Knee arthroplasty

Aims of TKA

The primary aim of arthroplasty is to achieve:

• Weight-bearing line through knee centre

• Joint line perpendicular to the weight-bearing line

• Soft-tissue balance

• Restoring normal Q angle and joint alignment

Anatomic and mechanical axes

The valgus cut angle is the angle between the femoral anatomical and mechanical axes. The normal anatomical axis or tibiofemoral angle measures 5°–6° of valgus.

The mechanical axis, or weight-bearing line, is the line from the centre of the hip to the centre of the tibiotalar joint; it typically measures 1.2° of varus. Hence, 60% of weight goes through the medial compartment.

Femoral roll-back

Femoral roll-back is the posterior shift in the femoral–tibial contact point in the sagittal plane as the knee flexes.

Aetiology of arthritis

• Idiopathic

• Post-traumatic

• Avascular necrosis

• Inflammatory arthritis

Contraindications to TKA

• Infection

• Neurogenic genu recurvatum

• Deficient quadriceps mechanism (polio)

Constraint ladder within knee implant design

• PCL-retaining (cruciate-retaining, or CR)

• PCL-substituting (posterior-stabilized, or PS)

• Unlinked constrained condylar implant (varus-valgus constrained, or VVC) provides anteroposterior and varus-valgus stability (substitute for deficient collaterals), e.g. CCK, TC3

• Linked, constrained condylar implant (rotating-hinge knee, RHK). Rarely indicated. Used for global instability (total collateral disruption/recurvatum) and severe distal femoral bone loss, osteolysis/fracture

Posterior cruciate ligament (Leo Whiteside)

• A major stabilizing ligament in the normal and pathological knee

• It tightens the flexion space only

• It is a secondary mediolateral stabilizer in flexion

• The only mediolateral stabilizer after releasing collateral ligaments

• PCL function cannot be corrected by polyethylene post