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Orthopaedic Implants & Principles

🔩 Orthopaedic Implants & Principles

Fixation biomechanics, antibiotic prophylaxis, total hip replacement, implant failure, and DVT prophylaxis — core perioperative orthopaedics.

Principles of Fracture Fixation

The fundamental principle of orthopaedic fixation is providing sufficient mechanical stability to allow biological healing. Stability requirements depend on the healing pathway chosen — primary (direct) bone healing requires absolute stability; secondary (indirect) healing via callus requires relative stability. Choosing the wrong fixation type disrupts the intended biology.

🔵 Absolute Stability → Primary Bone Healing

  • No movement at fracture site (strain <2%)
  • Direct cortical remodelling — Haversian remodelling
  • No visible callus on X-ray
  • Requires anatomical reduction + rigid fixation
  • Methods: Lag screws, compression plating (DCP), absolute rigidity
  • Indications: Intra-articular fractures (must be anatomical), simple diaphyseal fractures requiring anatomical reduction
  • Lag screw technique: gliding hole in near cortex, thread hole in far cortex → compression across fracture

🟠 Relative Stability → Secondary Bone Healing

  • Controlled micro-motion at fracture (strain 2–10%)
  • Indirect healing via periosteal callus (endochondral ossification)
  • Visible callus on X-ray — reassuring sign of healing
  • Biological fixation — preserve soft tissue envelope
  • Methods: Intramedullary nails, bridge plating, external fixators, cast immobilisation
  • Indications: Comminuted/multi-fragmentary fractures, diaphyseal fractures, open fractures (ex-fix initially)
  • Callus = biology working → do NOT disturb
Fixation Methods in Detail
🔩
Intramedullary Nails
Relative stability — load-sharing vs load-bearing
  • Load-sharing: Nail + bone share load together. Requires some cortical contact. Used in diaphyseal fractures with cortical contact (e.g., tibial nail, femoral nail). Allows early weight-bearing.
  • Load-bearing: Nail bears all load. Used when cortical contact is absent (severely comminuted). Nail takes 100% of load → stress risers at locking screw holes.
  • Static locking: Both proximal + distal locking screws — prevents shortening and rotation. Initial locking mode.
  • Dynamic locking: Removal of one set of locking screws to allow controlled axial micromotion → stimulates callus. Done if healing delayed (dynamisation).
  • Reaming: Widens medullary canal, allows larger nail, improves endosteal blood supply (destroys initially, then promotes via extraosseous collaterals). Reaming may cause fat embolism.
  • Common nails: Femoral (antegrade/retrograde), tibial, humeral, PFNA (proximal femoral nail antirotation — for trochanteric NOF fractures)
🔧
Plates & Screws
Versatile — compression to bridging
  • Dynamic Compression Plate (DCP): Oval screw holes allow screw eccentrically placed → tightening pulls fracture fragments together (compression). Absolute stability.
  • Locking Compression Plate (LCP): Screws lock into plate at fixed angle — like internal fixator. Works without plate-bone contact (preserves periosteum). Relative stability. Used in osteoporotic bone, periarticular fractures.
  • Bridge plate: Spans comminuted zone — does not fix intermediate fragments. Relative stability / biological fixation.
  • Buttress plate: Resists shear / prevents angular collapse. Used in metaphyseal fractures (distal tibia, proximal tibia).
  • Lag screw principle: Near cortex overdrilled (gliding hole) → screw threads only grip far cortex → tightening compresses fracture. Can use cortical screw as lag screw with overdrilling.
🏗️
External Fixation
Temporary or definitive
  • Indications: Open fractures (temporary stabilisation before definitive surgery), damage control orthopaedics (polytrauma), infected nonunion, pelvic ring injuries, soft tissue compromise
  • Damage Control Orthopaedics (DCO): Ex-fix in first 24–48 hours → definitive fixation after physiological optimisation (>5 days). Reduces ‘second hit’ phenomenon in polytrauma.
  • Complications: Pin site infection, pin loosening, stiffness, delayed conversion to IM nail (increases infection risk if left >2 weeks)
  • Ilizarov / circular frame: Fine wire tensioned frames — allows distraction osteogenesis (Ilizarov effect). Used for nonunion, limb lengthening, deformity correction.
🦴
Arthroplasty (Joint Replacement)
When fixation is not appropriate
  • Hemiarthroplasty: One joint surface replaced (e.g., femoral head only in displaced intracapsular NOF fractures in elderly). Faster, less blood loss than THR. Austin-Moore (uncemented) vs Thompson’s (cemented — preferred).
  • Total joint replacement: Both surfaces. THR, TKR, TSR.
  • Cemented vs cementless: See THR tab.
  • Revision arthroplasty: For failed primary replacements. More complex — bone loss, scarring, infection risk.

Biomechanical Principle — Strain Theory (Perren)

Perren’s strain theory: Strain = change in gap / original gap length. Granulation tissue tolerates up to 100% strain. Cartilage: up to 10%. Bone: only 2%. Therefore, a small fracture gap with rigid fixation = very low strain = bone can heal. A large gap with rigid fixation = high strain per unit length = bone cannot bridge. A comminuted fracture with relative fixation distributes strain over many fragments = each sees low strain despite motion. This is why biological bridging fixation works for comminuted fractures.

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