Acetabular component Flashcards
What is the orientation of the normal acetbulum?
45 degrees relative to the coronal and sagittal planes of the body and faces slightly backwards
What are the different features to be considered in acetabular design?
Size of the head and cup Metal backing plate or not Thickness of HDP layer Outer dimension of the acetabulum Acetabular cup can be placed more centrally by deepening the original socket
How does the contact pressure change with the size of the head?
Contact pressure increases as size of head decreases
What surface angle is recommended to minimise contact pressure on the cup?
At least 120 degrees
What is the radial clearance between the head and cup?
The difference in the radius between the cup and head (cup always slightly bigger)
Why must the radial clearance be reduced when the thickness of the HDP cup is reduced or a stiffer material is used?
To spread the point contact load over a greater area in order to avoid excessive contact stress on the HDP
What are the advantages of a larger outer cup diameter?
Better anchorage to the pelvis
Greater fricitonal torque
How can cups be secured in place?
Cement
Fixed with screws
Push fitted
Threaded stem which is screwed into a female thred cut into the bone
What are the advantages of a metal backing cup?
Helps to hold the plastic in place
Reduces plastics tendency to creep and distort
Thereby avoiding high contact stresses and focal wear on the HDP
What is the disadvantage of a threaded cup?
Bone resorption resulting from the high stress concs in the region of the sharp threads
What are the disadvantages of a metal backing cup?
Increases head-cup contact pressure - depends on thickness of the HDP layer (thinner layers give higher contact pressure)
What can cause cup loosening?
Mechanical overstressing
Biological reaction to the ingress of HDP wear particles - leads to resorption of the trabecular bone at the bone-cement interface
How can stesses at the bone-implant interface be reduced?
Retaining the subchondral bone Using a thick layer of HDP Using a thick layer of cement Using a metal backed cup ALL CREATE A STIFFER STRUCTURE
What have clinical trials of a stiffer acetabular component shown?
Not shown any improvement - therefore loosening is probably due to mainly biological effects of HDP wear particles
How can cup position affect loads at the joint?
If cup moved nearer to midline - lower load at the hip joint
