Patients with spinal deformity or a stiff spine dislocate after THA at far higher rates, and the Lewinnek safe zone fails to predict who. This prospective multicentre study validates the Hip-Spine Classification, a four-group preoperative system based on spinal alignment and pelvic mobility. It asks whether classifying patients and tailoring cup position lowers dislocation in this high-risk group.
When you plan a THA, do not assume the supine pelvis tells the whole story. The Lewinnek safe zone is unreliable here, with 58% of dislocators sitting inside it.
The mental model is functional, not static. A normal spine rolls the pelvis back when sitting, which opens and anteverts the cup to clear the flexing femur. A stiff spine cannot do this, so a conventionally placed cup impinges and dislocates.
Learn the two thresholds cold: PI-LL > 10° is flatback deformity, and < 10° sacral slope change standing to sitting is a stiff spine. Combine them into the four groups, with 2B (stuck sitting) being the highest risk at 93.2% survival.
This is the spine half of the modern revision and primary THA dislocation workup. When a patient presents with recurrent instability and a well-looking cup on AP pelvis, get standing and seated lateral spinopelvic imaging before blaming the implant.
Patients with spinal deformity or a stiff spine dislocate after THA at far higher rates, and the Lewinnek safe zone fails to predict who. This prospective multicentre study validates the Hip-Spine Classification, a four-group preoperative system based on spinal alignment and pelvic mobility. It asks whether classifying patients and tailoring cup position lowers dislocation in this high-risk group.
When you plan a THA, do not assume the supine pelvis tells the whole story. The Lewinnek safe zone is unreliable here, with 58% of dislocators sitting inside it.
The mental model is functional, not static. A normal spine rolls the pelvis back when sitting, which opens and anteverts the cup to clear the flexing femur. A stiff spine cannot do this, so a conventionally placed cup impinges and dislocates.
Learn the two thresholds cold: PI-LL > 10° is flatback deformity, and < 10° sacral slope change standing to sitting is a stiff spine. Combine them into the four groups, with 2B (stuck sitting) being the highest risk at 93.2% survival.
This is the spine half of the modern revision and primary THA dislocation workup. When a patient presents with recurrent instability and a well-looking cup on AP pelvis, get standing and seated lateral spinopelvic imaging before blaming the implant.