STEMS
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- Cemented Advantages: Allows version adjustment, lower intraoperative fracture risk, reduced end-of-stem pain, antibiotics can be added to cement
- Cemented Disadvantages: Potentially longer operation time, risk of cement implantation syndrome
- Cementless Advantages: Promotes biologic ingrowth
- Cementless Disadvantages: Higher perioperative fracture risk, thigh pain, increased stress shielding
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- Collarless ➔ allows subsidence
- Polished ➔ prevents bonding
- Tapered (double taper AP and ML) ➔ allows subsidence ➔ converts shear forces to compression stresses ➔ countered by hoop stresses generated by cement mantle and femoral canal ➔ increases stability
- Load transfer occurs through hoop stresses
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- Collar ➔ prevents subsidence and stress shielding
- Rough surface ➔ promotes cement-bone bonding
- Cylindrical shape
- Load is transferred to bone by shear stress at stem-cement interface
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- Composed of two different materials - stainless steel core with polyacetal resin exterior
- Developed as an attempt to reduce stress shielding
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- It converts longitudinal stress into shear stress, similar to a well-bonded cementless implant
- Rama: "These are high shear stresses, low compressive stresses and medium tensile stresses."
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- For exams, Charnley = composite beam
- 4 generations: Polished → Matt → Matt → Polished
- 1st gen - polished, flat back
- 2nd gen - round back with matt surface (vanquished)
- 3rd gen - Matt surface with cobra flange
- 4th gen - polished triple taper C stem
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- Coronal, Sagittal and Axial tapers
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- I will choose taper slip because this is what I am trained in
- Both stem philosophies have shown excellent results in several European registries, but their modes of failure differ:
- Composite beam - aseptic loosening
- Taper Slip - slightly higher risk of periprosthetic fracture
- Overall, NJR showed a modest survival advantage of the most popular taper-slip design over all other groups (97.9% vs 97.5%) - practically the same
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6534220/
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- I will choose taper slip because this is what I am trained in
- Both stem philosophies have shown excellent results in several European registries, but their modes of failure differ:
- Composite beam - aseptic loosening
- Taper Slip - slightly higher risk of periprosthetic fracture
- Overall, NJR showed a significant survival advantage of the most popular taper-slip design over all other groups of patients (97.9% vs 97.5%) - practically the same
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- Improves quality and homogeneity of distal cement mantle
- Allows subsidence into air-filled centralizer
- Prevents "end bearing" of distal metal tip on cement to prevent cement cracking
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- Reduces effective joint space
- Cement is loaded in compression rather than shear compared to composite beam
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- Both are unipolar implants
- Austin Moore implant - has holes (OOOO); uncemented
- Thompson implant - cemented with no holes
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- Also known as Robert Mathys (RM) isoelastic stems
- Developed to overcome mismatch between stiff stem and more elastic bone; concept of isoelasticity was introduced
- Composed of polyesterate resin with stainless steel core; uncemented implant
- Require screws at the proximal side to anchor the implant
- Very poor outcomes. 69% survival at 10 years! 48% survival rates at 15 years!
Poor results from the isoelastic total hip replacement.pdf1649.5KB
CEMENTLESS Stems
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- 2 types of bone growth
- In-Growth = Porous
- Stronger
- Criteria (rule of 50) – pore size 50 micrometer, porosity (pore density) 50%, micromotion < 50 micrometer, gap < 50 micrometer
- On growth = micro divots created by grit blasting
- 2 types of insertion
- Press fit = under ream 1 size
- Line-to-line (also called Scratch fit or interference fit) = ream to same size
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- Osteoinductive; not an adhesive
- Hydroxyapatite is for bony ongrowth only. Not for bony ingrowth.
- HA is gradually replaced by bone
- Problem → coating delamination leading to third body wear and osteolysis
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- By fit - press fit or line-to-line
- By surface growth - in or on growth
- By shape
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- Classified by area of fit (Khanuja Classification) into 3 main types:
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- Type 1 = single wedge - only ML taper
- Type 2 = double wedge - AP + ML taper
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- Type 3 = longer stem
- Type 6 = Anatomic stem
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- Type 4 = Long stem Cylindrical fully coated
- Type 5 = Modular
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- Mechanical load bypass of proximal femur because ingrowth present throughout diaphysis, resulting in osteopenia and bone loss
- This is due to Hoek's law which states that when 2 springs of different stiffness are loaded next to each other, more force will be loaded through the stiffer spring and bypasses the less stiff proximal bone.
- Occurs in extensively coated stems.
- DOES NOT occur in taper slip stems.
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- Hooke's law states that when 2 springs of different stiffness are loaded next to each other, more force will be loaded through the stiffer spring.
- Thus, a stem with higher modulus will experience more loading
- Extent of porous coating. Proximal coated stems will reduce stress shielding
- Collar will transmit forces to medial proximal calcar
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- Patchy coating allows debris to track down the smooth sides - leads to increased effective joint space.
- Circumferential coating can seal off the canal
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- Assess as per Glassman and Engh 1992
- Divides fixation modes into 3; implications = will determine ease of extraction
- If Stable and fibrous = cannot be removed by simple disimpaction ➔ need ETO and burr
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- If no new radiolucencies or reactive lines, it can be associated with a stable stem.
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- Collared
- Bowed to fit into the diaphysis
- Hole at the top for easier removal
- Fits by "diaphyseal scratch fit," requiring 2 cortical diameters
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- Left side - Bony ingrowth
- Right side - Bony ongrowth with HA coating. The layer of hydroxyapatite coating has been stripped off during explant, so bone is no longer attached to it.
TRUNNION
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- It is a method of mounting two rotating machine parts together
- Cone-in-cone structure, self-locking through an interference fit, providing both axial and rotational stability
- The Trunnion is the male part (the core), while the Bore is the female part (the hole)
- Aims to achieve cold welding between the two components
- Critical locking angle is less than 7°
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- Stryker Exeter - taper angle is 5° 40' (V40)
- Each degree is divided into 60 minutes, each minute into 60 seconds
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- It describes the male trunnion (cone)
- 12mm is the proximal diameter, 14mm is the distal diameter
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- There can be "negative mismatch" in the design features resulting in more trunnionosis
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- It is a form of FRETTING CORROSION
- It is a form of Mode 4 Wear (between 2 unintended surfaces)
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- Large diameter head > 36mm
- Large offset/ Long neck (increases moment arm)
- Varus positioning
CUPS
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- Cemented cups theoretically fail at higher rates due to shear forces.
- Cement is strong in compression, weak in tension and shear
- Original Charnley designs used cemented cups, but the concept of biological integration has led to increased use of cementless cups today
- Frank Van Praet et al. Systematic review -
- "Both cemented and hybrid models provide excellent results in both the short and long term.
- However, the widespread preference for cementless fixation of the acetabulum cannot be explained by superior survival of cementless or hybrid models. Irrespective of age, cemented fixation of the acetabulum remains the gold standard to which other techniques should be compared."
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- PMMA Pods/spacer beads to prevent bottoming out phenomenon (ensuring cement remains between bone and implant)
- Flange at rim to aid in pressurization
- Grooves to increase stability within the cement mantle
- Wire markers - placed in anterior hemisphere of cup
- Crescent shape - good version
- Hemisphere - 0 version
- Olive shape - retroversion
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- Hemispherical cup +/- lip
- Constrained Cup [for neuromuscular conditions]
- = Head locked in cup
- Higher constraint = increased wear
- Dual Mobility Cups aka Tripolar aka (Modular Dual Mobility MDM by Stryker)
- Femoral head that moves within a PE component, which also moves within a shell
- Consider in patients with prior spine fusion/dislocating THR
- Greater ROM and stability, smaller head leads to less wear
- ➖ Higher constraint = increased wear
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- This is only applicable for MOM THR because of the hard nature of the bearing
- First gen MOM THR typically had Equatorial contact due to manufacturing limitations. I.e., Head is larger than cup; so articulation occurs at the equator (widest portion of head)
- This resulted in friction and boundary lubrication as it prevents "fluid entrainment" between cup and head
- Leads to fluid "lockout" state, causing high friction and wear
- This led to implant seizing and subsequent premature loosening
- Polar Contact allows sufficient clearance for fluid entrainment/ingress ➔ Optimizes lubrication (fluid film)
- Can be high conformity = good [midpolar contact]
- Or low conformity = leading to point loading (not good)
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- Immediate stability is achieved by press fit (conferring hoop stresses), screws, and in older designs, spikes and pegs
Bearings
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- The head appears more radiolucent than the stem, indicating ceramic rather than metal
- Additionally, with CoC, the holes on the acetabulum are less clearly visible
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One gives freely, yet grows all the richer; another withholds what he should give, and only suffers want. Proverbs 11:24