Implant and Fracture Biomechanics

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

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.

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.

Perren's interfragmentary strain theory is the unifying concept linking construct stability to the tissue that can form in the fracture gap, and examiners expect it.

Strain at the fracture is the relative change in gap width under load — the interfragmentary movement divided by the original gap width. A tissue can only form and survive in the gap if the local strain stays below the strain tolerance of that tissue:

Two clinically vital corollaries follow:

• A gap that is too small can paradoxically fail to heal. With a tiny gap, even a small absolute movement produces a very high percentage strain that exceeds bone's tolerance — so an imperfectly stable simple fracture with a hairline gap can progress to nonunion. The answer is either true absolute stability (compress the gap towards zero motion) or deliberate relative stability with a larger effective gap.
• Comminution helps healing. Spreading the same total motion across many fracture lines means each individual gap sees only a fraction of the strain. This is why multifragmentary fractures bridged with a flexible construct heal readily by callus, whereas a single transverse gap is far less forgiving.

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.

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

Stiffness is inversely proportional to working length cubed: Doubling the working length reduces stiffness by 8-fold.

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

The tension band principle converts the tensile (distraction) force on the convex side of an eccentrically loaded bone into compression at the far cortex. An implant (wire, plate or band) placed on the tension side resists distraction; as the bone is loaded, the fragments are driven together on the opposite cortex.

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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.

Applying Biomechanics to Fixation Decisions

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

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.

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 bending strength roughly 8-fold (section modulus proportional to diameter³) and bending/torsional rigidity roughly 16-fold (area and polar moment of inertia proportional to diameter⁴). Screw pullout strength depends on thread engagement; bicortical screws are 2-3× stronger than unicortical fixation.