Knee Pain

Knee pain is a common presentation to primary care physicians. The diagnosis can be challenging as the pain may arise from the joint, surrounding tissues, or referred to the joint from distant structures. The differential diagnosis of knee pain is broad and categorizing the various diagnoses related to the timing (acute or chronic) is useful. A thorough understanding of pertinent anatomy, appropriate physical examination, and common problems of the knee joint are essential for diagnosis and proper management of knee pain. Exercise-based therapy is often the 1st-line management of many knee disorders, but surgical intervention is warranted for specific diagnoses.

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Overview

Epidemiology

  • ↑ Prevalence of knee pain in the population 
  • ↑ Obesity rates
  • Knee osteoarthritis (OA) accounts for the majority of the total OA burden.
  • Women > men
  • A common complaint of runners and athletes

Anatomy of the knee joint

  • Largest and most complex joint of the human body
  • Synovial hinge joint
  • Supporting structures include:
    • Joint capsule
    • Lateral and medial menisci
    • Multiple ligaments for mobility and stability of the knee

Categories of knee pain

  • Acute knee pain:
    • Trauma
    • Acute exacerbation of chronic knee pain
    • Infection
    • Rheumatological problem
  • Chronic knee pain:
    • Overuse
    • OA
    • Tendinopathies
    • Patellar instability
    • Patellofemoral pain syndrome (PFPS)
  • Knee pain without trauma:
    • Infection
    • Rheumatological etiology
    • Malignancy

History

Obtaining a clear history is essential to making the correct diagnosis and guiding the examination:

  • Location for common problems:
    • Anterior: 
      • PFPS
      • Patellar tendonitis
      • Osgood-Schlatter disease
    • Medial: 
      • Medial collateral ligament (MCL) sprain
      • Medial meniscus tear
      • Pes anserine bursitis
    • Lateral: 
      • Lateral meniscus tear
      • Iliotibial band syndrome (ITBS)
  • Exacerbating and alleviating factors
  • Presence of swelling/effusion
  • Mechanical symptoms: consider meniscus or cartilage lesion
  • Previous surgery/procedures
  • Systemic signs/symptoms:
    • Fever
    • Chills
    • Erythema
  • Remember: Knee pain may be referred from hip pathology (especially in children).

Physical examination

The exam is directed by the duration and type of injury (acute or chronic) and the age of the individual. Comparison of the healthy knee is important:

  • Observation:
    • Previous surgical scars
    • Stance and gait:
      • Limp
      • Inability to bear weight
    • Angular deformities:
      • Genu varum (bowleg)
      • Genu valgum (knock-knee)
    • Swelling, erythema, ecchymosis
    • Ability to perform a deep squat
    • Muscle atrophy
  • Range of Motion (ROM): 
    • Flexion: 0–135 degrees
    • Compare to the opposite side.
    • Evaluate the hip.
  • Palpation:
    • Generally performed with the knee at 90 degrees of flexion while seated or supine with 20–30 degrees of flexion
    • Joint line palpation:
      • Meniscus injury
      • Collateral ligaments
      • OA
      • Fat pad
    • Anterior:
      • Patella
      • Patella ligament
      • Quadriceps tendon
      • Tibial tubercle
      • Prepatellar bursa
      • Fat pad
    • Medially: 
      • MCL
      • Pes anserine tendons
      • Bursa 
    • Laterally: 
      • Lateral collateral ligament (LCL)
      • ITB
      • Proximal fibula
    • Posterior:
      • Popliteal cyst (Baker cyst)
      • Popliteal artery
    • Excessive warmth
    • Effusion:
      • Ballottement of the patella
      • “Fluid wave” for smaller effusions
    • Strength and neurovascular testing:
      • Lower extremity neurological exam
      • Assessment of distal pulses
Table: Special exam maneuvers when examining an individual with knee pain
InjuryManeuver
ACL injury
  • Anterior drawer test:
    • Flex knee to 90 degrees and translate the tibia with anteriorly directed force.
    • Laxity in anterior translation indicates ACL injury.
  • Lachman test:
    • Most sensitive test for ACL rupture
    • Flex knee to 30 degrees and translate the tibia with anteriorly directed force.
    • Indicates ACL injury
  • Pivot shift:
    • Individual supine with fully extended knee
    • Place an internal rotation and valgus force on the proximal tibia while flexing the knee.
    • “Clunk” with flexion indicates ACL injury.
PCL injury
  • Posterior drawer test:
    • Knee at 90 degrees of flexion
    • Assess for ↑ posterior tibial displacement compared to the opposite leg.
  • Quadriceps active test:
    • Ask the individual to raise the foot off the table while supine.
    • Assess for posterior subluxated tibia moving anteriorly (indicates injured PCL).
MCL injuryValgus stress test for medial instability:
  • Performed at 0 degrees of flexion and 30 degrees of flexion
  • Assess for pain and laxity.
LCL injuryVarus stress test for lateral instability:
  • Performed at 0 degrees of flexion and 30 degrees of flexion
  • Assess for pain and laxity.
Meniscal tear
  • McMurray test: Palpable or auditory click while flexing and extending the knee suggests a meniscal tear.
  • Joint line tenderness (most sensitive test)
  • Generally, meniscus tests have ↓ sensitivity.
ITBS
  • Tenderness where ITB crosses the lateral femoral condyle
  • Noble compression test: maneuver to replicate symptoms
PFPS
  • Pain during squatting
  • Tenderness to the patellar retinaculum
  • Assessment of patellar glide
  • “J” sign: lateral movement of the patella at full extension
ACL: anterior cruciate ligament
PCL: posterior cruciate ligament
MCL: medial collateral ligament
LCL: lateral collateral ligament
ITBS: iliotibial band syndrome
ITB: iliotibial band
PFPS: patellofemoral pain syndrome
Four exams used to determine the cause of knee pain

Four exams to determine the cause of knee pain: anterior drawer test, Lachman test, pivot test, and McMurray test

Image by Lecturio.

Imaging

  • Used as an adjunct to the history and physical examination
  • Plain X-ray clinical decision-making tools:
    • The Ottawa knee rules and the Pittsburgh knee rules are helpful in the acute care setting to decide if an individual should receive imaging of the knee.
    • Ottawa knee rules apply to individuals > 2 years of age with knee pain/tenderness in the setting of trauma (images not needed if all negative):
      • > 55 years of age
      • Point tenderness at the head of the fibula
      • Point tenderness of the patella
      • Flexion of the knee < 90 degrees
      • Inability to walk 4 weight-bearing steps
    • Pittsburgh knee rules apply to all individuals with an acute injury in the past week, without prior knee surgery or ED evaluation (images not needed if all negative):
      • < 12 years of age
      • > 50 years of age
      • Inability to walk 4 weight-bearing steps
  • MRI:
    • Important in the assessment of soft tissue injuries of the knee
    • Not indicated for initial evaluation
  • Ultrasound: 
    • Assess soft tissues around the knee (e.g., tendons, collateral ligaments).
    • May be helpful for aspiration/injections

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Differential Diagnosis of Knee Pain

The assessment of knee pain is challenging. One approach to the differential diagnosis is to group the various conditions based on the acuteness of the presentation, the presence or absence of effusion, and the precipitating activities.

Traumatic knee injuries

Table: Causes of traumatic knee injuries with significant swelling
ConditionMechanismFeaturesDiagnosis
ACL tear
  • Sudden change in direction
  • Rapid deceleration
  • Landing from a jump
  • Frequently noncontact
  • Audible “pop” at the time of injury
  • Rapid swelling (hemarthrosis)
  • Joint instability
  • Positive Lachman test
  • Positive anterior drawer test
  • Positive pivot shift test
  • MRI to confirm
PCL tear
  • Direct blow to the proximal anterior tibia
  • E.g., dashboard injury, landing on a flexed knee
  • Hyperextension injury
  • Posterior knee pain
  • Loss of terminal knee flexion
  • Positive posterior drawer test
  • Positive quadriceps active test
  • MRI to confirm
Meniscus tear
  • Twisting mechanism with the foot planted
  • Associated meniscus cartilage degeneration
  • Tearing/popping sensation
  • Locking/catching
  • ↓ ROM
  • Joint-line tenderness
  • Positive McMurray test
  • MRI to confirm
Patellar dislocationPivoting or sudden change in direction
  • Lateral displacement
  • Deformity usually noted
  • Apprehension
  • Laxity
  • Effusion
  • Clinical diagnosis
Patellar ligament tear
  • Sudden, forceful stop or start (eccentric mechanism)
  • Trying to prevent a fall
  • Inability to extend
  • Infrapatellar swelling
  • Superior patella displacement
  • Clinical diagnosis
  • Ultrasound or MRI to confirm
Quadriceps tendon tear
  • Sudden forceful stop or start (eccentric mechanism)
  • Trying to prevent a fall
  • Inability to extend
  • Suprapatellar swelling
  • Inferior patella displacement
  • Clinical diagnosis
  • Ultrasound or MRI to confirm
Intra-articular fracture
  • High-speed trauma
  • Contact sports
  • Large effusion
  • Lipohemarthrosis
  • X-ray
  • CT for further evaluation
ACL: anterior cruciate ligament
PCL: posterior cruciate ligament
Table: Causes of traumatic knee injuries with minimal swelling
ConditionMechanismFeaturesDiagnosis
MCL sprain
  • Valgus force to the knee
  • Most commonly injured knee ligament
  • Medial knee pain
  • Feeling of instability
  • Positive valgus stress test
  • MRI to confirm
LCL sprainVarus force to the knee
  • Lateral knee pain
  • Feeling of instability
  • Positive valgus stress test
  • MRI to confirm
MCL: medial collateral ligament
LCL: lateral collateral ligament

Atraumatic knee injuries

Table: Causes and features of atraumatic knee pain with significant effusion
ConditionFeatures
Knee osteoarthritis
  • Articular cartilage loss and bone remodeling
  • Risk factors:
    • > 50 years of age
    • Women > men
    • ↑ BMI
    • Previous injury or infection
  • History:
    • Insidious onset of activity-related pain
    • Brief stiffness after inactivity or in the morning (gelling)
    • Slowly ↓ ROM
  • Exam:
    • Crepitus with motion
    • Joint-line tenderness
    • Possible effusion and warmth
  • Diagnosis:
    • Clinical diagnosis
    • X-ray confirmation
  • Radiographs: joint-space narrowing, sclerosis, osteophytes, and subchondral cysts
Crystal arthropathy
  • Acute episode of:
    • Severe pain
    • Swelling
    • Potentially large effusion
    • Erythema in 1 or more joints
  • Diagnosis by synovial fluid analysis:
    • Gout: urate crystals (negative birefringence)
    • CPPD or pseudogout:
      • CPPD crystals
      • Chondrocalcinosis on X-ray
Septic arthritis
  • Infection of 1 or more joints
  • Most commonly bacterial, but includes fungal and viral infections
  • Risk factors:
    • ↑ Age
    • IV drug abuse
    • DM
    • Immunosuppression
    • Prosthetic joint
    • Previous surgery
  • Most common organisms:
    • Staphylococcus aureus
    • Streptococcal species
  • Presentation:
    • Acute pain
    • Large effusion
    • ↑ Warmth
    • ↓ ROM without a history of trauma
    • Systemic signs and symptoms
  • Diagnosis:
    • Arthrocentesis:
      • Gram stain
      • Cell count
      • Culture
      • Evaluation for crystals
    • Blood cultures
    • CBC and inflammatory markers
Rheumatoid arthritis
  • Presentation:
    • Symmetric polyarthritis of the hands and feet
    • Progressive articular deterioration
  • Involves the knee joint in ⅓ of individuals
  • Exam:
    • Large effusions
    • Thickening of the synovium
ROM: range of motion
CPPD: calcium pyrophosphate dihydrate deposition disease
DM: diabetes mellitus
CBC: complete blood count
Table: Atraumatic knee pain presenting without significant joint effusion (by location)
ConditionFeatures
Anterior knee pain
Tibial tubercle apophysitis (Osgood-Schlatter)
  • Apophysitis of the developing tibial tuberosity
  • Presentation:
    • Pain over the tibial tuberosity in adolescents often associated with a recent growth spurt
    • Exacerbated by sports/activity, improves with rest
  • Exam: tenderness and prominence at the tibial tubercle, no effusion
  • X-ray not routinely indicated
SCFE
  • Slippage of the proximal femoral epiphysis on the femoral neck at the proximal femoral physis (growth plate)
  • Common in children with endocrine disorders and obesity
  • Referred pain to the ipsilateral knee, limp
Prepatellar bursitis
  • Inflammation and swelling of the prepatellar bursa
  • Etiology: pressure from excessive kneeling (“housemaid’s knee”)
  • 3 primary categories:
    • Traumatic: acute or chronic trauma (noninfectious)
    • Infectious: Staphylococcus aureus most common
    • Crystal disease: gout
Patellofemoral pain syndrome
  • Common
  • Seen in runners/cyclists
  • Other risk factors:
    • Women
    • Dynamic valgus
    • Patellar instability
  • Presentation:
    • Dull
    • Vague pain under the patella
    • Occurs climbing stairs or after prolonged sitting
  • Diagnosis: based on clinical presentation
  • Radiographs: generally not indicated
Medial knee pain
Pes anserine bursitis
  • Inflammation of the bursa between the tibia and the 3 tendons of the pes anserine region:
    • Semitendinosus
    • Gracilis
    • Sartorius muscles
  • Seen with obesity and overuse
  • Pain along the anteromedial tibia
Lateral knee pain
ITBS
  • Insidious onset of lateral knee pain
  • Related to overuse
  • Primarily in runners, but also with exercise
  • Pain when the ITB crosses over the lateral femoral condyle
Posterior knee pain
Popliteal (Baker) cyst
  • Most common mass in popliteal fossa
  • Cyst arises in association with OA or inflammatory arthritis of the knee.
  • Result of extrusion of fluid from the knee joint space into the semimembranosus and gastrocnemius bursa
  • Presentation:
    • Posterior knee pain
    • Swelling
    • Stiffness
    • Feeling of mass behind the knee
  • Diagnosis made on clinical exam (US or MRI)
  • Complications include:
    • Pseudothrombophlebitis: acute cyst rupture resembling DVT
    • US can rule out DVT and confirm a popliteal cyst.

SCFE: slipped capital femoral epiphysis
ITBS: iliotibial band syndrome
ITB: iliotibial band
OA: osteoarthritis
US: ultrasound
DVT: deep vein thrombosis

Referred pain

  • Individual presents with:
    • Vaguely localized knee pain
    • No focal tenderness
    • No inflammatory signs
  • Associated with disorders affecting the lumbar spine, sacroiliac joint, or hip joint:
    • Pain is referred from the hip (i.e., OA in adults, slipped capital femoral epiphysis (SCFE) in adolescents).
    • Radicular pain: most commonly the L5, S1 nerve roots
  • Uncommon causes of knee pain:
    • Benign and malignant bone tumors
    • The knee is a common site.
    • Presentation: 
      • Often nonspecific symptoms
      • Mimics common musculoskeletal disorders
      • Regional or localized pain
      • Fevers and swelling of soft tissue
      • Night pain
      • Pathological fractures
    • The main determinants in the analysis of bone tumors include:
      • Morphology of bone lesion:
        • Well or ill-defined
        • Presence of sclerosis
      • Age of individual
    • Benign bone tumors:
      • The majority of bone tumors are benign and incidental findings.
      • May present with:
        • Swelling
        • Deformity
        • Pathological fractures
      • More common in younger age groups
    • Malignant bone tumors: 
      • Uncommon
      • Causes significant morbidity and mortality
      • Osteosarcoma is the most common bone cancer.
      • Bone-marrow cancers include multiple myeloma, lymphoma, and leukemia.

Management

  • Effective management of knee pain is dependent on the correct diagnosis. 
  • Many disorders causing knee pain can be treated nonoperatively.
  • Some injuries require operative management.
Table: General guidelines for managing knee pain
ConditionManagement
ACL injury
  • Initial management: RICE
  • Consider nonoperative management in:
    • Older, sedentary individuals
    • Individuals willing to modify activity
  • Consider operative management:
    • Associated injuries
    • Active young adults
    • Individuals with recurrent instability after a trial of PT
  • Both autograft and processed allograft tissues are used for reconstruction.
PCL injuries
  • Frequently injured in combination with other ligaments
  • Initial management: RICE therapy
  • Lesser-grade and partial tears are nonoperative.
  • Higher-grade and full-thickness tears need surgery.
Meniscal injuries
  • Initially: ROM, ice/elevation, and crutches to limit weight bearing
  • Arthroscopic evaluation for persistent symptoms, “locked” knee, a large/complex tear, and other associated injuries
MCL injury
  • Most commonly injured knee ligament
  • Isolated MCL injury is often treated nonoperatively:
    • RICE initially with weight bearing as tolerated
    • Hinged knee brace with protected motion
  • Often associated with other ligamentous injuries (ACL tear)
Osteoarthritis
  • Exercise therapy (preferably non-weight bearing) is the cornerstone to management (e.g., swimming, water aerobics, cycling).
  • Weight loss (if overweight)
  • Glucosamine/chondroitin (limited evidence of efficacy)
  • Topical treatments: NSAIDs, capsaicin
  • Pain management: NSAIDS, acetaminophen
  • Intra-articular glucocorticoids (limited length of benefit)
  • Biologics (limited evidence)
  • Surgical intervention
Crystal arthropathy: gout and CPPDGout management:
  • Patient education:
    • Diet
    • Elimination of alcohol
    • Weight loss
  • Manage acute attacks:
    • Colchicine or other NSAIDs
    • Consider glucocorticoids: oral or intra-articular
  • Urate lowering therapy: long-term prophylactic medications for recurrent attacks:
    • Allopurinol
    • Febuxostat
    • Uricosuric agent: probenecid
CPPD (pseudogout):
  • Initial treatment: aspiration and intra-articular glucocorticoids
  • Oral glucocorticoids may be considered.
  • NSAIDs
  • Prophylaxis:
    • Colchicine
    • NSAIDs
    • Methotrexate
Septic arthritis
  • Emergent aspiration
  • Debridement and irrigation of the joint
  • IV antibiotics
PFPS
  • Exercise therapy (quadricep and hip strengthening)
  • Avoid pain-aggravating activities.
  • Adjuncts to exercise therapy:
    • Patellofemoral bracing
    • Kinesiotaping (limited evidence)
  • Surgical interventions lack evidence.
Popliteal (Baker) cyst
  • Cyst arises secondary to underlying joint disease.
  • If no response to intra-articular injection, consider direct cyst aspiration and injection.
  • Surgical excision: very rare
ACL: anterior cruciate ligament
RICE: rest, ice, compression, and elevation
PT: physical therapy
PCL: posterior cruciate ligament
ROM: range of motion
MCL: medial collateral ligament
NSAIDs: non-steroidal anti-inflammatory drugs
CPPD: calcium pyrophosphate dihydrate crystal deposition disease
PFPS: patellofemoral pain syndrome

Clinical Relevance

  • The knee joint is made up of articulations between the femur, tibia, and patella bones. The knee joint is one of the largest and most complex joints of the human body. The knee is classified as a synovial hinge joint, which primarily allows for flexion and extension. The supporting structures include a joint capsule, the lateral and medial menisci, and multiple ligaments, which help to ensure the mobility and stability of the knee.
  • Osteoarthritis is due to cartilage destruction and changes of the subchondral bone. The risk of developing the disorder increases with age, obesity, and repetitive joint use or trauma. Individuals have joint pain, stiffness, and decreased ROM. Physical exam may reveal crepitus with joint motion and osteophyte formation. The diagnosis is clinical and supported with radiographic joint findings. Management is conservative.
  • Rheumatoid arthritis is a symmetric, inflammatory polyarthritis. Rheumatoid arthritis is a chronic, progressive, autoimmune disorder most commonly presenting in middle-aged women with joint swelling, pain, and morning stiffness. The diagnosis is by the presence of rheumatoid factor, anti-cyclic citrullinated peptide antibodies, and image findings. Management involves antirheumatic drugs, biologic agents, and physical therapy. 
  • Gout is a metabolic disease associated with elevated serum uric acid levels and abnormal deposits of monosodium urate in tissues. Gout is often familial and initially characterized by painful, recurring, monoarticular swollen joints, followed by chronic deforming arthritis. Management is with NSAIDs, steroids, or urate-lowering therapy.
  • Calcium pyrophosphate crystal deposition disease (CPPD) occurs secondary to the deposition of calcium pyrophosphate dihydrate crystals in the periarticular tissues of joints and soft tissues. The disease, also known as pseudogout, can produce joint inflammation similar to gout. Management is with colchicine, NSAIDS, and methotrexate.
  • Slipped capital femoral epiphysis (SCFE) is an orthopedic disorder of early adolescence characterized by the pathological “slipping” or displacement of the femoral head, or epiphysis, on the femoral neck. The disorder is due to a combination of biomechanical and endocrine factors. Diagnosis is made with an X-ray of the hip. Treatment ranges from conservative to surgical and prognosis depends on the severity of the slip or displacement.

References

  1. Covey, C. (2020). Approach to the adult with unspecified knee pain. UpToDate. Retrieved June 9, 2021, from https://www.uptodate.com/contents/approach-to-the-adult-with-unspecified-knee-pain
  2. Beutler, A. & Fields, K. (2021). Approach to the adult with knee pain likely of musculoskeletal origin. UpToDate. Retrieved June 9, 2021, from https://www.uptodate.com/contents/approach-to-the-adult-with-knee-pain-likely-of-musculoskeletal-origin
  3. Modarresi, S. & Matilda C. (2020). Radiologic evaluation of the acutely painful knee in adults. UpToDate. Retrieved June 9, 2021, from https://www.uptodate.com/contents/radiologic-evaluation-of-the-acutely-painful-knee-in-adults
  4. Evans, J., & Nielson, J. L. (2021). Anterior cruciate ligament knee injuries. StatPearls. Retrieved June 9, 2021, from http://www.ncbi.nlm.nih.gov/books/NBK499848/
  5. Naqvi, U., & Sherman, A. L. (2021). Medial collateral ligament knee injuries. StatPearls. Retrieved June 9, 2021, from http://www.ncbi.nlm.nih.gov/books/NBK431095/
  6. Yaras, R. J., O’Neill, N., & Yaish, A. M. (2021). Lateral collateral ligament knee injuries. StatPearls. http://www.ncbi.nlm.nih.gov/books/NBK560847/
  7. Martin, S. N., & DeWeber, K. (2019). Lateral collateral ligament injury and related posterolateral corner injuries of the knee. UpToDate. Retrieved June 10, 2021, from https://www.uptodate.com/contents/lateral-collateral-ligament-injury-and-related-posterolateral-corner-injuries-of-the-knee
  8. MacDonald, J. & Rodenberg, R. (2019). Posterior cruciate ligament injury. UpToDate. Retrieved June 10, 2021, from https://www.uptodate.com/contents/posterior-cruciate-ligament-injury
  9. McQuillen, K. K. (2018). Musculoskeletal disorders. In R. M. Walls MD, R. S. Hockberger MD & Gausche-Hill, Marianne, MD, FACEP, FAAP, FAEMS (Eds.), Rosen’s emergency medicine: Concepts and clinical practice (pp. 2201-2217.e2).

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