Updated: 1/21/2019

Catastrophic Wear & PE Sterilization

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Review Topic
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Questions
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https://upload.orthobullets.com/topic/5002/images/poly_moved.jpg
https://upload.orthobullets.com/topic/5002/images/subsurface line_moved.jpg
Introduction
  • Refers to macroscopic premature failure of polyethylene (PE) due to
    • excessive loading
    • mechanical loosening
  • Catastrophic failure is most commonly seen in TKA
    • in contrast to osteolytic failure that is usually seen in THA
    • catastrophic can also be seen in shoulder and hip replacement, but less common
  • Primary variables that lead to catastrophic wear include
    • PE thickness
    • articular surface design
    • kinematics
    • PE sterilization
    • PE machining
PE thickness
  • Introduction
    • PE insert width is usually defined as the maximal thickness of the PE insert and metal tray
    • therefore a PE insert labeled as 8mm, may only have a "true" PE of only 4-5 mm at the thinnest point, assuming the metal tray is ~ 2 mm thick
  • Cause of Failure
    • PE thickness <8mm
      • leads to loads transmitted to localized area of PE which exceed PE's inherent yield strength
      • thickness of < 8mm associated with catastrophic PE failure
  • Solution
    • keep thinnest portion of PE >8mm
      • avoid having to use a PE insert of less than 8mm by making a more aggressive tibial cut
        • in younger more active patients surgeons may tend to try to preserve more bone for future revision but the increased activity combined with thinner PE will increase risk of catastrophic failure
Articular surface design
  • Introduction
    • two general designs in total knee prosthesis include  
      • a deeper congruous joint (deeper cut PE) without rollback 
        • less anatomic
        • maximizes contact loads
        • decreases contact stress
      • a flat tibial PE that improves femoral rollback and optimizes flexion,
        • more anatomic
        • PCL sparing
        • increases contact stress and catastrophic failure 
  • Cause of Failure
    • flat designs of tibia PE 
      • low contact surface area leads to high contact stress load in areas of contact
  • Solution
    • increase congruency of articular design
      • higher contact surface area leads to lower contact stress load
      • newer prosthesis designs sacrifice rollback and have a more congruent or "dished" fit between the femoral condyle and the tibial insert in both the sagittal and coronal plane in order to decrease the contact stress
Kinematics
  • Introduction
    • variables that affect kinetics include
      • knee alignment
        • varus alignment of knee associated with catastrophic PE failure
      • femoral rollback 
        • optimizes flexion at the cost of increasing contact stress and increased risk of catastrophic failure
  • Cause of failure
    • excessive femoral rollback
      • dyskinetic sliding movements of femur on tibia causes surface cracking and wear
  • Solution
    • Perform medial release to avoid varus malalignment
    • Decrease contact stress by minimizing femoral rollback
      • use a more congruous joint design
      • increase posterior slope of tibia
      • use PCL substituting knee for incompetent PCL or dyskinetic femoral rollback
      • to compensate for the lack of rollback, newer designs move the point of contact (where femoral condyle rests) more posterior and have a steeper posterior slope to aid with flexion
PE Sterilization
  • Radiation
    • gamma radiation is the most common form of polyethylene sterilization
    • oxidation vs. cross linking
      • presence of oxygen determines pathway following free radical formation
        • oxygen rich environment
          • PE becomes oxidized
            • leads to early failure due to      
              • subsurface delamination
              • pitting
              • fatigue cracking
        • oxygen depleted environment
          • PE becomes cross linked
            • improved resistance to adhesive and abrasive wear
            • decrease in mechanical properties (decreased ductility and fatigue resistance)
            • greater risk of catastrophic failure under high loads
          • methods to obtain
            • packing via argon, nitrogen
            • packing in vacuum environment
  • Solution
    • irradiate PE in inert gas or vacuum to minimize oxidation 
PE Fabrication
  • Introduction
    • cutting tools can disrupt chemical bonds of PE
  • Fabrication methods
    • ram bar extrusion and machining
      • UHMWPE powder fed into heated chamber, ram pushes powed into heated cylinder barrel, forming a cylindrical rod, cut into 10ft lengths for sale
      • implants are machined from the cylindrical bar stock
    • sheet compression molding
      • UHMWPE powder introduced into large 4' x 8' rectangular container to make sheets up to 8" thick
      • implants are machined from these molded sheets
    • direct compression molding/net shape
      • UHMWPE powder placed into a mold the shape of the final component, which is heated
      • the net shape implant is removed and packaged
      • no external machining involved, implants have highly glossy surface finish
      • lower wear rates (50% wear rate of machined products)
        • slow, expensive
  • Cause of failure
    • machining shear forces cause subsurface region (1-2mm) stretching of PE chains
      • especially in amorphous regions > crystalline regions
    • leads to subsurface delamination and fatigue cracking
      • can show classic white band of oxidation in subsurface 1-2mm below articular surface 
  • Solution
    • use direct-compression molding of PE
      • performed by molding directly from PE powder to the desired product
      • results in less fatigue crack formation and propagation compared to ram bar extrusion 
    • avoid machining of articular surface
 

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Questions (23)
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(OBQ10.111) Which of the following manufacturing techniques of UHMWPE results in the lowest susceptibility to fatigue crack formation and propagation in joint arthroplasty bearings? Review Topic

QID: 3205
1

Ram bar extrusion with secondary machining into the desired product

10%

(264/2741)

2

Hot isostatic pressing into bars with secondary machining

2%

(66/2741)

3

Irradiation with 10 Mrad of radiation achieiving a polyethylene crystallinity of >99%

15%

(420/2741)

4

Direct compression molding from PE powder to the desired product

63%

(1729/2741)

5

Addition of calcium stearate to the polyethylene resin followed by compression molding into bars with secondary machining into the desired product

9%

(242/2741)

ML 3

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PREFERRED RESPONSE 4

(OBQ10.220) The polyethylene cup shown in Figure A has been sterilized in packaging that contains oxygen. Which of the following statements is true about the process of polyethylene sterilization? Review Topic

QID: 3319
FIGURES:
1

Irradiation of polyethylene in argon quenches free radicals

9%

(198/2184)

2

Polyethylene sterilization with ethylene oxide causes cross-linking of the polymer and minimizes oxidation following implantation

12%

(254/2184)

3

Oxidation occurs following polyethylene implantation regardless of sterilization process

57%

(1235/2184)

4

Irradiation of polyethylene in nitrogen quenches free radicals

8%

(165/2184)

5

Polyethylene sterilization with gas plasma causes cross-linking of the polymer and minimizes oxidation following implantation

15%

(317/2184)

ML 3

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PREFERRED RESPONSE 3

(OBQ04.222) In evaluating methods of polyetheylene sterilization for hip arthroplasty, gamma-irradiation in air compared to irradiation in an inert substance results in which of the following? Review Topic

QID: 1327
1

No difference in regards to outcome

1%

(28/2068)

2

Higher rate of cross-linking when irradiated in air

3%

(52/2068)

3

Lower rate of oxidation when irradiated in air

2%

(33/2068)

4

Accelerated wear and failure when irradiated in air

93%

(1929/2068)

5

Better wear resistance and longevity when irradiated in air

1%

(17/2068)

ML 1

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PREFERRED RESPONSE 4

(OBQ06.215) A 55-year-old male undergoes a revision total knee arthroplasty of an implant that is only 3 years old. At the time of surgery, the tibial polyethylene liner shows catastrophic delamination and cracking. What is the most likely cause of this extensive, accelerated wear of the polyethylene liner? Review Topic

QID: 226
1

Sterilization in ethylene oxide

2%

(64/2619)

2

Gamma irradiation of the polyethylene liner in the presence of air

95%

(2478/2619)

3

Gamma irradiation of the polyethylene liner with vacuum packaging

1%

(27/2619)

4

Gamma irradiation of the polyethylene liner in nitrogen

1%

(30/2619)

5

Gamma irradiation of the polyethylene liner in argon

0%

(13/2619)

ML 1

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PREFERRED RESPONSE 2

(OBQ09.108) Polyethylene gamma irradiation in nitrogen gas results in which of the following? Review Topic

QID: 2921
1

Annealing

21%

(455/2196)

2

Gas plasma

3%

(70/2196)

3

Uncross-linking

9%

(189/2196)

4

Free radicals

66%

(1456/2196)

5

Remelting

1%

(17/2196)

ML 2

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PREFERRED RESPONSE 4
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