Bone in osteoarthritis and osteoporosis

The hip joint is a major site of musculoskeletal morbidity in the elderly. With over 70,000 osteoporotic hip fractures every year in the UK, 20% of whom die shortly after, this represents a huge personal and social cost. Identifying individuals who are at high risk of hip fracture would enable early targeted prevention strategies and hence prevent or delay fracture and its devastating consequences. Forty million Americans are estimated to have osteoporosis or osteopenia with a subsequent increased risk of hip fracture. Osteoporosis alone has an estimated cost to the US national health system of $33.5 billion every year. The cost of each hip fracture in the UK is estimated to be over £12,000 and accounts for vast majority of the £1.7 billion costs for osteoporotic fractures (NOP?). Worldwide the incidence of hip fractures is expected to more than triple from 1.66 million in 1990 to 6.26 million in 2050, and in the European Union, an increase from 414 000 to 972 000 cases per annum is expected over the next 50 years.

About 50,000 hip replacements (and as many knees) are also performed every year in the UK and 193,000 in the US for osteoarthritis. Currently there are no reliable procedures for identifying early disease and measuring its progression, nor are there any methods for measuring joint degeneration quantitatively and reproducibly. The consequences are that it is difficult to identify patients early for intervention and there is no reliable method for monitoring the effects of disease modifying agents.

Sufferers from osteoarthritis and osteoporosis occupy about half of the orthopaedic beds in the UK. Despite this, these two diseases are still not well understood, and much research into the physiology, mechanics and composition of bone is still necessary to provide a better understanding of the processes involved.

Bone

Bone may be described as a complex composite; in fact it is a naturally occurring fibre-reinforced material. It is formed as a cartilaginous precursor, in which the main structural protein is collagen. This becomes impregnated with calcium phosphate mineral, in the form of small, poorly-crystalline apatite crystals, which considerably increases the strength and stiffness of the material. There are two bone structures: cortical and cancellous (or trabecular). Cortical bone makes up about 80% of the mass of the skeleton and forms the outer shell of bone. It is dense and has a slow turnover rate. Cancellous bone has a spongy texture and hence is less stiff. It is found primarily at the ends of long bones (epiphyses and metaphyses) and in the interior of short bones such as vertebrae.

Changes in bone are very significant in diseases such as osteoporosis and osteoarthritis. In osteoporosis there is a reduction in the amount of trabecular bone resulting in an increased risk of fracture, especially of the hip, wrist and spine. Osteoarthritis results in an increase in the amount of trabecular bone. In previous studies we showed that there was little change in the material properties of the bone matrix in osteoporotic bone, simply a reduction in the amount of trabecular bone. However, we found that osteoarthritic bone was less well mineralised and appeared to have a more porous texture. Techniques used to date include Thermogravimteric analysis-Mass Spectrometry, powder x-ray diffraction (room and high-temperature), Mercury Intrusion Porisimetry, micro-indentation, electron probe microanalysis and FTIR spectroscopy. This has also involved analysis of a variety of animal bones with a range of organic:mineral ratios and the analysis of synthetic bone replacement materials. (some links can be made with existing pages)

Collagen

Collagen is a triple-helical molecule, about 300 nm long and 1.5 nm diameter. It assembles into fibres with a relatively disordered side-to-side packing of molecules but a regular axial structure. Molecules are staggered length-wise such that there is a characteristic axial repeat of about 67 nm. These structures can grow up to nearly 1 mm wide and of unknown length though commonly they are 10-100 um across. In conjunction with other matrix components they form all the body’s supporting tissues - tendon, ligament, cartilage, skin, blood vessels etc. Like ropes, collagen fibres are strong in tension but cannot sustain compression. These tissues can be considered as fibre-composite materials in which the collagen fibres provide reinforcing to the weak matrix. Cross-links between the molecules are essential for the tensile strength of collagen fibres. Initial formation of reducible cross-links, largely based on lysine, is followed by maturation of non-reducible pyridinium and pyrrolic cross-links.

Collagen and mineralization

There are more than 20 genetically different types of collagen. The most common is type I and this is found in bone, skin, ligaments and tendon and many other skeletal tissues. In type I collagen the triple-helix comprises three so-called alpha chains, two a1 chains and one a2 chain. In Aberdeen, a G to T polymorphism in the Sp1 promoter region was identified which was related to a reduced bone mineral density (Grant et al. 1996). Further work showed that cells from patients who were heterozygous for this polymorphism over-expressed the a1 chain so that instead of the ratio of a1: a2 chains being synthesized being 2:1, it was found to be 2.36 (Mann et al. 2001). We proposed that this would form a small fraction of homotrimeric collagen molecules, instead the correct heterotrimer, and that this could be the source of the increased bone fragility. Individuals with an inactivating mutation in the gene for the a2 chain have severe osteogenesis imperfecta and a similar phenotype has been found in the oim/oim mouse, which has no a2 chains. Mice which are heterozygous for the oim mutation have collagen fibrils comprising a mixture of homo- and heterotrimers and they have an osteoporotic phenotype. The polymorphism was found to result in reduced mineralization of bone nodules in vitro, supporting this hypothesis (Stewart et al. 2005). We have however, failed to find any intact homotrimer in tissue extracts from heterozygous individuals even though dissociated, monomeric collagen a chains are found in the expected ratios using SDS-PAGE. Other studies have also found altered ratios of monomers in osteoporotic and osteoarthritic patients with the same Sp1 polymorphism but, it needs to be noted, most have not looked for, and none has found, intact homotrimer. The reason for this is not yet clear.

References

Grant, S. F. A., Reid, D. M., Blake, G., Herd, R., Fogelman, I., & Ralston, S. H. 1996, Reduced bone density and osteoporosis associated with a polymorphic Sp1 binding site in the collagen type I alpha 1 gene, Nat.Genet., 14, 203-205.

Mann, V., Hobson, E. E., Li, B., Stewart, T. L., Grant, S. F., Robins, S. P., Aspden, R. M., & Ralston, S. H. 2001, A COL1A1 Sp1 binding site polymorphism predisposes to osteoporotic fracture by affecting bone density and quality, J Clin.Invest, 107, 899-907.

Stewart, T. L., Roschger, P., Misof, B. M., Mann, V., Fratzl, P., Klaushofer, K., Aspden, R., & Ralston, S. H. 2005, Association of COLIA1 Sp1 Alleles with Defective Bone Nodule Formation In Vitro and Abnormal Bone Mineralization In Vivo, Calcif Tissue Int, in press.