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TKR implant selection and bearing design

Implant choice, and cruciate-retaining versus posterior-stabilised TKR designs.

19 questions 2 source pages 1 fact-check flags

Images appear with the first question taken from each source page — tap a question to open it.

19 questions
Q1What factors determine the choice of implant in TKR?▸
  • Constraint
  • Modularity of the tibial component
  • Bearing fixation
  • Type of PE insert
  • Cemented vs cementless
  • +/- patella
Q2How are TKR implants classified by constraint?▸
  • Unconstrained: CR, P sacrificing, P stabilise (substitute)
  • Constrained: non-hinged / hinged (or rotating hinge)
  • Also fixed vs mobile bearing; metal back vs all PE
Q3What is a non-hinged constrained prosthesis?▸
  • A constrained prosthesis without an axle connecting the tibial and femoral components
  • Used in soft tissue / bony defect
  • Increased stability but more stress on the interface
Q4What are the pros and cons of a mobile bearing in TKR?▸
  • No benefit proven
  • Pros: theoretically increased contact area leading to less wear; theoretically replicates native knee kinematics
  • Cons: spinout, more back side wear
  • Used in hinged implants
Q5What are the features of an all-polyethylene tibial component?▸
  • Cheaper
  • Less modular
Q6What surgical approaches are used in TKR?▸
  • Medial parapatellar - extensile; patella perfusion impaired if lateral release needed
  • Lateral parapatellar - direct access to lateral structures; difficulties in everting patella
  • Smaller exposure: midvastus, subvastus
  • Larger exposure: quadriceps snip, VY / osteotomy
Q7How do you correct gap imbalance in TKR?▸
  • Tight in flexion and extension: decrease PE insert size, resect more proximal tibia
  • Tight in extension only: resect additional distal femur, posterior capsular release
  • Tight in flexion only: downsize femoral component, recess/release PCL, increase posterior tibial slope, release posterior capsule
Q8What are the problems of patellar resurfacing versus non-resurfacing in TKR?▸
  • Resurfacing: fracture, loosening, clunk, AVN
  • Not resurfacing: residual quads weakness (poor moment arm, patellar height not restored), maltracking, residual anterior knee pain
Q9What is the evidence on routine patellar resurfacing in TKR?▸
  • Zmistowski (Rothman) J Arthroplasty 2019: should NOT routinely replace the patella
  • Teel J Arthroplasty 2019 meta-analysis: function no difference, non-resurfacing = more reoperations
Q10How do you reduce transfusion in TKR?▸
  • Pre-op: correct coagulopathy, stop medications, correct anaemia
  • Intra-op: normovolaemic haemodilution, permissive hypotension, transamin, tourniquet, bone wax
  • Cell saver for revision cases
  • Post-op: transamin, no drain
Q11What alignment deviation is acceptable in TKR?▸
  • Accept 3 degrees deviation
  • Outliers have poor results
Q12What is the evidence on single stage versus sequential bilateral TKR?▸
  • Local AHNH study: no outcome difference; backed by Denmark group 2015 KSSTA Lindberg-Larsen
  • J Arthroplasty 2019 David Ward: simultaneous BTKA had significantly increased odds of multiple complications vs staged BTKA, but the absolute risk differences are minimal and complications are uncommon
  • Stanford J Arthroplasty 2022: simultaneous TKR increases the risk of establishing the second knee outside the mechanical neutral
Q13What is roll back in the knee?▸
  • Posterior translation of the femur with progressive flexion
Q14What are the prerequisites for CR TKR?▸
  • PCL intact
  • Varus/valgus <15 degrees
Q15What are the advantages of CR TKR?▸
  • PCL gives better proprioception
  • Femoral roll back - better ROM and improved lever arm for the quadriceps
  • PCL absorbs the AP and varus-valgus strain otherwise transferred to the bone-prosthesis interface
  • Preserves bone stock
  • Preserves the joint line
Q16What are the disadvantages of CR TKR?▸
  • Increased shearing + delamination leading to PE wear
  • Technically more difficult (poorer exposure, gap balancing, see –saw effect if flexion and extension gaps are both too tight)
  • Paradoxical roll forward if the PCL is defunctioned
Q17What are the advantages of PS TKR?▸
  • More conformity so less PE wear (delamination, pitting, fatigue)
  • Does not rely on the PCL for roll back
  • Technically easier
Q18What are the disadvantages of PS TKR?▸
  • More constraint - more stress at the bone-implant interface
  • More bone loss
  • Dislocation (cam jump)
  • Patellar clunk - superior patellar scar caught in the box in extension
  • Reduce ROM: cut PCL -> looser flexion gap -> thicker PE -> cut more distal femur -> elevate joint line -> patella baja
Q19What is the evidence comparing CR and PS TKR?▸
  • ROM similar
  • Proprioception similar
  • Australian joint registry 2021: PS 7.5% vs CR 6.1% revision at 15 years

Fact check

Zmistowski from Rothman Institute J Arthroplasty 2019 prospective RCT shows patella should not routinely be replaced — mischaracterised study design — It is a cost-utility analysis of 14 prospective RCTs, not itself an RCT; it concludes routine resurfacing of arthritis-free patellae is not cost-effective — source
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