Tuesday, October 6, 2026

Robotics in reverse shoulder arthroplasty – 16 questions


Title: lead_operating_room.jpg - Description: lead_operating_room.jpg

In brief

Robotic assistance is being introduced into reverse total shoulder arthroplasty, carried by its use in the knee and hip. It provides a unique and much needed opportunity to investigate the relationship between reverse total shoulder geometry and patient reported outcomes. A number of thought leaders are pioneering these efforts. The role of robotics in the clinical practice of shoulder arthroplasty is being actively explored.

At the International Congress on Shoulder and Elbow Surgery in Vancouver this September we discussed some questions about it, and a paper published since then adds another: whether randomized trials could detect a patient benefit at all [1]. The answers so far:

•      In the knee, robots improve alignment. However, three meta-analyses and a masked randomized trial of 339 patients found no clinically meaningful improvement in what patients report.

•      In the shoulder, the robot has been shown to be more precise in plastic glenoid models. No study has yet shown that it improves the outcomes patients experience.

•      The common causes of reverse arthroplasty failure (instability, infection, aseptic loosening and fracture) may not be primarily  problems of component position. 

•      The robot brings its own complications. For one experienced surgeon, operating time did not reach conventional levels until about the 23rd robotic case; the median US surgeon performs only 2 reverse arthroplasties a year.

•      The reduction in revision needed for the robot to pay for itself may be larger than the first-year revision rate for reverse total shoulders performed without the robot.

A masked trial of a few hundred patients answered the question about the benefit the patient receives from the robot in total knee arthroplasty. A similar trial could answer the same question for the shoulder. 

The questions, and the evidence that bears on each, follow.

1. Do robots improve outcomes in total knee arthroplasty?

Title: fig01_knee_trackers.jpg - Description: fig01_knee_trackers.jpg

Figure 1. Robotic total knee arthroplasty. The tracker arrays are fixed to the femur and tibia with bone pins.

Knee robotics are often offered as the precedent for shoulder robotics, so the knee evidence is the place to start. Three recent meta-analyses agree. Twelve randomized trials with 2,200 patients showed fewer alignment outliers with robotic total knee arthroplasty but no clinically important gain in function [2]. Twenty-five randomized trials with 5,614 patients showed modest and inconsistent differences in the Knee Society Score and pain, no difference in the Oxford Knee Score, and only two trials at low risk of bias [3]. Twenty prospective studies with 2,421 patients showed about 5 degrees more motion and better WOMAC scores with the robot but no difference in the Oxford Knee Score, KOOS, Forgotten Joint Score or EQ-5D; the authors judged the differences not clinically meaningful [4]. NICE allowed robot-assisted hip and knee surgery in the NHS only on condition that further evidence be collected [5].

The RACER-Knee trial, published in The Lancet in August, randomized 339 patients across ten UK hospitals and 33 surgeons [6]. Patients and assessors were masked, with sham incisions in the conventional group, and both arms had CT planning, so the trial isolated what the robot adds in the operating room. The robot came closer to the planned alignment, by 0.8 degrees on average, and added 10.5 minutes of operating time. At one year the Forgotten Joint Score differed by 1.5 points in favor of conventional surgery (95% CI −7.5 to 4.5), and the confidence interval did not reach the 12-point difference the trial was designed to detect; for pain, the Oxford Knee Score and EQ-5D, the confidence intervals did not reach previously reported minimal important differences. The robot cost more and had at most a 4% probability of being cost-effective at the thresholds tested. One more finding is worth noting. Patients who believed they had received the robot reported much better scores than the few who believed they had not. The authors suggest that patients with good results may have credited the robot, but either way, belief and reported outcome travel together, which is why unmasked comparisons of new technology deserve caution [32].

Two other knee studies are instructive. In the Glasgow randomized trial of robotic versus manual unicompartmental knee arthroplasty (139 patients randomized, 104 seen at five years), patient-reported outcomes did not differ, but reintervention occurred in none of the robotic group and in six (9%) of the manual group [7]. This result is often cited for robotics, but the authors found no alignment or component-position errors in any of the six: two were revisions, one for loosening after a fall and one for pain, and four were arthroscopies for pain, a possible meniscal tear or a possible infection. The trial was unmasked. A difference in reoperations that cannot be traced to positioning does not show that precision prevents failure. The trial’s five-year cost-utility analysis found the robot cost-effective in units performing at least 300 robotic cases a year, but that result rested on one manual-group revision for presumed infection, which the authors themselves call arguably a random event; without it, the robot was not cost-effective, and it reached cost neutrality only above 900 cases a year with no consumable costs [8]. The ten-year analysis was similar: the robot was cost-saving above 100 cases a year with all reinterventions counted, but only above 800 once the infected cases were removed [9]. In the Australian registry, with more than 300,000 total knees over a decade, nine-year revision was 5.2% without computer navigation and 4.6% with it, a difference that was not statistically significant overall; a significant reduction appeared only in patients under 65 (7.8% versus 6.3%) [10]. A third of a million knees could not establish an overall difference of half a percentage point.

Outside orthopaedics, robotic inguinal hernia repair spread widely before the RIVAL pilot trial randomized 102 patients to robotic or laparoscopic repair. At 30 days there were no differences in wound events, readmissions, pain or quality of life; the robotic operations took longer (median 76 versus 41 minutes), cost more than twice as much ($3,258 versus $1,421), and frustrated the surgeons more [11].

Proponents answer that patient-reported outcome measures are too blunt to detect the benefits of precision, that mean scores hide outliers, and that the absence of evidence is not evidence of absence [12]. The first two points are fair, and they argue for measuring what happens to individual patients, such as the proportion with a poor result, rather than for adopting the technology. The third point is true, but it cannot justify adoption: the burden of showing benefit rests with the technology that adds cost.

2. How sensitive are reverse arthroplasty outcomes to precision in positioning?

Title: fig02_dose_response.jpg - Description: fig02_dose_response.jpg

Figure 2. Two possible relationships between component malposition and revision risk. Left: each degree adds risk. Right: risk rises only beyond a threshold.



If every degree of malposition adds risk, tight targets are justified and a more precise tool should help. If risk rises only beyond a threshold, mild deviation does not matter, and the only benefit of a robot is avoiding gross outliers, which a careful surgeon with good exposure may already avoid. We do not know which curve describes reverse arthroplasty. Until we do, improvements in precision cannot be translated into expected patient benefit.

3. How wide are the goal posts? What is an “outlier”?




Figure 3. How wide are the goal posts? The acceptable window for component position may be narrow or wide, and it is rarely derived from outcomes.

Accuracy studies count “outliers” against thresholds that differ between studies and are rarely derived from outcomes. The most-cited figure is from a series of failed arthroplasties: among 60 reverse arthroplasties with usable radiographs before revision, 56 (93%) had at least one glenoid measurement outside predefined limits, most often superior tilt, with the acceptable inclination window set at 10 degrees [13]. When an independent reviewer was simply asked whether the glenoid was malpositioned, the answer was yes in 54%. No shoulders that were doing well were measured. Without the same measurements in successful shoulders, we cannot tell whether malposition causes failure or is simply common. A test would be straightforward: apply identical criteria to failed and successful shoulders and see where the goal posts actually need to be.

4. In a laboratory model, robotics outperformed patient-specific guides. Do differences of a few degrees or millimeters matter clinically?

Mean deviation from plan, synthetic glenoid models [14]

Deviation from plan

Manual

Patient-specific guide

Robotic

Version

9°

4°

1°

Inclination

9°

3°

2°

AP translation

2 mm

2 mm

0.3 mm

SI translation

2 mm

1 mm

0.7 mm

Outliers (>10° or >5 mm)

11

0

0

36 preparations in synthetic glenoid models (two glenoid designs) by 6 surgeons.

Title: fig04_guide_and_robot.jpg - Description: fig04_guide_and_robot.jpg

Figure 4. A patient-specific guide on a plastic glenoid model (left) and a shoulder robot (right).

In plastic glenoids prepared by experienced surgeons, the robot was more accurate than freehand preparation by 7 to 8 degrees, and more accurate than patient-specific guides by 1 to 3 degrees and about a millimeter [14]. Freehand preparation produced 11 outliers; patient-specific guides and the robot produced none. If only gross outliers matter (the threshold curve in item 2), the cheaper guide already removed them in this model. This is a surrogate, demonstrated in the laboratory. Whether differences of this size change what patients experience is the question in item 2, and it remains unanswered.

5. Can the dominant failure modes be addressed by more precise transfer of a preoperative plan?

Title: fig05_failure_modes.png - Description: fig05_failure_modes.png

Figure 5. Causes of failure of primary reverse arthroplasty: 2,086 failures among 69,222 arthroplasties in 42 studies [15]. Each percentage is a separately pooled estimate, so they do not add to 100%. Glenoid loosening is part of aseptic loosening.

A meta-analysis of 42 studies, with 2,086 failures among 69,222 primary reverse arthroplasties, attributed failures to instability (21%), aseptic loosening (19%), infection (19%), periprosthetic fracture (11%) and pain (under 1%). Glenoid loosening accounted for 4.6% of all failures (95% CI 2.1% to 9.6%) [15]. Infection is not a positioning problem. Instability depends on soft tissues, tensioning and patient factors as well as component position.

In a CORR Clinical Faceoff, it was estimated that robotic assistance might reduce surgical failures by 25%. An alternate estimate was that it could address at most a fraction of the failures attributed to glenoid loosening, perhaps 2% to 3% of all failures, consistent with the 4.6% that glenoid loosening represents in the meta-analysis above, and only if the difference between freehand and robotic positioning matters to the patient [16]. Neither estimate is data. The difference between them is the question a study should answer.



Figure 6. Oxford Shoulder Score outcome of 12,030 reverse arthroplasties in the UK National Joint Registry. An unsatisfactory score was below 29 [17].

Revision also misses most failures. Of 12,030 reverse arthroplasties in the UK National Joint Registry with a postoperative Oxford Shoulder Score, 27% had an unsatisfactory score (below 29), and fewer than 5% of those were revised. Patients with equally poor function after an anatomic total shoulder or a hemiarthroplasty were revised two to three times as often (11% and 14%) [17]. The threshold for revising a reverse is higher, so its low revision rate partly reflects reluctance to revise rather than success. Revision rate is therefore a poor measure of whether a technology helps.

6. Much of the accuracy data concerns the position of the guide pin. Do we know the ideal position?




Figure 7. A guide pin placed in a plastic glenoid model. Much of the accuracy data concern the position of this pin.

A robot or navigation system transfers a plan faithfully. Whether the plan is the right one is a separate question. In a matched comparison of navigated with non-navigated reverse arthroplasty, the authors themselves noted that a baseplate can be placed exactly as planned while there is no gold standard for the plan [18]. Precise execution of an unvalidated plan is precision without a demonstrated target.

Planning software is now used to choose the target. In a simulation of 49 planned reverse arthroplasties, impingement-free motion depended mostly on implant selection: humeral neck-shaft angle, glenosphere size, eccentricity and lateralization; baseplate version had less consistent effects [19]. The model considered only bone and implant contact, not soft tissues, and was not linked to how the patients actually did. These are choices of what to put in, not how precisely to put it there, and whether the simulated optimum produces better shoulders is unknown.

7. Does robotic arthroplasty introduce complications of its own?

Adverse events reported with robotic arthroplasty, FDA MAUDE database

Reported event

Hip

Knee

Surgical delay

146

73

Unexpected robot-arm movement during bone cuts

 

59

Intraoperative hardware failure

304

 

Incorrect component placement

95

 

Hip: 521 robotic total hip reports, 2017–2021 [20]. Knee: 204 robotic total knee reports [21]. MAUDE counts reports, not procedures, so these numbers cannot give a rate.

The FDA MAUDE database records adverse events with robotic hip arthroplasty: among 521 reports from 2017 to 2021, there were 304 intraoperative hardware failures and 95 instances of incorrect acetabular cup placement; the robot was abandoned in 13% of reports, surgery was delayed in 28% (by about 18 minutes on average), and patient injury was described in 14% [20]. MAUDE counts reports, not procedures, so these numbers cannot give a rate, but they show the kinds of harm the technology itself introduces. A second MAUDE analysis of 263 reports across robotic hip, knee and partial knee arthroplasty found that the commonest knee event was unexpected robot-arm movement during bone cuts (59 of 204 knee reports). Surgery was delayed in 99 reports, by 20 minutes on average and up to two hours; 31 cases were converted to manual surgery; and 68 patient injuries were reported, including fractures, a medial collateral ligament laceration, retained registration checkpoints and an electrical burn, 7 of which required reoperation [21]. The authors judged the technology generally safe, with the serious events related more to technique than to the device. In randomized knee trials, robotic cases were converted to conventional technique in 10% versus 2% for conventional cases, and readmissions were more frequent [3]. Tracker pins bring their own risks. A systematic review of 28 studies covering 15,004 robotic or navigated knee arthroplasties found pin-related complications in 0.95%, mostly wound problems and infections; pin-site fractures occurred in 0.16%, usually through the femoral pin site about two months after surgery, and were treated with intramedullary nailing or plating; two arterial injuries were reported [22]. These events are uncommon, but any benefit has to exceed them, and a shoulder robot needs its own pins in the scapula.

8. Does robotics improve the skills of the surgeon?

Title: fig08_self_driving_car.jpg - Description: fig08_self_driving_car.jpg

Figure 8. Does a self-driving car make the driver better?

A robot helps execute a cut or a pin placement. It does not supply judgment about indications, exposure, soft-tissue balance, or what to do when the unexpected happens. Does a self-driving car make the driver better? A surgeon who learns reverse arthroplasty with the robot may become dependent on it, and the day the robot fails or the anatomy does not match the plan, the surgeon’s own skills are what remain.

9. Could robotics help the low-volume surgeon?

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Figure 9. A surgeon with a draped robot in the operating room.

This is the strongest argument made for the technology. In Medicare data from 2016 to 2018, 7,097 surgeons performed 90,318 reverse arthroplasties: a mean of 6 per surgeon per year and a median of 2. Ninety-six percent of surgeons performed fewer than 29 a year, and their patients had higher 90-day readmission for infection and for dislocation [23]. Learning curves in robotic knee arthroplasty apply to operating time rather than accuracy: 7 to 11 cases in a meta-analysis [24], but 11 to 43 cases in a series of six surgeons, longer for those with lower volumes [25]. In a multicenter study of 146 surgeons, operating time reached steady state after 15 to 20 cases or more, and only about two-thirds of surgeons ever matched their manual operating times [26]. For robotic reverse arthroplasty, a single fellowship-trained surgeon’s first 10 robotic cases took about 30 minutes longer than conventional cases at the same center, and operating time reached the conventional benchmark at about the 23rd case [27]. At 6 reverse arthroplasties a year, 20 to 25 cases would take three to four years; at the median of 2 a year, more than a decade. And the Glasgow analysis found robotics cost-effective only at high volume [8], the opposite of the setting where it is said to help most. 

10. What determines the cost of acquiring and using a robot?

The costs of robotic arthroplasty, including capital, service, disposables, CT scans and personnel, are often set by non-disclosure agreements between the vendor and the hospital. Pricing may depend on anticipated case volumes, which could disadvantage lower-volume centers. Surgeons and patients cannot weigh a cost they are not allowed to see.

11. If there is an adverse outcome, who bears the responsibility?

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Figure 10. If a robot-assisted operation goes wrong, who is responsible?

If a robot-guided component is malpositioned, or if there is a robot-related complication leading to a poor patient outcome, who is responsible: the surgeon, the hospital that bought the system, or the company that built it? Is the surgeon the method or the robot the method? These questions have not been tested, and surgeons should know the answers before consenting patients.

12. Can routine reverse arthroplasty be performed well without a CT scan or a robot?

Title: fig11_first_fellowship_case.jpg - Description: fig11_first_fellowship_case.jpg

Figure 11. First case on the first day of fellowship: preoperative radiographs (left), with templating (center), and postoperative radiographs (right), performed without CT or robot.

Most reverse arthroplasties are done for cuff tear arthropathy or osteoarthritis with modest glenoid deformity. Careful radiographic templating, good exposure of the glenoid and attention to its anatomy produce reliable component placement in these routine cases, as this first fellowship case illustrates.

13. Can young surgeons become good at reverse arthroplasty without a robot?

Title: fig12_young_surgeons.jpg - Description: fig12_young_surgeons.jpg

Figure 12. Reverse arthroplasties performed by two young surgeons without robotic assistance.

Young surgeons with good training can achieve consistent component placement with standard instruments. The skills they build doing so are the ones they will need when the anatomy is unusual, the exposure is difficult, or the technology is not available.

14. Is robotics the best investment for improving the lives of the millions of people with symptomatic shoulder arthritis?

A break-even analysis frames the question simply. The reduction in revision a robot must achieve to pay for itself equals its added cost per case divided by the cost of a revision. Using an added robotic cost of $1,500 per case, a revision cost of $22,920, and a baseline early revision rate of 2.1%, the robot would need to prevent one revision for every 15 procedures, an absolute reduction of 6.55% [28]. The reported revision rate in the first year after reverse arthroplasty in US hospital billing data is 2.1%, about one revision in every 48 cases, with a mean hospital cost of $22,920 per revision [29]; in the UK National Joint Registry, one-year revision stayed below 2% from 2013 through 2021 [30]. The robot would have to prevent about three times as many revisions as occur using a non-robotic approach. Even a robot that prevented every early revision would repay only about $481 per case (2.1% of $22,920), less than a third of its added cost. The revision cost is a hospital cost that excludes professional fees; but even at the roughly $60,000 hospital cost of a revision for infection, the break-even reduction (2.5%) would still exceed the entire first-year revision rate. The analysis considered only first-year revision; the authors note that any value of more accurate positioning may appear later, in time to revision and in patient-reported outcomes, and that selective use in higher-risk anatomy or in high-volume centers already owning a robot may change the arithmetic [28].

Is spending substantial money on robotics the best way to improve the outcomes for the many patients with symptomatic shoulder arthritis?

15. Is there evidence that robotic assistance will improve the outcomes our patients now experience?




Figure 13. Percentage of 232 patients able to perform each Simple Shoulder Test function before conventional reverse arthroplasty (grey) and at two years (blue). Matsen, unpublished data.

This is the standard robotic arthroplasty must improve upon, and it is a moving one: in the UK registry, the mean improvement in the Oxford Shoulder Score six months after reverse arthroplasty rose from 15.8 points in 2013 to 20.3 in 2021, without robots [30]. The answer here is that without robotics most patients regain comfort and the ability to sleep, reach a shelf and lift a light object; fewer regain overhead lifting and throwing. A robot would need to raise these percentages, and the improvement would need to be large enough for patients to recognize. Published minimal clinically important differences for the ASES score after reverse arthroplasty range from 6.2 to 21 points, the widest range of any shoulder score, with 10.3 the value most often cited [31]. The Hurley analysis modeled robotic benefits of 1 to 5 points [1], below even the lowest of them.

16. Can randomized trials realistically detect a patient benefit from robotic reverse arthroplasty?

A trial designed to detect a small reduction in revision would need to be very large. With a 4% baseline revision rate, detecting a 1% absolute reduction would take 5,301 patients per arm, and a 0.5% reduction 22,663 [1]. The authors conclude that the field should therefore turn to intermediate endpoints, registries and subgroups. That reverses the usual burden of proof: a technology that adds cost needs to show that it helps, and the absence of a feasible trial is not evidence of benefit. An effect that takes tens of thousands of patients to detect is also an effect that few patients will ever experience.

The only comparison of patient outcomes after navigated and non-navigated reverse arthroplasty matched 113 shoulders of each [18]. The navigated patients started with better motion and scores, and their improvement after surgery did not differ on any measure. Revision was 0.9% versus 3.5% (P = .37). The navigated shoulders also received augmented baseplates far more often (96% vs 50%), and no postoperative imaging confirmed that navigation changed component position. This is what an underpowered comparison looks like: it demonstrates neither a benefit nor its absence.

RACER-Knee shows that the question can be answered. With 339 patients, it found no clinically meaningful benefit of robotic knee arthroplasty at one year, with confidence intervals narrow enough to be informative. A shoulder trial of comparable size could do the same, using an outcome patients recognize: the proportion with poor comfort and function. If one patient in five has that outcome after conventional reverse arthroplasty, about 220 patients per arm would give 80% power to detect a halving of that proportion, and would rule out a benefit of that size if none exists.

The question ahead

A trial of fewer than 500 patients may clarify whether the robot delivers a benefit patients would notice. Isn’t it time for this study?

 

Golden-cheeked Warbler

San Antonio

 

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The author of this post has no financial relationship with any orthopaedic company

References

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28. Menendez ME, Moverman MA, Schiffman CJ, Matsen FA 3rd. Is robotic-assisted reverse shoulder arthroplasty economically justified? A break-even analysis. J Shoulder Elbow Surg 2026;35(9):2313-2317. doi:10.1016/j.jse.2026.03.010

29. Corso KA, Smith CE, Vanderkarr MF, Debnath R, Goldstein LJ, Varughese B, Wood J, Chalmers PN, Putnam M. Postoperative revision, complication and economic outcomes of patients with reverse or anatomic total shoulder arthroplasty at one year: a retrospective, United States hospital billing database analysis. J Shoulder Elbow Surg 2025;34:e59-e71. doi:10.1016/j.jse.2024.05.009

30. O’Malley O, Davies A, Taghavi Azar Sharabiani M, Rangan A, Sabharwal S, Reilly P. Are we getting better over time? Clinical and patient-reported outcomes for reverse shoulder arthroplasty: a National Joint Registry cohort study. BMJ Open 2025;15:e096084. doi:10.1136/bmjopen-2024-096084

31. Yendluri A, Alexanian A, Lee AC, Megafu MN, Levine WN, Parsons BO, et al. The variability of MCID, SCB, PASS, and MOI thresholds for PROMs in the reverse total shoulder arthroplasty literature: a systematic review. J Shoulder Elbow Surg 2024;33:2320-2332. doi:10.1016/j.jse.2024.03.051

32. Menendez ME, Moverman MA, Namdari S, Matsen FA 3rd, Ring D. Modeling surgeon belief updating under bias: a Bayesian simulation in shoulder arthroplasty. JSES Int 2026;10:101399. doi:10.1016/j.jseint.2025.09.018

Tuesday, September 29, 2026

The Challenge of Intraoperative Posterior Instability in Anatomic Shoulder Arthroplasty The Magic of the Anteriorly Eccentric Humeral Head


While posterior decentering of the humeral head on the glenoid may be noted on a preoperative axillary “truth view” taken with the arm in a position of functional elevation


 

it is not always evident before shoulder arthroplasty. 

At surgery, after osteophyte resection and anterior releases, posterior instability may first appear during trialing. In the series of Hsu et al, 11 of the 33 shoulders that needed an anteriorly eccentric head had less than 5% posterior decentering before surgery, and decentering did not correlate with glenoid version. Leaving decentering unaddressed can lead to inferior outcomes [3].

Correcting glenoid version does not always settle the matter. After anterior asymmetric reaming of 62 B2, B3, or C glenoids, posterior subluxation had returned in 25 to 41 of them at a mean of 8.3 years, depending on how it was measured; glenoid loosening was more frequent when it persisted, although clinical scores were not worse [14]. Gerber et al, by contrast, found static subluxation reversed in 21 of 23 shoulders at a mean of 42 months [13]. Chamberlain et al reamed B2 glenoids to within about 10° to 15° of normal version and still found excessive posterior subluxation with trials in place in 32 of 114 shoulders [4]. The practical questions are how to recognize excess posterior laxity with trials in place, and what can be done about it on the humeral side.

 

Consider a very active 40-year-old man who did not want plastic in his shoulder and chose a ream and run.

Osteoarthritis


Posterior decentering on axillary “truth” view, minimal retroversion

 

Preoperative plan suggested a 50-20 head

 

Under anesthesia, the shoulder was stiff

 

 

The glenoid was biconcave

 

And was conservatively reamed to a monoconcavity

 

Trialing with the planned 50-20 head revealed excessive soft tissue tightness. Trialing with a 50-18 concentric head revealed excessive posterior translation. Trialing with an anteriorly eccentric head revealed excellent balance of mobility and stability.




Motion at the end of the case.

Assisted flexion started on day 1. He returned to his favorite activities.

 

Our approach, step by step

1. The intraoperative check

When to test

Once the osteophytes are removed, the soft tissues released, and the glenoid prepared, the shoulder is a different joint than it was before surgery. With the trial humeral component in place, the surgeon can evaluate its mobility and stability [4].

What to test

The 40/50/60 parameters are a useful guide: 40° of external rotation with the subscapularis approximated to its insertion on the lesser tuberosity, 50% posterior translation of the head on the glenoid under posteriorly directed pressure, and 60° of internal rotation with the arm abducted 90° [1]. More translation suggests too much laxity; failure to reach 40° of external rotation, 60° of internal rotation, or 150° of flexion suggests too much tightness.

 

The second test is posterior drop-back, or “shake and bake”: with trials in place, elevate the arm and give it a gentle shake. If the head drops back out of the glenoid, stability may need to be added [1, 2]. Clinically, posterior decentering is usually a problem when the arm is elevated; a shoulder that looks centered with the arm at the side may decenter during functional elevation [3, 11].

Thresholds used in published series

Published series have used different arm positions and thresholds for excessive posterior translation.

Source

Arm position and maneuver

Threshold for action

Matsen & Lippitt [1] (40/50/60 rule)

External rotation with subscapularis approximated; posterior translation of the head on the glenoid; internal rotation with the arm abducted 90°

Reconstruction should allow 40° ER, 50% posterior translation, and 60° IR

Matsen & Lippitt [1]; Clinton et al [2]

Forward elevation of the arm with trials in place

Head drops back out of the glenoid on elevation

Hsu et al [3]

Passive elevation of the arm

More than 50% posterior subluxation of the humeral head

Chamberlain et al [4]

Arm resting in adduction and neutral rotation; posterior translation

Head translates more than 100% posteriorly and/or rests more than 50% subluxated on the glenoid

Szolomayer et al [15] (posterior capsular plication)

Anterior–posterior translation in neutral rotation with trials in place, after sizing components

Translation greater than 50% with trial components sized preoperatively

 

2. Balancing mobility and stability on the humeral side

The steps run from least to most consequential. Repeat the Section 1 checks after each.

2.1 Confirm head size and height

First confirm that the initial trial head matches the resected head in diameter and thickness and that the neck cut is appropriate. In cadavers, head size and articular conformity each affected laxity and motion [7].

In a shoulder that was very stiff before surgery, the surgeon may deliberately choose a thinner head for motion rather than to reproduce the anatomy [17]. If this “understuffing” allows excessive posterior translation, the steps below are usually sufficient to restore stability.

2.2 If the trial concentric head translates excessively


consider replacing it with a head that is anteriorly eccentric.


The articular surface then sits centered in the glenoid while the humeral shaft and tuberosities remain in their posterior position [1, 4, 5].

 

The ream and run. As shown in the case presented above, the eccentric head is often needed in the young, active man who chooses a ream and run. Preoperative radiographs not infrequently show posterior decentering,


 

and without a polyethylene glenoid component, nothing on the glenoid side adds stability. Here the anteriorly eccentric head is particularly useful [3].

After a ream and run with an anteriorly eccentric head, the prosthetic head is centered on the glenoid.


A few more examples; there are many more.

 

Laboratory evidence. In 14 cadaveric total shoulders with 10° or 20° of glenoid retroversion created by eccentric reaming, Kim et al tested each head in the anatomic and the anterior offset position. At 20°, anterior offset increased the force needed to translate the head 10 mm posteriorly by 64% and the energy by 75%, and shifted contact anteriorly. At 10°, force increased by 26% and the contact changes were not significant [5].

A finite element model found a modest anterior shift in the center of pressure (about 1.4 mm) and a more anteromedial muscle force vector [6].

Clinical evidence. Hsu et al used a 4-mm anteriorly eccentric head when posterior subluxation exceeded 50% on passive elevation in 33 shoulders: 24 ream and runs and 9 total shoulders, 16 with a rotator interval plication [3]. Decentering on functional axillary views fell from 10.4% to 0.9%, and the Simple Shoulder Test improved from 4.8 to 10.0 at a minimum of two years. None was revised for instability; two were revised for pain and stiffness. Without any deliberate attempt to change it, retroversion fell from 19.8° to 15.5° (P = .001) [3].

Chamberlain et al used the technique in 20 total shoulders with B2 glenoids (mean retroversion 31° in the 15 with CT), with partial version correction by anterior reaming, a pegged all-polyethylene glenoid, and no plication [4]. At about four years, the SST improved from 4.9 to 9.8 and the ASES score from 32.7 to 86.2; 18 of 20 reached the MCID for the SST and 19 of 20 for the ASES [4, 12]. Decentering fell from 9.9% to 0.5%, with none more than 5% posterior. No glenoid was loose at 24 months, and there were no revisions or instability. 

The goal is a centered head, not an anteriorly displaced one: centering was maintained within 5% in all but one of the shoulders of Hsu et al [3] and in all of those of Chamberlain et al [4].

 

Technical points. Keep the head height correct relative to the greater tuberosity while inserting the offset component [4]. The shift is limited by the offset built into the head, 2 to 4 mm, and is often less than the maximum [3, 5]. The head overhangs anteriorly and may leave posterior cut surface uncovered; Chamberlain et al left that bone when it did not engage the glenoid in 40° of external rotation [4]. The subscapularis must drape over the overhang, which lifted it in cadavers [5]. Every lesser tuberosity osteotomy in the series of Chamberlain et al healed [4], and Hsu et al found no clinical subscapularis failure after peeling it [3].

 

2.3 Add a rotator interval plication when the eccentric head alone is not enough

If posterior translation is still excessive with the definitive humeral component in place, plicate the rotator interval [1]. In cadavers, imbricating the interval reduced posterior and inferior translation, and external rotation [8]; Hsu et al added it in about half their shoulders [3]. Recheck that 40° of external rotation remains [1].

2.4 Posterior capsular plication

Posterior capsular plication tightens the redundant posterior capsule to hold the head forward, rather than accommodating the posterior humerus. Kim et al describe its efficacy as questionable [5], and at revision for posterior instability it has often failed [16].

Szolomayer et al plicated 19 of 138 total shoulders with one or two purse-string stitches when translation in neutral rotation exceeded 50% with anatomically sized components. Seven of the 14 with a preoperative axillary view showed posterior subluxation [15].

At a mean of 8.9 months, forward elevation improved from 91° to 131° and external rotation from 15° to 51°. One patient needed an arthroscopic release for adhesive capsulitis, and none of the 12 contacted at a mean of 45 months reported dislocation, loss of motion, or further surgery [15].

2.5 A thicker head is a last resort

A thicker head reduces laxity by overstuffing, and in cadavers larger heads reduced motion as well [7]. It is an option for drop-back [1], but it trades instability for stiffness and higher joint loads.

2.6 Changing humeral version is not supported

With the glenoid component in 15° of retroversion, anteverting the humeral component by 15° did not increase resistance to posterior translation [9], and in another cadaver study humeral rotation did not compensate for glenoid version [10]. Re-cutting or re-cementing the humerus adds morbidity for a benefit that has not been demonstrated.

2.7 When humeral-side steps fail

If the head still drops back after sizing, anterior offset, and plication, look to the glenoid for a residual posterior facet or incomplete concave reaming in a ream and run, and reconsider whether the posterior cuff is competent. If instability remains, a glenoid component may need to be considered.

2.8 Confirming the result

Document centering on a standardized axillary view with the arm elevated in the plane of the scapula, where decentering shows itself [3, 11]; a view with the arm at the side may look centered. Chamberlain et al did not state their arm position, so their values may not compare directly with those of Hsu et al [3, 4].

 

Demonstrated and inferred

These reports are uncontrolled case series with short-to-intermediate follow-up [4]. No study has compared the eccentric head with version correction, augmented glenoids, or reverse arthroplasty in shoulders with the same intraoperative decentering.

 

Demonstrated: in cadavers with 20° of glenoid retroversion, anterior offset of the head increases resistance to posterior translation, with smaller effects at 10° [5], while anteverting the humeral component does not [9]; two case series report recentering on functional axillary views with SST gains well above the MCID [3, 4, 12]; posterior subluxation often returns after asymmetric reaming alone [14]; and posterior capsular plication at revision fails often [16].

Inferred: that the intraoperative thresholds in the table identify the shoulders that need treatment; that the eccentric head, rather than the accompanying glenoid preparation, plication, or the rest of the operation, produces the recentering; and that recentering itself produces the improvement in function. 

Conclusions from our practice

1.     Trial intraoperatively: mobility and stability cannot be reliably predicted before surgery.

2.     Posterior instability is a particular risk in the ream and run, which has no glenoid component to add stability; much of our experience with the anteriorly eccentric head comes from this procedure.

3.     If a concentric trial head translates excessively, consider an anteriorly eccentric head.

4.     If posterior instability persists, consider a rotator interval plication.

5.     With this approach, residual posterior instability, glenoid loosening, and subscapularis failure have been rare, and patient-reported outcomes have consistently exceeded the MCID.

6.     Because our patients have done well with this method, we have not conducted randomized comparisons with other approaches.

 

It’s about balance

Black necked Stilt

 

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References

1.      Matsen FA 3rd, Lippitt SB. Current technique for the ream-and-run arthroplasty for glenohumeral osteoarthritis. JBJS Essent Surg Tech. 2012;2(4):e20. doi:10.2106/JBJS.ST.L.00009

2.      Clinton J, Warme WJ, Lynch JR, Lippitt SB, Matsen FA 3rd. Shoulder hemiarthroplasty with nonprosthetic glenoid arthroplasty: the ream and run. Tech Shoulder Elbow Surg. 2009;10(1):43-52.

3.      Hsu JE, Gee AO, Lucas RM, Somerson JS, Warme WJ, Matsen FA 3rd. Management of intraoperative posterior decentering in shoulder arthroplasty using anteriorly eccentric humeral head components. J Shoulder Elbow Surg. 2016;25(12):1980-1988. doi:10.1016/j.jse.2016.02.027

4.      Chamberlain AM, Orvets N, Patterson B, Chalmers P, Gosselin M, Salazar D, Keener JD. Total shoulder arthroplasty with an anterior-offset humeral head in patients with a B2 glenoid. JSES Int. 2020;4(3):638-643. doi:10.1016/j.jseint.2020.02.001

5.      Kim HM, Chacon AC, Andrews SH, Roush EP, Cho E, Conaway WK, et al. Biomechanical benefits of anterior offsetting of humeral head component in posteriorly unstable total shoulder arthroplasty: a cadaveric study. J Orthop Res. 2016;34(4):666-674. doi:10.1002/jor.23048

6.      Lewis GS, Conaway WK, Wee H, Kim HM. Effects of anterior offsetting of humeral head component in posteriorly unstable total shoulder arthroplasty: finite element modeling of cadaver specimens. J Biomech. 2017;53:78-83. doi:10.1016/j.jbiomech.2017.01.010

7.      Harryman DT 2nd, Sidles JA, Harris SL, Lippitt SB, Matsen FA 3rd. The effect of articular conformity and the size of the humeral head component on laxity and motion after glenohumeral arthroplasty: a study in cadavera. J Bone Joint Surg Am. 1995;77(4):555-563.

8.      Harryman DT 2nd, Sidles JA, Harris SL, Matsen FA 3rd. The role of the rotator interval capsule in passive motion and stability of the shoulder. J Bone Joint Surg Am. 1992;74(1):53-66.

9.      Spencer EE Jr, Valdevit A, Kambic H, Brems JJ, Iannotti JP. The effect of humeral component anteversion on shoulder stability with glenoid component retroversion. J Bone Joint Surg Am. 2005;87(4):808-814. doi:10.2106/JBJS.C.00770

10.    Nyffeler RW, Sheikh R, Atkinson TS, Jacob HAC, Favre P, Gerber C. Effects of glenoid component version on humeral head displacement and joint reaction forces: an experimental study. J Shoulder Elbow Surg. 2006;15(5):625-629. doi:10.1016/j.jse.2005.09.016

11.    Matsen FA 3rd, Warme WJ, Jackins SE. Can the ream and run procedure improve glenohumeral relationships and function for shoulders with the arthritic triad? Clin Orthop Relat Res. 2015;473(6):2088-2096. doi:10.1007/s11999-014-4095-7

12.    Tashjian RZ, Hung M, Keener JD, Bowen RC, McAllister J, Chen W, et al. Determining the minimal clinically important difference for the American Shoulder and Elbow Surgeons score, Simple Shoulder Test, and visual analog scale (VAS) measuring pain after shoulder arthroplasty. J Shoulder Elbow Surg. 2017;26(1):144-148. doi:10.1016/j.jse.2016.06.007

13.    Gerber C, Costouros JG, Sukthankar A, Fucentese SF. Static posterior humeral head subluxation and total shoulder arthroplasty. J Shoulder Elbow Surg. 2009;18:505-510. doi:10.1016/j.jse.2009.03.003

14.    Gauci MO, Ceccarelli R, Lavoue V, Chelli M, van der Meijden OAJ, Gonzalez JF, Boileau P. Total shoulder arthroplasty for primary glenohumeral osteoarthritis: does posterior humeral subluxation persist after correction of the glenoid version at 5 years minimum? J Shoulder Elbow Surg. 2024;33(7):e347-e355. doi:10.1016/j.jse.2023.11.010

15.    Szolomayer LK, Kuether J, Kassam HF, Mata Fink A, Regnell E, Kovacevic D, Blaine TA. Outcomes of total shoulder arthroplasty with posterior capsular plication. J Shoulder Elbow Arthroplast. 2019;3:1-5. doi:10.1177/2471549218822389

16.    Alentorn-Geli E, Wanderman NR, Assenmacher AT, Sperling JW, Cofield RH, Sánchez-Sotelo J. Revision anatomic shoulder arthroplasty with posterior capsular plication for correction of posterior instability. J Orthop Surg (Hong Kong). 2018;26(2). doi:10.1177/2309499018789527

17.    Matsen FA 3rd. Avoiding overstuffing: the kinematic total shoulder arthroplasty. Int Orthop. 2025;49(12):2845-2849. doi:10.1007/s00264-025-06688-w