Knee Pain

The knee is a vulnerable joint. It takes the full weight of the body and undergoes considerable stress in activities such as running and jumping. Stability is maintained by four ligaments and the muscles around the joint. It can be susceptible to a number of problems which can lead to pain.

The causes of a swollen knee are discussed in the separate article Knees That Swell. A swollen knee may also be painful.

• Joints: there are two joints in the knee:
  • Patellofemoral joint
  • Tibiofemoral joint (the joint that is usually referred to as 'the knee joint')
• Patella: the patella slides over the anterior surface of the knee in a sesamoid joint. The patellar tendon (also called patellar ligament) passes anteriorly to the patella. The medial retinaculum also gives support to the patella.
  
  
• Ligaments: stability to the tibiofemoral joint is provided by various ligaments:
  • Anterior cruciate ligament (ACL) - controls rotational movement and prevents forward movement of the tibia in relation to the femur. Runs between attachments on the front (hence anterior cruciate) of the tibial plateau and the posterolateral aspect of the intercondylar notch of the femur.
  • Posterior cruciate ligament (PCL) - prevents forward sliding of the femur in relation to the tibial plateau. Runs between attachments on the posterior part (hence posterior cruciate) of the tibial plateau and the medial aspect of the intercondylar notch of the femur.
  • Medial collateral ligament - prevents lateral movement of the tibia on the femur when valgus (away from the midline) stress is placed on the knee. Runs between medial epicondyle of the femur and the anteromedial aspect of the tibia. Also has a deep attachment to the medial meniscus.
  • Lateral collateral ligament - prevents medial movement of the tibia on the femur when varus (towards the midline) stress is placed on the knee. Runs between lateral epicondyle of the femur and head of the fibula.
• Menisci: the medial and lateral menisci are located within the knee joint, attached to the tibial plateau. They help to protect the articular surfaces by absorbing some of the forces transmitted through the knee. They also help to stabilise and lubricate the knee.

Common acute knee injuries that can lead to knee pain include:

• Medial or lateral meniscal injury
• Medial or lateral collateral ligament injury
• Anterior cruciate ligament injury
• Posterior cruciate ligament injury
• Acute patellar trauma: fracture or dislocation
• Patellar tendon rupture

If there is no history of acute injury/trauma, one way of assessing knee pain is by determining the site of the pain and then looking at differential diagnoses for pain in that area. So, knee pain can be divided into:

• Global knee pain
• Anterior knee pain
• Medial knee pain
• Lateral knee pain
• Posterior knee pain

Global knee pain

The causes of global pain in the knee can be divided into different aetiologies:

• Primary arthritis in the knee
  • Osteoarthritis (OA): tends to be slow and insidious and occurs in the elderly although it can affect younger people, especially if obese. Crepitus is common. X-ray may show reduction of the joint space because of loss of cartilage. There is a separate article entitled Osteoarthritis.
  • Rheumatoid arthritis (RA): tends to be faster in onset and there will almost certainly be other joints involved, especially the MCP joints. Morning stiffness is marked. Indices of inflammation such as ESR and CRP are often raised but positive rheumatoid factor is a later feature. If in doubt refer, as early treatment with DMARDs reduces long-term joint damage. In childhood, RA can present as Still's disease. There is a separate article entitled Rheumatoid Arthritis.
  • Crystal arthropathies: a rapid onset of knee pain is typical of gout and pseudogout. Gout does not often affect the knee initially but pseudogout can. Diagnosis is by demonstration of crystals in fluid aspirated from the knee. There are separate articles entitled Gout and Pseudogout.
• Seronegative arthropathies
  There is a separate article entitled Seronegative Arthropathies. There may also be signs or symptoms of other underlying diseases. Seronegative arthropathies include:
  • Ankylosing spondylitis
  • Reiter's syndrome
  • Enteropathic arthritis
  • Psoriatic arthritis
  • Behcet's syndrome
  • Juvenile idiopathic arthritis
• Infective causes
  • Septic arthritis: the patient will be systemically unwell. Fluid should be aspirated from the knee and sent for culture. Antibiotics should be started. A hot, red joint is usually septic arthritis, gout or pseudogout. It may sometimes occur in psoriatic arthropathy and is rare in RA.
  • Osteomyelitis: can affect the knee. There is a separate article entitled Osteomyelitis.
• Rare diseases causing knee pain
  • Haemochromatosis: this can lead to knee pain.
  • Rheumatic fever: now rare in the UK. Arthritis occurs in 80% of patients. It usually involves multiple large joints, particularly the knees, ankles, elbows, and wrists and flits from site to site.
  • Spontaneous haemarthrosis: may occur in coagulation disorders, especially haemophilia.
  • Familial Mediterranean Fever: recurrent episodes of joint pain occur. Knees, ankles and wrists are most commonly affected. Joints are normal between attacks. Around 2% develop chronic arthritis that can lead to joint destruction.
• Disease of bone around the knee
  • Osteosarcoma: usually affects children. The commonest sites are around the knee or proximal humerus. The most frequent presenting symptom of osteosarcoma is pain, especially with activity.¹ There is often delay in making the diagnosis although it seems to have little effect on outcome.² There is a separate article entitled Bone Tumours.
• Referred pain (usually from the hip)

Anterior knee pain³

Common causes include:

• Patellofemoral syndrome: also known as chondromalacia patellae. There is pain in and around the patella. There is a separate article entitled Chondromalacia patellae.
• Patellar tendinopathy: used to be known as "jumper's knee". Affects young athletes in sports that require repetitive running, kicking, and jumping but can occur in any sportspeople. There is pain at the inferior pole of the patella during activities such as climbing stairs, running uphill, and jumping. Treatment is rest, activity modification, NSAIDs and a strengthening exercise programme for the calf, quadriceps and gluteal muscles. Rarely, surgery is needed.
• Fat pad impingement: the infrapatellar fat pad is impinged between the patella and the femoral condyle due to a direct blow to the knee. Treatment includes patellar taping to relieve impingement.
• Patellofemoral instability (or recurrent patellar subluxation): more common in females, patellar hypermobility with apprehension and pain when the patella is pushed laterally are found on examination. Treatment can include bracing and crutches to reduce weight-bearing. Exercises to strengthen vastus medialis obliquus are needed. Surgery may be required if conservative management fails.

Other causes can include:

• Referred pain from the hip
• Osteochondritis dissecans
• Slipped capital (upper) femoral epiphysis (See our dedicated record)
• Perthes' disease
• Tumour
• Prepatellar bursitis or infrapatellar bursitis
• Patella stress fracture
• Osgood-Schlatter's disease
• Sinding-Larsen-Johansson syndrome

Lateral knee pain⁴

Common causes include:

• Iliotibial band friction syndrome: occurs due to friction between the iliotibial band and the underlying lateral epicondyle of the femur. Produces lateral knee pain in cyclists, dancers, long-distance runners, football players, and military recruits. There is tenderness over the lateral epicondyle of the femur 1-2 cm above the lateral joint line. Flexion/extension of the knee can reproduce symptoms. It is more likely with poor muscles, lax ligaments and poor training regimes.⁵ Treatment includes NSAIDs, massage, stretching, muscle strengthening and correction of predisposing factors (e.g. more common in downhill running). Steroid injection and surgery are rarely needed.
• Lateral meniscus problem (tear, degeneration, cyst)

Other causes include:

• Common peroneal nerve injury
• Slipped capital (upper) femoral epiphysis
• Perthes' disease
• Patellofemoral syndrome (see above)
• Osteoarthritis
• Referred pain from lumbar spine

Medial knee pain⁴

Common causes include:

• Patellofemoral syndrome (see above)
• Medial meniscus problem (tear, degeneration, cyst)

Other causes include:

• Tumour
• Slipped capital (upper) femoral epiphysis
• Perthes' disease
• Referred pain from hip/lumbar spine
• Medial collateral ligament injury
• Osteoarthritis

Posterior knee pain⁴

Common causes include:

• Knee joint effusion
• Referred pain from lumbar spine or patellofemoral joint

Other causes include:

• Baker's cyst
• Deep vein thrombosis
• Peripheral vascular disease
• Posterior cruciate ligament injury

• A careful history and examination may give an indication as to the cause of knee pain. Please refer to the separate article Knee Assessment (History and Examination) for further details, including how to examine the knee.
• Investigations should be appropriate to the suspected cause and are discussed in the same article and also in the individual articles according to suspected diagnosis.
• Diagnostic knee joint aspiration is discussed in the article Knee Injections and Aspirations.
• Arthroscopy is a minimally invasive procedure that can be used for both diagnosis and sometimes treatment. Torn cartilage can be removed and even anterior cruciate ligament reconstruction undertaken.

This should be appropriate to the underlying cause.

• Strong muscles around the knee give good protection against injury and therefore help to prevent some causes of knee pain.
• Physical activity and the avoidance of obesity delay the onset of osteoarthritis.
• Always encourage warm-up before exercise. The evidence base to support warm-up may not be strong^6,7 but it is traditional good advice.