Updated: 2/5/2017

THA Implant Fixation

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https://upload.orthobullets.com/topic/5003/images/pedestal_formation.jpg
https://upload.orthobullets.com/topic/5003/images/dorr.jpg
https://upload.orthobullets.com/topic/5003/images/press_vs._cemented.jpg
https://upload.orthobullets.com/topic/5003/images/valgus_stem.jpg
https://upload.orthobullets.com/topic/5003/images/proximally_porous_coated.jpg
Introduction
  •   Types of fixation
    • cement fixation
      • polymethylmethacrylate (PMMA)
    • biologic fixation (cementless fixation)
      • bone ingrowth
      • bone ongrowth
  • History
    • cemented fixation
      • first described by Gluck in 1891
      • Charnley popularized technique in 1950s
        • used cement borrowed from dentists
      • failures in 1980s thought to be due to "cement disease"
        • driving force to perfect cementless techniques
    • cementless fixation
      • used throughout 1900s, with varying results
      • in 1983, FDA approved Anatomic Medullary Locking (AML) implant
        • first microporous surface with potential for bone ingrowth
      • proximally coated stems designed shortly thereafter due to concerns of thigh pain and osteolysis
  • Prevalence of fixation technique
    • increasing trend towards cementless fixation
    • 93% of THA in United States in 2012 were cementless
Indications
  •  Dorr classification attempts to guide indications for cemented or uncemented femoral component fixation.
Dorr Classification
Ratio (inner canal diameter at midportion of lesser trochanter divided by diameter 10 cm distal)
Characteristics Suggested Femoral Component Fixation
Type A
<0.5
Cortices seen on both AP and lateral XR
Uncemented
Type B
0.5 to 0.75
Thinning of posterior cortex on lateral XR Uncemented
Type C
>0.75
Thinning of cortices on both views Cemented
 
Cement Fixation
  • Mechanism
    • acts as grout by producing interlocking fit between surfaces
  • Indications
    • femoral component
      • elderly patients
        • deeper penetration of cement in osteopenic patients provides excellent fixation 
      • irradiated bone
        • bone ingrowth potential is limited with press-fit components in irradiated bone
      • "stovepipe femur"
        • also known as Dorr C femur  
        • enlarged metaphyseal region and lack of supporting isthmus make cementless fixation difficult
    • acetabular component
      • controversial
        • cemented acetabular component fails at a higher rate than press-fit 
          • cement resists shear poorly
  • Technique
    • cementing techniques have evolved with time
      • 1st generation
        • hand-mixed cement
        • finger packed cement
        • no canal preparation or cement restrictor
      • 2nd generation
        • cement restrictor placement
        • cement gun
        • femoral canal preparation
          • brush and dry
      • 3rd generation
        • vacuum-mixing to reduce cement porosity
        • cement pressurization
        • femoral canal preparation
          • pulsatile lavage
    • cement fixation optimized by
      • limited porosity of cement
        • leads to reduced stress points in cement
      • cement mantle > 2mm
        • increased risk of mantle fractures if < 2mm mantle
      • stiff femoral stem
        • flexible stems place stress on cement mantle
      • stem centralization
        • avoid malpositioning of stem to decrease stress on cement mantle
      • smooth femoral stem 
        • sharp edges produce sites of stress concentration
      • absence of mantle defects
        • defined as any area where the prosthesis touches cortical bone with no cement between
        • creates an area of higher concentrated stress and is associated with higher loosening rates
      • proper component positioning within femoral canal 
        • varus or valgus stem positioning increases stress on cement mantle
  • Radiographic analysis
    • Barrack and Harris grading system
      • grade A
        • complete filling of medullary canal
        • "white-out" of cement-bone interface
      • grade B
        • slight radiolucency of cement-bone interface
      • grade C
        • radiolucencies > 50% of bone-cement interface or incomplete cement mantles
      • grade D
        • gross radiolucencies and/or failure of cement to surround tip of stem
Biologic Fixation
  • Mechanism
    • 2 different types
      • ingrowth
        • bone grows into porous structure of implant
      • ongrowth
        • bone grows onto the microdivots in the grit blasted surface
  • Indications
    • femoral component
      • younger patients
      • older patients with good bone stock
      • revision total hip arthroplasty
        • cemented femoral stems have lower success rates in the revision setting
    • acetabular component
      • all situations except
        • poor acetabular bone stock
        • irradiated bone
  • Technique
    • methods
      • press fit technique
        • slightly larger implant than what was reamed/broached is wedged into position
      • line-to-line technique
        • size of implant is the same as what was reamed/broached
        • screws often placed in acetabulum if reamed line-to-line
    • biologic fixation is optimized with
      • pore size 50-300um
        • preferably 50-150um 
      • porosity of 40-50%
        • increased porosity may lead to shearing of metal
      • gaps < 50um
        • defined as gap space between bone and prosthesis
      • micromotion < 150um
        • increased micromotion may lead to fibrous ingrowth
      • maximal contact with cortical bone
    • types of coating
      • porous-coated metallic surfaces
        • allows bone ingrowth fixation
        • extent of coating
          • proximal coating only
            • less distal stress shielding
          • extensively coated stem
            • produces more stress shielding of proximal bone
            • useful for revision arthroplasty where proximal bone stock may be compromised
      • grit blasted metallic surface
        • allows bone ongrowth fixation
        • all grit blasted stems are extensively coated
          • fixation strength is less than with porous coated stems, necessitating greater area of surface coating
      • hydroxyapatite (HA)
        • osteoconductive agent used as an adjunct to porous-coated and grit blasted surfaces
        • may allow more rapid closure of gaps between bone and prosthesis
          • has shown shorter time to biologic fixation in animal models, but no advantage clinically in humans 
  • Radiographic analysis
    • signs of a well-fixed cementless femoral component
      • spot-welds
        • new endosteal bone that contacts porous surface of implant
      • absence of radiolucent lines around porous portion of femoral stem
      • proximal stress shielding in extensively-coated stems
      • absence of stem subsidence on serial radiographs
    • signs of a well-fixed cementless acetabular component
      • lack of migration on serial radiographs
      • lack of progressive radiolucent lines
      • intact acetabular screws
Complications of Implant Fixation
  • Aseptic loosening
    • causes
      • poor initial fixation
      • mechanical loss of fixation over time
      • particle-induced osteolysis
    • clinical presentation
      • acetabular loosening
        • groin/buttock pain
      • femoral loosening
        • thigh pain 
        • start-up pain
    • evaluation
      • sequential radiographs
      • bone scan
    • treatment
      • revision of loose components 
  • Stress shielding
    • definition
      • proximal femoral bone loss in the setting of a well-fixed stem
    • risk factors
      • stiff femoral stem
        • most important risk factor
      • large diameter stem
      • extensively porous coated stem
      • greater preoperative osteopenia
    • clinical implications
      • clinical implications of proximal stress shielding unknown
    • treatment
      • no specific treatment is necessary
  • Intraoperative fracture
    • risk factors
      • use of press fit technique
    • treatment
      • acetabular fracture
        • stable cup
          • add screws for additional fixation
        • unstable cup
          • remove cup, stabilize fracture, and reinsert cup with screws
      • femur fracture
        • proximal femur fracture
          • stable prosthesis
            • limit weight-bearing
            • consider cerclage cables/wires
          • unstable prosthesis
            • remove prosthesis, stabilize fracture, reinsert new stem that bypasses fracture by two cortical diameters
 

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Questions (4)

(OBQ08.16) What is the range of pore size of cementless porous implants to allow for optimal bony ingrowth? Review Topic

QID: 402
1

Less than 1 micron

2%

(61/2764)

2

50 to 400 microns

78%

(2166/2764)

3

1,000 to 5,000 microns

11%

(299/2764)

4

10,000 to 50,000 microns

8%

(213/2764)

5

100,000 to 500,000 microns

0%

(12/2764)

ML 2

Select Answer to see Preferred Response

PREFERRED RESPONSE 2

(OBQ05.17) Which of the following has been shown to increase the rate of failure of cemented femoral components in total hip arthroplasty? Review Topic

QID: 54
1

Stems that are precoated with polymethylmethacrylate

57%

(2051/3567)

2

Calcar contact of the collar

9%

(329/3567)

3

Smoother implant corners

5%

(173/3567)

4

Cement mantle of 2 millimeters

9%

(336/3567)

5

Stem material with a Young's modulus higher than 115 GPa

19%

(660/3567)

ML 4

Select Answer to see Preferred Response

PREFERRED RESPONSE 1

(OBQ04.253) In animal models, which of the following is true when comparing hydroxyapatite(HA)-coated femoral stems to identical non-HA porous-coated stems after implantation? Review Topic

QID: 1358
1

Grit-blasted stems have decreased rates of loosening

5%

(92/2006)

2

Hydroxyapatite-coated stems have shorter time to biologic fixation

84%

(1680/2006)

3

Harris hip scores are higher after porous-coated stem insertion

2%

(45/2006)

4

Transient thigh pain is increased after hydroxyapatite-coated stem insertion

5%

(106/2006)

5

Porous-coated stems show increased rates of calcar atrophy

4%

(72/2006)

ML 2

Select Answer to see Preferred Response

PREFERRED RESPONSE 2

(OBQ05.261) Osteopenia has what effect on the strength of the bone-cement interface in comparison to normal bone? Review Topic

QID: 1147
1

no effect

5%

(149/2762)

2

improved mechanical integrity (higher fracture resistance)

56%

(1547/2762)

3

diminished mechanical integrity (low fracture resistance)

34%

(934/2762)

4

reduced depth of cement penetration into bone

3%

(72/2762)

5

less affected by cement pressurization

2%

(49/2762)

ML 4

Select Answer to see Preferred Response

PREFERRED RESPONSE 2
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