Instability after TKA

Instability after total knee arthroplasty (TKA) is one of the leading indications for revision surgery, accounting for approximately 15–20% of all revisions in major registries. Instability encompasses a spectrum from subtle flexion instability causing patient dissatisfaction to frank coronal plane instability causing the patient to fall. Understanding the anatomical planes of instability, the mechanical and technical causes, and the reconstructive options is essential. Instability can be classified by the direction of instability (coronal — varus/valgus; sagittal — anterior/posterior; global/multidirectional) and by the underlying mechanism (ligamentous laxity, component malposition, gap imbalance, or implant design mismatch).

• Coronal plane instability (varus-valgus): caused by imbalance between the medial and lateral soft tissue envelopes; in a well-balanced TKA, the medial and lateral gaps are equal in both flexion and extension; if one side is lax relative to the other, varus (lateral laxity) or valgus (medial laxity) instability results; causes — (1) inadequate intraoperative soft tissue balancing (the most common cause — medial or lateral release was insufficient or excessive); (2) asymmetric tibial resection (too much resection on the lax side deepens the gap further); (3) polyethylene wear (wearing of the tibial insert flattens the articular surface and progressively loses the constraint provided by the tibial post and lip geometry); (4) late collateral ligament attrition; clinical features — patient describes the knee `giving way` sideways; varus or valgus stress testing in extension reveals the laxity
• Sagittal plane instability — flexion instability: the most common and most frequently missed form of instability after TKA; occurs when the flexion gap is larger than the extension gap (the polyethylene insert that balances the extension gap is too thin for the flexion gap); in flexion, the knee subluxes anteriorly or posteriorly (depending on the implant type) under load; patients complain of anterior knee pain, difficulty rising from chairs, and a sense of giving way in mid-flexion; the diagnosis is often delayed because examination in full extension appears stable — stress testing must be performed at 90° of flexion; causes — (1) undersized femoral component (smaller femoral component = smaller AP dimension = larger flexion gap); (2) over-resection of the posterior femoral condyles; (3) posterior tibial slope excessive (increases the effective posterior translation of the tibia relative to the femur in flexion)
• Extension instability: the extension gap is too large for the insert thickness; the knee feels `loose` in full extension — the patient may hyperextend; causes — (1) too much tibial resection (deepens both extension and flexion gaps proportionally, but if the tibial cut is excessive, the remaining gaps are too large for any available insert); (2) too much distal femoral resection (raises the joint line distally, deepening the extension gap); (3) inadequate MCL/LCL repair or insufficiency; management — increase insert thickness (if both gaps are proportionally large); distal femoral augment (if distal femoral over-resection is the cause — raises the femoral component distally, reducing the extension gap preferentially)

• Posterior-stabilised (PS) TKA: the standard cam-and-post mechanism substitutes for the PCL and provides sagittal stability; the tibial post engages the femoral cam during flexion, preventing posterior tibial subluxation; however, the PS mechanism provides NO coronal (varus-valgus) constraint — the knee relies entirely on the collateral ligaments for coronal stability; a standard PS TKA tibial insert is therefore NOT the answer for coronal plane instability after TKA
• Constrained condylar knee (CCK): a taller, wider tibial post that provides both sagittal AND coronal constraint; the increased height and width of the post engages the femoral cam housing, limiting varus-valgus motion; CCK is the standard revision implant for moderate coronal instability (functioning but attenuated collateral ligaments); it transfers more stress to the bone-implant interface than PS — hence longer stems are required to off-load this stress to the diaphysis; CCK is NOT appropriate when the collateral ligaments are completely absent — the post-cam mechanism would fracture under the forces imposed
• Rotating hinge knee (RHK): a fully linked prosthesis — the femoral and tibial components are mechanically coupled by a hinge mechanism that allows flexion/extension and internal/external rotation but resists varus/valgus and AP translation; used when collateral ligaments are completely deficient (post-trauma, multiple revisions, tumour resection, complex neuropathic joint); the rotating (vs fixed) hinge design reduces rotational torque transmission to the bone-implant interface, improving implant longevity vs old fixed hinge designs; nevertheless, RHK carries high complication rates (infection, aseptic loosening) and is reserved as a salvage procedure; the GMRS (Global Modular Replacement System) and Legion Hinge are contemporary examples

• Instability after TKA: ~15–20% of revision TKA indications; coronal (varus/valgus), sagittal (flexion/extension), or multidirectional; diagnosis requires stress testing in BOTH full extension AND 90° flexion
• Flexion instability: the most commonly missed type; stable in extension but unstable at 90° flexion (anterior drawer); causes — undersized femoral component, excessive tibial slope, over-resected posterior condyles; fix — larger femoral component, posterior condylar augments, reduce slope, thicker insert
• Coronal instability: inadequate balancing or PE wear most common; >10° varus/valgus opening = significant; thicker insert (mild); CCK (moderate collateral laxity — functioning but attenuated ligaments); RHK (absent collaterals)
• PS TKA: cam-post provides sagittal stability; NO coronal constraint; do not confuse PS with CCK — PS does not treat varus/valgus instability
• CCK: taller/wider tibial post; coronal AND sagittal constraint; for moderate coronal instability with functioning collaterals; requires longer stems (off-load increased bone-implant stress); NOT for absent collateral ligaments
• Rotating hinge (RHK): fully linked prosthesis; resists varus/valgus + AP; allows rotation (reduces rotational torque vs fixed hinge); for absent collaterals, multiple failed revisions, tumour; salvage procedure; high complication rates
• Extension instability: hyperextension in standing; excessive distal femoral or tibial resection; distal femoral augment preferentially reduces extension gap; thicker insert if symmetric gap enlargement
• Gap balancing principle: flexion gap too large → larger femoral component + posterior condylar augments + reduce tibial slope; extension gap too large → distal femoral augment; both gaps too large → thicker tibial insert
• Minimum constraint principle: always use minimum constraint necessary to achieve stability; excessive constraint → increased bone-implant interface stress → accelerated loosening