Human bones are hard and rigid structures that develop during life as cartilaginous tissue in fetal life is gradually replaced with bone tissue. Eventually this tissue undergoes calcification which makes it hard and strong. Throughout life the bone is undergoing remodeling due to the action of bone cells like osteoblasts and osteoclasts. The bone tissue is maintained by another bone cell known as an osteocyte. In order to make bone, osteoblasts keep depositing osteoid tissue which becomes calcified. Simultaneously osteoclasts are resorbing this bone. However, the rate of deposition and resorption are kept in balance so that there is not an excess nor deficiency of bone mineral. To maintain normal bone mineralization, the body needs an adequate supply of calcium and phosphate. This is derived from the food (dietary intake) when it is absorbed from the gut. Vitamin D plays an important role in calcium absorption from the gut and also on bone mineralization. When there is a deficiency of vitamin D, these processes are affected and leads to diseases of the bones.
What is osteomalacia and rickets?
Osteomalacia and rickets are conditions characterized by defective bone mineralization due to to vitamin D deficiency leading to softening of the bones. Rickets occurs in children and osteomalacia in adults. Therefore osteomalacia is sometimes referred to as “adult rickets”. Vitamin D deficiency is largely linked to two factors – poor dietary intake of the vitamin and/or lack of exposure to sunlight. However, other diseases may affect vitamin D absorption and production. Less commonly, rickets and osteomalacia can also occur with a decreased dietary intake of calcium and phosphorus over a very long period but this is uncommon. Although rickets and osteomalacia are not commonly seen in developed countries, it is more prevalent among the elderly and frail and particularly among women who are fully covered (from head-to-toe) and live in northern latitudes.
Several sterol compounds make up vitamin D but the most important is cholecalciferol (vitamin D3). Most of the other compounds are very similar to vitamin D3 and derived from the diet. Vitamin D3 (cholecalciferol) is manufactured when 7-dehydrocholesterol, a substance that is normally in the skin, is exposed to sunlight. Irrespective of whether these vitamins are derived from sunlight exposure, food or through supplements, it still has to undergo further hydroxylation in the liver and kidney before it becomes biologically active. Cholecalciferol is first hydroxylated to 25-hydroxycholecalciferol in the liver and then to 1,25-dihydroxycholecalciferol (calcitriol) in the kidney. Calcitriol is the most biologically active form of vitamin D.
Calcium and Phosphate
Calcitriol has three important actions in the body with regard to calcium and phosphorus. Firstly calcitriol increases absorption of calcium and phosphorus from the gut. Secondly it promotes reabsorption of phosphate in the kidney. Lastly, calcitriol acts on bone to release calcium and phosphate. By these effects, calcitriol increases the calcium and phosphate levels in the blood and tissue fluids. The presence of calcium and phosphorus in these fluids promotes bone calcification which is also facilitated by calcitriol.
Parathyroid hormone (PTH) is secreted from the parathyroid gland and regulates the calcium levels in the blood. When the calcium levels drop in the blood, PTH acts in several ways to restore these levels. It indirectly increases osteoclastic activity leading to release of calcium from the bones, increases absorption of calcium from the gut and the reabsorption of calcium from the tubules (kidney) so that calcium is not lost in the urine.
Vitamin D deficiency leads to a reduction in calcium and phosphate levels and with the lack of calcitriol, there is a decrease in bone calcification. In order to compensate, the parathyroid glands increases PTH secretion. This, however, further exacerbates the condition by causing a release of calcium from the bone. Phosphate reabsorption from the tubules are not influenced by PTH or any other hormonal factor and therefore cannot be stemmed. In fact, PTH serves to promote phosphate loss.
Prolonged PTH secretion which causes increased osteoclastic activity weakens the bones. The osteoblasts do increase its normal activity and lay down new bone tissue but without calcium and phosphates, calcification cannot occur. This new uncalcified bone tissue gradually replaces the older calcified bone which is being resorbed by the osteoclasts. Progressive deminaralization of the bone in this manner occurs over a long period of time. In children it affects normal bone growth and therefore there is a delay in normal growth and abnormalities in bone growth. In adults the bone growth is complete but bone remodeling is abnormal leading to softening of the bones that are easily fractured.
Causes of Osteomalacia and Rickets
Osteomalacia and rickets is mainly caused by vitamin D deficiency. This may not always be due to a reduced dietary intake or sunlight exposure (classical vitamin D deficiency). A number of conditions may contribute to the deficiency despite adequate intake of vitamin D in the food and sufficient sunlight exposure. These conditions may include :
- Gastrointestinal diseases like celiac disease and gastrointestinal surgery like partial/complete removal of the stomach (gastrectomy) which affects proper digestion of foods to release vitamin D (maldigestion) or absorption of vitamin D from the gut (malabsorption).
- Liver or kidney diseases which affects the hydroxylation of vitamin D into its more biologically active forms.
- Certain medication can contribute to osteomalacia like those drugs used to treat seizures, bile acids, certain antibiotics and biphosphonates used for osteoporosis.
- Substances like aluminium and fluoride if consumed in high doses.
- Genetic disorders which may cause :
- Vitamin D-resistant rickets type I and II
- Hypophosphatemic rickets
- Tumors like tumor-induced hypophosphatemic rickets
- Children and the elderly
- Covering of the face and body most of the time, particularly in individuals living in northern latitudes
- Darker-skinned individuals
- Breastfeeding infants for long periods of time without vitamin D supplementation
- Family history
- Individuals who do not venture outdoors.
Signs and Symptoms
Without normal calcification, bone tissue cannot undergo calcification which makes it hard and rigid. The clinical presentation varies to some extent in rickets and osteomalacia since it affects children and adults respectively. The signs and symptoms may vary depending on the degree of bone involvement. In the early stages, osteomalacia is asymptomatic but gradually pain develops and there is difficulty with normal movement. With rickets, the delay and abnormal bone development is not detected until a period of time has passed or deformities become obvious. The bones are unable to withstand the force or strain it normally would as the body mass increases in the growing child.
- Delayed growth.
- Abnormalities / deformities :
- Bowed legs (genu varum).
- Abnormal curvature of the spine.
- Protruding breastbone (‘pigeon chest’).
- Thickening at the wrists and ankles.
- Delayed closure of the anterior fontanelles (skull).
- Bone pain particularly in the spine, legs and pelvis.
- Muscle weakness.
- Bone pain worse in the legs, hips, pelvis, lower spine and ribs.
- Abnormal curvature of the spine.
- Dental deformities.
- Muscle weakness particularly in the arms and legs.
- Changes in gait – waddling.
- Hyporeflexia – reduced or absent reflexes.