Adult Reconstruction

Adult Reconstruction
Savyasachi C. Thakkar
Louis C. Okafor
BIOMATERIALS
  • Wear mechanisms
    • Radiostereometric analysis is the most sensitive method to measure wear in total hip arthroplasty (THA).
    • Polyethylene (PE) wear
      • 6-mm minimum thickness for ultrahigh-molecular-weight PE in total knee arthroplasty (TKA) and THA
      • Types of wear
        • Adhesive wear—primary wear mechanism for the hip; osteolytic process of wear
        • Abrasive wear—a hard third body gets between two surfaces, leading to scratches, gouges, and scoring marks on the worn surface
        • Third-body wear
      • Wear rate is measured by either volumetric (volume) or linear wear (depth of wear into the PE).
        • Non-cross-linked PE has linear wear of 0.1 to 0.2 mm/y, and highly cross-linked has lower linear wear rate, smaller particles, and reduced toughness.
        • Younger and more active patients have more wear.
        • At risk for osteolysis if linear wear rate is more than 0.1 mm/y
    • Ceramic wear
      • Ceramic-on-ceramic has the lowest wear rate.
      • Stripe wear is unique to ceramic heads, in which femoral head liftoff separation from cup during ambulation leads to wear in form of markings on the head.
    • Metal-on-metal wear
      • In fluid-film lubrication, metal-on-metal has less wear than metal-on-PE.
      • Ability to achieve fluid-film lubrication is inhibited by component position.
        • Acetabulum with abduction greater than 55° leads to edge loading.
      • Titanium is too soft to use for bearing surface and leads to higher failure rates.
  • Osteolysis
    • Wear particles act via receptor activator of nuclear factor kappa-B ligand pathway to stimulate bone resorption.
    • Effective joint space
      • Circumferentially coated femoral implants can seal the rest of the femur from wear particles and osteolysis.
      • Acetabular components with screws are all part of the effective joint space.
  • Cross-linking/sterilization
    • Irradiation should be done in an inert environment to create cross-links and minimize free radicals.
      • Free radicals can be quenched with annealing or remelting.
      • Remelting leads to a decrease in the amount of free radicals compared to annealing but increases the risk for crack propagation.
    • Irradiation in air leads to greatest amount of oxidation and worse outcomes.
      • Increased shelf-life of these products is associated with more rapid failure.
      • Oxidation causes increased elastic modulus and decreased strength.
    • Increased irradiation leads to more cross-links and less volumetric wear but lower tensile and fatigue strength.
  • Metals
    • Nickel allergy cannot use cobalt chrome, which is made from chromium, molybdenum, and cobalt.
    • Zirconium femoral head failure is due to its material properties, which often leads to component fracture.
  • Polymethylmethacrylate (PMMA) components
    • Methacrylate—liquid monomer
    • PMMA—polymer powder
    • N,N-dimethyl-p-toluidine—activator
    • Benzoyl peroxide—initiator of the reaction
HIP RECONSTRUCTION
  • Anatomy
    • Deep branch of medial femoral circumflex artery is critical to maintain femoral head vascularity.
    • Dysplastic hips have excessive femoral neck anteversion and a posterior greater trochanter.
    • Marfan syndrome has increased prevalence of acetabular protrusio.
    • Sciatic nerve crosses anterior to the piriformis between ischial tuberosity and greater trochanter (Figure 2.1).
    • Acetabulum is the safe zone for screws (Figure 2.2).
      Figure 2.1 Sciatic nerve anatomy at hip. From Helfet DL, Sen MK, Bartlett CS, et al. Acetabular fractures: extended iliofemoral approach. In: Wiss DA, ed. Master Techniques in Orthopaedic Surgery: Fractures. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013:885-910.
      Figure 2.2 The acetabular quadrant system can be used to identify the “safe zone” for screw fixation. Placing hardware in the superior-posterior quadrant minimizes risk. From Klein GR, Levine HB. Neurovascular injury following total hip arthroplasty. In: Callaghan JJ, Rosenberg AG, Rubash HE, et al., eds. The Adult Hip: Hip Arthroplasty Surgery. 3rd ed. Philadelphia, PA: Wolters Kluwer; 2016:1062-1076.
  • Diseases
    • Ankylosing spondylitis
      • Pelvic positioned in fixed hyperextension that leads to exaggerated anteversion in standing and can put at risk for anterior dislocations.
    • Osteonecrosis
      • Associated with steroids, alcohol, and HIV—Most common reason in the United States is alcohol abuse.
      • Treatment with bisphosphonates in early stages decreases risk of collapse of the femoral head.
      • Core decompression for precollapse
    • Sickle cell disease
      • Tight medullary canals and may need to be reamed for femoral component placement
    • Rheumatoid arthritis
      • Discontinue biologics that affect tumor necrosis factor-alpha pathway (ie, etanercept) 1 to 2 weeks preoperatively
      • Can continue methotrexate
      • Protrusio acetabuli common in rheumatoid hip
    • Femoral neck fractures
      • Healthy, active, elderly—THA
      • Sick, inactive, elderly—hemiarthroplasty
Surgical Treatment
  • Arthrodesis
    • Preferred fusion position is 20° hip flexion, 0° abduction, and 5° external rotation (ER).
    • In conversion to a THA, abductor function is predictive of better postoperative walking ability.
  • Approaches
    • Anterior (Smith-Petersen approach)—lateral femoral cutaneous nerve at risk
      • Superficial internervous plane: sartorius (femoral nerve) and tensor fascia lata (superior gluteal nerve)
      • Deep internervous plane: indirect head of rectus femoris (femoral nerve) and gluteus medius (superior gluteal nerve)
    • Lateral
      • Partial removal of gluteus medius insertion from greater trochanter
      • Lower rate of posterior hip dislocation since short external rotators are not detached
    • Posterior
      • Psoas protects the anterior retractor from causing damage to the iliac vessels.
      • Highest risk of dislocation
  • THA implants
    • Bearings
      • Metal-on-metal
      • Metal-on-poly
      • Ceramic-on-poly
      • Ceramic-on-ceramic (ceramic has fracture risk)
    • Fixation
      • Acetabulum: cemented versus cementless (standard)
      • Femur
        • Cemented (risk of embolism during cement pressurization)
          • At least 2-mm mantle is necessary and cement restrictor 2 cm distal to stem.
          • Third-generation cementing uses cement mixing in vacuum, cement pressurization, and cleaning of femoral canal with pulse lavage.
        • Cementless—Prior radiation for cancer increases aseptic loosening risk.
          • Bony ingrowth requires surface with pores 50 to 150 µm, 40% to 50% porosity, and <50 µm gaps—Fibrous ingrowth may occur if these conditions are not achieved.
          • Fixation location metaphyseal or diaphyseal—Diaphyseal has increased stress shielding of greater trochanter and proximal femur and may lead to greater trochanter fracture. For example, anatomic medullary locking stem with significant stress shielding of proximal femur.
    • Implant positioning considerations
      • Offset
        • High offset also correlates with decreased joint reaction force.
        • If the offset is too low, abductor weakness and hip instability can result.
      • Leg length
        • Increased neck length increases leg length and offset.
        • Large amount of leg lengthening can lead to sciatic palsy.
          Figure 2.3 Hip implant positioning. Inadequate offset leads to worse lever arm for abductors and positive Trendelenburg sign. A, abductor moment arm; Ab, abductor force; B, moment arm of body weight; JRF, joint reaction force; W, body weight. Adapted from Mirza SB, Dunlop DG, Panesar SS, et al. Basic science considerations in primary total hip replacement arthroplasty. Open Orthop J. 2010;4:169-180.
      • Stability
        • Increase soft-tissue (abductor) tension by increasing offset (Figure 2.3).
          • Inadequate offset leads to worse lever arm for abductors and positive Trendelenburg sign.
        • Increased head-to-neck ratio
        • Female sex is an independent risk factor for dislocation
        • Posterior dislocation—flexion, adduction, and internal rotation (IR)
        • Anterior dislocation—extension and ER
  • Vancouver classification of periprosthetic fractures (Figure 2.4)
    Figure 2.4 Vancouver classification of periprosthetic fractures. From Nauth A, Stevenson I, Smith MD, Schemitsch EH. Fixation of periprosthetic fractures about/below total hip arthroplasty. In: Parvizi J, Rothman RH, eds. Operative Techniques in Joint Reconstruction Surgery. 2nd ed. Philadelphia, PA: Wolters Kluwer; 2016:258-269.
    • Type A—fracture around trochanter; treatment is conservative or surgical if associated with a loose stem
      • Type AG—around greater trochanter
      • Type AL—around lesser trochanter
    • Type B—fracture around or just distal to stem
      • Type B1—stable stem; treated with open reduction and internal fixation (ORIF) with cerclage, struts, and plate
      • Type B2—loose stem; treated with long-stem revision with or without ORIF
      • Type B3—loose implant with substantial bone loss; treated with revision and structural allograft
    • Type C—fracture well below the implant; treated with ORIF with plate systems
  • THA complications
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Dec 19, 2019 | Posted by in ORTHOPEDIC | Comments Off on Adult Reconstruction

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