KneeHipAnkleFootHandSpineThe thesisElbowWristGirdleCervicalPartner vault
Pre-op instrument · shoulder

Every glenoid has a correction budget. Most plans never state it.

The glenoid has travelled some number of degrees from where it started. The reamer can bring back ten to fifteen of them before you are medialising into the vault. The difference between those two numbers decides the case, and it is the number that usually goes unwritten: it is what tells you whether this is a standard glenoid, an augment, or a reverse.

Budget this glenoid → The hip instrument →
Retroversion, subluxation, biconcavity and cuff status · gives the Walch type, the correction budget, and the residual the construct has to supply
The envelope · CT with three-dimensional glenoid planning

The shoulder's envelope is distance travelled, not motion.

The knee measures opening across flexion and the hip measures tilt across posture. The shoulder measures something static: how far the glenoid has eroded away from the premorbid surface it started as. That premorbid glenoid, reconstructed from the vault, is this joint's fixed reference.

The glenoid

Off the axial CT, referenced to the scapular plane.
24°
Positive is retroverted. Over 25° is dysplastic territory.
72%
50% is a centred head. 45–55% counts as centred; 70% or more is B3 territory.

The reference and the gate

Premorbid version, and the soft tissue that overrides everything.
5°
Reconstructed from the vault where your planning software supports it; otherwise estimated.
15°
Most surgeons cap corrective reaming here. Many reach for an augment beyond 10°.
The soft-tissue gate
Open — the bone budget decides this case
With a working cuff, the glenoid arithmetic below is what chooses the construct.
Walch B2

Beyond the reamer.

The correction budget
14° needed, 14° reamable

Log this shoulder →What the tiers mean →
The four tiers · and the gate above them

The bone decides the case only when the cuff works.

This is where the shoulder genuinely differs from the knee and the hip. Those two are decided by geometry. The shoulder has a soft-tissue gate sitting above the geometry: an incompetent cuff sends the case to a reverse no matter how correctable the glenoid is. The budget matters underneath that gate, not above it.

Tier A · concentric

Nothing to correct.

Centred head, version near premorbid, cuff intact. A standard glenoid on an anatomic reconstruction. The Walch A shoulder is the one that behaves.

The Walch classification →
Tier B · reamed correction

The reamer reaches it.

Deformity inside the safe limit and inside the threshold where most surgeons still ream rather than augment. Anatomic, with the version corrected on the way in.

Eccentric reaming for posterior wear →
Tier C · beyond the reamer

The construct supplies the rest.

Correcting this much by reaming alone medialises into the vault. An augmented baseplate or graft carries the correction the reamer should not. Anatomic only if the cuff is genuinely intact and the residual is small.

Vault perforation after eccentric reaming →
Tier D · reverse

Either the cuff is gone or the residual is.

Anatomic reconstruction on a B2 glenoid carries about a 10.5% complication rate, most often recurrent posterior subluxation, even where revision stays near 2.4%. When the numbers do not close, the reverse is the honest answer.

Anatomic versus reverse for B2 →
The numbers under the budget

Where each threshold comes from, and how firm it is.

B3 by definition
≥15° or ≥70%
Monoconcave posterior wear with that retroversion or that subluxation, or both, Walch and colleagues
Where surgeons stop reaming
78% cap at 15°
And 73% reach for an augment beyond 10°. ⚠ Vendor-published survey — a practice pattern, not evidence
Anatomic on a B2
10.5% complication
Revision only 2.4%, but complications most often recurrent posterior glenohumeral subluxation
Reverse geometry targets
40–65 / 75–95°
Distalisation and lateralisation shoulder angles. ⚠ Contested — later series find only marginal correlation with outcome
The fastest-growing arthroplasty
+67%
One projection put 2025 shoulder volume around 174,810 procedures, with revision climbing alongside
No mandate reaches this joint
none
TEAM covers lower-extremity joint replacement and four other episodes. The 2027 specialty model covers heart failure and low back pain. Neither includes the shoulder
The honest difference from the knee and hip pages. Those two sit inside a mandatory federal episode and the clock does the arguing. This one does not, and pretending otherwise would be the easiest lie on this site. What carries the shoulder instead is that the implant-class decision is close to irreversible in practice, the volume is growing faster than either other joint, and the cost of getting it wrong lands on an ambulatory centre with thin margins.
What is estimated rather than measured. Medialisation here is geometry, not a measurement: glenoid half-width times the tangent of the correction. The reaming limit is a practice pattern with a vendor-survey component, not a validated threshold, which is why it is a slider you can set to your own. Premorbid version is estimated unless your planning software reconstructs it from the vault. The reverse geometry targets are contested. Not a cleared device, not clinical advice.
Three joints, one structure

Measure the envelope, position first, escalate only on the residual.

 KneeHipShoulder
The envelopeCompartment opening under load, across flexionFunctional pelvic tilt, across postureDistance travelled from the premorbid glenoid
The fixed referenceConstitutional alignmentPelvic incidencePremorbid (paleo) glenoid
Position firstComponent angles inside the alignment boundariesCup anteversion inside the functional windowEccentric reaming inside the safe limit
The residualLaxity positioning cannot balancePostures no cup orientation satisfiesDegrees the reamer should not take
The escalationConformity, lip, stepDual mobilityAugment, graft, or reverse
Mandatory episodeTEAMTEAMnone
Instance one

The knee.

Ten-item clinic rating, functional alignment first, geometry only on the residual.

The knee instrument →
Instance two

The hip.

Two lateral films give the cup window. A stiff pelvis collapses it to a few degrees.

The hip instrument →
Log this shoulder

Put the first row in.

The measurements above are attached. Contact details only; we reply with the ledger format and the other two instruments.

Stored as telemetry for calibration, not as a clinical record. No patient identifiers are collected.