Biomechanics of Plate Contouring
In general, an orthopaedic plate can be contoured in three planes as follows: (1) in line with the holes (bending), (2) perpendicular to the holes along the long axis of the plate ("on the flat"), and (3) in torsion (twisting). The amount of force required to contour in these planes is dependent on the plate's moment of inertia, which is related to the amount of material contained along the axis of the desired contour. It therefore follows that differences in the plate shape can have a significant impact on the surgeon's ability to contour them. Plate thickness is one of the more straightforward factors that influences contouring. Many thin plates (eg, tubular plates and minifragment plates) can be bent by hand or altered after they have been applied to a bone with the use of a nonlocking screw. Other implants, such as straight compression plates and stout precontoured plates, require more force and specialized benders to manipulate. Using a nonlocking screw to attempt contouring with these implants may induce malreduction, rather than altering the implant's shape. Contouring a plate on the flat also requires more force than bending in the plane of the holes because more metal is contained within this plane.[5,6] Some plates have design features to facilitate easier bending on the flat. Commonly referred to as "reconstruction" plates, these implants have material removed from between the holes, decreasing the moment of inertia (Figure 1).
Photograph showing two types of plates. On the left, a tubular style plate is relatively thin and malleable, allowing for autocontouring when nonlocking screws are used. On the right, a reconstruction plate has reduced plate width (material relief) between the holes, resulting in a decreased moment of inertia for bending on the flat.
Plate material also affects contourability. Most fracture plates are made from stainless steel, titanium, or titanium alloy.[5,7–9] Typically, the titanium used in orthopaedic implants has an elastic modulus that is about half that of stainless steel.[5,9] This can result in decreased overall construct stiffness compared with stainless steel[8–10] which may or may not be desirable, depending on the fracture being treated, the mode in which the plate is being used, and the degree of stress modulation desired by the surgeon. However, a material's elastic modulus describes its ability to resist change under load only in the elastic region of a stress-strain curve (ie, under loads that are not strong enough to produce permanent deformity of the implant). Contouring, by definition, results in plastic (permanent) deformation of the implant and depends on the material's malleability. Stainless steel implants are often more malleable than titanium, meaning they can undergo more plastic deformation before reaching the point of ultimate failure. Practically, this means that a stainless steel plate might be more resistant to contouring than a titanium plate under initial loads, but once the surgeon applies enough force to introduce permanent contour, the stainless plate can tolerate more shape change before breaking. Carbon fiber implants are also available for fracture fixation, although they are less commonly used. Plates made of this material cannot be contoured because they are not malleable.[11–14]
J Am Acad Orthop Surg. 2020;28(14):585-595. © 2020 American Academy of Orthopaedic Surgeons