Load Sharing | Stress Shielding | Fracture Healing Mechanics
Fixation Construct Types
Critical Must-Knows
- Wolff's Law: Bone adapts to mechanical stress by remodeling architecture
- Stress shielding occurs when implant bears majority of load, leading to bone resorption
- Working length of construct determines flexibility (longer = more flexible)
- Screws fail by pullout or shear; plates fail by bending or fatigue fracture
- Torsional rigidity proportional to diameter to the 4th power
Clinical Pearls
- "AO principles: Reduction, fixation, preservation of blood supply, early mobilization
- "Locked plating converts screws into fixed-angle device (more like external fixator)
- "Composite beam effect: Plate and bone together stronger than sum of parts
- "Elastic modulus mismatch causes stress concentration at implant-bone interface
Clinical Imaging
Imaging Gallery
Critical Implant Biomechanics Exam Points
Load Sharing vs Load Bearing
Load sharing = implant and bone share load (bridge plating). Load bearing = implant carries all load (comminuted fracture, bone loss). Determines implant selection.
Working Length
Distance between nearest screws on each side of fracture. Longer working length = more flexible construct = more callus but more implant stress.
Stress Shielding
Bone resorption from implant bearing load. Rigid plates reduce bone stress by 50-80%. Leads to refracture risk after implant removal.
Screw Mechanics
Pullout strength proportional to thread engagement. Bicortical screws 2-3x stronger than unicortical. Stripping torque defines maximum tightness.
At a Glance
Fracture fixation biomechanics follows the AO principles (FREP): anatomic or relative Fracture reduction, Rigid fixation appropriate to fracture pattern, Early mobilization, and Preservation of blood supply. Constructs provide either absolute stability (compression plating, lag screwsâno interfragmentary motion, direct bone healing) or relative stability (bridge plating, intramedullary nailsâcontrolled micromotion, callus formation). Stress shielding occurs when rigid implants bear 50-80% of load, causing bone resorption per Wolff's Law and risking refracture after implant removal. The working length (distance between nearest screws on each side of fracture) determines construct flexibilityâlonger working length increases flexibility but also implant stress. Critical geometric relationships include: doubling implant diameter increases strength 4-fold and stiffness 8-fold (torsional rigidity proportional to diameterâŽ). Screw pullout strength depends on thread engagement; bicortical screws are 2-3Ă stronger than unicortical fixation.
FREPAO Principles of Fracture Fixation
| F | Fracture reduction Anatomic (articular) or relative (diaphyseal) alignment |
| R | Rigid fixation Absolute vs relative stability based on fracture pattern |
| E | Early mobilization Prevent stiffness, maintain function during healing |
| P | Preservation of blood supply Minimize soft tissue stripping, preserve periosteum |
| F | Fracture reduction Anatomic (articular) or relative (diaphyseal) alignment | E | Early mobilization Prevent stiffness, maintain function during healing |
| R | Rigid fixation Absolute vs relative stability based on fracture pattern | P | Preservation of blood supply Minimize soft tissue stripping, preserve periosteum |
Hook:FREP your fracture: Follow AO principles for successful fixation!
STRIPPEDScrew Failure Modes
| S | Shear Transverse force across screw shaft (highest stress at bone-implant interface) |
| T | Tension Pullout force along screw axis (resisted by threads) |
| R | Rotation Torsional failure during insertion or loosening |
| I | Interface Bone-screw interface failure from osteoporosis or over-torquing |
| P | Pullout Thread stripping from inadequate purchase or excessive tension |
| E | Elastic mismatch Stress concentration from modulus difference |
| D | Debris Particulate wear leads to osteolysis around screw |
| S | Shear Transverse force across screw shaft (highest stress at bone-implant interface) | I | Interface Bone-screw interface failure from osteoporosis or over-torquing | D | Debris Particulate wear leads to osteolysis around screw |
| T | Tension Pullout force along screw axis (resisted by threads) | P | Pullout Thread stripping from inadequate purchase or excessive tension | ||
| R | Rotation Torsional failure during insertion or loosening | E | Elastic mismatch Stress concentration from modulus difference |
Hook:Don't get STRIPPED: Know how screws fail to prevent fixation failure!
STABLEFactors Affecting Fracture Healing with Implants
| S | Stability Absolute (direct healing) vs relative (callus formation) |
| T | Tissue perfusion Preserve blood supply, minimize stripping |
| A | Alignment Anatomic reduction for articular, acceptable for diaphyseal |
| B | Biology Patient factors (age, smoking, diabetes, medications) |
| L | Load transmission Implant stiffness matches healing phase (dynamic compression) |
| E | Early motion Mobilization stimulates healing without disrupting fixation |
| S | Stability Absolute (direct healing) vs relative (callus formation) | A | Alignment Anatomic reduction for articular, acceptable for diaphyseal | L | Load transmission Implant stiffness matches healing phase (dynamic compression) |
| T | Tissue perfusion Preserve blood supply, minimize stripping | B | Biology Patient factors (age, smoking, diabetes, medications) | E | Early motion Mobilization stimulates healing without disrupting fixation |
Hook:Keep it STABLE: Control these factors for optimal fracture healing!
Overview and Introduction
Introduction to Implant Biomechanics
Successful fracture fixation requires understanding the biomechanical interaction between implant, bone, and healing tissue. The choice of fixation construct determines the stability type (absolute vs relative), which in turn influences the healing mechanism (primary vs secondary).
Key Biomechanical Principles:
- Load sharing: Implant and bone distribute load together
- Load bearing: Implant carries all load (comminuted fractures)
- Stress shielding: Excessive implant rigidity leads to bone resorption
- Working length: Distance between screws determines construct flexibility
Concepts and Fundamental Principles
Wolff's Law and Bone Adaptation
Wolff's Law states that bone adapts its structure to the mechanical demands placed upon it. Increased stress stimulates bone formation; decreased stress leads to resorption. This principle underlies stress shielding after rigid internal fixation.
Stress Shielding Mechanism
- Rigid implant bears majority of load
- Bone stress reduced by 50-80%
- Remodeling leads to cortical thinning, porosity
- Refracture risk after implant removal
Clinical Implications
- Bridge plating preferred over compression plating in some cases
- Locked plates act as internal fixators, less stress shielding
- Gradual load sharing as fracture heals
- Delayed removal allows bone adaptation before unloading implant
Mechanical Properties of Implants
Elastic modulus mismatch between implant and bone creates stress concentration at the implant-bone interface, particularly at screw holes. This can lead to peri-implant fractures.
Load Sharing vs Load Bearing
Definitions
- Load Sharing: Implant and bone both transmit load across the fracture site. Bone contributes to mechanical stability. Examples: Bridge plating of simple fractures, intramedullary nailing with cortical contact.
- Load Bearing: Implant carries all or most of the load. Bone contributes minimally due to comminution, bone loss, or non-union. Examples: Locked plating of segmental defects, arthroplasty, massive allografts.
Load Sharing vs Load Bearing Constructs
| Feature | Load Sharing | Load Bearing | Clinical Example |
|---|---|---|---|
| Bone contribution | Significant (50%+) | Minimal (under 20%) | Simple vs comminuted fracture |
| Implant stress | Lower, distributed | Higher, concentrated | Bridge plate vs locking plate with gap |
| Failure risk | Bone failure more likely | Implant fatigue fracture risk | Refracture vs plate breakage |
| Healing requirement | Callus formation essential | Biological healing may not occur | Hypertrophic vs atrophic non-union |
Recognize Load-Bearing Scenarios
Load-bearing constructs require stronger implants and carry higher failure risk. If bone cannot contribute (segmental defect, severe comminution, infection with bone loss), consider stronger constructs (double plating, reconstruction nail, arthroplasty) or biological augmentation (bone graft, BMP).
Working Length and Construct Stiffness
Working Length Defined
Working length is the distance between the nearest screws on either side of the fracture site. It determines the flexibility of the construct.
- Short working length = rigid construct = less callus formation = higher implant stress
- Long working length = flexible construct = more callus = lower implant stress but higher strain at fracture
Short Working Length
Indications:
- Articular fractures requiring anatomic reduction
- Metaphyseal fractures with good bone quality
- Fractures where callus is undesirable
Advantages: Maximum stability, minimal motion
Disadvantages: Stress shielding, higher implant stress, less biological stimulus
Long Working Length
Indications:
- Diaphyseal fractures amenable to relative stability
- Osteoporotic bone requiring load distribution
- Fractures where callus formation is desirable
Advantages: Load distribution, biological healing, lower implant stress
Disadvantages: More motion at fracture, potential for delayed union if too flexible
Stiffness is inversely proportional to working length cubed: Doubling the working length reduces stiffness by 8-fold.
Screw Biomechanics
Pullout Strength
Screw pullout strength depends on:
- Thread engagement: Deeper threads = more surface area
- Outer diameter: Larger diameter = more bone engagement
- Bone density: Osteoporotic bone has 50-70% lower pullout strength
- Cortical vs cancellous: Cortical provides majority of holding power
Screw Types and Mechanics
| Screw Type | Mechanism | Advantage | Disadvantage |
|---|---|---|---|
| Cortical screw | Fine threads, cut own path | Maximum holding in cortical bone | Poor purchase in cancellous bone |
| Cancellous screw | Coarse threads, self-tapping | Good purchase in metaphyseal bone | Weaker in pure cortical bone |
| Locking screw | Threads engage plate, fixed angle | No compression on bone, unicortical OK | Cannot compress fracture, more expensive |
| Lag screw | Gliding hole, compression across fracture | Absolute stability, interfragmentary compression | Requires precise technique, can overdistract |
Implant Materials and Stiffness
Material Properties That Matter
The elastic (Young's) modulus of an implant determines how much load it carries relative to bone. A stiffer implant shields the bone from stress; a more compliant implant transfers more load to healing bone. Fatigue strength governs how many loading cycles an implant survives before failure - a key consideration in load-bearing constructs spanning a fracture gap.
Modulus and Behaviour of Common Implant Materials
| Material | Elastic Modulus (GPa) | Key Property | Biomechanical Implication |
|---|---|---|---|
| Cortical bone | 15-20 | Reference value | Implants are far stiffer, driving stress shielding |
| Cancellous bone | 0.1-2 | Highly variable with BMD | Poor screw purchase in metaphysis when osteoporotic |
| Titanium alloy (Ti-6Al-4V) | ~110 | Lower modulus, biocompatible, MRI-friendly | Less stiff than steel, lower stress shielding, notch-sensitive |
| Stainless steel (316L) | ~200-210 | High stiffness and ductility | Stiffer construct, more stress shielding, allows contouring |
| Cobalt-chrome | ~210-240 | High wear and fatigue resistance | Very stiff; used for bearing surfaces and stems |
Modulus mismatch
The order of stiffness is cobalt-chrome greater than stainless steel greater than titanium greater than cortical bone. The larger the modulus mismatch between implant and bone, the greater the stress concentration at the bone-implant interface (e.g. at the last screw hole) and the more stress shielding under the plate.
Controversies and Areas of Uncertainty
Optimal construct stiffness
Very stiff bridging constructs can suppress callus and lead to delayed/nonunion (especially distal femur). Strategies to "soften" constructs - far cortical locking, longer working length, titanium plates, fewer screws near the fracture - aim to allow healthy micromotion, but the ideal target stiffness is still debated.
Routine implant removal
Whether to remove plates after union (to reverse stress shielding and reduce refracture risk) is contested. Removal carries its own risks (refracture through screw holes, neurovascular injury) and is increasingly reserved for symptomatic hardware rather than performed routinely.
Unicortical vs bicortical locking
Unicortical locked screws reduce dissection but provide less torsional and pullout resistance; the balance between soft-tissue preservation and mechanical security remains case-dependent.
Reaming in nailing
Reamed nailing improves fixation and union in closed fractures but the marginal benefit in open tibial fractures is small; debate continues over the embolic and biological cost of reaming versus the mechanical gain.
Clinical Relevance and Applications
Applying Biomechanics to Fixation Decisions
Fracture Pattern Determines Construct:
- Simple fractures: Absolute stability via compression plating, lag screws
- Comminuted fractures: Relative stability via bridge plating, nailing
- Articular fractures: Anatomic reduction + absolute stability
Implant Selection Considerations:
- Match implant stiffness to fracture personality
- Longer working length for comminuted patterns (more flexible)
- Bicortical screws for maximum pullout strength
- Locked plates act as internal external fixators
Avoiding Complications:
- Stress shielding: Use less rigid constructs when possible
- Implant failure: Ensure adequate working length and screw density
- Refracture after removal: Gradual loading, delay high-impact activities
Evidence Base and Key Studies
Strain Theory & the Scientific Basis of Biological Internal Fixation
- Interfragmentary strain theory: tissue at the fracture gap can only form if local strain stays below the strain tolerance of that tissue (granulation tissue tolerates high strain, lamellar bone very little)
- Flexible (relative) stability induces callus; rigid fixation of a small gap with even minimal motion produces high strain and impairs direct healing
- The internal fixator splints rather than compresses, preserving periosteal blood supply and enabling minimally invasive percutaneous osteosynthesis (MIPO)
- Bone loss under plates is attributed to stress shielding and necrosis-induced remodelling rather than to plate-bone contact alone
Internal Plate Fixation: Stress Shielding & Cortical Porosis
- Rigid plates cause cortical porosis, delayed bridging and refracture after plate removal
- Histomorphometry showed necrosis predominantly in the periosteal cortex and porosis in the endosteal cortex, with no positive correlation between them
- Evidence favoured stress shielding (not interference with cortical perfusion from plate-bone contact) as the dominant cause of bone loss
- Motivated axially compressible plate designs that reduce stress shielding during remodelling
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Stress Shielding (~2 min)
"A patient returns 2 years after femoral shaft fracture fixation with a plate. X-rays show cortical thinning beneath the plate. Explain the mechanism and management."
Scenario 2: Working Length Selection (~3 min)
"You are plating a comminuted mid-shaft tibial fracture. Discuss how you would determine the working length of your construct and the biomechanical rationale."
Scenario 3: Fixation Failure in Osteoporotic Bone (~3 min)
"An 82-year-old woman has a distal femoral fracture fixed with a non-locking plate. At 6 weeks the screws have pulled out and the construct has collapsed into varus. Explain why this failed and how you would revise it."
MCQ Practice Points
Screw Pullout Strength Question
Q: What is the most important factor affecting screw pullout strength? A: Outer thread diameter and depth of engagement. Bicortical purchase increases strength 2-3x compared to unicortical. Bone density is also critical (osteoporotic bone has 50-70% reduced pullout strength).
Working Length Question
Q: How does doubling the working length affect construct stiffness? A: Reduces stiffness by 8-fold (stiffness is inversely proportional to working length cubed). Longer working length = more flexible = more callus but higher implant stress.
Stress Shielding Question
Q: What percentage of stress is reduced in bone beneath a rigid plate? A: 50-80% stress reduction. This leads to bone resorption (Wolff's Law) and refracture risk if plate removed before remodeling (12-18 months).
AO Principles Question
Q: What are the four AO principles of fracture fixation? A: FREP: Fracture reduction, Rigid fixation (absolute or relative stability), Early mobilization, Preservation of blood supply.
Guidelines, Registries & Global Practice
Global Epidemiology
- Fractures requiring fixation are among the commonest procedures in orthopaedics worldwide; the global incidence of fragility fractures is rising sharply with population ageing, increasingly shifting fixation challenges into osteoporotic bone.
- Implant-related complications (loss of fixation, nonunion, peri-implant fracture, fatigue failure) are a leading reason for unplanned reoperation after fracture surgery globally.
- Osteoporosis disproportionately affects fixation in older adults and in low- and middle-income settings where bone-protection therapy and implant choice may be limited.
Side-by-Side Guideline Frameworks
How Major Frameworks Approach Fixation Biomechanics
| Body (Region) | Core Position | Practical Emphasis |
|---|---|---|
| AO Foundation (global) | Anatomical reduction, stable fixation matched to pattern, blood-supply preservation, early motion | Absolute vs relative stability framework; biological internal fixation |
| BOA / BOAST (UK) | Standards for open fractures and fragility-fracture care emphasising soft tissues and timely definitive fixation | Construct that permits early weight-bearing in the elderly |
| AAOS (US) | Evidence-based clinical practice guidelines (e.g. hip fracture) supporting stable fixation and early mobilisation | Implant selection guided by fracture stability and bone quality |
| EFORT / European consensus | Promotes fragility-fracture networks and fixation strategies tailored to osteoporotic bone | Locked/augmented constructs and orthogeriatric co-management |
Across frameworks the biomechanical principles are universal - they differ mainly in emphasis on system-level care (open-fracture timing, orthogeriatric pathways) rather than in the underlying mechanics.
Registry Evidence
- Arthroplasty and implant registries - NJR (UK), AJRR (US), AOANJRR (Australia), SHAR (Sweden), the Norwegian and New Zealand registries - track implant survival and revision and have repeatedly shown how design and fixation choices affect longevity. The same registry methodology increasingly captures fracture-fixation devices and peri-implant fractures.
- Registry data reinforce that construct and material choice (e.g. fixation mode, modulus, bearing) measurably influence revision rates over time.
High- vs Limited-Resource Practice Variation
- Well-resourced settings: full range of locking plates, multiple alloys, intraoperative imaging, cement/augmentation and orthogeriatric pathways are available, allowing precise tailoring of stiffness and stability.
- Limited-resource settings: reliance on a narrower implant range, more external fixation and conventional plating, and emphasis on robust, low-cost constructs; sound biomechanical reasoning (working length, screw purchase, load sharing) becomes even more important when implant options are constrained.
IMPLANT AND FRACTURE BIOMECHANICS
Clinical summary
Key Concepts
- âąWolff's Law: Bone adapts to stress (increased stress = formation, decreased = resorption)
- âąStress shielding: Implant bears load, bone resorbs (50-80% stress reduction)
- âąWorking length: Distance between nearest screws (longer = more flexible)
- âąLoad sharing: Bone and implant share load (vs load bearing: implant carries all)
AO Principles (FREP)
- âąFracture reduction: Anatomic (articular) or relative (diaphyseal)
- âąRigid fixation: Absolute stability (compression) or relative (bridge plating)
- âąEarly mobilization: Prevent stiffness during healing
- âąPreservation of blood supply: Minimize stripping, biological fixation
Screw Mechanics
- âąPullout strength: Bicortical 2-3x stronger than unicortical
- âąDiameter effect: Pullout proportional to diameter squared
- âąOsteoporotic bone: 50-70% reduced holding power
- âąFailure modes: Pullout (tension), shear, stripping, fatigue
Working Length
- âąShort working length: Rigid, less callus, higher implant stress
- âąLong working length: Flexible, more callus, load distribution
- âąStiffness inversely proportional to length cubed (2x length = 8x less stiff)
- âąOptimal: 2-3 screw holes each side for diaphyseal fractures
Implant Properties
- âąStainless steel: 210 GPa modulus (10x bone)
- âąTitanium: 110 GPa modulus (5x bone)
- âąCortical bone: 20 GPa modulus
- âąElastic mismatch creates stress concentration at interface