By Karl Maritato, DVM, ACVS

Over the past decade or so, locking plates have gained tremendous popularity amongst veterinary surgeons. Locking plates are great, but there is also a lot of confusion and misconception around their proper use, which can lead to unnecessary and avoidable complications for the patient. A proper foundation in the biomechanics of both non-locking (compression) and locking plates is critical to their correct use. In this article, we will discuss an overview of the biomechanics of how these plate systems work, as well as give some examples of their uses.

The highest-level difference between non-locking plates (here forth referred to as DCP for dynamic compression plate) and locking plates (LP) is the mechanism by which the plates interact with the bone. With DCP, the force of friction between the plate and the bone, by way of the compression of the plate onto the bone surface by the screw tightening, is what creates the stiffness of the construct. It is essentially a "load-sharing" construct. When a plate is placed on a bone, as each screw is tightened into the bone, the friction created by the thread-bone interface allows the screw to pull the plate down to the bone and tighten it to the bone surface. As it is tightened, the force of friction at the plate-bone interface creates the strength of the construct.

With LP, the concept of a load-sharing construct based on friction between the plate and the bone does not apply. The easiest way to think of LP are to consider them "internal fixators". The screws have a special set of threads that lock into the plate. As the screws are placed though the plate, into the bone, there is no compression of the plate onto the bone, because the screws lock into the plate instead of the screws pulling the plate down onto the bone. In this situation, a fixed angle construct is created, where the forces are carried by the implant and not shared, as in the case with DCP. A helpful visual is to think of a classic external fixator where the pins and bar travel out and down external to the limb. The forces travel down the bone, across the pin, down the bar and then back the other pin to the bone again. This is exactly what happens with LCP. The weight bearing forces travel from the bone to the screw to the plate and then back again at the other end. Bypassing the bone where the plate is. This explains LCPs popular use for comminuted fractures where load sharing either isn’t possible or is very complex to recreate.

One of the things that has driven the development and refinement of locking plates is the paradigm shift of rigid anatomic fixation of fractures to minimally invasive alignment and relative stability.

According to initial AO principles of fracture fixation, the goal was essentially to piece back together the entire fracture and rigidly stabilize it with a DCP. Over time, research revealed that to do this, one causes significant trauma to the soft tissue and blood supply to the bone, which can and does result in healing problems for the fracture. This led to the development of a limited contact DCP (LC-DCP). The LC-DCP has a scalloped underside to the plate, which reduces how much of the plate is compressing onto the periosteum, reducing blood supply damage.

To further limit blood supply damage, as well as reduce the need for complete anatomic reduction, research began focusing on locking implants. Because the plate is not compressed to the bone, and can support forces in a bridging fashion not requiring anatomic reconstruction, the plates can be applied with significantly reduced trauma to the soft tissues and blood supply.

This also led to the refinement of minimally invasive plate osteosynthesis (MIPO). Because complete anatomic reconstruction is not the goal with locking implants, they are well suited to fracture repair through small incisions at the ends of the long bones.

Other important differences between DCP and LP are the angulation in which the screws can be placed. With DCP, the plate holes are oval, and various angles can be achieved for the screw direction. This is particularly useful near joints and fracture fragments. For most LP, the construct is locked at a fixed angle of 90 degrees to the plate. There are a couple of LP systems that are considered polyaxial, which allows for angulation of the screws while still being locked into the plate.

Lastly, there are some LP that are considered combi-hole, in which the screw hole can accommodate both locking and non-locking screws. There are specific uses for these types of plates, particularly near joints.

Complete detail of the biomechanics behind different fracture-use scenarios is beyond the scope of this article. However, a "Birds Eye" visual of standard fracture-plate configurations is reasonable.

Essentially, the most common use for locking plates is comminuted long bone fractures. These are frequently deployed minimally invasively through small incisions to not disturb the blood supply. Since anatomic reconstruction of the many pieces is not desirable, nor indicated, with locking plates for construct strength to be achieved, minimal manipulation and damage occurs.

Simple two-piece long bone fractures, whether transverse, oblique or spiral, are excellent examples in which the bone cylinder can easily and efficiently be rebuilt, which is ideal for standard DCP and LC-DCP configurations.

In summary, there is a common misconception that locking plates are “stronger” than traditional non-locking plates. This is not the case. It is not their respective strengths that differentiate them, but their mechanism of function and how that correlates to their proper usage. With improper knowledge, understanding and application of LCP to specific fractures, failure and patient morbidity can occur.

For a complete understanding of locking plates and their use, I recommend the ACVS Foundation textbook Locking Plates in Veterinary Orthopedics.