Factors affecting the selection of an ideal ointment base

                                    Ideal ointment base                                    

                                                                      Factors affecting the selection of an ideal ointment base

A- dermatological factors

B- Pharmaceutical factors
A-dermatological factors 

1- Absorption and penetration

Absorption: entry in the blood stream i.e systemic absorption

Penetration: transfer through the skin

The problems of absorption and penetration are:

a- only oint. Base penetrate deep into the skin

b- mainly medicament absorbed into blood stream

c- paraffins don't penetrate the skin
Animal and vegetable fats and oils normally penetrate the skin

d- substances which are soluble in both oil and water are readily absorbed
e- water soluble drugs are absorbed from water soluble bases

f- o/w emulsion release the medicament more readily than the greasy w/o
2- Effect on skin function

Greasy bases interfere with skin function such as heat radiation and sweat. o/w emulsion and other water miscible bases produce cooling effect rather than heating effect and readily mixed with skin secretions

3- Miscibility with skin secretion

Bases miscible with skin secretion  rapid release of the medicament

4- Compatibility with skin secretion

Used bases should have pH about 5.5 because this the normal pH of the skin
5- Free from irritant effect

Used bases should be non irritant and of high standard purity specially for eye ointments

6- Emollient properties

Ointments should keep the skin moisture

7- Ease of application and removal

The applied ointments should be easily applied as well as removed from the skin. Emulsion bases are more preferable as they are softer and spread more readily over the skin. They can be easily removed by simple washing with water
B-Pharmaceutical factors  

1- Stability

To increase the stability antioxidant and antiseptic should be added according to the need depending upon the constituent of the preparation.

2- Solvent properties

Insoluble medicaments should be finely powdered and distributed uniformly inside the base using small portion of oils.
3- Emulsifying properties

Depends upon the amount of water that must be added to the formulation.

4- Consistency

the consistency can be adjusted by using suitable proportion of high melting substances and low melting point substances
Preparation of ointments

By either trituration or fusion

1- Trituration method

The most common method used for preparation of ointments. The medicament is finely powdered and triturated with small amount of the base till homogenous dispersion is obtained. Then followed by addition of the remaining base with good mixing

2- Fusion method

It is used when the ointment contains a number of solid ingredients such as bees wax, cetyl alcohol….etc
In this case it is necessary to melt these ingredients. The melting of ingredient take place in decreasing order of melting points. The medicament is then added to the melted ingredients and stirred thoroughly until cooling and formation of homogenous product.

Liquid should be heated to nearly the same temperature of the melted base before adding it the melted bases.
Other additive in ointments

Preservatives, antioxidants, perfumes and chelating agents may be incorporated in the ointment.

The commonly used preservatives are :

1) Phenolics: phenol (0.2-0.5%) , chlorocresol ( 0.075- 0.12%) .

2) Methyl parapen (0.02-0.03%), propyl parapen (0.02-0.3%).

3) Benzyl alcohol (< or = 3%).

preservatives prevent the bacterial growth when ointments stored for long time.

Antioxidants prevent the oxidation decomposition of the ingredients such as:

a) Lipophilic antioxidants: e.g., BHA & BHT

b) Hydrophilic antioxidants (dissolved in aqueous phase) such as sodium metabisulfite or sodium Sulfite.

Humectants such as glycerin and propylene glycol and sorbitol may be added to prevent the loss of moisture.

Pigments such as iron oxide may be added to give ointments a cosmetic like appearance specially in formulation intended for application to the face.

Perfumes may be also incorporated to give ointment a pleasant odour.
Packaging and storage of ointments

Packaging

Ointment jar and collapsible tubes of different sizes, shapes and capacities are used for storage of ointments. Amber glass containers are used for light sensitive preparations.

Storage

Ointments must be stored at an optimum temperature otherwise separation of phases may take place which is very difficult to remix to get uniform product