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Metabolic disorders of bone are those conditions that adversely affect the formation and maintenance of bone. Each of these conditions may be seen in bones throughout the body, including the bones of the feet and lower extremity.
Before diving into some of the various metabolic bone diseases, it's worth spending a moment on what bone is exactly, and how bone is regulated.
Bone is basically made up of two things: an organic component known as bone matrix or osteoid, and an inorganic component--calcium, which exists largely in the form of a compound called calcium hydroxyapatite, Ca10(PO4)6OH2. (Abnormal concentrations of the calcium hydroxyapatite crystal is a disease unto itself, and is discussed on our web page on crystalline deposition disorders.)
You can think of the osteoid as the frame of the bone, with an internal composition sort of like a honeycomb. Inside the holes of the honeycomb your body deposits calcium. The osteoid is what gives bone its form and shape; the addition of calcium is what gives bone its hardness and rigidity.
The amount of calcium in the bone is not constant. When you're born, for example, your bones consist mostly of matrix, with relatively little calcium. As your skeleton matures, more and more calcium is added to the matrix, and the bone hardens or "ossifies". (We'll use the term "ossification" to refer to the normal process of the bones' hardening and reserve the term "calcification" to refer to the abnormal deposition of calcium into soft tissues that shouldn't have calcium deposits--such as calcification of muscle (myositis ossificans) or tendon (calcific tendinosis).
Once the bone is fully formed and matured, the amount of calcium in the bone tends to be relatively stable, but there is still a constant process of the body's removing and adding calcium from the bone. The cells that take remove calcium are known as "osteoclasts"; the cells that put calcium into bone are known as "osteoblasts". You may be able to guess, then, that a bone tumour with a name like "osteoblastoma" would tend to be made up of an abnormal number of osteoblasts, thereby causing an abnormal production of bone.
The osteoblasts, osteoclasts and the process of calcium regulation and bone metabolism are controlled largely by three major chemicals in the body. The big player in affecting the removal of calcium from bone is the Parathyroid Hormone (PTH). Opposing the actions of the PTH are two compounds: calcitonin, which acts to slow the removal of calcium from bone, and vitamin D, which actively puts calcium and phosphate back into bone.
While what I've written above is overly simplified, it should give you an idea of some of the basic ideas of bone metabolism, so let's move on to the disorders of bone.
Metabolic Disorders of Bone
Disorders of diminished bone density
The catch-all term for the various disorders causing diminished bone density is osteoporosis or osteopenia. While both terms refer to the general loss of bone density regardless of cause, many people seem to use the word osteoporosis with the connotation of diminished bone density exclusively in the case with the elderly and with post-menopausal females. While these are the two most frequent causes of diminished bone density, in reality, there may be many other causes, including inactivity, diabetes, smoking, kidney problems, the use of steroids, excessive intake of caffeine, excessive intake of phosphate, diminished intake of calcium, fair skin, leukemia, multiple myeloma, and a host of other potential causes.
Most of the time diminished bone density is asymptomatic, but the weakened state of bone tends to cause fractures that don't occur in normal bone. The most well-known location of a fracture from osteoporosis is the hip, but fractures from osteoporosis may occur in any bone. The foot is a common location for such fractures, as it undergoes a great deal of stress with each step. For more information on this topic, please visit our web page on osteoporosis.
When bone density diminishes because of a lack of vitamin D, it may have one of two names. Rickets is bone deformation in children as a result of lack of vitamin D. Osteomalacia is the adult version of rickets. These are two specific forms of general osteoporosis or osteopenia. Both are characterized by transverse lines (known as Looser's lines) of diminished bone density on X-ray.
Scurvy is a result of a lack of vitamin C, which results in abnormal bone formation. Once common for men at sea for long periods of time, (where vitamin C in foods may be hard to obtain), this condition is now quite rare in developed nations. It is now seen in those with poor nutrition and in under-developed nations.
As we learned above, Parathyroid Hormone is what regulates removal of calcium from bone. Hence, it follows that conditions that result in too much of this hormone (hyperparathyroidism) tends to cause diminished bone density.
Because the kidneys play such a strong role in bone metabolism, (Parathyroid hormone acts there, and vitamins C and D, calcium and phosphorus are absorbed there), kidney failure will frequently affect bone metabolism. This general condition is known as renal osteodystrophy, and is characterized by a disorder known by a disorder known as osteitis fibrosa. Osteitis fibrosa is a condition of diminished bone density, with concurrent deposition of fibrous tissue in bone. Osteitis fibrosa is known by several other names--osteitis fibrosa cystica, von Recklinhausen's Disease of Bone. Bones affected with osteitis fibrosa are often discoloured brown, hence they're often called by still another name, "Brown Tumour".
Osteogenesis imperfecta (Ekman-Lobstein
Disease) is a genetic disease causing abnormally
low numbers of osteoblasts. Frequently seen in childhood, this condition
causes multiple fractures of bone around the body. This condition must be
differentiated from child abuse.
Some types of bone tumours, both benign and malignant, may also diminish bone density. Other types of tumours add density to bone. If you're interested in this topic, please visit our web page on bone tumours.
Disorders of increased bone density
As mentioned earlier, Parathyroid Hormone is what regulates removal of calcium from bone. You can imagine, then, that conditions that result in too little of this hormone (hypoparathyroidism) tends to cause increased bone density.
Osteopetrosis (Albers-Schonberg Disease) is a result of abnormal osteoclast function. It gives the appearance of diffuse, dense bone on X-ray. Interestingly, the bones in this condition are actually quite weak.
Melorrheostosis is a condition of increased thickening of the cortex (outer shell) portion of bone. It's cause is unknown.
Osteopoikilosis is a condition of localized spots of increased bone density within a bone. Common in the foot, this condition must be differentiated from the osteoblastoma tumour mentioned above, which may give a similar appearance.
Bone Islands are similar to osteopoikilosis, only occurring as a singular lesion.
Paget's Disease, a disease of simultaneous destruction and repair of bone, results in a thickened, disorganized appearance on X-ray. This condition is discussed on its own web page.
Finally, some types of bone tumours, both benign and malignant, may cause abnormally dense bones. Other types of tumours diminish bone density. If you're interested in this topic, please visit our web page on bone tumours.