Locking Plate-Application Restrictions and Limitations

The use of locking plates has largely expanded the scope of application of plate internal fixation of fractures. However, their use must be rationalized and optimized due to potential pitfalls and limitations. In this article, we will look at the application considerations, challenges of removal, and limitations, 3 aspects of locking plate application.

01.What Special Features And Difficulties Should Be Noted In Applying Locking Plates?

Steps and Challenges in Reducing Fractures with Locking Plates

The steps for resetting a fracture are standardized. Locking plates do not reset fractures. 

Once placed in the bone segment, adding more screws will not move it. If a locking plate that only accepts locking nails is used, 

this means that the plate can only be locked after the fracture has been set.

Since locking plates allow for bone healing without loss of initial repositioning, 

the primary cause of malunion of locking plates is incorrect initial repositioning. 

And, poor repositioning due to inadequate mechanics can lead to poor healing as the bone plate ruptures due to delayed healing or nonhealing.

Repositioning without the use of locking plates is particularly difficult when performing minimally invasive

 procedures because bone exposure is very limited. It requires various traction procedures (traction tables, retractors),

 various percutaneous repositioning forceps, and Kirschner pins to manipulate the bone fragments and temporary fixation. 

Prior to the application of locking plates and locking nails, it is crucial to check the reset by fluoroscopy.

Conversely, when using a locking plate that also has standard screw holes, 

a standard traction screw can be placed in the standard holes for initial repositioning on the plate.

 Bone fragments are placed against the plate. If the plate conforms to the anatomy, it can be used as a reset guide. 

Locking nails ensure stable results without altering the initial reset. This order of insertion (standard screws, then locking screws) is important (Figure 4).

Fig. 4 First insert the standard screws and tighten them.

Lack Of Tactile Feedback When Tightening Locking Pegs

There is no tactile feedback when tightening the locking head screws. In fact, 

tightening of the locking nail occurs simultaneously in the cortical or cancellous bone and in the metal of the locking plate. For this reason,

 it is easy for the physician to mistakenly assume that the locking nail is holding well in the cortical or cancellous bone (Figure 3).

Fig. 3 Working length of locking screws based on bone type and number of cortices.

Self-Tapping Locking Screws

The use of self-tapping locking screws means that there is no tactile feedback during drilling or tightening as they occur simultaneously.

Their mechanical properties are similar to those of single cortex locking screws during single cortex applications. If they are too long, 

they will contact the undrilled second cortex, resulting in incorrect positioning of the locking nail in the locking plate.

During bicortical applications, they may be too short, making them mechanically equivalent to single-cortical locking nails. 

If they are too long, they will extend beyond the cortex and may damage critical structures on the other side of the plate.

The correct locking nail length can only be obtained by measuring the desired length after drilling or verifying it by fluoroscopy.

Features Of Uniaxial Locking Screws

The main disadvantage of uniaxial locking nails is that their orientation is predetermined. 

They may have another implant or prosthetic stem in their path, making insertion impossible or limiting them to unicortical fixation.

For anatomical locking plates used in limbs with uniaxial locking nails with a fixed orientation, 

optimized for anatomical and biomechanical reasons, there is a risk of intra-articular locking nail placement. 

A typical example is a distal radius fracture. This risk is even greater when the locking plate is in close proximity to the joint or when the anatomy is substandard. 

The absence of an intra-articular fracture must be confirmed by fluoroscopy.

MIPO Technologies

The minimally invasive percutaneous osteosynthesis (MIPO) technique involves subcutaneous and/or submuscular 

and extraperiosteal insertion of a bone plate through a small opening into the bone after sliding, without exposing

 the fracture site. This allows for smaller incisions, less surgical site morbidity, and makes the procedure more “biologic”

 because there is no need to expose each bone fragment and there is no interference with soft tissue, periosteal vascularization, or fracture hematoma.

It can be accomplished with a locking plate and a specially designed instrument that allows the plate 

to be manipulated and passed through the skin to easily locate the locking nail holes in the plate. 

Fluoroscopic images must be taken at each step to verify progress. Each step of this technique is challenging. The first challenge is to reset the fracture prior to fixation.

 The locking plate must then be properly centered along the length of the bone, otherwise the alignment of the locking plate will be asymmetric (Figure 5). In addition, 

the locking plate must be perfectly parallel to the cortex of the bone it is designed to follow and as close to the bone as 

possible without greatly reducing the stiffness of the structure. During the final locking step, it is difficult to ensure that

 the conduits of the screws are properly aligned on the locking plate and that the locking nails are properly engaged during tightening.

Figure 5 Eccentric positioning of the locking plate and lack of haptic feedback during screw tightening.

Potential Skin Injuries When Using Outer Ankle Locking Plates

The use of locking plates to fix external ankle fractures has been associated with abnormally high rates of skin necrosis. 

The thickness of these subcutaneous locking plates puts pressure on the skin and interferes with its vascular distribution and healing. 

Something similar may happen when locking plates are used for hawksbill fractures.

Risk Of Osteoporosis And Screw Extraction

In osteoporotic bone, locking nails help reduce the risk of screw pullout or withdrawal. 

The construct is not sufficiently stiff because of the thinner bone cortex and the reduced density of the trabeculae.

In this case, locking plate fixation is always stronger and better anchored when using an evanescent or convergent monolithic construct (Figure 3).

Summary:

1. locking screws do not allow the fracture on the bone plate to be reset.

2. the fracture must be reset before adding a locking screw.

3. percutaneous fixation for fracture reduction requires locking plate instrumentation. the MIPO technique is more demanding.

02. Challenges Of Removing Locking Plates

Removing the locking plate once the fracture has healed is challenging and unpredictable, 

but the situation can be resolved. The biggest challenge is loosening the locking screw.

In some cases, the threads on the head of the locking nail are damaged during insertion 

(multiple tightening and loosening, screwdriver pattern is damaged and not perfectly hexagonal, screw insertion is done with a power drill), 

which means it cannot be unscrewed. Therefore, it is best to prevent this complication by immediately replacing any screw with a 

damaged head pattern during implantation, using a complete screwdriver and fully tightening the screw by hand (not with an electric drill).

 Using screws made of stronger materials can help minimize this problem.

In most cases, there is mechanical locking or jamming between the locking nail threads and the threaded hole in the locking plate. 

This is most commonly seen with 3.5mm diameter titanium hex pattern single shaft locking screws. There is no single mechanism

 for interference. Screws are often initially over-tightened due to failure to use the torque wrench provided in the instrument kit, 

which can alter the threads on the locking peg and locking plate. In other cases, 

failure to use or use of an incorrect drill guide resulted in the screws not being aligned when tightened, 

which caused the screws to jam. In order to minimize the risk of jamming during the initial fixing process,

 it is essential to use all available instruments: drill guides and sockets, torque wrenches in full integrity mode when tightening the locking nails.

The MIPO technique carries a high risk of incorrect placement of the alignment guide, 

as there is no direct view of the locking plate. Incorrect alignment of the drill guide means that the hole drilled for 

the locking nail and the insertion of the locking nail will also be incorrect. There is also a risk of damaging the head pattern of the

 locking nail when the screwdriver is not properly engaged with the screw.

For these reasons, before removing the locking plate, the surgeon must be aware that it may not be 

possible to loosen the locking nail, thus requiring the use of a high-quality hexagonal screwdriver and additional instrumentation.

When the locking nail cannot be loosened, or the head pattern is damaged, 

the first step is to place a screw extractor (a tapered screwdriver with inverted threads) into the head of the screw;

 this may be sufficient to loosen the screw. Another option is to cut the locking plate on either side of the locking nail and use

 it as a screwdriver to loosen the entire structure. If the screw still cannot be loosened, the locking plate can be loosened by 

drilling it with a drill, destroying the head of the locking nail, or by cutting around the plate to loosen the locking nail. After that, 

a vise can be used to remove the stakes of the locking pegs. If it still cannot be loosened (because it is integrated into the bone or does not protrude enough), 

it can be removed with a ring drill (Fig. 6).

Figure 6 Hints and tips for removing locking screws stuck in the board.

All of these problems can prolong the surgery, can cause soft tissue abrasion due to released metal fragments, and carry a risk of infection. 

The use of a ring drill increases the risk of perioperative fractures.

Summary:

1. The challenge of removing locking nails occurs primarily with 3.5mm hex head locking titanium screws.

2.The best way to avoid this problem is to use all provided instruments when inserting the screw. These difficulties can be resolved by using the appropriate tools.

03. What Are The Application Limitations Of The Locking Plate?

Clavicle plate fracture and osseous nonunion

By ensuring that the structure is not overly stiff due to insufficient working length of the locking plate or an excessive number of locking nails (Fig. 7), the risk of the locking plate breaking below the screw holes or at the screw/bone plate junction can be reduced.

Figure 7 Bone healing was achieved after 60 days by varying the number and position of locking screws and increasing the elasticity of excessively stiff structures.

The diagnosis of bone nonunion is usually confirmed by plate breakage. 

Late breakage of a locking plate or locking nail is timely as micromotion can occur leading to bone healing.

In simple fractures requiring compression, which depends on the type of fracture rather than the bone involved, 

a rigid structure in which the two fragments do not touch can lead to non-healing and fatigue failure of the plate. 

The combination of a rigid splint + locking nail + traction at the fracture site results in bone nonunion.

A variation of this is the simultaneous rupture of the locking nail underneath its attachment to the plate, 

which is also due to an overly rigid structure. This causes one end of the plate to pull out “in one piece” and healing is not achieved (Figure 8).

Figure 8 Secondary failure of an overly stiff and unbalanced structure: too many locking screws were used distally and the proximal splice plate was not long enough.

Therefore, fixation of intracapsular fractures of the hip with locking plates can lead to bone nonunion because the structure is too rigid to impinge on the fracture site. 

Without the micromotion required for healing, all the load is carried by the implant and it eventually fails.

asymmetric healing tissue

Periosteal bone scabs may be asymmetric, 

especially in fractures of the distal 1/3 of the femur. Micromotion due to elasticity permits the 

uniform development of fracture healing tissue that occurs only on the corresponding surfaces of the locking plate/nail construct.

 To control this risk, the working length of the locking plate should be increased, either by using more flexible titanium plates or by using new locking nail designs. 

Conversely, overly flexible constructs can lead to hypertrophic bone nonunion.

Plastic deformation of steel plates

Placing the plate as close to the cortex as possible reduces the risk of plastic deformation in the middle of the plate;

 when the distance between the plate and the cortex exceeds 5 mm,

 the structural strength is greatly reduced and the risk of plastic deformation of the plate and failure of the titanium plate is high.

Late fractures of the diaphysis or metaphysis at the end of a locking plate

The risk of late fracture at the end of the locking plate diaphysis or metaphysis, 

especially in osteoporotic bone, can be reduced by inserting a single cortical locking nail or a standard bicortical screw at the end of the plate to reduce the stresses in this region.

Mechanical Failure

The following conditions increase the risk of mechanical failure of the locking plate:

1. Fixation of humeral diaphysis fractures requires the use of four locking nails on either side of the fracture site to resist torsion and increase mechanical failure ;and

2. Fixation of epiphyseal fractures is difficult because they are often unstable, 

especially because the fracture site cannot be compressed by the locking nails and the bone is osteoporotic;

3. comminuted intra-articular and extra-articular comminuted fractures of the epiphysis are unstable

 (e.g., distal femur fractures, bicondylar tibial plateau fractures, distal radius fractures);

4. medial comminution of metaphyseal fractures tending to displace to inversion (e.g., proximal humerus, proximal femur, and proximal tibia fractures).

 Locking plates anchored to the lateral aspect of the bone provide a robust structure that is often sufficient 

to stabilize these fractures without the need to add console-type plates medially or to add bone while maintaining a biologic fracture environment. 

Stability is dependent only on the locking plate/locking nail interface, 

which is most stressed after reset when the epiphysis remains inverted or when the medial console is not reconstructed. The locking plate may then fail secondary to fatigue.

Therefore, fixation of bicondylar tibial plateau fractures using locking plates only on the lateral side must be considered depending on the type.

 For proximal humerus fractures, the number of fracture blocks, loss of medial support,

 and inversion of the epiphysis for fixation are known risk factors. To minimize the risk of construct failure, 

certain locking nails will be mechanically supported to compensate for the absence of medial support in the reduction of externally translated fractures

Biological Failure of Locking Plates

The biological failure modes of locking plates are screw cut out and fracture or impingement of the locking nail. 

These risks are greater when bone osteoporosis is present in the skeleton, 

which means that early rehabilitation and return to weight bearing must be done carefully before bone healing is achieved.

(1) Removal of Locking Screws

Screw extraction corresponds to the “total” and simultaneous removal of the locking nail from the bone at one or both ends of the plate. In some cases, 

the locking nail is extracted with a piece of bone around it.

In the epiphyseal region, the one-piece locking plate structure usually provides adequate stability due to the dispersed or convergent locking nail anchorage, 

and the three-dimensional structure increases the resistance to screw extraction from the cancellous bone.

In the diaphyseal region, converging and dispersed locking nails and constructs with longer locking plates have better pullout strength . 

This type of construction is more suitable for periprosthetic fractures. In osteoporotic bone, 

bicortical stem screw fixation is superior to monocortical screw fixation. For periprosthetic fractures, flat-head unicortical screws help avoid contact with intramedullary implants.

These fixation failures are associated with poor bone quality, even if the structure is mechanically intact.

(2) Cut-out or impingement of locking screws

Cut-out or impingement of locking nails with intra-articular penetration may occur in the cancellous epiphyseal region.

These displacements are displacements of epiphyseal fragments of low-mass bone displaced around the fixation locking nail.

 This results in loss of epiphyseal fracture reduction. In the best case, the epiphyseal locking nail impinges and

 penetrates the cancellous bone. In the worst case, the epiphyseal locking nail exits the epiphysis and travels into the joint.

These two complications occur most often in proximal humerus and distal radius fractures. 

For locking plate fixation of proximal humerus fractures, it is recommended that the length of the 

epiphyseal locking nail be limited to minimize the risk of ingrowth and secondary joint penetration.

These fixation failures are due to inadequate bone quality and large initial displacement of the fracture fragments prior to reduction, 

even if the structure is mechanically intact.

Joint Rehabilitation and Immediate Weight Bearing

Rehabilitation and weight bearing are only permitted after perfect fixation has been achieved and verified on postoperative X-rays.

Biomechanical studies have shown that in normal bone, if the gap between the fragments is less than 1 mm, 

weight bearing is possible without risk. after 1 million cycles, the stiffness is the same as that of normal bone, which is sufficient for healing to take place.

When structurally sound, locking plates and fixed-angle locking nails allow for early return to 

weightbearing because the load is transferred directly from the locking nail to the locking plate, with no risk of fixation failure at the nail-plate junction.

However, when the axis of the multiaxial locking peg is not perpendicular to the locking plate, early weight bearing is not allowed.

For MIPO, early weightbearing is allowed for extra-articular, simple and/or simple comminuted fractures. 

Very long specific structures are sufficiently flexible with alternating bicortical locking nails and openings for load absorption and distribution.

Conclusion:

1.Biomechanical studies have evaluated various types of constructs and their mechanical properties.

The literature helps to validate the theoretical hopes associated with this type of fixation. 

However, recent literature also highlights the technical difficulties and failures associated with locking plates.

2.The main reason for failure is inadequate planning of the surgical technique, 

which is very demanding, especially when performing minimally invasive procedures.

3.The fracture must be reset first, without locking the screws to the plate,

as indirect resetting of the plate by locking the screws is not possible.

4.The structure must be of the correct length and strength, 

which means that the surgeon must be familiar with the principles and rules that guide the use of these plates. 

The structure must therefore be elastic, with a limited number of regularly spaced locking screws that alternate with empty holes.

5.Despite the better theoretical initial stability of locking plates, 

fixation of the structure is limited by the complexity of the fracture, the quality of the reduction, and the biological quality of the bone.

6.If the structure is mechanically intact, the quality of the bone is good and the fracture is extra-articular, 

a patient with sufficient autonomy may be allowed to bear weight on the fractured limb. In many cases, locking plate fixation permits early rehabilitation.