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Facts & Figures

Addressing knee instability and activity limitation post TKR


Up to 32% of patients have reported knee instability with activity limitation and pain six months after TKR. 1


The concept of ‘mid-flexion instability’ is defined as a condition in which the TKR component is stable in both extension and 90◦ of flexion but is unstable between the two positions. 2 It is more common in cruciate retaining (CR) Knee designs and has been identified as one of the causes of dissatisfaction following CR TKR. 2  The Medial Congruent (MC) bearing, which articulates with a CR femur, was designed to improve stability by enhancing the conformity and contact area with the medial femoral condyle and incorporates an anterior lip that provides anterior constraint. 2

This study compared the intraoperative kinematics of an MC Knee with those of a CR Knee using an image-free navigation system. Both bearings articulate with the same design of CR femur. The PCL can also be retained with the MC bearing, and PCL retention with a bony island was attempted in all cases. 2

Observations of interest:

  • The MC bearing restored physiologic AP kinematics more in mid-flexion than the CR bearing under PCL-retaining conditions 2
  • Mid-flexion AP stability was greater with the MC bearing than with the CR bearing 2
  • In the CR group the femoral position relative to the tibia was significantly anterior at 45°, 60°, and 90° flexion compared to the preoperative condition 2
  • The authors acknowledged that this paradoxical motion or mid-flexion instability is more common in CR knee designs, and can be one of the causes of dissatisfaction after CR TKR 2
  • The femoral position in the MC group at 45°, 60°, and 90° flexion did not differ significantly from that of the preoperative condition (paradoxical anterior sliding was prevented with the MC bearing) 2
  • Femoral rollback in deep flexion was maintained with the MC bearing 2

According to the authors, these results indicate that the MC bearing could prevent mid-flexion instability in CR TKR

  1. Li, C, Y., et al. Literature review of the causes of pain following total knee replacement surgery: prosthesis, inflammation and arthrofibrosis. EFORT Open Rev. (2020); 5: 534-543. DOI: 10.1302/2058-5241.5.200031 Find the source here.
  2. Tsubosaka M, Ishida K, Kodato K, Shibanuma N, Hayashi S, Kurosaka M, Kuroda R, Matsumoto T. Mid-flexion stability in the anteroposterior plane is achieved with a medial congruent insert in cruciate-retaining total knee arthroplasty for varus osteoarthritis. KSSTA. 2020 Mar 10. doi: 10.1007/s00167-020-05927-4. Find the source here.

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