The Problem of Under- or Oversizing

ofTotal knee replacement

Ahmed Alqatub4th year ICMS Ortho

Mr Mahmood Shuhab WahabConsultant orthopedic

surgeon

• 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.

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.

• 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.

• 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.

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”

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

Undersizing the femoral component in a posterior referenced system anterior notching

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

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

Femur problems

Increased flexion of the femoral component to fill flexion gap

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

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

Under sized

Lateralisation of femoral TKR implant, if implant does not cover

mediolateral width

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.

Tibia problems

• In order to achieve correct rotational

alignment, a posterolateral overhang

of <2 mm can be necessary with the use

of symmetrical implants

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

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.

• 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

Patella problems

• Cranial overhang of an all-poly patellar replacement

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.

• 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

• 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.

• Thank you