Instability is associated with almost 50% of early revisions and 10-20% of all revisions. Instability is caused by a multitude of underlying issues.  Gap imbalance, coronal imbalance (failed soft tissue tensioning, or ligament rupture/attenuation), component malpositioning (ie internal rotation), extensor mechanism failure, or component loosening. 

Patients feel their knee “giving way” or “buckling”, particularly with stairs.  Patients may also complain of stiffness because of secondary guarding.  A locking sensation is more common with patellar maltracking.  A varus thrust (the knee buckles laterally upon heel strike) with lateral instability or a valgus thrust with medial instability.  It is important to rule out muscle weakness before homing in on instability.  Persistent quad weakness due to poor rehab or inability to fully extend the knee that prevents a patient from "locking-out" their knee while standing can appear similar to instability.  

The onset of instability is important for diagnosis.  Gap Imbalance, iatrogenic collateral ligament injury, and component malpositioning (size, internal rotation) present early postop, while aseptic loosening occurs late, as can attritional ligament instability (such as PCL rupture in a CR-TKA).  Extensor mechanism injury presents as acute onset of instability.  

Instability is more of a symptom than a diagnosis,and identifying the underlying cause is key to successful treatment. 

Gap IMBalancE & ligament instability

Bone cuts and ligament stability influence each other so it can be somewhat challenging to isolate one cause.  Examining the varus/valgus stability in 0°, 30°, and 90° can give insight about cause of instability.

Isolated collateral ligament insufficiency can be identified when either the medial or lateral side opens in flexion and extension. A CCK “contrained condylar knee” (the POST is wider and therefore has greater congruence with the CAM, thus limiting coronal motion) is used.  Most CCK implants require a larger box cut to accommodate the wider post and thus implant revision is needed.  Many surgeons also add stems to the CCK implant because as it is more constrained, it assumes greater stress during ambulation, and therefore transmits more force to the bone-cement interface.  Adding a stem will increase the surface area for this stress and decrease aseptic loosening.  However, some implants are “partially constrained” meaning the constrained post is wider than a standard post but not completely congruent with the cam and therefore there is some wiggle room (maybe 5° as opposed to only 1° of motion for the standard CCK)  , and therefore, only the poly liner needs to be exchanged.  The CCK offers stability in the coronal plane, but not in the saggital plane, therefore balanced flexion and extension gaps must be maintained.  The CCK is sometimes used with the superficial MCL is damaged during surgery (ie released off the tibia) and it acts as an internal splint, giving stability in the first 6 weeks until the soft tissues heal and then give further stability.  A hinge implant is used with complete ligament instability.  If the MCL is completely missing, a CCK will experience excessive stress over time and will eventually fail due to poly wear or fracture.  Therefore a hinge is recommended for cases of complete MCL deficiency.  Current hinge designs are a rotating platform, which adds some increased motion at the articular surface.  A fixed hinge transmits excessive force to the bone-cement interface and causes early failure.  Additionally there is a segmental type hinge for large bone defects versus a condylar-type hinge for use in cases of good bone stock and deficient soft tissue, which is preferred because of better load distribution. 

Mid-flexion instability is a recognized problem with a controversial underlying etiology.  The soft tissues in TKA are balanced during surgery at 0° and 90°, but think about the more complete arc of motion, and the potential for changes in soft tissue tensioning between these two points.  Some believe that stability should be checked not only at 0° and 90° but also at 30° and 60°.  Its possible that the greater articular congruence at 0° may mask ligament instability in early flexion, which can be identified by checking knee stability in more points of motion.  Other theories on mid-flexion instability see an association in TKA cases that had a normal extension gap and loose flexion gap, which were corrected by increasing distal femoral resection.  When such a bone cut is used to balance soft tissue tension mismatch at 0°, the soft tissue mismatch is only solved at 0° but persists in mid-flexion because the true problem was a loose flexion gap, not a tight extension gap.  

Midflexion instability is seen in people that never feel confident in their knee.  They report that it gives way rising from chair, going down stairs or with walking.  They often experience recurrent knee effusions, complain of anterior knee pain, and wear some type of knee brace.  The key to fixing is restoration of posterior offset.   This can be issue with anterior referencing guide.  The flexion gap is particularly at risk in valgus knees due to an attenuated MCL and bony erosion of the posterior condyle.  Its also a concern in PCL retaining knee (CR-TKA) due to changes in the tibial slope cut. Treatment: Revision surgery to restore the flexion gap typically with posterior condylar augments +/- distal femoral augments.

Patellar Instability & component malposition 

Typically caused by malrotation of components. For a complete explanation of component rotation, see the Basic Concepts Chapter, but in brief review, overly internally rotating or medializing the components will change the Q angle, thus increasing the lateral subluxing force. The Q angle is the direction of patellar tracking (which change based on component positioning) relative to the vector of pull from the quad muscle (remember the quad is fixed from a more proximal position so it does not change).  If the patella tracks in line with the pull of the quad, the Q angle is small (there is not conflict in trajectory).   Patients with patellar instability typically report a locking sensation or a deficient extensor mechanism (both can present as buckling) as well as vague anterior knee pain with intermittent swelling, patellar crepitus.  The best method to diagnose patellar maltracking is with CT scan.  While there is no numerical indication for revision surgery, some studies have correlated degree of Internal Rotation with risk for symptoms.  1-4° of IR cause lateral tracking and lateral patellar tilt; 3-8° cause subluxation, 7-17° may cause patellar dislocation or fracture.   Reports on early TKA designs cited patellar complications of 12-24%, yet developments in femoral component design deepened the trochlear groove, created asymmetric femoral condylar flanges (augmenting the lateral side) and graduated the anterior-to-distal transition.  Currently, reports of patellar issues are rare. 

Treatment: Identify malrotated component and revise if significant discomfort that fails to improve with physical therapy. Alternatively consider lateral release. 




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