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.
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