Knee Injuries
Summary
The knee is a complex hinge joint dependent on ligamentous stability. Injuries are extremely common in sports and trauma. The key to diagnosis is the History (Mechanism) and the timing of swelling. While fractures require X-rays, the majority of significant knee pathology (ACL/Meniscus) is soft tissue, requiring MRI. Knee dislocation is a limb-threatening emergency. [1,2]
Key Injuries
- ACL Rupture: 70% of sports knee injuries. Pivot/Twist.
- Meniscal Tear: Rotation on a flexed knee. "Locking".
- MCL/LCL Sprain: Direct blow (Valgus/Varus).
- Patellar Dislocation: Kneecap shifts laterally.
- Knee Dislocation: Tibio-femoral separation. VASCULAR EMERGENCY.
- Fracture: Tibial Plateau / Patella.
Clinical Pearls
The "Pop" and the "Swell":
- Immediate Swelling (less than 1-2 hours): This is a Haemarthrosis (blood in joint). 80% chance of ACL Rupture (if no fracture).
- Delayed Swelling (12-24 hours): This is a Synovial Effusion (reactive fluid). 80% chance of Meniscal Tear.
The Dashboard Inquiry: A direct blow to the anterior tibia (e.g., knee hitting dashboard in car crash, or falling on flexed knee) drives the tibia backwards, rupturing the PCL (Posterior Cruciate Ligament).
A Normal X-ray means nothing: For ligament/meniscal injuries, the X-ray is normal. If the patient has a "Pop", instability, and haemarthrosis, they have an ACL rupture until proven otherwise, even if X-ray is clear.
Demographics
- ACL: Young athletes (15-30). Females > Males (4:1) due to Q-angle and hormonal laxity.
- Meniscus: Young (Acute tears) vs Elderly (Degenerative tears).
- Fractures: Bimodal (Trauma in young, Osteoporosis in old).
Mechanisms
- ACL: Non-contact Valgus + Internal Rotation (Cutting/Pivoting).
- MCL: Valgus force (rugby tackle from side).
- Meniscus: Shear stress (grinding) between femur and tibia.
- Patella Dislocation: Quadriceps contraction with knee in slight flexion + Valgus.
History Grid
| Injury | Mechanism | Sound | Swelling | Ability to play on? |
|---|---|---|---|---|
| ACL | Twist/Pivot | "POP" | Rapid | No (Unstable) |
| Meniscus | Twist/Squat | Click? | Delayed | Yes (initially), then locks |
| MCL | Valgus blow | Tearing | Localised | Often Yes (strapped up) |
| Dislocation | High Energy | - | Massive | No |
Stability Tests
- ACL:
- Lachman's Test: 30° flexion. Anterior draw. (Most sensitive).
- Anterior Drawer: 90° flexion. (Less sensitive due to muscle spasm).
- PCL:
- Posterior Sag Sign: 90° flexion. Tibia sags backwards.
- MCL/LCL:
- Stress in 30° flexion (isolates ligament).
- Meniscus:
- McMurray's: Rotate and extend. (Pain/Click).
- Thessaly's: Twist on one leg (More sensitive).
- Joint Line Tenderness: Very sensitive.
1. Ottawa Knee Rules (For X-Ray)
Perform X-Ray ONLY if:
- Age >55.
- Tenderness at Fibular Head (Peroneal nerve risk) or Patella (isolated).
- Inability to flex to 90°.
- Inability to weight bear (4 steps) immediately and in ED. Sensitivity ~99%.
2. MRI
- Gold Standard for all soft tissue injuries (ACL/Meniscus).
- Indicated if: Mechanical symptoms (Locking), Instability, or unclear diagnosis after 4-6 weeks conservative care.
3. Aspiration
- If tense haemarthrosis causes severe pain, aspirating blood can relieve symptoms. (Lipohaemarthrosis = Fat globules in blood = Intra-articular fracture).
Management Algorithm
ACUTE KNEE INJURY
↓
APPLY OTTAWA KNEE RULES
┌───────┴───────┐
POSITIVE NEGATIVE
↓ ↓
X-RAY SOFT TISSUE INJURY?
↓ ↓
FRACTURE? ASSESS SWELLING
┌───┴───┐ ┌─────┴─────┐
YES NO RAPID DELAYED
↓ ↓ (ACL?) (Meniscus?)
ORTHO RE-EVAL ↓ ↓
(MRI?) RICE RICE
BRACE PHYSIO
MRI (OP) REVIEW
Specific Eponymous Injuries
- Segond Fracture: Avulsion fracture of lateral tibial plateau. Pathognomonic for ACL Tear.
- Unhappy Triad (O'Donoghue): ACL + MCL + Medial Meniscus. (Blown knee).
Treatment Strategies
- ACL:
- Conservative (Physio) for non-athletes / copers.
- Reconstruction (Hamstring/Patellar tendon graft) for athletes / unstable knees.
- MCL:
- Almost always Conservative (Hinged brace 6 weeks). Heals well due to blood supply.
- Meniscus:
- Peripheral (Red zone): Repair.
- Central (White zone - avascular): Debridement (Menisectomy).
- Degenerative: Physiotherapy (Surgery has no benefit over placebo).
- Patella Dislocation:
- Reduce (Extension + Medial pressure).
- Physiotherapy (VMO strengthening).
- Surgery (MPFL reconstruction) if recurrent.
- Knee Dislocation: 30-40% have Popliteal Artery injury. Must measure ABPI. if less than 0.9 -> CT Angio. Also Common Peroneal Nerve palsy (Foot drop).
- Osteoarthritis: Long term sequela of menisectomy or ACL rupture (50% at 20 years).
- Arthrofibrosis: Stiffness if operated too early (before swelling settles).
- ACL: 9 months return to sport post-op. Re-rupture rate 5-10%.
- MCL: 6-8 weeks heal time.
- Meniscus: Good outcomes, but higher OA risk.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Knee Imaging | Ottawa | Use rules to reduce radiation/cost. |
| ACL Mgmt | BOAST / AAOS | Early MRI for athletes. Pre-hab before Rehab. |
| Degenerative Meniscus | NICE | Do NOT arthroscope degenerate knees (ineffective). |
Landmark Evidence
1. KANON Trial (NEJM)
- Compared Early ACL Reconstruction vs Rehab + Delayed Reconstruction.
- Result: No difference in outcomes at 2 or 5 years. Supports frequent conservative management.
What is the ACL?
The Anterior Cruciate Ligament is a rope inside the knee that stops the shin bone sliding forwards. It is crucial for twisting/turning sports (football, skiing) but less important for walking/cycling.
Do I need surgery?
Not necessarily. Many people can live normal lives without an ACL by strengthening the hamstring muscles to do the job instead. We usually only operate if you want to return to pivoting sports or if the knee keeps giving way.
What is a meniscal tear?
The meniscus is the shock-absorber cushion. A tear is like a rip in a rubber washer. A loose flap can get stuck in the hinge, causing the knee to "lock".
Can I walk on it?
If it's an ACL, usually yes (once pain settles). If it's a fracture, no. Listen to your body.
Primary Sources
- Stiell IG, et al. Implementation of the Ottawa Knee Rule for the use of radiography in acute knee injuries. JAMA. 1995.
- Frobell RB, et al. A randomized trial of treatment for acute anterior cruciate ligament tears. N Engl J Med. 2010. (The KANON Trial).
- Sihvonen R, et al. Arthroscopic partial meniscectomy versus sham surgery for a degenerative meniscal tear. N Engl J Med. 2013.
Common Exam Questions
- Diagnosis: "Rapid swelling + Pop?"
- Answer: ACL Rupture.
- Sign: "Posterior Sag?"
- Answer: PCL Rupture.
- Anatomy: "Nerve at risk in fibular head fracture?"
- Answer: Common Peroneal Nerve (Foot drop / sensation dorsal foot).
- Guideline: "X-Ray indication?"
- Answer: Can't weight bear, age >55, fibular head tenderness.
Viva Points
- Segond Fracture: What is it? Avulsion of the ALL (Anterolateral Ligament) from lateral tibia. Why important? 100% association with ACL tear.
- Lipohaemarthrosis: Fat and Blood in the joint (seen on horizontal beam lateral X-ray). Signifies an intra-articular fracture (marrow fat has leaked out).
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.