Abstract
Instability currently represents the most frequent cause for revision total knee replacement. Instability can be primary from the standpoint of inadequately performed collateral and/or posterior cruciate ligament balancing during primary total knee replacement or it may be secondary to malalignment secondary to loosening which can develop later progressive instability. Revision surgery must take into consideration any component malalignment that may have primarily contributed to instability.
Care should be given to assessing collateral ligament integrity. This can be done during physical examination by radiological stress testing to see if the mediolateral stress of the knee comes to a good endpoint. If there is no sense of a palpable endpoint, then the surgeon must assume structural incompetency of the medial or lateral collateral ligament or both. In posterior cruciate retaining knees, anteroposterior instability must be assessed.
For instability, must revisions will require a posterior cruciate substituting design or a constrained condylar design that are unlinked. However, if the patient displays considerable global instability, a linked, rotating platform constrained total knee replacement design will be required. Recent data has shown that the rotating hinges work quite well in restoring stability to the knee with maintenance of the clinical results over a considerable length of time.
During revision surgery, laminar spreaders may be utilised to assess the flexion and extension spaces after the tibial platform is restored. If a symmetric flexion and extension space are achieved, then the collateral ligaments are intact. Depending on the remaining existing bone stock, a posterior stabilised or constrained condylar unlinked prosthesis may be used for implantation. If there is considerable asymmetry or a large flexion/extension mismatch, then a rotating hinge design should be utilised.
Intramedullary stems should be utilised in most cases when bone integrity is suspect and insufficient. Currently, stems should be placed cementless to permit easier future revision. Cementing the stems is only recommended if there is lack of intramedullary isthmic support. However, revision of fully cemented revision implants may be quite difficult later.
Infection should be ruled out by aspiration off of antibiotics prior to any revision operation, especially if loosening of the components represents the cause of instability. The surgeon should attempt to restore collateral ligament balance whenever possible as this yields the best clinical result.
