Most hips that dislocate were aimed inside the safe zone.
Fifty-eight percent of dislocated hips had the cup within Lewinnek. The safe zone was never the problem. The problem is that a cup is aimed once, in one position, and the patient then stands up and sits down. If the pelvis moves the way it should, almost any reasonable cup works. If it does not, the window you have to hit is a few degrees wide, and nothing on a supine film tells you that.
The hip's envelope is posture, not flexion.
Pelvic incidence is fixed morphology and never changes. Sacral slope changes when the patient sits. The difference between those two sacral slopes is the whole story, and it takes two films to see it.
Fixed morphology
The two postures
The ordinary hip.
Two questions: is the spine aligned, and does the pelvis move?
Flatback is pelvic incidence minus lumbar lordosis over 10 degrees. Stiff is under 10 degrees of sacral-slope change from standing to seated. Those two yes-or-no answers give you the group, and the group gives you the construct.
The ordinary hip.
The pelvis tilts back when the patient sits, which quietly adds the anteversion they need in flexion. Your target window is wide and a standard bearing is right.
The 2021 Otto Aufranc Award paper →The protective tilt is missing.
Alignment looks normal, so nothing flags this hip on a standard workup. But the pelvis will not rotate when they sit, so the cup has to carry anteversion it would otherwise have been given for free.
Spinopelvic motion, a simplified approach →The frame is shifted, the motion is intact.
The patient stands in more posterior tilt than their anatomy suggests. Plan from the functional frame rather than the anatomic one and the motion still protects them.
Spinopelvic challenges in primary THA →The three-times hip.
Deformed and immobile. Roughly triple the dislocation rate. Dual mobility is the honest default, and the reason to compute the target anyway is to find out when you can safely decline it.
Decreased instability in 2B patients →A cup is aimed once. A patient changes position all day.
Every degree the pelvis tilts backwards adds about seven tenths of a degree of functional cup anteversion. A mobile pelvis supplies roughly twenty degrees of that automatically the moment the patient sits. A stiff one supplies almost none, and the cup has to have been built that way from the start.
Predict it, then find out how right you were.
The knee instrument predicts a laxity envelope and gets scored against the measured knee. This does the same with posture: the predicted functional target against the cup you actually delivered and where the hip actually sat afterwards. One programme, two joints, one calibration record that belongs to the surgeon.
The knee, already live.
Compartment opening under load across flexion, functional alignment first, geometry only on the residual. Same structure, different envelope.
The knee instrument →See the residual drive geometry.
The knee studio shows what happens when positioning runs out. The hip's version of running out is a cup window a few degrees wide.
Envelope studio →Every prediction hashed.
Predictions and outcomes anchored on the proof layer so a sceptic can recompute the calibration rather than trust it.
hashcare verify →The record travels with the surgeon.
The surgeon signs the plan and owns the calibration history. It is not the robot vendor's data and it is not the hospital's.
SurgeonValue →Put the first row in.
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