Anatomy of the knee

• 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 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 (MCL) - prevents lateral movement of the tibia on the femur when valgus (away from the midline) stress is placed on the knee. Runs between the medial epicondyle of the femur and the anteromedial aspect of the tibia. Also has a deep attachment to the medial meniscus.
  • Lateral collateral ligament (LCL) - prevents medial movement of the tibia on the femur when varus (towards the midline) stress is placed on the knee. Runs between the 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.

History

• Pain (see 'Causes of knee pain', below).
• A 'popping' or 'snapping' sound may suggest rupture of a ligament.
• Swelling: rapid swelling (0-2 hours) suggests haemarthrosis which may, for example, be due to anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) rupture, and patellar dislocation. Gradual swelling (6-24 hours) suggests an effusion which may be due to meniscal injury. Swelling over a 24-hour period, with no history of trauma, could be due to septic arthritis or an inflammatory arthritis. See also separate article Knees That Swell.
• Locking or clicking suggests a loose body and may be due to meniscal injury.
• The knee giving way suggests instability (eg ACL injury) or muscle weakness.
• Also ask about previous history of knee injury, other past medical history, occupation and level of exercise.

Causes of knee pain

Acute knee injury

• Knee cartilage injuries: medial or meniscal injury.
• Knee ligament injuries: medial collateral ligament (MCL), lateral collateral ligament (LCL), posterior cruciate ligament (PCL), anterior cruciate ligament (ACL).
• Leg fractures and dislocations: knee fractures and dislocations, distal femoral fractures, proximal tibial and fibular fractures.
• Patellar tendon rupture.

Global knee pain

• Monoarthritis.
• Polyarthritis: osteoarthritis, rheumatoid arthritis.
• Crystal arthropathies: gout, pseudogout.
• Seronegative arthropathies, eg ankylosing spondylitis, Reiter's syndrome, enteropathic arthritis, psoriatic arthritis, Behçet's disease, juvenile idiopathic arthritis.
• Infective causes: septic arthritis, osteomyelitis.
• 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] See separate article Bone Tumours.
• Referred pain (usually from the hip).
• Rare causes, eg haemochromatosis, rheumatic fever, spontaneous haemarthrosis (may occur in coagulation disorders, especially haemophilia), familial Mediterranean fever.

Anterior knee pain

Common causes include:

• Patellofemoral pain syndrome (also known as chondromalacia patellae).
• 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 include:

• Referred pain from the hip, eg slipped upper (capital) femoral epiphysis, Perthes' disease.
• Osteochondritis dissecans.
• Bone tumour.
• Prepatellar bursitis or infrapatellar bursitis.
• Patellar stress fracture.
• Osgood-Schlatter disease.
• Sinding-Larsen Johansson disease.

Lateral knee pain

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. Treatment includes non-steroidal anti-inflammatory drugs (NSAIDs), massage, stretching, muscle strengthening and correction of predisposing factors (eg 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, patellofemoral syndrome, osteoarthritis, referred pain from the hip or the lumbar spine.

Medial knee pain

Common causes include:

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

Other causes include: tumour, referred pain from the hip or the lumbar spine, MCL injury, osteoarthritis.

Posterior knee pain

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, PCL injury.

Examination

Examination of gait

Always remember to watch the patient in standing and in walking.

Get the patient to lie comfortably on a couch. Pain or apprehension will make examination difficult. Always examine and compare both knees.

Inspection

• Look at the patient: a pyrexial and unwell patient may have septic arthritis.
• Look at the joint: is it swollen, red, or hot?
• Examine for muscle wasting: compare with the other side.

Examine for an effusion

• This is unnecessary if swelling is gross.
• Eliciting a patellar tap: extend the knee and empty the suprapatellar pouch by applying pressure from the palm of your hand above the knee. This will push fluid underneath the patella, lifting it. Maintain this pressure. Next, press down on the patella with the fingers of the other hand and the patella will be felt to move down and touch ('tap') the underlying bone if an effusion is present.
• The massage (bulge) test: with the knee in extension, use the palm of your hand to massage any fluid in the anteromedial compartment of the knee into the suprapatellar pouch. Next, stroke the lateral side of the joint and the lateral side of the suprapatellar pouch. This will push any fluid present back into the anteromedial compartment. Look for a fluid impulse.

Examine for tenderness

Palpation should include:

• The medial and lateral joint line - palpate with the knee in 30° flexion.
• The patellofemoral joint.
• The medial collateral ligament (MCL) and the lateral collateral ligament (LCL).
• The popliteal fossa. This may be easier with the patient supine. Look for Baker's cyst, deep vein thrombosis, gastrocnemius pathology, popliteal artery aneurysm.^[2]

Examine range of movement

• Examine active and passive flexion and extension.
• The examining hand on the kneecap may detect crepitus. This usually suggests osteoarthritis but is also found in chondromalacia patellae.
• Full range of movement is from 3° of hyperextension to 140° of flexion.^[2] For most activities of daily living 115° of flexion is needed.^[2]
• Always compare both knees.
• Fixed flexion deformities can be due to torn cartilage or loose body.

Examine stability

MCL and LCL

The valgus and varus stress tests can be used.^[3]

• Flex the knee by 30°.
• Hold the ankle firmly between your arm and your side.
• With your other hand putting pressure above the knee, attempt to adduct and abduct the knee joint.
• More than minimal movement is abnormal.

Anterior cruciate ligament (ACL)

The anterior drawer test:

• Flex the knee to 90°.
• Hold the position by sitting on the patient's foot.
• Ensure that the hamstring muscles are relaxed.
• With both hands, grasp below the knee and pull the tibia forward.
• Compare the degree of movement with the other side.
• Excessive movement may indicate ACL disruption.

Lachman's test:

• Flex the knee to 15-20°.
• Hold the lower thigh in one hand and the upper tibia in the other.
• Push the thigh in one direction and pull the tibia in the other.
• Reverse the direction, pushing the tibia and pulling the thigh, and look for increased movement or laxity between the tibia and the femur.

Pivot shift test:
This test is difficult to perform and is generally not recommended for use by GPs.^[3]

• Hold the patient's heel with one hand.
• Internally rotate the foot and the tibia and, at the same time, apply an abduction (valgus) force at the knee.
• Flex the knee from 0° to 30° whilst applying this force and still holding the foot and tibia in internal rotation.
• Try to detect any palpable or visible reduction between the femur and the tibia.

Posterior cruciate ligament (PCL)

Posterior drawer test:

• Perform the same examination as the anterior drawer test but pushing backwards in relation to the tibia instead of pulling forwards.
• Compare the degree of movement with the other side.

Posterior sag test:

• Flex both knees to 90°.
• Look at the position of the tibia in relation to the femur.
• If there is rupture of the PCL, the position will be relatively posterior.

Other tests

McMurray's test for meniscal injury

This test is no longer recommended because the diagnostic accuracy is low and it is thought that it may exacerbate the injury.^[3]

• Flex the patient's hip and knee to 90°.
• Hold the heel with the right hand and steady the knee with the left.
• Slowly extend the knee, using the right hand and, at the same time, palpate the joint line with the left hand. Perform this with the tibia in external and then internal rotation at the various stages of flexion.
• A positive test is when a 'clunk' is felt with associated pain.

Patellar apprehension test to assess stability of the patella

• The patient should be lying on their back with the knee extended.
• Apply pressure to the medial side of the patella.
• Keep this pressure applied whilst passively flexing the knee to 30°.
• Look for any lateral patellar movement and any 'apprehension' from the patient.

Investigations

• Knee joint aspiration can be both diagnostic and therapeutic. See separate article Joint Injection and Aspiration.
• X-ray may reveal fracture of any of the bones or the calcium pyrophosphate crystals of pseudogout.
• Damage to cartilage or ligaments can be demonstrated by MRI or arthroscopy.
• The Direct Access Magnetic resonance imaging: Assessment for Suspect Knees (DAMASK) trial looked at the influence of early access to MRI of the knee, compared with referral to an orthopaedic specialist, on GPs' diagnoses and treatment plans for people with knee problems. The trial found that access to MRI did not significantly alter their diagnoses or treatment plans but it did significantly increase their confidence in these decisions.^[4]