Bursitis Knee: Types, Infection Red Flags and What the Evidence Shows

Key Takeaways

Kneeling work is associated with knee bursitis: self-reported bursitis was 20% in carpet layers versus 6% in comparison workers who seldom kneel,[1] and prepatellar bursitis was the most frequent knee injury in one collegiate wrestling cohort.[2]
Septic bursitis can be hard to spot clinically; in a small mixed cohort of elbow and kneecap bursitis, a bursal fluid white cell count above 2 × 10⁹/L had a sensitivity of 94% and a specificity of 79% for infection.[3]
In one ultrasound study, pes anserine bursitis was found in 20% of knees in patients with symptomatic knee osteoarthritis.[4]
In a retrospective cohort of hospitalised septic bursitis, most of it treated surgically, total antibiotic duration was not associated with cure.[5]
In one small trial in pes anserine bursitis, a corticosteroid injection reduced pain more at one month, while shockwave therapy gave lower pain at three months.[6]
In my clinical experience, prepatellar bursitis is the clearest form to recognise, whereas infrapatellar bursitis and bursitis at Gerdy’s tubercle can be hard to separate from tendon problems without imaging.

Introduction: Bursitis Knee

A soft, swollen lump over the kneecap or a sore spot just below the inner knee looks like a minor problem, but knee bursitis has documented occupational and sporting associations. In a study of 112 carpet and floor layers, bursitis was self-reported by 20% of carpet layers compared with 6% of millwrights and bricklayers, who seldom kneel.[1]

Knee bursitis is not one condition. Pressure, direct impact and infection are recognised factors in bursitis in front of the kneecap (prepatellar). Pes anserine bursitis on the inner side of the upper shin behaves differently and has been documented in patients with knee osteoarthritis. Their different locations and presentations call for site-specific assessment.

This article looks at bursitis knee problems from both a clinical and a sports medicine perspective: who gets which type, how an irritated bursa is told apart from an infected one, and what the research shows about injections, antibiotics, shockwave therapy and surgery.

What Is Knee Bursitis and Who Gets It?

A bursa is a small, fluid-filled sac that reduces friction between skin, tendon and bone. The knee has several of them. Two sites often involved in bursitis knee problems are in front of the kneecap (the prepatellar and superficial infrapatellar bursae) and on the inner upper tibia beneath the pes anserinus tendons.

Prepatellar bursitis: a pressure problem

Ultrasound studies show more frequent changes in the front of the knee in workers who kneel. When 96 randomly selected carpet and floor layers were compared with 72 house painters, thickening of the prepatellar or superficial infrapatellar bursa was found in 49% of the floor layers but in only 7% of the painters. Ten floor layers also had fluid in these bursae, and that finding was associated with pain when kneeling.[7]

The same pattern appears in self-reported symptoms. In the carpet-layer study above, carpet layers also reported knee fluid aspiration more often (32% vs 6%), and how often a worker used a “knee kicker” (a tool struck with the knee to stretch carpet) was the only statistically significant predictor of bursitis.[1]

Pes anserine bursitis: an osteoarthritis companion

Pes anserine bursitis was relatively uncommon in one selected MRI cohort. In a review of 509 knee MRI studies from 488 orthopaedic patients with suspected internal derangement, its prevalence was 2.5%.[8]

The picture changes in people with knee osteoarthritis. In 85 patients with primary knee osteoarthritis (170 knees examined), ultrasound showed pes anserine bursitis in 20% of knees (34/170). It was significantly more common in women, its prevalence rose with age, and it differed significantly between osteoarthritis grades.[4]

In my clinical experience, prepatellar bursitis is the most common and the most clear-cut form of knee bursitis. It is usually easy to recognise as a distinct, often soft swelling directly on top of the kneecap. Other bursae around the knee are less obvious. Infrapatellar bursitis is felt near the insertion of the patellar tendon, and pes anserine bursitis is felt below the knee on the inner side, which I see from time to time in clinic. Strictly speaking, pes anserine bursitis is extra-articular rather than a problem of the knee joint itself, but patients naturally experience it as knee pain. I also occasionally see bursitis over Gerdy’s tubercle on the outer side of the upper tibia.

Why Bursitis Knee Pain Develops: Pressure, Impact and Infection

Impact in sport

Wrestling provides a documented sporting example. In a six-year retrospective study of the University of Iowa wrestling team, prepatellar bursitis was the most frequent knee injury: 13 of 136 wrestlers developed an initial case, which represented 21% of all initial knee injury episodes. Most cases developed gradually, most occurred in the off-season, and direct impact during takedowns was suspected as the most frequent cause.[2]

Infection through the skin

The superficial position of the prepatellar bursa is clinically relevant, as skin injury is commonly reported in septic cases. In a series of 49 septic bursitis episodes, 27% involved the prepatellar bursa, Staphylococcus aureus was the most common pathogen (78%), and skin breakage, trauma and/or occupational risk factors were associated with 92% of prepatellar episodes.[9] A 2022 systematic review that pooled prepatellar cohorts found S. aureus in 48 of 61 patients (78.7%).[10]

Many of my patients with prepatellar bursitis work on their knees, for example as installers. In this group I also see erysipelas for the same reason: small breaks in the skin over the knee can let bacteria in, which may lead to septic bursitis or to erysipelas. Repeated loading on its own, without a skin break, can instead lead to aseptic bursitis.

In practice, trauma-related bursitis is also fairly common. In my clinical experience, after a knock to the knee the bursitis usually develops with some delay. If the swelling appears immediately or very soon after the blow, the bursa itself may be filled with blood-stained fluid.

Pes anserine bursitis mimics other knee problems

Pes anserine bursitis is easy to mislabel. In the MRI series above, its most common clinical presentation was pain along the medial joint line that mimicked a medial meniscal tear.[8]

How Knee Bursitis Affects Function and Training

For athletes, the initial episode of prepatellar bursitis may cost little time, but recurrence is the real problem. In the wrestling cohort, median time lost for the first episode was 4 days. However, eight of the 13 wrestlers had 20 recurrences between them, and recurrences and surgeries added significantly to total time lost. Prognosis appeared better in wrestlers with no previous history of bursitis.[2]

Septic bursitis can take a long time to settle. In one septic bursitis case series, cellulitis next to the affected bursa was present in 89% of episodes, and clinical resolution took a mean of more than 5 weeks, sometimes up to 20 weeks.[9]

Pes anserine bursitis was associated with worse function in one knee osteoarthritis cohort. Patients with knee osteoarthritis plus pes anserine tendino-bursitis had significantly higher initial WOMAC scores and timed up-and-go times than patients with knee osteoarthritis alone.[11]

In practice, patients with infrapatellar bursitis often describe knee pain when climbing stairs, or difficulty getting down into a squat. The problem is usually related to overuse. In my patients, the background is often gym training with squats, after which the knee has become painful.

Diagnosing Bursitis Knee Problems: Is It Infected?

The key clinical question in prepatellar bursitis is whether the bursa is infected. Clinical signs help but are not reliable on their own.

In 30 cases of olecranon and prepatellar bursitis, 10 of which were septic, fever, tenderness, cellulitis around the bursa and skin involvement were more common in the septic cases. A high leukocyte count, a low bursal-to-serum glucose ratio and a positive Gram stain of the bursal fluid distinguished septic from non-septic bursitis.[12]

A later comparative study of 36 patients (28 olecranon and 8 prepatellar bursitis) found that septic cases usually presented earlier and with more pain, erythema, warmth and tenderness, but only some had mild fever, with a maximum recorded temperature of 37.5 °C. A bursal fluid white cell count above 2 × 10⁹/L had a sensitivity of 94% and a specificity of 79% for detecting septic bursitis.[3]

Absence of obvious infection does not rule it out. In the wrestling cohort, which included only a small number of septic cases, the authors reported no clinical evidence of infection in 50% of the cases and, in this 1986 study, advocated Gram stain and culture, and therefore aspiration, in all cases of prepatellar bursitis.[2]

Septic bursitis is the trickier problem, and early recognition matters. In my clinical experience, a difficult differential diagnosis is erysipelas over the knee, which is surprisingly common in clinic. From a clinical perspective, erysipelas is usually more widespread: the redness and warmth extend beyond the bursa, and the area is evenly red and hot. On the other hand, bursitis can also occur together with erysipelas, and in my experience this is not unusual. In both situations, bending the knee may be difficult.

In my clinical experience, diagnosing septic bursitis often requires aspiration. Fortunately, the prepatellar bursa is fairly easy to aspirate, because the swelling is clearly on top of the kneecap rather than beneath it, and it is usually not a joint effusion. If there were also a joint effusion, I would consider aspirating the knee joint itself as well, but in my experience the joint is often calm.

After a single blunt trauma, when the swelling is superficial over the kneecap and there is no haemarthrosis, I tend to be cautious about aspirating or injecting the bursa because of the risk of infection. From a clinical perspective, haematoma inside the bursa could become infected, so I would consider aspiration mainly when it seems necessary. A haemarthrosis is a separate matter.

From a clinical perspective, particular caution may be warranted when the patient has systemic symptoms or fever together with redness, swelling and warmth, as this generally raises suspicion of a bacterial process. In my view, appropriate work-up may then include inflammatory markers, aspiration with sampling and culture, and in my practice often blood cultures. In practice, these patients are often managed in hospital emergency departments, and when I work in occupational health, I may refer them onward.

For medial knee pain, imaging can prevent unnecessary procedures. The MRI study authors suggested that accurately diagnosing pes anserine bursitis on MRI may help prevent unnecessary arthroscopy,[8] and the osteoarthritis study concluded that pes anserine bursitis is easily evaluated with ultrasound.[4]

From a clinical perspective, the differential diagnosis is harder outside the prepatellar bursa. Because of its location, infrapatellar bursitis can be difficult to distinguish from runner’s knee (patellofemoral pain), and in my experience the pain is most often felt at the very top of the tibia, just below the kneecap. Patellar tendinopathy is another difficult differential, since it is felt in much the same spot, and it can be hard to tell whether the pain comes from the tendon itself or from the bursa beneath it. I have often found MRI helpful in clarifying whether the problem is patellar tendinopathy, runner’s knee or infrapatellar bursitis, and sometimes a non-specific knee effusion can also mimic these conditions. Similarly, bursitis at Gerdy’s tubercle can be difficult, and sometimes impossible, to distinguish from an insertional tendinopathy on clinical examination alone.

Bursitis Knee Treatment: Injections, Antibiotics and Surgery

Non-infected prepatellar bursitis

A 2022 systematic review of prepatellar bursitis management included 10 studies (N = 702). In aseptic bursitis, recurrence was 5/79 (6.33%) after surgery and 2/7 (28.57%) without surgery, a difference that was not statistically significant. No statistically significant difference was detected between endoscopic and open bursectomy in recurrence after one year or in surgical complications, and 80% of endoscopically treated patients were pain-free at follow-up. The review authors named heterogeneity and sample size as the principal limitations. The review authors suggested that surgery be considered for recalcitrant aseptic bursitis.[10]

Septic prepatellar bursitis

The antibiotic studies cited here largely concern hospitalised patients. In a retrospective study of 343 infectious bursitis episodes (106 patellar), S. aureus accounted for 85% of cases with an identified pathogen. Total antibiotic duration was not associated with cure, and in multivariate analysis only immunosuppression was linked to recurrence (OR 5.6). The authors suggested that adjuvant antibiotic therapy in severe infectious bursitis requiring hospitalisation might be limited to 7 days in non-immunosuppressed patients. These findings do not establish a universally adequate antibiotic duration for septic bursitis.[5]

The prepatellar-specific data point the same way. In the 2022 systematic review, recurrence was not significantly more frequent in patients treated with antibiotics for less than eight days than in those treated for eight days or more, although this small comparison could not establish equivalence.[10]

When surgery is needed, a one-stage procedure was a feasible option in one trial. A randomised trial of 164 hospitalised patients with septic bursitis (34 at the patella) compared one-stage bursectomy with primary closure against a two-stage approach, with 7 days of postoperative oral antibiotics in both arms. Treatment failure occurred in 10% vs 16% (P = .23), and median hospital stay was 4.5 vs 6.0 days in favour of the one-stage approach. The wound dehiscence advantage was demonstrated for elbow bursitis specifically.[13]

Pes anserine bursitis

Injection and physical therapy modalities were both associated with improvement. In a trial of 60 patients with knee osteoarthritis and pes anserine tendino-bursitis randomised to physical therapy modalities or a corticosteroid injection, both treatments significantly improved all measured parameters at eight weeks, with no statistically significant difference between groups; the trial had no untreated control group.[11]

Adding an anti-inflammatory tablet after the injection may not help. In a double-blind trial of 64 patients given 20 mg of triamcinolone acetonide, adding 7 days of oral meloxicam did not significantly change the success rate at three weeks (50% vs 40.6%, p = 0.62).[14]

Shockwave therapy showed lower pain at three months in one small trial. In an open-label randomised trial (53 patients analysed), an ultrasound-guided injection of 40 mg methylprednisolone with 1 mL of 1% lidocaine reduced pain more than three weekly shockwave sessions at one month, but pain was significantly lower in the shockwave group at three months; baseline pain differed between the groups. Within this three-month follow-up, the authors described the injection’s effect as temporary.[6]

In practice, infrapatellar bursitis and the conditions that mimic it are generally managed conservatively in any case, with avoidance of squatting and anti-inflammatory pain medication. When the diagnosis is bursitis, rest is often the first step, and active physiotherapy may be helpful after that. I have not injected corticosteroid into infrapatellar bursitis myself. Because the true diagnosis around the infrapatellar region, Gerdy’s tubercle or the pes anserinus may also be an enthesopathy, I would be hesitant to inject corticosteroid into these areas blindly.

Practical Bursitis Knee Recommendations

Consider reducing repeated direct pressure. Kneeling and knee-kicker use were identified as occupational risk factors that offer opportunities for prevention.[1]
Treat a hot, red swelling after a scrape as possible infection. Skin breakage, trauma and/or occupational risk factors were associated with 92% of septic prepatellar episodes in one series.[9]
Do not rely on clinical signs alone. Septic cases may show only mild fever or no clinical evidence of infection.[2][3]
Use fluid analysis when infection is a question. Bursal fluid cell count, bursal-to-serum glucose ratio and Gram stain helped distinguish septic from non-septic bursitis in studies.[12][3]
Be particularly vigilant with immunosuppressed patients. Immunosuppression was the only factor linked to recurrence of septic bursitis in multivariate analysis.[5]
Consider pes anserine bursitis in inner-knee pain with osteoarthritis before assuming a meniscal tear.[8][4]
Weigh short-term options. In one small trial, a corticosteroid injection relieved pes anserine pain more at one month and shockwave therapy at three months; in another, physical therapy modalities showed no statistically significant difference from injection at eight weeks.[11][6]
Reserve surgery for recalcitrant cases. For aseptic prepatellar bursitis, the review authors recommend considering surgery when the bursitis is recalcitrant.[10]

For patients who work on their knees, a sick leave may sometimes be needed, and I may suggest some form of knee protection, although many of them already use knee pads at work. I would consider referring difficult cases to an orthopaedic surgeon with a fairly low threshold.

Conclusion: Knee Bursitis Is Two Different Problems

Bursitis knee complaints split into two main patterns. Prepatellar bursitis is associated with pressure and impact: bursitis was self-reported by 20% of carpet layers versus 6% of comparison workers,[1] and it was the most frequent knee injury in one collegiate wrestling cohort.[2] An important risk is infection, which may present without fever; aspiration with fluid analysis helps confirm it.[2][3] Pes anserine bursitis was found in about one in five knees with symptomatic osteoarthritis in one ultrasound study,[4] and in small trials it improved with injection, physical therapy modalities or shockwave therapy, with different short-term time courses.

This matches what I see in practice: patients with prepatellar bursitis often work on their knees, and erysipelas is an important differential diagnosis. Around the patellar tendon and Gerdy’s tubercle the picture is less clear-cut, and MRI is often what settles the diagnosis.

In my view, the practical message is simple: protect the front of the knee from repeated pressure, take a hot, red swelling seriously, and give the inner knee a proper look before blaming the meniscus.

Bursitis Knee: Types, Infection Red Flags and What the Evidence Shows

Table of Contents

Key Takeaways

Introduction: Bursitis Knee