What is the anatomy of the joint causing my knee pain, specifically related to my age and activity level?

Knee Joint Anatomy Diagram and Pain Sources

I cannot provide an actual visual diagram in this text format, but I can describe the key anatomical structures of the knee joint that are most likely causing your pain based on your age and activity level.

Primary Anatomical Structures of the Knee Joint

The Tibiofemoral Joint (Main Knee Joint)

The medial (inner) compartment is the most common source of knee pain, bearing 70-80% of your body weight during walking because your center of mass sits medial to the knee joint center 1, 2.

Key structures include:

• Medial femoral condyle (rounded end of thighbone on the inner side) - most commonly affected by subchondral insufficiency fractures in middle-aged to elderly females 1, 2
• Medial tibial plateau (top surface of shinbone on inner side) - bears majority of load 1
• Medial meniscus (C-shaped cartilage cushion on inner side) - tears are extremely common, with the majority of people over 70 having asymptomatic tears 1, 3
• Articular cartilage - smooth covering on bone ends that wears down with osteoarthritis 1

The Patellofemoral Joint (Kneecap Joint)

This joint consists of the patella (kneecap) articulating with the patellar groove (trochlea) of the femur 4, 5.

Critical components:

• Patella - largest sesamoid bone in your body, sits in front of knee 4
• Trochlear groove - V-shaped channel on front of femur where patella glides 4
• Patellar cartilage - frequently damaged in patellofemoral pain syndrome, especially in active adults under 40 6, 7
• Medial patellofemoral ligament - restrains lateral patellar displacement 1

Age-Specific Pain Sources

If You're 45 Years or Older:

Osteoarthritis is the primary cause of knee pain, affecting 50% of those ≥65 years and 85% of those ≥75 years 8.

The medial compartment degenerates first because it handles the majority of load 1, 2. Key pain generators include:

• Bone marrow lesions (BML) - areas of bone swelling visible on MRI that strongly correlate with pain intensity 1
• Synovitis and joint effusion - inflammation of joint lining that correlates significantly with frequent knee pain 1, 2
• Subchondral bone changes - bone remodeling beneath cartilage 1
• Meniscal tears - present in most older adults but often asymptomatic; the likelihood of a tear in a painful versus painless knee is not significantly different in patients 45-55 years old 1, 3

If You're Under 40 Years and Physically Active:

Patellofemoral pain syndrome affects approximately 25% of active individuals in this age group 6, 7.

Pain originates from:

• Patellar maltracking - abnormal movement of kneecap in its groove 1
• Cartilage overload - excessive pressure on patellar cartilage during activities 1, 9
• Hoffa's fat pad - inflammation of fat tissue behind patellar tendon 1, 2
• Medial plica - irritated synovial fold on inner knee 1, 2

Supporting Soft Tissue Structures

These structures stabilize the joint and commonly contribute to pain:

• Patellar tendon - connects kneecap to shinbone; prone to tendinopathy in jumping athletes 1
• Medial collateral ligament - inner side stabilizer 1
• Anterior cruciate ligament (ACL) - central stabilizer preventing forward tibial shift 1
• Quadriceps muscle - primary active stabilizer; weakness increases OA risk 1
• Hamstrings and gastrocnemius - additional active stabilizers 1

Clinical Correlation

Your specific pain pattern determines which structure is most likely involved:

• Anterior knee pain worsening with squatting or stairs = patellofemoral joint (91% sensitive for patellofemoral pain) 6, 7
• Medial joint line tenderness matching activity pain = medial meniscus or medial compartment OA 2, 6
• Pain with less than 30 minutes morning stiffness in patients >45 = knee osteoarthritis (95% sensitive, 69% specific) 6
• Pain with knee rotation and extension = meniscal tear (McMurray test: 61% sensitive, 84% specific) 6

Common Pitfall to Avoid

Do not assume meniscal tears seen on MRI are causing your pain if you're over 45 years old - the majority of people over 70 have asymptomatic tears, and bone marrow lesions plus synovitis better correlate with actual pain 1, 3. The imaging finding may be incidental rather than causative 1.