Clinical Applications of Biomechanics

Clinical Applications of Biomechanics - Bones & Bolts

• Fracture Stability Principles:
  • Absolute Stability: No motion at fracture site. Goal: Primary bone healing (interfragmentary strain, $\epsilon = \Delta L / L_0$, < 2%). Achieved via compression (e.g., lag screws, compression plates).
  • Relative Stability: Controlled motion. Goal: Secondary bone healing via callus (strain 2-10%). Achieved via splinting, IM nails, bridging plates, external fixators.
• Load & Implant Dynamics:
  • Load-sharing: Implant & bone share forces (e.g., IM nail). Promotes callus.
  • Load-bearing: Implant carries entire load (e.g., bridging plate for comminuted fracture).
  • Stress Shielding: Bone resorption if implant is too rigid, reducing bone stress.
• Key Implants & Techniques:
  • Plates: Compression (DCP), neutralization, buttress, bridging functions.
  • Screws: Lag technique for interfragmentary compression.
  • Intramedullary Nails: Load-sharing; for long bone diaphyseal fractures.
  • External Fixators: For open fractures, severe soft tissue damage.
• 📌 AO Principles: ARSA (Anatomic Reduction, Stable Fixation, Atraumatic Technique, Active Mobilization).

⭐ Dynamic compression plating (DCP) converts torsional and bending forces into axial compression across the fracture site.

Clinical Applications of Biomechanics - Motion Makers

• Arthroplasty Biomechanics:

  • THR: Target acetabular inclination 40±10°, anteversion 15±10° for stability & ROM.
  • TKR: Aim for neutral mechanical axis, optimal component alignment & ligament balance.

• Implant Materials & Properties:

  Material	Young's Modulus (GPa)	Wear Resistance	Key Features
  CoCr	~200-230	Good	MoM issues, ion release
  Ti alloys	~100-120	Moderate	Lower stiffness, biocompatible
  UHMWPE	~1	Fair	📌 'POLY'ethylene wears, XPE wear ↓
  Ceramics	~300-400	Excellent	Brittle, fracture risk, squeaking

• Wear Mechanisms: Adhesive, abrasive, fatigue, corrosive (tribocorrosion).

• Fixation & Loosening:

  • Cemented (PMMA) vs. Cementless (porous coating for bone ingrowth).
  • Stress shielding: Implant-induced bone resorption due to ↓ load.
  • Aseptic loosening: Primary long-term failure mode.

⭐ Osteolysis secondary to wear debris (particle disease) is a major cause of long-term failure in total joint arthroplasty.

Clinical Applications of Biomechanics - Column Control

• Denis Three-Column Theory: Assesses spinal stability. 📌 All My Patients (Ant, Mid, Post).
  • Anterior: Anterior longitudinal ligament (ALL), anterior 1/2 vertebral body & annulus.
  • Middle: Posterior longitudinal ligament (PLL), posterior 1/2 vertebral body & annulus.
  • Posterior: Posterior elements (pedicles, facets, lamina, spinous process), ligament complex.
  • Instability if ≥2 columns disrupted.
• Mechanisms of Spinal Injury: Axial compression, flexion, extension, rotation, shear, distraction.
• Spinal Instability Indicators:
  • Loss of >50% vertebral body height.
  • Angulation >20-30°.
  • Progressive neurological deficit.
• Biomechanics of Spinal Instrumentation:
  • Pedicle screws & rods: Strongest posterior fixation, load bearing.
  • Cages (interbody): Restore height, promote fusion, anterior load sharing.
  • Plates: Anterior/lateral stabilization.
  • Load sharing: Implant + graft share loads until fusion.
  • Fusion concepts: Achieve arthrodesis (bony union).

⭐ The 'tension band' principle is frequently applied in spinal fixation to counteract flexion forces and promote stability.

Clinical Applications of Biomechanics - Step & Support

• Normal Gait Cycle:
  • Phases: 📌 Stance (IC, LR, MSt, TSt, PSw), Swing (ISw, MSw, TSw).
  • Stance Phase Details:

    Phase	Event	Muscle(s)
    IC	Heel strike	Tib. Ant.
    LR	Wt. accept	Quads, Glutes
    MSt	Single support	Glut. Med.
    TSt	Heel off	Gastroc/Soleus
    PSw	Toe off	Gastroc/Soleus

  • Parameters: Cadence 90-120 steps/min; Step length ~70-82 cm; Speed 1.2-1.4 m/s. $F_{GRF}$.
• Pathological Gaits:
  • Trendelenburg: Gluteus medius weakness → pelvic drop.
  • Antalgic: Pain-induced → shortened stance phase.
• Orthotics: External devices.
  • Principles: Three-point pressure for control/correction.
  • Materials: Thermoplastics. demonstrating force application)
• Prosthetics: Artificial limbs.
  • Principles: Alignment, socket fit, energy storage & return.

⭐ A three-point pressure system is a fundamental biomechanical principle used in many orthotic designs to control movement or provide correction.

High‑Yield Points - ⚡ Biggest Takeaways

• Stress shielding causes bone loss around stiff implants, risking loosening.
• Wolff's Law: Bone remodels based on applied stress, impacting healing and implant stability.
• Joint reaction forces influence implant longevity and osteoarthritis progression.
• Biomechanics guides fracture fixation choices: screws, plates, nails for optimal stability.
• Gait analysis aids in diagnosing and managing neuromuscular and musculoskeletal disorders.
• Tendon/ligament failure mechanisms (e.g., ACL) are key to understanding sports injuries.
• Ergonomics applies biomechanics to prevent work-related musculoskeletal disorders (WMSDs).