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