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Updated: Apr 18 2024

Orthopaedic Implants

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  • Introduction
    • Characteristics of orthopaedic implants depends on
      • material properties
      • structural properties
  • Screws
    • Definitions
      • pitch
        • distance between threads
      • lead
        • distance advanced with one revolution
      • screw working distance (length)
        • defined as the length of bone traversed by the screw
      • outer diameter
      • root (inner) diameter
      • bending strength is proportionate to inner (minor) diameter^3
      • pullout strength is proportionate to outer (major) diameter^2
        • maximized by
          • large outer diameter difference
          • fine pitch
        • pedicle screw pullout most affected by quality of bone (degree of osteoporosis)
    • Types of screws
      • cortical screws
      • cancellous screws
      • locking screws
  • Plate Properties
    • Overview & definitions
      • a load-bearing device that is most effective when placed on the tension side
      • plate working distance
        • the length between the 2 screws closest to the fracture on each end of the fracture.
        • decreasing the working distance increases the stiffness of the fixation construct
    • Structural properties
      • bending rigidity proportional to thickness to the 3rd power
      • titanium has Young's modulus of elasticity that most closely approximates cortical bone
    • Biomechanics
      • absolute stability
        • constructs heal with primary (intramembranous/Haversian) healing
        • must eliminate micromotion with lag screw fixation
        • must be low strain at fracture site with high fixation stiffness
      • relative stability
        • constructs heal with secondary (endochondral) healing
        • strain rates must be <10%, or fibrous union will predominate
    • Plate functions
      • Compression
      • Buttress
      • Tension band
      • Bridging
      • Protection
  • Plate Variations
    • Concave plates
      • placing a concave bend on a plate is useful in transverse fractures to ensure compressive forces occur on both the far and near cortices of the fracture
    • Compression plates
      • compression plates work by placing a cortical screw eccentrically into an oval hole in the plate
      • place the cortical screw eccentrically AWAY from the fracture in order to achieve compression 
    • Locking plates
      • advantages of locking plates
        • locked plate/screw constructs compared to non-locked plate/screw constructs result in less angulation in comminuted metaphyseal fractures
      • indications for locking plate technology
        • indirect fracture reduction
        • diaphyseal/metaphyseal fractures in osteoporotic bone
        • bridging severely comminuted fractures
        • plating of fractures where anatomical constraints prevent plating on the tension side of the bone (e.g. short segment fixation).
      • locking plate screw biomechanics
        • bicortical locking screws have significantly more resistance to all applied forces, with resistance to torsion increased the most (versus unicortical)
          • far cortical locking screws may decrease construct stiffness enough to promote interfragmentary motion and callus formation
        • unicortical locking screws have less torsion fixation strength than non-locking bicortical constructs
      • percutaneous locking plates
        • application has less soft-tissue stripping but higher chance malunion
      • hybrid locked plates
        • utilize locking and nonlocking screws in order to assist with fracture reduction (nonlocking screws) as well as provide a fixed angle construct (locking screws).
      • locking plate construct stiffness and stability increases with:
        • bicortical locking screws
        • increased number of screws
        • screw divergence from screw hole < 5 degrees
        • longer plate
    • Bridging plates
      • provides relative stability, relative length and alignment
      • preserves the blood supply to the fracture fragments as the fracture site is undisturbed during the operative procedure
        • this theoretically improves secondary bone healing
      • allows some motion at fracture site; relative stability leads to callus formation
  • Intramedullary nails
    • Overview
      • a load-sharing device
    • Structural Properties
      • stiffness
        • torsional rigidity
          • defined as amount of torque needed to produce torsional (rotational) deformation
          • proportional to the radius to the 4th power
          • depends on
            • shear modulus
            • polar moment of inertia
          • increased by reaming
          • decreased by slotting of nail
        • bending rigidity
          • proportional to the radius to the 4th power for a solid nail
            • area moment of inertia of a solid cylinder {\displaystyle I_x = \frac{\pi}{4} r^4}
          • proportional to the radius to the 3rd power (approximately) for a hollow nail
            • area moment of inertia of a hollow cylinder {\displaystyle I_x = \frac{\pi}{4} \left({r_2}^4-{r_1}^4\right)}
              • where r1 is inner radius and r2 is outer radius
            • for thin cylinders, {\displaystyle r\equiv r_{1}\approx r_{2}}{\displaystyle r \equiv r_1 \approx r_2}and {\displaystyle r_{2}\equiv r_{1}+t}{\displaystyle r_2 \equiv r_1+t}. and {\displaystyle I_{x}=I_{y}\approx \pi {r}^{3}{t}}
          • depends on
            • material properties
              • Young modulus of elasticity of material
            • structural properties
              • area moment of inertia
              • length
    • Radius of curvature
      • intramedullary nail radius of curvature is greater (straighter) than the radius of curvature of the femur
    • Interlocking options
      • dynamic locking-->axially and rotationally stable fractures
      • static locking-->axially and rotationally unstable fractures
      • secondary dynamization for nonunion
        • remove proximal interlocking screw or move proximal interlocking screw from the static to dynamic slot
  • External fixators
    • Factors that increase stability of conventional external fixators
      • contact of ends of fracture
      • larger diameter pins (most important)
      • additional pins
      • decreased bone to rod distance
      • pins in different planes
      • increasing size or stacking rods
      • rods in different planes
      • increased spacing between pins
    • Factors that increase stability of circular (Ilizarov) external fixators
      • larger diameter wires
      • decreased ring diameter
      • olive wires
      • extra wires
      • wires cross perpendicular to each other
      • increased wire tension
        • tensioned wires produce more axial compression with less interfragmentary shear than half pins
      • placement of two central rings close to fracture
      • increased number of rings
  • Total Hip Implants
    • Structural Properties
      • rigidity depends on length and radius of femoral stem
    • Biomechanics
      • place femoral component in neutral or slight valgus to reduce moment arm and stress on cement
      • increasing femoral offset does the following
        • advantages
          • increase abductor moment arm
          • decreases joint reaction forces 
          • reduces abductor force required for normal gait
        • disadvantages
          • increased strain on implant
          • increases strain on medial cement mantle
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