Compression Plating: Principles

Compression plating is a technique of internal fixation in which a plate is used to generate active compression across an anatomically reduced fracture, producing absolute stability — the abolition of interfragmentary motion — so the bone heals directly, without callus. It is one of several modes in which a plate can function, and it sits at one end of a spectrum of stability: at the other end, bridge plating deliberately provides only relative stability, encouraging healing by callus. Choosing between the two is a core decision in operative fracture management and one of the most reliably asked topics at the ST3 Trauma & Orthopaedics interview.

What is the difference between absolute and relative stability?

The distinction is mechanical, and it dictates the biology of healing. Perren's interfragmentary strain theory (J Bone Joint Surg Br, 2002) explains why: the tissue that forms in a fracture gap depends on the strain (change in gap length divided by gap width) that tissue experiences.

• Absolute stability means no detectable motion between fragments under physiological load (interfragmentary strain conventionally quoted as below about 2%). Lamellar bone can form directly across the fracture, healing by direct (primary) cortical healing — osteonal "cutting cones" cross the fracture line and remodel it. No callus forms; radiographically the fracture line simply fades.
• Relative stability means controlled micromotion at the fracture (strain broadly in the 2–10% range). The fracture heals indirectly (secondary healing) through the familiar cascade of haematoma, soft callus, hard callus and remodelling. Callus is the expected — and desired — radiographic finding.

Two practical corollaries are worth stating at interview. First, callus appearing around an absolute-stability construct is a warning sign: it implies the construct is not as stable as intended. Second, very high strain (a mobile, distracted simple fracture) inhibits any tissue differentiation and leads to non-union — which is why matching the fixation strategy to the fracture pattern matters more than the implant itself.

What are the modes of plate function?

The same plate can be applied in different mechanical modes; the mode, not the plate, defines the construct. The AO Surgery Reference describes the following functions:

A single construct may combine modes — for example, a lag screw placed through a plate that then functions in protection mode.

How does a dynamic compression plate actually achieve compression?

The dynamic compression plate (DCP), introduced by the AO group in 1969, builds the compression mechanism into the screw hole. Each hole is shaped as an inclined and transverse cylinder — the "spherical gliding principle". When a screw is deliberately inserted eccentrically (a "load" screw, drilled through the load guide at the end of the hole furthest from the fracture), the spherical undersurface of the screw head slides down the inclined ramp as it is tightened. Because the screw is fixed in bone, the plate translates horizontally along the bone by approximately 1 mm, driving the fragment towards the fracture and compressing it. The limited-contact DCP (LC-DCP) refined the design with an undercut footprint to reduce periosteal compromise while retaining the same hole mechanics.

Good practice for a standard diaphyseal compression plating, as described in the AO Surgery Reference technique for a transverse radial shaft fracture, is:

• Anatomically reduce the fracture — compression plating is only legitimate on an anatomically reduced simple fracture.
• Overbend (prebend) the plate so its centre stands 1–2 mm off the reduced fracture. A perfectly flat plate compresses only the near cortex and gaps the far cortex; the prebent plate acts as a spring and compresses the far cortex as the screws are tightened.
• Insert the first screw in neutral mode in one main fragment, then the load screw eccentrically in the opposite fragment to generate compression; remaining screws go in neutrally. An articulated (push–pull) tensioning device is an alternative when more compression or plate length is needed.
• Secure enough cortices: in the forearm, "three bicortical screws are required in each main fracture fragment due to the high torsional stresses".
• For oblique fractures, add interfragmentary compression with a lag screw — through or outside the plate — with the plate then protecting it; position the plate so the apex of the oblique fragment is captured in the plate's axilla to prevent it gliding away.

Note that a locking plate applied with locked screws alone cannot generate interfragmentary compression: locked screws fix the bone at whatever position it sits. If compression is the goal with a locking plate, it must be achieved first (lag screw, cortical screws in the dynamic holes of a combi-hole, or a compression device) before locking screws are added.

When should I choose compression rather than bridging?

The question to ask of every fracture is: what healing do I intend, and can I achieve the mechanics that healing requires without destroying the biology?

Choose compression (absolute stability) when:

• The pattern is simple (two main fragments — transverse, short oblique or spiral) and can be anatomically reduced without extensive soft-tissue stripping.
• The bone tolerates no deformity: the forearm shaft, which behaves functionally like a joint, and intra-articular fractures, where anatomical reduction and absolute stability of the articular surface are mandatory.
• Treating selected non-unions, where stability plus biological stimulus is the strategy.

Choose bridging (relative stability) when:

• The fracture is multifragmentary/comminuted — anatomical reduction of every fragment is impossible without devascularising them.
• Biology takes priority: compromised soft tissues, segmental injury, or a metadiaphyseal zone suited to minimally invasive plate osteosynthesis (MIPO), restoring only length, alignment and rotation.

The classic errors — and favourite interview probes — are the mismatches. Bridging a simple fracture concentrates all deformation at a single narrow fracture line: strain is very high, callus cannot bridge it, and the plate fatigues and fails in the race between union and implant failure (a short working length makes this worse). Conversely, attempting rigid compression of a comminuted fracture demands wide exposure and stripping, kills the local blood supply and invites non-union and infection. Perren's synthesis of this — "biological internal fixation", choosing a new balance between stability and biology — is the intellectual basis of modern bridge plating.

What goes wrong, and what should I do about it?

• Non-union and implant failure: a broken plate is a symptom of non-union, not usually its cause. Reassess the mechanics (was the mode wrong?), the biology (stripping, smoking, infection) and the patient; revision typically means correcting whichever was at fault, with compression and bone graft for atrophic non-union as good practice.
• Fracture-related infection: manage as per the Sep 2019 BOAST on Fracture Related Infections — deep sampling before antibiotics ("5 samples should be taken from around the fracture site for microbiological culture using separate sterile instruments and a no touch technique for each"), a documented plan, and consultation of a bone-infection MDT for late, recurrent or non-responding infections and infected non-unions. A stable implant can often be retained until union in early infection; unstable metalwork must be addressed.
• Surveillance: interpret radiographs against the intended healing mode — expect an invisible union (fading fracture line, no callus) after compression plating, and progressive bridging callus after bridge plating. The wrong radiographic picture for the construct should prompt review, not reassurance.

Key points

• Absolute stability abolishes interfragmentary motion and produces direct (primary) cortical healing without callus; relative stability permits micromotion and heals by callus. Perren's strain theory links the two.
• A plate is defined by its mode of application: compression, protection, buttress, bridging or tension band — not by its name.
• The DCP achieves compression through eccentric "load" screws sliding down an inclined hole (about 1 mm of glide); overbend the plate 1–2 mm so the far cortex is compressed too.
• Compress simple fractures, the forearm and articular surfaces; bridge comminution and protect the biology.
• Mismatching mode and pattern — bridging a simple fracture or rigidly compressing comminution — is the classic route to non-union and implant failure.
• Suspected fracture-related infection is managed to the Sep 2019 BOAST: deep sampling before antibiotics and MDT involvement for complex cases.