Funtional factors affecting surface shape & degree of motion constraint

Question 1

What does constraint mean?

Answer 1

Relationship between tibial and femoral bearing surface geometries (more constrained = less freedom of movement)

Question 2

What is essential for an anatomically shaped replacment to work?

Answer 2

Retain ligaments during surgery and made to function with the prosthessis

Question 3

What prosthesis is used if there are no ligaments intact?

Answer 3

Hinged prosthesis

Question 4

What freedom does a hinged prosthesis allow?

Answer 4

Constrains the motion to a single axis of rotation with total stability

Question 5

What are the issues with a hine?

Answer 5

Has no ‘give’ under lateral and long axis rotational loading (can transmit high shear forces)

Question 6

Which 2 prosthesses have been designed to allow some axial rotation at hinge to relieve stresses?

Answer 6

Spherocentric

Attenborough

Question 7

In which condition does the ACL tend to be either destroyed or so attenuated that is is of no mechanical value?

Answer 7

ACL

Question 8

Which cruciate ligament is more often preserved?

Answer 8

PCL

Question 9

If the PCL is removed what must be incorporated into prosthesis design?

Answer 9

Posterior stabilisation mechanism (enable femur to rotate on the tibial plateau without sliding too far posteriorly)

Question 10

What are the theoretical advantages of retaining the PCL?

Answer 10

Provides some degree of antero-posterior stability

May preserve some proprioceptive activity

Question 11

Which activity in particular benefits from PCL retention?

Answer 11

Walking on stairs

Question 12

What are the disadvantages of retaining the PCL?

Answer 12

constricts a free surgical dissection of the posterior capsule (which may limit full extension)
Enourages fem component to slide over tibial (wear) - removal allows more occngruent surfaces (reduce HDP wear)
Removal may facilitate deformity correction

Question 13

What wass found to be the only significant difference in retaining or removing the PCLin a recent study?

Answer 13

Retention offer better ROM

Question 14

What happens if the PCL is too loose?

Answer 14

Forward movement of the femur on the tibia (so the normal rollling back motion no longer works)
Possible compression of the 2 joint surfaces posteriorly (generating high contact stresses)

Question 15

Why is the tibial component normally slopped back by about 10 degrees?

Answer 15

To encourage the femoral component to roll back on the tibial component