Difficulty: Introductory | Prerequisites: Basic skeletal and muscular system terminology.
The knee is one of the most complex and heavily loaded joints in the body, and understanding its structure is foundational to almost everything else in musculoskeletal anatomy and sports medicine. This material covers the four bones of the knee, the two menisci, the four major ligaments, and the muscle groups responsible for knee movement. If you are coming into this cold, you should be comfortable with directional terms (medial, lateral, anterior, posterior) and know the difference between a ligament and a tendon before starting.
The knee joint is formed by the femur, tibia, fibula, and patella. It is stabilised by four key ligaments (ACL, PCL, MCL, LCL) and cushioned by two menisci. Movement is driven primarily by the quadriceps (extension) and hamstrings (flexion), with several accessory muscles handling rotation.
Femur
The thigh bone, the longest bone in the body. It forms the upper half of the knee joint. Think of it as the structural pillar that transfers your body weight down into the knee.
Tibia
The shinbone, the larger of the two lower-leg bones. It bears the majority of weight below the knee. In simple terms, this is the bone you would feel if you ran your hand down the front of your lower leg.
Fibula
The calf bone, the thinner bone running alongside the tibia on the lateral side. It does not bear much weight but serves as an attachment point for muscles and ligaments.
Patella
The kneecap, a sesamoid bone embedded in the quadriceps tendon. It sits in front of the knee and acts as a fulcrum, increasing the mechanical advantage of the quadriceps during extension.
Tibiofemoral joint
The articulation between the femur and tibia. This is the main weight-bearing joint of the knee. In simple terms, this is "the knee joint" most people picture.
Patellofemoral joint
The articulation between the patella and the femur. This joint lets the kneecap glide smoothly during flexion and extension.
Proximal tibiofibular joint
The articulation between the tibia and fibula, just below the knee. It allows a small amount of movement that accommodates ankle rotation.
Meniscus (plural: menisci)
Fibrocartilage disks sitting on the tibial plateau that increase joint stability and cushion compressive forces. Think of them as shock absorbers between the femur and tibia.
Medial meniscus
The C-shaped meniscus on the inner side of the knee. It is attached to the MCL, which is why MCL injuries and medial meniscus tears often occur together.
Lateral meniscus
The O-shaped meniscus on the outer side of the knee. It is more mobile than the medial meniscus and slightly less prone to injury.
Anterior Cruciate Ligament (ACL)
A ligament inside the knee that prevents the tibia from sliding forward relative to the femur and resists excessive internal rotation. In simple terms, it stops your shinbone from drifting ahead of your thigh bone when you plant and pivot.
Posterior Cruciate Ligament (PCL)
A ligament inside the knee that prevents the tibia from sliding backward relative to the femur. Think of it as the ACL's counterpart, resisting the opposite direction of translation.
Medial Collateral Ligament (MCL)
A ligament on the inner side of the knee that stabilises against valgus (inward-collapsing) forces. In simple terms, it stops the knee from buckling inward when hit from the outside.
Lateral Collateral Ligament (LCL)
A ligament on the outer side of the knee that stabilises against varus (outward-bowing) forces. It stops the knee from gapping open on the lateral side.
Valgus force
A force that pushes the knee inward, opening the medial side. A blow to the outside of the knee creates a valgus stress.
Varus force
A force that pushes the knee outward, opening the lateral side. A blow to the inside of the knee creates a varus stress.
Quadriceps
The four-muscle group on the front of the thigh responsible for knee extension: rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis.
Hamstrings
The three-muscle group on the back of the thigh responsible for knee flexion and hip extension: biceps femoris, semitendinosus, and semimembranosus.
The knee is classified as a hinge joint, though it also permits a small degree of rotation
Four bones contribute to its structure: the femur, tibia, fibula, and patella
Three articulations exist within the knee complex:
Tibiofemoral (femur on tibia) – primary weight-bearing surface
Patellofemoral (patella on femur) – guides kneecap tracking during movement
Proximal tibiofibular (tibia on fibula) – minor joint, accommodates rotational forces from the ankle
Two menisci sit on the tibial plateau, one medial and one lateral
The medial meniscus is C-shaped, less mobile, and attached to the MCL
The lateral meniscus is O-shaped and more mobile
Functions:
Increase the contact area between femur and tibia, distributing load more evenly
Cushion compressive stresses during weight-bearing activities
Contribute to joint stability by deepening the tibial plateau surface
The ACL and PCL cross inside the joint (hence "cruciate," from the Latin for "cross")
ACL: resists anterior tibial translation and excessive internal rotation. This is the ligament most commonly discussed in sports injury contexts
PCL: resists posterior tibial translation. Less commonly injured than the ACL, typically damaged by a direct blow to the front of the tibia (e.g. dashboard injury)
Neither the ACL nor the PCL can be palpated directly during physical examination
The MCL and LCL sit on the medial and lateral sides of the knee, respectively
MCL: resists valgus stress. Commonly injured in contact sports when the knee is struck from the lateral side
LCL: resists varus stress. Less commonly injured than the MCL
The quadriceps group has four muscles, all converging into the quadriceps tendon and then the patellar tendon:
Rectus femoris (the only quad that crosses both the hip and knee joints)
Vastus medialis (important for patellar tracking, especially the VMO portion)
Vastus intermedius (deepest of the four)
Vastus lateralis (largest of the four)
The hamstrings are the primary knee flexors:
Biceps femoris (lateral hamstring, also performs external tibial rotation)
Semitendinosus
Semimembranosus
Accessory knee flexors include: gracilis, sartorius, gastrocnemius, popliteus, and plantaris
External rotation of the tibia: biceps femoris
Internal rotation of the tibia: popliteus, semitendinosus, semimembranosus, sartorius, gracilis
The interplay between quadriceps and hamstrings is why strength-and-conditioning programmes emphasise balanced training of both groups. A hamstring-to-quadriceps strength ratio that falls too low is a known risk factor for ACL injuries, particularly in athletes who sprint, cut, and decelerate frequently.
Students often think the knee is a simple hinge with only flexion and extension. It also permits a small but clinically important degree of internal and external tibial rotation.
The fibula is sometimes dismissed as irrelevant to the knee. It forms the proximal tibiofibular joint and is an important attachment site for the LCL and the biceps femoris tendon.
Students frequently confuse valgus and varus. Valgus collapses the knee inward (think "knock-kneed"), varus bows it outward.
The medial and lateral menisci are not identical in shape or mobility. Knowing the C-shape versus O-shape distinction and the medial meniscus's MCL attachment matters for understanding injury patterns.
⚠️ Be able to name all four bones and three articulations of the knee.
⚠️ Know the specific function of each of the four major ligaments (ACL, PCL, MCL, LCL) and the direction of force each resists.
⚠️ Understand why the ACL and PCL cannot be palpated (they are intra-articular).
⚠️ Be able to list all four quadriceps muscles and all three hamstring muscles by name.
⚠️ Know which muscles contribute to internal versus external tibial rotation.
True or false: The patella articulates with both the femur and the tibia. (False – it articulates only with the femur.)
Fill in the blank: The ________ meniscus is C-shaped and attached to the MCL. (Medial)
True or false: The PCL prevents the tibia from moving forward relative to the femur. (False – that is the ACL. The PCL prevents posterior translation.)
Fill in the blank: The only quadriceps muscle that crosses both the hip and knee joints is the ________. (Rectus femoris)
True or false: Valgus force pushes the knee outward. (False – valgus force pushes the knee inward, stressing the medial side.)
Q: Name the four bones that form the knee joint complex.
A: Femur, tibia, fibula, and patella.
Q: What are the two functions of the menisci?
A: They increase joint stability and cushion compressive stresses on the knee.
Q: Which ligament prevents anterior tibial translation, and which prevents posterior tibial translation?
A: The ACL prevents anterior tibial translation. The PCL prevents posterior tibial translation.
Q: What distinguishes the MCL from the LCL in terms of the force each resists?
A: The MCL resists valgus (inward-collapsing) forces, while the LCL resists varus (outward-bowing) forces.
Q: Which muscle is the primary external rotator of the tibia?
A: The biceps femoris.
Q: Why is the rectus femoris unique among the quadriceps?
A: It is the only quadriceps muscle that crosses both the hip joint and the knee joint, meaning it contributes to both hip flexion and knee extension.
This material connects directly to the study of knee injuries (Part 2 of these notes), because understanding which ligament resists which force tells you which ligament is most likely damaged in a given injury mechanism. It also links to biomechanics and kinesiology units, where the lever-arm advantage provided by the patella and the agonist-antagonist balance of quadriceps and hamstrings become central topics.
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