The Lewinnek safe zone is a static coronal cup target that does not always prevent dislocation after THA. This study asked whether cups placed inside the Lewinnek zone actually land in the patient-specific functional safe zone defined on standing and sitting lateral spinopelvic radiographs. It also identified which preoperative measures predict falling outside that functional zone.
When a THA dislocates despite "perfect" cup angles, this paper explains why: the Lewinnek zone is a coronal snapshot, but the cup actually moves with the pelvis during sitting and standing. The key mental model is compensation. A stiff or fused spine cannot tilt the pelvis out of the way, so the femur must move more, driving the trochanter into the pelvis and levering the head out.
For boards and for the clinic, the screening tool is the standing and sitting lateral spinopelvic radiograph, from which you derive the combined sagittal index (AI + PFA). The highest-risk patient is one with low pelvic incidence, a stiff pelvis (ΔSS <11°), and prior spinal fusion. In that patient consider a dual-mobility construct rather than chasing a coronal cup target.
The practical lesson: stop trusting cup angles alone and start assessing functional, patient-specific spinopelvic motion.
The Lewinnek safe zone is a static coronal cup target that does not always prevent dislocation after THA. This study asked whether cups placed inside the Lewinnek zone actually land in the patient-specific functional safe zone defined on standing and sitting lateral spinopelvic radiographs. It also identified which preoperative measures predict falling outside that functional zone.
When a THA dislocates despite "perfect" cup angles, this paper explains why: the Lewinnek zone is a coronal snapshot, but the cup actually moves with the pelvis during sitting and standing. The key mental model is compensation. A stiff or fused spine cannot tilt the pelvis out of the way, so the femur must move more, driving the trochanter into the pelvis and levering the head out.
For boards and for the clinic, the screening tool is the standing and sitting lateral spinopelvic radiograph, from which you derive the combined sagittal index (AI + PFA). The highest-risk patient is one with low pelvic incidence, a stiff pelvis (ΔSS <11°), and prior spinal fusion. In that patient consider a dual-mobility construct rather than chasing a coronal cup target.
The practical lesson: stop trusting cup angles alone and start assessing functional, patient-specific spinopelvic motion.