This study examines why late dislocations occur after THA in patients with no obvious component malposition. It prospectively evaluates spinopelvic motion using lateral standing and sitting spine-pelvis-hip radiographs in 20 patients with late dislocation (mean 8.3 years post-op). The central question: is spinopelvic imbalance a causative mechanical factor?
When a THA dislocates years after surgery with components in acceptable position, the standard AP pelvis is not enough — you are missing the diagnosis.
Get lateral standing and sitting spine-pelvis-hip radiographs. A standing sacral slope below 30° with hip hyperextension points to anterior dislocation risk from fixed posterior pelvic tilt. A Δsacral slope below 10° (nearly no pelvic motion) points to posterior dislocation risk from spinopelvic stiffness.
This paper is why we ask about lumbar spine history before THA revision. A patient with prior lumbar fusion who dislocates posteriorly is not failing because of the implant. They are failing because the pelvis cannot tilt posteriorly during sitting, forcing the hip into hyperflexion.
The practical takeaway: treat the impingement, not just the implant. At revision, push the hip to extremes of motion on the table and burr any osseous impingement on the trochanter before relying on a constrained liner. Because constrained liners in the setting of unaddressed bony impingement have a well-documented high failure rate.
This study examines why late dislocations occur after THA in patients with no obvious component malposition. It prospectively evaluates spinopelvic motion using lateral standing and sitting spine-pelvis-hip radiographs in 20 patients with late dislocation (mean 8.3 years post-op). The central question: is spinopelvic imbalance a causative mechanical factor?
When a THA dislocates years after surgery with components in acceptable position, the standard AP pelvis is not enough — you are missing the diagnosis.
Get lateral standing and sitting spine-pelvis-hip radiographs. A standing sacral slope below 30° with hip hyperextension points to anterior dislocation risk from fixed posterior pelvic tilt. A Δsacral slope below 10° (nearly no pelvic motion) points to posterior dislocation risk from spinopelvic stiffness.
This paper is why we ask about lumbar spine history before THA revision. A patient with prior lumbar fusion who dislocates posteriorly is not failing because of the implant. They are failing because the pelvis cannot tilt posteriorly during sitting, forcing the hip into hyperflexion.
The practical takeaway: treat the impingement, not just the implant. At revision, push the hip to extremes of motion on the table and burr any osseous impingement on the trochanter before relying on a constrained liner. Because constrained liners in the setting of unaddressed bony impingement have a well-documented high failure rate.