TKR Implants
Key knee biomechanics to replicate: - Femoral roll back = posterior translation of femur on tibia in flexion
- This means the tibial surface must be relatively flat and less conforming
- In PS knees, this is achieved with the femoral cam coming into contact with tibial post
- Screw Home Mechanism
What is the condylar compromise? - The "condylar compromise" - A balance between constraint, kinematics and contact stresses
- The principle is that more normal kinematics requires lower implant constraint (e.g., flat tibial tray), but this increases contact stress
- To reduce contact stress, we need more implant conformity - this increases constraint and thus reduces kinematics
- Unicondylar
- Mobile Bearing design
- Cruciate Retaining
- Pros – bone preserving, more proprioception with PCL, avoids post-cam impingement
- PS aka Cruciate sacrificing
- Pros – easier to balance, more congruent joint surface
- Cons – cam jump, tibial post wear
- Anterior Stabilized
- Uses CR component and PCL is removed
- Instead, PE has raised anterior lip that is more conforming and resists anterior translation
- (-) More conforming PE → higher surface area → higher PE wear debris
- Constrained condylar knee (PS with post) - LCCK (legacy constrained condylar knee)
- Rotating Hinged
- F/E hinged
All-PE vs metal backed modular design in tibial component? - All-PE (+) - lower costs, no back side wear (so no osteolysis), thicker PE
- All-PE (-) - no modularity (fewer options), no liner exchange procedure for PJI, potential difficulty to remove, though theoretically allows thicker PE to be used
What materials are used in TKR implants? - Femur - Cobalt chrome
- Tibia - Titanium (or cobalt chrome)
- Patella - Titanium
Single vs Multi-radius femoral components? - Multi-radius design features 2 radii of curvature:
- Large anterior radius
- Small posterior radius
- Multi-radius designs theoretically improve femoral roll back during flexion
What is a Medial Pivot Knee? - A design with more conforming medial component and less conforming lateral compartment, placing the center of rotation on the medial aspect of the knee
- Purpose: To replicate the screw home mechanism
- Used when high implant-bone interface stresses exist in constrained implants → distributes stresses over larger surface area
- Pros:
- Resist shear forces
- Increase stability by reducing micromotion
- Reduce tibial lift-off (experienced in varus/valgus instability)
- Cons:
- Stress shielding → reduced bone density → theoretical risk of loosening, fracture, and end-of-stem pain
- Makes revision more difficult; less bone stock
Types of patella components? - Symmetrical dome vs asymmetrical anatomical design
- All PE components are used as previous metal-backed prostheses had high failure rates in the 1980s
- Design evolution - multiple small pegs instead of a single large peg
Femoral trochlea component design evolution? - More anatomic trochlear groove
- Broad and deep groove to accommodate patellar component throughout ROM
When do you use hinged implants? Indications for Hinged TKR? - Global instability from infection/trauma
- Hyperextension instability (e.g., Post-polio)
What do you know of the Knee Arthroplasty Trial? - JBJS 2009 Multicenter RCT of 2353 patients with 116 surgeons
- Compared patellar resurfacing, mobile bearing and metal backing
- 2-year results showed no difference in functional outcomes for these 3 factors
IMPLANT Choice
There are 3 key components in TKR implant choice: CR vs PS, Patellar resurfacing, and type of bearing.
What TKR implant PS vs CR will you choose? - I will use a CR knee because this is what I am trained in, UNLESS the patient has inflammatory arthritis, deficient PCL, or previous patellectomy.
- + Bone preserving procedure with no need for box cut; preserves proprioceptive properties of the PCL.
- CR also shows consistent good results in national registries.
- AOJNRR (Australian Orthopaedic Association National Joint Replacement Registry) 15-year data shows significantly lower revision rates in CR 7% vs 8%
What bearing surface for TKR? - I will use a normal UHMWPE (non-HXL) due to the different loading mechanisms compared to the hip:
- Hip - high congruity with large surface area of contact. Superior shear resistance of XLPE is offset by its brittleness due to large surface area.
- Knee - Round on flat surface with low surface area of contact, which may predispose to contact stress and loading resulting in catastrophic failure.
However, I recognize the improving wear properties of newer materials. HXLPE is becoming more popular in the Australian registry, rising to more than 60% in the last 10 years. AOJNRR shows better revision rates (4.9% vs 7.8%) with HXLPE. However, UK NJR (United Kingdom National Joint Registry) shows conflicting results: - Thomas Partridge et al. 2020 JBJS NJR study
- No difference in revision rates at 12 years between XLPE vs Conventional PE. However, certain patient subsets (< 60 yo, BMI > 35) and second generation XLPE demonstrated better survival.
- Due to increased cost, routine use of HXLPE may not be justified.
Will you do patellar resurfacing? - I will do selective resurfacing for patients with severe patellar wear or rheumatoid arthritis.
- I am aware of the risks of this procedure.
- The KAT (Knee Arthroplasty Trial) did not find any difference in outcomes with resurfacing up to 2 years.
- Meta-analysis of RCTs (JBJS 2012 Pilling et al.) showed no difference in anterior knee pain, functional scores, or satisfaction.
- However, AOJNRR found lower rate of revision compared to procedures without resurfacing.
- Technique:
- Ensure remnant thickness not less than 12mm
- Position – medialize it to prevent instability
Fixed or mobile bearing for TKR? - I use fixed bearing TKR with higher articular congruity of the polyethylene
- Based on AOJNRR, fixed bearings have significantly lower revision rates than mobile bearings (8.3% vs 9.5%)
- No difference in outcomes was found in the KAT study
Cemented vs uncemented TKR? - I will use cemented TKR
- Supported by AOJNRR data - showing lower revision rates at 18 years (7% vs 10%)
What type of UKA will you do? - Fixed bearing [ZUK] because this is what I am trained in
- + Lower risk of bearing dislocation
- Additionally supported by the AOJNRR - showing no significant difference in revision rates
Exit Exams MCQ QUIZ
According to the principle of the 'condylar compromise' in TKR implant design, what is the direct consequence of increasing implant conformity to reduce contact stress?
A. It decreases constraint, allowing for more normal kinematics.
B. It improves femoral roll back without needing a cam-post mechanism.
C. It increases constraint and thus reduces kinematics.
D. It eliminates the risk of polyethylene wear debris.
C. More implant conformity is needed to reduce contact stress, but this in turn increases constraint and reduces the normal kinematics of the knee.
What is the primary purpose of a Medial Pivot Knee design?
A. To reduce shear stresses by allowing the tibial insert to rotate on the tibial platform.
B. To provide maximal constraint for global instability.
C. To replicate screw home mechanism
D. To achieve femoral roll back via a cam and post mechanism.
C. This design uses a more conforming medial compartment and a less conforming lateral one to place the center of rotation medially, mimicking the natural screw home mechanism.
Which of the following is listed as a primary advantage of a Cruciate Retaining (CR) TKR implant compared to a Posterior Stabilized (PS) implant?
A. It is a bone-preserving procedure as no box cut is needed.
B. It is easier to achieve soft tissue balance.
C. It provides a more congruent joint surface.
D. It eliminates the risk of cam jump and tibial post wear.
A. CR implants do not require the central box cut in the femur that PS implants need for the cam-post mechanism, thus preserving more bone stock.
What is a potential downside of using stems in TKR components, as identified in the source material?
A. They increase tibial lift-off in cases of varus/valgus instability.
B. They are unable to effectively resist shear forces.
C. They cause stress shielding, which can reduce bone density.
D. They increase micromotion at the implant-bone interface.
C. By distributing stresses over a larger area and bypassing the proximal bone, stems can shield this bone from normal loading, potentially leading to reduced density.
Why might an all-polyethylene (All-PE) tibial component be chosen over a metal-backed modular design?
A. It has no back side wear, which eliminates a source of osteolysis.
B. It offers greater modularity with more size and thickness options.
C. It is made of titanium for better osseointegration.
D. It allows for easy liner exchange during revision for infection.
A.
'No back side wear (so no osteolysis)' is a significant advantage ('pro') of the All-PE tibial component.
The Knee Arthroplasty Trial (KAT) compared several factors in TKR. What did its 2-year results show regarding patellar resurfacing, mobile bearings, and metal backing?
A. Patellar resurfacing significantly reduced anterior knee pain.
B. There was no difference in functional outcomes for these three factors.
C. Mobile bearings had significantly better functional outcomes.
D. Metal-backed components had a lower revision rate.
The 2-year results of this large multicenter RCT showed no difference in functional outcomes for patellar resurfacing, mobile bearing, and metal backing.
📖
The wicked are overthrown and are no more, but the house of the righteous will stand. Proverbs 12:7
