TKA Complications: Instability

Why the knee is unstable. Most TKA instability results from gap imbalance: a mismatch between the flexion and extension gaps, or global ligamentous laxity. Component malpositioning is the underlying cause.

How it is named. Instability is described by the position in which the knee is loose: in flexion, in extension, in mid-flexion (30-60 degrees) or globally, in all positions. Each pattern has its own causes, set out under Classification.

Two gaps. The stability of a TKA depends on equal and balanced flexion and extension gaps. Knowing which structures contribute to each gap is the basis of both preventing instability and treating it.

The extension gap is assessed with the knee fully extended, at 0 degrees. The MCL and LCL are its primary stabilisers, with the posterior capsule, and a tight posterior capsule limits the extension gap.

The flexion gap is assessed at 90 degrees of flexion. The collaterals contribute to stability in flexion as well, and in a cruciate-retaining (CR) knee the retained PCL is a major flexion stabiliser.

What changes each gap. The biomechanical principles:

• Distal femoral resection primarily affects the extension gap
• Posterior condylar resection primarily affects the flexion gap
• Femoral sizing affects the flexion gap: a larger femur gives a smaller flexion gap
• Posterior tibial slope affects the flexion gap: more slope gives a larger flexion gap
• The collateral ligaments contribute to both gaps

Revision for instability frequently uncovers metaphyseal bone loss that dictates the reconstruction. The Anderson Orthopaedic Research Institute (AORI) classification grades the defect separately for the femur (F) and tibia (T) and maps directly onto the fixation strategy.

History. The timing of symptoms helps classify the instability. Establish:

• Giving-way episodes, and whether they come on stairs, walking or rising
• Pain location and character
• Time from the primary operation
• Previous surgery

Examination. Document both the degree of laxity and the position in which it occurs, comparing with the contralateral knee.

• Varus and valgus stress at 0 and 30 degrees
• Anterior and posterior drawer
• Recurvatum
• Range of motion
• Gait

The differential. The patient who reports "giving way" or recurrent effusion does not always have mechanical instability. Distinguishing true ligamentous or gap instability from its mimics changes management entirely.

Radiographs. Weight-bearing AP, lateral and skyline views, with full-length standing films for alignment. Component position and limb alignment are central to diagnosing instability, malalignment and loosening. On the films, assess:

• Component position
• Joint-line position
• Polyethylene wear
• Alignment

Stress views may demonstrate the laxity itself.

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CT. Indicated when component malrotation is suspected and for preoperative planning, and critical for rotational assessment. It measures:

• Femoral rotation, referenced to the surgical transepicondylar axis (medial sulcus to lateral epicondyle) and cross-checked against the posterior condylar axis and Whiteside's (anteroposterior) line
• Tibial rotation, referenced to the tibial tubercle (the Akagi line, from the PCL centre to the medial third of the tubercle)
• Component position and combined rotational mismatch

Berger's thresholds. Combined femoral and tibial internal rotation correlates with the severity of patellofemoral complication, and well-functioning knees sit in combined external rotation:

• Roughly 1-4° of combined internal rotation: patellar tilt and maltracking
• 3-8°: subluxation
• 7-17°: frank dislocation or late patellar failure

Why rotation matters for stability. Internal femoral rotation also opens the medial flexion space, linking malrotation to mid-flexion instability. That is why malrotation must be corrected at revision rather than simply adding constraint.

Examination under anaesthesia. Performed at revision, EUA quantifies laxity in all positions, compares the flexion gap with the extension gap and identifies the unstable arc. It guides the intraoperative decisions.

The choice. Treatment escalates from a conservative trial through polyethylene exchange to component revision, and the level of constraint is matched to the degree of instability.

Conservative trial. For mild instability, the poor surgical candidate, and the recent operation that should be given time to stabilise. A hinged knee brace, quadriceps strengthening physiotherapy and activity modification have limited success for true mechanical instability.

Polyethylene exchange. Exchange to a thicker insert, considering a more constrained option, is indicated for well-fixed, well-positioned components with mild to moderate laxity, or where polyethylene wear is identified. On its own it addresses only minimal laxity.

Component revision. For moderate to severe instability, component malposition, or a failed polyethylene exchange. The principles are to correct component position, balance the gaps and increase constraint as needed.

The constraint ladder. Implants provide varying degrees of intrinsic constraint:

Balancing the gaps. Identify the imbalance, correct it, and balance the gaps before closing:

Stems and augments. Stems are used in most revision cases, to bypass stress risers and improve fixation. Metal augments replace deficient condyles where bone has been lost.

Why the joint line matters. Restoring the native joint line is one of the most important and most often missed goals of revision. Raising it, for example by over-resecting the distal femur or over-stuffing the tibia to fill a gap, shifts the flexion-extension kinematics and is a recognised cause of mid-flexion instability.

Pseudo-patella baja. Joint-line elevation also leaves the patella low. The patellar tendon length is unchanged, but because the joint line has moved proximally the patella now sits low relative to the new joint line, causing anterior knee pain, reduced flexion and extensor dysfunction. This is distinct from true patella baja, a genuinely shortened or scarred patellar tendon.

Finding the native joint line. The landmarks:

• About 1 cm proximal to the fibular head
• About 1.5 cm distal to the medial epicondyle and about 2.5 cm distal to the lateral epicondyle
• About 3 cm distal to the adductor tubercle
• The meniscal scar and the inferior pole of the patella also help

Prevention at revision. Confirm gap balance before final cementation, and assess stability through the full range of motion under anaesthesia. Use adequate stem fixation for constrained implants, document why the constraint level was chosen, and consider a staged approach if there is any concern about infection.

Follow-up. Regular clinical and radiographic review identifies recurrent symptoms early, and a recurrent effusion should lower the threshold for aspiration. Long-term outcome depends on the initial balance.

Prognostic factors. A correct diagnosis, appropriate constraint and good bone stock predict a better outcome. Global instability, multiple prior surgeries and poor soft tissues predict a worse one.

Global epidemiology of TKA instability:

Instability is consistently reported as the second most common indication for TKA revision after aseptic loosening, accounting for roughly 15-20% of all revision TKAs across multi-centre and registry datasets. It is disproportionately an early failure: more than a third of all revisions occur within 2 years of the index procedure, and around half of these early revisions are attributable to instability, malposition or fixation failure - all surgeon-dependent technical factors.

National joint registry signals (cross-registry consensus):

Registries agree on three points: instability is largely preventable at the index operation, adequate constraint at revision reduces re-revision for instability, and younger patients carry a higher re-revision risk.

Side-by-side guidance from major societies: