Patellar Apprehension Test and Fairbank's Sign: Patellofemoral Instability Guide
In short
Recurrent kneecap slipping or knee buckling often signals lateral patellar instability and medial patellofemoral ligament (MPFL) rupture. Learn how Fairbank patellar apprehension test identifies instability.
In this guide (8 sections)
Pathomechanics of Lateral Patellar Instability
The patella tracks within the trochlear groove of the distal femur, restrained by the medial patellofemoral ligament (MPFL) and the dynamic pull of the vastus medialis obliquus (VMO).
Following an acute lateral dislocation, the MPFL tears in over 90 percent of cases. Subsequent fear of re-dislocation leads to instability, apprehension, and avoidance of pivoting sports.
Performing the Patellar Apprehension Test (Fairbank's Sign)
The patient lies supine with the quadriceps fully relaxed and knee flexed to 20 to 30 degrees:
- Initial Setup: A knee flexion angle of 20 to 30 degrees brings the patella onto the entrance of the femoral trochlea.
- Lateral Displacement Force: The clinician places both thumbs on the medial border of the patella and applies a firm, smooth lateral-directed translation force.
- Positive Apprehension Sign: The patient suddenly contracts their quadriceps, reaches to grab the examiner hand, or exhibits visible facial distress and fear of imminent dislocation.
The Moving Patellar Apprehension Test (Ahmad Protocol)
The Moving Patellar Apprehension test is performed in two parts: first, moving the knee from 0 to 90 degrees flexion with lateral patellar pressure (eliciting apprehension); second, repeating the motion with medial patellar pressure (which eliminates apprehension). A positive test carries 100 percent sensitivity and 88 percent specificity for patellar instability.
Clinical Stability Grading
| Translation Quadrants | Normal vs Laxity | Clinical Implication | |
|---|---|---|---|
| 1 Quadrant (25% patellar width) | Normal physiological glide | Intact MPFL and balanced lateral retinaculum | |
| 2 Quadrants (50% width) | Mild hypermobility / laxity | May respond well to VMO neuromuscular strengthening | |
| 3 to 4 Quadrants (>75% width) | Frank patellofemoral instability | Complete MPFL rupture; high risk of recurrent dislocation |
Frequently Asked Questions
Can patellar instability be managed conservatively after a first dislocation?
Yes. If radiographs confirm no osteochondral loose bodies, a 6 to 12 week physiotherapy program focusing on VMO motor control, hip abductor strengthening, and patellar taping prevents recurrent episodes.
What is trochlear dysplasia?
Trochlear dysplasia is a congenital anatomical variation where the femoral groove is abnormally shallow or flat, eliminating the bony barrier against lateral patellar dislocation.
MPFL Anatomy and Trochlear Dysplasia
The medial patellofemoral ligament (MPFL) provides up to 60 percent of the total restraining force against lateral patellar translation from 0 to 30 degrees of knee flexion. Beyond 30 degrees, the patella engages deeply within the femoral trochlear groove. Patients with shallow trochlear dysplasia or patella alta experience recurrent subluxation.
VMO and Neuromuscular Control Retraining
Physiotherapy targets the vastus medialis oblique (VMO) alongside the hip external rotators to eliminate dynamic knee valgus. Closed-chain mini-squats with an elastic band around the distal thighs train the gluteus medius to prevent inward femoral collapse during athletic landings.
High-Risk Sports on Society Courts
Fast-paced weekend badminton and football on hard society concrete courts involve rapid cutting maneuvers that trigger patellar subluxation. Wearing a supportive lateral J-brace during early return to sport provides mechanical guidance and restores patient confidence.
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