Patellar tendonitis is diagnosed primarily through a physical exam, not imaging. The hallmark finding is tenderness when pressing on the bottom tip of the kneecap, combined with a history of pain that worsens with jumping, squatting, or other activities that load the knee. Imaging like ultrasound or MRI can confirm the diagnosis in unclear cases, but most people get a definitive answer in a single clinical visit.
Where the Pain Shows Up
The key diagnostic feature of patellar tendonitis is pain and tenderness at the very bottom edge of the kneecap, right where the patellar tendon attaches. This is consistent across nearly all cases. In one study of patients with confirmed patellar tendonitis, all 10 had tenderness localized to this exact spot. The pain typically gets worse with activities that put force through the tendon: jumping, running, climbing stairs, or squatting deeply. Many people notice that pain flares at the start of exercise, fades once they’ve warmed up, then returns after the session ends or the next morning.
This “warm-up effect” is a useful clue during diagnosis. It separates patellar tendonitis from problems like a cartilage tear, where pain tends to stay constant or worsen throughout activity. The Blazina classification system grades the condition into four stages based on how much pain disrupts your activity:
- Grade I: Mild pain only after physical activity
- Grade II: Pain at the start of activity that improves with warming up but returns afterward, with no drop in performance
- Grade III: Pain during and after activity, with a noticeable decline in performance
- Grade IV: Partial or complete tendon tear
Your clinician will ask about the timeline, what makes it better or worse, and what activities you do. Jumping sports like basketball and volleyball are the most common triggers, which is why the condition is often called “jumper’s knee.”
The Physical Exam
A clinician will press along the bottom of your kneecap while your leg is straight and your thigh muscles are relaxed. This is when tenderness is most noticeable. Then they’ll repeat the test with your knee bent or while you tighten your quadriceps. If the tenderness drops significantly when the tendon is under tension, that strongly supports a diagnosis of patellar tendonitis. This decrease happens because tensing the tendon essentially shields the inflamed attachment point from the pressure of the examiner’s thumb.
The 2025 Dutch multidisciplinary guideline on anterior knee pain identifies three clinical tests that, combined with a history of load-related pain at the bottom of the kneecap, confirm the diagnosis:
- Palpation test: Direct tenderness at the bottom tip of the kneecap
- Royal London Hospital test: Tenderness that decreases when the knee is bent to about 30 degrees (putting the tendon under stretch)
- Single-leg decline squat: Pain at the patellar tendon when squatting on one leg on a downward slope
You don’t need all three to be positive. The guideline states that load-dependent pain at the bottom of the kneecap plus at least one positive test is sufficient for a clinical diagnosis.
The Single-Leg Decline Squat
This functional test is especially useful because it replicates the kind of loading that triggers patellar tendon pain in real life. You stand on the affected leg on a 25-degree decline board and slowly squat down. A study of volleyball players found that those who reported pain specifically at the patellar tendon during this test (rather than vague knee pain elsewhere) had measurably thicker tendons and more signs of tendon damage on imaging. About 12% of the athletes in the study had patellar tendon pain during the test, and their imaging findings matched. Athletes who reported pain in other parts of the knee during the squat looked no different from pain-free athletes on imaging, which highlights why the location of pain matters as much as its presence.
When Imaging Is Used
Most cases of patellar tendonitis don’t require imaging for diagnosis. But when symptoms are ambiguous, when treatment isn’t working, or when a clinician needs to rule out a tear, ultrasound and MRI are the standard tools.
Both methods look for increased tendon thickness, changes in internal structure, and new blood vessel growth within the tendon. A study of university athletes established that a patellar tendon thicker than 7 mm is a reliable cutoff for diagnosing patellar tendonitis. At that threshold, ultrasound correctly identified 81% of confirmed cases and correctly ruled it out in 96% of healthy tendons. MRI performed even better, catching 100% of cases while correctly ruling it out in 89%.
Ultrasound has the advantage of being cheaper, faster, and available in many sports medicine and physical therapy clinics. It also allows real-time examination while you bend and straighten your knee. MRI provides more detailed images of the tendon’s internal structure and is better at detecting subtle damage, but it’s more expensive and typically reserved for cases where the diagnosis remains uncertain or surgery is being considered. The Dutch guideline recommends imaging confirmation before any surgical intervention.
Conditions That Look Similar
Several other knee problems cause pain in the front of the knee and can be mistaken for patellar tendonitis. Getting the right diagnosis matters because treatment differs significantly for each.
Infrapatellar fat pad impingement causes pain just below the kneecap, but the tenderness sits to the sides of the patellar tendon rather than directly at the tendon’s attachment. Clinicians test for this using the Hoffa test: pressing firmly beside the tendon with the knee slightly bent, then straightening the knee. If pain increases with straightening, the fat pad is likely the issue. Patellar tendonitis, by contrast, produces pain most prominently with eccentric loading (the lowering phase of a squat, for example) and localized right at the bottom of the kneecap.
Sinding-Larsen-Johansson syndrome is common in adolescents aged 10 to 12 and affects the same spot as patellar tendonitis: the bottom pole of the kneecap. It’s a growth-plate injury rather than a tendon problem, and the two are difficult to distinguish by physical exam alone. An X-ray can usually settle the question by showing fragmentation or irregularity at the growth plate.
Patellofemoral pain syndrome produces a more diffuse ache around or behind the kneecap, especially with prolonged sitting, stair climbing, or squatting. The tenderness is usually around the edges of the kneecap rather than at its bottom tip. If someone has been treated for patellar tendonitis for 12 weeks without improvement, current guidelines recommend reconsidering whether patellofemoral pain is actually the diagnosis.
Other conditions on the differential include plica syndrome (a fold of tissue getting pinched inside the joint), Osgood-Schlatter disease (pain at the bump below the kneecap where the tendon attaches to the shinbone, common in adolescents), and iliotibial band tightness, which causes pain on the outer side of the knee. A careful exam that pinpoints the exact location and behavior of the pain can usually sort through these possibilities without advanced testing.
What Happens After Diagnosis
Once patellar tendonitis is confirmed, treatment centers on a structured loading program for the tendon, typically starting with exercises like slow, heavy squats on a decline board. Progress is usually evaluated at 6 and 12 weeks using standardized pain and function scores. If pain and function haven’t improved after 12 weeks of consistent exercise therapy combined with activity modification, additional treatments are considered.
Surgery is a last resort. Current guidelines recommend it only after at least 6 months of non-operative treatment has failed to relieve persistent, disabling symptoms, and only when imaging confirms structural tendon damage. The vast majority of people with patellar tendonitis recover without surgical intervention.