Knee Ligament & Meniscal Injuries

The knee is a hinge-modified condylar joint stabilised by four principal ligaments and two fibrocartilaginous menisci. Soft-tissue knee trauma is a perennial NEET PG favourite, tested through clinical-test recognition, injury mechanisms, MRI signs, and the repair-versus-resect decision. This sheet packs the high-yield matter into a single read.

Functional anatomy & stabiliser map

The knee is stabilised by a coordinated set of static (ligamentous) and dynamic (muscular) restraints. Understanding which structure resists which force is the master key to every clinical test.

Structure	Primary restraint to	Clinical test	Key fact
ACL (anterior cruciate)	Anterior tibial translation, rotation	Lachman, anterior drawer, pivot shift	Most commonly injured; poor intrinsic healing
PCL (posterior cruciate)	Posterior tibial translation	Posterior drawer, posterior sag, quadriceps active	Strongest knee ligament; "dashboard injury"
MCL (medial collateral)	Valgus stress	Valgus stress at 30° flexion	Most common collateral injury; heals well
LCL (lateral/fibular collateral)	Varus stress	Varus stress at 30° flexion	Part of posterolateral corner
Medial meniscus	Shock absorption, secondary AP stability	McMurray, Apley grinding	Less mobile, firmly attached to MCL → torn more often
Lateral meniscus	Shock absorption	McMurray	More mobile; discoid variant occurs here

• The ACL has two bundles: anteromedial (AM) — tight in flexion, and posterolateral (PL) — tight in extension.
• ACL blood supply is from the middle geniculate artery; its poor synovial-bathed vascularity explains its failure to heal, justifying reconstruction rather than repair.
• The medial meniscus is C-shaped and relatively fixed (attached to deep MCL and capsule); the lateral meniscus is more O-shaped and mobile (the popliteus tendon separates it from the LCL).
• The peripheral red-red zone of the meniscus is vascular (perimeniscal capillary plexus, geniculate arteries) and heals; the central white-white zone is avascular and does not.

High-yield: ACL is the most commonly injured knee ligament; PCL is the strongest. Medial meniscus is torn more often than lateral because it is firmly tethered to the MCL.

Mechanisms of injury

1. ACL → Non-contact deceleration with valgus + external rotation (pivoting in football, basketball, skiing). The athlete typically hears/feels a "pop", develops rapid haemarthrosis within hours (acute haemarthrosis = 70% ACL tear), and feels the knee "give way."
2. PCL → Dashboard injury (anterior force on flexed proximal tibia in road traffic accidents) or a fall onto the flexed knee with plantarflexed foot.
3. MCL → Direct valgus blow to lateral knee (clipping injury in contact sport).
4. LCL / posterolateral corner → Varus + hyperextension force, often with peroneal (common fibular) nerve traction.
5. Meniscus → Twisting on a flexed, weight-bearing knee (squatting and turning). Degenerate tears occur in older patients with trivial trauma.

High-yield: Acute, tense haemarthrosis appearing within hours of a twisting sports injury = ACL rupture until proven otherwise. A slow-onset effusion over 24–48 h favours a meniscal tear (no major vascular structure bleeding).

The "Unhappy / Terrible Triad of O'Donoghue"

A valgus-external-rotation blow classically injures three structures together — remember "ACL, MCL, Medial meniscus." Note: modern series suggest the lateral meniscus is in fact more frequently involved in acute ACL ruptures, but the classic exam answer remains the medial meniscus.

Clinical examination — the named tests

This is the densest exam region. Learn the position, motion, and what a positive result indicates.

Test	Position / manoeuvre	Positive finding → diagnosis	Notes
Lachman	Knee 20–30° flexion, pull tibia anteriorly	Excess anterior glide, soft/absent end-point → ACL	Most sensitive clinical ACL test
Anterior drawer	Knee 90° flexion, pull tibia forward	Anterior translation → ACL	Less sensitive than Lachman in acute knee
Pivot shift	Extension → flexion with valgus + internal rotation	Reduction "clunk" of subluxed lateral tibia → ACL	Most specific; best under anaesthesia
Posterior drawer	Knee 90°, push tibia backward	Posterior translation → PCL	Most accurate PCL test
Posterior sag (Godfrey)	Hips & knees 90°, observe tibial profile	Tibia drops back → PCL	Avoids false-positive "anterior drawer"
Quadriceps active test	Knee 90°, contract quadriceps	Posteriorly sagged tibia reduces forward → PCL	—
Valgus stress @30°	Abduction force on tibia	Medial opening → MCL	At 0° (full extension) implies severe (MCL + cruciate/capsule)
Varus stress @30°	Adduction force on tibia	Lateral opening → LCL/PLC	At 0° implies cruciate + capsular involvement
Dial test	Prone, external rotation @30° & 90°	>10° asymmetry at 30° only → PLC; at both → PLC + PCL	Posterolateral corner
McMurray	Flex fully, rotate + extend	Palpable click + pain → meniscal tear	Ext rotation → medial; Int rotation → lateral
Apley grinding	Prone, knee 90°, axial compression + rotation	Pain → meniscus (vs distraction pain → ligament)	Apley distraction differentiates ligament
Thessaly	Weight-bearing on one leg, rotate at 20° flexion	Pain/locking → meniscus	High sensitivity for meniscal tears

High-yield: Lachman = most sensitive for ACL; Pivot shift = most specific for ACL. Posterior drawer = best for PCL. Commit this trio to memory — it is asked repeatedly.

Flow — Approach to an acute traumatic knee: History (mechanism + pop + effusion timing) → Inspection/effusion → Palpate joint line & collaterals → Lachman/pivot (ACL) → Posterior drawer/sag (PCL) → Valgus/varus stress (collaterals) → McMurray/joint-line tenderness (meniscus) → MRI to confirm → Arthroscopy (gold standard, also therapeutic).

Grading of collateral / ligament sprains:

• Grade I — microscopic tear, tender, no laxity.
• Grade II — partial tear, laxity with firm end-point.
• Grade III — complete tear, gross laxity, no end-point.

Investigation of choice

• Plain radiographs (AP + lateral): first-line to exclude fracture. Look for the Segond fracture — a small avulsion off the lateral tibial plateau (lateral capsular ligament) that is pathognomonic of ACL rupture. The reverse Segond (medial plateau avulsion) associates with PCL + medial meniscus injury. An avulsed tibial spine in children = ACL avulsion.
• MRI — investigation of choice for soft-tissue knee injury (sensitivity & specificity >90% for ACL and menisci). Non-invasive, defines associated chondral/bone-bruise pattern.
• Arthroscopy — gold standard (directly visualises and simultaneously treats).

Structure	Classic MRI sign
ACL tear	Discontinuity/non-visualisation of fibres; bone bruise at lateral femoral condyle + posterolateral tibia (pivot-shift kissing contusion); "empty notch"
PCL tear	Increased signal/thickening; normally taut ligament becomes wavy
Meniscal tear	Linear high signal reaching the articular surface (Grade III); "double PCL sign" (bucket-handle medial meniscus), "ghost meniscus", flipped fragment
MCL	Oedema/disruption along the medial collateral

High-yield: Segond fracture (lateral tibial plateau avulsion) = ACL injury marker. Double-PCL sign on MRI = displaced bucket-handle tear of the medial meniscus.

Meniscal tear patterns

Tear morphology drives the treatment decision and is examined directly.

• Longitudinal / vertical — peripheral, can repair if in red zone.
• Bucket-handle — a displaced longitudinal tear; the central fragment flips into the notch → true locked knee (block to full extension). Surgical emergency-ish; repair if possible.
• Radial / parrot-beak — disrupts the circumferential collagen → hoop stresses lost → poor prognosis.
• Horizontal cleavage — degenerate, older patients.
• Flap, complex, degenerative.
• Root tear — detachment of meniscal root; functionally equivalent to total meniscectomy → rapid arthritis; increasingly repaired.

Discoid meniscus: congenitally broad meniscus, usually lateral; presents in children with a "snapping knee" (Wrisberg variant); classified by Watanabe (complete, incomplete, Wrisberg-ligament type).

High-yield: A young patient with a locked knee that cannot fully extend after a twisting injury = displaced bucket-handle meniscal tear.

Management & drug of choice

General acute care

Initial management of any soft-tissue knee injury follows PRICE/POLICE (Protect, Optimal Loading, Ice, Compression, Elevation). Analgesia is with paracetamol and NSAIDs; NSAIDs are the symptomatic drug of choice for pain and effusion. There is no specific pharmacotherapy that heals ligaments — management is rehabilitation and selective surgery.

ACL

• Reconstruction (not primary repair) because of poor healing.
• Indications: young/active patient, instability ("giving way"), high-demand sport, associated repairable meniscal/chondral injury.
• Conservative (physiotherapy, quadriceps/hamstring strengthening) for older, low-demand, or sedentary patients.
• Graft choices: Bone–patellar tendon–bone (BPTB) — strong, bony-to-bony healing, but anterior knee pain; hamstring (semitendinosus–gracilis) — less donor morbidity, common; quadriceps tendon; allograft.
• Timing: reconstruct after the acute phase, once swelling settles and range of motion returns, to reduce arthrofibrosis.

PCL

• Isolated grade I–II → conservative (quadriceps rehabilitation; quads dynamically reduce posterior sag).
• Reconstruct for grade III, combined injuries, or bony avulsion (fix the avulsion).

MCL

• Heals well — managed conservatively (hinged brace + rehab) even for grade III isolated tears. Surgery reserved for combined instability or distal avulsion (Stener-like).

LCL / posterolateral corner

• Surgical repair/reconstruction, often early (within ~3 weeks), because the PLC heals poorly and chronic injury causes varus thrust and ACL/PCL graft failure.

Meniscus — repair vs meniscectomy

The modern principle is "save the meniscus" — every effort to repair, because meniscectomy accelerates osteoarthritis.

Favour REPAIR	Favour (partial) MENISCECTOMY
Peripheral red-red / red-white zone	Central white-white (avascular) zone
Longitudinal/vertical tear	Radial, complex, degenerate tears
Younger patient, acute tear	Older patient, irreparable morphology
Tear <3 cm, stable knee	Tear pattern not amenable to suture
Concurrent ACL reconstruction (improves healing)	—

• Partial meniscectomy (resect only the unstable fragment) is preferred over total meniscectomy whenever resection is unavoidable.
• Total meniscectomy → predictable early osteoarthritis (Fairbank changes: ridging/osteophytes, joint-space narrowing, flattening of the femoral condyle).
• Meniscal transplantation is an option in the young, symptomatic, previously meniscectomised knee.

High-yield: Repair tears in the vascular peripheral (red) zone; resect (partial meniscectomy) tears in the avascular central (white) zone. Total meniscectomy → Fairbank changes and accelerated OA.

Complications

• ACL deficiency: recurrent instability, secondary meniscal/chondral damage, early osteoarthritis.
• Post-meniscectomy: Fairbank changes, OA.
• Reconstruction complications: arthrofibrosis ("cyclops lesion" — localised fibrous nodule blocking extension), graft failure, tunnel malposition, donor-site morbidity (patellar tendon).
• PLC injury: common peroneal nerve palsy (foot drop), chronic varus thrust.
• Knee dislocation (multi-ligament): orthopaedic emergency — high risk of popliteal artery injury; mandatory vascular assessment ± CT angiography / ABI even if pulses present.

High-yield: Multi-ligament knee injury / dislocation → always assess the popliteal artery (intimal tears can present with normal initial pulses) and the common peroneal nerve.

Key differentials

• Patellar dislocation — lateral; positive apprehension test; haemarthrosis can mimic ACL.
• Patellofemoral pain / chondromalacia patellae — anterior knee pain, no instability.
• Meniscal vs ligamentous effusion timing — fast tense haemarthrosis (ACL/fracture) vs slow effusion (meniscus).
• Osteochondral fracture / loose body — locking that is intermittent and positional.
• Tibial spine avulsion (children) — the paediatric "ACL equivalent."
• Septic / inflammatory arthritis — non-traumatic effusion; aspirate.
• Plica syndrome — medial snapping, mimics meniscus.

Mnemonics & eponyms

• Lachman = Lots of sensitivity; Pivot = Pinpoint specificity (ACL).
• Terrible/Unhappy Triad: "AMM" — ACL, MCL, Medial meniscus.
• "Red repairs, White resects" — meniscal zone treatment.
• Eponyms: Segond fracture (ACL), O'Donoghue's triad, Fairbank changes (post-meniscectomy OA), Godfrey sag test (PCL), Cyclops lesion (post-ACL reconstruction), Wrisberg variant discoid meniscus.

Recently asked / exam angle

• Most sensitive test for ACL → Lachman; most specific → pivot shift (these alternate as the stem).
• Investigation of choice for meniscal/ligament injury → MRI; gold standard → arthroscopy.
• Segond fracture is associated with → ACL tear (and reverse Segond → PCL + medial meniscus).
• Strongest ligament of knee → PCL; most commonly injured → ACL.
• Double-PCL sign / "double anterior horn" / flipped fragment → bucket-handle tear of medial meniscus.
• Which meniscus is more commonly torn and why → medial, because it is tethered to the MCL (less mobile).
• Dashboard injury → PCL.
• Best initial graft properties (BPTB vs hamstring) — image/feature-based MCQs.
• Mechanism MCQ: non-contact deceleration + pivot with a pop and immediate haemarthrosis → ACL.
• Discoid meniscus → lateral, snapping knee in a child.
• Management: isolated MCL grade III → conservative; isolated ACL in young athlete → reconstruction; avascular-zone meniscal tear → partial meniscectomy.
• Image-based: MRI sagittal showing ACL fibre discontinuity, or coronal showing displaced bucket-handle ("double PCL").

Rapid revision

1. ACL = most commonly injured; PCL = strongest knee ligament.
2. Lachman most sensitive, pivot shift most specific, for ACL.
3. Posterior drawer / sag (Godfrey) = best for PCL; dashboard injury mechanism.
4. Valgus stress @30° = MCL; varus stress @30° = LCL/PLC; testing at 0° implies severe combined injury.
5. Acute tense haemarthrosis within hours of pivot injury = ACL rupture (with a "pop").
6. Segond fracture (lateral tibial plateau avulsion) is pathognomonic of ACL; reverse Segond → PCL.
7. MRI = investigation of choice; arthroscopy = gold standard (diagnostic + therapeutic).
8. Medial meniscus torn more often (tethered to MCL); discoid meniscus is usually lateral.
9. Bucket-handle tear → locked knee + "double-PCL sign"; classic young-twist presentation.
10. Red zone repair, white zone resect; prefer partial over total meniscectomy.
11. Total meniscectomy → Fairbank changes and accelerated osteoarthritis.
12. ACL = reconstruct (BPTB/hamstring graft); isolated MCL/PCL grade I–II = conservative; PLC = early surgery; knee dislocation → check popliteal artery + peroneal nerve.