglobin level over the previous 10 years was negatively correlated with BMS (r ¼ 0.41; p ¼ 0.026). In conclusion, these findings represent the first demonstration of compromised BMS in patients with T2D. Furthermore, our results confirm previous studies demonstrating low bone turnover in patients with T2D and highlight the potential detrimental effects of prolonged hyperglycemia on bone quality. Thus, the skeleton needs to be recognized as another important target tissue subject to diabetic complications. © 2014 American Society for Bone and Mineral Research. KEY WORDS: TYPE 2 DIABETES; MICROINDENTATION TESTING; BONE MATERIAL STRENGTH; HRPQCT; BONE TURNOVER
Introduction
T
ype 2 diabetes (T2D) is one of the most common chronic diseases worldwide.(1,2) As changing lifestyles lead to increased obesity,(3) the prevalence of T2D will continue to grow and the economic public health burden will worsen significantly.(1,2) Indeed, the direct medical costs of T2D are estimated at over $116 billion annually in the United States alone.(1) Globally, 285 million people have T2D, and this number is predicted to increase to 439 million by 2030.(2) Although patients with T2D are at significant risk for premature mortality and morbidity from macrovascular disease, retinopathy, nephropathy, and neuropathy,(2) there is growing evidence that T2D is also an independent risk factor for fragility fractures at skeletal sites such as the hip, spine, and distal forearm.(4–8) These
findings are perhaps surprising given that patients with T2D often have normal or increased dual‐energy X‐ray absorptiometry (DXA)‐derived bone mineral density (BMD),(9) even when normalized for body mass index (BMI).(10) Recently, however, using data from three large prospective observational studies in the United States (Study of Osteoporotic Fractures [SOF]; Osteoporotic Fractures in Men Study [MrOS]; and Health, Aging, and Body Composition [ABC] Study), Schwartz and colleagues(11) demonstrated that patients with T2D have a higher fracture risk for a given femoral neck BMD T‐score and age or for a given FRAX probability (defined by the World Health Organization’s Fracture Risk Algorithm [FRAX] score(12)). These results have since been corroborated by data from a large clinical database in Canada.(13) Collectively, these findings indicate that BMD and FRAX underestimate fracture risk in T2D patients and
Received in original form August 22, 2013; revised form September 14, 2013; accepted September 26, 2013. Accepted manuscript online October 1, 2013. Address correspondence to: Sundeep Khosla, MD, College of Medicine, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA. E‐mail:
[email protected] Additional Supporting Information may be found in the online version of this article. For a Commentary on this article, please see Jepsen and Schlecht (J Bone Miner Res. 2014;29:784–786. DOI: 10.1002/jbmr.2189). Journal of Bone and Mineral Research, Vol. 29, No. 4, April 2014, pp 787–795 DOI: 10.1002/jbmr.2106 © 2014 American Society for Bone and Mineral Research
787
suggest that other factors are responsible for this increased fracture risk.(14) Although DXA is commonly used in fracture risk assessment, it provides little information about bone microarchitecture and material composition—the primary determinants of bone “quality.”(15,16) Fortunately, quantification of bone microarchitecture has become possible with the advent of high‐resolution peripheral quantitative computed tomography (HRpQCT), although this technique cannot measure bone material strength (BMS), an often neglected, but nonetheless important, component of bone quality.(15,16) Progress in understanding how BMS might be changed in T2D has been hampered by the invasive nature of the direct measures previously needed to quantify this property in vivo, which has rendered routine clinical assessments impractical. However, recent advances in microindentation technology have now made it safe to quantify BMS in humans with minimal discomfort.(17,18) Indeed, previous studies have used bone microindentation testing to show that bone material properties are worse in hip fracture(19) and atypical femoral fracture(20) patients. However, whether patients with T2D have deficits in BMS as compared to nondiabetic controls is not known. Therefore, because the underlying skeletal abnormalities in T2D have not been identified, we performed in vivo microindentation testing of the tibia to directly measure BMS in 60 postmenopausal women (age range, 50–80 years) including 30 patients diagnosed with T2D for >10 years and 30 age‐matched, nondiabetic controls. In addition, we measured regional BMD by DXA and bone microarchitecture of the distal radius and tibia by HRpQCT. Thus, the aims of this study were to: (1) determine whether BMS, bone imaging parameters (derived from DXA and HRpQCT), and/or bone turnover are altered in patients with T2D compared to age‐matched, nondiabetic controls; and (2) examine the associations of BMS with duration of T2D and circulating levels of glycated hemoglobin.
Subjects and Methods Subjects This cross‐sectional study was approved by the Mayo Clinic Institutional Review Board, and informed written consent was obtained from all participants. We recruited 60 normal postmenopausal women (serum follicle stimulating hormone [FSH] >20 IU/L), including 30 patients diagnosed with T2D for >10 years and 30 age‐matched, nondiabetic controls, by letters from an age‐stratified (50–80 years) random sample of Olmsted County, MN, USA, residents, augmented by newspaper and website advertisements. All potential subjects were rigorously screened for coexisting disease and excluded if they had low body stores of vitamin D (serum 25‐hydroxyvitamin D of 3 m, motor vehicle accidents, etc.) or pathological fractures (eg, due to Paget’s disease, myeloma, metastatic malignancy) were excluded.
788
FARR ET AL.
Further, subjects were excluded if undergoing treatment for blood clots or coagulation defects, or treatment with any of the following drugs: corticosteroids (>3 months at any time or >10 days within the previous year), anticonvulsant therapy (within the previous year), pharmacological doses of thyroid hormone (causing thyroid stimulating hormone [TSH] to decline below normal), adrenal or anabolic steroids, aromatase inhibitors, calcitonin, calcium supplementation >1200 mg/d (within the preceding 3 months), bisphosphonates, estrogen or selective estrogen receptor modulator (SERM) (within the past year), parathyroid hormone (PTH), sodium fluoride, teriparatide, or thiazolidinediones (TZDs). We also ascertained a complete list of current and past medications, including the preparation, duration, and dose of diabetic medications (eg, biguanides, insulin, dipeptidyl peptidase‐4 [DPP‐4] inhibitors, sulfonylureas, a‐glucosidase inhibitors, glucagon‐like peptide analogs and agonists, meglitinides, and sodium glucose cotransporter‐2 [SGLT‐2] inhibitors), as well as b‐ blockers, angiotensin‐converting enzyme (ACE) inhibitors, angiotensin II receptor blockers (ARBs), calcium channel blockers, statins, thiazides, proton pump inhibitors, and selective serotonin reuptake inhibitors (SSRIs), because these drugs may affect bone turnover. A summary of these medications is provided in Supplementary Table 1. All patients with T2D were on at least one oral diabetic medication; thus, none were being treated with diet alone. Clinical details in the medical records were reviewed to determine if subjects met study criteria. Between February 2012 and March 2013, fasting morning blood was drawn from potential candidates in the outpatient Clinical Research Unit (CRU) at the Mayo Clinic (Rochester, MN, USA) and serum was analyzed for determination of vitamin D (total 25‐hydroxyvitamin D), calcium, phosphorus, alkaline phosphatase, aspartate transaminase, creatinine, TSH, and FSH. Inclusion criteria required that all subjects were within the normal reference range for each of these parameters, except for FSH; menopause was defined as a serum FSH level >20 IU/L and no menses for >1 year. In addition, we measured serum glycated hemoglobin; inclusion criteria required that controls had a normal glycated hemoglobin level (4.0% to 5.9%), whereas patients were required to be diagnosed with T2D for >10 years according to the American Diabetes Association criteria(22) (ie, glycated hemoglobin level 6.5%). In the patients with T2D, we also ascertained the average glycated hemoglobin level over the previous 10 years from clinical records for use as an indicator of past glycemic control. All subjects were residents of Olmsted County; this community is highly characteristic of the U.S. white population but underrepresented with respect to persons of African or Asian ancestry.(23) Reflecting the ethnic composition of Olmsted County, 98% of the sample was white.
Study protocol All subjects were interviewed for fracture, medical, and medication history; these data were supplemented by medical records and cross‐checked with redundant sources (radiographic reports, primary care or other health care provider notes, etc.) for accuracy. In addition, smoking and alcohol habits were ascertained during the interview. Anthropometric data were collected on all subjects wearing light‐weight clothing and no shoes. Weight was obtained using an electronic scale (Model 5002; Tronic, Inc., White Plains, NY, USA) and height was measured using a customized stadiometer (Mayo Section of Engineering). We assessed BMS (primary end point) from in vivo microindentation testing at the midshaft of
Journal of Bone and Mineral Research
the nondominant anterior tibia as detailed below (power calculations are provided in the Supplementary Materials). Secondary endpoints included the DXA‐derived and HRpQCT‐ derived bone parameters, as well as the serum biochemical markers of bone turnover. BMD of the hip, radius, lumbar spine (L1–L4), and total body regions was measured by DXA; cortical and trabecular bone microarchitecture was assessed from HRpQCT images of the nondominant distal radius and tibia, but data from two radius scans (0 T2D; 2 control) were excluded because of motion artifact. Our research team has extensive experience using DXA and HRpQCT;(24–26) these methods are detailed in the Supplementary Materials.
Bone microindentation testing The OsteoProbe Reference Point Indenter (Active Life Scientific Inc., Santa Barbara, CA, USA) is a handheld microindentation instrument designed for in vivo BMS measurements.(17,18) Supplementary Fig. 1 shows the device positioned over the testing site. Primary components of the device include an impact mechanism, a displacement transducer, and a sterilized stainless steel disposable probe with a 90‐degree conical tip (375 mm diameter; 2 mm). The indentations are small (mean IDI from impact SD, 187 24.0 mm; range, 154–323 mm), yet large enough to create cracks (ie, microfractures) in the bone.(17,18) Thus, BMS is a direct measure of fracture resistance insofar as the farther the probe indents the bone (higher the IDI from impact), the more easily the bone is fractured (lower the BMS). This technology is well validated(17,18) and a similar device has been used in previous studies involving animals(27–29) and humans.(19,20) For example, in a study of human cadaveric bone specimens, IDI from impact was inversely correlated with crack growth toughness (r ¼ 0.90), and scanning electron microscope images of cracks induced by microindentation and by experimental fractures were virtually identical.(19) In our laboratory, the within‐subject (n ¼ 10 controls; mean age SD: 63.1 8.9 years; range, 50–76 years) precision error (coefficient of variation [CV]) of the OsteoProbe was 1.65% for BMS. It should be noted that patients who have a significant skin disorder, bruising, local edema, or infection, as well as those undergoing treatment for blood clots or severe coagulation defects should not be exposed to this
Journal of Bone and Mineral Research
procedure. Based on our experience with 60 human subjects (30 patients with T2D and 30 nondiabetic controls), the procedure causes minimal discomfort (only during the local anesthesia injection) and no complications have been observed. Thus, this procedure poses nonsignificant risk to humans.
Serum bone turnover measurements Fasting morning blood was drawn and stored at 80°C. Bone formation was assessed by serum amino‐terminal propeptide of type I collagen (P1NP; mg/L) as measured by RIA (interassay CV,
