Knee Bursitis: What the Swelling Means and What to Do About It

The knee has approximately 11 bursae, fluid-filled sacs that sit between bones, tendons and skin to reduce friction during movement. They are normally flat and contain a small amount of synovial fluid. When a bursa becomes irritated or inflamed, it fills with excess fluid, producing swelling and pain. This is bursitis: not a joint problem in the usual sense, but an irritation of one of the structures around the joint.

Two types account for the vast majority of knee bursitis: prepatellar bursitis (in front of the kneecap) and pes anserine bursitis (on the inner knee, below the joint line). They have different causes, different presentations, and different management strategies. Understanding which bursa is involved is the first step, because treating the wrong one is a waste of effort.

What Is Happening

Prepatellar bursitis affects the bursa that sits between the kneecap and the skin. It is the most common superficial bursitis in the body. The classic cause is repeated or prolonged kneeling, which compresses the bursa against the patella. It is historically called “housemaid’s knee” and is common in occupations that require kneeling: carpet layers, plumbers, gardeners, roofers [1]. Direct trauma to the front of the knee, such as a fall, can also cause it. The mechanism is straightforward: the bursa is a thin-walled sac designed to reduce friction, and when it is subjected to sustained pressure or impact, the lining becomes inflamed and produces excess fluid.

The bursa swells, sometimes dramatically. The swelling is directly over the kneecap, and the knee joint itself feels normal underneath. Range of motion is usually preserved unless the swelling is very large. You can often distinguish prepatellar bursitis from an intra-articular effusion (fluid inside the joint) by palpation: with bursitis, you can feel the fluid sac moving over the kneecap, while the joint itself has a normal feel when flexed and extended.

The key clinical distinction is between non-infected (aseptic) and infected (septic) bursitis. Septic bursitis produces redness, warmth, fever, and pain that is disproportionate to the swelling. It requires medical treatment with antibiotics and often aspiration [1]. The skin over the prepatellar bursa is thin and vulnerable to abrasion, and any break in the skin combined with the proximity of the bursa creates a route for bacterial entry. Septic bursitis is not rare, and assuming all prepatellar bursitis is benign without considering infection is a mistake.

Pes anserine bursitis affects the bursa at the insertion of the pes anserinus (the conjoined tendon of the sartorius, gracilis and semitendinosus muscles) on the medial tibial plateau. The pes anserinus, named for its goose-foot shape, is where three muscles that cross both the hip and the knee converge on the inner tibia. The bursa sits beneath this insertion, cushioning it against the bone. The point of tenderness is specific: approximately 5 cm below the medial joint line, on the inner surface of the tibia [2].

Risk factors include obesity, knee osteoarthritis, diabetes, valgus (knock-knee) alignment, and tight hamstrings [3]. The association with osteoarthritis is particularly strong: medial compartment arthritis changes the loading pattern of the knee and increases the stress on the medial structures, including the pes anserinus insertion. An MRI study of 320 symptomatic knees found pes anserine bursitis in 2.5 percent, with a strong association with medial compartment osteoarthritis [5]. It is more common in women than men, likely related to wider pelvic anatomy producing a greater valgus angle at the knee.

The mobility-stability lens is relevant here. The knee is a stability joint, designed to flex and extend reliably. When the hip (a mobility joint above) is stiff or the hip abductors are weak, the knee absorbs more valgus load during weight-bearing. This increased inward angulation loads the medial structures of the knee, including the pes anserinus, more than they are designed to tolerate. Addressing only the local bursitis without considering the hip contribution often leads to recurrence.

What the Evidence Supports

For prepatellar bursitis:

The first-line treatment is removing the cause. If the bursitis is from kneeling, stop kneeling, or use knee pads when kneeling is unavoidable. Compression wrapping and ice can reduce swelling. NSAIDs may provide symptomatic relief [1]. Most non-infected cases resolve within two to four weeks once the aggravating activity stops.

Aspiration is performed when the swelling is large, uncomfortable, or when septic bursitis must be ruled out. The aspirated fluid is sent for cell count, Gram stain and culture. Clear, straw-colored fluid with a low white cell count suggests aseptic bursitis. Turbid fluid, an elevated white cell count, or positive Gram stain indicates infection. If the bursitis is non-infected, corticosteroid injection after aspiration has been used, though the evidence for this is limited and recurrence is common [4]. A systematic review found that corticosteroid injection reduced recurrence rates compared to aspiration alone, but the quality of the available studies was low.

Chronic prepatellar bursitis that does not respond to conservative measures may require surgical bursectomy, but this is uncommon. Surgery is reserved for cases with repeated episodes of symptomatic effusion that fail at least three to six months of conservative management, or for chronic septic bursitis that does not resolve with antibiotics [1].

For pes anserine bursitis:

Hamstring stretching is the most commonly recommended first-line treatment. The logic is that tight hamstrings increase the tension across the pes anserinus insertion, and reducing that tension reduces bursal irritation [2]. Corticosteroid injection is frequently used and provides short-term relief, though controlled trials are limited. Addressing the modifiable risk factors (weight loss if overweight, managing underlying osteoarthritis) is important for preventing recurrence.

Hip abductor strengthening deserves consideration as part of the management, particularly when valgus knee alignment is present. Strengthening the gluteus medius reduces the dynamic valgus load at the knee during weight-bearing, which reduces the stress on the medial knee structures. This is the same principle that applies to patellofemoral pain and IT band syndrome: the hip controls the knee’s frontal plane position.

Ice applied to the tender point after activity may reduce symptoms. Activity modification, specifically avoiding deep squatting, stairs and prolonged walking while symptoms are acute, allows the inflammation to settle.

The condition is sometimes under-diagnosed because the pain overlaps with medial knee osteoarthritis. Both produce medial knee pain, but the tenderness locations are different. Osteoarthritis produces tenderness at the joint line itself. Pes anserine bursitis produces tenderness below the joint line, at the tibial insertion of the pes anserinus. Pressing directly on the bony prominence about 5 cm below the medial joint line and reproducing the patient’s familiar pain is the most reliable clinical sign [5]. In patients with both conditions, treating the bursitis specifically, alongside the osteoarthritis management, can produce more relief than either approach alone.

Exercises to Try

Hamstring stretch (standing). Place the affected leg on a low step or stool, knee straight. Hinge forward at the hips, keeping the back flat, until you feel a stretch in the back of the thigh. Hold for 30 seconds. Three repetitions, twice daily. This is the primary exercise for pes anserine bursitis. The stretch should be felt in the mid-belly of the hamstring, not behind the knee.

Hamstring stretch (supine). Lie on your back near a doorway. Place the affected leg up against the door frame, knee straight, while the other leg lies flat through the doorway. Adjust your distance from the frame to control the intensity. Hold for 30 seconds. This version removes the tendency to round the lower back that sometimes limits the standing version.

Quad set. Sit with the affected leg extended. Press the back of the knee into the floor by tightening the quadriceps. Hold for 5 seconds. 10 repetitions. This maintains quadriceps activation without loading the bursa. The goal is to keep the VMO (the inner portion of the quadriceps just above the kneecap) firing, as it is often inhibited by knee pain.

Straight leg raise. Lie on your back, one knee bent with the foot flat. Keep the other leg straight and raise it to the height of the opposite knee. Hold for 3 seconds. Lower slowly. Three sets of 10. This strengthens the quadriceps and hip flexors without bending the knee.

Side-lying hip abduction. Lie on the uninvolved side. Raise the top leg to about 30 degrees, keeping it straight and toes pointing slightly downward. Hold for 2 seconds. Three sets of 12. Hip abductor weakness contributes to valgus knee loading, which aggravates pes anserine irritation. This exercise addresses the proximal cause, not just the local symptom.

When to See a Doctor

See a doctor if:

• The swelling is red, warm, or accompanied by fever (possible septic bursitis, which requires urgent treatment)
• Swelling has not improved after 2 to 3 weeks of activity modification
• Pain is severe or worsening
• There was a puncture wound or skin break over the bursa (increased infection risk)
• Inner knee pain has not responded to 6 to 8 weeks of stretching and activity modification
• You develop pain or swelling in the calf, which may indicate a different condition

When to see a doctor. If joint pain follows a significant injury, is accompanied by swelling, giving way, locking, or loss of range of motion, or if you have neurological symptoms (numbness, tingling, weakness), see a doctor or physical therapist. They can assess whether imaging or specialist input is appropriate.