Plating and Nailing – Principles and Mechanics of Implants

Internal fixation is not merely the placement of metal across a fracture. The implant, fracture geometry, bone quality, reduction technique and biological environment together determine the mechanical behaviour of the construct. A successful fixation strategy must provide sufficient stability for fracture healing without unnecessarily disturbing fracture biology.

Plates and intramedullary nails achieve stability through fundamentally different mechanical principles. Plates are generally positioned away from the mechanical axis of the bone and may function as compression, neutralisation, buttress, bridge or tension-band devices. Intramedullary nails lie closer to the mechanical axis and usually function as load-sharing devices, making them particularly advantageous for many diaphyseal fractures.

• Restore appropriate length, alignment and rotation.
• Restore articular congruity where the joint surface is involved.
• Provide stability appropriate for the desired mode of bone healing.
• Preserve blood supply to bone and soft tissues.
• Permit safe mobilisation of the limb and adjacent joints.
• Minimise complications related to prolonged immobilisation.
• Allow fracture healing without fatigue failure of the implant.

The objective is therefore not to create the stiffest possible construct. The objective is to create a construct with appropriate stability for that particular fracture.

Understanding the distinction between absolute and relative stability is central to fracture fixation.

Articular fractures generally demand anatomical reduction and stable fixation because even small residual incongruity may adversely affect joint mechanics. In contrast, multifragmentary diaphyseal fractures are commonly treated using relative stability while preserving the fracture biology.

Simple fracture → think anatomical reduction and compression when appropriate. Comminuted fracture → think alignment, biology and relative stability.

Interfragmentary strain describes the relative deformation occurring across a fracture gap. It can conceptually be expressed as:

A very small fracture gap undergoing a given amount of movement experiences greater strain than a larger fracture zone experiencing the same movement.

This concept helps explain why multifragmentary fractures can tolerate relative motion better than a simple fracture with a narrow fracture line. In comminution, deformation is distributed over a larger fracture zone.

• Bone formation requires a relatively low-strain environment.
• Cartilage and fibrous tissue can tolerate progressively greater deformation.
• Excessive movement may prevent progression to stable bone union.

Therefore implant mechanics should not be considered independently from the size and geometry of the fracture gap.

A load-sharing construct allows both the bone and implant to transmit mechanical load. A load-bearing construct transfers a greater proportion of the load through the implant because the bone itself provides little structural support.

Implant breakage is often the final manifestation of a biological or mechanical failure of fracture healing rather than simply a defective implant. If union does not occur, repetitive cyclic loading may eventually cause fatigue failure.

• Compression: forces push fracture fragments together.
• Tension: forces pull fragments apart.
• Shear: fragments slide parallel to each other.
• Bending: creates compression on one cortex and tension on the opposite cortex.
• Torsion: produces rotational forces around the long axis of the bone.

Implants must resist combinations of these forces until the fracture acquires sufficient biological stability.

Common orthopaedic implant materials include stainless steel and titanium alloys. The mechanical behaviour of an implant depends not only on the material but also its geometry, cross-sectional dimensions and manufacturing characteristics.

• Strength: ability to resist failure under load.
• Stiffness: resistance to elastic deformation.
• Elasticity: ability to return to original form after unloading.
• Plastic deformation: permanent deformation following loading beyond the elastic limit.
• Ductility: ability to deform substantially before fracture.
• Fatigue strength: ability to tolerate repeated cyclic loading.

The elastic modulus of titanium is lower than that of stainless steel and therefore relatively closer to cortical bone. Titanium also demonstrates favourable biocompatibility. However, clinical implant selection depends on multiple factors and cannot be reduced to material alone.

Orthopaedic implants are subjected to repetitive cyclic loading during walking and daily activity. Even stresses below the single-load failure strength of an implant may eventually produce fracture after a sufficiently large number of cycles.

This phenomenon is known as fatigue failure.

Fatigue failure becomes especially important when:

• The fracture fails to unite.
• A plate continuously bridges a large defect.
• There is inadequate medial or opposite cortical support.
• The implant is undersized.
• Stress concentration is created around empty holes or abrupt changes in construct stiffness.
• Patient loading exceeds the mechanical capacity of the fixation.

A plate may perform several different mechanical functions depending on how it is applied. The same implant may behave differently in two different fractures.

Before applying a plate, the surgeon should ask:

• Do I want absolute or relative stability?
• Is the fracture simple or comminuted?
• Is anatomical reduction necessary?
• Is the plate functioning as compression, neutralisation, buttress, bridge or tension band?
• Does the bone have adequate cortical contact to share load?
• Should conventional screws, locking screws or a hybrid construct be used?

Compression plating is designed to generate interfragmentary compression across a simple fracture. This increases stability by increasing friction between the fracture surfaces and limiting interfragmentary motion.

Dynamic compression plates use specially shaped screw holes. When a screw is placed eccentrically within the dynamic compression unit, tightening of the screw causes the plate to translate relative to the bone and produces axial compression across the fracture.

Compression plating is most appropriate when:

• The fracture is simple enough to reconstruct anatomically.
• Good bone contact can be obtained.
• Absolute stability is desired.
• The biological cost of direct reduction is acceptable.

Compression works best when the fracture surfaces can actually transmit load.

A lag screw generates interfragmentary compression by allowing the screw threads to engage only the far fragment while the near fragment is allowed to glide beneath the screw head.

For a fully threaded cortical screw used as a lag screw:

• A gliding hole is created in the near cortex.
• A smaller threaded hole is drilled in the far cortex.
• The far cortex is tapped where appropriate.
• When tightened, the screw head compresses the near fragment toward the far fragment.

Partially threaded cancellous screws may generate lag effect naturally if the threads are entirely beyond the fracture line.

Lag screws are typically inserted as perpendicular as practically possible to the fracture plane to maximise compression and minimise shear.

A neutralisation plate protects a primary interfragmentary fixation, usually lag screws, from bending, torsional and shear forces.

A classic example is an oblique or spiral fracture reduced and compressed using lag screws followed by application of a plate to neutralise physiological loads.

Lag screw provides compression; the neutralisation plate protects that fixation.

A buttress plate supports a fracture fragment and prevents axial displacement or collapse. This is particularly important in metaphyseal and periarticular fractures where an articular fragment may otherwise slide or collapse under load.

Examples include selected tibial plateau, distal radius and ankle fracture patterns.

An antiglide plate is positioned so that the plate mechanically blocks a fracture fragment from sliding in the direction produced by physiological forces.

The tension-band principle involves converting tensile forces acting on one side of a bone into compressive forces at the fracture site.

The implant is placed on the tension side of the bone. Under physiological loading, tensile forces are resisted by the implant and compression is generated across the opposite cortex or fracture surface.

Classic applications include selected fractures of the patella and olecranon. Plates can also function biomechanically as tension-band devices when positioned on the tension surface.

Bridge plating is used principally for multifragmentary fractures where reconstruction of every intermediate fragment would cause excessive soft-tissue stripping and compromise blood supply.

The main proximal and distal fragments are aligned with restoration of:

• Length
• Mechanical axis
• Rotation

Intermediate fragments are usually left undisturbed. The plate spans the comminuted zone and functions as an internal splint.

Healing occurs predominantly through secondary callus formation.

Bridge plating treats the fracture as a biological zone rather than as multiple individual fragments that must all be anatomically reduced.

In a conventional plate construct, stability largely depends on friction generated between the plate and bone.

Tightening a conventional screw pulls the plate toward the cortex. Screw preload compresses the plate onto the bone, and friction between plate and bone resists displacement.

Therefore conventional plating requires reasonable screw purchase and contact between plate and bone.

Locking screws mechanically engage the plate. The screw head locks into the plate hole, producing a fixed-angle construct.

Unlike conventional plating, stability does not primarily depend on compression of the plate against the underlying cortex.

Each locked screw behaves somewhat like a small fixed-angle beam connected to the plate. Failure generally requires failure of the screw-bone interface, screw, plate or entire fixed-angle construct rather than simply sliding of the plate over the bone.

Advantages are particularly relevant in:

• Osteoporotic bone.
• Metaphyseal regions.
• Short periarticular fragments.
• Bridge plating.
• Situations where preserving periosteal blood supply is important.

Locking plates should not automatically be considered superior to conventional plates. Their mechanical advantages are useful only when incorporated into a sound fixation strategy.

Because locking screws rigidly connect to the plate and stability does not require compression of the plate against bone, a locking plate may function biomechanically like an external fixator positioned underneath the soft tissues.

The plate therefore does not necessarily have to be compressed tightly against the periosteal surface. This property forms the basis of many minimally invasive plate osteosynthesis techniques.

Many locking compression plates incorporate combination holes allowing either a conventional screw or locking screw to be inserted.

This permits a hybrid strategy in which conventional screws may initially bring the plate toward the bone or generate compression, after which locking screws provide fixed-angle stability.

Once locking screws have fixed the plate relative to bone, attempting to use additional conventional screws to pull the plate significantly toward the cortex may generate undesirable stresses or alter reduction. The sequence of screw insertion should therefore be planned.

Filling every plate hole can produce an unnecessarily stiff construct, particularly during bridge plating.

Excessive stiffness may reduce interfragmentary movement to a level that limits callus formation, while simultaneously creating abrupt stress transitions near the ends of the plate.

Plate length, screw distribution and number of screws should therefore be selected according to fracture mechanics rather than simply maximised.

In bridge plating, plate working length broadly refers to the segment of plate spanning the fracture between the innermost fixation points on either side.

Increasing working length generally:

• Reduces construct stiffness.
• Allows greater elastic deformation.
• Distributes plate bending over a longer segment.
• May reduce peak stress concentration around the fracture.

Decreasing working length generally increases stiffness but concentrates deformation over a shorter plate segment.

However, working length cannot be considered in isolation. Fracture gap, plate material, plate thickness, screw configuration, bone quality and loading pattern all contribute to construct behaviour.

Longer plates can distribute forces over a larger segment of bone. In bridge plating, a long plate with strategically spaced screws may provide a more favourable mechanical environment than a short plate densely packed with screws.

Important principles include:

• Avoid unnecessarily short plates in long comminuted zones.
• Avoid indiscriminately filling every plate hole.
• Obtain adequate fixation in the principal proximal and distal fragments.
• Consider bone quality and expected loading.
• Avoid creating an abrupt stress riser at the end of the implant where possible.

Screws can provide fixation by compressing a plate onto bone, generating interfragmentary compression, fixing a fragment directly or locking into a plate.

Important screw terminology includes:

• Core diameter: diameter of the central shaft beneath the threads.
• Outer/thread diameter: maximum diameter across the threads.
• Pitch: distance between adjacent threads.
• Lead: distance travelled axially during one complete revolution.
• Thread depth: difference between outer and core radii.

Cortical screws generally have finer threads, whereas cancellous screws typically possess deeper, wider threads designed for lower-density cancellous bone.

Screw fixation depends heavily on bone quality and the amount of bone engaged by the screw threads.

Pull-out resistance is influenced by:

• Bone mineral density.
• Thread geometry.
• Outer screw diameter.
• Length of thread engagement.
• Cortical versus cancellous purchase.
• Quality and direction of drilling.
• Repeated insertion or stripping of the screw hole.

In poor-quality bone, simply tightening a conventional screw harder does not improve fixation and may strip the bone-screw interface.

Bicortical conventional screws obtain purchase in both cortices and generally provide greater resistance to pull-out than unicortical conventional fixation.

Locked constructs may permit selected unicortical fixation because the screw is mechanically connected to the plate. However, the adequacy of fixation depends on bone, implant geometry, fracture location and loading conditions.

Intramedullary nails are load-sharing devices positioned close to the mechanical axis of long bones. Their central location provides favourable resistance to bending compared with an eccentrically located implant.

Modern nails can control:

• Axial loading.
• Bending.
• Rotation.
• Length.

Interlocking screws are particularly important in unstable, comminuted and segmental fractures, because an unlocked nail alone may not adequately control shortening or rotation.

Intramedullary fixation is often used with relative stability and secondary fracture healing.

Bending moment depends on the force applied and the perpendicular distance of that force from the axis about which bending occurs.

Because an intramedullary nail lies near the long axis of the bone, its lever arm relative to the mechanical axis is smaller than that of an eccentrically placed plate.

Consequently:

• The bending moment acting on the implant is reduced.
• Load can be shared more efficiently with the surrounding bone.
• The construct is mechanically well suited to long-bone diaphyseal loading.

Nail closer to mechanical axis → shorter lever arm → lower bending moment.

The resistance of a cylindrical implant to bending is strongly influenced by its radius. Small increases in nail diameter can therefore produce disproportionately large increases in bending stiffness.

Clinically, this means a larger-diameter nail is generally considerably stiffer in bending than a smaller nail of otherwise similar design.

However, nail diameter must be balanced against canal dimensions, cortical thickness, insertion safety and the biological consequences of reaming.

Nail cross-sectional geometry influences torsional stiffness, bending stiffness and insertion characteristics.

Historically, different nail designs used slots or open cross-sections to permit elastic deformation during insertion. Modern nails are generally designed to combine adequate torsional and bending resistance with space for interlocking mechanisms.

Reaming enlarges the medullary canal and permits insertion of a larger nail. This usually increases nail-bone contact and allows use of an implant with greater bending strength.

Mechanical advantages of reaming include:

• Ability to insert a larger-diameter nail.
• Greater construct stiffness.
• Improved fatigue resistance of the nail.

Reaming also produces biological effects, including temporary disturbance of endosteal blood supply and deposition of reaming debris at the fracture site. The clinical implications vary according to the bone, soft-tissue injury and patient physiology.

Proximal and distal interlocking screws connect the nail mechanically to the bone and allow the nail to control fracture length and rotation.

Their importance increases when cortical contact at the fracture site is poor, as in:

• Comminuted fractures.
• Segmental fractures.
• Metaphyseal extension.
• Fractures with substantial shortening tendency.

Interlocking screws may become sites of high stress, particularly when the fracture provides little load sharing. Screw breakage can therefore occur when union is delayed.

Static locking prevents substantial axial translation between the nail and bone and is used when maintaining length is essential.

It is commonly appropriate for unstable, comminuted or segmental fractures where uncontrolled shortening would otherwise occur.

Dynamic locking allows controlled axial motion while maintaining rotational control. Some nail designs achieve this through a dynamic slot.

Dynamisation traditionally refers to conversion of a statically locked construct into one that permits increased axial compression at the fracture site, commonly by removing a selected locking screw.

The intended effect is to increase load transfer through the fracture and stimulate compression during weight bearing.

However, dynamisation is not universally beneficial. In an unstable fracture it may result in:

• Shortening.
• Loss of alignment.
• Rotation.

The effective working length of an intramedullary construct is influenced by the distance over which the nail is unsupported by bone and by the position of locking screws relative to the fracture.

A fracture with good cortical contact provides greater load sharing and effectively supports the nail near the fracture site. A highly comminuted fracture leaves a longer unsupported segment of nail, increasing bending stresses.

This explains why implant fatigue failure is more likely when a nail bridges a persistent segmental defect or nonunion.

The entry point is not merely a technical step. An incorrect entry point may alter the path of the nail and produce angular or translational deformity.

This is particularly important in proximal and distal metaphyseal fractures where the canal is wide and the nail does not automatically centre itself.

Malalignment may therefore result even if the nail appears to pass adequately through the medullary canal.

Poller screws are strategically positioned screws placed adjacent to the path of an intramedullary nail, particularly in a wide metaphyseal canal.

They reduce the effective width of the canal, guide the nail and improve control of alignment.

Mechanically, a blocking screw acts as an artificial cortical wall.

Poller screw = narrow the effective canal and guide the nail away from the direction of deformity.

Although intramedullary nails can assist fracture alignment during insertion, they should not be expected to correct every deformity automatically.

Before locking, the surgeon should specifically assess:

• Length.
• Coronal alignment.
• Sagittal alignment.
• Rotation.
• Fracture distraction.

Once interlocking screws are inserted, an unrecognised rotational or angular deformity becomes mechanically fixed.

Excessive fracture distraction is mechanically and biologically undesirable. A distracted fracture reduces load sharing and increases the mechanical demand on the implant.

Persistent distraction may also impair progression toward union despite apparently adequate implant positioning.

Therefore after nail insertion and locking, the fracture site should be assessed carefully for unintended distraction.

MIPO combines plate fixation with biological principles traditionally associated with closed fracture treatment.

Instead of exposing the entire fracture, the plate is introduced through limited incisions and advanced along the bone while preserving the soft-tissue envelope around the fracture.

The goals include:

• Preserving fracture haematoma.
• Preserving periosteal blood supply.
• Avoiding unnecessary stripping of intermediate fragments.
• Restoring length, alignment and rotation indirectly.
• Providing relative stability.

MIPO is therefore conceptually similar to bridge plating rather than traditional direct anatomical reconstruction of every fragment.

Modern fracture surgery increasingly recognises that fixation must respect the biology of fracture healing. Perfect radiographic reconstruction of every fragment is not necessarily desirable if it requires extensive stripping of viable soft tissue.

Biological fixation emphasises:

• Preservation of vascularity.
• Indirect reduction where appropriate.
• Minimal disturbance of the fracture zone.
• Long implants with appropriate screw distribution.
• Relative stability for multifragmentary fractures.

Bone adapts to its mechanical environment. If a very stiff implant carries a disproportionately large amount of physiological load for a prolonged period, the underlying bone may experience reduced mechanical stimulus.

This phenomenon is termed stress shielding and may contribute to local reduction in bone density.

Stress shielding should be distinguished from stress concentration, which refers to local elevation of stress around changes in geometry such as implant ends, screw holes or cortical defects.

A stress riser is a region where mechanical stress becomes concentrated because of a change in geometry or stiffness.

Examples include:

• Drill holes.
• Empty screw holes near heavily loaded regions.
• End of a rigid plate.
• Previous screw tracks.
• Transition between two implants.
• Large cortical defects.

Peri-implant fractures may occur near these stress transitions, particularly in osteoporotic bone.

Plates may require contouring to match bone anatomy or achieve the desired mechanical effect. However, repeated bending and reverse bending can weaken the implant by introducing local plastic deformation and surface damage.

Excessive contouring near critical screw holes should therefore be avoided when possible.

Precontoured anatomical plates reduce the requirement for extensive intraoperative bending but do not eliminate the need to assess whether the implant actually fits the individual patient's bone.

Whenever an implant fails, the important question is not simply “Why did the metal break?” but:

Why was the implant still carrying substantial load when fatigue failure occurred?

Yes. In fractures intended to heal by callus formation, excessive construct stiffness may reduce the mechanical stimulus necessary for robust secondary bone healing.

This is particularly relevant in locked bridge plating, where a short plate, numerous screws and fixation immediately adjacent to the fracture may produce a very rigid construct.

Conversely, excessive flexibility may result in uncontrolled movement and failure to progress toward union.

The desired goal is therefore neither maximal rigidity nor maximal flexibility, but controlled mechanical stability appropriate for the fracture pattern.

In periarticular fractures, fixation strategy can be considered in two stages:

1. Reconstruct the articular surface where appropriate.
2. Connect the reconstructed articular block to the diaphysis.

The joint surface may require absolute stability, while the metaphyseal component can sometimes be managed using relative stability.

Thus a single fracture can contain regions requiring different mechanical strategies.

Direct reduction involves exposure and manipulation of the fracture fragments themselves. It is particularly useful when precise anatomical reduction is required.

Indirect reduction uses traction, ligamentotaxis, external manipulation, reduction tools or implant-assisted techniques to restore alignment without exposing the fracture zone extensively.

Indirect reduction is a key component of biological bridge plating and intramedullary nailing.

Ligamentotaxis refers to indirect reduction of fracture fragments through tension in intact soft tissues and ligamentous attachments.

Axial traction can restore alignment by tensioning the surrounding soft-tissue envelope. Ligamentotaxis is useful in several periarticular fractures but cannot reliably correct every impacted or rotated articular fragment.

• Using a short plate to bridge a long comminuted segment.
• Filling every hole of a locking plate without considering construct stiffness.
• Placing screws unnecessarily close to a comminuted fracture zone.
• Attempting anatomical reconstruction of devascularised intermediate fragments.
• Leaving a fracture distracted after intramedullary nailing.
• Ignoring rotational malalignment before locking the nail.
• Using an incorrect nail entry point.
• Assuming locking screws compensate for poor reduction.
• Using inadequate fixation in short metaphyseal fragments.
• Failing to restore opposite-column or cortical support when mechanically necessary.
• Excessive soft-tissue stripping in pursuit of a perfect radiograph.

• Absolute stability produces minimal interfragmentary movement and promotes primary bone healing.
• Relative stability permits controlled movement and typically produces callus through secondary healing.
• Lag screws produce interfragmentary compression by allowing the near fragment to glide while threads purchase the far fragment.
• A neutralisation plate protects lag screw fixation from bending and torsional forces.
• A buttress plate prevents collapse or sliding of a fragment.
• Bridge plating preserves intermediate fragments and functions as an internal splint.
• Conventional plates depend mainly on plate-bone friction; locking plates depend on a fixed-angle screw-plate construct.
• A locking plate therefore need not be compressed firmly against the bone to obtain mechanical stability.
• In bridge plating, increasing plate working length generally decreases construct stiffness.
• Filling every hole in a locking plate may produce an excessively stiff construct.
• Intramedullary nails are mechanically advantageous because they lie close to the mechanical axis.
• Increasing nail diameter produces a substantial increase in bending stiffness.
• Interlocking screws control rotation and length in unstable fractures.
• Static locking controls shortening; dynamic locking permits controlled axial motion.
• Poller screws function as artificial cortical walls and help guide the nail in a wide metaphyseal canal.
• Fracture distraction decreases load sharing and increases stress on the implant.
• Persistent nonunion converts an implant into a repeatedly loaded mechanical bridge and predisposes to fatigue failure.
• The implant is not a substitute for correct reduction.
• In comminuted fractures, preserve biology and restore length, alignment and rotation rather than dissecting every fragment.
• The goal is not maximal stiffness—the goal is appropriate stability.

1. Define the fracture personality: simple, comminuted, articular, metaphyseal or diaphyseal.
2. Decide the desired healing mode: primary healing with absolute stability or secondary healing with relative stability.
3. Restore what matters: joint congruity where necessary and length, axis and rotation throughout the limb.
4. Choose the mechanical function: compression, neutralisation, buttress, bridge, tension band or intramedullary load sharing.
5. Design the construct: choose appropriate implant length, diameter, screw type, working length and screw distribution.
6. Preserve biology: avoid unnecessary periosteal stripping and disturbance of viable fracture fragments.
7. Check the final mechanics: alignment, rotation, fracture gap, implant position and adequacy of fixation.

Good fracture fixation is a balance between mechanics and biology. The implant should provide enough stability for healing while the surgeon preserves enough biology for healing to occur.