A stratified research which actually proved that Locking plates do have their demerits too is summarised here as the next DNB Theory examination can have this as a short note or may be even a long one too :
Q. Locking plates failure / Tips to Use Locking plates :
So here it goes :
Before we start the topic we must understand one concept :
“Things are going to fail, as they always do, through the weak link,”
Modes of failure :
Locking plate failures :
1. Mechanical (bending stresses on the plate, the screw-to-plate junction)
2. Biological (the screw-to-bone interface, bone quality)
Sometimes the plates don’t fail, the screw–bone construct fails. “In a gap situation, locked constructs favor plate fatigue whereas unlocked constructs favor screw loosening.”
With unlocked fixation, the bone is stressed and screw loosening can occur in a sequential pattern. Screws used in a locked construct can also fail sequentially. An additional mode of failure with locked screws is junctional failure—failure at the point where the screw is locked into the plate
Although locked screws can be an advantage in osteoporotic bone, poor bone quality is another reason locking plates can fail. In certain instances, even though the screws maintain their position to the plate, the bone can pull out.
“Screws can eat their way through the proximal humerus, for instance” , “because the quality of the bone is insufficient to prevent that collapse.”
A final mode of locking plate failure is periprosthetic failure. The stiff construct that is created, particularly if all locking screws are used, “puts a very sharp transition of stress at the end of the implant” .
Technical tricks to avoid failures:
Locked plating, has been a major advance in improving fixation in mechanically and/or biologically challenged environments. To avoid failure when using locking plates it’s important to remember the basic principles of fracture management, including strain bone healing and internal fixation.
“Simple fracture patterns are best treated with a compressive reduction technique and absolute stability; they are not appropriate for bridge plating,”
“Violating that principle and placing locked fixation across a simple fracture gap leads to a very high strain environment at the fracture and on the implant, potentially leading to a nonunion or a construct failure.”
“Remember that compression across a fracture improves the ultimate strength of the bone-implant construct.”
Although bridge plating is not indicated for simple fractures, it may be appropriate for comminuted fractures where flexible bridge plate application can help in callus formation. In the absence of definitive rules for plating comminuted fractures, surgeons must understand working length, screw density, and construct stiffness. “Aim for a plate length that is greater than 2 or 3 times the working length of the fracture and a screw density ratio of 0.4 to 0.5, meaning that fewer than half of the holes will be filled by screws.”
The order of screw insertion is also critical—surgeons should avoid adding unlocked screws after locked screws have been placed, he says. Doing so can “induce a tremendous amount of angular stress on the adjacent locking screw and effectively eliminate many of the cycles of the implant.”
Following are the tips for treating specific fracture patterns with locked plating:
- Tibial plateau fractures: Do not assume that a locked lateral implant is sufficient fixation for a bicondylar fracture. Provide buttressing support for the posteromedial fragment, and do not rely on the lateral locked implant alone.
- Proximal humerus fractures: Screws need to be within 5 mm of the subchondral bone. Medial calcar support screws are critical for preventing varus failure.
- Distal femur fractures: Understanding the relationship of the anatomic and mechanical axes to the implant and to the anatomy of the distal femur is the key to achieving an accurate indirect reduction of the fracture.
Finally, he concluded, “Remember that construct strength is more about the reduction than it is about the implant; the fracture compression, the bone-to-bone contact, and construct strength that is created by that bone contact are substantially more important than the implant being used.”
Salvage of locking plate failures :
“Locked plating is a relatively new technique with a significant learning curve.”. As a result, technical errors do occur—poor reductions, distractions, failure to compress, and failure to lag.
“A locked plated can’t be blamed for failure if it’s not used properly”. For example, “a proximal humeral plate on the distal tibia without a reduction is not likely to work—it violates the basic principles of fracture surgery.”
“We get infatuated with new technology and tend to overlook proven treatments that have been used for years”. “Be careful about placement, and make sure the plate is in the right location.”
It is recommended to use bicortical screws whenever possible, although locking unicortical screws may be more appropriate in selected fracture patterns or locations. In the epi-metaphyseal region, he recommends using the longest screws possible.
So , this is the explanation if one needs to write in an examination , but for more details please click on the following link for illustrations :
Article Courtesy : AAOS
Question courtesy : Dr. Dhananjaya Sabat , Assistant Professor , Dept. of Orthopaedics , MAMC , New Delhi