The concept of locking plates

Abstract

After a short historical review of locking bone plates since their inception more than a century ago to the success of the concept less than 15 years ago with today's plates, the authors present the main locking mechanisms in use. In the two broad categories - plates with fixed angulation and those with variable angulation - the screw head is locked in the plate with a locknut by screwing in a threaded chamber on the plate or by screwing through an adapted ring. The authors then provide a concrete explanation, based on simple mechanical models, of the fundamental differences between conventional bone plates and locking plates and why a locking screw system presents greater resistance at disassembly, detailing the role played by the position and number of screws. The advantages of epiphyseal fixation are then discussed, including in cases of mediocre-quality bone. For teaching purposes, the authors also present assembly with an apple fixed with five locking screws withstanding a 47-kg axial load with no resulting disassembly. The principles of plate placement are detailed for both the epiphysis and diaphysis, including the number and position of screws and respect of the soft tissues, with the greatest success assured by the minimally invasive and even percutaneous techniques. The authors then present the advantages of locking plates in fixation of periprosthetic fractures where conventional osteosynthesis often encounters limited success. Based on simplified theoretical cases, the economic impact in France of this type of implant is discussed, showing that on average it accounts for less than 10% of the overall cost of this pathology to society. Finally, the possible problems of material ablation are discussed as well as the means to remediate these problems.

Biomechanical Analysis of Locking Reconstruction Plate Using Mono- or Bicortical Screws

“…This allows a better fit on more complex bony contours, or bones with irregular anatomic shapes, but makes it weaker compared with dynamic compression plates of equivalent size 3,4,6 . Several mechanical and biological advantages have been attributed to the locking screw-plate, including axial and angular stability due to its locking mechanism between plate and screw, preservation of the periosteal blood supply, reduced requirement for anatomical plate contouring, and possibility the system's elasticity to stimulate bone healing 1,2,[7][8][9][10] . These advantages have stimulated the modification or conversion of various traditional bone plates into locked systems, including reconstruction plates, by changing especially the screw-hole geometry.…”

“…However, there are some details that can influence the stability and resistance of the system, including the material used to manufacture the plate and screws, locking screw design, number and position of the screws, and type of mechanism for locking the screw into the plate 7,8,[10][11][12][13][14] . The locking mechanisms include screw head with threads that match with threads into the plate hole or into adapter, and threaded locknut or bushing into which the screw head is locked 10,14 .…”

“…The locking mechanisms include screw head with threads that match with threads into the plate hole or into adapter, and threaded locknut or bushing into which the screw head is locked 10,14 . On the other hand, the importance of the screw length -monocortical or bicortical -as a determining factor in fixation stability differs between a conventional plate and a locking plate 7,10 . Monocortical locking head screw has pullout resistance approximately 70% of the bicortical locking head screw, and pullout resistance similar to a conventional bicortical screw of comparable diameter 15,16 .…”

“…Monocortical locking head screw has pullout resistance approximately 70% of the bicortical locking head screw, and pullout resistance similar to a conventional bicortical screw of comparable diameter 15,16 . Because locked screws are under minimal tensile preload, the inclusion of the opposite bone cortex to obtain maximum purchase is apparently not critical 10,17,18 . The stabilising effect of the second cortex is substituted by the locking system 18 .…”

“…The stabilising effect of the second cortex is substituted by the locking system 18 . However, this may vary depending on the fracture site and bone quality 1,8,10 . In osteoporotic bone, the use of monocortical screws may not be sufficient to maintain stability, since the working length of the screw is dependent on the thickness of the bone cortex 7,8,12 .…”