Sunday, February 22, 2026

How much precision do we need to pay for in shoulder arthroplasty?

In a recent series of posts, we've discussed the relationship of patient outcomes from shoulder arthroplasty to "component malposition" and to the use of costly technologies directed at precision in executing a preoperative plan (e.g. robotics, navigation, patient specific instrumentation). 

Does component malposition lead to revision of shoulder arthroplasty? points out that in order to understand the relationship of different degrees of malposition to outcome it is necessary to assess not only malpositions in failed arthroplasties but also malpositions in successful arthroplasties.

Humeral and glenoid component malposition in revised shoulder arthroplasty - Part I - what might have been done differently? recognizes that major component malpositions can cause failure, but questions whether precision technologies are necessary to avoid clinically significant malpositions.

Intraoperative navigation for shoulder arthroplasty - where are we going? suggests (1) The question each surgeon must ask is whether intraoperative navigation addresses problems experienced by patients in their own practice and (2)The question the field of shoulder surgery must ask is which specific patient problems (instability? fracture? stiffness? pain?) occur due to positioning errors that: (a) surgeons using standard techniques cannot adequately control, and (b) navigation systems would prevent. 

What should be the role of robotics in shoulder arthroplasty? asks "to what degree will the application of robotics to execute a preoperative plan address the primary causes of shoulder arthroplasty failure - infection, instability, acromial fracture, limited motion, and component loosening?"

 
Recognizing that the use of robotics, navigation and patient specific instrumentation all are based on a preoperative plan created from a 3D CT scan, CT-free planning for reverse total shoulder arthroplasty - why and how to do it shows a non-CT method for preoperative planning that is applicable to a large percentage of cases of reverse total shoulder arthroplasty cases.

Finally The Bad B2 Pandemic and How I perform a kinematic anatomic shoulder arthroplasty : what is the appropriate amount of stuffing? remind us that while image-based preoperative planning provides a useful preview to the surgery, the final choice of implants and positioning needs to be based on intraoperative assessment of soft tissue balance - something that preoperative images cannot reveal.

So, if precision means carrying out a preoperatively-generated plan, how much precision do we need to get the clinical outcome we want for our patient? 

For fun, let's consider a comparison of two watches:
(1) the world's most accurate watch


and (2) one of the world's cheapest watches


Here are some points of comparison





The F-91W $19.61 is less precise than the $7,400 Citizen Caliber 0100: ± 15 sec/month vs ± 1 sec/year (just as the conventional approach to the placement of shoulder arthroplasty components is less precise than technology-based placement). The question is whether the difference is of importance to the wearer (or the patient); does the 377 times increase in price produce a tangible benefit?

The Casio F-91W costs $19.61 on Amazon, where it is currently the #1 Best Seller in Men’s Wrist Watches — more than 3,000 sold in the past month, 57,798 customer reviews, 4.6 stars. It has a 7-year battery. It has been in continuous production since 1989. It has no Bluetooth, no Wi-Fi, no radio receiver, no GPS, and no wireless capability of any kind. There are four buttons on the case. That is the entirety of its interface with the outside world.
Its quartz oscillator drifts approximately fifteen seconds per month. Yet it gets its owner to every meeting, every appointment, every surgical case on time. The reason is straightforward: its imprecision is correctable: the wearer presses two of those four buttons while looking at an external time source. Similarly in the operating room we frequently adjust our preoperative shoulder arthroplasty component selection and positioning based on our intraoperative observations, rather than being fixed to our preoperative plan.
 
The Citizen Caliber 0100 costs seven thousand four hundred dollars. Its quartz oscillator vibrates at 8,388,608 Hz, temperature-compensated every sixty seconds. Its certified annual accuracy is ±1 second. It is, by the consensus of the watchmaking world, the most precise autonomous wristwatch ever produced. But it cannot be corrected because its extraordinary precision is entirely self-referential. It measures its own performance against its own internal standard, derived from its own crystal, monitored by its own circuitry. When that internal standard diverges from external reality — due to a subtle manufacturing variation, an unforeseen temperature excursion, a factor its algorithm did not anticipate — the watch has no mechanism to detect the divergence and no capacity to correct for it. There are no buttons to press. There is no external reference to consult. It proceeds with precision, faithfully, in whatever direction it was last pointed.

Technologies such as robotics, navigation and patient specific instrumentation face the same constraint. They strive to execute the preoperative plan with absolute fidelity. The plan was derived from a CT scan acquired days before surgery, segmented by an algorithm, calibrated to a population average, and fixed at the moment the case was planned. In the operating room, the technology cannot read the bone quality beneath the reamer. It cannot sense the soft tissue tension that requires adjustment. It cannot recognize that the patient’s anatomy, once exposed, differs in a clinically relevant way from the CT model it was given. It proceeds with precision, whether or not that precision is directed at the target that will yield the optimal outcome for the patient.

The F-91W, reset by a human being attending to ground truth, is the more honest model of what excellent surgery actually looks like: an imperfect instrument, actively corrected, continuously coupled to the reality it is meant to serve. There needs to be a human in the loop!


How much precision is needed?

Anna's Hummingbird
Matsen Backyard
 2022



Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link)
The total shoulder arthroplasty (see this link)
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link)
Shoulder rehabilitation exercises (see this link). 


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