Overview
Knee language can be confusing — the same term might describe a structure, a movement or an injury pattern. This glossary collects the terms you will see across InjuryAtlas's knee pages and explains each one in plain English, alphabetically, so you never have to guess what a word means mid-article.
Each definition is short on purpose. Follow the links for the full picture on any structure or condition.
A
ACL (anterior cruciate ligament): One of the main ligaments inside the knee, crossing through the centre of the joint. It helps control forward movement and rotation of the shinbone, and it is commonly injured during pivoting, cutting or awkward landings. See the ACL anatomy page.
Articular cartilage: The smooth, white tissue covering the ends of the bones inside a joint. It lets the bones glide past each other and is the tissue that wears down in osteoarthritis.
Avulsion: An injury where a tendon or ligament pulls a small piece of bone away from its attachment.
B
Baker's cyst: A fluid-filled sac at the back of the knee, usually a sign that extra joint fluid is backing up, often because of arthritis or a meniscus problem.
Bipartite patella: A kneecap that has an extra bone fragment that never fused during childhood. It is harmless and can look like a fracture on X-ray.
Bursa: A small, fluid-filled sac that reduces friction between moving structures, such as between skin and bone or tendon and bone.
Bursitis: Inflammation or swelling of a bursa, such as prepatellar bursitis (swelling in front of the kneecap, often from kneeling).
C
Cartilage: A tough, flexible connective tissue. In the knee it covers the bone ends (articular cartilage) and forms the menisci.
Comminuted fracture: A fracture where the bone breaks into three or more pieces.
Crepitus: A clicking, popping, cracking or grinding sensation felt or heard during knee movement. Painless clicks and creaks are common and not usually a sign of damage by themselves.
Cruciate ligaments: The two crossing ligaments inside the knee — the ACL and PCL. They provide most of the knee's front-to-back stability.
E
Effusion: Swelling inside a joint caused by extra fluid. A swollen, tense knee is often described as having an effusion.
Extension: Straightening a joint. Knee extension is straightening the knee.
Extensor mechanism: The quadriceps muscle, quadriceps tendon, kneecap and patellar tendon working together to straighten the knee. See the quadriceps tendon anatomy page.
F
Femur: The thighbone, the long bone of the upper leg that forms the top half of the knee joint.
Flexion: Bending a joint. Knee flexion is bending the knee.
Fracture: A break in a bone.
G
Giving way: A feeling that the knee suddenly shifts, collapses or "goes out" under you. It can come from ligament injuries, muscle weakness or other knee problems.
H
Hamstring tendons: The tendons of the hamstring muscles, which run down the back of the thigh and attach around the knee. They can be a site of tendon pain at the back of the knee or high in the thigh.
Hemarthrosis: Blood inside a joint, usually from a significant injury such as a fracture or ligament tear, causing rapid, tense swelling.
I
Iliotibial (IT) band: A long band of fascia running down the outside of the thigh from the hip to just below the knee. IT band syndrome is a common cause of pain on the outside of the knee in runners and cyclists. See the IT band anatomy page.
Infrapatellar: Below the kneecap.
L
Lateral: The side of the body or limb away from the midline — for the knee, the outer side.
LCL (lateral collateral ligament): The ligament along the outside of the knee that resists the knee bending outward.
Locking: A knee that gets stuck, often in a bent position, and will not fully straighten. It can be caused by a piece of torn meniscus or loose tissue catching in the joint.
M
MCL (medial collateral ligament): The ligament along the inside of the knee that resists the knee bending inward.
Medial: The side of the body or limb toward the midline — for the knee, the inner side.
Meniscus: One of the two C-shaped cartilage pads (medial and lateral meniscus) between the thighbone and shinbone that cushion and distribute load. See the medial meniscus anatomy page.
O
Osteoarthritis: A common joint condition where cartilage thins and the joint changes over time, causing pain, stiffness and sometimes swelling. It is an active joint process, not simply "wear and tear." See the knee osteoarthritis page.
Overuse injury: An injury that develops gradually from repeated load rather than a single event.
P
Patella: The kneecap, the small bone at the front of the knee that protects the joint and helps the thigh muscles straighten the knee. See the patella anatomy page.
Patellar tendon: The strong band connecting the kneecap to the shinbone. It is a common site of tendon pain (patellar tendinopathy).
PCL (posterior cruciate ligament): One of the main ligaments inside the knee, crossing behind the ACL. It resists the shinbone sliding backward and is injured most often by a blow to the front of the bent knee.
Pes anserine: The area on the inner knee where the tendons of the sartorius, gracilis and semitendinosus muscles attach. Pes anserine irritation can cause inner knee pain.
Popliteal fossa: The hollow area at the back of the knee.
Prepatellar: In front of the kneecap.
Q
Quadriceps: The large muscles at the front of the thigh that straighten the knee. Their tendon attaches to the kneecap — the quadriceps tendon.
Quadriceps tendon: The strong tendon connecting the quadriceps muscle to the top of the kneecap. See the quadriceps tendon anatomy page.
R
Range of motion: How far a joint can move, usually described in degrees of flexion and extension.
S
Sesamoid bone: A small bone embedded within a tendon. The patella is the largest sesamoid bone in the body.
Superior pole: The top edge of the kneecap, where the quadriceps tendon attaches.
T
Tendinopathy: A load-related problem in a tendon causing pain and impaired function. "Tendonitis" is an older term; persistent tendon pain is usually not explained by inflammation alone.
Tibia: The shinbone, the large bone of the lower leg that forms the bottom half of the knee joint.
Tibial tubercle: The bony bump on the front of the shinbone, just below the kneecap, where the patellar tendon attaches. It is the site of Osgood-Schlatter disease in growing adolescents.
V
Valgus: A position or force that pushes the knee inward (knock-kneed direction).
Varus: A position or force that pushes the knee outward (bow-legged direction).
Related Reading
References
- Consensus definitions based on standard medical and orthopaedic terminology used throughout the InjuryAtlas knee library and cited on linked pages.
This information is for education only and is not a medical diagnosis. InjuryAtlas does not replace an assessment by a qualified health professional. If you have severe pain, visible deformity, numbness, loss of function, or symptoms that are getting worse, seek medical care promptly.