Acute Knee Pain

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Clinical Features

Red Flags

The initial goal of assessment is to exclude red flag conditions, especially fracture.

Red Flags
  • Constitutional symptoms: Fevers, chills, night sweats, fatigue, or rash.
  • Fracture: history of significant trauma, known osteoporosis or risk factors for same, haematoma, pathological fractures associated with cancer or Paget's disease, significant pain, positive Ottawa knee rules
  • DVT: Acute infection, cancer, stroke or paralysis, previous DVT, CHF, pregnancy, dehydration, varicose veins, nephrotic syndrome, rheumatological disease, IBD, recent major surgery, hormonal treatment, combined oral contraceptives, prolonged immobility or air travel, positive Wells Score.
  • Peripheral arterial disease: intermittent claudication, elderly, hypertension, smoking history, diabetes, sedentary lifestyle, obesity, stroke, ischaemic heart disease, impaired ABPI.
  • Infection: fever, history of risk factors such as body penetration or immunosuppression.
  • Cancer: past history of cancer, elderly, weight loss, failure to improve, prolonged pain, palpable deformities
  • Paediatric: consider slipped capital femoral epiphysis
  • Rheumatological: prior history, multiple joint involvement.

The knee is the most common peripheral site of cancer. The most common peripheral site of cancer is the distal femur, followed by the proximal tibia, and then the proximal humerus.

The Wells Score can be used in the appropriate clinical setting of acute non-traumatic acute knee pain to assess for the possibility of DVT.



The initial question should be whether the pain began following a traumatic event. Pain immediately following injury raises the possibility of structural damage to the knee such as fracture, microfracture, bone bruise, ligamentous tear, meniscal tear, and chondral injury.

Enquire about the nature of the trauma. Posterior cruciate injuries often occur with a direct blow to the flexed knee. Collateral ligament injuries can occur with varus or valgus strain, usually as a contact injury. Meniscal injuries often occur with twisting forces. For ACL tears, in men it is often from anteroposterior loading, while in women it is often from twisting.

Trauma also includes repetitive injuries and pain can be delayed here. Possible conditions here are stress fractures of the patella, stress fractures of the tibio-femoral condyles, chondral dessication, pre-patellar bursitis, hamstring tendon injury, and patella tendon injury.

The closer the pain onset is to the inciting event, the more likely it is that there is significant structural damage.

In the absence of trauma there should be some suspicion of an underlying serious disease process such as inflammatory joint disease, infection, or neoplasia. However the absence of trauma is not sufficient in itself to investigate for these conditions. with imaging

Pain Location

The primary pain site is classified into anterior, medial, lateral, posterior, and internal. There may be multiple pain sites and the pain may be diffuse or vague.

Anterior knee pain can involve any anterior structure, with the patellofemoral joint being the most common. Pain can arise from the patella, patellar tendon and its entheses, the supra-, pre-, and infra-patellar bursae, the fat pad, and the distal quadriceps.

Medial knee pain can arise from the medial collateral ligament, the medial meniscus or other intra-articular structure, the pes-anserine insertion, or other bony, ligamentous, tendinous, or adnexial structure.

Lateral knee pain can arise from the lateral collateral ligament, the lateral meniscus, or other intra-articular structure, the biceps insertion, the ilio-tibial band, the superior tibio-fibular joint, or other bony, ligamentous, tendinous, or adnexial structure.

Posterior pain can arise from the joint itself, or from extra-articular structures including the popliteal bursa.

Diffuse or vague pain may be secondary to injury of an intra-articular structure, rheumatological disease, infection, or from referred pain.

Pain severity is only weakly associated with clinical findings and disability.

Association with Activity

Pain that gets worse over the course of the day is consistent with many forms of mechanical knee pain. While pain and stiffness that is worse after rest is consistent with inflammatory causes.

Pain at rest, night pain, and pain that is unchanged by activity are often thought of as red flag symptom. However night pain is very common in hip and knee osteoarthritis. It also may not a sole phenomenon of "end-stage OA." [1] In a young adult, child, or older adult with normal plain films, night pain could indicate infection or neoplasm.

Pain that is worse at rest suggests anterior knee disorders including pre-patellar bursitis.

Knee Swelling

Swelling can be secondary to inflammatory and mechanical processes. Inflammatory causes are more likely if there is morning stiffness, rest pain, night pain, and relief on walking. Mechanical causes are more likely with pain on weight bearing, and pain that is worse as the day goes on.

The causes of a swollen knee include septic arthritis, crystal arthritis, trauma, haemarthosis, hydrarthrosis, and inflammatory arthritis.

A swollen hot knee may indicate infection, however aseptic bursitis and a resolving haemarthrosis can also cause some warmth.

A patient with an acute traumatic haemarthrosis should be regarded as having internal knee derangement or bony injury until proven otherwise.[2]However without instability there is no discriminating symptom to differentiate those with ACL ruptures. Trauma can also result in serous effusions.

An acutely swollen or erythematous knee in the absence of significant trauma invites consideration of septic arthritis, crystal arthritis, haemarthrosis, rheumatoid arthritis, and sero-negative spondyloarthropathies. Diagnostic aspiration is often indicated.

Mechanical Symptoms

Mechanical symptoms include locking, block to extension, popping, clicking, giving way, and snapping.

Locking is thought to be due to impingement of a structure that is abnormally located between the joint surfaces with movement. Possible causes are meniscal tears, ACL rupture, loose body, osteochondral lesions, fat pad fibrosis and adhesions.

Block to extension is a non-specific symptom that also suggests some sort of impingement.

Popping is uncommon. Feeling a pop at the time of trauma has no relationship with ACL rupture.

Giving way is thought to be due to internal derangement or muscle weakness. It isn't specific to ACL rupture. Giving way can also occur with reflex muscular action, and patellar subluxation or dislocation.

Snapping can be due to the gracilis and semitendinosus tendons popping over the medial tibial condyle.

There is no clinically significant difference in the prevalence of various symptoms in those with knee pain who have or don't have meniscal tears.

Neurological Features

It is uncommon for neurological conditions to present as primary knee pain. Neurological pain is usually more widely distributed that knee pain associated with mechanical impairments. Neurological pain is usually sharp, lancinating, and shooting in quality.

Injuries to peripheral nerves including popliteal, femoral, sciatic, or local cutaneous nerves can present as knee pain with neurological deficits that correspond to the action of the affected nerve.

Lumbar and sacral nerve root irritation can cause knee pain of a radicular type with or without a neurological deficit.

CRPS can occur in the knee. The most reliable indicator is pain out of proportion to clinical findings. Some do not have vasomotor symptoms.

Prior Knee History

Past knee injury predicts future knee injury. Enquire about type of injury, the duration of disability that occurred, and any rehabilitation that was undertaken. Frequently new onset of knee pain is a result of a complication of an old or concurrent injury.

One example is in those with chronic Achilles tendionopathy who alter their running gait and develop patellofemoral pain.

In those who have had prior surgical repair, the repair can fail leading to recurrence of the original problem such as ACL graft failure.

Also in those with previous surgery, there is often a degree of deconditioning with persistent muscle weakness, which predisposes the individual to new injuries.

Physical Examination

Fractures: The tuning fork test for fracture detection may have some validity for ruling out but not ruling in fractures. Place a 128-Hz tuning fork over the bony prominence distal to the suspect fracture. The vibration of the tuning fork is thought to cause the fracture site to move resulting in significant pain.[3]

Peripheral Vascular Examination: A vascular examination should be performed assessing for distal pulses. If there is suspicion for peripheral vascular disease then perform an ankle brachial pressure index (ABPI) measurement.

Ottawa Knee Rules

Ottawa knee rules are 100% sensitive for fractures. A knee series is required with any of the following findings

  • Age 55 or order OR
  • Isolated tenderness of the patella (no bone tenderness of knee other than patella) OR
  • Tenderness of the head of the fibula OR cannot flex to 90 degrees OR
  • Unable to bear weight both immediately and in the emergency room department for 4 steps (unable to transfer weight twice onto each lower limb regardless of limping)



If there is a large effusion consider aspirating the joint for diagnostic and therapeutic purposes. If a haemarthrosis is present then this is consistent with internal knee derangement. If there is a fat layer on top of the blood (lipohaemarthrosis) then consider fracture.


Plain Films

Differential Diagnosis

Knee Pain Differential Diagnoses

  • Baker's or popliteal cyst
  • Popliteus Tendinopathy
  • Popliteal artery aneurysm
  • Popliteal artery entrapment
  • Posterior capsule tear
  • Posterior cruciate ligament tear
  • Tibiofemoral dislocation



  1. โ†‘ Woolhead et al.. Night pain in hip and knee osteoarthritis: a focus group study. Arthritis care & research 2010. 62:944-9. PMID: 20191575. DOI.
  2. โ†‘ Maffulli et al.. Acute haemarthrosis of the knee in athletes. A prospective study of 106 cases. The Journal of bone and joint surgery. British volume 1993. 75:945-9. PMID: 8245089. DOI.
  3. โ†‘ Toney et al.. Using Tuning-Fork Tests in Diagnosing Fractures. Journal of athletic training 2016. 51:498-9. PMID: 27384015. DOI. Full Text.