Ointments, Ointment bases and Ointment preparations

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A semisolid preparation usually containing medicinal substances and intended for external application to the skin or mucous membranes.
o Semi-solid preparations of hydrocarbons (petrolatum, mineral oil, paraffin, synthetic hydrocarbons)
o Strong emollient effect makes it useful in dry skin conditions
o Occlusive effect enhances penetration of active drug and improves efficacy especially in thickened, lichenified skin.
o Provides a protective film on the skin (useful to irritant dermatitis) greasy, sticky, retains sweat (therefore, not suitable in wet weepy dermatitis, hairy areas, skin prone to folliculate, or hot weather conditions)
o Contains no water and does not require a preservative.

Ointment bases:
There are five types of ointment bases which are differentiated on the basis of their physical composition. These are:
i. oleaginous bases
ii. absorption bases
iii. water in oil emulsion bases
iv. oil in water emulsion bases
v. water soluble or water miscible bases

Selection of the base to use in the formulation of an ointment depends on careful assessment of a number of factors, including the following:
 Desired release rate of the drug substance from the ointment base.
 Desirability of topical or percutaneous drug absorption.
 Desirability of occlusion of moisture from the skin.
 Stability of the drug in the ointment base.
 Effect, if any, of the drug on the consistency or other features of the ointment base.
 Desire for a base easily removed by washing with water.
 Characteristics of the surface to which it is applied.
Preparation of ointments:
Ointments are prepared by two general methods depending primarily on the nature of the ingredients.

1. Incorporation of preparation of ointments:

The components are mixed until a uniform preparation is attained. On a small scale, as in extemporaneous compounding, the pharmacist may mix the components using a mortar and pestle, or a spatula may be used to rub the ingredients together on an ointment slab (a large glass or porcelain plate or pill tile). Some pharmacists use nonabsorbent parchment paper to cover the working surface; being disposable, the paper eliminates cleaning the ointment slab. If using an ointment parchment pad, it is best to not allow too long a contact of the ointment with the parchment, as it may soften and tear.

Others will use an ointment mill, an electronic mortar and pestle, or a device called an “Unguator” which allows a pharmacist to place the ingredients in a plastic ointment jar with a special lid that allows for a mixing blade to be used to mix the ingredients in the dispensing container. These devices can be controlled manually or via computer software.

 Incorporation of Solids.

When preparing an ointment by spatulation, the pharmacist works the ointment with a stainless steel spatula having a long, broad blade and periodically removes the accumulation of ointment on the large spatula with a smaller one. If the components of an ointment react with metal (as does iodine), hard rubber spatulas may be used. The ointment is prepared by thoroughly rubbing and working the components together on the hard surface until the product is smooth and uniform. The ointment base is placed on one side of the working surface and the powdered components, previously reduced to fine powders and thoroughly blended in a mortar, on the other side. A small portion of the powder is mixed with a portion of the base until uniform. Geometric dilution is continued until all portions of the powder and base are combined and thoroughly and uniformly blended.
It often is desirable to reduce the particle size of a powder or crystalline material before incorporation into the ointment base so the final product will not be gritty. This may be done by levigating or mixing the solid material in a vehicle in which it is insoluble to make a smooth dispersion.
The levigating agent e.g., mineral oil for bases in which oils are the external phase, or glycerin for bases in which water is the external phase should be physically and chemically compatible with the drug and base. The levigating agent should be about equal in volume to the solid material. A mortar and pestle are used for levigation. This allows both reduction of particle size and dispersion of the substance in the vehicle. After levigation, the dispersion is incorporated into the ointment base by spatulation or with the mortar and pestle until the product is uniform.
Solids soluble in a common solvent that will affect neither the stability of the drug nor the efficacy of the product may first be dissolved in that solvent (e.g., water or alcohol) and the solution added to the ointment base by spatulation or in a mortar and pestle. The mortar and pestle method is preferred when large volumes of liquid are added, because the liquid is more captive than on an ointment slab.
For incorporating a gummy material, such as camphor, pulverization by intervention can be used. The material is dissolved in a solvent and spread out on the pill tile. The solvent is allowed to evaporate, leaving a thin film of the material onto which the other ingredient or ingredients are spread. The material is then worked into the ingredients by trituration with a spatula.

 Incorporation of Liquids.

Liquid substances or solutions of drugs are added to an ointment only after due consideration of an ointment base’s capacity to accept the volume required. For example, only very small amounts of an aqueous solution may be incorporated into an oleaginous ointment, whereas hydrophilic ointment bases readily accept aqueous solutions. When it is necessary to add an aqueous preparation to a hydrophobic base, the solution first may be incorporated into a minimum amount of a hydrophilic base and then that mixture added to the hydrophobic base. However, all bases, even if hydrophilic, have their limits to retain liquids, beyond which they become too soft or semiliquid.
Alcoholic solutions of small volume may be added easily to oleaginous vehicles or emulsion bases. Natural balsams, such as Peru balsam, are usually mixed with an equal portion of castor oil before incorporation into a base. This reduces the surface tension of the balsam and allows even distribution of the balsam throughout the base.
Ointment or roller mills can be used to force coarsely formed ointments through stainless steel or ceramic rollers to produce ointments uniform in composition and smooth in texture. Small ointment mills also find use in product development laboratories and in small-batch manufacture or compounding.

2. Fusion:
By the fusion method, all or some of the components of an ointment are combined by being melted together and cooled with constant stirring until congealed. Components not melted are added to the congealing mixture as it is being cooled and stirred. Naturally, heat-labile substances and any volatile components are added last, when the temperature of the mixture is low enough not to cause decomposition or volatilization of the components.

Substances may be added to the congealing mixture as solutions or as insoluble powders levigated with a portion of the base. On a small scale, fusion may be conducted in a porcelain dish or glass beaker. On a large scale, it is carried out in large steam-jacketed kettles. Once congealed, the ointment may be passed through an ointment mill (in large-scale manufacture) or rubbed with a spatula or in a mortar to ensure a uniform texture.

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