Patient factors (overall health, diagnosis, comorbidities, sex, bone quality, cuff status, and steroid use) have a major effect on the results of reverse total shoulder arthroplasty (RSA). Most patients do well, but there are risks of persistent pain, poor function, dissatisfaction, infection, dislocation, baseplate loosening, and acromial and scapular spine stress fractures. Take the last of these as an example. In the ASES multicenter series, acromial and scapular spine stress fractures complicated 2.1% (about 1 in 48) of reverse arthroplasties done for osteoarthritis with an intact cuff, and 5.2% (about 1 in 19) of those done for the common indications of cuff tear arthropathy or massive cuff tear [33]. Risk factors include inflammatory arthritis, female sex, osteoporosis, steroid medication, and prior cuff surgery [32]. These factors are largely beyond the surgeon’s control. The surgeon does control what happens in the operating room: antibiotic prophylaxis, open wound time, soft tissue management, and the design, size, and placement of the implants.
In this post we focus on how the reconstructed RSA geometry relates to the outcome the patient experiences. We draw on clinical studies that relate geometry to patient-reported outcomes and complications, not on laboratory or simulation studies. Understanding this relationship helps guide the surgeon toward a target geometry, whether or not the surgeon uses patient-specific instrumentation, virtual or augmented reality, or robotics. We plan preoperatively from biplane radiographs or 3D CT reconstructions. The plan is a starting point, and the surgeon often modifies it based on experience and on what is found at surgery: soft tissue tension, bone quality, contact between the humerus and the acromion or glenoid, and contact between the polyethylene liner and the glenoid bone. The surgical product is the RSA geometry actually achieved in the patient’s shoulder.
Each surgeon has a unique method: their approach to selecting patients for RSA, optimizing the patient’s health, evaluating the shoulder, selecting and positioning the arthroplasty components, and assessing their patients’ comfort, function, and complications. Each surgeon will have patients who experience adverse outcomes. Each adverse outcome offers the surgeon the opportunity to iteratively improve their method by asking, “What might I have done differently to give this patient a better result?” In the AI parlance of the day, we can call this recursive self improvement.
What the literature has to say about RSA geometry
Data from the publications are confusing and contradictory, in part because the measurement methods differ from study to study and often do not measure what they are named for.
Here’s the high level summary of what we think we know regarding the target for RSA geometry
Neutral-to-inferior baseplate inclination. Inferior glenosphere position, with overhang to prevent notching. Less glenoid lateralization in the fragile patient, recognizing that lateralization does reduce notching. Humeral-side offset restored to the patient’s own anatomy and not beyond. Lengthening limited to what the deltoid requires. Version anywhere from neutral to 30°. A lower neck–shaft angle where notching is the concern, recognizing that the angle in the patient depends on the liner and the stem alignment. The subscapularis repaired when the tissue allows. And a glenosphere center of rotation farther from the acromion than from the greater tuberosity. Three of these are inferences from a thin and conflicting literature rather than demonstrated thresholds: neutral-to-inferior inclination rests on a single study whose text and table disagree; restraint in glenoid lateralization on two studies, against three that found no harm; and the limit on lengthening on studies that measured distalization in different ways and disagree in direction. The center-of-rotation relationship rests on two small series, one of them our own. None of these geometries has been shown to change what patients report by a clinically important margin.
Now into the weeds.
Glenoid inclination
Inclination is shown as the β angle, drawn between the scapular spine and the baseplate; larger values indicate more superior tilt.
Complications. In 97 shoulders from one surgeon, 13% reported instability of some kind, including 4% with a dislocation requiring reduction [1]. A greater change toward superior inclination from before to after surgery was associated with instability (odds ratio 1.08 per degree). The paper does not agree with itself on the postoperative β angle: its text reports higher odds of instability with a larger angle (odds ratio 1.15), while its regression table gives 0.92 and the unstable shoulders had the smaller mean angle, 81° against 88°. In that series no subscapularis was repaired, every glenosphere was 36 mm, and lateralized glenospheres were excluded.
Function. Function was not examined in relation to β angle. Patients reporting any instability had lower final ASES scores than those reporting none [1].
Inferior glenosphere overhang
Overhang means the millimeters of glenosphere extending past the inferior glenoid rim.
Complications. Two analyses of the same registry, using the same implant system, show overhang moving two outcomes in opposite directions. In one, covering 470 shoulders with 26 fractures (5.5%), each additional millimeter of overhang was associated with a 19% increase in the odds of acromial or scapular spine fracture [2]. In the other, greater overhang was protective against scapular notching (P < .001) among 517 shoulders followed at least 2 years, in which notching occurred in 10.8% [3].
Function. Function was not examined in relation to overhang. Notching was associated with an ASES score lower by 7 points at 2 years and with forward flexion lower by 13° [3].
Glenoid-sided lateralization
None of the studies below measured glenoid lateralization directly. Some used the manufacturers’ nominal values for the components, some did not specify the measurement method, and some used the lateralization shoulder angle. This shortcoming greatly confounds the interpretation of the literature.
Acromial and scapular spine stress fracture. Several studies consider glenoid lateralization, and they do not agree.
Eisenberg et al. did not measure glenoid lateralization; they used the manufacturer’s specifications for the components. They reported that after a single surgeon began using more medialized components in higher-risk patients, fracture rates fell from 9.2% to 2.3% overall, from 10.7% to 3.1% in patients with two or more risk factors, and from 14.0% to 3.3% in those with three or more [4]. Components that lateralized more than 5 mm carried an odds ratio of 14.5, with a 95% confidence interval of 1.75 to 119.6. The analysis has shortcomings: it rests on only 13 fractures, the design compares surgeries performed during one period with those performed in another, the later group had less time in which a fracture could appear, and the counts in the tables do not add to the reported totals.
Moverman et al. did not measure glenoid lateralization. They defined glenoid lateral offset as the sum of the offsets contributed by the glenosphere, baseplate and bone graft, but did not state how these were measured. Each additional millimeter of this sum was associated with a 6% increase in the odds of acromial (but not scapular spine) fracture [5]. In a separate propensity-matched radiographic subset of 181 fractures and 358 controls they measured the lateralization shoulder angle, which reflects global, not only glenoid, lateralization. A greater increase in that angle and a greater postoperative angle were associated with fracture; the effect sizes appear under global lateralization below. The authors state that the implant associations are much weaker than the patient ones.
Pak et al. did not measure glenoid lateralization; they measured global lateralization, but did not resolve the glenosphere contribution to it [2]. Instead they used the manufacturer’s implant specifications to characterize “glenoid side lateralization”. They found no association between metallic glenoid lateralization of up to 8 mm and fracture when a 135° inlay humeral component was used [2]. Global lateralization and baseplate inclination were likewise not associated
King et al. compared 102 shoulders with a glenosphere lateralized by 4 mm with 102 matched shoulders with the standard glenosphere of the same medialized-glenoid, lateralized-humerus system [6]. Lateralization was not measured. Acromial or scapular fractures occurred in 1% with the lateralized glenosphere and 3% with the standard (P = .31). A meta-analysis of 16 comparisons of lateralized with nonlateralized glenospheres, bony or metallic, found fewer complications overall with lateralization (odds ratio 0.38) and similar rates of acromial and scapular spine fracture [7]. The analysis grouped implants by category rather than by measured lateralization, and its authors note heterogeneity and low levels of evidence among the included studies.
Notching. The more lateralized glenoid implants had lower rates of notching. In 517 shoulders, notching fell from 14.7% at 0 to 4 mm of metallic lateralization to 8.6% at 6 mm and 7.1% at 8 mm (0 to 4 mm versus 8 mm, P = .030) [3]. In women, 6 mm reduced notching compared with 4 mm or less (5.3% versus 15.6%; P = .016); in men, the reduction at 8 mm did not reach significance (7.6% versus 13.6%; P = .161). The authors attribute the difference to scapular size. With 4 mm of added glenosphere offset in one system, notching fell from 9% to 2% (P = .03) [6], and the meta-analysis found lower odds of notching with lateralized glenospheres (odds ratio 0.14) [7].
Function. Kirkham et al. did not measure glenoid lateralization. They found that a greater increase in the lateralization shoulder angle was associated with a better Subjective Shoulder Value, each degree multiplying the ratio SSV/(100 − SSV) by 1.01 (95% CI 1.00 to 1.02) [8]. That analysis was exploratory, was not corrected for multiple comparisons, and is not anchored to a minimal clinically important difference. With 4 mm of added glenosphere offset, ASES, Constant, UCLA, SST, SPADI, pain and motion did not differ [6]. The meta-analysis found a Subjective Shoulder Value higher by 6 points and pain lower by 1 point with lateralized glenospheres, and no difference in ASES, Constant or SST [7].
Measuring center-of-rotation lateralization directly. Lateralization of the glenosphere center of rotation (COR) from the glenoid bone surface can be measured directly on a true AP radiograph with magnification correction. The COR is the center of a circle fit to the glenosphere. This measurement includes the glenosphere geometry, the baseplate thickness, and the contribution of bone graft and augments when used. [9]
T
Humeral-sided lateralization
Humeral side alone. Greater lateral humeral offset, measured as the distance between a line down the center of the humeral shaft and a parallel line through the glenosphere-humeral interface, was associated with fewer fractures (OR 0.74, 95% CI 0.56 to 0.97, and 0.68 in the postoperative model) [5]. The paper labels this odds ratio per millimeter, but its own Figure 2 shows the same ratio across the interquartile range of offset, 6.7 to 11.9 mm, which makes the effect closer to 6% lower odds per millimeter. The analysis does not consider the lateralizing effect of the glenosphere, so it relates fracture to part of the displacement rather than all of it.
Global lateralization. Every other measure in this section spans the distance from scapula to humerus, and several are called humeral lateralization. None separates the humeral contribution from the glenoid contribution.
Pak et al. did not measure humeral lateralization in their study; they measured only global lateralization, from the baseplate to the lateralmost humerus, which was not associated with fracture with a 135° inlay humeral component (OR 0.61, 95% CI 0.23 to 1.66) [2]. The vendor’s specification for the humeral offset entered in the same regression, was likewise not associated (OR 0.87, 95% CI 0.72 to 1.05).
Their notching study divided the lateralization shoulder angle into glenoid (GLA) and humeral (HLA) contributions, splitting it at the lateralmost point of the glenosphere [3], and related them to motion and the ASES score, not to acromial/spine fracture. Each additional degree of humeral lateralization angle was associated with lower forward flexion by 0.57° (P = .007); a higher HLA was associated with a lower ASES score (P = .035); neither passed the authors’ own Bonferroni threshold [3]. The pieces carry the names glenoid and humeral, but both share the vertex at the acromion, and both change with distalization and with the size of the scapula; neither measures the humerus alone. Here again an angle stands in for a linear dimension.
Kirkham et al. split the same angle at the same glenoid pivot point [8], the landmark named by Schippers et al. [16], and found that a greater “humeral lateralization angle” was associated with reoperation for baseplate failure or instability (OR 1.10 per degree), as was a greater postoperative lateralization shoulder angle, the global measure it subdivides (OR 1.11 per degree); both rest on 5 events [8].
In 12 acromial fractures and 48 matched controls, the distance from the glenoid to the greater tuberosity, measured through the center of rotation, was shorter in the fracture cases (48.0 versus 51.7 mm, P = .026) [10]. That runs opposite to the acromion–tuberosity relationship discussed below, in which risk rises as the tuberosity reaches farther than the acromion. The same paper reports greater lengthening in the fractured shoulders, so those tuberosities sat higher rather than farther out; the two measures run in different directions.
What Haidamous et al. call humeral lateralization, measured as shown below, did not differ between 26 scapular spine fractures and 400 controls in a series of three implants (52.8 versus 53.9 mm, P = .362) [11]. That measure spans scapula to humerus; it is global, not humeral.
In 860 shoulders with one 155° onlay system, the 16 fractures of the scapular spine (Levy III) had a larger postoperative lateralization shoulder angle (LSA) than the controls (89° versus 83°), while Levy I and II fractures showed no radiographic predictor [12]; that analysis made many comparisons without correction.
Global lateralization, measured by the lateralization shoulder angle (LSA), was associated with more fractures: both a greater increase in the LSA from before to after surgery (OR 1.42) and a greater postoperative LSA (OR 1.76) [5]. These are also labeled per degree, but the paper’s Figure 2 shows the odds of fracture only doubling across a 37° range of change in the angle, about 2% per degree; like the offset figure, they appear to be reported across the interquartile range. The LSA is a poor stand-in for humeral lateralization on two counts: it uses an angle to measure a linear dimension, and it does not separate the contribution of the glenoid from that of the humerus.
Measured instead in millimeters from the glenoid to the lateral humerus, postoperative global lateralization did not differ between 47 patients with acromial fractures and 141 matched controls, all with one lateralized-glenoid, 135° inlay system (50.3 versus 50.1 mm, P = .862) [13]. Notably this did not separate the contribution of the humeral and the glenoid components.
Function. Thirty patients were studied with pre- and postoperative CT and an isokinetic dynamometer at a mean of 2.4 years [14]. Greater postoperative lateral humeral offset (LHO, measured on two-dimensional axial CT images as the distance between the medial edge of the base of the coracoid process and the most lateral point of the humerus; despite its name, a global measure anchored on the scapula) predicted greater strength in every plane tested (all P ≤ .019). Offset carried more than 5 mm lateral to that patient’s preoperative value was associated with less strength in abduction (β −7.1), flexion (β −7.6), extension (β −13.5), and internal rotation (β −7.2), and larger deviations from preoperative offset predicted poorer internal rotation motion. The greatest external rotation motion occurred in patients whose version stayed within 10° of native and whose offset was at or medial to native. No patient-reported outcome was collected. This measure does not isolate the contribution of the humeral component, and it references the coracoid rather than the glenoid bone face.
Of the measures reported in this section, only one isolates the humeral contribution. Two others are labeled humeral but are subdivisions of a global angle, and the rest measure the whole distance from scapula to humerus, several of them under the name humeral lateralization. This variability confounds any attempt to synthesize the effect of humeral lateralization.
Measuring the humeral contribution directly. The humeral contribution to lateralization can be measured on a true AP radiograph with magnification correction, as the distance from the lateralmost aspect of the glenosphere to the greater tuberosity. [9]
Distalization
Complications. Each centimeter of increase in acromiohumeral distance was associated with a 121% increase in the odds of fracture (OR 2.21, 95% CI 1.33 to 3.68) [2].
Using the distalization shoulder angle (DSA) instead, Moverman et al. found no association [5]. In the 47 fractures and 141 controls of Polisetty et al., the postoperative acromiohumeral interval was 27.8 mm in both groups, and the increase from before surgery did not differ (P = .448) [13]. Again, an angle stands in for a linear dimension.
In a three-implant series, the postoperative acromiohumeral distance was 37.5 mm in shoulders with scapular spine fractures and 33.7 mm in those without (P = .042 in the text, .021 in the table) [11]. The onlay stem in that series produced 10 mm more acromiohumeral distance than the two inlay stems, and fractures followed 11.9% of onlay and 4.7% of inlay arthroplasties (P = .043).
In the Mayo series of Werthel et al., arm lengthening was 24.5 mm in shoulders with acromial fractures and 17.8 mm in controls (P = .004), perhaps because the shoulders with fractures started with more superior migration [10].
In the 155° onlay series, shoulders with scapular spine (Levy III) fractures had less distalization, not more: acromiohumeral distance 33 versus 38 mm (P = .049) and distalization shoulder angle 45° versus 53° [12]; acromial (Levy I and II) fractures showed no radiographic predictor.
These studies measured distalization in different ways and their results do not agree in direction, so a limit on lengthening is an inference rather than a demonstrated threshold. Plain films yield different numbers than CT scans. In 31 shoulders, humeral distalization and center-of-rotation distalization read 4.2 and 5.1 mm shorter on standing radiographs than on CT-based 3D models of the same shoulders, a difference the authors attribute to arm and scapular position; DA and DGT differed by less than 0.5 mm [37].
Function. A higher postoperative distalization shoulder angle was associated with a slightly lower Subjective Shoulder Value [8]. The size of the effect is small and its relation to the minimal clinically important difference is not reported.
Is it reasonable to use angles to measure distance?
Lateralization and distalization are linear: millimeters of displacement. The lateralization shoulder angle and the distalization shoulder angle are angles [15]. Both are taken from one triangle whose corners are the superior glenoid tubercle, the inferolateral acromion, and the superior aspect of the greater tuberosity. The lateralization shoulder angle is the corner at the acromion; the distalization shoulder angle is the corner at the glenoid tubercle.
Five issues follow from that construction.
• The two angles describe shape, not displacement. They are two corners of one triangle, so together they fix its shape while saying nothing about its size. A model that enters both is estimating the shape of the reconstruction, not how far the humerus moved. Kirkham found only weak correlation between the lateralization and distalization angles in their cohort, where earlier series reported a strong inverse correlation [8].
• Neither angle isolates one dimension. Moving the humerus laterally opens the corner at the acromion and closes the corner at the glenoid tubercle; moving it distally does the reverse. A change in either angle can come from lateralization, from distalization, or from both.
• An angle is dimensionless, so patient size is built into it. The same millimeters of lateral displacement subtend a smaller angle in a large scapula than in a small one. A per-degree odds ratio therefore mixes displacement with the size of the shoulder. Scapular size is the same variable invoked to explain why women and men required different amounts of metallic lateralization to reduce notching [3]. Normalizing to size could be a virtue rather than a fault, since the same displacement may matter more in a small scapula; but it makes the angle a measure of displacement relative to size, which is not the quantity the surgeon enters into the plan.
• The angles cannot be converted back to millimeters. Doing so requires a side length of the triangle, such as the distance from the acromion to the glenoid tubercle, which these studies do not report. So a per-degree odds ratio gives the surgeon no target to enter into the plan.
• The landmarks are not the same from study to study. Moverman takes the lateralization shoulder angle to the most lateral border of the greater tuberosity and the distalization shoulder angle to its most superior border, and uses the most lateral border of the acromion [5]; other descriptions use the inferolateral edge of the acromion and do not distinguish the two tuberosity points [8,15]. The division into glenoid and humeral parts, by contrast, is shared: both studies that report it split the angle at the lateralmost point of the glenosphere, which Schippers et al. named the glenoid pivot point [3,8,16].
The combined effects of distalization and lateralization
One proposed mechanism for acromial/spine stress fractures after RSA is impingement of the greater tuberosity on the undersurface of the acromion when the arm is abducted. The risk of this impingement can be estimated on an AP radiograph by comparing the distance from the glenosphere center of rotation to the acromion (AC) with its distance to the greater tuberosity (CT).
In one surgeon’s practice, both displaced Levy IIB fractures had AC at or below CT; the two Levy I fractures, which healed without displacement, did not, and the matched controls averaged 5.6 mm of clearance [9]. Kawashima et al., who had proposed the relationship from an in vivo study of 11 shoulders [36], tested it at another center, with another implant, a medialized-glenoid, lateralized-humerus onlay system, in 526 shoulders [17]. After propensity matching, 1 of 240 shoulders with AC equal to or greater than CT fractured the acromion or scapular spine (0.4%), against 6 of 120 with AC less than CT (5.0%, P = .006); subacromial notching occurred in 0% and 10.8%. The fracture rate difference was significant for lateral fractures (Levy I and IIA; 0 versus 3, P = .037) but not for medial fractures (IIB, IIC and III; 1 versus 3, P = .110); the paper’s text reports the group totals, 1 of 240 and 6 of 120, as if they were the lateral-fracture counts. ASES, SPADI and Simple Shoulder Test did not differ between the groups [17].
Humeral version
Complications and function. Humeral version is one of two elements of this geometry tested in a randomized trial; the other is the neck–shaft angle, below. Wiater et al. randomized 66 patients to neutral or 30° of retroversion; at 2 years the groups did not differ in rotation, elevation, abduction, strength, complications, pain, ASES, or PROMIS-10 [18]. A CT study of 30 patients found the most external rotation when postoperative version stayed within 10° of native [14].
Humeral neck–shaft angle
In one onlay system the 145° and 135° options are each built from a stem and an angled liner, 132.5° plus 12.5° or 127.5° plus 7.5°, so the liner chosen sets part of the angle [19]. In the one randomized trial, a single inlay cup was locked at 135° or 155° before insertion and the humerus was cut with a guide at the matching angle [20]. Stem alignment changes it further: in that series stems sat a mean of 2.3° to 2.8° away from their nominal angle, with standard deviations of 2.2° to 3.0° [19].
Complications. Gobezie et al. randomized 100 primary reverse arthroplasties, done by one surgeon with an otherwise identical implant and a neutral glenosphere, to 135° or 155° [20]. At 2 years scapular notching was seen in 21% at 135° and 58% at 155° (P = .009); stress fractures numbered none and one.
Neyton et al. found scapular notching in 53% of 73 shoulders with a 145° angle and in 30% of 30 with 135° (P = .028; odds ratio 4.1 after adjustment for glenoid lateralization and inferior eccentricity) [19].
Two registry comparisons of competing designs, a 155° inlay, a 135° inlay with a 4 mm lateralized glenosphere, and a 145° onlay with a 3 mm lateralized baseplate, found notching in 50%, 24% and 14% of shoulders with cuff tear arthropathy [21] and in 22%, 16% and 9% of shoulders with osteoarthritis [22]. Each design changes the angle, the stem and the glenoid offset together, so the angle cannot be separated from the rest. Acromial fractures were few: three in 226 shoulders [21] and none in 97 [22]. A systematic review of 11 studies (971 shoulders) reported the most fractures with 135°, the most revisions with 155° and the most infections with 145° [23].
Function. In the randomized trial, ASES score (74 versus 78, P = .446), Simple Shoulder Test (8 versus 7), SANE, pain, flexion and external rotation did not differ between 135° and 155° [20]. The trial was powered to detect a 10° difference in flexion, not a difference in what patients report. In the onlay series, Constant score (66.1 versus 68.2), Subjective Shoulder Value (76.5% versus 83.1%, P = .167) and motion did not differ between 145° and 135° [19]; with 30 shoulders in the 135° group, a modest difference could have been missed. In cuff tear arthropathy, the three designs did not differ in Constant score, Subjective Shoulder Value or SPADI at 2 years, while the 145° onlay design had more flexion and abduction [21]. The discussion of that paper reports a 7-point Constant difference (P = .03) against the 155° design where its table gives P = .088; on either P value the 7-point difference falls below the 8-point minimal clinically important difference the authors cite. In osteoarthritis, the Subjective Shoulder Value was 95% and 93% with the two lateralized designs and 90% with the 155° design (P = .046), with no difference in Constant score. Those groups differed before surgery: the supraspinatus was intact in 56% of the 145° onlay group and in 11% to 12% of the others, and 98% of the 135° group were women. [21,22].
Liner capture
Complications and function. Capture, the liner depth divided by the glenosphere radius, is a property of the implant. Two surveys of commercial liners found that a “standard” liner in one system can capture more of the glenosphere than a “retentive” liner in another [24,25], and that capture varies with the neck–shaft angle but not with glenosphere size [24]. We found no clinical study relating capture to what patients report. In one onlay system, “constrained” liners were used in 4.4% of shoulders that fractured and 5.0% of those that did not [26]; constraint, however, is a liner category, not a measured capture.
Studies without patients, a computer model of impingement-free motion [27], two surveys of liner dimensions [24,25], a cadaver and computer study of acromial strain [28] and a finite-element model of acromial stress [29], are listed in the references but not discussed here.
The subscapularis, an important non-imaged variable
Complications. A systematic review pooled three retrospective studies and reported an odds ratio of 2.48 for instability when the subscapularis was not repaired [30]. Whether the tendon can be repaired depends partly on tissue quality, muscle excursion and implant size, which are not accounted for in that comparison. A review of instability recommends repair particularly with a medialized prosthesis [31].
Function. Not studied in these reports.
The implant is the implant; its position depends on the surgeon
Much of this literature sorts reverse shoulders by catalog category: inlay or onlay, 135° or 145°, lateralized or medialized glenosphere. The patient, however, receives the geometry that results from how those parts were placed. An onlay tray seated deeply on a generous humeral resection behaves like an inlay; an inlay component that is not fully seated behaves like an onlay. The glenoid side works the same way: 2 mm of extra reaming removes 2 mm of the lateralization the implant specifications promised, and asymmetric reaming changes inclination with no change in the implant. In one registry the surgeons sometimes cut the humerus deeper for the onlay design, depending on soft-tissue tension, and recut at their discretion [21]; the resection plane was set by the surgeon’s intraoperative judgment in the companion study [22].
It helps to separate three layers. The first is the implant: neck–shaft angle, tray geometry, glenosphere diameter and offset, insert thickness. These are implant specifications, known before the operation. The second is the placement: the level and angle of the humeral resection, how deeply the stem or tray is seated, glenoid reaming depth and asymmetry, baseplate seating, bone graft, version and inclination. These are surgeon decisions, and most are recorded in no registry. The third is the geometry the patient ends up with: where the center of rotation sits relative to that patient’s glenoid, and how far the humerus has moved laterally and distally from where it started. Only the third layer loads the acromion, tensions the deltoid, and determines whether the components impinge. The first two are inputs to it.
One of the studies measured placement directly. Polisetty et al. measured an inlay–onlay index, the position of the humeral component relative to the anatomic neck, in 47 patients with acromial fractures and 141 controls matched for sex, indication and age, all operated on by one surgeon with one lateralized-glenoid, 135° inlay system [13]. The index averaged 1.1 mm in the fracture group and 1.7 mm in the controls, with standard deviations near 4 mm in both (P = .413). Within a single system labeled “inlay,” seating varied by several millimeters from one patient to the next. Neither the index, global lateralization, the acromiohumeral interval, the delta angle, the critical shoulder angle, nor the morphology or arthritis of the acromioclavicular joint on CT differed between the groups. With 47 fractures, this study shows the absence of a demonstrated association, not the absence of an effect.
In many series, comparisons are made with respect to implant type, rather than the final RSA geometry. Roche et al. reported a fracture rate of 1.52% in 9079 shoulders with one “medialized-glenoid, lateralized-humerus onlay system” [26] and attributed the difference from the 5.1% of Polisetty’s lateralized-glenoid inlay series [13] and the 3.9% of the ASES multicenter predictors series [32] mainly to implant design. Within Roche’s single system, the specifications for humeral tray and liner offsets were not associated with fracture; a smaller glenosphere (38.6 versus 39.5 mm) and more baseplate screws (3.8 versus 3.7) were [26].
Schneller et al. applied machine learning to predict acromial and scapular spine fractures, entering the implant as a design category — medialized-distalized, lateralized, or lateralized-distalized — rather than as measured geometry [34]. However the model performs, what it learns about the implant is the label on the box.
The implant category is a label on the box; the geometry is what the surgeon builds with it. A study that reports only the category answers a question about categories. A study that reports catalog millimeters without reaming depth and seating reports a partial sum. To learn what geometry does for patients, we need the placement recorded, the resulting geometry measured on the postoperative film, and the outcome reported by the patient.
What do the numbers in these papers mean?
A figure such as “5 mm of glenoid lateralization” has no meaning until we know what was measured and from where. Is it the distance from the bone surface to the center of rotation, from the baseplate to the center of rotation, from the bone to the lateral aspect of the glenosphere, or from the baseplate to that lateral aspect? Different studies answer differently, and some report degrees rather than millimeters. The table below gives the definition used by each study that reports a lateralization or distalization measure.
Study | Glenoid-sided lateralization | Humeral-sided lateralization | Distalization |
Eisenberg [4] | Millimeters of center-of-rotation offset taken from implant specifications. Zero is where a standard nonwedged baseplate would sit with full backside contact on that patient’s glenoid. Bone surface to center of rotation. | Radiographic perpendicular distance from the lateral greater tuberosity to the glenoid bone–implant interface, with the manufacturer’s glenoid value subtracted. | Acromiohumeral interval: inferior acromial cortex to the top of the greater tuberosity, preoperative versus postoperative. |
Pak, fracture [2] | Metallic offset from implant specifications, 0 to 8 mm in 2-mm increments, through the baseplate, the glenosphere, or both; augments counted as 2 mm. Global lateralization measured separately, perpendicular from the baseplate to the lateralmost humerus. | Not measured on the radiograph. Catalog humeral offset (metallic spacer and polyethylene) recorded, and global lateralization, which includes the humeral contribution. | Acromiohumeral distance on the Grashey view, from the undersurface of the acromion to the most superior greater tuberosity, referenced to the humeral axis; change from before to after surgery. Glenosphere overhang measured from the bottom of the glenoid face to the inferior glenosphere. |
Pak, notching [3] | Same metallic scheme as above. Also the lateralization shoulder angle, divided into glenoid and humeral contributions by splitting the angle at the lateralmost point of the glenosphere. | Humeral contribution to the lateralization shoulder angle, by the same split. Degrees. | Distalization shoulder angle, in degrees, plus glenosphere overhang in millimeters. |
Moverman [5] | Whole cohort: total glenoid lateral offset, defined as the sum of the glenosphere, baseplate and bone-graft lateral offsets. Matched radiographic analysis: lateralization shoulder angle, in degrees. | Lateral humeral offset: the distance between two parallel lines, one down the center of the humeral shaft and one through the tangential interface point of the glenosphere and humeral implants. | Distalization shoulder angle and its change, in degrees. |
Kirkham [8] | Glenoid lateralization angle, in degrees, splitting the lateralization shoulder angle at the glenoid pivot point, the lateralmost point of the glenosphere [16]. | Humeral lateralization angle, in degrees, by the same split. | Distalization shoulder angle [15], modified distalization shoulder angle (medial calcar rather than greater tuberosity), and glenoid and humeral distalization angles, all in degrees. |
Charles [14] | Not measured separately. Lateral humeral offset here is a global measure that includes the glenoid contribution. | Lateral humeral offset on axial CT: distance from the medial edge of the base of the coracoid to the most lateral point of the humerus, postoperative compared with that patient’s preoperative value. | Distance from the center of the distal humeral epicondyles to the intersection of the humeral shaft vector with the plane of the acromion, on 3D models. Reported but not used as a predictor. |
Matsen [9] | Center-of-rotation lateralization: glenoid bone surface to the glenosphere center of rotation. Glenosphere thickness: glenoid bone surface to the lateralmost glenosphere, including baseplate and augments. | Humeral component of global lateralization: global lateralization (glenoid bone surface to the tip of the greater tuberosity) minus glenosphere thickness. | Humeral distalization: lateralmost acromion to the tip of the greater tuberosity, before and after surgery, with the difference reported. |
Schneller [34] | No measurement. Implants were grouped as medialized-distalized, lateralized, or lateralized-distalized by design category. | Same design categories. | Same design categories. |
King [6] | Catalog offset: glenosphere lateralized 4 mm versus the standard glenosphere, whose center of rotation is 2 mm lateral to the glenoid. | Same system in both groups; not measured. | Not measured. |
Werthel [10] | Not measured separately. | Glenoid–baseplate interface to the center of rotation plus center of rotation to the greater tuberosity; acromion to greater tuberosity. | Arm lengthening: superior cortex of the acromial base to the humeral head center before surgery and to the center of the component epiphysis after. |
Haidamous [11] | Center-of-rotation offset: original humeral head center to the humeral cup center. | Humeral lateralization by the method of Levy. | Acromiohumeral distance, perpendicular to the humeral shaft. |
Kriechling [12] | Center of rotation to the lateral acromion. | Lateral acromion to greater tuberosity; center of rotation to greater tuberosity; lateralization shoulder angle. | Acromiohumeral distance, deltoid length and distalization shoulder angle. |
Kawashima [17] | Not measured; medialized-glenoid system. Inferior overhang: inferolateral glenoid edge to the most inferior glenosphere, parallel to the center peg. | Glenosphere center to the lateral tip of the greater tuberosity (CT), compared with glenosphere center to the lateral undersurface of the acromion (AC). | Not measured separately; AC and overhang reflect it. |
Neyton [19] | Recorded as present or absent: bone graft or lateral eccentric glenosphere. | Neck–shaft angle by stem and liner; stem alignment measured on the radiograph. | Not measured. |
Freislederer [21] | Theoretical global offset from implant specifications; lateralization shoulder angle. | Lateral humeral offset: distance between two lines parallel to the humeral shaft, one through the superior glenoid tubercle and one through the lateral greater tuberosity. | Distalization shoulder angle and inferior glenosphere overhang. |
Imiolczyk [22] | Theoretical global offset from implant specifications; global lateral offset on the radiograph. | Glenoid to lateral greater tuberosity along the axis of the scapular spine, before and after surgery, the difference reported as net lateralization. | Distalization shoulder angle and inferior glenosphere overhang. |
Polisetty [13] | Not measured separately. The system’s glenosphere center of rotation is lateral to the glenoid by 2 to 10 mm by design. | Global lateralization on radiographs, glenoid to lateral humerus, before and after surgery. Inlay–onlay index: position of the humeral component relative to the anatomic neck, in millimeters. | Acromiohumeral interval, before and after surgery. |
Roche [26] | No radiographic measurement. Glenosphere diameter, expanded glenosphere and augmented baseplate use from the operative record. | No radiographic measurement. Catalog humeral tray and liner offsets. | Not measured. |
Several points follow from this table. Eisenberg [4] and both Pak studies [2,3] report the same kind of quantity, a millimeter offset taken from implant specifications, but Eisenberg states his reference point and Pak does not, so even these numbers cannot be assumed to be comparable. The angles reported by Moverman [5] and Kirkham [8] cannot be converted into millimeters, so a per-degree odds ratio gives no threshold for the plan. The glenoid and humeral contributions to the lateralization shoulder angle, at least, are built the same way in the two studies that report them: both split the angle at the lateralmost point of the glenosphere [3,8,16]. Lateral humeral offset is another matter: Moverman measures it on the radiograph as the distance between a line down the center of the humeral shaft and a parallel line through the glenosphere–humeral interface [5], which isolates the humeral side, while Charles measures it on axial CT from the coracoid base to the most lateral point of the humerus [14], which includes the glenoid contribution. One name, two quantities.
The evidence at a glance
What the plan aims for | Complication evidence | Evidence on function |
Neutral-to-inferior inclination | Change toward superior inclination associated with instability [1] | Not studied |
Inferior glenosphere overhang | Odds of fracture higher by 19% per millimeter [2]; overhang protective against notching, P < .001 [3] | Notching associated with ASES lower by 7 points and forward flexion lower by 13° [3] |
Restrained glenoid lateralization in the fragile patient | Fracture: 9.2% to 2.3% before and after medialization [4]; OR 1.06 per millimeter of summed glenoid offset, acromial fractures only [5]; OR 1.42 and 1.76 for LSA, a global measure, apparently across its interquartile range [5]; no association up to 8 mm of metal [2]; 1% versus 3% with and without 4 mm added offset [6]; similar in meta-analysis [7]. Notching: 6 mm or more protective [3]; 2% versus 9% [6]; OR 0.14 [7] | Greater change in LSA, a global measure, associated with slightly better SSV, per degree [8]; less notching associated with better ASES and motion [3]; no difference with 4 mm added offset [6]; SSV 6 points higher in meta-analysis [7] |
Humeral-side offset restored to native | Lateral humeral offset, the one humeral-only measure, associated with less fracture, OR 0.74 across the interquartile range [5]. Global measures, several labeled humeral: shorter glenoid-to-tuberosity distance in 12 fractures [10]; no difference in 26 fractures [11]; humeral lateralization angle, a subdivision of the global LSA, associated with reoperation, OR 1.10, as was the global LSA itself, OR 1.11, on 5 events [8] | Strength and motion with a global offset measure in 30 patients; no patient-reported outcome [14]. Humeral lateralization angle associated with less forward flexion and lower ASES, neither passing Bonferroni correction [3] |
Lengthening limited | OR 2.21 per centimeter of added acromiohumeral distance [2]; greater distance or lengthening in fractures [10,11]; distalization angle not associated [5]; acromiohumeral interval did not differ in one 135° inlay system [13]; less distalization in scapular spine fractures with a 155° onlay [12] | Higher distalization shoulder angle associated with slightly lower SSV [8] |
Humeral version 0° to 30° | No difference, randomized [18] | No difference in ASES or PROMIS-10, randomized [18] |
Lower neck–shaft angle | Notching 21% at 135° and 58% at 155°, randomized, neutral glenosphere [20]; less notching at 135° than 145° within one system, confounded by glenoid lateralization and period [19]; notching also lower with complete lateralized designs [21,22] | No difference in ASES, SST, SANE or motion, randomized [20]; no difference in Constant, SSV or motion within one system [19]; no difference in Constant, SSV or SPADI across designs in cuff tear arthropathy [21]; SSV higher by 3 to 5 points in osteoarthritis with unequal groups [22] |
Liner capture | Constrained liner use similar in fractured and unfractured shoulders [26] | Not studied |
Subscapularis repaired | Pooled OR 2.48 for no repair, retrospective [30] | Not studied |
AC greater than CT on the postoperative film | Both displaced Levy IIB fractures had clearance at or below zero; their controls averaged 5.6 mm [9]. In 360 matched shoulders at another center, fracture 0.4% with AC ≥ CT versus 5.0% with AC < CT, on 7 fractures [17] | No difference in ASES, SPADI or SST [17] |
What would move this forward
Five things. The first is open to every surgeon today; the second would let each surgeon’s lessons be compared and pooled; the other three are in ascending order of difficulty.
• Learn from each of our own patients. The studies above come from surgeons around the world and frame the questions. Each patient, however, receives the product of one surgeon’s method: how that surgeon selects and prepares patients, assesses the shoulder, chooses and places the components, and learns what the patient reports about comfort, function and complications. Every adverse outcome raises the question, “What might I have done differently to give this patient a better result?” One way to answer it is to compare the geometry of that shoulder with matched shoulders from the same practice that did well [9]. Made a habit, this introspection lets each surgeon improve the method recursively, one patient at a time. It needs no trial, registry or new technology; only the postoperative film, the patient’s own report, and the willingness to look.
• Agree on what to measure. Individual surgeons learn faster when their measures can be compared with those of colleagues. A meeting of leading shoulder surgeons, modeled on the 2018 International Consensus Meeting on Musculoskeletal Infection, whose shoulder group agreed by vote on a definition of periprosthetic shoulder infection [38], could settle by structured discussion and vote a short list: which radiographic view, which landmarks and distances, how placement is recorded, and which patient-reported outcome at what interval, with its minimal clinically important difference. The product would be a one-page protocol any surgeon could apply to their own films and patients, and that a registry could collect across practices. The American Shoulder and Elbow Surgeons (ASES) Revision Shoulder Arthroplasty and Periprosthetic Joint Infection (PJI) Multicenter Research Group shows that this second step works: surgeons at many centers collect data prospectively on consecutive revision arthroplasties and classify each case by the consensus definition [39]. Each surgeon’s loop would then add to a shared body of evidence, without anyone having to randomize the many interacting variables of reverse arthroplasty.
• Publish the reliability of whatever set is agreed. Whatever set is used, each landmark should carry one name and one point; distances should be calibrated against the known diameter of the glenosphere rather than reported as raw millimeters; and quantities referenced to the glenoid face should be measured before and after against a scapular landmark the operation does not touch. Two blinded observers and an intraclass correlation for every measure. This requires no new patients and would let the studies already published be compared with one another. For the acromion and tuberosity distances, one group has shown what this looks like: two observers, repeated readings, intraclass correlations above 0.9, and radiographic values checked against 3D models of the same shoulders [36,37].
• Report the placement, not only the part. Reaming depth, seated tilt and resection level are known in the operating room and omitted from nearly every report. Without them a millimeter figure is a partial sum, and two cohorts using the same catalog number are not studying the same geometry. The inlay–onlay index of Polisetty et al. shows that humeral seating can be measured on the routine postoperative film [13]. Without such reporting the question cannot be pooled: a meta-analysis of 25 studies and more than 100,000 shoulders could not analyze implant design or lateralization, because the studies reported them too inconsistently to combine [35].
• Anchor the outcome to the patient. Every association here would be more useful if the outcome were what the patient reports at 2 years and the effect were placed against the minimal clinically important difference.
The AC–CT relationship rests on two retrospective series: 526 shoulders with one implant at one center [17] and four fractures in one surgeon’s practice [9]. Both found an association with fracture, but not for the same fractures: lateral fractures in the first, displaced Levy IIB fractures in the second. What remains is for surgeons to measure the relationship on their own postoperative films, alongside their fractures and what their patients report, and see whether it holds in their hands.
Planning software now lets us specify this geometry to the degree and the millimeter. What it cannot tell us is how well the RSA geometry served the patient. For that, each of us can measure what we built and, at 2 years, ask each patient, How are you doing? The answer can shape the next operation.
Keeping our eye on what’s most important
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References
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