Locking Plates: Do They Prevent Complications?
A Pertinent Primer On How Locking Plates Function
As the demand to preserve the surgical site “biology” increased in priority and minimally invasive and indirect bridging fixation became popular, locking plate technology emerged. Locking plates are also essential for stable fixation in osteopenic or pathologic bone, and are less compressive to the periosteal blood supply of the bone. The locking of the screw to the plate controls axial orientation and creates a single-beam construct that is very stable. This single-beam construct functions when there is no motion between the components of the beam (the plate and screws) and the bone. This construct is four times stronger in comparison with the load-sharing constructs in which motion occurs between each individual component of the screw-plate configuration.11 Load-bearing plates require ideal circumstances of “good” bone to be able to permit enough screw torque to utilize single beam constructs.12
These single-beam constructs act as fixed angle devices, which enhance fracture fixation in circumstances of comminuted fractures or in instances of poor bone quality since bone to plate compression is not necessary for stability. The locking plate construct converts shear stress to compressive stress at the screw to bone interface. Therefore, the strength of fixation in locking plates is equal to the sum of all the screw-bone interfaces. This differs from the non-locking plates in which the strength of the fixation depends on each individual screw’s axial stiffness or pullout resistance.13
Locking plates act as “internal external fixators” due to the inherent angular and axial stability, and because of their close proximity to the bone and fracture site.12 The screw lengths for locking plates are 10 to 15 times shorter than for external fixators, thus greatly increasing fixation rigidity. The fixation rigidity is a direct function of the screw material, length and diameter as well as the material and dimensions of the plate.
Acting as “internal fixators,” locking plates do not rely on plate to bone compression or friction forces to obtain stability, which allows the preservation of the local blood supply to the bone. This preservation of the local blood supply in theory leads to more rapid bone healing, decreased bone resorption, decreased incidence of delayed or non-union, and secondary loss of reduction. This also avoids stress shielding below the plate, preventing local bone necrosis and infection.14-16
The mechanical principles for fracture fixation offer completely different biological environments for bone healing between locked plates and compression plates. The compression plate creates an environment that promotes primary bone healing through absolute stability and anatomic reduction. Since locking plates function as internal fixators, they provide an environment conducive to secondary bone healing. The choice of fixation often depends on the fracture pattern, fracture location and quality of the bone. Initially, surgeons reserved locking plates for indirect fracture reduction, osteoporotic bone and comminuted fractures.
In foot and ankle surgery, there has been an increase in locking plate use for trauma, reconstruction and elective surgeries. These plates have anatomic site-specific designs that increase ease of use. There has also recently been a push for immediate weightbearing due to the stability of locking plates. This in theory leads to decreased morbidity.