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He is also chairman of this department. He has also been chairman of the Committee of Scientific Advisors of the IOF, a position which he held for two mandates, and is a former president of the Swiss Association against Osteoporosis. Professor Rizzoli is the Editor of the journal Bone and Associate Editor of Osteoporosis International , and has authored more than articles and book chapters.
The relative proportions of the two types of bone vary considerably among different skeletal sites: the cancellous: cortical bone ratio is about 75 : 25 in the vertebra, 50 : 50 in the femoral head and 95 : 5 in the shaft or diaphysis of the radius [ 1 ].
In the cortical bone, the periosteum is the outer fibrous structure of all bones, which contains the blood vessels that nourish the bone, nerve endings, osteoblasts and osteoclasts, and which is anchored to the bone by Sharpey's fibres that penetrate into the bone tissue. The endosteum is a membranous sheath that constitutes the inner surface which is in direct contact with the marrow, and that also contains blood vessels, osteoblasts and osteoclasts. A particular feature of the bone is its ability to adapt its shape and size in response to mechanical loads.
This mechanical adaptation is generated by a process known as modelling, in which bones are shaped or reshaped by the independent action of osteoblasts and osteoclasts. Modelling occurs vigorously not only during growth, but also, in the adult, in response to a mechanical load such as in tennis players in whom the radius of the playing arm has a thicker cortex and a larger external diameter than the contralateral radius.
Conversely, rapid bone loss may be induced by the unloading of the skeleton during bed rest or space flight [ 3 ].
Bone modelling differs from bone remodelling, because in this process bone formation is not coupled with prior bone resorption. The modelling process is less frequent than the remodelling one, but it does occur in normal subjects [ 4 ] and may be increased by some pathological states [ 5 , 6 ].
Another feature of the human bone is the process of remodelling, a surface-based phenomenon that involves the removal of a quantum of bone by osteoclasts followed by the deposition of new bone by osteoblasts in the cavity formed [ 2 ]. The resorption phase refers to the osteoclastic resorption that is regulated by local cytokines and systemic hormones [ 11—14 ].
During this phase, specific types of proton pumps and other ion channels in the osteoclast membrane transfer hydrogen ions to the resorbing compartment, and this acidic solution dissolves the mineral component of the matrix while a number of lysosomal enzymes are secreted and digest the organic phase of the matrix. Resulting from this process, saucer-shaped resorption cavities are created on the surface of the cancellous bone Howship's lacunae , and cylindrical tunnels form within the cortex [ 1 ].
Resorption is first accomplished by multinucleated osteoclasts and later by mononucleated cells [ 1 , 15 , 16 ].
This phase concludes with osteoclast apoptosis and is followed by reversal [ 1 , 2 , 17 ]. During the reversal phase, the resorption lacuna is inhabited by mononuclear cells monocytes, osteocytes liberated from the bone by osteoclasts, and pre-osteoblasts recruited to initiate the formation phase of the cycle. It is during this phase that coupling mechanisms resorption always followed by formation must work in an efficient and balanced manner. In the absence of efficient coupling and bone balance, each remodelling transaction would result in a net loss of bone.
Bone remodelling units on the periosteal surface of cortical bone produce a slightly positive bone balance so that with ageing, the periosteal circumference increases. On the other hand, remodelling units on the endosteal surface of cortical bone are in negative balance so that the marrow cavity enlarges with age.
In addition, the balance is more negative on the endosteal surface than on the perisoteal surface, which results in age-related cortical thickness decline. The bone balance is also negative on cancellous surfaces, resulting in an age-related gradual thinning of the trabecular plates [ 18 ] Fig. In the two-step formation process, the osteoblasts initially synthesize the collagenous organic matrix, and then regulate its mineralization by secondary nucleation on contact with pre-existing mineral [ 19 ].
At the end of each remodelling cycle, a new osteon, a bone structural unit BSU , has been created [ 1 ].
The process of bone remodelling is equivalent in cancellous and cortical bone. At the macroscopic level, the normal cortical bone appears dense and solid, whereas cancellous bone is a lace-like structure of interconnected trabecular plates and bars surrounding marrow-filled cavities. At the light microscope level, both cortical and cancellous bone is composed of BSUs or osteons [ 1 ]. The normal trabecular bone is composed of internal rods or plates that form a 3D branching lattice oriented along the lines of stress.
The trabecular interstices of the axial skeleton are the primary repository of red bone marrow, therefore trabecular bone lies in close proximity with the marrow-derived cells that participate in bone turnover. Bone loss initially starts at the bone surfaces; therefore, changes in bone mass occur earlier and to a greater extent in trabecular bone than in skeleton regions that are primarily cortical. Osteoporosis is a systemic disease defined as a reduction in bone mass associated with an impaired bone architecture: disruption of trabecular continuity by trabecular perforation, resulting in reduced connectivity of the trabecular bone structure, increased bone fragility and increased fracture risk; and thinning and increased porosity of the cortices occur, with the conversion of the normal plate-like trabeculae into thinner rod-like structures Fig.
These changes result from the combination of the increased osteoclastic activity and the reduced osteoblast function that characterizes postmenopausal osteoporosis. Besides conventional radiographs, bone densitometry has long been the standard technique to assess bone mineral content despite the fact that this technique provides important information about osteoporotic fracture risk.
René Rizzoli Menopause is the time in a woman’s life when reproductive capacity ends. This period may be associated with a large variety of symptoms affecting the cardiovascular and urogenital systems, as well as skin, hair and bone. Sex hormone deficiency leads to accelerated bone. Atlas of Postmenopausal Osteoporosis Third Edition René Rizzoli Division of Bone Diseases, Department of Rehabilitation.
Recent clinical investigations indicate that BMD only partly explains bone strength and show limitations of BMD measurements in assessing fracture risk and monitoring the response to therapy [ 21—27 ]. As new products and methods have been developed by molecular and cellular research focusing on bone fragility, it became essential to develop effective and sensitive non-invasive means by which early changes in the fracture repair process can be detected [ 20 ].
FEM: finite element microscopy model. These currently available advanced imaging modalities help to investigate bone fragility and to define the skeletal response to innovative therapies and assess the biomechanical relationships [ 20 ]. QCT allows separate analysis of the trabecular and cortical compartments. The analysis of cortical bone, in particular at the hip, is important to estimate fracture risk, and this technique has been utilized in several clinical trials [ 28 , 29 ].
MicroCT is a technique particularly adapted to 3D analysis of human iliac crest bone biopsies, investigating evolution of trabecular structure under treatment Fig.
A Placebo and B strontium ranelate therapy. Despite the progress made with these techniques, certain issues remain, such as the important balances between spatial resolution and sampling size, or between signal-to-noise and radiation dose or acquisition time, which need to be considered further, as do the complexity and expense of the methods vs their availability and accessibility. The relative merits of these sophisticated imaging techniques must be weighed with respect to their applications as diagnostic procedures, requiring high accuracy or reliability, compared with their monitoring applications, requiring high precision or reproducibility [ 21 ].
Fundamentally, a fracture occurs when the external force or load applied to a bone exceeds its strength.
Whether or not a bone will be able to resist the fracture [ 22 ] depends on the amount of bone present, the spatial distribution of the bone mass, the cortical and trabecular microarchitecture and the intrinsic properties of each of the bone components. Imaging techniques that can measure one or more of the determinants of bone strength may enhance clinical management of osteoporosis and enhance new drug development [ 22 ].
Several novel non-invasive techniques for the assessment of bone quality and strength are currently being investigated in clinical studies. They aim to quantify various determinants of bone strength such as 3D bone geometry, volumetric bone density, microarchitecture and properties of the bone matrix [ 22 ].
Finite element analysis, by combining bone geometry with material characteristics to predict bone strength, holds promise as a biomechanically based technique for fracture assessment, and imaging modalities capable of assessing trabecular architecture may be particularly useful in assessing subtle treatment-based changes in bone strength [ 22 ].