Updated: 6/9/2021

THA Implant Fixation

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Evidence
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Images
https://upload.orthobullets.com/topic/5003/images/pedestal_formation.jpg
https://upload.orthobullets.com/topic/5003/images/cement_mantle.jpg
https://upload.orthobullets.com/topic/5003/images/valgus_stem.jpg
https://upload.orthobullets.com/topic/5003/images/spot_weld.jpg
https://upload.orthobullets.com/topic/5003/images/l tha.jpg
https://upload.orthobullets.com/topic/5003/images/cemented stem resized.jpg
https://upload.orthobullets.com/topic/5003/images/stress  shielding.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.
      • Ratio is calculated as canal diameter 10 cm distal to midportion of lesser trochanter divided by inner canal diameter at midportion of lesser trochanter
      • Dorr Classification
        Ratio 
        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
          • 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
Flashcards (2)
Cards
1 of 2
Questions (4)

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

QID: 402
1

Less than 1 micron

3%

(104/3796)

2

50 to 400 microns

76%

(2881/3796)

3

1,000 to 5,000 microns

12%

(474/3796)

4

10,000 to 50,000 microns

8%

(295/3796)

5

100,000 to 500,000 microns

1%

(20/3796)

L 2 D

Select Answer to see Preferred Response

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

QID: 54
1

Stems that are precoated with polymethylmethacrylate

56%

(2393/4247)

2

Calcar contact of the collar

9%

(397/4247)

3

Smoother implant corners

5%

(195/4247)

4

Cement mantle of 2 millimeters

11%

(449/4247)

5

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

19%

(792/4247)

L 4 D

Select Answer to see Preferred Response

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

QID: 1147
1

no effect

5%

(179/3402)

2

improved mechanical integrity (higher fracture resistance)

56%

(1894/3402)

3

diminished mechanical integrity (low fracture resistance)

34%

(1164/3402)

4

reduced depth of cement penetration into bone

3%

(95/3402)

5

less affected by cement pressurization

2%

(58/3402)

L 4 D

Select Answer to see Preferred Response

(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?

QID: 1358
1

Grit-blasted stems have decreased rates of loosening

5%

(136/2491)

2

Hydroxyapatite-coated stems have shorter time to biologic fixation

82%

(2053/2491)

3

Harris hip scores are higher after porous-coated stem insertion

2%

(59/2491)

4

Transient thigh pain is increased after hydroxyapatite-coated stem insertion

5%

(131/2491)

5

Porous-coated stems show increased rates of calcar atrophy

4%

(98/2491)

L 2 D

Select Answer to see Preferred Response

Evidence (16)
VIDEOS & PODCASTS (7)
CASES (1)
EXPERT COMMENTS (29)
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