Implant technology Unit 6a Flashcards
Function of bones?
Support and protect internal organs, carry load and enable locomotion
What 3 factors determines the location and mode of fracture?
The geometry and structure of the bone
The loading mode e.g. compression, bending, etc.
The loading rate
Why do torsional fractures tend to occur at the distal tibia rather than proximal?
Because the polar moment of inertia is smaller distally compared to proximally
What fracture type is seen after a pure bending force is applied?
Transverse fracture
What fracture type is seen in pure compression?
Oblique fracture
What fracture type is seen in combined compression and bending?
Butterfly type fracture
What fracture type is seen in pure torsion?
Spiral fracture
What fracture type is seen in high energy fractures?
Comminuted fracture
Summarise the fracture process?
Energy is delivered to the limb
The energy is transferred via the soft tissue to the bone which absorbs the energy
The bone break and energy is released back to the soft tissue
The broken bone and damaged tissues bleed and cause a build up of blood around the damaged area. Such a blood collection is termed a haematoma
An acute inflammatory response occurs around the damaged area which causes pain to the victim and commences the processes which lead to healing
What is the process of secondary (natural) bone healing?
Weeks 0-2: Haematoma is invaded by macrophages which clear up dead and damaged tissues and the haematoma
2-6: New capillaries grow into the fracture haematoma bringing with them cells of healing e.g. osteoblasts, fibroblasts, etc.
6-12: New bone tissue is laid down in the endosteal space from the residual living bone and eventually the two ends are reunited as a ball of provisional “callus”
Up to 12 months: Callus continues to form woven bone which gradually remodels to form a cortex
Up to 2 years: The callus matures so that the trabecular pattern is reformed and the bone is able to cope with stresses it encounters
What is primary bone healing?
When there is no micromotion taking place, the fracture heals without external callus formation. New Haversian systems grow directly across the fracture gap. It is quicker than secondary bone healing but the bone isn’t initially as strong
What is Wolff’s law?
Bone in a healthy person or animal will adapt to the loads under which it is placed
Why is the rigidity of a recently fractured bone with a callus similar to the rigidity of the normal bone?
Although it’s Young’s modulus is reduced, the second moment of inertia is increased to compensate. As the callus hardens, the cross-sectional area decreases again
Fracture management involves reduction followed by holding. How can a fracture be reduced?
Anaesthesia is often provided which relaxes the muscles which otherwise are in spasm and would make the process very difficult.
Closed reduction - manipulation of the fracture fragments
Open reduction - the fracture site is opened up surgically and fracture fragments are restored to alignment directly
Give 3 examples each of internal and external fracture holding?
External:
- Plaster of Paris
- Traction
- External fixator
Internal:
- Plates and screws
- Compression screw
- Rods and nails
What is plaster of Paris made of?
Calcium sulphate - it is extracted in crystal form and heated to remove the water and make calcium sulphate hemihydrate. When this compound is mixed with water, it forms crystals again and sets to a solid. It is an exothermic reaction and so care must be taken when applying to avoid damage to the patient’s skin
What is a hard coated bandage?
Calcium sulphate hemihydrate is dissolved in ether which contain no water. Starch is added and the whole paste in coated on bandage. The wet bandage is then dried
What effect does the starch have?
It doesn’t alter the strength of the cast, however it does speed up the setting process
What other factors influence the speed at which the plaster of Paris sets?
The temperature of the water used
Retarders - slow the process e.g. alum and borax
The plaster of Paris consists of two types of crystals. What effect does each have?
Some are long and sharp and others are shorter. The longer crystals occur naturally as alabaster (type of rock). They give the finished cast a hard quality. The smaller crystals give the cast a softer feel
How does the plaster of Paris function? (2 ways)
By encasing the limb in a rigid exoskeleton it provides support to the soft tissues which in turn support the broken bone
By moulding the cast against the fracture it is possible to obtain a 3-point fixation system
Disadvantages of a plaster of Paris?
If the casting is prolonged, the encased joints become stiff and muscles waste through disuse. This prolongs overall rehabilitation
What is functional or cast bracing?
A combination of casting and hinges
When are the braces suitable for application?
After 2-3 weeks when the soft tissue injury has settled and there isn’t swelling
