Chapter: 4

Fluoride and Biological Tissues

Fluorine being an electronegative element and having a negative charge (F-) is attracted by positively charged ions like calcium (Ca++). Bones and teeth having highest amount of calcium in the body, attract the maximum amount of fluorine which is deposited as calcium fluoro-apatite. At the same time the unbound calcium, for reasons unknown, is lost from certain locations in the same tissue.

The incidents of fluorosis is higher in tropical and subtropical countries probably on account of higher drinking water consumption (Srikantia, 1977). It is observed that in adults, exposed to high fluoride ingestion, the hydroxyl bonds of the hydroxy apatite material in bone are partly replaced by fluorides. It is surmised that in order to immobilise fluorine from the circulating fluoride phase in the body (blood and cellular fluids), the bodyís defence mechanism fixes excess fluorides into hydroxy apatite material of the bone by replacement of OH- by F- (Teotia and Teotia, 1992) irreversibly till the exposure continues. In the process, the rate of synthesis of bone material (hydroxy apatite) is considerably increased leading to excessive bone formation or Osteosclerosis, a basic symptom of subjects suffering from skeletal fluorosis.

Effect of fluoride on bones, skeletal system and skeletal fluorosis

The effects of the fluoride on the bone depends on the type of bone and its organic and inorganic constituents. There are 200 distinct bones in the body. They are found in different regions in the body as follows:

The spine or vertebral column (Sacrum and coccyx included)

26

Bones in the head (Cranium)

8

Face

14

Hyoid bone, sternum and ribs

26

Upper extremities

64

Lower extremities

62

Total

200

The bones are grouped into 4 classes on the basis of shape and size.

Long bones

These are found in the limbs. A long bone consists of a shaft (diaphysis) and two extremities (epiphysis). The bones belonging to this class are: Humerus, Radius, Ulna, Femur, Clavide, Metacarpal and Metatarsal and Phalanges. The shaft (diaphysis) is a hollow cylinder, the wall of which is made-up of a special type of bone i.e. compact bone/ Densebone/ Cortical bone. The epiphyses i.e. two ends of the long bone are made up of a spongy bone/or Cancellous bone. with a thin covering of a compact bone substance.

Short bones

These bones are Cancellous/ spongy throughout except at their surfaces where there is a thin crust of compact bone substance.

Flat bones

These bones are composed of two thin layers of compact tissues, enclosing a variable quality of Cancellous bone.

Irregular bones:

The irregular bones are the vertebrae, sacrum, coccyx. Their structure is same like other bones with an outer layer of compact with spongy/bone within Cancellous.

Based on the microscopic structure bones are classified into Cancellous and cortical types.

Cancellous bones

Cortical bones

Porous

compact

Osteons not well defined

Osteons well defined

Copious blood supply

Scanty blood supply

Haversian system does not exist

Haversian system well developed

Larger surface area is exposed to blood supply due to the presence of trabecular bone surfaces and bone engulfing bone marrow

Limited surface area exposed to the blood supply due to compactness of the bone

 

Characteristic structural changes in fluorosed bones

Skeletal fluorosis is not easily recognisable until the disease has developed to an advanced stage. Excessive quantities of fluoride when deposited in the skeleton is more in Cancellous bone compared to cortical bone. Changes in the bone will then be revealed through radiographs. Maximum ill effects of fluoride are detected in the neck, spine, knee, pelvic and shoulder joints. It also affects small joints of the hands and feet. The usual complaints of the patients, viz., pain in the neck, back, joints and rigidity begins in regions where Cancellous bones predominates. With increased severity of skeletal fluorosis, pain is associated with rigidity and restricted movement of cervical and lumber spine, knee and pelvic joints as well as shoulder joints.

When calcium fluor-apatite deposition takes place in the bone, the bone density and bone mass get increased. In the backbone, the perforations through which nervous and blood vessels pass through, are constricted, leading to excess pressure on nerves and blood vessels culminating in paralysis and excruciating pain.

Dental fluorosis

Dental fluorosis is an aesthetic and social problem besides being a health problem. The discoloration of teeth may change the colour from white, yellow brown to black. The discoloration may be in spots, streaks invariably horizontal in orientation, as during development new layers of the matrix are added horizontally.

Parts of a tooth

  1. Crown. Visible into the oral cavity.
  2. Root. Embedded in the jaw bone.

Structural parts of teeth

Enamel The outermost covering of the tooth, is the hard structure in the body, with inorganic compounds, mainly with the calcium salts. Enamel protects the tooth beside giving colour and lustre.

Normally healthy dental enamel is semi-transparent, smooth and milk white in appearance. However, appearance of white opaque patches on the enamel may be indications of initial phases of dental fluorosis. In extreme cases of fluorosis not only does the entire dental enamel turn opaque white to brown, but the teeth tend to break off easily and even their shape may begin to be affected.

Dentin It is also a calcified structure forming bulk of the tooth and is constituted of collagen fibres.

Cementum Another calcified structure covering the root portion.

Pulp The central part of the tooth contains nerves and blood vessels serving nutrition to the tooth and carries sensation.

Periodontal ligament A bond of connective tissues attaches tooth to the alveolar bone. It carries pain, pressure sensation and allow movement of tooth within physiological limits. The root of the tooth is well protected by gingiva or gum.

In dental fluorosis the discoloration will be away from the gums and on the enamel surfaces and it can never be removed as it is an integral part of tooth matrix. Calcium rich constituents of teeth, viz., enamel and Dentin have strong affinity for fluoride during formation of teeth. Fluoride combines with calcium during the mineralisation of teeth forming calcium fluoro-apatite crystals.

Enamel matrix is laid down in incremental lines before and after birth. Hence, dental fluorosis is invariably seen as horizontal lines or bands on the surface of teeth and never as vertical bands. It may also appear as spots.

Teeth commonly affected are 1) central incisors 2) lateral incisors and 3) molars of the permanent dentition. Fluorosis affects both the inner and outer surfaces of teeth.

Teeth affected by fluorosis being poorly calcified (hypo-mineralised) loose enamel under the normal masticatory stress. Enamel has no regenerative capacity. Once it is lost, it is lost forever. The Dentin is then exposed. Cavities formed in Dentin spreads much faster and involves the pulp easily, leading to loss of teeth.

The teeth once affected by dental fluorosis cannot be reverted to normal. But the disclosed teeth can be masked by bleaching and or by other methods.

Dental fluorosis is prevalent in children who are born and brought up in an endemic area for fluorosis. It can occur both in milk teeth and permanent teeth.

Effects of fluoride/ fluorosis on soft tissues/ organs/ systems

The conventional belief that fluoride affects only bone and teeth has been negated in recent years, as the evidence on the involvement of the soft tissues/ organs/ systems of the body are convincing. Although radiographs taken on the fluorosed individuals do reveal that ligaments do calcify, very little attention was paid in past to understand the extent of soft tissue involvement in fluorosis.

Convincing evidence now demonstrate the damage or involvement of: 1) Skeletal muscle 2) Erythrocytes 3) Gastro-intestinal systems as well as 4) Ligaments in human fluorosed patients. There are evidences of involvement of other organs and systems of the animal models, viz., kidney, liver, adrenal gland and reproductive organs.

Studies have shown that skeletal muscle is directly involved in fluorosis. Muscle involvement was earlier considered as ìsecondaryî effect due to neuronal involvement. The electron microscopic observations and biochemical data suggests that there is primary muscle destruction in fluorosis. It is evident from patient of fluorosis that they suffer from muscle weakness, loss of muscle energy and cannot carryout normal routine work.

Fluoride toxicity, fluorosis and its effects on red blood cells

As red blood cell membrane is an important structural entity which lodges the chemical factors responsible for blood group substances, considerable work on membrane structure and function has been carried out. It is now known that when fluoride is ingested, it will also accumulate on erythrocyte membrane, besides other cells, tissues and organs. The erythrocyte membrane in turn loses calcium content. The membrane which is now deficient in calcium content, becomes pliable and is thrown into folds. The RBCs attain the shape of an amoeba with pseudopodia like folds projecting in different directions. Such RBCs are termed as Echinocytes. The Echinocytes will be found in large numbers, depending upon the extent of fluoride poisoning and duration of exposure to fluoride. The RBCs, in human beings, have life span of 120-130 days, the Echinocytes undergo phagocytosis (eaten-up-by macrophages) and are eliminated from the circulation. This would mean that RBCs in individuals exposed to fluoride poisoning shall not live their entire life span of 120-130 days, but are likely to be eliminated as Echinocytes. This would lead to low haemoglobin levels in patients chronically ill due to fluoride toxicity.

Effects of fluoride poisoning on the gastro-intestinal mucosa

Acute abdominal pain, diarrhoea, constipation, blood in stool, bloated feeling (gas), tenderness in stomach, feeling of nausea (flu-like symptoms) and mouth sores, loss of appetite are common complaints due to fluoride toxicity.

Fluoride is known to combine with HCl of the stomach and is converted to hydrofluoric acid. Hydrofluoric acid is highly corrosive. The stomach and intestinal lining (Mucosa) is destroyed with loss of microvilli (the structure which is responsible for absorbing the nutrients from food), drying up and cracking of the cell surface and mucus (the slimy substance required for comfortable bowl movements) production is hampered.

Neurological manifestations

Nervousness, depression, tingling sensation in fingers and toes, excessive thirst (Polydypsia) and tendency to urinate frequently (Polyurea) are controlled by certain regions of brain that appears to be adversely affected.

Allergic manifestations

Very painful skin rashes which are perivascular inflammations, pinkish red or bluish red round or oval shaped spots on the skin prevalent in women and children that fade and clear up in 7-10 days can also occur.

Urinary tract manifestations

Urine may be much less in volume yellow-red in colour and itching in the genitals may occur.

Ligaments and blood vessels calcification

A unique feature of the disease is that soft tissues like ligaments, blood vessels tends to harden and calcify and blood vessels get blocked. Calcified ligaments and blood vessels can be seen in radiographs.

Chapter: 5