Biomechanics and joint replacement of the hip

Question 1

Femoral rollback and slide mechanism

Answer 1

The femoral rollback is the posterior translation the femur with progressive flexion.

In the native knee rollback is controlled by the anterior cruciate ligament and posterior cruciate ligament, so for good TKA is important to improve quadriceps function and range of knee flexion by preventing posterior impingement during deep flexion.

For that reason CR(CRUCIATE - RETAININIG) prostheses have posterior displacement of femoral condyles similar to a native KNEE.
PS(POSTERIOR-STABILIZED) protheses tibial component contacts the femoral cam causing posterior displacement of the femur.

Biomechanical function of femoral rollback
I. To increase lever arm of quadriceps
Ii. To allow clearance of the femur from tibia in deep flexion

Question 2

Screw home mechanism of knee extension

Answer 2

definition
tibial externally rotates 5 degrees in the last 15 degrees of extension

cause
medial tibial plateau articular surface is longer than lateral tibial plateau
relevance

“locks” knee decreasing the work performed by the quadriceps while standing

Question 3

Q angle

Answer 3

Intersection of lines joining centre of patella with anterior superior iliac spine and tibial tubercle

Normal Q angle 5-20 degree

Woman have increased Q angle compared with men

Angle greater >20 degree= patellofemoral instability

Question 4

Biomechanics of knee arthroplasty

Answer 4

First condylar TKR = cruciate sacrificing
Surface is conforming (radii of tibial and femoral surfaces similar
Purely static mechanical concept

PCL retaining
Low conforming
Round on flat design to allow femoral rollback
Disadvantage:
I. Lift off
ii . Slamdown
Iii. Edge loading of polyethylene, resulting in increased contact stress and wear

PCL substituting/ posterior stabilised
Increased tibial conformity

Question 5

Biomechanical goals of TKR

Answer 5

Restore mechanical axis
Bone cuts perpendicular to mechanical axis
Preserve level of joint line
Balance the ligament
Ensure rigid durable fixation

Question 6

Factors in TKR design that increase the probability of loosening

Answer 6

Flexible implant
Small contact area
Load transfer at edge of contact in unbalanced knee
Features that concentrate stress eg: Peripheral tibial tray pegs