There are some serious and possibly life-threatening causes of knee problems that result in swelling - especially in combination with other symptoms.
Serious Swelling

Here are types of knee swelling that are potential medical emergencies - they require urgent diagnosis and proper treatment:

1. You suffer a knee injury or injuries and the joint swells up like a balloon

Those types of swelling are commonly caused by:

• fracture of the thigh bone (femur), leg bones (tibia, fibula) and knee cap (patella)
• dislocated kneecap (patella)
• tear in the joint covering (synovial membrane)
• tear in the cruciate ligaments, either anterior (ACL) or posterior (PCL)
• Osteochondritis Dessicans
• patellar tendon tear (Osgood-Schlatter disease in teenagers and kids) with possible tendon rupture
• quadriceps tendon tear or rupture
• quadriceps or hamstring muscles tear

A large swollen knee is usually a serious symptom. You should seek immediate medical help and get the proper treatment for any knee injury.

Mild Swelling

Mild swelling is usually the result of extra joint fluid caused by inflammation (not blood) from knee problems such as:

• Osteoarthritis (common knee arthritis) from degenerative joint disease
• Pre-arthritis, also called Patellar Femoral Syndrome (PFS) and Chondromalacia
• damaged cartilage, such as medial or lateral meniscus tears
• quadriceps tendonitis

Symptom #2: Locking This is when your knee gets stuck in one position and won't move. There are two types of joint locking - Real and Pseudo (fake).

Real Locking In Real Locking, your knee joint gets stuck and will not flex or extend until you deliberately move or manipulate it. If you can't unlock your knee, get to a qualified medical doctor immediately for evaluation and proper knee injury treatment.
Real Locking is caused by damaged cartilage, usually from either a medial or lateral meniscus tear; or a loose body getting lodged in between your thigh bone (femur) and your leg bone (tibia). This is why you need to manipulate your joint until the cartilage or loose body gets unstuck. Real Locking is potentially damaging to your knee cartilage and may cause early Osteoarthritis (common wear-and-tear knee arthritis).

Pseudo Locking In Pseudo Locking, your knee feels stuck but will move without manipulation - usually unlocking slowly with some knee pain. This knee pain symptom usually stems from damaged, rough or arthritic cartilage located on the backside of the knee cap (patella). Pseudo Locking is typical of Osteoarthritis (common knee arthritis) and Pre-arthritis (also called Chondromalacia and Patellar Femoral Syndrome - PFS).

Symptom #3: Aching and Pain

Pain Location The knee pain location is not quite as helpful for diagnosis as you would think - some tissues in the knee have poor nerve supply. Pain can therefore be felt in one place while your actual knee injury, or knee problems, are really somewhere else. For example, knee pain on the inside (medial) may be from local structures such as the medial collateral ligament or the medial meniscus - but, that is also a common place to feel referred pain that is actually coming from the back of the knee cap (patella).

Pain on the inside, outside, around, below or behind the kneecap (patella) and the back of your knee ... could be from any of the following serious causes:

• bone fracture
• knee cap (patella) dislocation
• a tear in the joint covering (synovial membrane)
• a tear in the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL)
• Osteochondritis Dessicans
• patellar tendon tear
• quadriceps tendon tear

Where is your pain? Inside (Medial) Knee Pain Inside, or medial pain, is a common location. Causes include:

• medial collateral ligament sprain or tear
• medial meniscus injury with tear
• knee Osteoarthritis (common knee arthritis)
• Pre-arthritis (aka Patellar Femoral Syndrome - PFS, Chondromalacia and Runner's Knee
• fracture of the leg bone (tibia) or thigh bone (femur)
• dislocation of the knee cap (patella)

Outside (Lateral) Knee Pain This is a less-common location. Causes include:

• lateral collateral ligament sprain or tear
• lateral meniscus injury with tear,
• common knee arthritis (Osteoarthritis)
• Pre-arthritis (Patellar Femoral Syndrome, Chondromalacia, and Runners Knee)
• a fracture of the leg bone (tibia) or thigh bone (femur) or fibular head
• Iliotibial Band Syndrome

Pain Around, Below or Behind the Kneecap (Patella) This location is quite deceptive because the kneecap is poorly innervated. As a result, you may feel referred pain somewhere else, other than in the real spot. Common causes include:

• common knee arthritis (Osteoarthritis)
• Pre-arthritis (Patellar Femoral Syndrome - PFS Chondromalacia and Runners knee)
• patella fracture
• patella dislocation
• patellar tendonitis
• Bursitis

Pain at the Back of the Knee This is a less-frequent location for pain and swelling. Common causes include:

• Baker's Cyst
• popliteus muscle tear
• lateral meniscus tear, usually posterior horn
• medial meniscus tear, usually posterior horn
• plantaris tear with rupture
• common knee arthritis (Osteoarthritis)

Knee Pain on Stairs, Standing, Sitting This is quite common and classic for the following conditions:

• Osteoarthritis (common knee arthritis)
• Pre-arthritis (Patellar Femoral Syndrome - PFS, Chondromalacia and Runners Knee)

Knee Pain That Worsens with Activity - especially with running

• Osteoarthritis (common knee arthritis) with articular cartilage damage
• Pre-arthritis (Patellar Femoral Syndrome - PFS, Chondromalacia, Runners Knee)
• tear of the medial or lateral meniscus
• Iliotibial Band Syndrome
• Bursitis
• chronic sprain or tear of the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), with instability
• Osteochondritis Dessicans
• chronic medial collateral ligament sprain or tear
• chronic lateral collateral ligament sprain or tear

Symptom #4: Buckling

Knee Buckling This means you take a step and your knee buckles under you. Buckling, like locking, can be Real or Pseudo (fake).

Real Buckling This is usually from a complete anterior cruciate ligament tear with instability. The ACL holds your knee bones together when you move. If your ACL is torn, the thigh bone (femur) can slide over top of the leg bone (tibia), causing you to buckle and fall.

Pseudo Buckling This is when your knee buckles from pain, not instability. Usually, pseudo buckling is from:

• Osteoarthritis (common knee arthritis)
• Pre-arthritis (Patellar Femoral Syndrome - PFS, Chondromalacia, Runners Knee)

Those are the four common knee injury symptoms (swelling, locking, pain and buckling) with their associated diagnosis. The next step is to confirm your diagnosis with testing, if needed.

Hopefully, the above information will help you find an answer for your knee pain.