Assessment of Skeleton Health Tuan Van Nguyen and Nguyen Dinh Nguyen Garvan Institute of Medical Research Sydney, Australia

Overview Background Normal bone and bone remodelling Bone loss and age Definitions Measurements of bone strength: Bone mass and DXA, QUS Bone turnover markers

Background Aging population: fastest growing age group Osteoporosis and osteoporotic fracture: age-related disorders Osteoporosis and osteoporotic fracture: Common Cause serious disability and excess mortality Major economic burden on healthcare system

Residual lifetime risk of different diseases Men Women To put in perspective I would like to make cross comparision LR for different diseases (Source: Nguyen ND et al, 2006, under review process)

Survival probability and fracture Women Men Cumulative survival rate Age (y) (Soure: Center J, Nguyen TV et al., Lancet 1999;353:878-82)

Burden of Osteoporotic fractures Annual cost of all osteoporotic fractures: $20 billion in USA and ~$30 billion on EU1. Worldwide direct and indirect cost of hip fracture: US$131.5 billion2. (Sources: 1Cummings et al., Lancet 2002;359:1761-67; 2Johnell O, Am J Med 1997;103:20S-26)

Cortical and Trabecular Bone Cortical Bone 80% of all the bone in the body 20% of bone turnover Trabecular Bone This slide illustrates the location of cortical and trabecular bone using the uCT image of an iliac crest biopsy. Cortical bone (also called compact bone) is the dense outer layer of bone; it is found predominantly in the shafts of the long bones in the arms and legs. Cortical bone comprises about 75-85% of the total bone in the body, but because of the relative small remodelling surface area, participates in only ~20% of bone turnover at any given moment. Remodeling in cortical bone occurs at the periosteal (outer) surface, the endosteal (inner) surface, and within the haversian canal system. Trabecular bone, although comprising a much smaller percentage of the total bone in the body, has a very large surface area due to its lace-like structure, and participates in ~80% of bone turnover at any given moment. Remodelling occurs on the surface of the trabeculae. 20% of all bone in the body 80% of bone turnover

Cortical (Compact) Bone 80% of the skeletal mass Provides a protective outer shell around every bone in the body Slower turnover Provides strength and resists bending or torsion

Trabecular (Cancellous) Bone 20% of the skeletal mass, but 80% of the bone surface. less dense, more elastic, and higher turnover rate than cortical bone. appears spongy found in the epipheseal and metaphysal regions of long bones and throughout the interior of short bones. constitutes most of the bone tissue of the axial skeleton (skull, ribs and spine). interior scaffolding maintains bone shape despite compressive forces.

Distribution of Cortical and Trabecular Bone Thoracic and 75% trabecular Lumbar Spine 25% cortical 1/3 Radius >95% Cortical Femoral Neck 25% trabecular 75% cortical This slide depicts the relative proportions of cortical and trabecular bone at various skeletal sites. Loss of trabecular bone would predispose to vertebral compression fractures, whereas loss of cortical bone would clearly predispose a patient to fractures at the hip and radius. Trabecular bone mass begins to decrease in early adulthood, occurring sooner than the decline in cortical bone. However, it has also been reported by Riggs and Melton (NEJM 14: 1776-1686, 1986) that the acceleration of trabecular bone loss at the time of menopause is not as pronounced as the loss of cortical bone. Remodeling of trabecular and cortical bone is influenced by the environment of the bone remodeling surfaces. Bone remodelling cells on the trabecular surface are in contact with the cells of the marrow cavity and are likely influenced by their interaction with osteotropic cytokines. Cortical bone remodeling is more likely to be influenced by osteotropic hormones such as PTH and 1a,25, dihydroxyvitamin D3 due to the greater distance of the remodeling surface from the marrow cavity. Ultradistal Radius 25% trabecular 75% cortical Hip Intertrochanteric Region 50% trabecular 50% cortical

How does bone loss happen? Bone is a living, growing, tissue Healthy bones are not quiescent. They are constantly being remodeled. This is not simply a problem of bony destruction, but imbalance between the formation and destruction of bone.

Bone remodeling cycle Endosteal sinus Monocyte Pre-osteoclast Osteocyte Osteoclast Macrophage Pre-osteoblast Osteoblast Bone-lining cell Osteoid New bone Old bone

Bone remodeling cycle Pre-osteoblasts Monocytes Osteoblasts Osteoclasts Osteocytes

Bone loss Bone formation Bone resorption Bone formation

Bone mass declines with age Remodeling occurs at discrete foci called bone remodeling units (BRUs). Number of active BRUs  with age   bone turnover. Osteoblasts not able to completely fill cavities created by osteoclasts and less mineralized bone is formed. Endosteal bone loss partially compensated by periosteal bone formation  trabecular thinning.

Relative Influence of Inner and Outer Diameters on Bone Strength This figure illustrates the impact of changing the Moment of Inertia on bone strength. Starting from the initial state (A), a moderate degree of endosteal resorption (expansion of the marrow space by 3 units) is balanced by application of 1 unit to the periosteal surface, resulting in no change in bending strength. Lee CA, Einhorn TA. The Bone Organ System, Form and Function. In Marcus R, Feldman D, Kelsey J, Eds. Osteoporosis, 2nd Edition. Academic Press, San Diego, 2001 Volume 1, pp. 3-20. (Adapted from Lee CA, and Einhorn TA. Osteoporosis 2nd Ed. 2001)

Gain and loss of Bone throughout the lifespan Pubertal Growth Spurt Menopause BMD Resorption Formation Age (Years)

Relationship between BMD and Age (VN 2006, unpublished data)

Definition of Osteoporosis (WHO) A systematic skeleton disease characterized by: low bone mass microarchitectural deterioration of bone tissue consequent increase in bone fragility and susceptibility to fracture Consensus Development Conference: Diagnosis, Prophylaxis, and Treatment of Osteoporosis, Am J Med 1993;94:646-650. WHO Study Group 1994.

Osteoporosis Normal Osteopenia Osteoporosis

Normal bone Osteoporosis

Definition of Osteoporosis (NIH) Osteoporosis is defined as a skeletal disorder characterized by: compromised bone strength predisposing a person to an increased risk of fracture. bone strength primarily reflects the integration of bone density and bone quality. (Source: NIH Consensus Development Panel on Osteoporosis JAMA 285:785-95; 2001)

BONE STRENGTH BONE MINERAL BONE QUALITY DENSITY Bone architecture Gram of mineral per area Bone turnover Bone size & geometry

Bone Quality Architecture Turnover Rate Damage Accumulation Degree of Mineralization Properties of the collagen/mineral matrix ( NIH Consensus Development Panel on Osteoporosis. JAMA 285:785-95; 2001)

Bone mass, Bone mineral density (BMD) Bone mass = the amount of bone tissue as the total of protein and mineral or the amount of mineral in the whole skeleton or in a particular segment of bone. (unmeasurable) BMD = the average concentration of mineral per unit area  assessed in 2 dimensions (measurable)

Effect of Size on Areal BMD BMC AREA BMD 1 1 1 1 1 1 2 2 8 4 2 2 3 3 This slide illustrates the impact of body size on the standard area BMD measurement. Assume the existence of a stone quarry in which all the stone has a true mineral density of 1 gram per cubic centimeter. Assume further that 3 cubes are mined from this quarry, and the cubes are 1, 2, and 3 centimeters on a side. If all cubes are placed on a tray and undergo DXA scanning, the resulting BMC, Area, and BMD results are given in the slide. It can be seen that larger objects are reported as having a systematically increased “BMD,” and smaller objects are systematically low. This means that interpretation of areal BMD measurements in very tall (>5 ft 10 inches) or very short (< 5 ft 2 inches) persons will require some adjustment. Carter DR, Bouxsein ML, Marcus R. New approaches for interpreting projected bone densitometry data. J Bone Miner Res 1992;7:137-145. 27 9 3 3 “TRUE” VALUE = 1 g/cm3 (Adapted from Carter DR, et al. J Bone Miner Res 1992)

Bone Densitometry Non-invasive test for measurement of BMD Major technologies Dual-energy X-ray Absorptiometry (DXA) Quantitative Ultrasound (QUS) Quantitative Computerized Tomography (QCT) Many manufacturers Numerous devices Different skeletal sites

DXA (or DEXA)

DXA (or DEXA) Gold-standard” for BMD measurement Measures “central” or “axial” skeletal sites: spine and hip May measure other sites: total body and forearm Extensive epidemiologic data Correlation with bone strength in-vitro Validated in many clinical trials

DXA Technology Detector (detects 2 tissue types - bone and soft tissue) Very low radiation to patient. Very little scatter radiation to technologist Patient Collimator (pinhole for pencil beam, slit for fan beam) Photons X-ray Source (produces 2 photon energies with different attenuation profiles)

DXA: BMD scan Spine Hip Total body

DXA: Femoral neck BMD

DXA: Lumbar spine BMD

DXA: Hip BMD: Results

Which Skeletal Sites Should Be Measured? Every Patient Spine L1-L4 (L2-L4) Hip Total Proximal Femur Femoral Neck Trochanter Some Patients Forearm (33% Radius) If hip or spine cannot be measured Hyperparathyroidism Very obese

BMD measurement: subject to variability In vivo/in situ BMD inaccuracy: effect of bone structure, bone size and shape, and extra-osseous soft tissue Measurement error: within subject and between-subject variations. Between machine variation.

In vivo/In situ BMD inaccuracy REGION OF INTEREST Lateral region Lateral region Bone region Trabeculaae + Marrow Extra-Osseous Fat+Lean tissue Cortical region X-RAY PATHS (Adapted from Bolotin HH, Med Phys 2004;31:774-88)

In vivo/In situ BMD inaccuracy Under-/or over-estimate BMD (%) Normal Osteopenia Osteoporosis Typical lumbar vertebral bone site ~25 ~35 Up to 50 Distal radius, femur ~20 Trabecular-free sites (mid-shaft femur, mid-shaft radius…) <2 Individual Type of bone (Source: Bolotin HH, Med Phys 2004;31:774-88)

Source of variability in BMD measurements Number of measurements per subject required to increase the reliability of measurement for a given coefficient of reliability (Source: Nguyen TV et al., JBMR 1997;12:124-34)

Standard error of rate of change in BMD Individual Group (Source: Nguyen TV et al., JBMR 1997;12:124-34)

Source of variability in BMD measurements Group level: Intra-subject estimation error could contribute about 90% of the variability component   power of study, and underestimate the RR (BMD-fracture). Individual level: false +ve & false –ve error rates of diagnostic BMD.  measurement error by multiple measurement.  long-term intra-subject variation by:  the length of follow-up and/or  the frequency of measurements. Studies with 3-5 yrs of follow-up: optimal “cost benefits”. More than 2 measurements/year: not improve the precision appreciably. (Source: Nguyen TV et al., JBMR 1997;12:124-34)

“True” level and “True” biological change of BMD Factors affect to BMD level and BMD change: Invivo/in situ BMD inaccuracy Random error Measurement errors: intra- and between-subject variability Systematic errors Effect of regression-toward-the mean (Sources: Bolotin HH, Med Phys 2004;31:774-88; Nguyen TV et al., JBMR 1997;12:124-34; Nguyen TV et al, JCD 2000;3:107-19)

“True” level and “True” biological change of BMD BMD level: Good agreement between observed and true values Individual with low BMD: 20% false +ve and false –ve of diagnosis of osteoporosis. BMD change: Overall average increase in BMD of 2%: no conclusion of significant change for an individual. An observed  of at least 5.5% or  of at least 7.5%: could be a significantly biological change. (Source: Nguyen TV et al, JCD 2000;3:107-19)

BMD Values From Different Manufacturers Are Not Comparable Different dual energy methods Different calibration Different detectors Different edge detection software Different regions of interest

Peripheral BMD Testing Accurate & Precise What it can do Predict fracture risk Tool for osteoporosis education What it cannot do Diagnose osteoporosis Monitor therapy A “normal” peripheral test does not necessarily mean that the patient does not have osteoporosis. WHO criteria do not apply to peripheral BMD testing.

Quantitative Ultrasound (QUS) Broad-band ultrasound attenuation or ultrasound velocity No radiation exposure Cannot be used for diagnosis Preferred use in assessment of fracture risk

Bone Quality Architecture Turnover Rate Damage Accumulation Degree of Mineralization Properties of the collagen/mineral matrix ( NIH Consensus Development Panel on Osteoporosis. JAMA 285:785-95; 2001)

Cortical and Trabecular Bone Cortical Bone 80% of all the bone in the body 20% of bone turnover Trabecular Bone This slide illustrates the location of cortical and trabecular bone using the uCT image of an iliac crest biopsy. Cortical bone (also called compact bone) is the dense outer layer of bone; it is found predominantly in the shafts of the long bones in the arms and legs. Cortical bone comprises about 75-85% of the total bone in the body, but because of the relative small remodelling surface area, participates in only ~20% of bone turnover at any given moment. Remodeling in cortical bone occurs at the periosteal (outer) surface, the endosteal (inner) surface, and within the haversian canal system. Trabecular bone, although comprising a much smaller percentage of the total bone in the body, has a very large surface area due to its lace-like structure, and participates in ~80% of bone turnover at any given moment. Remodelling occurs on the surface of the trabeculae. 20% of all bone in the body 80% of bone turnover

Relevance of Architecture This figure illustrates the loss of bone strength due to trabecular thinning (loss of quantity) and perforation of trabeculae (loss of architecture). The strength of a trabecular strut is proportional to the square of its radius. Preservation of architecture is essential for bone strength. Normal Loss of Loss of Quantity Quantity and Quantity and Architecture Architecture

Bone Architecture Trabecular Perforation The effects of bone turnover on the structural role of trabeculae Risk of Trabecular Perforation increases with: Because much of the strength of trabecular bone is caused by its microarchitecture, the loss of trabecular connectivity and other adverse structural consequences of increased osteoclastic activity clearly could predispose to fractures of the vertebrae and other skeletal sites that have a high content of cancellous bone. Riggs & Melton. JBMR (2002) 17:11-14 Increased bone turnover Increased erosion depth Predisposition to trabecular thinning

Bell et al. Calcified Tissue Research 1: 75-86, 1967 Structural Role of Trabeculae Compressive strength of connected and disconnected trabeculae 1 16 X The compressive strength of connected trabeculae is 16-fold greater than disconnected trabeculae. Bell et al. Calcified Tissue Research 1: 75-86, 1967

Resorption Cavities as Mechanical Stress Risers Normal Osteoporotic Resorption cavities occur on trabecular surfaces in normal bone. In osteoporotic bone, the cavities are more numerous and may be deeper. Resorption cavities in bone can act as mechanical stress risers, which decrease overall bone strength. This concept is illustrated on the right with the picture of the cane. The notch in the cane acts as a stress riser, to decrease the overall strength of the cane. (Adapted from Parfitt A.M. et al. Am J Med 91, Suppl 5B: 5B-34S)

Hip strength indice CSMI (cm4): Cross-sectional moment of inertia CSA (cm2): Cross sectional area Z (cm3): Section modulus= CSMI/distance from the centre of the mass to the superior neck margin. Cstress (N/mm2): Compressive stress on the superior surface of the FN during a fall on the greater trochanter. Calculated by combining CSMI and CSA. FND (cm): Femoral neck Diameter Buckling ratio= radius/thickness

Cross-Sectional Moment of Inertia CSMI = /4 (r4 outer – r4 inner) Area (cm2) 2.77 2.77 2.77 CSMI (cm4) 0.61 1.06 1.54 Bending Strength 100% 149% 193% Courtesy of M. Bouxsein

Bone strengh indice: summary Not well-studied Derived from BMC, BMD, and several assumptions Used in research field.

Bone Turnover Markers Components of bone matrix or enzymes that are released from cells or matrix during the process of bone remodeling (resorption and formation). Reflect but do not regulate bone remodeling dynamics.

Urinary Markers of Bone Resorption Marker Abbreviation Hydroxyproline HYP Pyridinoline PYD Deoxypyridinoline DPD N-terminal cross-linking telopeptide of type I collagen NTX C-terminal cross-linking telopeptide of type I collagen CTX (Source: Delmas PD. J Bone Miner Res 16:2370; 2001)

Serum Markers of Bone Turnover Marker Abbreviation Formation Bone alkaline phosphatase ALP (BSAP) Osteocalcin OC Procollagen type I C-propeptide PICP Procollagen type I N-propeptide PINP Resorption N-terminal cross-linking telopeptide of type I collagen NTX C-terminal cross-linking telopeptide of type I collagen CTX Tartrate-resistant acid phosphatase TRAP (Source: Delmas PD. J Bone Miner Res 16:2370, 2001)

Bone Turnover Effects Bone Quality Very low turnover excessive mineralization and the accumulation of microdamage Very high turnover  accumulation of perforations and a negative bone balance There is a complex relationship between bone turnover and bone quality. Suppression of bone turnover is not always good. Long-term excessive suppression of bone turnover may result in increased skeletal fragility as a result of increased brittleness of bone. An increase in bone brittleness is due to excessive mineralization which may result in the formation of microcracks and accumulation of microdamage. Microdamage accumulation is a result of the absence of normal repair mechanisms in bone with very low bone turnover.

Summary Osteoporosis and osteoporotic fractures are common among aging population “Gold standard” of assessment skeleton health is BMD via DXA machine. BMD measurement is subject to bias and errors. Additional measure of bone health: QUS (BMD), bone strength indice and bone turnover markers.

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