From the Guidelines
Increasing conformity between the tibial and femoral components in total knee arthroplasty (TKA) can lead to improved joint stability, but may also increase the risk of aseptic loosening and reduced range of motion, as evidenced by recent studies 1.
Key Considerations
- Higher conformity designs provide greater stability by restricting motion and distributing contact forces over larger areas, which reduces polyethylene wear and may extend implant longevity.
- However, increased conformity can limit the natural rotational movement of the knee, potentially causing increased stress at the bone-implant interface that can lead to early loosening, as noted in studies on aseptic loosening 1.
- The ideal conformity level depends on patient-specific factors including age, activity level, and ligament integrity, with younger, more active patients potentially benefiting from lower conformity designs and older or less active patients requiring higher conformity for stability.
Balancing Conformity and Mobility
- Surgeons must balance the need for stability with the need for reasonable motion and minimizing wear and loosening risks, often selecting moderate conformity designs.
- Recent updates on imaging after TKA highlight the importance of considering aseptic loosening, osteolysis, and instability in the evaluation of knee pain and periprosthetic lucency 1.
Clinical Implications
- The relationship between conformity, stability, and loosening is complex, and surgeons should consider individual patient factors and the potential tradeoffs of higher conformity designs.
- Osteolysis, a leading cause of late TKA revision, can occur secondary to particle debris and is more common in cementless TKA, emphasizing the need for careful component selection and monitoring 1.
From the Research
Effects of Increasing Conformity
- Increasing conformity of a fixed tibial bearing component and femoral component in total knee arthroplasty (TKA) can lead to reduced polyethylene wear and contact stresses, as seen in mobile-bearing TKA designs 2, 3.
- Mobile-bearing TKA designs are designed to correct rotational mismatch between femoral and tibial components, providing high conformity and low contact stress 4, 5.
- Studies have shown that mobile-bearing TKA designs can provide less tibiofemoral contact stresses under tibiofemoral malalignment conditions and less wear rate in in-vitro simulator tests 3.
Rotational Mismatch and Clinical Outcomes
- Postoperative rotational mismatch between the femur and tibia can cause poor clinical results following TKA, and mobile-bearing prostheses can suppress this mismatch better than fixed-bearing prostheses 5.
- Rotational mismatch between the femur and tibia was significantly less in mobile-bearing TKA than in fixed-bearing TKA, and this was associated with better patient-reported outcome measurements 5.
- External rotation of the tibial component and the femoral component was positively correlated with better functional outcome, as measured by the Knee Society score 6.
Implant Design and Patient Selection
- The choice between fixed-bearing and mobile-bearing TKA designs may depend on patient factors, such as age and activity level, with mobile-bearing designs suggested for younger or higher-demand patients 3.
- The fixed-bearing design with an all-polyethylene tibial component may be suitable for relatively inactive, elder patients, while the mobile-bearing design may be more suitable for patients who require more normal kinematics response after joint replacement 3.