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Updated: Jul 19 2021

THA Templating

  • Introduction
    • Definition
      • the process of anticipating the size and position of implants prior to surgery
    • Importance
      • allows surgeon to anticipate potential difficulties
      • to reproduce hip biomechanics
      • minimizes leg length inequality
    • Accuracy
      • 52-98% accurate +/- one size
      • related to experience and practice
    • Steps
      • obtain appropriate radiographs
      • record vital patient information on template (age, height, weight, etc)
      • establish radiographic landmarks
      • establish limb length discrepancy
      • template acetabular component
        • do this first to determining center of rotation of new hip
      • template femoral component
    • Tips
      • best to achieve a good template with sizes in the middle range of the component system
      • different system may be a better choice if this cannot be achieved
  • Radiographic Analysis
    • Necessary radiographs
      • AP pelvis
        • centered over pubic symphysis
      • AP hip
        • taken with 10-15 degrees of internal rotation
          • places femoral neck parallel to cassette
          • external rotation on radiographs will
            • falsely decrease offset
            • create valgus appearing femoral neck
            • falsely decrease femoral canal diameter
      • frog lateral hip
    • Magnification
      • 20% is standard
        • most templates account for this
      • magnification markers are helpful
    • Secondary assessment of radiographs
      • pelvic obliquity
        • may be secondary to spinal deformity
        • may cause leg-length issues
      • acetabular retroversion
        • makes appropriate positioning of acetabular component more difficult intraoperatively
  • Radiographic Landmarks
    • Femoral side
      • medullary canal
      • greater trochanter
      • lesser trochanter
      • saddle point
        • most distal part of the junction between the superior aspect of the femoral neck and the greater trochanter
    • Acetabular side
      • acetabular roof
      • tear drop
        • created by superposition of the most distal part of the medial wall of the acetabulum and the tip of the anterior/posterior horn of acetabulum
    • Pelvis
      • ischial tuberosities
        • important to determine limb length discrepancy
  • Establish Limb Length Discrepancy
    • Steps
      • on AP pelvis, draw horizontal line connecting the ischial tuberosities
        • ensure the line extends beyond the medial femoral cortices bilaterally
      • mark the top (proximal-most point) of both lesser trochanters on the AP pelvis radiograph
      • measure the distance between the inter-tuberosity line and the line drawn at the most proximal aspect of the lesser trochanters
    • Alternatives
      • a line connecting the teardrops may be used instead of the ischial tuberosity line
        • this may be more accurate
  • Template the Acetabulum
    • Steps
      • place appropriately sized acetabular template with roughly 40 degrees of abduction
        • medial border of cup should approximate the ilioischial line and lie close to the teardrop
          • a medial cup will decrease joint reactive forces and decrease force required by abductors to maintain a level pelvis
        • inferior border of cup should be at level of inferior teardrop line
      • mark center of rotation of acetabular component
  • Template the Femur
    • Steps
      • choose an appropriate sized femoral implant to fill medullary canal
      • insertion depth is determined to optimize limb length inequality
        • placing the new femoral head center of rotation superior to the acetabular center of rotation will lengthen the limb
        • placing the new femoral head center of rotation inferior to the acetabular center of rotation will shorten the limb 
        • mark the intended femoral neck resection level
          • use lesser trochanter for posterior approach
          • use saddle point (see above) for anterior approaches
      • restore offset
        • may be restored by
          • choosing a stem with more or less offset
          • choosing a stem with a different neck-shaft angle
          • modifying the length of the femoral neck
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