The Soft Bones and Porous Bones

Soft Bones and Porous bones

             PKGhatak,MD

The soft bone develops due to a failure of normal mineralization of bones, when it happens in childhood known as Rickets and in adults called Osteomalacia. Porous bone develops due to excess removal of the protein matrix of bone in relation to new bone formation. In medicine, this is called Osteoporosis, commonly referred to as brittle bones.

New bone tissue continuously forms and the dead bone is removed and this process is called remodeling and the process continues all through the lifetime. The rate of new bone tissue formation is faster till about midlife, then after midlife and menopause in women, the new bone formation lags behind the removal process. Whereas, Osteopenia is another term when both minerals and matrix are lost resulting in smaller bone mass.

The growth process of the bones of arms and legs is different from the growth of the skull, scapula and pelvis. In the case of limb bones, cartilage structures grow initially, followed by the deposition of calcium and phosphate. A plate of cartilage at each end of the bone remains sandwiched between the shaft and the two ends, this is called the growth plate. The growth plate continues to divide and grow in length till the mid to late 20s when the growth period ends. The width of long bones increases due to new bone formation underneath the bone covering called the periosteum. The flat bones of the skull, wing bones and pelvis are laid down as connective tissue plates followed by calcium-phosphate mineralization. The enlargement of the size of flat bones is due to the growth of membrane around the margins and when fully ossified called suture lines.

Soft bones: - Rickets and Osteomalacia.

Children with short statures, bowed legs, deformed head, pigeon breast, rachitic rosary chest, knobby knees and wrists, hunched back, lordosis and crooked pelvis, etc. are well documented in old Chinese and Greek literature. We know these children today as rickets. The word rickets was taken from an old English word Wrickken, meaning twisted. In 1645 Dr. David Whistler described English children with similar deformities, and he called it the Malady of English Children. In 1919 Dr. Edward Mellanby discovered vitamin D and its role in bone formation. The deficiency of vitamin D and calcium in diet in childhood was very common due to poverty when children were raised on mothers' milk alone and in an environment where sun rays were obscured by a thick layer of coal burning smoke. It is still prevalent in poor countries. Besides diet and sunshine, growth hormone, parathyroid hormone, calcitonin, thyroxine, and adrenocorticoids are also involved in bone development and growth. Deficiency of any of these hormones, malabsorption, and liver and kidney failure are some of the other causes of rickets.

Osteomalacia:

Deficiency of vitamin D and calcium, phosphorus and magnesium from any cause in adults will lead to a negative balance of calcium and phosphorus; this results in the mobilization of minerals from bones and bones become weak and soft. Symptoms of osteomalacia are few or absent at the beginning, a bone fracture without an injury may be the first symptom. Diffuse bone pain, weakness of muscles of thigh and arms, difficulty in climbing stairs, and waddling gait may be present.

Osteomalacia is more common in women and in pregnancy, nursing home patients, prolonged bed rest from debilitating diseases, long term use of anti seizure drugs, renal diseases, and vegans are more prone to osteoporosis. Malabsorption syndrome including celiac disease, certain malignancies and gastrointestinal disorders, resection of small bowel and post gastric surgery may result in Osteomalacia. Hereditary vitamin D and phosphate deficiency is a rare cause of osteoporosis.

Treatment of Rickets and Osteomalacia is straightforward. An adequate supply of vitamin D and calcium, phosphorus, and magnesium should be the first order of business, and then adequate supply must be maintained for the rest of life. Treatment of the underlying case where possible must accompany.

Osteoporosis:

Unlike osteomalacia, osteoporosis is very common. It is estimated that people over 70 years have some degree of osteoporosis detected by the Bone Mineral Density (BMD) test and reported as T-Score. T- 1 is normal, and T- 2.5 or over is destined for fractures.

To comprehend osteoporosis, a step back is necessary. Bone is a dense but living tissue, bone cells are Osteoclasts and Osteoblasts. The outer layer of bone is very dense and called the Cortex, inside the bone is a cavity filled with Cancellous bones and bone marrow cells. The cortex and cancellous matrix are linked together to provide strength and stability of bones. Osteoblast, when activated, lays down new bone and osteoclast dissolves minerals by secreting hydrochloric acid locally and digests collagen of the matrix by an enzyme collagenase. The cell surface is loaded with receptors for hormones, vitamins, prostaglandins and other chemicals. When the receptors bind with appropriate chemicals, it either stimulates a specific cell function or suppresses it. Osteoblast and osteoclast communicate with each other by chemical signals. After midlife, the activities of osteoclasts are not matched by the slower speed of the activities of osteoblasts. This results in a weaker bone which is prone to compression fractures of the spine and neck of the femur with minor trauma.

Conditions leading to Osteoporosis:

The white race, female sex, small stature, sedentary life, alcohol, cigarette smoking, prolonged bed rest, space flights, prolonged use of corticosteroids, falling blood levels of sex hormones, food fetishists, thyrotoxicosis, hyperactive parathyroid, heparin, certain oral diabetic pills, certain antidepressant drugs, rheumatoid arthritis. Diabetes, liver disease, vitamin C deficiency, genetic disorders involving collage formation, etc. are some of the causes of osteomalacia. Osteoporosis may coexist with osteomalacia.

Symptoms:

In the early stage, the patients are symptom free, pain from compression fracture of the spine may be the first symptom. Subsequent symptoms are all related to deformities of bones from fractures and pain in muscles and connective tissues.

Treatment and prevention:

Until recently vitamin D and calcium supplements were the mainstays of treatment. Now, it is well demonstrated that this has failed to reverse osteoporosis, moreover, renal stones and high serum calcium may lead to heart disease and other complications.

At present vitamin D 800 iu to 1000 iu along with 1000 mg of calcium daily is recommended.

Other commonly prescribed medications:

1. Bisphosphonates, a synthetic analog of pyrophosphate, are given orally but IV formulation is also available. It inhibits osteoclast mediated bone resorption. GI side effects of bisphosphonates are significant and many patients are unable to continue this drug. The long term use is associated with subtrochanteric fracture of the femur and necrosis of the jaw bone.

2. Raloxifene, an estrogen receptor modulator, is useful in post-menopause patients. It stimulates osteoblasts in new bone formation. Side effects are edema, hot flashes, and deep vein thrombosis.

3. Strontium ranelate. Available as tablets, it increases bone density and strength. Its use is associated with an increase in heart attacks and deep vein thrombosis.

4. Denosumab, an antibody to RANKL. The RANKL is a signal pathway to osteoclasts by osteoblasts which is the first signal in the process of bone resorption. The drug is given by subcutaneous injections every 6-month interval. Pain and cellulitis at the injection sites are reported.

5. Teriparatide, a recombinant human parathormone peptide. Given subcutaneous injection daily. It increases new bone formation and bone density. It may cause high serum and urine calcium, nausea, headache and dizziness are common.

6. Calcitonin, derived from salmon fish, is given as an injection or by nasal spray. Calcitonin is a good pain reliever in compression fractures. Prolonged use is associated with liver cancer.

7. Anti-sclerostin antibody, is osteocyte secreted protein, given by IV, that increases bone density in post-menopausal women.

8. Several other Androgen and estrogen modulators are in the process of evaluation.

 

 

edited 2020.

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