The purpose of fixation
Fixation creates a mechanical environment that permits healing while maintaining acceptable alignment and function.
The desired construct depends on whether the aim is:
- absolute stability with minimal interfragmentary motion
- relative stability with controlled motion and callus formation
Load sharing and load bearing
Load-sharing constructs allow bone and implant to carry load together.
Load-bearing constructs make the implant carry a larger proportion of load, particularly when bone contact is absent or comminution prevents meaningful load transfer.
Plates
Compression plating
Used to generate absolute stability in appropriate simple fracture patterns. Compression reduces interfragmentary motion and favours direct healing.
Bridge plating
Used for comminuted fractures where preserving biology is important. The plate spans the fracture zone and provides relative stability.
Working length
Working length is the portion of the construct that is able to deform around the fracture. Increasing working length generally makes a plate construct less stiff and distributes strain over a larger segment. Excessive flexibility, however, can lead to instability or fatigue failure.
A conventional plate can function in compression, neutralisation, buttress or bridging mode depending on application.
In bridge plating:
- the plate spans the comminuted zone
- fracture fragments are not stripped unnecessarily
- fixation is concentrated in the main proximal and distal segments
- plate length and screw distribution influence construct stiffness
A short working length generally produces a stiffer construct and higher local plate strain. A longer working length increases flexibility but may also increase motion excessively if taken too far.
Locked plates behave mechanically more like internal fixators. Stability is created by the fixed-angle relationship between screw and plate rather than by plate-to-bone friction alone. They are particularly useful in poor bone, periarticular segments and bridging constructs, but they are not immune to fatigue failure or poor reduction.
Intramedullary nails
An intramedullary nail is positioned close to the mechanical axis and is mechanically efficient in bending.
Important factors include:
- nail diameter
- cross-sectional geometry
- material
- fracture gap
- locking configuration
- working length
Increasing nail diameter usually increases bending and torsional rigidity markedly.
A nail is close to the mechanical axis and therefore has a smaller bending moment than an eccentrically placed plate. Nails are load-sharing devices when there is useful cortical contact and can function as relative-stability constructs in comminuted fractures.
Key variables include:
- nail diameter
- canal fit
- number and position of locking screws
- fracture level
- working length
- cortical contact
Reaming can increase nail diameter and alter the mechanical environment, but also has biological effects that must be understood.
Hollow versus solid sections
For bending and torsion, material placed farther from the neutral axis contributes disproportionately to rigidity. This explains why a hollow section can use material efficiently while maintaining substantial stiffness.
Screws
Screw fixation depends on:
- outer diameter
- core diameter
- thread design
- bone quality
- insertion technique
- length of purchase
Pull-out resistance generally improves with greater thread engagement and better-quality bone.
External fixation
Construct stiffness is influenced by:
- pin diameter
- pin spread
- number of pins
- distance of connecting rod from bone
- number and arrangement of rods
- fracture gap
Bringing the frame closer to bone generally increases stiffness, provided soft tissues remain safe.
External fixator stiffness depends on:
- pin diameter
- number of pins
- pin spread
- distance of connecting bar from bone
- number and arrangement of rods
- frame geometry
Bringing the bar closer to bone generally increases stiffness. Wide pin spread within each main fragment improves control.
Interfragmentary strain
Strain depends on change in fracture gap relative to the original gap. A small absolute movement across a very small gap can produce high strain.
The construct should provide a strain environment compatible with the intended mode of healing.
Stress shielding
A very stiff implant can carry a large proportion of load, reducing mechanical stimulus to adjacent bone. The clinical importance depends on implant, location, duration and biological setting.
Fatigue failure
Implants may fail if cyclic loads continue before sufficient biological union develops. A broken implant is often a sign that the fracture construct and biology failed to achieve union before fatigue life was exhausted.
Start with the desired healing mode
Fixation should be selected according to the fracture pattern and biological goal.
Absolute stability minimises interfragmentary motion and is used where direct healing and precise restoration are required, especially simple articular fractures and selected simple diaphyseal fractures. Interfragmentary compression can be produced with lag screws, compression plating or related techniques.
Relative stability permits controlled micromotion and secondary healing with callus. It is typical of intramedullary nailing, external fixation and bridge plating of comminuted fractures.
Trying to obtain absolute stability in a highly comminuted fracture by extensive stripping can damage biology without achieving a durable construct.
Lag screws
A lag screw creates interfragmentary compression by allowing threads to engage only the far fragment while the near fragment glides. Compression increases friction at the fracture interface and reduces shear.
The screw should be orientated to maximise useful compression across the fracture plane. In oblique fractures, screw position and direction affect resistance to shear.
Strain theory
Interfragmentary strain is the change in fracture-gap length divided by the original gap length. Small gaps can experience very high strain with modest movement. This is one reason a tiny residual gap under a flexible construct may be biologically unfavourable while a larger comminuted zone can tolerate similar absolute movement.
Construct failure
When fixation fails, ask whether the cause was:
- poor reduction
- inadequate implant choice
- insufficient fixation
- stress concentration
- poor bone
- excessive gap
- infection
- non-union causing fatigue
- patient-related loading or host factors
FRCS synthesis
The best answers explain why the construct creates the intended mechanical environment. Naming an implant is not enough. Describe how load is transmitted, where motion occurs, what healing pattern is expected and how the construct might fail.