1. Evidence
1.1. Fixation selection
1.1.1. Cup - porous coated cementless preferred
1.1.2. Stem - both cement and cementless accepted
2. Complications
2.1. Femoral stem Breakage
2.1.1. Via Cantillever bending
2.1.2. Stem fixed distally, loose at the top
2.1.3. Cyclic bending stress
2.1.4. Fracture occur in middle portion when taper and becomes thin
3. Concepts
3.1. Femoral Stress shielding
3.1.1. Proximal femoral bone density loss observed over time in presence of a solidly fixed implant
3.1.2. Due to modulus mismatch between stem (stiffer) and femoral cortex (less stiff)
3.1.2.1. Hoek’s Law- when 2 springs are placed next to each other and loaded, more force is transmitted through stiff spring and less through flexible spring
3.1.3. Factors affecting stem stiffness
3.1.3.1. Stem diameter (most important)
3.1.3.1.1. Radius ^4
3.1.3.2. Material - CoCr stiffer than titanium
3.1.3.3. Geometry - solid and round stems more stiff
3.2. New node
3.3. Femoral Stem Loading
3.3.1. Proximal bone loading
3.3.1.1. Occurs in proximal porous coated implants
3.3.1.2. Mechanical load is transferred to metaphysics and proximal diaphysis
3.3.1.3. Thus, proximal bone density maintained with proximal porous-coated implants
3.3.2. Distal bone loading
3.3.2.1. Mechanical load bypasses proximal femur because porous ingrowth is present throughout diaphysis
3.3.2.2. Occurs in extensive porous coating
3.3.2.3. In well-fixed cementless stem, there is endosteal consolidation of bone near end of stem (spot weld)A
3.3.2.4. Also happens in well-fixed cemented stem as load distributed throughout mantle and more load bypasses proximal femur
3.3.2.5. New node
4. Stability
4.1. Component design
4.1.1. All affects Prosthetic ROM which has 2 parts
4.1.1.1. Primary Arc Range
4.1.1.2. Lever range
4.1.2. Factors
4.1.2.1. Alignment
4.1.2.1.1. PAR must be centered within patient's functional hip range
4.1.2.1.2. Malalignment results in a stable and unstable side
4.1.2.1.3. Components
5. Compression Hoop Stresses
6. Mantle defect: Area where prosthesis touches the bone - an area of significant stress concentration
7. Fixation Techniques
7.1. Press-fit technique
7.1.1. Bone prepared such that slightly oversized implant is wedged into position
7.1.2. Cup - typically 1mm larger in size, press fit against acetabular rim; screws not required
7.1.3. Stem - has gradual taper design; for On-growth cementless fixation
7.1.4. Cx
7.1.4.1. Fractures usually due to underreaming
7.1.4.1.1. Cup - if stable add screws, if unstable remove and stabilise and reinsert with screws
7.1.4.1.2. Femur - typically proximal in calcar. If stable, limit WB if unstable, remove and stabilise with cerclage wires or cables and reinsert.
7.2. Line-to-line technique
7.2.1. Bone prepared such that contour of bone same size as implant
7.2.2. Cup - inserted with screws
7.2.3. Stem - has extensive porous coating that achieves frictional fit/ interference fit/ scratch fit
7.2.4. Cx
7.2.4.1. Fractures typically at femur distal stem tip
7.3. Use of Hydroxyapatite
7.3.1. Ca10 (PO4)6 (OH)2
7.3.2. Osteoconductive effects - allow more rapid closure of gaps between bone and prosthesisC
7.3.3. Clinically shortened time to biological fixation
7.3.4. Success determinants
7.3.4.1. High cystallinity
7.3.4.2. Optimal thickness
7.3.4.3. Surface Roughness - increased M-HP interface.
8. Methods of Fixation
8.1. Cement (PMMA)
8.1.1. Works by PMMA acting as a grout, with microinterlocking with endosteal bone
8.1.2. 5 Keys to cementing success
8.1.2.1. Porosity reduction - via vacuum mixing
8.1.2.2. Pulsatilla lovage - clean, dry bone allows better cement interdigitation
8.1.2.3. Pressurization of cement before insertion - enhances cement interdigication with bone
8.1.2.4. Stem Centralizing with distal stem centralizer - maintains uniform mantle with no mantle defect
8.1.2.5. Stiff stem - lessens bending stress upon cement mantle. Co-Cr and SS stems perform better than titanium
8.1.3. Cement in THA?
8.1.3.1. Cement fatigues with cyclic loading
8.1.3.2. Cemented cup fails faster than cemented stem due to increased tension and shear forces (theta angle)
8.1.3.3. Should not use cement in young active individuals
8.2. Cementless
8.2.1. 2 types
8.2.1.1. Bone On-growth
8.2.1.1.1. Requires Gritty surface
8.2.1.1.2. Always requires a press-fit fittingC
8.2.1.1.3. Cx
8.2.1.2. Bone In-growth
8.2.1.2.1. Requires Porous surface
8.2.1.2.2. Keys to optimal in-growth