TKR Design

Question 1

Describe the designs of a TKR?

Answer 1

• Unconstrained
  
  • Posterior cruciate retaining (PCR)
  • Posterior crucitate subsituting (PS)
• Constrained
  
  • Non hinged
  • hinged
• fixed vs mobile bearing

Question 2

What is femoral rollback?

Answer 2

• The posterior translation of the femur with progressive flexion

Question 3

What is the important of femoral rollback and what controls it?

Answer 3

• Improves quadriceps function and range of knee flexion by preventing posterior impingment during deep flexion
• controlled iin native knee by acl and pcl

Question 4

How is rollback implicated into design of prothesis?

Answer 4

• Both PCL retaining and PCL subsituting designs allow for femoral rollback
• PCL retaining
  
  • native PCL promotes posterior displacement of femoral condyles similar to native knee
• PCL substituting
  
  • tibial post contacts the femoral cam causing posterior displacment of the femur

Question 5

What is constraint?

Answer 5

• The ability of a prosthesis to provide varus -valgus and flexion-extension stability in the face of ligamentous laxity or bone loss
• NB in ligamenotus laxity/bone loss normal crucite retaining or posterior stabilising implants may not provide enough stability

Question 6

Can you descibe the least to the most constraint TKR?

Answer 6

• Cruciate retaining ( PCL)
• PCL sacrificing
• Non hinged- (varus-valgus constrained)
• rotating- hinge

Question 7

What is modularity?

Answer 7

• The ability to augment a standard prosthesis to balance soft tissue and or restore bone loss

Question 8

How can modularity be achieved in TKR?

Answer 8

• Metal tibial baseplate with modular polyethylene inserts
  
  • more expensive then all poly tibial component
  • has an equivalent rate of aseptic loosening cf all PE tibial components
• Metal augmentation for bone loss
• modular femoral and tibial stems

Question 9

what are the adv and dis to modularity iin TKR?

Answer 9

• ADV
  
  • ability to customise implant intraoperatively
• DIS
  
  • increased rates of osteolysis in modular components
  • backside PE wear
    
    • micromotion between tibial baseplate and undersurface of PE insert that occurs during loading

Question 10

Describe the cruciate retaining TKR?

Answer 10

• Minimal constraint device that relies on intact PCL to provide stability
• Indications
  
  • arthritis with minimal bone loss, minimal soft tissue laxity . intact PCL.
  • Varus <15o, valgus <10o
• Xrays no box cut is seen on lateral cf PS

Question 11

What are the adv/dis of cruciate retaining TKR?

Answer 11

ADV

• Avoids tibial post cam impingement/dislocation that may occur in PS knee
• more closely resembles normal knee kinematics
• less distal femur needs to be cut cf PS
• Imporved proprioception with preservation of native PCL

DIS

• Tight PCL may cause accelerated polyethylene wear
• loose or ruptured PCL -> flexion instability and subluxation

Question 12

Describe the Posterior stabilised knee replacement?

Answer 12

• PCL subsitiuting
• Slightly more constrained that sacrificised the PCL
• femoral component contains a cam that engages the tibial PE post during flexion
• PE inserts are more congruent or deeply dishes

Question 13

What are the indications for posterior stabilised TKR?

Answer 13

• Previous patellectomy
  
  • reduced risk of potential anteroposterior instability in setting of weak extensor mechanism
• Infammatory arthritis
  
  • infammatory arthritis may lead to late PCL rupture
• Deficient or absnet PCL
• xrays
  
  • show outline ot box on lateral xray

Question 14

What are the adv/dis of a posterior stabilised TKR?

Answer 14

ADV

• easier to balance the knee with absent PCL
• arguably more ROM
• easier surgical exposure

DIS

• CAM Jump
  
  • with loose flexion gap or hyperextension, the cam can rotate over the post and dislocate
  • tx with closed reduction using anterior draw
  • revise to address loose flexion gap
• Tibial post PE wear
• Patellar clunk syndrome
  
  • scar tissue gets caught in box as knee moves into extension
  • tx with arthroscopic vs open resection of scar tissue
  • additional bone is cut from distal femur to balance extension gap

Question 15

Describe the design of the constrained non hinged TKR?

Answer 15

• Constrained prosthesis without axle connecting tibial and femoral components ( non hinged)
• large tibial post and deep femoral box provide
  
  • varus-valgus stability
  • rotational stability
• Indications for use
  
  • LCL attenutation/deficiency
  • MCL attenutation/deficiency
  • flexion gap laxity
  • moderate bone loss in the setting of neuropathic arthropathy

Question 16

What are the adv/dis of Constrained non hinge TKR?

Answer 16

ADv

• Prothesis allows stability in the face of soft tissue (ligamentous) or bony deficiency

Dis

• More femoral resection
  
  • necessary to accomdate large box
• aseptic loosening
  
  • as a result of increased constraint

Question 17

Describe the design of a constrained hinged tkr?

Answer 17

• most constrained device with linked femoral and tibial components
• tibial bearing rotates around a yoke on the tibial platform ( rotating hinge)
  
  • decreases overall level of constraint
• Indication
  
  • Global ligamentous deficiency
  • hyperextension instability
    
    • polio and tumour resections
  • resection of tumour
  • massive bone loss in setting of neuropathic joint

Question 18

What are the adv and dis of constrained hinged tkr?

Answer 18

Adv

• Prothesis allows stability in the face of soft tissue ligamentous or bony deficiency

Dis

• aseptic loosening
  
  • as a result of increased constraint
  • large amount of bone resection required

Question 19

Can you describe the design of a mobile bearing TKR?

Answer 19

• Minimally constrained prosthesis where the polyethylene can rotate on the tibial baseplate
• PCL is removed at time of surgery
• indications
  
  • young, active

Question 20

What are the adv and dis of a mobile bearing TKR?

Answer 20

ADV

• Theoretically reduced PE wear
• increased contact area reduced pressure placed on PE ( pressure= force/area)

Dis

• Bearing spin out
  
  • occurs as a result of loose flexion gap
  • tibia rotates behind femur
  • inital closed reduction
  • final revision to address loose flexion gap