Knee Joint Anatomy & Biomechanics

Joint Capsule: Definition & Spatial Relationships

• "Bag of bones": fibrous envelope that surrounds tibio-femoral + patello-femoral joints.
• Spatial classification used in texts can be confusing; adopt a clinically clearer rule-of-thumb:
  • Extra-capsular / Outside the capsule
  • Tibial (medial) collateral ligament (MCL) – blends with capsule but still considered extra-capsular.
  • Fibular (lateral) collateral ligament (LCL) – clearly outside.
  • Oblique popliteal ligament.
  • Arcuate popliteal ligament.
  • Intra-articular / Inside the joint cavity
  • Anterior & posterior cruciate ligaments (ACL, PCL).
  • Medial & lateral menisci.
  • Synovial lining does not always follow the fibrous capsule → adds to classification confusion.
• Clinical shortcut: “Everything I can palpate outside = extra-capsular; everything else = intra-articular.”

Static Ligamentous Restraints

1. Medial (Tibial) Collateral Ligament – MCL

• Origin/Insertion: medial femoral epicondyle → medial tibial flare; deep fibers attach to medial meniscus.
• Functions
  • Primary: resist $\text{valgus}$ stress (tibia abducting).
  • Secondary: limit tibial external rotation & anterior translation when knee flexed.
• Position of greatest stress: $25°–30°$ flexion (capsule slack → MCL is isolated).
• Clinical exams
  • Valgus test at $0°$ (capsule + meniscus also taut).
  • Valgus test at $30°$ (MCL primary).

2. Lateral (Fibular) Collateral Ligament – LCL

• Origin/Insertion: lateral femoral epicondyle → fibular head.
• Functions: resist $\text{varus}$ stress & tibial internal rotation.
• Injuries rare because normal gait/athletics rarely apply pure varus with foot fixed.

3. Anterior Cruciate Ligament – ACL

• Bundles: anteromedial & posterolateral.
• Primary restraint to anterior translation of tibia (or posterior translation of femur).
• Also limits excessive tibial internal rotation & valgus stress.
• Highest strain ≈ $20°–30°$ flexion → Lachman position (hence red-flagged in slides).
• Common mechanism: non-contact, valgus + internal rotation at $30°$ flexion ("unhappy triad": ACL + MCL + medial meniscus).

4. Posterior Cruciate Ligament – PCL

• Restrains posterior translation of tibia (or anterior translation of femur).
• Surgical repair uncommon → proximity to popliteal artery & tibial nerve; “juice not worth the squeeze.”
• Conservative management: keep knee’s centre of rotation anterior (e.g., year in cowboy boots or weight-lifting shoes – plantar-flexed ankle).

5. Posterior Capsule / Oblique & Arcuate Popliteal Ligaments

• Assist PCL in preventing hyper-extension & excessive tibial external rotation.

Dynamic Stability: Muscle & Fascial Supports

• Anterior: Quadriceps (especially vasti) oppose posterior sag, provide compression.
• Posterior: Hamstrings limit anterior tibial glide; attach to menisci (see below).
• Lateral: Iliotibial band (ITB) + biceps femoris augment LCL.
• Medial “Pes Anserine” Sling (tri-planar):
  • Sartorius (anterior)
  • Gracilis (medial)
  • Semitendinosus (posterior)
    → counters valgus & rotary loads where ligamentous support weaker.

Menisci

• Semilunar fibro-cartilage; wedge-shaped (thicker periphery).
• Attachments to hamstrings
  • Lateral meniscus ↔ biceps femoris.
  • Medial meniscus ↔ semimembranosus.
    → During flexion hamstrings pull menisci posteriorly, preventing impingement.
• Primary functions
  • Increase contact area ⇒ reduce pressure $P = \frac{F}{A}$.
  • Shock absorption (load distribution).
  • Guide centre of rotation (anterior ↔ posterior shift).
  • Provide proprioceptive input (loss → OA progression after meniscectomy).
• Vascularity: peripheral 1/3 only; nutrition by osmosis & diffusion, accentuated by motion.
• Injury mechanics
  • Posterior horn tears: deep loaded flexion >155° (heavy squat).
  • Stress test: knee $≈30°$ flexion + varus/valgus load.
• Post-repair rehab: avoid isolated hamstring curls (shear), prefer co-contractions (quad + hamstring) such as straight-leg raises.

Patella & Patello-femoral Joint

• Sesamoid embedded in quadriceps tendon → lengthens moment arm, acts as pulley.
• Without patella, tibia would impinge on femoral condyles; knee extension $25° \rightarrow 0°$ would require enormous force.
• Arthrokinematics
  • Knee extension: patella glides superiorly.
  • Knee flexion: patella glides inferiorly.
  • Closed-pack (max congruence) ≈ full flexion; open-pack = full extension.
• Clinical clue: patient sitting with knee fully extended to ease pain → patello-femoral pathology likely.

Arthrokinematics & Screw-Home Mechanism

• Tibio-femoral open-pack ≈ $25°–30°$ flexion (capsule lax).
• Closed-pack = full extension (ligaments & capsule taut).
• Osteokinematic paradox (“screw home”):
  • Last $≈10°$ of extension → tibia externally rotates & abducts because lateral femoral condyle is shorter ⇒ medial condyle continues moving.
  • Unlocking for flexion: popliteus internally rotates tibia (or externally rotates femur in closed chain).
• Manual therapy “condylar glides”: one hand pushes anterior on medial tibial plateau & posterior on lateral (or vice-versa) to recreate coupled rotation without stressing ACL/PCL.

Range of Motion & Functional Benchmarks

• Full physiologic flexion (male adult): $135°–145°$ (soft tissue “muscle” end-feel).
• Activities of daily living (ADL):
  • Level walking: $≈75°$ flexion.
  • Sit↔stand: $≥90°$.
  • Low chair / toilet: $≥105°$.
  • Deep squat / kneeling: >130°.
• OA clients often present at $105°–120°$; emphasis on regaining flexion in rehab.

Alignment Metrics

Q-Angle

• Draw ASIS → mid-patella & tibial tubercle → patella; angle between lines.
• Typical adult values $≈12°–18°$ (women > men).
• Merely an observation; not diagnostic by itself.

Hip–Knee Coupling

• Coxa vara (smaller femoral neck-shaft angle) ↔ genu valgum.
• Coxa valga (larger neck-shaft angle) ↔ genu varum.
• Exam questions often test these paired relationships – normal ranges must be memorised.

Personal example (lecturer)

• Childhood genu valgum: knees touching, medial malleoli 5 inches apart; underwent growth-plate stapling to straighten – hindsight suggests progressive exercise loading might have sufficed (ethical reflection on paediatric orthopaedic surgery).

Common Injury Mechanisms & Tests

• ACL: quick deceleration + valgus + tibial IR at $≈30°$ flexion (open-pack, capsule slack).
  • Lachman (20–30°), Anterior Drawer (90°).
• PCL: dashboard injury / fall on flexed knee; Posterior Drawer (90°).
  • Managed with plantar-flexed footwear + ham/glute strengthening.
• MCL: blow to lateral knee; valgus stress tests at 0° & 30°.
• LCL: blow to medial knee; varus stress tests.
• Posterior horn meniscus: loaded flexion >155° (heavy squats) → avoid deep loaded squats post-repair.

Rehabilitation & Exercise Selection Principles

• Recognise closed-chain paradox: two-joint muscles (rectus femoris, hamstrings) simultaneously act at hip & knee → no purely “quad” or “hamstring” phase in squats.
• Post-meniscal repair: favour quad–hamstring co-contractions (e.g., straight-leg raise) over isolated hamstring curls.
• Post-ACL: early squats with vertical tibia (Spanish squat) to minimise anterior shear; restore screw-home mechanics over ~4 months.
• Post-PCL: hip-dominant patterns (RDLs, standing hip extension machine) increase hamstring activation without posterior tibial translation.
• Biomechanical variables to manipulate ("10 essentials"): duration, rate, direction, environment, lever type, load magnitude, stability demand, ROM, sensory input, task complexity.

Proprioception, OA & Insurance Realities

• Meniscectomy → diminished proprioceptive feedback → altered loading patterns → knee OA.
• Latest evidence: final rehab phase for OA & post-surgery = proprioceptive / perturbation training; often cut short by insurance (Medicare often allows only $13–18$ visits/year at 4 units/visit).
• Ethical duty: provide intentional, high-value care within visit limits; may prioritise HEP (home exercise programmes) for neuromuscular training.

Key Take-Home Checklist for Exam & Clinic

1. Know what each ligament limits and the knee position that stresses it most.
2. Memorise normal vs pathological alignment ranges for hip & knee (coxa/genu, Q-angle).
3. Understand screw-home mechanism & popliteus role.
4. Distinguish loose-pack vs close-pack for tibio-femoral vs patello-femoral joints.
5. Recall meniscus muscle attachments & why isolated ham curls are restricted after repair.
6. Apply load/area concept $P = \frac{F}{A}$ to meniscal shock absorption.
7. Recognise presentation clues:
   • Sitting with knee ~30° flex → tibio-femoral pain.
   • Sitting with knee extended → patello-femoral pain.
8. Couple hip alignment (coxa vara/valga) with knee posture (genu valgum/varum).
9. Insurance constraints demand strategic visit planning – proprioception often sacrificed; advocate for patients.
10. “Show me” approach: palpate structures directly under point of pain, then narrow hypothesis list before special tests.