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Updated: Feb 18 2024

TKA Prosthesis Design

Images
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
https://upload.orthobullets.com/topic/5019/images/cone augment.jpg
https://upload.orthobullets.com/topic/5019/images/cr illustration.jpg
https://upload.orthobullets.com/topic/5019/images/cr radiogaph.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
            • exhibits paradoxical anterior translation in the first 40 degrees of flexion
          • 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
    • Fixation
      • options include
        • cemented
          • proven survivorship and function
          • high viscosity cement has longer working time
        • cementless 
          • trabecular surface allows for long term biologic fixation 

  • 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
        • resection of PCL increases the flexion gap in relationship to extension gap so posterior must be matched to avoid flexion-extension mismatch
      • 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
      • chronic collateral (MCL) insufficiency in low demand, elderly patients
      • hyperextension instability
        • seen in polio or tumor resections
      • resection for tumor
      • massive bone loss in the setting of a neuropathic joint
      • gross flexion/extension mismatch despite appropriate soft tissue releases  
    • 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
      • decreased rates of osteolysis
    • Disadvantages
      • lose 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
  • Patient Specific Instrumentation
    • Design
      • femoral and tibial cutting block instrumentation based on imaging specific to patient's anatomy
    • Indications
      • no clear indications at this time
    • Advantages
      • less instrumentation to process peri-operatively
    • Outcomes
      • no obvious cost benefit from PSI
      • no obvious benefit in postoperative TKA alignment
      • no obvious benefit in outcomes or patient satisfaction
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