Acquisition protocol for CupCalc — for staff and radiology technologists

Imaging Acquisition Protocol

How to obtain your
measurements

CupCalc requires six radiographic measurements from three standard acquisitions. Consistent patient positioning is critical — small errors in sacral slope or proximal femoral angle propagate into meaningfully different cup targets.

Acquisition 1

Standing Lateral Spinopelvic Film

Three measurements are taken from the standing lateral view: pelvic incidence, sacral slope in standing, and proximal femoral angle in standing. This film captures the patient's functional upright pelvic posture.

Standing patient positioning

Patient Positioning

Patient stands upright, arms crossed over chest or resting on a support bar to avoid obscuring the pelvis. Feet shoulder-width apart, flat on floor. The cassette captures the full lateral spinopelvic silhouette from L1 to mid-femur. Ensure both femoral heads are superimposed.

PI Pelvic Incidence
Pelvic Incidence measurement diagram

Left: perpendicular from S1 endplate center · Center: line to bicoxofemoral axis · Right: resulting PI angle

How to Measure

  1. 1Identify the center of the S1 superior endplate on the lateral film.
  2. 2Draw a line perpendicular to the S1 endplate at its midpoint.
  3. 3Draw a second line from that midpoint to the center of the bicoxofemoral axis (midpoint between both femoral head centers).
  4. 4The angle between these two lines is the Pelvic Incidence.
Key point: PI is a fixed anatomic constant — it does not change between standing and sitting. Measure it once on the standing film and use the same value throughout.
SSstand Sacral Slope — Standing
Sacral Slope standing measurement diagram

Left: horizontal reference · Center: line along S1 endplate · Right: resulting SS(stand) angle

How to Measure

  1. 1Identify the S1 superior endplate on the standing lateral film.
  2. 2Draw a true horizontal reference line.
  3. 3Draw a line along the superior surface of the S1 endplate.
  4. 4Measure the angle between these two lines — this is the sacral slope in standing.
Key point: SS(stand) reflects pelvic tilt in the upright functional position. It is the primary mobility input for CupCalc's spinopelvic classification algorithm. A normal value is typically 35–45°.
PFAstand Proximal Femoral Angle — Standing
Proximal Femoral Angle standing measurement diagram

Left: line from femoral head to S1 · Center: line parallel to femoral shaft · Right: resulting PFA(stand)

How to Measure

  1. 1On the standing lateral film, identify the center of the femoral head and the center of the S1 endplate.
  2. 2Draw a line connecting these two points.
  3. 3Draw a second line parallel to the anterior cortex of the femoral shaft.
  4. 4Measure the angle between these two lines — this is PFA(stand).
Key point: PFA(stand) quantifies femoral extension in the upright position. Combined with PFA(sit), it defines the total functional arc the reconstructed hip must accommodate without impingement.
Acquisition 2

Sitting Lateral Spinopelvic Film

Two measurements are taken from the sitting lateral view: sacral slope sitting and proximal femoral angle sitting. The delta between standing and sitting sacral slope (ΔSS) is the core input for mobility classification.

Sitting patient positioning

Patient Positioning

Patient sits on a firm, flat surface (not a soft chair). Thigh-trunk angle should be 100–110°, not 90° — this mimics the functional seated position in a chair. Back straight, arms crossed over chest, feet flat on the floor. Avoid lumbar flexion or slumping. This positioning standard is critical for reproducible ΔSS values.

SSsit Sacral Slope — Sitting
Sacral Slope sitting measurement diagram

Left: horizontal reference · Center: line along S1 endplate · Right: resulting SS(sit) angle

How to Measure

  1. 1Identify the S1 superior endplate on the sitting lateral film.
  2. 2Draw a true horizontal reference line.
  3. 3Draw a line along the superior surface of the S1 endplate.
  4. 4Measure the angle — this is SS(sit).
ΔSS = SS(stand) − SS(sit). This value drives mobility classification. ΔSS < 10° = stiff spine (high dislocation risk). ΔSS 10–30° = normal mobility. ΔSS > 30° = hypermobile. Note: 32.5% of patients change mobility class postoperatively without spine surgery — treat classifications as probabilistic estimates. [6]
PFAsit Proximal Femoral Angle — Sitting
Proximal Femoral Angle sitting measurement diagram

Left: line from femoral head to S1 · Center: line parallel to femoral shaft · Right: resulting PFA(sit)

How to Measure

  1. 1On the sitting lateral film, identify the center of the femoral head and center of the S1 endplate.
  2. 2Draw a line connecting these two points.
  3. 3Draw a line parallel to the anterior femoral cortex.
  4. 4Measure the angle — this is PFA(sit).
Key point: PFA(sit) quantifies the patient's flexion demand. A larger PFA(sit) indicates greater flexion arc requirement from the hip — which directly informs the impingement risk calculation and the minimum safe anteversion target.
Acquisition 3

AP Pelvis Radiograph — Standing

The AP pelvis provides the anterior pelvic tilt measurement, which is used to correct functional cup anteversion targets for each patient's individual pelvic orientation.

APT Anterior Pelvic Tilt
Anterior Pelvic Plane measurement diagram

Left: vertical from pubic symphysis · Center: line from pubic symphysis to ASIS · Right: resulting APP/APT angle

How to Measure

  1. 1Identify the pubic symphysis and both anterior superior iliac spines (ASIS) on the standing AP pelvis film.
  2. 2Mark the midpoint between both ASIS.
  3. 3Draw a line from the pubic symphysis to this ASIS midpoint — this is the anterior pelvic plane (APP).
  4. 4Measure the angle between the APP and a true vertical line.
  5. 5If the ASIS are anterior to the pubic symphysis, the tilt direction is anterior. If the ASIS are posterior to the pubic symphysis, the tilt direction is posterior.
Key point: APT directly modifies the functional anteversion target. Patients with anterior pelvic tilt require more cup anteversion to maintain the functional target in the standing position. Enter the direction (anterior/posterior) and magnitude in CupCalc.
📐 Intraoperative AP Correction: All anteversion values reported by CupCalc are referenced to the standing AP pelvis radiograph. If targeting cup position intraoperatively using an AP hip radiograph (rather than AP pelvis), add 4° to the functional anteversion target to account for the projection difference. [Wan Z et al., Clin Orthop 2009]