Constraints in knee replacement

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Epidemiology

  • Instability causes 25% of TKR revisions.

Aetiology

  • Generalised soft-tissue laxity.
  • Inadequate flexion-extension gap balancing.
  • Improper component alignment or positioning.
  • Specific ligamentous insufficiency.

Instability results when the available ligaments and soft-tissue structures, in combination with the prosthesis design and limb alignment, are insufficient to provide the necessary stability for adequate function.

Terminology and level of constraint

In order of increasing constraint from minimum to maximum constraint:

  • PCL retaining prosthesis.
  • PCL substituting – Posterior stabilised either deep dish or cam and post prosthesis.
  • Box and cam for sagittal constraint, but no varus/valgus constraint.
  • Unlinked constrained: this prosthesis provides varus/valgus constrained. It has high tibial post, which Permits 2-3° of varus/valgus and 2-3° of rotation. This type of prosthesis requires a stem extension to aid transmission of the increment in stresses generated by the increase constraint.
  • Rotating hinge: Link between the tibial and femoral components that restrict varus/valgus, but the tibial bearing is a rotating platform. This type of prosthesis is regarded as fully constrained devise where there is no translation nor varus/valgus movement allowed.

Reported (unproven) pros and cons

Cruciate retaining (CR)

Cruciate sacrificing (PS)

Pros

  • Proprioception
  • Bone stock preservation
  • Guide to joint line reconstruction
  • More normal kinematics – stair climbing
  • ­ quads strength
  • ¯ shear forces at bone-implant interface
  • Fewer patellar complications (patellar clunk syndrome)

Pros

  • Easier ligament balancing
  • Greater versatility in severe deformity
  • More predictable kinematics
  • Higher ROM
  • Avoids risk of PCL rupture

Cons

  • PCL failure
  • Less predictable rollback
  • Ligament balancing difficulties

Cons

  • ­ shear at interface
  • Potential for jumping the cam – flexion instability
  • Patellar clunk syndrome
  • More bone resection

 

NO difference :

  • Function
  • Patient satisfaction
  • Survivorship

Indications for Posterior stabilized knee (PS)

  • Quads weakness
  • Patellectomy
    • Insall CORR 2000 – better functional and pain scores using PS implant
  • Inflammatory arthropathy
    • Conflicting data – Hanyu showed 10 year 93% of the PCL in RA patients, AND 6% late dislocation rate in the PS group.
    • Others have shown inferior results with cruciate retaining (CR) in this group.
  • Varus > 15°
    • Laskin CORR 1996 – 10 year retrospective review of > 100 knees done.
    • Group A had > 15° varus and CR TKR, Group B > 15° varus and PCL post, Group C < 15° varus and CR TKR.
    • Group A at 10 years had - ¯ ROM, ­ “lateral wedge sign”, ­ medial tibial pain, ­ radiolucent lines, ¯ survival (72% Vs 92%).
    • No difference between group B and C.
  • “Severe” valgus – No data
    • That after which coronal balance is achieved results in an incompetent MCL.
    • Suggested - MCL grade 2 or greater incompetence.