The document discusses challenges with sizing components in total knee replacement. It notes that undersizing the femoral component in a posterior-referenced system can cause notching of the anterior cortex, while oversizing can cause patellofemoral overstuffing. For the tibial component, undersizing risks loosening due to lack of cortical coverage, while oversizing risks soft tissue irritation from overhang. Both under and oversizing the patellar component can cause pain; undersizing leaves bone uncovered while oversizing irritates surrounding tissues. Proper sizing of all components is important for reconstructing knee kinematics and avoiding post-operative pain.

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

2. • Femoral size is determined by the antero-
posterior 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. Increased flexion of the femoral component to fill flexion gap

12. Femur problems

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. 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.

16. • 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

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

18. 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.

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. Tibia problems

22. 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 uncov-
ered and can cause persistent pain.

23. • 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

24. Patella problems

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

26. • Thank you