principles of Knee replacement, talking about the problems with sizing in TKR

1. The Problem of Under- or
Oversizing
of
Total knee replacement
Ahmed Alqatub
4th year ICMS Ortho
Mr Mahmood Shuhab Wahab
Consultant orthopedic
surgeon

2. • Femoral size is determined by the AP
dimension as the reconstruction of the
posterior condylar offset is important for
reconstructing knee kinematics.
• A mediolateral overhang of more than 3 mm
irritates the capsule, the iliotibial band (ITB) or
collateral ligaments and causes pain.

3. Tips
• Undersizing the tibial component can cause
early loosening due to missing cortical
coverage.
• A medial overhang can cause irritation of the
medial collateral ligament (MCL) and medial
capsule; a lateral overhang can irritate the
lateral capsule, the ITB or the popliteus
tendon.

4. • Oversizing the femoral component in a
posterior referenced system leads to
overstuffing of the patellofemoral joint and
increased patellofemoral pressure.
• Undersizing the femoral component in a
posterior referenced system means notching
of the anterior cortex and a possible risk of
fracture.

5. • Oversizing the patellar component is
uncommon and causes irritation of the
surrounding soft tissues.
• Undersizing leaves bone and/or cartilage
uncovered and can cause persistent pain.

6. Sizing the Femoral Component
• The most common problem is adequate sizing
of the femoral component, as this is
influenced by the underlying principles,
“anterior referencing” or “posterior
referencing”

7. Oversizing the femoral component in a posterior referenced system
overstuffing of patellofemoral joint

8. Undersizing the femoral component in a posterior referenced system anterior
notching

9. Oversizing the femoral component in an anterior referenced system tightness
of flexion gap

10. Undersizing the femoral component in an anterior referenced system flexion
gap instability

11. Femur problems

12. Increased flexion of the femoral component to fill flexion gap

13. Femoral undersizing with a posterior referenced TKR system: notching of the
anterior femoral cortex

14. Undersizing of the femoral component with an anterior referencing TKR system:
On the left side correct femoral sizing with restoration of the posterior condylar offset.
On the right side undersizing of the femoral component with decreased posterior condylar offset and consecutive flexion gap
instability and decreased range of motion

15. Lateralisation of femoral TKR
implant, if implant does not cover
mediolateral width

16. Sizing the Tibial Component
• Undersizing the tibial component can cause early
loosening due to missing cortical coverage
• A medial overhang of the tibial component can
cause irritation of the MCL and medial capsule
• A pronounced lateral overhang can irritate the
lateral capsule, the iliotibial band or the
popliteus tendon.
• Only 26 % of the tibial components achieved
optimal fit, whereas 49 % showed posterolateral
overhang.

17. Tibia problems

19. • In order to achieve
correct rotational
alignment, a
posterolateral overhang
of <2 mm can be
necessary with the use
of symmetrical implants

20. • Oversizing of the tibial
component with medial
overhang and consecutive
irritation of the medial
collateral ligament

21. Sizing the Patellar Replacement
• Oversizing the patellar component in diameter
is very uncommon and can cause irritation of
the surrounding soft tissues (retinacula,
tendons)
• Undersizing the patellar component in
diameter leaves bone and/or cartilage
uncovered and can cause persistent pain.

22. • The most common problem in sizing the
patellar component is reconstruction of the
pre- operative patellar thickness
• If the remaining bone is less than 10–12 mm,
the risk of fracture increases
• If bone resection is less than the patellar
implant (usually 8–10 mm), overstuffing of the
patellofemoral joint can cause pain

23. Patella problems

24. • Cranial overhang of an
all-poly patellar
replacement

25. Recommendation
• As a general rule, upsizing is recommended in
a posterior referenced system, and downsizing
is recommended in an anterior referenced
system, if the measurement is in between
sizes.

26. • In the case that the chosen femoral size
doesn’t cover the whole mediolateral width, a
lateralisation rather than medialisation of the
implant is recommended in order to improve
patellar tracking.
• A mediolateral overhang of more than 3 mm
irritates the capsule, the ITB or collateral
ligaments and causes pain

27. • In order to achieve correct rotational
alignment, a posterolateral overhang of <2
mm can be necessary with the use of
symmetrical implants
• Proper sizing of the tibia prevent early
loosening or postop knee pain.
• Patella proper sizing.

28. • Thank you