What is the diagnosis and management for a female patient with left knee pain and mild swelling after running a marathon, with no significant past medical history?

Diagnosis: Post-Marathon Knee Pain – Most Likely Patellofemoral Pain Syndrome

This female patient most likely has patellofemoral pain syndrome (PFPS), the most common running-related knee injury, which characteristically presents with anterior knee pain and mild swelling after endurance activities like marathon running. 1

Clinical Reasoning

Primary Diagnosis: Patellofemoral Pain Syndrome

• PFPS is the most common cause of knee pain in runners, accounting for retropatellar or peripatellar pain that worsens with activities requiring knee flexion under load, such as running and stair climbing. 1
• The presentation of left knee pain with mild swelling after a marathon is classic for PFPS, as this condition is triggered by repetitive loading of the patellofemoral joint during endurance activities. 1
• Among running-related musculoskeletal injuries, patellofemoral pain syndrome has the highest prevalence proportion at 16.7%, making it the single most common specific pathology in runners. 2
• Knee injuries account for the highest proportion of running-related injuries, with incidence and prevalence rates consistently showing the knee as the most frequently injured anatomical site. 3, 2

Supporting Evidence from Running Injury Epidemiology

• Medial tibial stress syndrome (9.4% incidence), patellofemoral pain syndrome (6.3% incidence), and plantar fasciitis (6.1% incidence) are the three most common running injuries by incidence proportion. 2
• The knee, ankle, and lower leg account for the highest proportion of injury incidence in runners, with the knee being the predominant site. 2
• Two-thirds of running injuries are training-related, suggesting that rapid increases in mileage or intensity (such as marathon participation) are significant risk factors. 4

Differential Diagnoses to Consider

Other Anterior Knee Pathologies

• Anterior knee pain during activities like stair climbing strongly suggests patellofemoral disorders, including early cartilage loss, patellar tendinopathy, or fat pad impingement. 1
• Patellar tendinopathy should be considered if pain localizes specifically to the inferior pole of the patella rather than diffusely around the kneecap. 1

Meniscal Pathology (Less Likely)

• Meniscal tears are unlikely in this asymptomatic marathon runner, as the prevalence of meniscal tears in marathon runners (9%) is actually lower than in sedentary persons (16%). 5
• Marathon runners have no higher prevalence of meniscal abnormalities than the general population, with 53% showing normal meniscal signal on MRI. 5

Degenerative Changes (Less Likely Given Age and History)

• Active marathon runners have significantly lower arthritis prevalence (8.8%) compared to the matched U.S. population (17.9%), making osteoarthritis unlikely in an active marathoner without prior injury history. 6
• Running duration, intensity, mileage, and number of marathons completed do not correlate with increased arthritis risk in active marathoners. 6

Diagnostic Approach

Initial Assessment

• Obtain anteroposterior and lateral knee radiographs first to exclude fractures, osteoarthritis, osteophytes, and loose bodies, even though these are unlikely given the clinical presentation. 1
• Assess for knee effusion on physical examination, as presence of effusion guides treatment approach and may indicate more significant pathology. 1
• Pain that localizes behind or around the kneecap and worsens with knee flexion under load confirms PFPS. 1

When to Consider Advanced Imaging

• MRI is not initially indicated unless radiographs show abnormalities or symptoms fail to improve with appropriate treatment after 3 months. 1, 7
• MRI should be reserved for persistent symptoms after conservative management, as it may reveal early cartilage changes even with normal radiographs. 1

Critical Pitfall to Avoid

• Do not attribute all knee pain solely to the knee joint – evaluate hip and lumbar spine clinically, as hip pathology commonly refers pain to the knee and must be evaluated if knee imaging is normal. 1

Management Protocol

First-Line Treatment (3-6 Months)

• Individualized knee-targeted exercise therapy (with or without hip strengthening) should be delivered following assessment of symptom severity and irritability. 1
• Education should underpin all interventions, providing rationale for treatment and building confidence that PFPS is a benign, self-limiting condition. 1
• Prefabricated foot orthoses should be prescribed when patients respond favorably to treatment direction tests. 1
• Taping may be used as an adjunct to exercise delivery to facilitate pain-free movement during rehabilitation. 1

Activity Modification

• Reduce running volume and intensity temporarily while maintaining cardiovascular fitness through alternative activities that do not load the patellofemoral joint (swimming, cycling). 4
• Avoid rapid changes in training program, as this is a significant risk factor for running injuries. 4
• Gradual return to running should follow a structured progression, starting with short-duration running periods (30-60 seconds) interspersed with walking. 8

Pharmacological Management

• NSAIDs (such as naproxen) may be used to reduce pain and swelling, but usage should be limited as they may suppress the natural healing process. 8, 9
• Naproxen has been shown to be effective for musculoskeletal pain with onset of pain relief beginning within 1 hour and lasting up to 12 hours. 9
• Care should be taken with NSAID usage after an ankle or knee injury, as they may interfere with tissue healing despite providing symptomatic relief. 8

When Conservative Management Fails

• If symptoms persist after 3-6 months of structured exercise therapy, consider intra-articular corticosteroid injection, though this is rarely necessary for PFPS. 7
• Surgery should be reserved for cases that do not respond to thorough and comprehensive exercise-based treatment, which is exceedingly rare for PFPS. 8

Prognosis and Prevention

Expected Outcome

• PFPS typically responds well to conservative management with structured exercise therapy and activity modification over 3-6 months. 1
• Most runners can return to full activity following appropriate rehabilitation and gradual progression of running load. 8

Prevention of Recurrence

• Maintain consistent training volume without rapid increases in mileage or intensity, as training errors are the primary cause of running injuries. 4
• Continue knee and hip strengthening exercises even after symptoms resolve to prevent recurrence. 1
• Monitor for pain during or following running as any pain suggests overloading and requires temporary reduction in training load. 8