A constrained-condylar fixed-bearing total knee arthroplasty is stabilised by the medial soft tissues

Abstract

Purpose: Revision constrained-condylar total knee arthroplasty (CCK-TKA) is often used to provide additional mechanical constraint after failure of a primary TKA. However, it is unknown how much this translates to a reliance on soft-tissue support. The aim of this study was therefore to compare the laxity of a native knee to the CCK-TKA implanted state and quantify how medial soft-tissues stabilise the knee following CCK-TKA.

Methods: Ten intact cadaveric knees were tested in a robotic system at 0°, 30°, 60° and 90° flexion with ± 90 N anterior-posterior force, ± 8 Nm varus-valgus and ± 5 Nm internal-external torques. A fixed-bearing CCK-TKA was implanted and the laxity tests were repeated with the soft tissues intact and after sequential cutting. The deep and superficial medial collateral ligaments (dMCL, sMCL) and posteromedial capsule (PMC) were sequentially transected and the percentage contributions of each structure to restraining the applied loads were calculated.

Results: Implanting a CCK-TKA did not alter anterior-posterior laxity from that of the original native knee, but it significantly decreased internal-external and varus-valgus rotational laxity (p < 0.05). Post CCK-TKA, the sMCL restrained 34% of the tibial displacing load in anterior drawer, 16% in internal rotation, 17% in external rotation and 53% in valgus, across the flexion angles tested. The dMCL restrained 11% of the valgus rotation moment.

Conclusions: With a fully-competent sMCL in-vitro, a fixed-bearing CCK-TKA knee provided more rotational constraint than the native knee. The robotic test data showed that both the soft-tissues and the semi-constrained implant restrained rotational knee laxity. Therefore, in clinical practice, a fixed-bearing CCK-TKA knee could be indicated for use in a knee with lax, less-competent medial soft tissues.

Level of evidence: Controlled laboratory study.

Keywords: Constrained condylar prosthesis; Medial collateral ligament; Revision total knee arthroplasty; Robotic testing; Semi-constrained implant; Stability; Total knee replacement.

Fig. 1

The Attune semi-constrained revision TKR with high-conforming fixed bearing. The tests were performed without stem extensions in the intact cadaveric bones

Fig. 2

Anterior (positive) and posterior (negative) tibiofemoral translation of the native knee and CCK-TKA implanted knee in response to ± 90  N anterior–posterior force (mean ± standard deviation, n = 10)

Fig. 3

Internal (positive) and external (negative) tibial rotation of the native knee and CCK-TKA implanted knee in response to ± 5 Nm internal–external torque (mean ± standard deviation, n = 10)

Fig. 4

Varus (positive) and valgus (negative) angulation of the native knee and CCK-TKA implanted knee in response to ± 8 Nm varus-valgus moment (mean ± standard deviation, n = 10)

Fig. 5

Percentage contributions of the deep medial collateral ligament (dMCL), superficial MCL (sMCL), anterior fibres of the sMCL and posteromedial capsule (PMC) in resisting 90  N anterior (left) and posterior (right) forces in the CCK-TKA implanted knee (mean ± standard deviation, n = 10)

Fig. 6

Percentage contributions of the deep medial collateral ligament (dMCL), superficial MCL (sMCL), anterior fibres of the sMCL and posteromedial capsule (PMC) in resisting 5 Nm internal (left) and external (right) tibial rotation torque in the CCK-TKA implanted knee (mean ± standard deviation, n = 10)

Fig. 7

Percentage contributions of the deep medial collateral ligament (dMCL), superficial MCL (sMCL), anterior fibres of the sMCL and posteromedial capsule (PMC) in resisting 8 Nm varus (left) and valgus (right) moment in the CCK-TKA implanted knee (mean ± standard deviation, n = 10)