Читать книгу Synopsis of Orthopaedic Trauma Management - Brian H. Mullis - Страница 41

A. Implant material

1. Titanium alloy (Ti-6Al-4V ELI or Ti-6Al-7Nb)—choose for applications requiring lower stiffness and higher strength: Young’s modulus of elasticity, E = 105 to 120 GPa, tensile yield strength, σ_Y ≥ 760 MPa.

2. Stainless steel (AISI Type 316L)—choose for applications requiring higher stiffness and lower strength: Young’s modulus of elasticity, E = 193 GPa, tensile yield strength, σ_Y ≥ 490 MPa.

B. Considerations for plate fixation

1. Working length:

a. Fracture working length is the distance between the closest points of fixation or the distance between screws immediately adjacent but on opposite ends of the fracture (▶Fig. 4.3).

b. Working length is the most important factor affecting construct stiffness, strain at the fracture site, and stresses in the implant components.

c. Increasing the working length decreases construct stiffness and increases interfragmentary strain. Omitting screws immediately adjacent to the fracture reduces bending stresses on the plate near the fracture line and reduces the risk of premature fatigue failure. This also decreases axial and torsional stiffness thus allowing higher strains during weight bearing, so should be undertaken with caution in simple fractures if direct healing is intended.

2. Screw type:

a. Screws are often subjected to bending loads. Screw bending stiffness, or flexural rigidity, depends on the choice of material (titanium alloy or stainless steel) and screw diameter, with larger-diameter screws being exponentially stiffer than smaller-diameter screws.

b. Stability of nonlocking screws is dependent upon bone quality and friction between the bone ends (lag screw) or the plate–bone interface. Target compressive force for nonlocking screws is 3 N.

c. Locking screw stability is dependent upon the plate–screw locking mechanism.

d. Locking screw push-out strength is decreased if the screw is inserted off-axis.

e. Locking screws increase construct stiffness compared to nonlocking screws. Placing a locking screw at the end of a plate in osteoporotic bone creates a stress riser that can result in a peri-implant fracture.

f. Unicortical locking screws (screws placed into the near cortex only) are less stiff than bicortical locking screws.

g. Far cortical locking screws allow axial motion and decrease stiffness. By engaging only the far cortex while the near cortex is relatively overdrilled, these screws allow symmetric motion at the near and far cortex as the locking screw is able to bend. This theoretically leads to symmetric callus formation.

3. Screw number:

a. Increasing the screw number (plate screw density) increases construct stiffness and construct strength.

Fig. 4.4 (a) Anteroposterior radiograph of a simple femoral shaft fracture shows stress concentration over a single empty screw hole (high screw density) that may result in fatigue failure. (b) Anteroposterior radiograph of a tibia metaphyseal fracture stabilized with a compression plate. Distribution of screws through the plate decreases peak stress adjacent to the fracture line.

i. Three screws on either side of a fracture maximize axial stiffness.

ii. A fourth screw on either side of a fracture increases torsional stiffness compared to three screws.

b. Increasing the screw number increases stress concentrations in the plate near the fracture site and can lead to fatigue failure with prolonged weight bearing in cases of delayed union or nonunion, especially when there is no bony contact for load sharing (▶Fig. 4.4).

c. Plate screw density is the ratio of the number of screws inserted to the number of holes in the plate.

i. Ideal screw density for comminuted fractures is > 0.5.

ii. Ideal screw density for simple fractures is < 0.3.

iii. Screw density has a greater effect on stiffness in simple fractures than in comminuted fractures.

4. Plate length:

a. Longer plates decrease the stress across the construct and increase bending flexibility (deflection) proportionally to the plate length.

b. Longer plates decrease pullout load at each screw.

c. Longer plates decrease peak stresses adjacent to the fracture line and therefore decrease risk of implant fatigue failure (more important for bridging constructs but also applicable to simple fractures; ▶Fig. 4.4b).

d. For bridging constructs, the plate length should approach three times the fracture working length.

i. Longer plates afford lower screw density and balanced fixation which results in better distribution of stress across the construct rather than concentrating stress at empty screw holes over the fracture (▶Fig. 4.4).

ii. Shorter plates require increased screw density and concentrate stress at the fracture and any open screw holes.

iii. Short plate constructs are reserved for simple fractures that are fixed with interfragmentary compression (▶Fig. 4.5).

5. Plate thickness:

a. Increasing plate thickness increases bending stiffness (flexural rigidity) to the third power.

b. Thicker plates with increased prominence may cause soft tissue irritation.

Fig. 4.5 (a, b) Simple transverse fractures fixed with interfragmentary compression and short constructs. Notice direct healing of fracture at 3 months with no callous formation. Direct bony contact protects the plate from stress during loading. The same construct with a residual fracture gap would create a high stress concentration at the plate near the fracture site due to the short construct and high screw density.

c. Thicker plates may cause stress shielding of the underlying bone leading to resorption and a greater stress concentration at the terminal ends of the plate–bone interface. It may also cause symptomatic implants, peri-implant fracture, and greater refracture rate after implant removal.

C. Considerations for intramedullary nailing—reamed, locked intramedullary nails (IMN) are the standard of care for most diaphyseal adult fractures. Advances in nail design, interlocking screw configuration, and angularly stable interlocking screws have enhanced the biomechanical properties of IMN, and therefore extended their indications to metaphyseal and simple intra-articular fractures.

1. Nail geometry:

a. Slotted: A slotted nail increases friction between nail and endosteal bone through radial compression of the nail, but at the expense of torsional and bending rigidity. Historically, these nails were designed to obtain better stability within the bone before interlocking was developed.

b. Terminal slotting: This clothespin-shaped relief slot may be found at the terminal end of a nail. This type of nail is designed to decrease rigidity and lessen the stress concentration at the terminus of the nail.

c. Fluting: Fluting along the working length increases torsional interference between nail and bone, and decreases flexural rigidity which may be important especially in larger-diameter nails.

d. Cannulation: Most modern nails utilize a cannulated design to facilitate nail insertion over a guide wire without compromising size of the outer nail diameter.

e. Diameter: Nail diameter is chosen to suit patient anatomy and ensure good cortical contact after reaming (if done). A larger diameter increases bending stiffness (flexural rigidity) and torsional stiffness in proportion to (r⁴ outer–r⁴ inner) for cannulated nails. Larger nails typically accommodate larger screws and so have reduced risk of early construct fatigue failure.

f. Length: Nail length is chosen to suit patient anatomy, except in short nails which produce stiffer constructs due to their shorter working length and which typically terminate in the isthmus.

Fig. 4.6 Screw in the dynamic slot. When loading a length unstable fracture, the nail will migrate proximally around the screw in the proximal end of the slot.

g. Anterior bow of the femur: Mismatch between nail and femur anterior bow increases point contact and frictional fit. The point contact can result in malreduction, iatrogenic fracture, or anterior cortical perforation.

2. Reaming:

a. Allows a larger-diameter nail with benefits stated above (i.e., increased stiffness, larger screws, reduced risk of fatigue failure).

b. Increases the area of direct contact between endosteal bone and nail, which can help minimize undesirable interfragmentary shear and increase load sharing by the bone to decrease risk of fatigue failure.

3. Interlocking screws:

a. Static screws: Screws placed in tightly fitting holes allow very little relative movement and this provides axial and rotational stability for the construct.

b. Dynamic screw: A single screw placed in a short slot can allow axial shortening for use in intraoperative fracture compression or as postoperative intervention in cases of delayed union or nonunion (▶Fig. 4.6). In length-stable fractures, this allows compression with weight bearing and may stimulate healing.

c. Screw diameter: Larger-diameter screws have increased bending stiffness (flexural rigidity) and reduced risk of fatigue failure. Choosing a large diameter increases bending strength of screw and reduces construct failure especially in length unstable fractures.

d. Angularly stable screws: Some implant systems have design features that enable mechanical coupling between one or more screws and the nail body to increase the rigidity of the construct, particularly in torsion. Examples include threaded or partially threaded screw holes, polymer bushings in the screw holes or sleeves added to the screw prior to insertion, and locking or compression endcaps.

e. Number of screws: Addition of a third screw increases stiffness in the proximal tibia metaphysis. This benefit is not observed in the distal tibia.

f. Screw distance to fracture: Screws positioned closer to metaphyseal fractures afford greater rotational control, but do not increase axial stability.

g. Screw orientation: Oblique interlocking screws increase stability of proximal one-third tibia constructs but not distal one-third tibia constructs.