High Yield Overview

IMPLANT AND FRACTURE BIOMECHANICS

Load Sharing | Stress Shielding | Fracture Healing Mechanics

4xstrength increase with double diameter

8xstiffness increase with double diameter

50-80%stress shielding with rigid plate

3mmoptimal interfragmentary gap

Fixation Construct Types

Absolute Stability

PatternNo interfragmentary motion, direct bone healing

TreatmentCompression plating, lag screws

Relative Stability

PatternControlled micromotion, callus formation

TreatmentBridge plating, intramedullary nails

Flexible Fixation

PatternElastic deformation, secondary healing

TreatmentExternal fixation, elastic nails

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

Examiner's Pearls

"
AO principles: Reduction, fixation, preservation of blood supply, early mobilization
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Locked plating converts screws into fixed-angle device (more like external fixator)
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Composite beam effect: Plate and bone together stronger than sum of parts
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Elastic modulus mismatch causes stress concentration at implant-bone interface

Clinical Imaging

Imaging Gallery

HIPS model for testing of lag screw fixation strength: a) unstable fracture model; and b) HIPS base fixture, shown in cross-sectional view and in assembly with material test system for application of — HIPS model for testing of lag screw fixation strength: a) unstable fracture model; and b) HIPS base fixture, shown in cross-sectional view and in asseCredit: Kouvidis GK et al. via J Orthop Surg Res via Open-i (NIH) (Open Access (CC BY))

a) DHS single lag screw, and b) Endovis dual lag screws with self-drilling and self-tapping screw tip.Credit: Kouvidis GK et al. via J Orthop Surg Res via Open-i (NIH) (Open Access (CC BY))

Lag screw failure modes: a) DHS varus collapse and subsequent cut-out failure, b) lag screw bending in absence of cut-out, and c) axial migration of distal Endovis screw, leading to cut-out despite mi — Lag screw failure modes: a) DHS varus collapse and subsequent cut-out failure, b) lag screw bending in absence of cut-out, and c) axial migration of dCredit: Kouvidis GK et al. via J Orthop Surg Res via Open-i (NIH) (Open Access (CC BY))

X-ray showing vertical shear intra-capsular fractured neck of femur with comminution (AP view)Credit: Mukherjee P et al. via Strategies Trauma Limb Reconstr via Open-i (NIH) (Open Access (CC BY))

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 screws—no interfragmentary motion, direct bone healing) or relative stability (bridge plating, intramedullary nails—controlled 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 flexibility—longer 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 diameter⁴). Screw pullout strength depends on thread engagement; bicortical screws are 2-3× stronger than unicortical fixation.

Mnemonic

FREPAO Principles of Fracture Fixation

Fracture reduction

Anatomic (articular) or relative (diaphyseal) alignment

Rigid fixation

Absolute vs relative stability based on fracture pattern

Early mobilization

Prevent stiffness, maintain function during healing

Preservation of blood supply

Minimize soft tissue stripping, preserve periosteum

Memory Hook:FREP your fracture: Follow AO principles for successful fixation!

Mnemonic

STRIPPEDScrew Failure Modes

Shear

Transverse force across screw shaft (highest stress at bone-implant interface)

Tension

Pullout force along screw axis (resisted by threads)

Rotation

Torsional failure during insertion or loosening

Interface

Bone-screw interface failure from osteoporosis or over-torquing

Pullout

Thread stripping from inadequate purchase or excessive tension

Elastic mismatch

Stress concentration from modulus difference

Debris

Particulate wear leads to osteolysis around screw

Memory Hook:Don't get STRIPPED: Know how screws fail to prevent fixation failure!

Mnemonic

STABLEFactors Affecting Fracture Healing with Implants

Stability

Absolute (direct healing) vs relative (callus formation)

Tissue perfusion

Preserve blood supply, minimize stripping

Alignment

Anatomic reduction for articular, acceptable for diaphyseal

Biology

Patient factors (age, smoking, diabetes, medications)

Load transmission

Implant stiffness matches healing phase (dynamic compression)

Early motion

Mobilization stimulates healing without disrupting fixation

Memory 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

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

AO Principles of Fracture Fixation

Rüedi TP, Murphy WM • AO Foundation (2007)

Key Findings:

Four core principles: Fracture reduction, stable fixation, preservation of blood supply, early mobilization
Absolute stability (compression, lag screws) for simple fractures
Relative stability (bridge plating, nailing) for comminuted fractures
Biological fixation minimizes soft tissue disruption

Clinical Implication: AO principles remain foundation of modern fracture fixation. Biological plating has shifted away from rigid compression in favor of relative stability.

Limitation: Principles evolve with technology; locked plating changes traditional AO concepts.

Stress Shielding and Bone Remodeling

Uhthoff HK, et al • J Bone Joint Surg Br (2006)

Key Findings:

Rigid plate fixation reduces cortical stress by 50-80%
Bone resorption occurs beneath plate within 6-12 weeks
Refracture rate 5-20% after plate removal if done before remodeling
Locked plates show less stress shielding than conventional plates

Clinical Implication: Delayed plate removal (12-18 months) allows bone remodeling. Bridge plating or locked plating preferred for diaphyseal fractures to minimize stress shielding.

Limitation: Animal studies; human data limited by ethical constraints.

Working Length and Construct Failure

Stoffel K, et al • Bone (2003)

Key Findings:

Biomechanical study: Longer working length distributes strain over more screws
Short working length concentrates stress at screws nearest fracture
Fatigue failure more common with short working length in load-bearing scenarios
Optimal working length = 2-3 screw holes on each side for most diaphyseal fractures

Clinical Implication: Balance rigidity (short working length) with load distribution (long working length) based on fracture pattern and bone quality.

Limitation: Cadaveric study; does not account for bone healing and load sharing over time.

Locked Plating Biomechanics

Smith WR, et al • J Orthop Trauma (2007)

Key Findings:

Locked screws create fixed-angle construct (like internal fixator)
Does not require plate-bone compression (preserves periosteal blood supply)
Unicortical screws acceptable in some scenarios (reduces soft tissue dissection)
Higher rate of non-union if no cortical contact (over-reliance on implant)

Clinical Implication: Locked plating allows biological fixation but requires cortical contact for load sharing. Not a substitute for reduction.

Limitation: Clinical outcomes vary; technique-sensitive for achieving balance between stability and biology.

Screw Pullout Strength in Osteoporotic Bone

Patel PS, et al • Injury (2007)

Key Findings:

Pullout strength reduced 50-70% in osteoporotic bone
Bicortical purchase increases strength 2-3 fold
Bone cement augmentation increases pullout strength by 30-50%
Larger diameter screws provide exponentially better purchase

Clinical Implication: Use bicortical screws, larger diameter, and consider cement augmentation in severe osteoporosis.

Limitation: Ex vivo testing; in vivo loading and healing differ.

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Stress Shielding (~2 min)

EXAMINER

"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."

EXCEPTIONAL ANSWER

This is **stress shielding**, a phenomenon where the implant bears the majority of load, reducing stress on the underlying bone. According to Wolff's Law, bone adapts to reduced stress by resorbing. The rigid plate has reduced cortical bone stress by 50-80%, leading to cortical thinning and porosity. This is a normal adaptive response but increases refracture risk if the plate is removed prematurely. Management involves counseling the patient that the bone needs time to remodel. If plate removal is desired, I would delay until at least 18-24 months post-injury to allow bone adaptation. I would warn about refracture risk (5-20%) and consider prophylactic measures such as protected weight-bearing after removal.

KEY POINTS TO SCORE

Define stress shielding: Implant bears load, bone resorbs

Reference Wolff's Law

Explain 50-80% stress reduction with rigid plates

Management: Delayed removal, counsel on refracture risk

COMMON TRAPS

✗Confusing stress shielding with implant failure or infection

✗Not mentioning Wolff's Law

✗Recommending immediate plate removal without considering remodeling time

LIKELY FOLLOW-UPS

"How long would you wait before removing the plate?"

"What is the refracture rate after plate removal?"

"How do locked plates differ in stress shielding?"

VIVA SCENARIOChallenging

Scenario 2: Working Length Selection (~3 min)

EXAMINER

"You are plating a comminuted mid-shaft tibial fracture. Discuss how you would determine the working length of your construct and the biomechanical rationale."

EXCEPTIONAL ANSWER

Working length is the distance between the nearest screws on either side of the fracture. It determines construct flexibility and load distribution. For this **comminuted mid-shaft fracture**, I would use a **bridge plating technique with a longer working length**. The rationale is that a longer working length distributes load over more screws, reducing stress concentration at any single screw. This creates a more flexible construct that allows micromotion and callus formation, which is essential for healing in comminuted fractures. I would aim for 2-3 screw holes (or 4-6 cortices) on each side of the fracture zone. The trade-off is that longer working length increases implant stress and strain at the fracture, but in this load-sharing scenario with expected callus formation, this is acceptable. Shorter working length would be too rigid, risk stress shielding, and concentrate stress at the fracture-adjacent screws.

KEY POINTS TO SCORE

Define working length accurately

Explain relationship between working length and construct stiffness

Justify longer working length for this comminuted fracture

Mention load distribution and callus formation benefits

COMMON TRAPS

✗Choosing short working length (articular fracture thinking)

✗Not explaining the biomechanical trade-offs

✗Forgetting that comminuted fractures need relative stability

LIKELY FOLLOW-UPS

"What is the relationship between working length and stiffness?"

"When would you use a short working length?"

"How does cortical contact affect your working length decision?"

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.

Australian Context

Australian Epidemiology and Practice

Implant Biomechanics in Australian Practice:

AO principles form the foundation of fracture fixation teaching in Australian orthopaedic training programs
Major trauma centres (Royal Melbourne, Westmead, Royal Adelaide, Royal Brisbane) manage complex fractures requiring sophisticated biomechanical understanding
Australian trauma registries contribute data on implant failure rates and modes

RACS Orthopaedic Training Relevance:

Implant and fracture biomechanics are core FRACS Basic Science examination topics
Viva scenarios frequently test AO principles, stress shielding, working length, and screw mechanics
Candidates must understand the difference between load-bearing and load-sharing constructs
Material properties (elastic modulus, yield strength) are commonly tested in written examinations

Australian Implant Supply:

TGA (Therapeutic Goods Administration) regulates all orthopaedic implants in Australia
Major implant suppliers (DePuy Synthes, Stryker, Smith+Nephew, Zimmer Biomet) provide comprehensive product ranges
Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) tracks implant performance for arthroplasty
Prostheses List determines private health insurance coverage for implants

PBS (Pharmaceutical Benefits Scheme) Considerations:

Bone stimulators and adjuncts (BMP, bone graft substitutes) have limited PBS coverage
Antibiotic cement is PBS-subsidised for arthroplasty procedures
rhBMP-2 available through Special Access Scheme for specific indications

eTG (Therapeutic Guidelines) Recommendations:

Antibiotic prophylaxis guidelines for implant surgery (cefazolin standard)
VTE prophylaxis protocols for fracture fixation surgery
Wound management and infection prevention in trauma surgery

Australian Research Contributions:

Australian researchers have contributed significantly to understanding of locked plating biomechanics
Melbourne Orthopaedic Group and other centres publish on fixation techniques
Collaboration with AO Foundation for ongoing research and education