Knee pain needs a clear pathway.

A whole-joint disease involving gradual change in cartilage, subchondral bone, synovium and surrounding structures. Radiographic change does not always match pain intensity.

• Gradual pain aggravated by walking, stairs or loading
• Brief morning or post-rest stiffness
• Swelling, reduced movement and difficulty rising from a chair
• Crepitus and, in advanced disease, altered knee alignment

Pain around or behind the kneecap associated with loading the patellofemoral joint. It is not synonymous with chondromalacia and can occur without visible cartilage damage.

• Anterior knee pain with stairs, running, jumping or squatting
• Pain after prolonged sitting with the knee bent
• Tenderness around the kneecap
• Crepitus may accompany movement but is not itself a diagnosis

Injury to articular cartilage, with or without involvement of the underlying bone. It may follow trauma, repetitive loading or osteochondritis dissecans.

• Deep joint pain during loading
• Recurrent swelling after activity
• Catching or locking when a fragment is unstable
• Restricted movement or difficulty with sport

A traumatic or degenerative tear of the medial or lateral meniscus. Traumatic tears often follow loaded rotation; degenerative changes may develop gradually and may also be asymptomatic.

• Medial or lateral joint-line pain
• Swelling that may develop several hours after injury
• Catching, painful clicking or locking
• Pain with squatting, rotation or change of direction
• Inability to fully straighten the knee with some displaced tears

A sprain or partial or complete tear of the ligament that limits anterior tibial translation and contributes to rotational stability. It often occurs during pivoting, deceleration or landing.

• A pop or popping sensation at injury
• Rapid swelling within the first hours
• Giving way or instability, especially during direction changes
• Loss of movement and inability to continue activity

Injury to the ligament limiting posterior tibial translation, often caused by a blow to the front of the shin with the knee bent or by hyperextension.

• Pain and swelling after trauma
• Pain at the back of the knee
• Difficulty descending stairs or decelerating
• Instability in larger or combined injuries

A sprain or tear of the ligament on the inner knee, usually caused by valgus force or contact to the lateral knee.

• Medial knee pain and tenderness
• Local swelling
• Pain with valgus load or direction change
• Medial instability in high-grade injury

Injury to the lateral collateral ligament and/or posterolateral stabilisers. It may follow varus force, hyperextension or high-energy trauma and often coexists with other ligament injury.

• Lateral or posterolateral pain and tenderness
• Swelling and difficulty weight-bearing
• Giving way, particularly during direction changes
• Foot numbness or weakness when the peroneal nerve is involved

Injury to two or more major ligaments, sometimes with joint dislocation. Blood vessels and nerves may be threatened, so urgent assessment is required.

• Marked deformity or instability after trauma
• Severe pain and swelling
• Inability to bear weight
• A cold or discoloured foot, weak pulse, numbness or weakness below the knee

A load-related condition of the patellar tendon, common in sports involving jumping and acceleration. It is not simply inflammation and usually develops gradually.

• Local pain at the lower pole of the patella or along the tendon
• Pain with jumping, landing, sprinting or squatting
• Pain or stiffness at the start of activity
• Focal tendon tenderness

A load-related problem of the quadriceps tendon above the kneecap, often seen with high volumes of jumping, running or strength work.

• Pain above the kneecap at the tendon attachment
• Pain with squats, stairs, jumping or resisted extension
• Stiffness after rest
• Focal tenderness above the upper patellar pole

A partial or complete disruption of the knee extensor mechanism. A complete rupture interrupts active knee extension and needs prompt orthopaedic assessment.

• A pop with immediate pain and swelling
• Inability to actively straighten the knee or perform a straight-leg raise
• A palpable gap above or below the kneecap
• Knee buckling during weight-bearing

The kneecap moves partly or completely out of the trochlear groove, usually laterally. It may be a first traumatic episode or recurrent instability influenced by anatomy and functional control.

• Feeling the kneecap shift or about to dislocate
• Visible deformity during acute dislocation
• Pain and swelling, often medially after reduction
• Apprehension during running or direction changes

Load-related lateral knee pain associated with repeated knee movement, commonly in runners and cyclists.

• Local lateral knee pain
• Pain beginning after a predictable running distance or duration
• Worse symptoms downhill or with repeated bending and straightening
• Occasional lateral snapping sensation

Irritation or inflammation of bursae over the kneecap or at the anteromedial tibia. It may relate to repeated pressure, impact, overload and, less commonly, infection.

• Visible swelling over the kneecap in prepatellar bursitis
• Anteromedial tenderness below the joint line in pes anserine bursitis
• Pain with kneeling, stairs or pressure
• Redness, heat and fever when infection is suspected

Irritation of the medial synovial fold or Hoffa fat pad, sensitive structures at the front of the knee. Diagnosis requires exclusion of more common causes.

• Anteromedial or infrapatellar pain
• Painful catching or clicking during bending and straightening
• Pain with prolonged standing, extension or squatting
• Tenderness near the patella or patellar tendon

Growth-related traction apophysitis at the tibial tubercle or lower patellar pole. These conditions affect active children and adolescents.

• Local pain below the knee with running, jumping or kneeling
• Tenderness and swelling at the tibial tubercle or lower patellar pole
• Improvement with reduced load and recurrence with activity
• A prominent tibial tubercle in Osgood–Schlatter disease

A collection of synovial fluid behind the knee, often secondary to an intra-articular condition such as osteoarthritis, inflammation or meniscal injury.

• Fullness or a lump behind the knee
• Tightness and restricted bending
• Pain after activity or prolonged standing
• Calf swelling after rupture, which needs urgent differentiation from thrombosis

Fractures of the patella, femoral condyles or tibial plateau after trauma. Fragility fractures may occur after minor loading when bone is weakened.

• Severe pain and swelling after trauma
• Inability or marked difficulty bearing weight
• Bruising, deformity or focal bone tenderness
• Inability to extend the knee in some patellar fractures

Joint inflammation caused by rheumatological disease, crystal deposition (as in gout), or infection. Septic arthritis is a medical emergency.

• Knee swelling, heat, pain and restricted movement
• Sudden severe onset in crystal or septic arthritis
• Redness and fever, especially with infection
• Other joints or prolonged morning stiffness in inflammatory disease

Pain perceived at the knee while the source is the hip, lumbar spine or a peripheral nerve. Local knee examination may be relatively normal.

• Knee pain accompanied by hip, back or thigh symptoms
• Numbness, tingling or weakness not matching a local knee structure
• Symptoms changing more with hip or spinal movement than knee movement
• In children, limping or knee pain without clear local findings warrants hip assessment