Locking Plates: Do They Prevent Complications?
Offering a closer look at the emergence and evolution of locking plates, these authors provide a thorough review of the literature to gauge the effectiveness of this fixation for first MPJ arthrodesis, calcaneal fractures and distal tibia fractures.
Plates and screws have been in use for bone healing to facilitate osteosynthesis since 1886.1 Researchers have studied these techniques of promoting primary bone healing with rigid stable internal fixation.
The founders of the Swiss American Study of Internal Fixation standardized the use of plates in the 1950s.2 The principles set forth by the American Orthopedic group included direct fracture exposure with anatomic reduction and rigid internal fixation.2 These conventional plating systems and methods have relied on the compressive force of the plate-to-bone interface to provide a stable construct. Ideally, the rigidity with this technique will lead to primary bone healing with no callus formation.
For many years, all the focus has been on the mechanical stability of the bone. Over the past 30 years, this philosophy has changed to addressing the “biology” of the fracture. More and more, attention and priority have shifted to the soft tissue envelope and the vascularity of the injury.
The amount of compression needed at the plate-to-bone interface to obtain stability diminishes the periosteal blood supply. This decreased blood to the injured bone has the potential to cause bone necrosis, delayed union or non-union of the bone, and an increased risk of infection and sequestration.3,4 The torque that surgeons use to insert the screws generates an axial preload and results in friction between the plate-bone interface. Accordingly, one must adequately contour the plate to sit directly against the bone. Within the plate, the screws can still toggle so authors have recommended bicortical screw fixation to prevent this toggling motion.5,6 Since this technique relies on the compression of the plate to bone for stability, good bone quality is required for solid fixation.
Primary bone healing can occur when strain levels at the fracture site are less than 2 percent. The definition of strain is the change in the length of the fracture gap divided by the length of the original fracture gap.7 Strain levels between 2 and 10 percent produce callus formation with bone healing, referred to as secondary bone healing. Strain levels greater than 10 percent result in no bone healing.7
That said, stable internal fixation usually provides less than 2 percent strain, resulting in primary bone healing.7 In contrast, splints, casts, intramedullary nails, bridging plates and external fixation usually lead to strain levels of 2 to 10 percent, leading to secondary bone healing with callus formation.7 Perren noted that “tissues cannot be produced under strain conditions which exceed the elongation at time of tissue rupture.”8
Plate-screw-bone constructs can act as either load-sharing or load-bearing devices. Neutralization plates function as load-sharing devices, neutralizing the effects of bending, rotation and axial forces on the fracture site.9,10 These plates cross a fracture that is already reduced and compressed with lag screws. In contrast to this are load-bearing buttresses and antiglide plates. These plates act as counter shear forces at the fracture site by converting the shear strain to axial compression forces.
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.
Sorenson and colleagues also assessed the success of a locked plate for the Lapidus fusion.20 In 19 out of 21 feet, surgeons used an interfragmentary screw with plates in the other two. It is also worth noting that 16 out of 21 feet received bone marrow aspirate. The findings denoted an average of 6.95 weeks to radiographic fusion, an average of two weeks to ambulation and a 9.52 percent rate of asymptomatic malunion. There was also a 0 percent rate of delayed union or non-union, a 0 percent rate of revision, and a rate of hardware removal of 4.76 percent.
In a cadaveric study, Scranton and coworkers looked at two different constructs for the first metatarsocuneiform joint arthrodesis.21 This study compared a locking compression plate to crossed screws. Researchers tested each modality on five cadaveric limbs. The plate proved to be a more rigid construct.
Cottom and coworkers performed a slightly different study of locking plates on the first metatarsocuneiform arthrodesis.22 They compared a low profile locking plate with a compression screw versus the same locking plate with a plantar interfragmentary screw. There were five cadaveric limbs in each group. The mean ultimate load of the locking plate with a plantar interfragmentary screw was statistically greater than the locking plate with an intra-plate compression screw.
Klos and colleagues compared a medial locking plate with a compression screw versus two crossed screws for the fixation of a first metatarsocuneiform joint arthrodesis.23 Each group consisted of four cadaveric limbs. Under cyclic loading conditions, the construct using a medial locking plate with an adjunct compression screw was superior to the construct using two crossed screws.
Gruber and coworkers also compared two types of fixation for a first metatarsocuneiform arthrodesis: a dorsomedial locking plate with a lag screw and crossed screws. Each group consisted of five cadaveric limbs. The researchers found no difference in rigidity between the two groups.24
Abbasian and colleagues analyzed three types of fixation for a calcaneal osteotomy: a lateral locking plate, a headless screw or a headed screw.26 In 67 osteotomies, 17 had fixation using a headed screw, 18 received a headless screw and the remaining 32 had lateral plate fixation. Overall, 47 percent of the headed screws, 11 percent of the headless screws and 6 percent of the lateral plates required removal to address symptoms that physicians suspected were due to the hardware. There was a 10 percent rate of wound complications in the lateral plate cohort. The incidences of local wound complications and radiological delayed union were higher in the group fixated with lateral locking plates. There was no significant difference in union rate among the three types.
In an analysis of locking plates on calcaneal fractures, Hyer and colleagues performed a two-year retrospective analysis of 17 calcaneal fractures treated with locking plates.27 Weightbearing began at approximately four to five weeks. Serial radiographic analysis occurred throughout the two years of the study. The authors concluded that early weightbearing could begin in patients with calcaneal fractures if surgeons employed locking plates for the fractures.
Blake and coworkers performed a biomechanical analysis of a locking and a non-locking plate on a cadaveric calcaneal fracture model.28 Surgeons created a Sanders IIB type calcaneal fracture in 10 matched pairs of cadaveric calcanei. Each pair had fixation with the same calcaneal reconstruction plate using either locking or non-locking screws in the same hole pattern. Specimens had axial loading for 1,000 cycles through the talus followed by load to failure. Statistical comparisons occurred between the locking and non-locking constructs on the displacements during cyclic loading as well as construct stiffness and load achieved at selected fragment displacements. Researchers found no mechanical advantage to locking technology for calcaneal fractures in their model.
Redfern and coworkers also compared a locking plate and non-locking plate for calcaneal fractures in five cadaveric limbs each.29 In a cadaver model of Sanders type IIB calcaneal fractures, locking plate fixation did not provide a biomechanical advantage over traditional non-locking plate fixation.
Eckel and colleagues analyzed four different lateral plate constructs for distal fibula fractures.30 Researchers divided 40 fresh frozen lower extremities into four groups. Simulating Weber B distal fibula fractures with an osteotomy, the study authors stabilized the fracture with one of four plate systems: a standard one-third tubular plate with an interfragmentary lag screw; a locked compression plate with a lag screw; a low-profile locking plate with a lag screw; or a non-locking plate. Researchers applied controlled monotonic bending and cyclic torsional loading, and quantified bending stiffness, torsional stiffness and fracture site motion. They found no significant differences in plate performance.
In a cadaveric model, Kim and colleagues compared a locking and conventional plating system for the fixation of the distal fragment of a distal fibula fracture.31 Overall, the data indicated that a locking plate construct with two distal unicortical screws was mechanically equivalent to standard plating with three distal screws. In addition, fixation with the standard plates depended on bone mineral density whereas the locking plate fixation was independent of bone mineral density. The authors state that the clinical implication of this study was that locking plates may be advantageous in patients with the most severe osteoporosis.
Another study by Ozkaya and coworkers analyzed the treatment of distal tibial fractures with locking and non-locking plates through a minimally invasive approach.32 In their study, 43 patients with closed fractures of the distal tibia metaphysis received either a stainless steel non-locking plate or a titanium locking plate. Minimally invasive medial plating with titanium locking plates resulted in prolonged secondary healing both in comminuted and simple fracture patterns in comparison to conventional stainless steel non-locking plates. Researchers utilized no free interfragmentary lag screws in this study.
Ahmad and colleagues analyzed percutaneous locked plating for fractures of the distal tibia.33 They reviewed 18 patients treated with locking plates. They found that distal tibial locking plates have high fracture union rates, minimum soft tissue complications and good functional outcomes. The literature shows similar fracture union and complication rates in locking and non-locking plates.
O’Neil and coworkers compared two types of fixation for a tibiotalocalcaneal arthrodesis.34 Researchers tested an intramedullary nail with a lag screw against a locking plate with a lag screw with six cadaveric limbs in each group. The locking plate construct showed higher final rigidity than the intramedullary nail construct.