The knee is made up of four bones. The femur or thighbone is the bone connecting the hip to the knee. The tibia or shinbone connects the knee to the ankle. The patella (kneecap) is the small bone in front of the knee and rides on the knee joint as the knee bends. The fibula is a shorter and thinner bone running parallel to the tibia on its outside. The joint acts like a hinge but with some rotation.
The knee is a synovial joint, which means it is lined by synovium. The synovium produces fluid lubricating and nourishing the inside of the joint. Articular cartilage is the smooth surfaces at the end of the femur and tibia. It is the damage to this surface which causes arthritis.
The femur (thighbone) is the largest and the strongest bone in the body. It is the weight bearing bone of the thigh. It provides attachment to most of the muscles of the knee.
The two femoral condyles make up for the rounded end of the femur. Its smooth articular surface allows the femur to move easily over the tibial (shinbone) meniscus.
The tibia (shinbone), the second largest bone in the body, is the weight bearing bone of the leg. The menisci incompletely cover the superior surface of the tibia where it articulates with the femur. The menisci act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation of the knee.
The fibula, although not a weight bearing bone, provides attachment sites for the Lateral collateral ligaments (LCL) and the biceps femoris tendon.
The articulation of the tibia and fibula also allows a slight degree of movement, providing an element of flexibility in response to the actions of muscles attaching to the fibula.
The patella (kneecap), attached to the quadriceps tendon above and the patellar ligament below, rests against the anterior articular surface of the lower end of the femur and protects the knee joint. The patella acts as a fulcrum for the quadriceps by holding the quadriceps tendon off the lower end of the femur.
The medial and the lateral meniscus are thin C-shaped layers of fibrocartilage, incompletely covering the surface of the tibia where it articulates with the femur. The majority of the meniscus has no blood supply and for that reason, when damaged, the meniscus is unable to undergo the normal healing process that occurs in the rest of the body. The menisci act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation of the knee. As secondary stabilizers, the intact menisci interact with the stabilizing function of the ligaments and are most effective when the surrounding ligaments are intact.
Anterior Cruciate Ligament (ACL)
The anterior cruciate ligament (ACL) is the major stabilizing ligament of the knee. The ACL is located in the center of the knee joint and runs from the femur (thigh bone) to the tibia (shin bone), through the center of the knee. The ACL prevents the femur from sliding backwards on the tibia (or the tibia sliding forwards on the femur). Together with the posterior cruciate ligament (PCL), ACL stabilizes the knee in a rotational fashion. Thus, if one of these ligaments is significantly damaged, the knee will be unstable when planting the foot of the injured extremity and pivoting, causing the knee to buckle and give way.
Posterior Cruciate Ligament (PCL)
Much less research has been done on the posterior cruciate ligament (PCL) because it is injured far less often than the ACL.
The PCL prevents the femur from moving too far forward over the tibia. The PCL is the knee’s basic stabilizer and is almost twice as strong as the ACL. It provides a central axis about which the knee rotates.
Collateral Ligaments prevent hyperextension, adduction, and abduction
- Superficial MCL (Medial Collateral Ligament) connects the medial epicondyle of the femur to the medial condyle of the tibia and resists valgus force
- Deep MCL (Medial Collateral Ligament) connects the medial epicondyle of the femur with the medial meniscus
- LCL (Lateral Collateral Ligament) entirely separate from the articular capsule, connects the lateral epicondyle of the femur to the head of the fibula and resists varus force
- ACL Repair (Anterior Cruciate Ligament Repair)
- Meniscus Repair
- Knee Arthroscopy
- Partial Knee Replacement
- Mako Robotic Arm Assisted Technology for Partial Knee Replacement
- Total Knee Replacement
- Mako Robotic Arm Assisted Technology for Total Knee Replacement
Click on the topics below to find out more from the orthopaedic connection website of American Academy of Orthopaedic Surgeons.
- ACL Injury: Should it be fixed?
- Activities After a Knee Replacement
- Additional Resources on the Knee
- Adolescent Anterior Knee Pain
- Arthritis of the Knee
- Care of the Aging Knee: Baby Boomers May Need Lifestyle Changes
- Cemented and Cementless Knee Replacement
- Deep Vein Thrombosis
- Frequently Asked Questions about Osteoarthritis of the Knee
- Goosefoot (Pes Anserine) Bursitis of the Knee
- Knee Arthroscopy
- Knee Arthroscopy Exercise Guide
- Knee Implants
- Knee Replacement Exercise Guide
- Kneecap (Prepatellar) Bursitis
- Meniscal Tear
- Meniscal Transplants
- Minimally Invasive Total Knee Replacement
- Nonsurgical Treatment Options for Osteoarthritis of the Knee
- Osgood-Schlatter Disease (Knee Pain)
- Osteonecrosis of the Knee
- Posterior Cruciate Ligament (PCL) Tear
- Rotating Platform/Mobile-bearing Knees
- Runner’s Knee (Patellofemoral Pain)
- Surgical Treatment of Osteoarthritis of the Knee
- The Knee
- Total Knee Replacement
- Unstable Kneecap
- Viscosupplementation Treatment for Arthritis