Knee Joint: Anatomy, Function and Conditions

What Is the Knee Joint?

The knee joint (Latin: Articulatio genus) is the largest and most complex joint in the human body. It connects the thigh bone (femur) with the shin bone (tibia) and the kneecap (patella). The fibula (calf bone) is located nearby but does not directly participate in the knee joint. The knee is classified as a hinge joint, primarily allowing bending (flexion) and straightening (extension), as well as slight rotational movement when the knee is bent.

Anatomical Structure

The knee joint has an exceptionally complex structure, consisting of several specialized components:

• Bones: The femur, tibia, and patella form the bony framework of the joint.
• Cartilage: Hyaline articular cartilage covers the joint surfaces, enabling smooth, low-friction movement.
• Menisci: Two crescent-shaped fibrocartilage pads -- the medial (inner) and lateral (outer) meniscus -- lie between the femur and tibia, acting as shock absorbers and improving joint congruence.
• Ligaments: The anterior and posterior cruciate ligaments (ACL and PCL) stabilize the joint internally. The medial and lateral collateral ligaments prevent excessive side-to-side movement.
• Joint capsule: A fibrous envelope surrounds the entire joint and contains synovial fluid, which lubricates and nourishes the joint structures.
• Bursae: Several fluid-filled sacs around the knee reduce friction between tendons, bone, and skin.
• Muscles and tendons: The quadriceps muscle at the front of the thigh extends the knee, while the hamstrings at the back of the thigh flex it. The quadriceps tendon and the patellar tendon connect the muscle to the kneecap and shin bone.

Function of the Knee Joint

The knee joint plays a central role in human movement and biomechanics:

• Movement: It allows flexion of approximately 130 to 150 degrees and full extension. In a flexed position, internal and external rotation of about 10 to 30 degrees is also possible.
• Load transmission: The knee bears the full weight of the body and transmits forces during walking, running, jumping, and climbing stairs.
• Stability: The coordinated action of ligaments, menisci, and muscles keeps the joint stable even under dynamic loads.

Common Conditions of the Knee Joint

Due to its complex structure and high mechanical demands, the knee joint is susceptible to a variety of injuries and diseases:

Osteoarthritis (Gonarthrosis)

Gonarthrosis is the most common knee condition and refers to the degenerative breakdown of articular cartilage. It causes pain, stiffness, and limited mobility, particularly in older adults.

Meniscus Tear

Meniscus injuries often result from sudden twisting motions or overuse. Symptoms include pain, swelling, and a feeling of the knee locking or giving way.

Cruciate Ligament Rupture

A tear of the anterior or posterior cruciate ligament is a common sports injury. It leads to joint instability and often requires surgical treatment.

Patellofemoral Pain Syndrome

Pain behind or around the kneecap, especially common in younger, physically active individuals, is referred to as patellofemoral pain syndrome.

Bursitis

Inflammation of the bursae around the knee can be caused by overuse, prolonged pressure, or infection. It typically presents with localized swelling and pain.

Rheumatoid Arthritis

In rheumatoid arthritis, the immune system attacks the synovial membrane lining the joint, leading to chronic inflammation, swelling, and progressive joint destruction.

Diagnosis of Knee Conditions

Several diagnostic methods are used to evaluate the knee joint:

• Physical examination: Inspection, palpation, and specific clinical tests such as the Lachman test (cruciate ligaments) and the McMurray test (menisci).
• X-ray: Used to assess bony structures and detect joint space narrowing in osteoarthritis.
• Magnetic Resonance Imaging (MRI): The gold standard for visualizing soft tissue structures including menisci, ligaments, and cartilage.
• Ultrasound: Useful for assessing bursae, tendons, and joint effusions.
• Arthroscopy: A minimally invasive procedure using a small camera inserted into the joint, often combined with therapeutic interventions.

Treatment Options

Treatment of knee joint conditions depends on the underlying cause, severity, and the individual profile of the patient:

Conservative Treatment

• Physiotherapy and targeted strengthening exercises for the surrounding muscles
• Pain management with anti-inflammatory medications (e.g., NSAIDs such as ibuprofen)
• Joint injections with corticosteroids or hyaluronic acid
• Orthopedic aids such as braces, orthoses, or insoles
• Weight reduction to relieve joint loading

Surgical Treatment

• Arthroscopy: Minimally invasive surgery to repair menisci or ligaments
• Cruciate ligament reconstruction: Replacement of the torn ligament using a graft
• Osteotomy: Surgical correction of malalignment (e.g., bow legs or knock knees) to redistribute joint loading
• Total knee arthroplasty (TKA): Replacement of the damaged joint surfaces with a prosthetic implant in cases of severe osteoarthritis

Prevention and Long-Term Joint Health

The following measures can help maintain a healthy knee joint over time:

• Regular exercise to strengthen the quadriceps and hamstring muscles
• Maintaining a healthy body weight
• Choosing low-impact sports such as swimming or cycling
• Proper warm-up before physical activity
• Wearing appropriate footwear and orthopedic insoles if needed

References

1. Standring S (Ed.). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd edition. Amsterdam: Elsevier; 2021.
2. Buckwalter JA, Saltzman C, Brown T. The impact of osteoarthritis: implications for research. Clinical Orthopaedics and Related Research. 2004;427(Suppl):S6-S15.
3. National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). Knee Problems. U.S. Department of Health and Human Services; 2023. Available at: https://www.niams.nih.gov