# Biomechanics of Implant Fixation

## Fundamental Biomechanical Concepts

### Forces Acting on Bone and Implants

Bones and their fixation devices are subjected to several types of mechanical forces. Axial loading produces compression or tension along the long axis of the bone. Bending creates compression on one cortex and tension on the opposite cortex simultaneously. Torsion generates rotational forces that produce shear stress within the material. Shear forces act parallel to a surface rather than perpendicular to it. The fundamental principle of fracture fixation is that the construct must resist the dominant deforming forces acting at each specific anatomic location.

### Stress, Strain, and Material Properties

Stress is defined as force per unit area (measured in Pascals or N/m2), while strain is the change in length divided by original length (a dimensionless ratio). Young's modulus (E), the ratio of stress to strain, defines a material's stiffness. This property varies enormously across orthopedic materials:

| Material | Young's Modulus (GPa) | Clinical Relevance |
|----------|----------------------|-------------------|
| Stainless steel (316L) | ~200 | Greatest stiffness; highest stress shielding |
| Cobalt-chrome | ~210 | Arthroplasty bearings |
| Titanium alloy (Ti-6Al-4V) | ~110 | Less stress shielding; MRI compatible |
| Cortical bone | 17-20 | Native load-bearing tissue |
| Cancellous bone | 0.1-2 | Metaphyseal/epiphyseal region |

Stainless steel has a modulus of approximately 200 GPa, titanium alloy about 110 GPa, cortical bone 17-20 GPa, and cancellous bone only 0.1-2 GPa. The yield point represents the stress beyond which permanent (plastic) deformation occurs, while ultimate strength is the maximum stress a material can withstand before failure. In clinical practice, the most common mode of implant failure is fatigue failure, which occurs under repetitive cyclic loading at stress levels well below the ultimate strength.

### Load Sharing vs. Load Bearing

Fixation constructs function in one of two biomechanical modes. In load sharing, both the implant and the bone carry mechanical forces together. This requires cortical contact (bone-to-bone continuity) at the fracture site, places lower stress on the implant, and reduces the risk of fatigue failure. A lag screw with a neutralization plate applied to a simple fracture pattern is a classic load-sharing construct. In load bearing, the implant carries all or nearly all of the mechanical load because there is no cortical contact due to bone loss or comminution. This places much higher stress on the implant with greater risk of fatigue failure. A bridge plate spanning a comminuted zone exemplifies load-bearing fixation. Load-bearing constructs must span sufficient working length and typically require larger or stronger implants.

### Stress Shielding

Stress shielding occurs when there is a significant mismatch in stiffness between an implant and the surrounding bone. The rigid implant bears an excessive proportion of the load, effectively unloading the bone beneath it. Per Wolff's law, unloaded bone undergoes resorption. This phenomenon is more pronounced with stiffer materials (stainless steel more than titanium) and is clinically observed as proximal femoral resorption around stiff femoral stems in total hip arthroplasty and bone thinning beneath rigid plates left in place long-term. Strategies to mitigate stress shielding include using titanium alloys (lower modulus), employing shorter plate spans, choosing load-sharing constructs when anatomy permits, and selective implant removal.

## Screw Biomechanics

### Screw Anatomy

Understanding screw design requires familiarity with several geometric parameters. The outer diameter is the major diameter measured across the threads. The core (root) diameter is the diameter of the central shaft without threads. Pitch is the distance between adjacent threads. Lead is the distance the screw advances with one complete rotation. Thread depth equals half the difference between outer diameter and core diameter.

### Cortical Screws

Cortical screws are designed for purchase in dense cortical bone. They feature smaller pitch with more threads per unit length, providing extensive thread contact with hard bone. Most cortical screws require pre-drilling and tapping before insertion. They are fully threaded, gripping along their entire length within bone.

### Cancellous Screws

Cancellous screws are optimized for the lower-density trabecular bone found in metaphyses and epiphyses. They have larger pitch, deeper threads, and a greater thread-to-core diameter ratio than cortical screws, all of which maximize holding power in soft bone. Many are partially threaded, with threads only at the tip, which creates a lag effect by compressing the near and far fragments together. Most cancellous screws are self-tapping.

### Lag Screw Technique

The lag screw technique achieves interfragmentary compression by ensuring that the screw threads engage only the far fragment while the near cortex has a glide hole that allows the screw shaft to slide freely. As the screw is tightened, it draws the two fragments together under compression. This can be accomplished using either a partially threaded screw or by overdrilling the near cortex when using a fully threaded screw. The screw generates maximum compression when oriented perpendicular to the fracture line, and maximum resistance to displacement when perpendicular to the bone's long axis. Since these two ideals rarely coincide, the optimal screw trajectory bisects the angle between these two directions.

### Screw Pullout Strength

Pullout resistance depends on the outer diameter of the screw, thread depth, length of thread engagement in bone, and bone density. Pullout strength is proportional to the total thread purchase area. Cancellous screw designs maximize this purchase in weaker bone through deeper threads and larger diameter. In osteoporotic bone, augmentation strategies to improve purchase include polymethylmethacrylate (PMMA) cement, calcium phosphate cement, upsizing to larger diameter screws, and using locking screw technology.

## Plate Biomechanics

### Plate Functions

| Function | Mechanism | Example Application |
|----------|-----------|-------------------|
| Compression | Applies axial compression via eccentric screw placement | Simple transverse fracture with DCP |
| Neutralization | Protects underlying lag screw from bending/rotation/shear | Lag screw fixation of spiral fracture |
| Buttress | Supports articular surface against shear/displacement | Tibial plateau fracture |
| Bridge | Spans comminuted zone; provides relative stability | Comminuted distal femur fracture |
| Tension band | Converts tensile forces to compression at opposite cortex | Femoral shaft with lateral tension plate |

Plates serve five distinct biomechanical functions depending on application. A compression plate applies axial compression across a fracture using the dynamic compression principle of eccentric screw placement in oval holes. A neutralization plate protects an underlying lag screw fixation from bending, rotation, and shear forces. A buttress plate supports an articular surface against shear and displacement (as in tibial plateau fractures). A bridge plate spans a comminuted zone without attempting interfragmentary fixation, providing relative stability and allowing callus formation. A tension band plate is applied to the tension side of a bone and converts tensile forces into compression at the opposite cortex.

### Dynamic Compression Plate (DCP)

The DCP achieves compression through its sloped oval screw holes. When a screw is placed eccentrically (toward the far end of the hole) and tightened, it slides down the inclined plane of the hole, translating the bone fragment toward the fracture and generating compression. The limited-contact DCP (LC-DCP) introduced undercuts on the plate's undersurface to reduce the contact area between plate and bone, thereby better preserving periosteal blood supply.

### Locking Plates vs. Non-Locking (Conventional) Plates

| Feature | Conventional (Non-Locking) Plate | Locking Plate |
|---------|----------------------------------|---------------|
| Mechanism of stability | Friction between plate and bone | Fixed-angle screw-plate construct |
| Bone quality dependence | High (requires good screw purchase) | Low (functions in osteoporotic bone) |
| Plate contouring | Must precisely match bone surface | Less critical (no compression against bone) |
| Load distribution | Concentrated at individual screws | Distributed across all screws simultaneously |
| Periosteal blood supply | Compromised by plate-bone compression | Better preserved (no compression) |
| Best applications | Simple fractures, good bone quality | Periarticular fractures, osteoporotic bone, bridge plating |
| Biomechanical analogy | Plate pressed against bone | Internal external fixator |

Conventional (non-locking) plates rely on friction between the plate and bone surface for stability. Screw tightening compresses the plate against the bone, and the resulting friction prevents motion. This system requires good bone quality for adequate screw purchase and demands precise plate contouring to match the bone surface. Locking plates represent a fundamentally different biomechanical concept: the screw heads thread directly into the plate holes, creating a fixed-angle construct that functions as an internal external fixator. Because stability does not depend on friction between plate and bone, locking plates are advantageous in osteoporotic bone, periarticular fractures, and bridge plating scenarios. They provide better resistance to pullout in poor-quality bone and do not need to be perfectly contoured to the bone surface (though excessive contouring should be avoided as it can cause primary reduction loss). The fixed-angle design distributes load across all screws simultaneously rather than concentrating stress at individual screws.

### Working Length

Working length is defined as the distance between the two screws closest to the fracture on either side. A shorter working length produces a stiffer construct, which is appropriate for simple fractures treated with compression. A longer working length produces a more flexible construct that allows controlled micromotion, promoting callus formation in comminuted fractures treated with bridge plating. The surgeon must balance the need for flexibility (to stimulate healing) against the need for stiffness (to prevent hardware failure).

### Plate Material Considerations

Stainless steel (316L) offers greater stiffness, higher strength, excellent corrosion resistance, and superior fatigue strength. Titanium alloy (Ti-6Al-4V) has a lower modulus that produces less stress shielding, better biocompatibility, MRI compatibility, and promotes better periosteal blood flow due to its surface properties. For most fracture applications, titanium is generally preferred.

## Intramedullary Nail Biomechanics

Intramedullary nails are load-sharing devices positioned along the mechanical axis of the bone. They resist bending and rotation effectively but are less effective at resisting axial shortening unless interlocking screws are used. Reaming the canal allows insertion of a larger-diameter nail (increasing stiffness) and generates autograft debris that deposits at the fracture site. Interlocking screws at both ends control rotation and maintain length. The nail diameter is the primary determinant of both bending and torsional stiffness. The working length of the nail (distance between the most proximal and distal interlocking screws) also affects overall construct stiffness.

## External Fixation Biomechanics

External fixation uses a pin-to-bar construct that creates a stabilizing frame outside the body. Construct stiffness can be increased by using larger-diameter pins, increasing the number of pins, decreasing the distance between pins and the connecting bar, placing pins closer to the fracture, using stiffer bar materials, or stacking multiple bars. Among these variables, pin diameter is the most important single factor because stiffness is proportional to the fourth power of the radius. External fixation is employed for temporary stabilization of open fractures, definitive treatment of certain periarticular fractures, and limb lengthening procedures.

## Tension Band Principle

The tension band principle applies to fractures where one cortex experiences tensile force during functional loading, such as the olecranon, patella, greater trochanter, and medial malleolus. A wire or plate placed on the tension side of the bone converts the tensile forces into compressive forces at the fracture site during loading. This principle requires an intact cortex on the opposite (compression) side to serve as a fulcrum. The classic technique uses two parallel K-wires with a figure-of-eight wire loop on the tension surface.

<image>A biomechanical diagram comparing load-sharing vs. load-bearing plate fixation. On the left, show a simple transverse fracture with cortical contact fixed with a compression plate and lag screw — arrows showing load distributed between plate and bone. On the right, show a comminuted fracture with no cortical contact fixed with a bridge plate — arrows showing all load carried through the plate. Include stress distribution diagrams for each construct.</image>

<image>An illustration demonstrating lag screw technique. Show a cross-section through two bone fragments. The near cortex has a glide hole (overdrilled), the far cortex has a threaded hole. As the screw is tightened, it compresses the two fragments together. Include vector arrows showing the optimal screw angle: perpendicular to fracture line for maximum compression, perpendicular to bone axis for maximum resistance to shear, and the ideal compromise angle bisecting both.</image>

<image>A comparative illustration of locking vs. non-locking plate constructs in osteoporotic bone. On the left, a conventional plate with screws toggling and pulling out of porotic bone under load — showing the reliance on friction between plate and bone. On the right, a locking plate with fixed-angle screws maintaining their position in porotic bone — showing how the construct functions as an internal external fixator with load distributed across all screws simultaneously.</image>

## Clinical Pearls

Fixation strategy must match the fracture personality: simple patterns require absolute stability through compression, while comminuted patterns require relative stability through bridge plating or nailing. In osteoporotic bone, locking plates outperform conventional plates because their pullout strength depends less on bone quality. Stress shielding is a real clinical phenomenon, and choosing titanium implants with appropriate plate length helps minimize it. The lag screw technique is the most efficient method for achieving interfragmentary compression and should be mastered for all simple fracture patterns. Working length is a critical variable that the surgeon must consciously choose: too stiff across a comminuted zone prevents callus formation, while too flexible leads to hardware failure. Pin diameter is the dominant factor in external fixator stiffness because of its fourth-power relationship to bending rigidity. The tension band principle absolutely requires an intact opposite cortex to function as a fulcrum; if both cortices are comminuted, the construct will fail. Every fixation decision should consider the biomechanical environment holistically: patient weight and compliance, bone quality, and expected time to healing.

## References

- Perren SM. Evolution of the internal fixation of long bone fractures. *J Bone Joint Surg Br*. 2002;84(8):1093-1110.
- Gautier E, Sommer C. Guidelines for the clinical application of the LCP. *Injury*. 2003;34 Suppl 2:B63-76.
- AO Principles of Fracture Management. Ruedi TP, Buckley RE, Moran CG, eds. 3rd ed. Thieme; 2018.
- Egol KA, Kubiak EN, Fulkerson E, et al. Biomechanics of locked plates and screws. *J Orthop Trauma*. 2004;18(8):488-493.