# Popliteal Cyst

> Popliteal cyst for NEET-PG Orthopaedics: Baker cyst between semimembranosus and gastrocnemius, its link to intra-articular disease, and rupture that mimics DVT.

- Canonical URL: https://prepelephant.com/topics/neet-pg/orthopaedics/popliteal-cyst
- Exam / course: NEET-PG · Subject: Orthopaedics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Apley's System of Orthopaedics and Fractures; Maheshwari's Essential Orthopaedics
- How to cite: "Popliteal Cyst", PrepElephant, https://prepelephant.com/topics/neet-pg/orthopaedics/popliteal-cyst

## Direct answer

A popliteal cyst, or Baker cyst, is a synovial swelling behind the knee that herniates through the posteromedial capsule between the semimembranosus and medial head of gastrocnemius. It usually reflects raised joint pressure from osteoarthritis, rheumatoid arthritis or a meniscal tear. Rupture can track fluid into the calf and mimic deep vein thrombosis, so Doppler ultrasound is essential before the swelling is accepted as a simple cyst.

## What you must remember

- **Location:** the gastrocnemius-semimembranosus bursa, which communicates with the knee joint through the posteromedial capsule.
- **Eponym:** named after William Morrant Baker, who described the lesion.
- **Mechanism:** raised intra-articular pressure, usually from effusion, pushes synovium out through the posteromedial capsule.
- **Associations:** osteoarthritis, rheumatoid arthritis, meniscal tears and other intra-articular disease, which must always be sought.
- **Rupture:** synovial fluid dissects into the calf, causing pain, swelling and redness that mimic deep vein thrombosis.
- **Investigation:** Doppler ultrasound shows the cyst and excludes thrombosis, while MRI assesses the underlying knee.
- **Management:** treat the underlying knee disease first; aspiration with steroid may relieve symptoms, but recurrence is common.

## A diagnostic approach that protects the calf

A swelling behind the knee is usually a Baker cyst, but thrombosis and aneurysm must be excluded before that label is accepted. The steps below put the vascular check ahead of the diagnosis.

1. **Assess the swelling:** a soft, fluctuant swelling in the popliteal fossa suggests a cyst, whereas a firm, solid mass needs a different differential.
2. **Check for pulsation:** an expansile, pulsatile swelling points to popliteal artery aneurysm, which needs vascular assessment rather than aspiration.
3. **Examine the calf:** unilateral calf pain, swelling and tenderness raise the possibility of cyst rupture or deep vein thrombosis.
4. **Image with Doppler ultrasound:** ultrasound shows the cyst, its communication with the joint and any thrombus in the popliteal or calf veins.
5. **Look for the cause:** MRI or arthroscopy assesses the meniscus, cartilage and synovium, because treating the source matters more than the cyst itself.
6. **Choose the treatment:** the underlying disease is treated first, aspiration with steroid is an option, and excision is considered only for persistent or recurrent symptoms.

## Where the exam tries to catch you

Popliteal cyst questions usually test the mimics, because the cyst is rarely the diagnosis that matters most.

- **Trap:** treating calf swelling with a popliteal cyst as a simple cyst without Doppler — **Correct:** a ruptured cyst can mimic deep vein thrombosis, so Doppler ultrasound must exclude thrombosis first.
- **Trap:** aspirating a pulsatile popliteal swelling — **Correct:** a pulsatile mass suggests popliteal artery aneurysm, which needs vascular assessment rather than aspiration.
- **Trap:** excising the cyst and ignoring the knee — **Correct:** the cyst reflects intra-articular disease, so the meniscus and joint must be examined to prevent recurrence.

## Frequently asked questions

### What is a Baker cyst?

A Baker cyst, or popliteal cyst, is a synovial swelling behind the knee that herniates through the posteromedial capsule. It lies between the semimembranosus and the medial head of gastrocnemius and is named after William Morrant Baker.

### Which knee conditions are commonly associated with a popliteal cyst?

Osteoarthritis, rheumatoid arthritis and meniscal tears are commonly associated with a popliteal cyst. Raised intra-articular pressure from these conditions pushes synovial fluid out of the joint and into the popliteal space.

### How does a ruptured popliteal cyst mimic deep vein thrombosis?

When a popliteal cyst ruptures, synovial fluid dissects down into the calf and causes pain, swelling and redness. These features mimic deep vein thrombosis, so Doppler ultrasound is needed to exclude thrombosis before treatment.

### What is the best imaging for a popliteal cyst?

Doppler ultrasound is the usual first-line investigation for a popliteal cyst, because it shows the cyst and excludes venous thrombosis. MRI is used to look for the intra-articular cause of the cyst.