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Instability After Total Knee Arthroplasty: Limits of Constraints

March 01, 2013

Contributors: Francisco Chana, MD, PhD; Javier Pereiro, MD; Antonio Rios-Luna, MD, PhD; Jose Manuel Rojo-Manaute, MD, PhD; Felipe Benito Del Carmen, MD; Homid Fahandez-Saddi, MD; Antonio J. Perez-Caballer, MD; Manuel Villanueva, MD, PhD; Manuel Villanueva, MD, PhD

Peer review has expired on this program. It is the viewer’s responsibility to determine the educational value of this historical content.
Peer review has expired on this program. It is the viewer’s responsibility to determine the educational value of this historical content.

The authors illustrate some of the most common causes and patterns of instability after a total knee arthroplasty (TKA) and identify the limits of constraint that might make surgical reconstruction procedures more predictable. Although instability represents one of the first global causes for revision knee arthroplasty, the difficulty of distinguishing between symptomatic instability and functional laxity may lead us to underestimate this silent epidemic. The most frequent causes of instability are lower limb malalignment, inadequate soft tissue balancing, inappropriate selection of primary knee implants, and implant malpositioning. For the femoral and tibial components, there are six degrees of freedom and twelve possible simple malpositions (and a myriad of combined malpositions) that create complex patterns of instability_all of which have some impact on ligament stability. The consequences of malalignment in TKA are ligament instability, soft-tissue overload, bone overload, polyethylene insert overload, and aseptic loosening. Additionally, TKA dislocation is a fearsome_but fortunately infrequent_consequence of instability. The jump height factor for a given design and the degree of laxity in flexion after a TKA determine whether this complication is likely to occur. Understanding the type of deformity can help the surgeon choose the lowest degree of constraint necessary to achieve a reproducible result and minimize symptomatic instability. Treatment of instability after TKA requires identifying and correcting of the components´ malpositions and soft tissue balancing as well as probably increasing the degree of constraint. However, just increasing the degree of constraint of the prosthesis does not obviate the need to achieve a correct knee alignment and implant position. Revision total knee arthroplasty must not be performed until the causes of instability have been identified and a plan has been formulated that will correct these causes and to prevent new mistakes.

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