Baker cyst or popliteal cyst is a soft-tissue swelling in the posterior aspect of the knee which contains gelatinous fluid. It is called Baker Cyst after Dr. William Morrant Baker who first described it. Most common location of Baker cyst is distal the popliteal crease under the medial head of the gastrocnemius muscle.
These conditions have also been referred in the past as gastrocnemio-semimembranosus bursae, semimembranosus burse, synovial cysts, posterior herniae of the knee joint.
The Baker cyst is unilateral mostly. It is almost twice as common in boys than the girls. In children, the cyst infrequently communicates with joint however in adults, intra-articular pathology is common and recurrence may recur if joint pathology is not corrected.
Baker cyst may be either bursal or hernial. The bursal cysts develop in bursa of the gastrocnemius-semimembranosus bursae. Hernial cysts are protrusion the capsule to lie outside.
Baker cyst is primary if there is no knee pathology involved. It is called secondary Baker cyst if there is an underlying knee problem.
The cysts are medially located mostly but rarely they are laterally located and occasionally extend into the calf of the leg.
Infiltration by lymphocytes, plasma cells, histiocytes, and even polymorphonuclear cells may be seen in varying degrees. Areas of metaplasia into cartilaginous and osteoid elements may also be there.
Clinical Presentation of Baker Cyst
Swelling in the posterior aspect of the knee is the presenting complaint. Knee stiffness and pain may also occur.
The examination reveals a swelling which is located distal to the popliteal crease, which becomes prominent when a knee is hyperextended but disappears upon flexion. The popliteal cyst may be soft to firm in consistency. Transillumination test is positive [ The passing of a strong beam of light through a part of the body for medical inspection.]
Imaging of Baker Cyst
Diagnosis of a popliteal cyst is usually evident on clinical examination. Conventional radiography in the anteroposterior, lateral, and oblique projections is done to check fluid density and to rule any other lesion that might give rise to popliteal swelling.
Ultrasonography helps in assessing the swelling and can distinguish between fluid and solid mass. Baker cysts are best shown by sagittal ultrasound image projection. Ultrasound can also be used to rule vascular tumors by looking at arterial pulsations. Color Doppler imaging can confirm the absence of vascular flow within the mass to exclude a popliteal artery aneurysm or cystic adventitial degeneration of a popliteal artery.
CT scanning can easily delineate secondary findings, such as intracystic osseous fragments, mass effect, wall thickening, and bony erosion.
MRI offers superior soft-tissue contrast resolution and helps to determine the extent and composition of the Baker cyst.
It is also able to differentiate between a benign Baker cyst and cystic tumors.
Computed tomography and MRI will show the lesion in a better way and can reveal its relationship to adjacent soft tissues and knee joint.
Arthrography is rarely indicated.
Lipoma, aneurysm, thrombophlebitis, neuroma, nerve ganglia, semimembranosus hypertrophy, and enlarged lymph nodes are other conditions to consider in the differential diagnosis.
Fibrosarcoma, synovial sarcoma, and fibrous histiocytoma, pigmented villonodular synovitis, rheumatoid arthritis, tuberculosis, brucellosis, or a pyogenic abscess may as well cause a cystic swelling in the Popliteal area.
Popliteal cyst usually arises in the medial part of the fossa. If the involvement is lateral, it could be something else.
Treatment of Baker Cyst
Support stockings, non-steroidal anti-inflammatory drugs, ice compression are supportive treatments
Fluid aspiration and steroid instillation may be tried but recurrence is common.
If a cyst is very large or painful and/or other treatments have not worked. Surgery may be carried out to treat an underlying problem at the same time – for example, repairing a meniscal tear.
Observation should be carried out on children for a period as Baker cyst is known to regress in children.