The knee is a vulnerable joint. It takes the full weight of the body and is subject to considerable additional forces with movement and activity (for example, running and jumping). Stability is maintained by four ligaments and the muscles around the joint. The knee is susceptible to a variety 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.

Anatomy of the knee

• Joints: there are two joints in the knee:
  • The patellofemoral joint.
  • The 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) - this controls rotational movement and prevents forward movement of the tibia in relation to the femur. It 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) - this prevents forward sliding of the femur in relation to the tibial plateau. It 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 (MCL) - this prevents lateral movement of the tibia on the femur when valgus (away from the midline) stress is placed on the knee. It runs between the medial epicondyle of the femur and the anteromedial aspect of the tibia. It also has a deep attachment to the medial meniscus.
  • Lateral collateral ligament (LCL) - this prevents medial movement of the tibia on the femur when varus (towards the midline) stress is placed on the knee. It runs between the lateral epicondyle of the femur and the 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.

Acute knee injury causing knee pain

There are a number of possible acute knee injuries which can cause knee pain. These may occur in combination depending on the force and mechanism of injury:

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

Other causes of knee pain

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 metacarpophalangeal (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 disease-modifying anti-rheumatic drugs (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
  • Behçet's disease
  • 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 most common 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'. This 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, non-steroidal anti-inflammatory drugs (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): this is 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 the 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 upper (capital) femoral epiphysis.
• Perthes' disease.
• Tumour.
• Prepatellar bursitis or infrapatellar bursitis.
• Patellar stress fracture.
• Osgood-Schlatter's disease.
• Sinding-Larsen Johansson disease.

Lateral knee pain⁴

Common causes include:

• Iliotibial band friction syndrome: this occurs due to friction between the iliotibial band and the underlying lateral epicondyle of the femur. It 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. it is 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 upper (capital) femoral epiphysis.
• Perthes' disease.
• Patellofemoral syndrome (see 'Anterior knee pain', above).
• OA.
• Referred pain from lumbar spine.

Medial knee pain⁴

Common causes include:

• Patellofemoral syndrome (see 'Anterior knee pain', above).
• Medial meniscus problem (tear, degeneration, cyst).

Other causes include:

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

Posterior knee pain⁴

Common causes include:

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

Other causes include:

• Baker's cyst.
• Deep vein thrombosis.
• Peripheral vascular disease.
• Posterior cruciate ligament (PCL) injury.

Assessment of knee pain

• A careful history and examination may give an indication as to the cause of knee pain. Please refer to the 'History' and 'Examination' sections in the separate article Knee Assessment for further details.
• 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 separate 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 (ACL) reconstruction undertaken.

Management

This should be appropriate to the underlying cause.

Prevention

• 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 (OA).
• Always encourage warm-up before exercise. The evidence base to support warm-up may not be strong but it is traditional good advice.^6,7

Document references

1. Mehlman CT et al, Osteosarcoma, Medscape, Apr 2010
2. Goyal S, Roscoe J, Ryder WD, et al; Symptom interval in young people with bone cancer. Eur J Cancer. 2004 Oct;40(15):2280-6. [abstract]
3. Clinical Sports Medicine. Revised second edition. McGraw-Hill. Chapter 24. Anterior Knee Pain
4. Clinical Sports Medicine. Revised second edition. McGraw-Hill. Chapter 25. Lateral, Medial and Posterior Knee Pain
5. Krivickas LS; Anatomical factors associated with overuse sports injuries. Sports Med. 1997 Aug;24(2):132-46. [abstract]
6. Pope RP, Herbert RD, Kirwan JD, et al; A randomized trial of preexercise stretching for prevention of lower-limb injury. Med Sci Sports Exerc. 2000 Feb;32(2):271-7. [abstract]
7. Hart L; Effect of stretching on sport injury risk: a review. Clin J Sport Med. 2005 Mar;15(2):113. [abstract]

Internet and further reading

• Knee Joint Menu, Wheeless' Textbook of Orthopaedics

Acknowledgements