Mobile Bearing TKR

Courtesy: Dr Maniar, Ashok Shyam TV, Ortho

Why Consider a Mobile-Bearing Knee?

Primary concern:

• Polyethylene wear:

  • At the articular surface

  • At the undersurface (backside wear)

Fixed-bearing knees have demonstrated:

• Significant articular wear.

• Undersurface micromotion leading to wear debris.

• Associated osteolysis and revision in mid-term follow-up.

---

Problem 1: Articular Surface Wear

Contact Stress Principle

• Contact stress is inversely proportional to contact area.

• Smaller contact area ? Higher contact stress.

• Higher contact stress ? Increased polyethylene wear.

Fixed-Bearing Knees

• Variable conformity.

• Often small contact areas.

• Many designs exceed optimal stress thresholds for polyethylene.

Constraint Dilemma in Fixed Bearings

• High conformity ? Large contact area:

  • But increased constraint.

  • Translates rotational forces to tibial fixation.

  • Risk of loosening.

• Low constraint ? Allows rotation:

  • But reduces contact area.

  • Increases contact stress and wear.

This creates a kinematic conflict.

---

Mobile-Bearing Concept

Mobile-bearing knees aim to resolve this conflict by:

• Providing a large articular contact area.

• Allowing rotational movement at the undersurface.

• Decoupling rotational stresses from the tibial fixation.

Result:

• Low contact stress.

• Low constraint force.

• Improved kinematic compatibility.

---

Laboratory Evidence

Studies demonstrate:

• 6–7 times less wear in mobile-bearing designs compared to fixed-bearing knees.

• Improved contact area throughout flexion range.

• Lower contact stress across motion arc.

---

In Vivo Observations

Clinical wear analysis shows:

• Extremely low polyethylene wear rates in mobile-bearing systems.

• Lower wear compared to many conventional fixed-bearing designs.

---

Problem 2: Undersurface (Backside) Wear

Fixed-Bearing Modular Knees

• Polyethylene insert locks into metal tibial tray.

• Micromotion occurs at insert–tray interface.

• Leads to:

  • Fretting wear.

  • Abrasion.

  • Biologically active polyethylene debris.

• Correlates strongly with osteolysis.

Studies show:

• Greater duration of implantation ? Increased backside wear.

• Statistically significant association between backside wear and osteolysis.

---

Why Undersurface Wear Is Reduced in Mobile Bearings

• Mobile-bearing trays typically:

  • Made of polished cobalt-chromium.

  • Smoother surface finish than titanium trays.

• Reduced surface roughness ? Reduced abrasion.

• Designed to permit intended motion rather than unintended micromotion.

---

Advancements in Fixed Bearings

• Improved locking mechanisms.

• Introduction of cobalt-chromium trays.

• Better manufacturing tolerances.

However:

• Long-term performance still under evaluation.

---

Additional Advantages of Mobile Bearings

• Greater tolerance for minor rotational malalignment.

• Potentially improved knee kinematics.

• More physiologic motion patterns.

---

Not All Mobile Bearings Are the Same

Unidirectional Mobile Bearing (Rotating Platform)

• Rotation allowed in one plane.

• Better wear characteristics.

• Stronger long-term data.

Multidirectional Designs

• Allow motion in multiple planes.

• Potential for increased polyethylene wear in some designs.

---

Long-Term Outcomes

Multiple studies report:

• Excellent survivorship at 10–20 years.

• Minimal or no osteolysis in well-designed rotating platform systems.

• High functional outcomes with good range of motion.

Reported outcomes include:

• Survivorship approaching 100% at mid-term follow-up in selected series.

• Average flexion around 120°.

• Sustained activity without osteolysis.

---

High-Flex and Posterior-Stabilized Mobile Bearings

• Rotating platform concept adapted to:

  • Posterior-stabilized designs.

  • High-flexion implants.

• Demonstrated favorable mid-term outcomes.

---

Summary: Why Favor Mobile-Bearing TKA?

Advantages include:

• Reduced articular surface wear.

• Reduced undersurface wear.

• Improved kinematic behavior.

• Lower contact stress.

• Excellent mid- to long-term survivorship in selected designs.

---

Key Take-Home Message

Mobility in tibial inserts may be:

• Polyethylene protective.

• Kinematically advantageous.

• Associated with durable long-term results in properly designed systems.