Overview Knee pain accounts for approximately one third of musculoskeletal problems seen in primary care settings. This complaint is most prevalent in physically active patients. Knee pain can be a source of significant disability, restricting the ability to work or perform activities of daily living. Chronic knee pain affects one in four people aged ≥55 years. Usually symptoms are mild to moderate. Osteoarthritis, presenting as activity related pain and limitation of movement, crepitus, and intermittent swelling in the absence of constitutional symptoms, is the commonest working diagnosis.

Classification

• Acute vs Chronic
• Congenital vs Acquired
• Traumatic vs Atraumatic

Differential Diagnosis

The next section will focus of the differential diagnosis of knee pain and will be divided into the following:

• Generalised Knee Pain
• Knee pain based on location
• Other causes of knee pain
• Common causes of knee pain by age group

Generalised Knee Pain

Acute knee pain (with swelling)

• Traumatic synovitis can occur from any severe injury
• Post-traumatic haemarthrosis is blood in the joint from trauma, resulting in intense swelling immediately after injury.
• Non-traumatic haemarthrosis is blood in the joint from clotting disorders or from tears from vascular lesions in the knee (i.e pigmented villonodular synovitis)
• Acute septic arthritis is the presence of bacteria in the synovial joint.
• Crystal arthropathy
  • Gout or Pseudogout
• Reactive arthritis -  also known as Reiter’s disease (triad: can't pee; can't see; can't climb a tree which means urethritis; conjunctivitis; arthritis)

Chronic knee Pain (with swelling)

• Osteoarthritis is a inflammatory condition due to degeneration of joint tissue
• Rheumatoid Arthritis is an systemic inflammatory disease
• Chronic infective arthritis - the most important condition to exclude is tuberculosis.

Knee Pain based on Location

Frontal Knee pain

• Prepatellar bursitis This fluctuant swelling is confined to the front of the patella and the joint itself is normal. It is an uninfected bursitis due to constant friction between skin and bone. As such, it is seen mainly in carpet layers, paving workers, floor cleaners and miners who do not use protective knee pads. Treatment consists of firm bandaging, and kneeling is avoided; occasionally aspiration is needed. In chronic cases the lump is best excised.
• Infrapatellar bursitis The swelling is below the patella and superficial to the patellar ligament, being more distally placed than prepatellar bursitis. Treatment is similar to that for prepatellar bursitis

• Quadriceps tendonitis
• Patella disorders
  • Patellar tendinopathy (Jumper's Knee)
  • Recurrent dislocation of the patella
  • Chondromalacia of the patella (Patellofemoral pain syndrome)
• Osgood-Schlatter's Disease

Posterior Knee Pain

• Semimembranosus bursa (Pes anserinus pain syndrome) The bursa between the semimembranosus and the medial head of gastrocnemius may become enlarged in children or adults. It presents usually as a painless lump behind the knee, slightly to the medial side of the midline and is most conspicuous with the knee straight. The lump is fluctuant and transilluminates. The knee joint is normal. A waiting policy, even if the lump causes an ache, is wise as it usually disappears with time.
• Baker's Cyst is a bulging of the posterior capsule and synovial herniation may produce a swelling in the popliteal fossa. It is usually caused by rheumatoid or osteoarthritis.Occasionally the ‘cyst’ ruptures and the synovial contents spill into the muscle planes causing pain and swelling in the calf – a combination which can easily be mistaken for deep vein thrombosis. The swelling may diminish following aspiration and injection of hydrocortisone; excision is not advised, because recurrence is common unless the underlying condition is treated.

Lateral Knee pain

• Lateral Meniscal tears
• Lateral collateral ligament injury
• Meniscal cyst This presents as a small, tense swelling, usually on the lateral side at or just below the joint line. Sometimes it is so tense that it can easily be mistaken for a bony lump. It is usually tender on pressure.
• Bony swellings Bony lumps (exostosis or bone tumours) arising in the metaphyses of the distal femur or proximal tibia may cause visible and palpable swelling on any aspect close to the joint. The diagnosis is revealed by x-ray examination and, if necessary, biopsy
• Illiotibial band syndrome

Medial Knee Pain

• Medial Meniscal tears
• Medial collateral ligament injury
• Meniscal cyst This presents as a small, tense swelling, usually on the lateral side at or just below the joint line. Sometimes it is so tense that it can easily be mistaken for a bony lump. It is usually tender on pressure.
• Bony swellings Bony lumps (exostosis or bone tumours) arising in the metaphyses of the distal femur or proximal tibia may cause visible and palpable swelling on any aspect close to the joint. The diagnosis is revealed by x-ray examination and, if necessary, biopsy
• Semimembranosus bursa (Pes anserinus pain syndrome) 
• Medial plica syndrome

Other causes of knee pain

• Loose bodies The knee joint offers a relatively capacious haven for loose bodies. These may be produced by
  • Injury (a chip of bone or cartilage)
  • Osteochondritis dissecans (which may produce one or two fragments)
  • Osteoarthritis (pieces of cartilage or osteophyte)
  • Charcot’s disease (large osteocartilaginous bodies, separated by repeated trauma in a joint that has lost protective sensation)
  • Synovial chondromatosis (cartilage metaplasia in the synovium, sometimes producing hundreds of loose bodies).
• Osteochondritis dissecans
• Stress fracture
• Referred pain syndrome (Usually from the hip)
  • Capital femoral epiphysis
  • Legg-Calve-Perthes disease)

Investigations depends on the history and examination and you differential diagnosis

• X-ray (at least 2 planes)
• CT scan
• FBC
• ESR/CRP
• Joint Aspiration
• Rheumatoid factor