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

Dec 19, 2019 | Posted by in ORTHOPEDIC | Comments Off on Adult Reconstruction
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