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Research Papers

Load Sharing Among Collateral Ligaments, Articular Surfaces, and the Tibial Post in Constrained Condylar Knee Arthroplasty

[+] Author and Article Information
Xiaonan Wang

Physiology, Biophysics, and
Systems Biology Program,
Weill Cornell Graduate School
of Medical Sciences,
New York, NY 10065

Aamer Malik

Department of Orthopaedic Surgery,
Hospital for Special Surgery,
New York, NY 10021

Donald L. Bartel, Timothy M. Wright

Department of Biomechanics,
Hospital for Special Surgery,
New York, NY 10021

Douglas E. Padgett

Department of Orthopaedic Surgery,
Hospital for Special Surgery,
535 East 70th Street,
New York, NY 10021
e-mail: padgettd@hss.edu

1Corresponding author.

Manuscript received October 29, 2014; final manuscript received May 2, 2016; published online June 16, 2016. Assoc. Editor: Paul Rullkoetter.

J Biomech Eng 138(8), 081002 (Jun 16, 2016) (8 pages) Paper No: BIO-14-1539; doi: 10.1115/1.4033678 History: Received October 29, 2014; Revised May 02, 2016

The normal knee joint maintains stable motion during activities of daily living. After total knee arthroplasty (TKA), stability is achieved by the conformity of the bearing surfaces of the implant components, ligaments, and constraint structures incorporated in the implant design. The large, rectangular tibial post in constrained condylar knee (CCK) arthroplasty, often used in revision surgery, provides added stability, but increases susceptibility to polyethylene wear as it contacts the intercondylar box on the femoral component. We examined coronal plane stability to understand the relative contributions of the mechanisms that act to stabilize the CCK knee under varus–valgus loading, namely, load distribution between the medial and lateral condyles, contact of the tibial post with the femoral intercondylar box, and elongation of the collateral ligaments. A robot testing system was used to determine the joint stability in human cadaveric knees as described by the moment versus angular rotation behavior under varus–valgus moments at 0 deg, 30 deg, and 90 deg of flexion. The angular rotation of the CCK knee in response to the physiological moments was limited to ≤1.5 deg. The primary stabilizing mechanism was the redistribution of the contact force on the bearing surfaces. Contact between the tibial post and the femoral box provided a secondary stabilizing mechanism after lift-off of a condyle had occurred. Collateral ligaments provide limited stability because little ligament elongation occurred under such small angular rotations. Compressive loads applied across the knee joint, such as would occur with the application of muscle forces, enhanced the ability of the bearing surfaces to provide resisting internal varus–valgus moment and, thus, reduced the exposure of the tibial post to the external varus–valgus loads. Our results suggest that the CCK stability can be refined by considering both the geometry of the bearing surfaces and the contacting geometry between the tibial post and femoral box.

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References

Figures

Grahic Jump Location
Fig. 1

Additional coronal constraint in the CCK implant over that of a conventional primary knee implant is provided in part by contact between the tibial post and the intercondylar femoral box

Grahic Jump Location
Fig. 2

The robot testing system is shown, including: (a) the robotic arm and load cell, (b) the digitizer, and (c) digitization points and tibial and femoral coordinate systems

Grahic Jump Location
Fig. 3

Three conditions of CCK TKA were tested to make force and moment measurements and, through the superposition principle, determine the relative contributions of the bearing surfaces, the post–box contact, and the collateral ligaments

Grahic Jump Location
Fig. 4

(a) A pressure sensor was secured on the tibial insert and (b) inserted in the knee joint during robot tests to measure contact between the bearing surfaces

Grahic Jump Location
Fig. 5

(a) A representative varus–valgus moment versus angular displacement curve at full extension for a knee with the CCK implanted is shown together with the contact pressure distribution from the pressure sensor at the initial position and at the transition points from the initial high-stiffness region to the low-stiffness region. (b) No significant differences were found between the varus and valgus angulations (±SD) of seven CCK knees under 200 N compression and 10 N·m varus 0 deg, 30 deg, and 90 deg of flexion, though angulation did significantly increase with flexion angle.

Grahic Jump Location
Fig. 6

(a) The varus–valgus moment versus angular rotation curve of an intact CCK specimen at 30 deg of flexion is shown together with the relative contributions from the bearing surfaces, the post–box contact, and the ligaments. The valgus moments are positive. (b) The primary stabilizing mechanism was contact across the bearing surfaces, which took the largest share of the load, followed by the post–box contact, and finally, the ligaments (average ± SD for seven knees tested at 10 N·m moment with 200 N compression at 0 deg, 30 deg, and 90 deg of flexion).

Grahic Jump Location
Fig. 7

(a) A representative varus–valgus moment versus angular rotation curve for a CCK knee specimen with 200 N and 10 N compressive loads applied across the joint demonstrates the additional stabilizing effect of the larger load. The valgus moments are positive. (b) The increased load sharing by the bearing surfaces with the additional compressive force across the joint occurred across all the seven specimens (average ± SD is shown for load sharing between the bearing surfaces, post–box contact, and the ligaments with 200 N and 10 N compression).

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