Savyasachi C. Thakkar
Louis C. Okafor
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
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-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.
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.
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.
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.
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
Benzoyl peroxide—initiator of the reaction
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).
Pelvic positioned in fixed hyperextension that leads to exaggerated anteversion in standing and can put at risk for anterior dislocations.
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
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
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.
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)
Partial removal of gluteus medius insertion from greater trochanter
Lower rate of posterior hip dislocation since short external rotators are not detached
Psoas protects the anterior retractor from causing damage to the iliac vessels.
Highest risk of dislocation
Ceramic-on-ceramic (ceramic has fracture risk)
Acetabulum: cemented versus cementless (standard)
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
High offset also correlates with decreased joint reaction force.
If the offset is too low, abductor weakness and hip instability can result.
Increased neck length increases leg length and offset.
Large amount of leg lengthening can lead to sciatic palsy.
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)
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
Recurrent dislocation with malpositioned components requires revision THA.
Late dislocation primarily caused by poly wear
Deep venous thrombosis
Limb length discrepancy
Most common reason for lawsuits after THA
Patient may feel it is greater than the actual difference due to weak hip abductors.
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