Hip Arthroplasty Flashcards
List the methods of implant fixation
- Cemented - polymethylmethacrylate (PMMA)
- Uncemented - biologic fixation
What are the two mechanisms of cementless fixation?
- Bone ingrowth - porous coating
- Bone ongrowth - grit coating
List the indications for cementless stems?
- High-activity patient - cement would cyclically fail over time
- Young male patient - high loading stress causes cement failure
How can cementing technique be optimised?
- Vacuum mixing - reduces porosity in cement, reducing stress points
- Pressurisation - enhances interdigitation with bone
- Pulsatile lavalge - alows better interdigitation
- Stem centralisation - maintains uniform cement mantle
- Stiff stem - reduces bending stress on cement mantle
How does implant design promote bone ingrowth?
- Optimal pore size
- Optimal pore depth
- Optimal metal porosity (40-80%)
- Minimise gap between metal and bone
- Minimal implant micromotion (initial rigid fixation must be achieved)
How is initial rigid fixation for cementless implants achieved?
- Press fit technique
- Line-to-line technique (requires screw supplementation)
Describe the press fit for cementless fixation
- Bone is prepared so that implant is oversized
- Wedge effect of implant creates compression hoop stress to hold implant in position
Describe the line-to-line technique of cementless fixation
- Bone is prepared so that contour is equal to implant
- Femoral stem has extensive porous coating to provide frictional fit
- Acetabulum is held in place with screws
Why is optimal porosity of cementless implants between 40-80%?
- Too little - weak fixation
- Too much - shearing of metal
What is femoral stress shielding?
Proximal femoral bone density loss observed over time in the presence of a solidly fixed stem
What causes femoral stress shielding?
Modulus mismatch between stem and femoral cortex
What factors affect stem stiffness?
- Stem diameter (+radius^4)
- Metal type
- Stem geometry
What is the minimum cement mantle thickness?
2mm
List the Gruen modes of cemented femoral stem failure
- Pistoning
- Medial mid-stem pivot
- Calcar pivot
- Bending cantilever
Desctribe pistoning in stem failure
Stem subsides within the cement or stem + cement subside within bone. Pistons in and out.
Describe medial mid-stem pivot failure
Stem fixed in the middle, proximal part tilts medially whilst distal part tilts laterally causing fracture of the mid-stem
Describe bending cantilever stem failure
Loss of proximal fixation with the distal end fixed, causes stem fracture
How may femoral bone be classified
Dorr Classification
Describe the Dorr Classification
- Type A = narrow canal with thick cortices (champagne flute)
- Type B = moderate cortical wall thickness
- Type C - wide canal with thin cortical walls (stove pipe canal)
What are the two types of cemented stem?
- Composite beam - all elements fixed
- Taper slip - bone interface fixed, stem-cement interface free to slip
Name the classification system for lucency zones in the femur
Gruen Zones
Describe Gruen Zones
1-7 from GT to LT
Name the classification system for lucency zones around the acetabulum
Delee Charnley Zones
