The history, examination, and image is 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.
Morgan H, Battista V, Leopold SS. Constraint in primary total knee arthroplasty. J Am Acad Orthop Surg. 2005 Dec;13(8):515-24. Review.
PMID:16330513 (Link to Abstract)
Gonzalez MH, Mekhail AO. The failed total knee arthroplasty: evaluation and etiology. J Am Acad Orthop Surg. 2004 Nov-Dec;12(6):436-46. Review.
PMID:15615509 (Link to Abstract)