Arthroplasty Flashcards
What are the types of prosthetic articular bearing interfaces?
Hard-on-soft and Hard-on-Hard
Of the metallic heads, which is considered the best? Name one alternative?
Cobalt-Chrome is the best
Ceramic is an alternative - but not Zirconia. This can change in-vivo into a weaker state
What is the best Hard-on-Soft bearing?
Cobalt-Chrome & polyethylene
For Hard on Soft bearings, what two factors are optimum wear based on?
Roughness of the head surface
Sphericity of head surface
What is the lubrication regiment for hard on soft bearings of the hip?
Boundary Lubrication
What is boundary lubrication?
Lubrication regiment for hard on soft bearings.
It is where the lubricant (aka synovial fluid) is not thick enough to prevent contact between asperites (high points on the bearing surfaces) but can seaprate the two surfaces enough to prevent severe wear
What is the advantage of a ceramic component?
Greater scratch resistance than Cobalt-Chrome
What are the disadvantages of ceramic components?
More brittle -> may lead to prosthetic breakage Low toughness (resistance to fracture)
What is th emajor factor in causing osteolysis and prosthetic failure in hard on soft prosthetic bearings?
Polyethylene wear
What are the factors associated with polyethylene wear?
PE manufacturing: Direct compression molding is best
PE sterilization after processing: Irradiation in oxygen free environment is best
Shelf life: best left for less than a couple years
What is direct compression molding of polyethylene? What are other types of manufacturing of PE?
Where the powder is pressed directly into the final shape. It creates the best wear results
Other types of manufacturing:
- Ram bar extrusion with secondary machining into final product
- Hot isostatic pressing into bars with secondary machining into final product
- Compression molding into bars with secondary machining into final product
How is highly cross-linked polyethylene produced?
By high-dose irradiation of ultra-high molecular weight polyethylene
(as opposed to low-dose irradiation)
Is high crystallinity good or bad for highly cross-linked polyethylene?
Bad - higher than 70% crystallinity associated with higher PE failure rates
- Best is 50-56%
What is the main advantage of highly cross-linked polyethylene?
Improved wear rates, theoretically improving (aka decreasing) osteolysis and implant survival
What are the disadvantages of highly cross-linked polyethylene?
Diminished mechanical properties
- Decreased tensile strength (pulling force to break)
- Decreased fatigue strength (max cyclic stress the material can withstand)
- Decreased fracture toughness (force to propogate a crack)
- Decreased ductility (elogation without fracture)
Basically makes it harder but more brittle. Effects exacerbated by edge-loading
Compare the size of the particles generated with highly cross-linked polyethylene versus regular ultra-high molecular weight polyethylene:
Highly cross linked creates smaller particles
What is the advantage of a hard-on-hard bearing?
Theoretically less structural bone damage and prosthetic failure from the polyethylene particulate-induced osteolysis of hard-on-soft (b/c there is no PE interface)
What is the particle size that has been shown to trigger an immune response?
0.2-0.7um
What is the size of the of particles from hard on hard bearings?
Smaller than that (0.015-0.12um)
What is the lubrication regiment for hard on hard bearings?
Mixed lubrication - meaning that half of the time it’s mixed lubrication and half of the time it’s hydrodynamic (fluid film) lubrication
When is the lubrication regimen for hard on hard bearings hydrodynamic? Boundary?
Hydrodynamic when in motion (aka walking)
Boundary when static
What is hydrodynamic lubrication?
When the fluid film completely separates the two bearing surfaces
What is radial clearance?
Difference in the radius of the head and cup
What are the different types of radial clearance?
Equatorial: head is larger than the cup and loading is on the edge (equator)
Polar: head smaller than cup and loading occurs at the tip (polar aspect) of the head
Midpolar: head just right for cup. Leading occurs at midpolar point
What is the optimum radial clearance? Why are the other bad?
Midpolar is the best. Allows ingress and egress of lubricant into the bearings.
In equatorial, the fluid is locked out, creating high friction
In polar, there is also high friction and wear b/c of point loading
What is Run-In wear?
Phenomenon of hard on hard bearings.
- Higher wear rate during the first 1 million cycles. The wear rate then stabilizes after that
What is edge loading?
Axial hip load is passed through the femoral head into the acetabular cup near the edge. It’s bad b/c increases load
By contrast, in if axial load is passed through the pole of the cup, the contact area is maximized and contact loads are low
What is stripe wear?
A visible stripe on the bearings showing the area of wear
- Generally occurs with high edge-loading
What is bone cement made out of?
methylmethacrylate
Generally speaking, when do you use cemented versus uncemented components in total hip arthroplasty?
Uncemented: young healthy individual with good bone stock/ingrowth potential
Cemented: older individual with poor bone stock/ingrowth potential
What is the mechanical advantage/disadvantage of cemented components?
Advantage: allows mechanical interdigitation with bone, particularly in patients with poor bone stock
Disadvantage: static interface allowing for no remodeling (as bony ingrowth in uncemented components would allow), making the interface more prone to mechanical failure
Describe 3rd generation cementing techniques
Cement factors: - Vacuum mixing for porosity reduction - Pressurization of cement Component factors - Precoated stem - Rough suface finish on the stem - Use of a stem centralizer - Canal preparation (brush and dry)
Describe the optimum cement mantle:
2/3 rule: 2/3 stem, 1/3 cement
- Intramedullary canal should be displaced 2/3 by the femoral stem and the other 1/3 by the cement
All else being equal, what will fail first: cemented femoral stem or cemented femoral acetabulum?
Cemented acetabulum
What is the interface used today for the acetabuluar cup?
Uncemented, backed up with screws if necessary
- Because cemented acetabuli fail faster
What is Ling’s technique for cementing?
Using highly polished tapered stem with square edges and no centralizer.
What are the two types of biologic fixation?
Ingrowth (provided by porous coated metallic stems)
Ongrowth (provided by grit-blasted metallic surface)
What is the preferred porosity to facilitate ingrowth?
50-150um up to 40-50% of the stem, not more as it will lead to increased shearing forces
What are 3 factors in porous coated surfaces affecting secure fit?
Porosity - best size is 50-150um up to 40-50% of the stem
Pore depth - deeper = more shear strength
Gaps - between prosthesis and bone must be <50um
What affects grit-blasted component’s ability to generate on-growth?
Surface roughness
What are two factors required for good biologic (uncemented) fixation of components?
Rigid fixation
Cortical bone seating
What is the press fit technique?
Bone is prepared 1-2mm smaller than the components, and then the components are fitted into the bone
What is the line to line fit technique?
Where bone is fitted to the same size as the components, but may require screws for additional fixation
What is cortical bone seating?
Implant being seated against cortical bone (as opposed to cancellous) for maximum stable ingrowth
What is the sequalae of too much motion in terms of biological fixation?
Will get fibrous ingrowth instead of bony ingrowth
- Bad as it is not strong and will lead to continued mechanical instability
What is a “spot weld”?
dfs
What is stress shielding?
Bone density loss observed over time in a solidly fixed implant
What is the mechanism behind stress sheilding?
Load passes through the material with greater stiffness, decreasing bone density (from decreased stress through bone)
What are the causes of stress shielding?
Stem stiffness - primary factor - increased with stiffness
Stem size: stiffness increases with radius of stem (by r^4)
Metal choice: again to do with stiffness
Stem geometry: Solid, rounded stems are stiffer
Give the prototypical scenario of stress shielding in THR:
Large diameter stem (>16mm), made of stiff (Co-Cr) alloy, with a rounded, solid, cyindrical shaft with an excessive porous coating
What has more stress shielding, partially or fully coated stems?
Fully
Does stress shielding affect the implant?
Not the primary one, but makes revision more difficult because of poor bone stock
What is the clinical picture of fat embolus syndrome?
Rapid onset of hypotension and hypoxia seen after insertion of a cemented implant
What is the pathophys of fat embolus syndrome?
Pressurization of the canal (by cement, IM rods), force fat out of bone and into venous circulation.
- It then goes into the lungs where it deposits in the capillary parenchymal tissue and prevents oxygenation
What are the major areas affecting hip stability after THR?
4 areas:
- Component design
- Component alignment
- Soft tissue tensioning
- Soft tissue function
What is the primary arc range?
Arc of the articulation before impingement occurs
What affects the primary arc range?
Head to neck ratio most important
- Larger head:neck ratio increases primary arc range
What is excursion distance?
Distance that, once impinged, the head must travel to become dislocated
- Usually 1/2 of the diameter of the femoral head
What happens to primary arc range with a constrained liner?
Drastically reduced
What is the appropriate anteversion for the acetabulum?
10-15deg
15-30 in the books
What is the appropriate theta angle (coronal tilt) of the acetabulum?
35-45deg
What is the key soft tissue complex to the hip?
Tension of the abductor complex (gluteus medius/minimus)
What are 2 factors affecting soft tissue tensioning?
Head offset
Neck length
- Must be fully restored to get appropriate tensioning
Why is soft tissue tensioning important?
Must be kept at appropriate tension to keep hip stable
Define true and apparent leg length discrepancy
True: actual leg length difference (aka anatomically measured)
Apparent: one perceived by the patient either due to change in leg length (back to normal) or due to posture (pelvic obliquity, scoliosis)
Describe the Paprosky classification for femoral bone loss:
Type 1: minimal metaphyseal bone loss and intact diaphyseal fixation
Type 2: extensive metaphyseal bone loss with intact diaphyseal fixation
Type 3a: severe metaphyseal bone loss with >4cm bone preservation for distal fixation
Type 3b: severe metaphyseal bone loss with <4cm bone preservation for distal fixation
Type 4: extensive metaphyseal and diaphyseal bone loss
What are the risks of peroneal nerve palsy after TKA?
Preoperative diagnosis of neuropathy
post-op epidural analgesia
Large valgus deformity
Excessive medial release
What are the major complications following TKA?
Infection Instability Stiffness Vascular injury Nerve palsy Extensor mechanism injury
What are the types of instability following TKA?
Axial instability (medial/lateral) Flexion instability (AP)
What are the causes of axial instability?
Axial instability
- If flexion and extension symmetric: thicker tibial liner
- If flexion and extension asymmetric: augmentation and component revision
What are the different types of total knee arthroplasties?
Unconstrained: cruciate retaining and cruciate substituting/stabilizing
Constrained: unhinged and hinged
What is the issue if both the ACL & PCL are sacrificed during TKA without stabilization?
Limited knee flexion - the knee is unstable in flexin and the femur could sublux/dislocate anterior to the tibia
Why is it important to have a PCL (native or prosthetic) in TKA?
- To prevent anterior dislocation of the femur on the tibia
2. To facilitate femoral rollback
What is femoral rollback?
When the knee flexes, the femur slides posteriorly on the tibia.
This moves the contact point on the tibia posteriorly and allows for more range of motion in flexion before impingement
When are posterior stabilized/substituting TKA’s indicated?
Anytime there is an issue or risk of an issue with PCL deficiency:
- Previous PCL injury
- Inflammatory arthritis (risk of late PCL rupture)
- Over-release of the PCL during knee ligament balancing
- Previous extensor mechanism injury (patellar tendon/patella damage), as this weakens the extensor mechanism and diminished the anterior restraint to anterior femoral subluxation
What is a mobile-bearing total knee arthroplasty?
One were the polyethylene is able to spin (aka be mobile) on the tibial plate
(vs. fixed bearing, where the PE is locked in - these are what we use)
What is the unique complication to a mobile bearing design?
Spinout
- When the polyethylene rotates beyond the normal constraints of the knee
What is thought to be the advantage of a mobile bearing joint?
Better congruity of the tibia and femur during all ranges
- Hasn’t really worked. They are equal to fixed bearing ones
What is the problem and solution in the following situation: Tight in extension & flexion
Problem: symmetric gap. Did not cut enough tibia
Solution: cut more proximal tibia
What is the problem and solution in the following situation: Loose in extension & flexion
Problem: symmetric gap. Cut too much tibia
Solution: thicker PE or metallic tibial augmentation
What is the problem and solution in the following situation: Extension OK, flexion loose
Prob: asymmetric gap. Cut too much posterior femur
Solution:
- Increase size of femoral component from anterior to posterior (go up to next size or fill posterior gap with cement or metal augmentation)
- Use thicker PE insert and readdress right extension gap
What is the problem and solution in the following situation: Extension tight, flexion OK
Prob: asymmetric gap. Did not cut enough distal femur or did not release enough posterior capsule
Solution:
- Release posterior capsule
- Take off more distal femoral bone
What is the problem and solution in the following situation: Extension OK, flexion tight
Prob: asymmetric gap. Did not cut enough posterior femur or PCL scarred in too tight. No posterior slop in tibial cut
Solution: Decrease size of femoral component from A to P (recut next smaller size)
- recess PCL
Check posterior slope of tibia and recut if req
What is the problem and solution in the following situation: Extension loose, flexion OK
Prob: Asymmetric gap. Cut too much distal femur or AP size too big
Solution: Distal femoral augmentation
- Smaller femur (A to P) and readdress as symmetric gap problem
- Use a thicker PE insert and readdress as tight flexion gap
What structures are released in TKA when you have a varus deformity?
Medial tight, so Medial release
- Medial osteophytes
- deep MCL
- Posterior corner with Semimembranosus
- Superficial MCL and pes anserinus
- PCL (rarely)
What structures are released in TKA in a valgus deformity?
Lateral tight, so lateral release.
- Osteophytes
- Lateral capsule
- IT band if tight in extension
- Popliteus if tight in flexion
- LCL
What is the preferred alignment of the femoral component in TKA?
Slight ER to the neutral axis
Why is IR femoral component in TKA bad?
Creates:
- Asymmetric flexion gap presenting as a stiff, painful knee
- Increased Q angle, predisposing to femoral patellofemoral pain and dislocation
What are 3 accepted landmarks to define the neutral femoral rotational axis?
Anteroposterior axis of the femur
Epicondylar axis
Posterior condylar axis
What is the preferred position of the tibial component in total knee arthroplasty?
Neutral to ER
Why must IR of the tibial component in TKA be avoided?
Results in an effective ER of the tibial tubercle and an increasd Q-angle
What is the preferred position of the patellar component in TKA?
Central or medialized
What is the minimum thickness of PE insert in TKA that must be used to avoid catastrophic failure?
8mm (of the tibial insert only, as many systems show thickness as the tibial insert + the metal tray)
Name the extensile exposures for TKA?
Lateral release (upside down happy face)
Quadriceps snip - Diagonal cut going proximally
Patellar turndown Diagonal cut going distally
Tibial tubercle osteotomy - cut medial to lateral & preserve the lateral side to keep blood supply
What is the major issue with patellar baja in total knee arthroplasty?
Impingement on the tibia during flexion, resulting in decreased flexion
What are the technical goals of total knee arthroplasty?
- Restore mechanical alignment (mechanical alignment of 0°)
- Restore joint line ( allows proper function of preserved ligaments. e.g., pcl)
- Balanced ligaments (correct flexion and extension gaps)
- Maintain normal Q angle (ensures proper patellar femoral tacking)
What is the classification for periprosthetic femur fractures around a TKA?
Su classification:
Su 1: fracture proximal to femoral component
Su 2: fracture originates at the proximal edge of the femoral component and extends proximally
Su 3: Any part of the fracture extends distal to the proximal edge of the femoral component
What is the classification system for periprosthetic tibial fractures around a TKA?
Felix classification I: Tibial plateau fracture II: Fracture adjacent to stem III: Fracture distal to stem IV: Fracture of tibial tuberosity
What is the classification for acetabular bone loss in a revision THA setting?
Paprosky Classification
I: Minimal deformity, rim intact
IIa: Superior bone lysis with intact superior rim
IIb: Absent superior rim, superolateral migration
IIc: Absent medial wall
IIIa: Bone loss from 10-2 around rim, superolateral cup migration
IIIb: Bone loss from 9-5 around rim; superomedial cup migration; pelvic discontinuity
Name the problem associated with each of the following symptoms:
groin pain
thigh pain
start-up pain
night painGroin pain: acetabulum
Thigh pain: femoral stem
Start-up pain: component loosening
Night pain: infection
Name the classification for femoral bone loss around a total hip arthroplasty
Paprosky classification:
I: minimal metaphyseal bone loss
II: extensive metaphyseal bone loss with intact diaphysis
IIIa: extensive metadiaphyseal bone loss, minimum of 4 cm of intact cortical bone in the diaphysis
IIIB: extensive metadiaphyseal bone loss, less than 4cm intact cortical bone in the diaphysis
IV: extensive metadiaphyseal bone loss with a non-supportive diaphysis
What is the indication for a constrained, non-hinged TKA prosthesis?
MCL attenuation
LCL attenuation or deficiency
Flexion gap laxity
What is the indication for a constrained, hinged TKA prosthesis?
- Global ligament deficiency (post-trauma or multiply revised knee)
- Hyperextension instability (polio)
- Resection of the knee for tumor or infection
- Charcot arthropathy (relative)
- Complete MCL deficiency (relative & controversial)
In reconstruction of the knee, which side should be reconstructed first and why?
Tibia - to restore the joint line
What are landmarks used for reconstruction of the knee joint line?
Height of the fibular head (1.5-2cm above the fibular head)
Contralateral knee
What are the contraindications to unicompartmental knee arthroplasty (UKA)?
- ACL deficiency
- Fixed varus deformity that cannot be corrected on clinical exam
- Previous menisectomy in the opposite compartment
- Knee flexion contracture
- Lack of knee flexion (>90 deg req)
- Inflammatory arthritis
- Significant tricompartmental disease
- Highly active patient or labourer
