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Unlinked constrained (varus-valgus constrained)
82%
3550/4335
Fixed bearing PCL-substituting (posterior-stabilized)
5%
213/4335
Mobile bearing PCL-substituting (posterior-stabilized)
4%
153/4335
PCL-retaining (cruciate-retaining)
1%
38/4335
Rotating-hinge constrained
8%
347/4335
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The history and intraoperative examination are consistent with an iatrogenic MCL injury that is irreparable. An unlinked constrained (varus-valgus constrained) prosthesis has a tall tibial post and a deep femoral box, which provide more inherent coronal plane stability than do standard cruciate retaining or cruciate-substituting prostheses. Because there is no axle connecting the tibial and femoral components, these implants are sometimes referred to as unlinked constrained implants. Morgan et al discuss in their Level 5 review that the added degrees of implant stability confer disadvantages. As the amount of constraint increases, stress transmitted to the modular implant-host or prosthesis-host interface also increases. The heightened stress may result in increased backside polyethylene wear in modular tibial components or in early implant loosening, and ultimately to failure. Therefore, a rotating-hinge constrained knee would offer sufficient stability for a MCL deficiency but offers more constraint than is necessary and appropriate. Gonzalez et al present a Level 5 reivew stating that the primary causes of failure of total knee arthroplasty include pain, postoperative stiffness, and instability. They state that medial-lateral instability can be a product of improper implant balancing or deficient medial or lateral collateral ligaments. Illustration A shows a varus-valgus unlinked constraint knee implant and Illustration B shows a rotating hinge constraint knee implant. Illustration C and D show a cruciate-retaining implant on the left and a cruciate-substituting implant with femoral box and tibial polyethylene post on the right. Illustration E depicts a cadaveric right knee with a MCL (sutured in picture) that has been transected during a tibial cut.
4.1
(16)
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