Anatomy

Anatomy and Biomechanics of the Knee

Anatomy

• Knee: A condylar joint.
• Patellofemoral Joint: A saddle joint.

Tibia

• 7–10° posterior slope.
• 3° varus to the anatomic axis.
• Lateral plateau higher than medial.
• Lateral plateau convex; medial plateau concave.
• Plateaus separated by the intercondylar eminence.
• Spinous processes arise from the intercondylar eminence.
• Gerdy’s tubercle: Attachment of the ITB.

Femur

• Medial femoral condyle (MFC) is larger and more distal.
• Lateral femoral condyle (LFC) is wider.
• Adductor tubercle is the proximal origin of MCL and MPFL, located just proximal to the medial epicondyle.
• PCL is on the medial wall of the notch; ACL is on the lateral wall.
• Just distal to the LFC’s medial edge lies the sulcus terminalis.

Patella

• Thickest articular cartilage in the body.
• Three facets:
  • Medial, Lateral, and Odd Facet (small and far medial).
  • Medial and lateral facets separated by a longitudinal ridge; the lateral facet is usually larger.
• Accessory patella typically found superior-laterally.

Wiberg Classification of Patella Morphology

1. Type 1: Medial and lateral facets equal; ridge central.
2. Type 2: Medial facet smaller; ridge medialized (most common).
3. Type 3: Medial facet tiny and far medialized, rendering the ridge almost absent.

Trochlea

• Lateral ridge is higher than the medial ridge (may be dysplastic).
• Dejour Index: Used for classification.

Fibula

• Does not contribute to the knee joint.
• Articulates with the lateral tibia at the fibular notch.
• Fibular styloid process: Attachment of LCL and biceps tendon.
• Common peroneal nerve runs beneath the biceps and around the fibular neck.

Vascular Anatomy

• Arterial anastomosis from femoral, popliteal, and tibial arteries:
  • Femoral artery → Popliteal artery → Anterior and Posterior tibial arteries.
• Branches:
  • Superior Geniculate (medial and lateral): Popliteal.
  • Inferior Geniculate (medial and lateral): Popliteal.
  • Middle Geniculate: Popliteal.
  • Descending Geniculate: Femoral.
  • Anterior Tibial Recurrent Arteries: Tibial.
• Inferior geniculates pass deep to their respective collateral ligaments.

Patella Blood Supply

• Geniculate anastomosis and anterior recurrent tibial artery.
• Blood enters the patella in the lower two-thirds.

Nerve Anatomy

• Sensory nerves:
  • Femoral (L2–L4), Obturator (L2–L4), and Sciatic (L4–S2).
• Important nerves:
  • Posterior Articular Nerve: Branch of the posterior tibial nerve (intra-articular structures).
  • Infra-patella Branch of Saphenous Nerve: Supplies skin over the anterior knee and tibia.

Ligaments

• Refer to the section on ligament injuries.

Extensor Mechanism

• Quadriceps Tendon:
  • Rectus femoris, vastus medialis, vastus intermedius, vastus lateralis.
• Medial and lateral retinaculae extend from the quadriceps.
• Patellofemoral ligaments are thickenings of the retinaculum.
• MPFL stabilizes the patella and attaches to the adductor tubercle.

Meniscus

• Medial: C-shaped; lateral: nearly circular.
• Lateral meniscus is more mobile (posterolaterally not attached at the popliteal hiatus).
• Connected to the capsule by coronary ligaments (more on the medial meniscus).
• Menisci are connected anteriorly by the intermeniscal ligament.
• Meniscofemoral Ligaments:
  • Humphrey’s (anterior) and Wrisberg’s (posterior) ligaments.
  • Arise from the lateral meniscus, pass around the PCL, and attach to the femur.
  • Alphabetical order: Humphrey (H) → PCL → Wrisberg (W).

Biomechanics

Gait

• Knee functions during gait:
  • Flexes to decrease impact at initial contact.
  • Extends in mid-stance for leg stabilization.
  • Flexes in the initial swing for foot clearance.

Gait Abnormalities

• Quads Avoidance Gait:
  • Weak quads or ACL deficiency.
  • Compensatory forward leaning to prevent knee buckling.
• Fixed Flexion Deformity (FFD):
  • Difficulty in stance; compensatory mechanisms occur in the opposite leg.

Joint Reaction Forces

• Tibiofemoral joint: 3× body weight during walking, 4× on stairs.
• Patellofemoral joint: 3× body weight on stairs, 7× during deep squats.

Ligament Kinematics

• Knee acts as a 4-bar linkage enabling rolling and sliding motion.
• ACL and PCL cross-over point: Center of axis of rotation (COR).
• ACL:
  • AM bundle tight in flexion; PL bundle tight in extension.
  • Ultimate tensile strength: 2000N.
• PCL:
  • Opposite bundle behavior; PM bundle tight in extension.
  • Stronger than ACL (UTN: 2500N).

Meniscus Biomechanics

• Anisotropic properties (load-dependent behavior).
• Dissipates 50% of the load in extension, 75% in flexion.
• Meniscectomy increases articular contact pressure proportionally.

Articular Cartilage

• Biphasic material (70% water content).
• Withstands repetitive loading via a strong proteoglycan matrix.
• Disuse or pathological loading damages cartilage integrity.