Updated: 4/15/2020

TKA Polyethylene Wear & Manufacturing

0%
Topic
Review Topic
0
0
0%
0%
Questions
23 23
0
0
0%
0%
Evidence
15 15
0
0
https://upload.orthobullets.com/topic/5002/images/poly_moved.jpg
https://upload.orthobullets.com/topic/5002/images/subsurface line_moved.jpg
Introduction
  • Overview
    • Catastrophic wear refers to macroscopic premature failure of polyethylene (PE) due to
      • excessive loading and mechanical loosening
      • Treatment involves revision TKA vs PE exchange
  • Epidemiology
    • Catastrophic failure is most commonly seen in TKA
      • in contrast to osteolytic failure that is usually seen in THA
      • catastrophic failure may occur in TSA and THA replacement, but less common
  • Pathophysiology
    • Primary variables that lead to catastrophic wear include
      • PE thickness
      • articular surface design and geometry
        • flat PE should be avoided as knee loads exceed yield strength of UHMWPE in flat design
        • goals of PE design
          • maximize contact area
          • minimize contact loads (force/area)
          • biplanar congruency is the best design
            • congruent in both coronal and sagittal planes
      • kinematics
        • sliding wear is bad for PE
          • occurs when ACL is sacrificed and PCL remains
            • most pronouced with CR knee design with a flat PE insert
            • least pronounced with PS or AS knee design with a congruent PE insert
      • PE sterilization
      • PE manufacturing
      • surgical technique 
        • tight flexion gap hastens sliding wear effect
        • tight PCL and anterior tibial slope amplify stress
Polyethylene thickness
  • Introduction 
    • PE insert thickness can be variable depending on manufacturer definition (e.g. some may list PE thickness as the combined thickness of the insert + tibial tray)
      • PE insert labeled as 8mm, may only have a "true" PE thickness of only 4-5 mm at the thinnest point with a ~3mm thick metal tray
  • Cause of Failure
    • PE thickness <8mm
      • leads to loads transmitted to localized area of PE which exceed PE's inherent yield strength (12-20 mPA)
      • thickness of < 8mm associated with catastrophic PE failure
      • data based on older studies/PE generations, may not be as applicable with modern manufacturing
  • Solution
    • maintain thinnest portion of PE >8mm
      • a more aggressive tibial cut may avoid having to use a PE insert of <8mm  
        • in younger patients, increased activity combined with thinner PE will increase risk of catastrophic failure
Articular surface design and geometry
  • 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 adequate bone cuts and/or releases 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
Polyethylene sterilization
  • Radiation
    • gamma radiation is the most common form of polyethylene sterilization       
      • results in oxidized PE which wears poorly and results in osteolysis 
    • 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 strength/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
    • Removal of free radicals     
      • thermal stabilization/remelting
        • removes free radicals formed during the radiation sterilization process for cross-linking
          • most effective means of removing free radicals as it occurs above the PE melting point
        • changes the PE from its partial crystalline state to its amorphous state
          • disadvantage is that it reduces the mechanical properties of the material
      • annealing
        • maintaines its mechanical property
        • less effective at removing free radicals as it occurs below the PE melting point
          • leaves the PE more susceptible to oxidation
  • Solution
    • irradiate PE in inert gas or vacuum to minimize oxidation 
Polyethylene manufacturing
  • 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
      • leads to variations in PE quality within the bar
    • calcium stearate additive 
      • leads to fusion defects in PE
    • 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 
      • best PE fabrication process
      • 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
    • PE chains are more susceptible to radiation resulting in greater oxidation in this region   
      • leads to subsurface delamination and fatigue cracking
        • can show classic white band of oxidation in subsurface 1-2mm below articular surface 
    • "Perfect storm" scenario for catastophic wear 
      • metal-backed tibial baseplate with bone-conserving tibial bone cut (thin PE)
      • flat bearing design in coronal plane (low contact area with high contact load)
      • PCL retention with flat PE insert (high sliding wear)
      • ram bar PE with calcium stearate additive (fusion defects in PE)
      • gamma radiation sterilization in air (weakened mechanical properties of PE)
      • machined PE surface (cutting-tool stretch effect upon the PE)
  • 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
 
 

Please rate topic.

Average 3.9 of 62 Ratings

Thank you for rating! Please vote below and help us build the most advanced adaptive learning platform in medicine

The complexity of this topic is appropriate for?
How important is this topic for board examinations?
How important is this topic for clinical practice?
Questions (23)
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK

You have 100% on this question.
Just skip this one for now.

(SAE13BS.48) What polyethylene processing step results in increased polyethylene wear and subsequent osteolysis? Review Topic | Tested Concept

QID: 8284
1

Gamma irradiation in air

78%

(212/272)

2

Remelting after gamma irradiation in nitrogen

4%

(12/272)

3

Heat annealing

7%

(19/272)

4

Ethylene oxide sterilization

9%

(24/272)

L 2 D

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(SAE13BS.70) How are free radicals removed from highly cross-linked polyethylene? Review Topic | Tested Concept

QID: 8309
1

Low-dose irradiation

20%

(55/282)

2

High-dose irradiation

19%

(54/282)

3

Oxidation

18%

(50/282)

4

Thermal processing

42%

(118/282)

L 4 D

Select Answer to see Preferred Response

Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK
Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK

You have 100% on this question.
Just skip this one for now.

(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 | Tested Concept

QID: 3205
1

Ram bar extrusion with secondary machining into the desired product

10%

(341/3589)

2

Hot isostatic pressing into bars with secondary machining

3%

(93/3589)

3

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

16%

(578/3589)

4

Direct compression molding from PE powder to the desired product

62%

(2228/3589)

5

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

9%

(324/3589)

L 3 C

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(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 | Tested Concept

QID: 3319
FIGURES:
1

Irradiation of polyethylene in argon quenches free radicals

10%

(257/2566)

2

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

12%

(301/2566)

3

Oxidation occurs following polyethylene implantation regardless of sterilization process

56%

(1426/2566)

4

Irradiation of polyethylene in nitrogen quenches free radicals

7%

(191/2566)

5

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

15%

(374/2566)

L 3 B

Select Answer to see Preferred Response

Question locked
Sorry, this question is for
PEAK Premium Subscribers only
Upgrade to PEAK

You have 100% on this question.
Just skip this one for now.

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

QID: 2921
1

Annealing

21%

(528/2481)

2

Gas plasma

3%

(80/2481)

3

Uncross-linking

8%

(210/2481)

4

Free radicals

66%

(1631/2481)

5

Remelting

1%

(21/2481)

L 2 A

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(SAE08OS.9) When comparing unaged, radiated, cross-linked, ultra-high molecular weight polyethylenes (XPE) treated with remelting or annealing, annealed XPE has Review Topic | Tested Concept

QID: 6371
1

fewer free radicals, better mechanical characteristics, and lower wear than remelted XPE.

41%

(86/212)

2

fewer free radicals, worse mechanical characteristics, and lower wear than remelted XPE.

13%

(28/212)

3

more free radicals, better mechanical characteristics, and equivalent wear than remelted XPE.

30%

(64/212)

4

more free radicals, worse mechanical characteristics, and equivalent wear than remelted XPE.

5%

(10/212)

5

equivalent free radicals, better mechanical characteristics, and lower wear than remelted XPE.

8%

(17/212)

N/A E

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(SAE07HK.60) When polyethylene is exposed to radiation and subsequently heated, certain chemical changes occur in the material. Which of the following statements best describes these changes? Review Topic | Tested Concept

QID: 6020
1

The process converts an otherwise interpenetrating networking structure of polymer chains into a linear, high molecular weight polyethylene macromolecule.

19%

(42/221)

2

The process increases the ductility of the material.

2%

(5/221)

3

The process leads to fewer particles that are larger in size than the untreated material.

3%

(7/221)

4

The process improves (lowers) the wear rate but may increase the risk of fracture.

55%

(122/221)

5

The process decreases the wear rate of the material, compared to untreated polyethylene, when tested against a rough counterface.

19%

(43/221)

L 4 E

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(SAE07HK.40) Sterilization of ultra-high molecular weight polyethylene by gamma irradiation in air will degrade its wear performance because of Review Topic | Tested Concept

QID: 6000
1

oxidation.

90%

(225/251)

2

melting.

0%

(0/251)

3

cross-linking.

8%

(19/251)

4

corrosion.

1%

(3/251)

5

creep.

1%

(3/251)

L 1 E

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(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 | Tested Concept

QID: 226
1

Sterilization in ethylene oxide

3%

(74/2874)

2

Gamma irradiation of the polyethylene liner in the presence of air

94%

(2714/2874)

3

Gamma irradiation of the polyethylene liner with vacuum packaging

1%

(33/2874)

4

Gamma irradiation of the polyethylene liner in nitrogen

1%

(30/2874)

5

Gamma irradiation of the polyethylene liner in argon

0%

(14/2874)

L 1 B

Select Answer to see Preferred Response

You have 100% on this question.
Just skip this one for now.

(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 | Tested Concept

QID: 1327
1

No difference in regards to outcome

2%

(43/2338)

2

Higher rate of cross-linking when irradiated in air

3%

(64/2338)

3

Lower rate of oxidation when irradiated in air

2%

(38/2338)

4

Accelerated wear and failure when irradiated in air

93%

(2163/2338)

5

Better wear resistance and longevity when irradiated in air

1%

(22/2338)

L 1 A

Select Answer to see Preferred Response

Evidences (28)
VIDEOS (0)
Topic COMMENTS (24)
Private Note