U01.11.024 Common knee conditions

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Learning Objectives

  • Identify the anatomical components of the Unhappy Triad and the mechanism of injury.
  • Distinguish between Prepatellar Bursitis and Popliteal Cysts by location and etiology.
  • Explain the relationship between chronic joint disease and Baker cysts.
  • Recognize the clinical importance of valgus force in knee trauma.

1. The “Unhappy Triad”

This classic sports injury occurs when a valgus force (lateral hit) impacts the knee while the foot is planted.

  • The Components:
    1. ACL: Anterior Cruciate Ligament.
    2. MCL: Medial Collateral Ligament.
    3. Medial Meniscus: Classically included because it is attached to the MCL.
  • Step 1 Fact Check: While the “classic” triad includes the medial meniscus, modern studies show that lateral meniscus involvement is actually more common in acute ACL/MCL injuries.
  • Presentation: Acute “popping” sensation, swelling (hemarthrosis), and significant joint instability.

2. Prepatellar Bursitis

Commonly known as “Housemaid’s Knee,” this is an extra-articular inflammation.

  • Location: Inflammation of the bursa located directly in front of the patella.
  • Mechanism: Repeated trauma or pressure from excessive kneeling (e.g., carpet layers, gardeners, or housemaids).
  • Presentation: Swelling and redness over the kneecap. The range of motion of the actual joint is often preserved unless the swelling is massive.

3. Popliteal (Baker) Cyst

A Baker cyst is not a true cyst, but rather a herniation or fluid collection in the posterior knee.

  • Location: Fluid collection in the gastrocnemius-semimembranosus bursa.
  • Mechanism: Usually communicates with the synovial space. It is secondary to chronic joint disease like Osteoarthritis (OA) or Rheumatoid Arthritis (RA).
  • Clinical Risk: If the cyst ruptures, it can cause acute calf pain and swelling that mimics a Deep Vein Thrombosis (DVT).

Clinical Notes & Step 1 Pearls:

  • Valgus vs. Varus: Valgus force comes from the outside (lateral) hitting in. The Varus force comes from the inside (medial), hitting out.
  • Lachman Test: This is the most sensitive physical exam maneuver for an ACL tear.
  • Bursa Communication: Unlike the prepatellar bursa, the popliteal bursa often communicates with the joint space, which is why joint effusions “leak” into a Baker cyst.

Activity: Knee Pathology Differential

Quick Mnemonics:

Unhappy Triad: M-A-M (MCL, ACL, Medial Meniscus).

Baker Cyst: Think of a Baker standing on their Back (Posterior) side all day.

Prepatellar: Pre-patellar = Pressure on the patella.

Activity