7.3 Knee Joint Assessment
Key Takeaways
- Lachman is the most sensitive bedside ACL test; anterior drawer is limited by hamstring spasm and effusion; pivot shift is more specific but often guarded (Prins).
- Posterior sag and posterior drawer diagnose PCL—do not ACL-reconstruct a posteriorly displaced tibia that feels 'anteriorly lax' only because it was hanging back.
- Valgus at 30° isolates MCL; valgus at 0° adds capsule/cruciate. Dial test: >10° ER at 30° only = PLC; at 30° and 90° = PLC + PCL.
- Meniscus is a cluster (joint-line tenderness, McMurray, Thessaly). Ottawa Knee Rule after trauma: age ≥55, isolated patellar tenderness, fibular-head tenderness, flex <90°, or inability to take four steps immediately and at evaluation.
- O'Donoghue unhappy triad is ACL + MCL + medial meniscus. Do not return a locked knee. Clarke/grind is poorly tolerated and is not the patellofemoral gold standard.
PA8 task 0202 at the knee rewards people who look at the tibia's rest position before they name a cruciate, who stress ligaments at the angle that isolates them, and who do not return a locked knee.
Cruciate Tests: Lachman, Drawer, Pivot, Sag
The ACL stops anterior tibial translation and is a rotatory restraint. The PCL stops posterior tibial translation. If the tibia is hanging posteriorly, an unreduced PCL can feel like an 'anterior drawer' when you pull the tibia to neutral—that is reducing a sag, not an ACL tear.
| Test | Structure / construct | Positive finding | What +/− actually means |
|---|---|---|---|
| Lachman (~20–30° flexion) | ACL | Increased anterior tibial translation versus the other side with a soft end-feel | Most sensitive bedside ACL test. Prins: Lachman is the most sensitive ACL test; pivot shift is the most specific. Hamstrings are slack at 20–30°, so spasm fools this less than the 90° drawer. |
| Anterior drawer (90°) | ACL | Anterior tibial translation at 90° | Limited in the acute knee: hamstring guarding and effusion. A 'negative drawer, positive Lachman' is still an ACL until proven otherwise. |
| Pivot shift | ACL (rotatory) | Tibia subluxes anteriorly in extension and reduces with a clunk around 20–40° flexion under valgus/IR | More specific, less sensitive, and often guarded. A painful athlete will not give you a textbook clunk. Do not force it. |
| Posterior drawer | PCL | Posterior tibial translation at 90° | Grade the step-off. Combine with sag. |
| Posterior sag (Godfrey) | PCL | With hips and knees at 90°, the tibial tubercle drops back versus the other side | The visual you must not miss. Do not ACL-reconstruct a PCL. |
| Quadriceps active test | PCL | From sag, quadriceps contraction reduces the tibia anteriorly | Confirms PCL insufficiency. |
Exam trap: calling a PCL sag an ACL tear because the anterior drawer felt loose after you pulled a posteriorly displaced tibia forward.
Collateral Ligaments and the Posterolateral Corner
Valgus stress:
- At 30° flexion: isolates the superficial MCL (primary medial restraint in flexion).
- At 0° (full extension): the ACL/PCL and posterior capsule share the load. Opening in full extension is a combined medial plus cruciate/capsular injury, not a simple grade-I MCL.
Varus stress:
- At 30°: LCL.
- At 0°: LCL plus cruciate/capsule. Isolated LCL rarely opens in full extension.
Posterolateral corner (PLC) (LCL, popliteus tendon, popliteofibular ligament). Dial test (prone or supine, compare ER of the tibia/foot):
- >10° more ER at 30° only: isolated PLC
- >10° more ER at 30° and 90°: PLC + PCL
- Increased ER at 90° only is the less common isolated-PCL rotation pattern
Varus thrust gait and varus opening at 0° should make you hunt PLC plus cruciate, not tape an 'LCL sprain' and return a knee that gives way in extension.
Meniscus, Patellofemoral, Extensor Mechanism
Meniscus is a cluster, not McMurray alone: history of twist, delayed swelling, mechanical locking or catching, joint-line tenderness (often the most sensitive single finding), McMurray (flexion-rotation; click or joint-line pain), and Thessaly (single-leg 20° flexion twist). One painful McMurray in an arthritic 50-year-old is not an automatic bucket-handle.
A locked knee that cannot extend is a bucket-handle meniscus (or osteochondral loose body) until proven otherwise. Do not return a locked knee. Do not force it straight on the sideline.
O'Donoghue unhappy triad: ACL + MCL + medial meniscus from valgus-plus-rotation (classically skiing or a cleat plant). Lateral meniscus is also commonly torn with ACL in modern series—know the classic triad for the exam and still examine the lateral joint line.
Patellofemoral
Clarke (patellar grind) is often poorly tolerated even in mildly irritable PF pain and is a crude compression. Prefer apprehension (lateral glide at 20–30° that the athlete stops), J-sign (lateral patellar tracking in terminal extension), and a step-down or squat that reproduces anterior pain. PF pain is a syndrome; it does not exclude cruciate or meniscus injury after trauma.
Extensor mechanism
Patellar tendon rupture: typically a younger jumper, cannot actively extend, palpable gap below the patella, patella rides high (patella alta). Quadriceps tendon rupture: typically older or after corticosteroid injection, gap above the patella, patella rides low. Both are surgical referrals. A complete gap and no active extension is not 'PF pain.'
Osteochondral injury and tibial plateau fracture follow a blow or twist with immediate hemarthrosis or inability to load. Plateau: lip hemarthrosis, valgus or varus deformity, inability to bear weight. These are Ottawa and imaging problems, not taping problems.
Ottawa Knee Rule
After blunt trauma or a fall, obtain radiographs if any of Stiell's items is present:
- Age ≥55 years
- Isolated patellar tenderness (no other bony tenderness)
- Tenderness at the fibular head
- Inability to flex to 90°
- Inability to bear weight (four steps—two on each limb, limp allowed) immediately after injury and at evaluation
The rule is a highly sensitive rule-out: if none of the items is present, a clinically important fracture is very unlikely. It does not rule out ACL, meniscus, or PLC. A locked, unflexable knee already fails Ottawa (cannot flex 90°) and should be imaged and referred.
Worked Scenario: Dashboard Sag versus Locked Twist
A soccer player is hit on the anterior tibia (dashboard-type blow). At 90° the tibial tubercle sags. Posterior drawer is positive. Someone pulls the tibia forward and shouts 'ACL' because the anterior drawer now moves. You reduce the sag, then Lachman has a firm end-feel. Dial is symmetric. Valgus and varus are stable.
Interpretation: PCL. Document sag and posterior drawer. Do not book an ACL reconstruction on a posteriorly displaced tibia. Isolated PCL is often nonoperative, but you still cannot miss a PLC (dial at 30° and 90°).
Second athlete: twist, immediate inability to extend, joint-line tenderness, McMurray not even possible because the knee is locked at 30°. He could not take four steps. Ottawa is already positive (flex <90°, no four steps). Do not return a locked knee. Splint in the position of comfort and refer for imaging—bucket-handle versus osteochondral loose body versus plateau.
Exam trap: calling a PCL sag an ACL tear, grinding the patella as your only PF test, or unlocking a meniscus on the sideline so the athlete can finish the game.
After a dashboard blow, the tibia sags posteriorly at 90°, the posterior drawer is positive, and a poorly performed anterior drawer feels 'loose.' Lachman has a firm end-feel once the tibia is reduced. What is the injury?
A skier has a valgus-plus-rotation injury. Valgus at 30° opens; valgus at 0° is stable; Lachman is soft; joint-line tenderness and McMurray are positive medially. What is the cluster?
After a twist, the knee is locked at 30° of flexion and cannot actively extend. Joint line is tender. Ottawa: he cannot flex to 90° and could not take four steps. What is the correct action?