Surface: polish, ingrowth or ongrowth mechanism, HA coated
Collar; and what/when it is used for
Q2Who was Charnley and what is he famous for?▸
Father of modern hip replacement
Charnley low friction arthroplasty (CLFA)
Introduction of antibiotics into cement, clean air enclosure, total body exhaust suits, instrument trays
Importance of bony compression in arthrodesis; closed treatment of common fractures
Q3What is Charnley's low friction hip arthroplasty design?▸
1962
ultra high molecular weight polyethylene cup (UHMWPE; pioneer to use); small femoral head 7/8 inch = 22.225mm based on the low frictional torque principle
Trochanteric osteotomy with lateral and distal repositioning
Medialisation of the cup; use of acrylic cement for load distribution
Q4Describe the classical Charnley stem and its generations.▸
Monobloc, 22.225mm head, collar, single taper with sharp corner, polished
Flat back -> round back (more cross-sectional area, matted) -> cobra flange -> C stem/triple taper
Q7Describe a cementless stem as presented in the lecture.▸
Modular stem
Collared
Proximal rough surface with porous coating
Not tapered; designed to encourage proximal ingrowth
Q8How would you describe this stem?▸
Monobloc stem used in cemented total hip arthroplasty
Made of Orton, a stainless steel alloy
Head 22.225mm with a collar to prevent subsidence
Looks like a round back generation with a matted surface
Charnley principle: low frictional torque arthroplasty, medialisation of hip centre, lateralisation of GT
Q9Why does the Charnley stem fail?▸
Small head neck ratio
Sharp edges
Fatigue failure at the anterolateral tension site in earlier generations
Loosening from micromotion between implant and cement (Gruen 1a failure)
Q10Describe the evolution of the Charnley stem.▸
Flat back (sharp corner, single tapered) -> round back (increased cross-sectional area to resist fatigue, matted surface) -> cobra flange -> C stem/triple taper
Four generations: polished - matt - matt - polished
Q11How would you describe the Exeter stem?▸
Modular, collarless stem used in THR or hemiarthroplasty
Made of orthinox
Double taper with a polished surface and round edges
Allows for controlled subsidence
Q12Compare the composite beam and taper slip design philosophies.▸
Charnley (composite beam): a rod in two tubes, relying on mechanical interlock at cement-bone and implant-cement interfaces
Load travels from the femoral head, bypasses the stem to the tip and to the bone; rough surface and collar minimise micromotion
The mechanism of failure is different: composite beam fails when micromotions occur at the prosthesis-cement interface; the rough stem will piston within the cement mantle, leading to wear debris circulation around the effective joint space and osteolysis
Exeter (taper slip): viscoelastic cement dissipates vertical force into hoop stress as the stem subsides, transferred along the whole stem; polished taper stems are inherently stable and fail when they rotate in the axial plane (RSA and retrieval studies)
Q13What is the evidence comparing Exeter and Charnley survival?▸
Kiran Acta Orthop Belg 2019
Minimum 15-year follow-up, 876 THR
Exeter 98%, Charnley 97%
Q14What is the centraliser made of and what is its key design feature?▸
Made of PMMA
Contains an air-filled distal void
Q15What are the functions of a PMMA centraliser with an air-filled distal void?▸
Allows the stem to subside into the void in a controlled fashion
Converts shear stresses into compressive forces with almost no tensile stress
Prevents end bearing of the stem tip on the cement mantle and cement fracture
Improves stem position and centralises the tip for an even cement mantle distally
Seals off the effective joint space and stem-cement interface
Q16How does the air-filled distal void affect stresses at the stem-cement interface?▸
Stem can subside into the void in a controlled fashion
Converts shear stresses into compressive forces
Almost no tensile stress on the mantle
Q17What is the consequence of end bearing of the stem tip on the cement mantle?▸
Direct end bearing causes cement fracture
Prevented by the air-filled distal void allowing controlled subsidence
Q18How does the centraliser improve the distal cement mantle?▸
Improves stem position
Centralises the tip of the stem
Creates an even cement mantle distally
Q19What does the centraliser seal off and why does this matter?▸
Seals off the effective joint space
Seals off the stem-cement interface
Prevents fluid flow that may lead to loosening
Q20What are the design features of the Muller stem?▸
Modular
Stainless steel
32mm head - relatively large head/neck ratio to decrease impingement; alumina ceramic head
Curved stem - easier insertion in anterior approach
Collar increases stress transfer to proximal medial femur
Q21What material is the Muller stem made of and is it modular?▸
Stainless steel
Modular design
Q22Why does the Muller stem use a 32mm head?▸
Relatively large head/neck ratio
To decrease impingement
Q23What is the purpose of the collar on the Muller stem?▸
Increases stress transfer to proximal medial femur
Decreases stress shielding
Q24Why is the Muller stem curved and how is it inserted?▸
Curved - easier insertion in the anterior approach
Sharp edges with diamond-shaped cross section
Inserted using the 1st generation cementation technique
Q25What type of stem is the isoelastic stem and what is it made of?▸
Cementless monobloc stem
Polyacetal resin with a stainless steel core
Q26What is the geometry of the isoelastic stem?▸
Quadrilateral cross section + fluted for rotational stability
Straight conical stem
Collared
Q27What is the surface of the isoelastic stem and how does it achieve fixation?▸
Crosshatched surface and conical recess for bone ingrowth
Skirted head, usually used with a cemented PE cup
Q28Why is the isoelastic stem designed to have a Young's modulus similar to bone?▸
Young's modulus similar to bone
Reduces stress shielding
Q29How does the isoelastic stem transmit force?▸
Medially through the collar to the medial calcar
Laterally through 2 screws at the greater trochanter
Q30What are the disadvantages of the isoelastic stem?▸
Less strong material - porosity cannot be small - poor bone ingrowth and micromotion
Generates polyacetal molecules which increase wear
Small head-neck ratio (thick neck to prevent fracture) - impingement
Q31What material is the monoblock stem made of?▸
Cobalt chrome – Molybdenum alloy
Q32What is the geometry of the monoblock stem?▸
Curved stem tapering in coronal and sagittal profile
Transverse dimension rectangular
Q33What surface features does the monoblock stem have?▸
Collared with fenestrations
Matted surface
Q34How is the monoblock stem fixed?▸
Cementless fixation with the 3 point fixation principle
No bone ingrowth unless bone graft is inserted into the fenestrations
Q35What are the modes of failure of the monoblock stem?▸
Loosening
Subsidence
Q36How can failure of the monoblock stem be prevented? (Yau, Injury 2003)▸
Only use in patients >75 years
Metaphyseal filling >75%
Fact check
McKee Farrar metal-on-metal prosthesis has a 10-year survival of 27% — likely timeframe error — The 27.5% survivorship figure is from the 808-hip Norwich series at 20 years (August et al. 1986); other series report 76% at 10 years and 74-84% at 20-28 years — (medium confidence) — source