Templating for Total Hip Arthroplasty in the Modern Age

Jonathan M. Vigdorchik, MD; Abhinav K. Sharma, BS; Seth A. Jerabek, MD; David J. Mayman, MD; Peter K. Sculco, MD

Disclosures

J Am Acad Orthop Surg. 2021;29(5):e208-e216. 

In This Article

Radiographic Technique and Templating

Radiographic Imaging

Preoperative planning before total hip arthroplasty (THA) involves evaluation of an AP radiograph of the pelvis obtained with the patient in either a standing, weight-bearing position, or supine on a table[5,6] (Figure 1). Standing radiographs can be advantageous for recapitulating the functional pelvic position during activity and in assessing lumbar spinal deformity but can be challenging to obtain proper positioning and a centered beam projection.[7] For supine imaging, position the hip at 10° to 15° of internal rotation to ensure accurate calculation of the true femoral offset and avoid offset underestimation and a valgus-appearing femoral neck.[4] True lateral views of the affected hip should be obtained to visualize the acetabulum and lateral aspect of the femur. Surgeons should consider the magnification of radiographs, with 20% magnification being typical when the radiograph tube is positioned 1 meter away and use magnification markers for greater precision.[4]

Figure 1.

Standard anteroposterior (AP) pelvis radiograph and full hipknee- ankle length biplanar (AP and lateral) radiographs obtained with EOS imaging demonstrating preoperative workup before total hip arthroplasty, ensuring that no rotation exists of the pelvis, the coccyx lies over the public symphysis, and the obturator foramen are open bilaterally.

In addition, novel imaging modalities such as EOS (EOS imaging, Paris, France) biplanar low-dose radiograph imaging can be used for obtaining full-body images. This enables the evaluation of the relationship between adjacent segments (spine, pelvis, and lower extremities) to be easily made, important for the assessment of hip-spine patients. If this imaging modality is not available, conventional standing and seated lateral radiographs should be obtained because measurement of spinopelvic parameters can still be done[7,8] (Figure 2). In addition, positioning the patient in a hip-flexed, seated posture for a lateral radiograph is a functional position that mimics the anatomic position in which THA dislocation commonly occurs in patients with stiff spines, providing additional impingement information for cup planning.

Figure 2.

EOS lateral views demonstrating typical and impaired pelvic mobility during transition from (A) standing to (B) sitting. Sacral slope is the angle between a horizontal reference line and a line parallel to the superior end plate of S1 and change in sacral slope between the standing and sitting positions on the lateral images is calculated during evaluation for spinal stiffness. A change in sacral slope of less than 10 degrees from standing to sitting defines a stiff spine. In the seated position, the pelvis rolls back, increasing the posterior pelvic tilt (seen in blue on figure) and decreasing the sacral slope, indicated by the red line spanning the superior plate of S1. The patient with impaired pelvic mobility and a stiff spine due to a spinal fusion does not demonstrate an increase in posterior tilt of the pelvis with a corresponding decrease in sacral slope in the transition from standing to sitting. The red line on the superior plate of S1 indicates the sacral slope, which does not change, regardless of the position. Sagittal imbalance is defined by a pelvic incidence (PI) minus lumbar lordosis (LL) difference of greater than 10. PI is the angle between a line drawn from the center of the femoral heads to the center of the S1 end plate, and another line drawn perpendicular to the S1 end plate on a standing lateral image (seen in orange on figure). LL is the angle between a line drawn at the superior end plate of L1 and another line drawn at the superior end plate of S1 on a standing lateral image (seen in green on figure).

Acetabular Templating

For templating the acetabular implant, the tear drop, acetabular roof, ilioischial line, and superolateral margin of the acetabulum should be identified. A horizontal reference line through the base of both teardrops can be used in assessing leg length discrepancies and offset.[4–6] Templating with an AP pelvis radiograph assumes that leg lengths are equal from the lesser trochanter, distally, underscoring the importance of a physical examination. Cup templating and sizing should be done to ensure that the medial border of the acetabular implant does not extend beyond the ilioischial line while allowing for lateral bone coverage at approximately 40° inclination.[4–6] In addition, oversizing acetabular components should be avoided because it increases risk of component overhang, which anteriorly can result in psoas tendinitis and increased risk of component impingement if the cup extends past the acetabular rim. Acetabular reaming and cup positioning will secondarily influence leg length and offset and must be accounted for in the preoperative plan. The center of rotation (COR) of the templated cup should be compared with the contralateral side by measuring the vertical distance from the horizontal reference line on each side.[4–6]

Femoral Templating

On the femoral side, the greater trochanter, lesser trochanter, and medullary canal should be identified. LLD can be measured using the vertical distance from the lesser trochanter to the horizontal reference line and compared with the true leg length measurements.[4–6] A best-fit circle can be placed over the femoral head, and the center of that circle estimates the femoral head COR. The distance between the femoral COR in relation to that of the templated acetabular COR indicates the increase or decrease in limb length. Most commonly, the templated femoral head COR will be superior to the acetabular COR by 3 to 5 mm which indicates a planned limb lengthening.[4] Appropriate offset restoration may require increasing or decreasing overall offset. For example, offset should increase from preoperative offset measurements in cases of acetabular protrusion and decrease in cases when the femoral head is lateralized from medial osteophyte hypertrophy or superolateral subluxation (osteonecrosis or developmental dysplasia of the hip). A templated femoral COR medial to the acetabular COR produces an increased offset and vice versa.[4] Importantly, a hip that is externally rotated because of severe arthritis or contracture may result in underestimated offset, and thus, either the contralateral side can be evaluated or it can be done intraoperatively after hip dislocation.[4] The level of neck osteotomy should then be determined and marked on the template and measured in relation to anatomic landmarks (lesser trochanter and piriformis fossa) to ensure reliable replication intraoperatively.[4–6]

Evaluating the Hip-spine Relationship and Templating in the Sagittal Plane

Evaluating the spine-pelvic-hip relationship will help determine whether the patient is at increased risk for postoperative hip instability because of altered hip-spine mechanics. Several reports have highlighted the increased risk for hip instability in patients with "stiff" lumbar spines and spinal malalignment.[7–9] However, most patients with stiff spines do not have instrumented fusions, and thus, it is important for surgeons to regularly evaluate the spinopelvic parameters to optimize component orientation, bearing selection, and potentially reduce the incidence of postoperative hip instability.[10]

Assessment of Dynamic Pelvic Motion

Evaluating patient-specific dynamic pelvic motion in the standing and sitting positions provides a basic understanding of overall pelvic mobility in the sagittal plane. When transitioning from a standing to sitting position, several linked changes occur in the spino-pelvo-hip kinetic chain. When standing, the lumbar spine has on average 40° to 60° of lumbar lordosis (LL), which decreases to 5° to 15° in the sitting position. As the lumbar spine flexes as it transitions from standing to sitting, the pelvis will "roll back" secondary to this linked kinetic chain. The amount of pelvic roll back can be measured as either a change in sacral slope (SS) or pelvic tilt between standing and sitting. The average SS is 40° while standing and 20° while sitting. Pelvic roll back has a powerful effect on increasing pelvic-hip clearance, reduces the amount of hip flexion required to achieve a sitting position, and secondarily reduces the likelihood of anterior acetabular rim or anterior inferior iliac spine bone impingement while also improving posterior-inferior acetabular coverage of the femoral head in the sitting position. A break in this kinetic chain can occur when the lumbar spine stiffens, reducing pelvic roll back and potentially increasing anterior compartment impingement while reducing posterior-inferior acetabular coverage of the femoral head.

Assessment of Overall Sagittal Balance

Recently, the role of sagittal balance in stability after THA and the importance of sagittal balance after spinal fusion have been studied. To measure the spinal sagittal balance, the difference between pelvic incidence and LL is calculated, with a difference greater than 10° qualifying as sagittal spinal deformity.[7] These spinopelvic measurements can be applied by surgeons through the use of the Hip-Spine Classification to characterize the nature of hip-spine pathology and guide intraoperative acetabular implant positioning[7,8] (Table 1). Theoretically, a combination of a stiff spine (SS change < 10°) and sagittal imbalance (pelvic incidence—LL > 10°) represents the highest anatomic risk cohort. These patients can be managed with a combination of increased anteversion, large femoral heads, or consideration for dual mobility for the additional head size and increased jump distance.[1,3] Increasing anteversion too much can, however, increase the potential for posterior impingement.

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