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 proportional to the radius to the 3rd power (approximately) for a hollow nail area moment of inertia of a hollow cylinder where r1 is inner radius and r2 is outer radius for thin cylinders, {\displaystyle r\equiv r_{1}\approx r_{2}}and {\displaystyle r_{2}\equiv r_{1}+t}. and 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