Reducing Plate Strain with Orthogonal Plating

The relationships between working length, construct stiffness, and implant stress and strain are important for the orthopedist to understand when applying a bone plate to a fracture so that healing is optimized and the risk of implant failure is minimized. The study by de Bruyn and colleagues in this edition[1] examines the impact of orthogonal plating on these parameters. I had the pleasure of discussing this study and its impact with Dr. de Bruyn during a “Bridging the Gap: Translating Clinical Research into Clinical Practice” symposium on 8 December 2023,[a] and now have the opportunity to review the findings of this study, and how they can influence our approach to fracture stabilization. The study compares stiffness and plate strain in fracture models stabilized with 3.5-mm locking plates (LCP) with short, medium, and long working lengths, with, and without, an orthogonal 2.7-mm LCP. As expected, for the 3.5-mm plate constructs, bending stiffness decreased and plate strain increased as working length increased. With the addition of the 2.7-mm LCP, the stiffness in four-point bending was equal between constructs and increased compared with the single-plate constructs. The increase was 40% for the short working length, 65% for the medium working length, and 80% for the long working length constructs. Plate strain was reduced after addition of the orthogonal plate. The strain in an orthogonally plated long working length construct reduced to a value similar to that of the single plated short working length construct. The orthogonally plated constructs were stiffer in torsion than their single-plated comparison group, except for the short working length constructs.

Two concepts intersect in the clinical application of bridging plates—optimal fracture gap strain, influenced by construct stiffness, and plate or screw fatigue, impacted by plate stress (directly related to strain) and applied load (patient activity). While we understand the basics of these concepts, for a particular fracture, we do not really know what the optimal construct stiffness should be so that there will be just the right amount of strain to accelerate callus formation and maturation. For an individual patient, we do not know what the applied loads will be. A low stiffness construct is at greater risk of bending, or failing. A construct that is very strong will also be very stiff, and the healing process may be impacted. So, we rely on our collective clinical experience, using bench studies like these to help understand the mechanical factors in play.

In highly comminuted fractures, a plate will have a long working length. Contrary to opinions in the past,[2] we know that strain, and, hence, stress will be higher in these plates. The yield load, where permanent deformation will occur, is also lower. To reduce the stress in the plate and strengthen the construct, an intramedullary pin or an orthogonal plate may be added.[1] [3] While direct comparison between these two studies is not possible, the increase in stiffness when an orthogonal plate is used appears to be greater. This is not unexpected, as the 2.7-mm plate is oriented side-on to the direction of bending, resulting in a large increase in the area moment of inertia of the implants. A disadvantages of orthogonal plating is the potential for greater “surgeon-inflicted” damage to the soft tissues, and to the bone fragments, due to the need to drill more holes. Using locking plates and minimally invasive approaches may help reduce the surgical damage.

One situation where orthogonal plating may be advantageous over a plate-rod construct is for increasing purchase in small proximal or distal fragments. An intramedullary pin will only add to construct stiffness and reduce plate stress if it is firmly engaged in both ends of the bone. If it is not, small deflections in the plate will just shift the pin a little, and it will not help reduce the stress on the plate or screws. For small fragments, it may not be possible to seat the pin well, and bicortical screw purchase becomes important.

I would like to thank Dr. de Bruyn and his colleagues for their contribution to our understanding of the mechanical impact of decisions we make regarding the stiffness and strength of constructs we apply in our patients.

Publikationsverlauf

Artikel online veröffentlicht:
16. Juli 2024

• References

• 1 de Bruyn BW, Glyde M, Day R, Hosgood G. Effect of an orthogonal locking plate and primary plate working length on construct stiffness and plate strain in an in vitro fracture-gap model. Vet Comp Orthop Traumatol 2024; 37 (04) 173-181
• 2 Moreno MR, Zambrano S, Dejardin LM, Saunders WB. Bone biomechanics and fracture biology. In: Johnston SA, Tobias KM. eds. Veterinary Surgery: Small Animal. 2nd ed.. St. Louis, MO: Elsevier; 2018: 646
• 3 Pearson T, Glyde M, Hosgood G, Day R. The effect of intramedullary pin size and monocortical screw configuration on locking compression plate-rod constructs in an in vitro fracture gap model. Vet Comp Orthop Traumatol 2015; 28 (02) 95-103