How a CT scan becomes a shoulder replacement: inside Mako planning.

The misconception about robotic shoulder surgery.

The most common misconception about robotic shoulder replacement is that the robot does the surgery. It does not. The surgeon does the surgery. The robot is an instrument — a very accurate, very expensive ruler — that allows the surgeon to execute a plan drawn up days before the patient ever arrives in theatre.

The operation is the plan. The robot is the instrument.

The CT scan is not imaging. It is the substrate for the plan.

Most patients come to a shoulder replacement consultation having had an X-ray. The X-ray is enough to confirm the diagnosis of glenohumeral arthritis. It is not enough to plan a shoulder replacement. The single most important investigation in the modern arthroplasty pathway is the high-resolution CT scan of the shoulder — and not because we are looking at the scan ourselves. We are looking at it through the planning software.

The CT dataset is converted into a patient-specific three-dimensional model of your scapula and humerus. The software then lets the surgeon do four things that simply cannot be done on an X-ray:

1. Measure glenoid version — the rotation of the socket relative to the scapula. Normal is 0° to –5° (slightly retroverted). An arthritic glenoid often sits at –15° or worse. The plan corrects this.
2. Measure glenoid inclination — the tilt of the socket. Excessive superior inclination is associated with early failure of reverse shoulder replacements.¹
3. Classify glenoid bone wear pattern — using the Walch classification (A1, A2, B1, B2, B3, C, D). Each pattern has a different planning implication.
4. Place the implant in 3D space — the size of the baseplate, the position of the fixation pegs, the line of bone removal. All decided before theatre.

By the time the patient is anaesthetised, the operation is no longer a series of intra-operative judgement calls. It is a plan being executed.

Why the glenoid is the rate-limiting variable.

The single most technically demanding component of a shoulder replacement is the glenoid (the socket on the scapula side). The glenoid is small, often worn unevenly by years of arthritis, and sits at the back of a deep surgical field. Even a few degrees of error in glenoid version or inclination shortens implant survival and worsens function.²

In reverse total shoulder replacement, baseplate position — version, inclination and lateralisation — sets the mechanics for the rest of the construct. Get it wrong and you trade scapular notching, early loosening, and limited active elevation for short-term gains. Australian and international registry data consistently identify glenoid-side complications as a leading driver of revision in shoulder arthroplasty.³ Accurate glenoid baseplate position is therefore the rate-limiting variable in long-term success — not the technique of the humeral side, not the choice of implant brand, not the speed of the surgeon. The glenoid.

This is where the robot earns its place. The robotic platform constrains the surgical instruments to the pre-operative plan, with millimetre-level accuracy. It is most valuable where the glenoid anatomy is hardest — significant wear, deformity, B2 or B3 patterns, small bone stock. In those shoulders, the difference between intended and achieved implant position is the difference that conventional instrumentation has struggled with for thirty years.⁴

What the robot actually does in the operating theatre.

The robot does not move on its own. It does not "perform" the operation. Specifically, four things happen in the theatre:

1. Registration. The surgeon registers the patient's anatomy to the pre-operative plan by mapping defined bony landmarks. The software aligns the plan with the patient in real time.
2. Haptic burring of the glenoid. The Mako shoulder platform does not use a central guide pin. Instead, the surgeon holds a high-speed burr at the end of the robotic arm, and the robot defines a three-dimensional haptic boundary around the planned volume of glenoid bone. Inside that volume the burr cuts freely; the instant it reaches the planned edge, the arm pushes back and the burr cannot advance. The amount and shape of bone removed is exactly what the plan called for — no more, no less — without the surgeon having to "aim" through a fixed pin trajectory.
3. Baseplate preparation and placement. Once the planned glenoid surface is burred, the baseplate fixation holes are prepared along the planned axes and the reverse baseplate is implanted into the corrected, prepared bone.
4. Verification. The implant trial is verified against the plan before the final component is fixed.

Everything else — the deltopectoral approach, the soft-tissue release, the humeral side preparation, the glenosphere and tray choice, the trial and tension, the wound closure — is conventional reverse shoulder surgery. The robot is the instrument that handles the single hardest step. The rest is hands, eyes, and judgement.

Why Mako is currently a reverse-only platform.

One point that gets lost in promotional material: the Mako shoulder application is approved and used for reverse total shoulder replacement only. It is not used for anatomic total shoulder replacement. The anatomic procedure continues to be performed with conventional, well-validated instrumentation.

This matters for two reasons. First, the patient who needs an anatomic replacement — a younger patient with arthritis and an intact rotator cuff — is not missing out by being offered a non-robotic operation. Second, the patient who needs a reverse replacement (cuff tear arthropathy, irreparable cuff disease, severe glenoid bone loss, complex proximal humerus fracture) is exactly the patient in whom baseplate position has the most influence on long-term outcome, which is where the platform earns its place.

Who is a candidate.

Most patients being considered for a reverse total shoulder replacement can be planned robotically. The technology is most valuable when one or more of these are true:

• The glenoid anatomy is abnormal — significant retroversion (B2 or B3 wear pattern), posterior bone loss, congenital dysplasia, or post-traumatic deformity.
• Bone stock is limited — small, worn or fragile glenoids where the margin for error is smallest.
• The reverse construct is at the edge of its tolerances — where baseplate version, inclination and lateralisation choices critically influence long-term survival.
• Revision surgery is planned — where prior surgery has altered the anatomy and the conventional landmarks are unreliable.

Robotic assistance is not a substitute for surgical experience — and not every reverse is a robotic case. The decision is individualised at the consultation, against your imaging and your specific glenoid anatomy.

The first on the Sunshine Coast.

In August 2025, Dr Coory performed the first Mako robotic-assisted total shoulder replacement on the Sunshine Coast — and the second in Australia — in partnership with Buderim Private Hospital.⁵ The platform is now in routine use for selected reverse arthroplasty cases at Buderim Private Hospital, Sunshine Coast University Private Hospital, and the new Maroochy Private Hospital in Maroochydore.

The conversation in clinic about whether to use it is not promotional. It is a planning conversation — CT findings, wear pattern, bone stock, what the realistic gain is for your specific shoulder.

What this means for your shoulder.

If you have been told you need a shoulder replacement, three questions are worth asking before the operation date is set:

1. What does my CT scan show about my glenoid wear pattern?
2. Is the plan being drawn up in 3D before the operation — or read from the X-ray on the day?
3. In my specific shoulder, does robotic-assisted glenoid preparation change the realistic outcome?

The answer to the third question is sometimes yes and sometimes no. A surgeon who can tell you which it is for your shoulder is doing the planning right.

References.

1. Boileau P, et al. Reverse shoulder arthroplasty: a review of recent literature. J Shoulder Elbow Surg. 2018;27(2):e122–e133.
2. Walch G, et al. Three-dimensional planning and use of patient-specific guides improve glenoid component position. J Shoulder Elbow Surg. 2015;24(2):302–309.
3. Australian Orthopaedic Association National Joint Replacement Registry. Shoulder Arthroplasty Annual Report. Adelaide: AOA; 2024. aoanjrr.sahmri.com
4. Verborgt O, et al. Computer-assisted and patient-specific guidance in shoulder arthroplasty: a systematic review of accuracy. J Shoulder Elbow Surg. 2020;29(7):1481–1492.
5. Buderim Private Hospital. Among first in the world to provide robotic shoulder surgery. News release, 8 August 2025. Read the source →