When you get injured casts and splints help to protect the broken bone and injured soft tissues. Doctors manipulate the fracture to align the broken bone and to stabilize they apply cast or splint. They hold the bones in place while they unite. They also help to reduce pain, swelling and muscle spasm.
In some instances following surgery splints and casts are applied to immobilize bone or joints.
Splints are nothing but “Half casts” and offer less stability than the casts. But splints can be malleable, adjustable and can be accommodated according to the type of injury, swelling etc.
Doctor decides which type of support suits according to the situation.
TYPES OF SPLINTS AND CASTS: 
Cast is custom made as they must fit according to the shape of injured limb. They can be made from Plaster of Paris or fiber glass. 
Plaster of Paris or fiber glass can be moulded according to individual needs to make custom made splints. Now a day’s readymade splints of various sizes and shapes, with Velcro straps are available which are comfortable and easier to use. 
Recently thermoplastic material is available which has advantage of light weight, good cosmetic appearance, from which casts and splints can be made.
Plaster of Paris: Plaster bandages are the traditional material used to make casts and splint. Plaster can be moulded better than fiber glass for some uses and cheap. Disadvantage is it is heavy and weaker than the counterpart.
Fiber glass: It is lighter in weight, strong enough to hold the weight of patient, and easy to apply.
APPLICATION: 
Initially after a fresh injury splints are applied, as the swelling subsides, full cast can be applied if necessary. Before application of cast or splint it is necessary to give a protective layer of soft cotton padding and extra padding to bony prominences. 
Both the materials are available in strips or rolls which are dipped in water and applied over the padded area. Splint or cast must fit the shape of the injured limb and also covers the joint above and below the broken bone.
In some instances as the swelling comes down cast may have to be reapplied. In some rare case the as the swelling comes down and fracture may loose the alignment, then it may be necessary to realign and reapply the cast. 
In some cases when the fracture is healing the cast may be replaced with splint to facilitate rehabilitative exercises.
CARE OF THE SPLINT OR CAST:
In the first 48 hours to 72 hours you may experience a sense of tightness inside the cast or splint which is caused due to injury. In case of splint doctor can help you to adjust the splint.
In order to decrease the swelling it is advised to keep the injured limb elevated, do some active movements in the toes or fingers. 
You should not wet, cut or insert any objects into the plaster. In the case of lower limb splints or casts you are not advised to walk on them.
In case if you want to have bath protect the cast or splint with a plastic cover or the waterproof plaster covers that are available in the market.
WARNING SIGNS OF TIGHT CAST OR SPLINT:
Report immediately to your doctor if you observe any of these
•Increased pain and the feeling that the splint or cast is too tight.
•Numbness and tingling in your hand or foot
•Excessive swelling below the cast. 
•Loss of active movement of toes or fingers
CAST REMOVAL
Your doctors will advice on the time of removal of the cast.
After the removal of the cast skin may be dry, and there will be layers of peeled off dead skin. These can be removed off with good wash with soap and application of moisturizers. You may also feel stiffness of the joints and some wasting of the muscles around the joint, which can be addressed with physiotherapy exercises after the removal of casts or splints.