Updated: 10/17/2018

TKA Prosthesis Design

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https://upload.orthobullets.com/topic/5019/images/Rollback illustration_moved.jpg
https://upload.orthobullets.com/topic/5019/images/waldius hinge.jpg
https://upload.orthobullets.com/topic/5019/images/mcintosh.jpg
https://upload.orthobullets.com/topic/5019/images/gunston.jpg
https://upload.orthobullets.com/topic/5019/images/total condylar.jpg
Introduction
  • Designs include
    • unconstrained
      • posterior-cruciate retaining (CR)
      • posterior-cruciate substituting (PS)
    • constrained
      • nonhinged
      • hinged
    • fixed versus mobile bearing
History
  • 19th century
    • interposition of soft tissues for reconstruction of articular surfaces
  • 1950s
    • Walldius designs first hinged knee replacement 
  • 1958 
    • MacIntosh and McKeever introduce acrylic tibial plateau prosthesis to correct deformity 
  • 1960s
    • Gunston introduces first cemented surface arthroplasty of knee joint 
  • 1970
    • Guepar develops a new hinged prosthesis based on design by Walldius that increases motion and decreases bone loss
  • ~1973
    • "total condylar prosthesis" is introduced which is first to resurface all three compartments (PCL sacrificing) 
Concepts in Prosthetic Design
  • Femoral rollback
    • definition
      • the posterior translation  the femur with progressive flexion
    • importance
      • improves quadriceps function and range of knee flexion by preventing posterior impingement during deep flexion
    • biomechanics
      • rollback in the native knee is controlled by the ACL and PCL
    • design implications
      • both PCL retaining and PCL substituting designs allow for femoral rollback
        • PCL retaining
          • native PCL promotes posterior displacement of femoral condyles similar to a native knee
        • PCL substituting
          • tibial post contacts the femoral cam causing posterior displacement of the femur
  • Constraint
    • definition
      • the ability of a prosthesis to provide varus-valgus and flexion-extension stability in the face of ligamentous laxity or bone loss
    • importance
      • in the setting of ligamentous laxity or severe bone loss, standard cruciate-retaining or posterior-stabilized implants may not provide stability
    • design implications
      • in order of least constrained to most constrained
        • cruciate-retaining
        • posterior-stabilized (cruciate-substituting)
        • varus-valgus constrained (non-hinged)
        • rotating-hinge
  • Modularity 
    • definition
      • the ability to augment a standard prosthesis to balance soft tissues and/or restore bone loss
    • options include
      • metal tibial baseplate with modular polyethylene insert 
        • more expensive than all-polyethylene tibial component
        • has an equivalent rate of aseptic loosening compared with all-polyethylene tibia component
      • metal augmentation for bone loss
      • modular femoral and tibial stems
    • advantages
      • ability to customize implant intraoperatively
    • disadvantages
      • increased rates of osteolysis in modular components
      • backside polyethylene wear  
        • micromotion between tibial baseplate and undersurface of polyethylene insert that occurs during loading
Cruciate-Retaining (CR) Design
  • Design
    • minimally constrained prosthesis that depends on an intact PCL to provide stability in flexion
  • Indications
    • arthritis with minimal bone loss, minimal soft tissue laxity, and an intact PCL
    • varus deformity < 10 degrees
    • valgus deformity < 15 degrees 
  • Radiographs
    • radiographs won't show box in the central portion of the femoral component as PS knees have (see PS knee radiographs)
  • Advantages
    • avoids tibial post-cam impingement/dislocation that may occur in PS knees
    • more closely resembles normal knee kinematics (controversial)
    • less distal femur needs to be cut than in a PS knee
    • improved proprioception with preservation of native PCL
    • newer poly-options can allow for PCL substitution via anterior-stabilized or ultra-congruent shapes in cases of PCL insufficiency without loss of functional results
  • Disadvantages
    • tight PCL may cause accelerated polyethylene wear
    • loose or ruptured PCL may lead to flexion instability and subluxation
Posterior Stabilized (PS) Design
  • Design
    • slightly more constrained prosthesis that requires sacrifice of PCL
    • femoral component contains a cam that engages the tibial polyethylene post during flexion
    • polyethylene inserts are more congruent, or deeply "dished"
  • Indications
    • previous patellectomy
      • reduces risk of potential anteroposterior instability in setting of a weak extensor mechanism
    • inflammatory arthritis
      • inflammatory arthritis may lead to late PCL rupture
    • deficient or absent PCL
  • Radiographs
    • lateral radiograph will show the outline of the cam, or box, in the femoral component
  • Advantages
    • easier to balance a knee with absent PCL
    • arguably more range of motion
    • easier surgical exposure
  • Disadvantages
    • cam jump
      • mechanism
        • with loose flexion gap, or in hyperextension, the cam can rotate over the post and dislocate
      • treatment
        • initial
          • closed reduction by performing an anterior drawer maneuver
        • final
          • revision to address loose flexion gap
    • tibial post polyethylene wear
    • patellar "clunk" syndrome
      • mechanism
        • scar tissue gets caught in box as knee moves into extension
      • treatment
        • arthroscopic versus open resection of scar tissue
    • additional bone is cut from distal femur to balance extension gap
Constrained Nonhinged Design
  • Design
    • constrained prosthesis without axle connecting tibial and femoral components (nonhinged)
    • large tibial post and deep femoral box provide
      • varus/valgus stability 
      • rotational stability
  • Indications
    • LCL attenuation or deficiency
    • MCL attenuation or deficiency
    • flexion gap laxity
    • moderate bone loss in the setting of neuropathic arthropathy
  • Radiographs
  • Advantages
    • prosthesis allows stability in the face of soft tissue (ligamentous) or bony deficiency
  • Disadvantages
    • more femoral bone resection
      • necessary to accommodate large box
    • aseptic loosening
      • as a result of increased constraint
Constrained Hinged Design
  • Design
    • most constrained prosthesis with linked femoral and tibial components (hinged)
    • tibial bearing rotates around a yoke on the tibial platform (rotating hinge)
      • decreases overall level of constraint
  • Indications
    • global ligamentous deficiency
    • hyperextension instability
      • seen in polio or tumor resections
    • resection for tumor
    • massive bone loss in the setting of a neuropathic joint 
  • Radiographs
  • Advantages
    • prosthesis allows stability in the face of soft tissue (ligamentous) or bony deficiency
  • Disadvantages
    • aseptic loosening
      • as a result of increased constraint
      • large amount of bone resection required
Mobile Bearing Design
  • Design
    • minimally constrained prosthesis where the polyethylene can rotate on the tibial baseplate
    • PCL is removed at time of surgery
  • Indications
    • young, active patients (relative indication)
  • Advantages
    • theoretically reduces polyethylene wear
      • increased contact area reduces pressures placed on polyethylene (pressure=force/area) 
  • Disadvantages
    • bearing spin-out
      • mechanism
        • occurs as a result of a loose flexion gap
        • tibia rotates behind femur
      • treatment
        • initial
          • closed reduction
        • final
          • revision to address loose flexion gap
All-polyethylene base plates
  • Design
    • tibial plate is a solid block of polyethylene as opposed to a metal tray with a poly insert.
  • Indications
    • no clear indications
  • Advantages
    • less expensive
    • decreased rates of osteolysis
  • Disadvantages
    • loose modular flexibility
  • Outcomes
    • studies show equivalent functional outcomes with decreased cost
Highly Congruent Liners
  • Design
    • medial compartment concavity allows lateral compartment to translate between flexion and extension
    • this creates a medial pivot 
  • Indications
    • no clear indications
  • Advantages
    • may better create native knee kinematics
  • Outcomes
    • studies show equivalent outcomes and survivorship in short and mid-term studies
 

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Technique Guides (3)
Questions (23)

(OBQ07.28) Figure A displays the preoperative radiographs of a 67-year-old obese, diabetic woman undergoing total knee arthroplasty. Following the bone resections, a PCL-substituting trial implant is placed. At full extension, 30 degrees of flexion, and 90 degrees of flexion, the knee is found to be tight laterally and loose medially. Sequential releases are made until the IT band, popliteus, and LCL have all been released, and a larger polyethylene trial is placed, but the knee continues to demonstrate medial laxity and lateral tightness. Which of the following should be done to achieve a successful outcome? Review Topic

QID: 689
FIGURES:
1

Surgical release of the lateral retinaculum

3%

(74/2373)

2

Surgical release of the lateral retinaculum and reconstruction of the medial patellofemoral ligament

2%

(49/2373)

3

Imbrication of the MCL and augmentation with allograft

5%

(115/2373)

4

Convert to a constrained TKA design

73%

(1743/2373)

5

Recut the tibia in valgus to improve balance

16%

(382/2373)

ML 2

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PREFERRED RESPONSE 4
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(OBQ04.143) A 45-year-old woman is scheduled to undergo a TKA. Which of the following implant designs theoretically reduces poylethylene wear and reduces bone-implant-interface stress? Review Topic

QID: 1248
1

Mobile-bearing TKA

83%

(857/1038)

2

Posterior stabilized fixed bearing TKA

3%

(34/1038)

3

Cruciate retaining fixed bearing TKA

10%

(99/1038)

4

Constrained TKA

2%

(20/1038)

5

Mobile-bearing hinged TKA

2%

(21/1038)

ML 2

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PREFERRED RESPONSE 1
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(SBQ07HK.1) A 57-year-old man complains of knee pain that is exacerbated with weight bearing and ambulation. He underwent surgery on his knee 10 years ago following a motor vehicle collision. On physical exam he has medial and lateral joint line tenderness and no instability. Radiographs are provided in figures A and B. Conservative therapy with NSAID's and viscosupplementation is initiated. If he continues to develop further degenerative changes and needs arthroplasty what type of implant should be utilized? Review Topic

QID: 1586
FIGURES:
1

Unicompartmental mobile bearing knee arthroplasty

6%

(108/1836)

2

Posterior cruciate retaining total knee arthroplasty

23%

(431/1836)

3

Posterior stabilized total knee arthroplasty

64%

(1179/1836)

4

Constrained nonhinged total knee arthroplasty

3%

(51/1836)

5

Constrained hinged total knee arthroplasty

3%

(61/1836)

ML 2

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PREFERRED RESPONSE 3

(OBQ07.46) In total knee arthroplasty, what would be a difference with use of the tibial component shown in Figure A compared to Figure B? Review Topic

QID: 707
FIGURES:
1

Increased ductility of the polyethylene

2%

(21/881)

2

Increased wear on the articular side of the polyethylene

6%

(49/881)

3

Less wear on the articular side of the polyethylene

6%

(56/881)

4

More wear at the backside of the polyethylene

68%

(602/881)

5

Less wear at the backside of the polyethylene

17%

(150/881)

ML 3

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PREFERRED RESPONSE 4

(SBQ07HK.6) The medial and lateral joint surfaces have different tibiofemoral geometry. How does this affect the kinematics of normal knee movement from full extension into flexion? Review Topic

QID: 1591
1

Tibia will externally rotate

26%

(986/3752)

2

Distal femur will pivot about a medial axis of the knee

52%

(1946/3752)

3

Distal femur will translate anteriorly on the tibia

8%

(307/3752)

4

Distal femur will pivot about a lateral axis of the knee

12%

(454/3752)

5

No effect

1%

(39/3752)

ML 4

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PREFERRED RESPONSE 2

(OBQ12.92) Which of the following types of prosthetic designs, seen in figures A-E, has been shown to have a high rate of loosening secondary to overconstraint? Review Topic

QID: 4452
FIGURES:
1

Figure A

1%

(58/3991)

2

Figure B

4%

(175/3991)

3

Figure C

85%

(3395/3991)

4

Figure D

5%

(209/3991)

5

Figure E

3%

(133/3991)

ML 2

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PREFERRED RESPONSE 3

(OBQ10.201) A posterior cruciate retaining total knee arthroplasty is contraindicated in all of the following patients EXCEPT? Review Topic

QID: 3294
1

52-year-old female with severe rheumatoid arthritis of the knee

3%

(88/2527)

2

73-year-old male with post-traumatic arthritis of the knee and prior patellectomy

8%

(203/2527)

3

67-year-old male with degenerative arthritis and 10 degree valgus deformity of the knee

75%

(1883/2527)

4

55-year-old male with post-traumatic arthritis of the knee 20 years after bicruciate ligament ruptures

8%

(195/2527)

5

63-year-old female with a chronic history of steroid treatment of systemic lupus erythematosus and an arthritic knee

6%

(148/2527)

ML 2

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PREFERRED RESPONSE 3

(SBQ07HK.7) A 55-year-old female with a long history of diabetes mellitus presents for evaluation of chronic knee pain. A current radiograph is shown in Figure A. Which of the following knee prostheses, shown in Figures B through E, would be most appropriate in the initial treatment of this patient? Review Topic

QID: 1592
FIGURES:
1

Figure B

53%

(779/1463)

2

Figure C

13%

(195/1463)

3

Figure D

12%

(181/1463)

4

Figure E

20%

(293/1463)

5

Figure F

1%

(9/1463)

ML 3

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PREFERRED RESPONSE 1

(OBQ05.265) Figure A demonstrates a total knee prosthesis design. Which of the following motions is constrained in this particular design: Review Topic

QID: 1151
FIGURES:
1

Complete anterior-posterior translation constraint only

10%

(183/1886)

2

Partial varus-valgus angulation constraint only

15%

(274/1886)

3

Partial varus-valgus angulation and partial internal-external rotation constraint

52%

(986/1886)

4

Complete internal-external rotation constraint only

1%

(26/1886)

5

Complete varus-valgus angulation and anterior-posterior translation constraint

22%

(411/1886)

ML 4

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PREFERRED RESPONSE 3
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