Abstract
Stability after TKA is essential for knee function and patient satisfaction. Stability may be marginally more important even than alignment because “stability” means there will be ONE alignment, whereas INSTABILITY means there will be many alignments of the joint, usually the worst one for any loading pattern. Whereas alignment results from the orientation and size of implants, stability depends on all of these, plus soft tissue integrity and in many cases, surgical alteration. Ligament releases (and rarely reconstructions) will certainly be required if alignment is changed with the arthroplasty. Instability may be a subtle or flagrant problem.
The “Instabilities” are:
- i.
Varus- valgus
- ii.
Plane of motion- Flexion
- iii.
Plane of Motion-Extension
Varus-valgus instability is the prototype and while it may originate exclusively from the failure of soft tissue, knee alignment and dynamic forces outside the knee joint such as hip abductor dysfunction, scoliois and tibialis posterior rupture may be implicated. A comprehensive approach will be needed.
Flexion instability, most simply stated results from a flexion gap that exceeds the dimensions of the extension gap. It will result most commonly after surgery for the patient with a fixed flexion contracture whose knee extends fully because a relatively thin polyethylene insert has been selected. So-called “mid-flexion” instability (implying stability in extension and flexion) has not yet been thoroughly characterised.
Extension instability includes all failures of the extensor mechanism (rupture, maltracking and weakness) which are better characterised as “buckling” under a separate topic. Recurvatum has received little attention but can generate the most destructive forces leading to knee arthroplasty failure. In general begins as a compensatory mechanism for relative extensor weakness.
All treatment of the unstable TKA must characterise the mode(s) of failure above and correct the underlying cause. Surgical technique will be extremely important, followed eventually by implant selection.
