Why Knee Special Tests Are Tested
Knee special tests form a key component of musculoskeletal OSCE stations. They test knowledge of anatomy, the ability to perform structured examination, and clinical reasoning in interpreting findings. The most commonly tested structures are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial and lateral menisci, and patella.
Before Special Tests — Standard Knee Examination
Always perform Look-Feel-Move before special tests:
- Look: swelling, deformity, muscle wasting (VMO), scars, erythema
- Feel: temperature (dorsum of hand), joint line tenderness (medial = MMT, lateral = LMT), effusion (patellar tap, bulge test)
- Move: active and passive ROM (normal 0–135°), crepitus
Testing for Effusion
Patellar tap (ballottement): Empty suprapatellar pouch with one hand (slide distally from 10 cm above patella), then tap the patella sharply with the other — a fluid "click" or patellar bounce = positive (moderate-large effusion)
Bulge test (sweep test): Stroke fluid out of medial gutter, then stroke lateral gutter — a fluid wave appearing in the medial gutter = positive (small effusion)
ACL Tests
Lachman Test (most sensitive for ACL)
- Patient supine, knee flexed 20–30°
- Stabilise the femur with one hand; grasp the proximal tibia with the other
- Draw the tibia anteriorly — feel for anterior translation and quality of endpoint
- Positive: excessive anterior translation with soft/absent endpoint = ACL rupture
- Sensitivity ~85%, specificity ~94%
Anterior Drawer Test (less sensitive than Lachman)
- Knee flexed to 90°, foot flat on couch (sit on foot to stabilise)
- Hands wrapped around proximal tibia, thumbs on tibial plateau
- Pull tibia anteriorly — note translation and endpoint
- Positive: >5 mm anterior translation compared to contralateral side
💡 Tip
The Lachman test is more sensitive than the anterior drawer because at 90° flexion, the posterior capsule and menisci may compensate for ACL insufficiency. Always perform Lachman first.
PCL Tests
Posterior Drawer Test
- Same position as anterior drawer (90° flexion)
- Push tibia posteriorly — note translation
- Positive: posterior translation = PCL injury
Posterior Sag Sign (Gravity Sign)
- Both hips and knees flexed to 90°, heels resting on couch
- Observe from the side — posterior sag of the tibial tubercle compared to the contralateral side = PCL rupture
Meniscal Tests
McMurray Test
- Patient supine, hip and knee fully flexed
- One hand on knee (feeling for a click), the other rotates the foot
- Medial meniscus: externally rotate the tibia and extend the knee — click/pain at joint line = positive
- Lateral meniscus: internally rotate the tibia and extend the knee — click/pain at joint line = positive
Thessaly Test (most sensitive for meniscal tears)
- Patient standing on the affected leg, slightly flexed (5° then 20°)
- Rotate the knee medially and laterally
- Positive: joint line pain or sensation of locking = meniscal tear
- Sensitivity ~90% at 20° flexion
Collateral Ligament Tests
Valgus Stress Test (medial collateral ligament — MCL)
- Knee at 0° and 30° flexion
- Apply valgus force (abduct the tibia) — medial joint line pain or opening = MCL injury
- 0° = posterior capsule also involved; 30° = pure MCL
Varus Stress Test (lateral collateral ligament — LCL)
- Apply varus force (adduct the tibia)
- Lateral joint line pain or opening = LCL injury
Patella Tests
Patella Apprehension Test
- Patient supine, knee extended
- Push patella laterally with thumbs while slowly flexing the knee to 30°
- Positive: patient resists or shows apprehension (grimace, attempt to straighten) = patella instability / previous dislocation
Patella Grind Test (Clarke's sign)
- Patient supine, push patella distally and ask patient to contract quadriceps
- Positive: pain = patellofemoral syndrome (unreliable — high false positive rate)
Frequently Asked Questions
"Why is the Lachman test more sensitive than the anterior drawer test for ACL rupture?"
At 90° flexion (the anterior drawer position), the posterior capsule, posterior horn of the medial meniscus, and hamstring muscles all act as secondary restraints to anterior tibial translation — they can compensate for ACL insufficiency, reducing anterior translation and giving a false negative. At 20–30° flexion (the Lachman position), these secondary restraints are relaxed and the ACL is the primary restraint — rupture is therefore unmasked. Additionally, haemarthrosis and pain often limit full knee flexion to 90°, making the Lachman test more practical in acute injuries. Sensitivity of Lachman is approximately 85–94% vs 55–75% for anterior drawer.
"How do you perform and interpret the McMurray test correctly?"
For the McMurray test, the patient lies supine with the hip and knee in full flexion. For the medial meniscus: externally rotate the foot and apply a valgus force while gradually extending the knee — a palpable or audible click at the medial joint line, reproduced with pain, is a positive result. For the lateral meniscus: internally rotate the foot and apply a varus force while extending the knee — a click at the lateral joint line is positive. A purely painful response without a click is less specific. The test has moderate sensitivity (~55–70%) but good specificity — a positive click with pain strongly suggests meniscal pathology and should prompt MRI.
"What is the posterior sag sign and what does it indicate?"
The posterior sag sign (gravity sign) is assessed with both hips and knees flexed to 90°, heels on the couch, and the examiner observing from the side. Normally, both tibial tubercles are at the same level. In PCL rupture, the tibia sags posteriorly under gravity on the affected side, making the tibial tubercle appear lower than the contralateral side. This is a subtle but specific sign of PCL injury. Always look for this before performing the posterior drawer test — if the tibia is already posteriorly subluxed, reducing it to neutral before the test will create a false impression of anterior translation (false-positive anterior drawer).
"What does a positive patella apprehension test indicate?"
A positive patella apprehension test — where the patient resists lateral displacement of the patella and shows signs of apprehension (grimacing, muscle guarding, or attempting to extend the knee) — indicates patellofemoral instability, most commonly as a result of previous patellar dislocation. The apprehension reflects the patient's fear of the patella dislocating again as it is pushed laterally. Predisposing factors include: trochlear dysplasia, increased Q angle, VMO weakness, tight lateral retinaculum, and patella alta. Management of recurrent instability includes physiotherapy (VMO strengthening), bracing, and in refractory cases, tibial tubercle transfer surgery.
"How do you test for knee effusion and what are the different tests?"
Two tests are used: the patellar tap (ballottement) for moderate-to-large effusions and the bulge test for small effusions. For the patellar tap: empty the suprapatellar pouch by firmly sliding your hand distally from 10 cm above the patella, then tap the patella sharply with your other hand — if fluid is present, the patella bounces off the femur with a palpable click. For the bulge test: stroke the medial gutter firmly to empty it of fluid, then stroke the lateral gutter — a fluid wave appearing in the medial gutter confirms a small effusion. Common causes of acute haemarthrosis (blood) include ACL rupture, patellar dislocation, peripheral meniscal tear, and fracture — blood stains the fluid yellow-brown on aspiration.
"What is the Thessaly test and why is it more sensitive than McMurray?"
The Thessaly test is a functional weight-bearing meniscal test performed with the patient standing on the affected leg with the knee slightly flexed (first at 5°, then at 20°). The examiner holds the patient's outstretched hands for support while the patient internally and externally rotates their body over the fixed foot, creating rotational torque across the knee. A positive test reproduces joint line pain or a sensation of locking/catching. The Thessaly test at 20° has sensitivity of approximately 90% and specificity of 97% for medial meniscal tears, and 92% sensitivity for lateral tears — significantly higher than McMurray. Its advantage is that it loads the joint dynamically, more closely replicating normal walking mechanics.
Related guides: Knee Examination OSCE · Lower Limb Examination OSCE · GALS Screening Examination OSCE · Musculoskeletal History OSCE · Ankle Examination OSCE