CT-free planning for reverse total shoulder arthroplasty - why and how to do it

The most recent Australian Orthopaedic Association's Joint Replacement Registry reveals that the use of CT scans prior to shoulder arthroplasty is increasing rapidly.

This use needs to be considered in light of five factors

(1)  Evidence is lacking that preoperative CT planning improves patient-reported outcomes, complication rates, or revision rates for common arthroplasty cases:

Use of Preoperative CT Scans and Patient-Specific Instrumentation May Not Improve Short-Term Adverse Events After Shoulder Arthroplasty: Results from a Large Integrated Health-Care System found "no reduction in the risk of aseptic revision was observed for patients having preoperative CT scans".

The influence of computed tomography preoperative planning on clinical outcomes after anatomic total shoulder arthroplasty: a matched cohort analysis found the differences in clinical outcomes between the CT and non-CT scan groups failed to meet a clinically significant threshold.

Templating in shoulder arthroplasty - A comparison of 2D CT to 3D CT planning software: A systematic review did not find evidence demonstrating differences in time, cost, functional outcomes, complications, or patient satisfaction.

3D CT-Based Preoperative Planning and Intraoperative Navigation in Reverse Shoulder Arthroplasty: Early Clinical Outcomes did not find statistically significant benefit of CT-based navigation with respect to complication rate, radiographical glenoid notching, and clinical outcomes

Assessing the Value to the Patient of New Technologies in Anatomic Total Shoulder Arthroplasty did not find evidence that the patient outcomes for shoulder arthroplasty were statistically or clinically improved with the use of advanced technologies, such as CT based planning.

(2) CT scanning and 3D planning increase the cost of healthcare by dint of their own costs. But they also initiate a "cost cascade" by driving the use of more expensive implants, and by driving the use of expensive "transfer technologies", such as patient specific instrumentation, virtual and augmented reality, and robotics. Because planning software packages are proprietary, they can drive the use of the implants associated the software, rather than the implants that may be more cost effective.

The influence of three-dimensional planning on decision-making in total shoulder arthroplasty. found 3-D planning influenced the choice of implant 

Impact of preoperative 3-dimensional planning and intraoperative navigation of shoulder arthroplasty on implant selection and operative time: a single surgeon's experience. found an almost three-fold increase in the use of the more expensive augmented components when using 3D preoperative planning 

Computer navigation re-creates planned glenoid placement and reduces correction variability in total shoulder arthroplasty: an in vivo case-control study. found that patients undergoing computer-assisted shoulder arthroplasty had more than twice as many augmented glenoid components as the conventional group 

However, none of these studies presented costs or clinical outcomes comparisons and thus it cannot be ascertained whether the use of augmented components was a cost-effective intervention or not.

Preoperative planning in reverse shoulder arthroplasty: plain radiographs vs. computed tomography scan vs. navigation vs. augmented reality concluded that further research is needed to determine the added value of these technologies in terms of improving clinical outcomes for the patients. The scarce evidence comparing short-term clinical outcomes of RSA with and without the use of these technologies show no or marginal benefits "Any benefit should be balanced against the increased costs and the 200 to 1,000-fold increase in radiation exposure associated with the CT scans in comparison with radiographs ".  

(3) Thus, In addition to its cost, CT scanning is associated with an increased lifetime risk of cancer

Computed Tomography for Preoperative Shoulder Arthroplasty Planning: Lifetime Malignancy Risk: The current reliance on preoperative shoulder CT for arthroplasty planning needs to be weighed against the potential lifetime cancer risks.

Projected Lifetime Cancer Risks From Current Computed Tomography Imaging found that at current utilization and radiation dose levels, CT examinations in 2023 were projected to result in approximately 103 000 future cancers over the course of the lifetime of exposed patients. If current practices persist, CT-associated cancer could eventually account for 5% of all new cancer diagnoses annually.

(4) Reverse shoulder arthroplasty (RSA) is a highly successful operation in the population-based study by the Australian Orthopaedic Association with 10 year revision rates under 6%. Note that these national data include all cases performed by all surgeons, not only "high volume" surgeons.

Complications and revision of reverse total shoulder arthroplasty found that 40% of revisions were for infection, 20% for instability/dislocation, 20% for loosening, and 10% for fracture. Evidence is lacking that CT scans, CT-based planning, and associated technologies have a substantial impact on these common reasons for RSA revision. 

Patient-Specific Instrumentation Versus a Free-Hand Technique for Glenoid Baseplate and Peripheral Screw Placement in Reverse Total Shoulder Arthroplasty Using the Exactech Implant System: A Multicenter Randomized Controlled Trial concluded "in primary rTSA, PSI did not improve glenoid baseplate positioning compared to a free-hand technique"

(5) In most cases, cost-effective preoperative planning can be efficiently carried out without a CT scan. 

Here's a way to it in under ten minutes using the Picture Archiving and Communication Systems (PACS) tools applied to two standardized preoperative plain films (AP view in the plane of the scapula and axillary view). This approach is generic and is not tied to any commercial company, allowing surgeons to have their choice of implant. It can be used by essentially all shoulder surgeons. It provides an approach for transferring the plan to the patient. It is cost-effective - an important feature, especially at this time when many are having difficulty affording health care.

Step 1, on the AP view, draw a line along the base of the supraspinatus fossa (yellow line in right hand image).

Step 2, draw a line segment  equal to the radius of the base of the glenosphere from the inferior glenoid at a right angle to the supraspinatus fossa line so that it intersects the glenoid face (yellow line on left image below). This intersection will be the starting point for the drill for the central scew of the baseplate. This point will be located at surgery by measuring the distance from this point to the inferior lip of the glenoid (yellow line on right image below). Note that the amount of bone that will be removed by reaming is represented by the triangle in between these two lines.

Step 3 Draw a line parallel to the supraspinatus fossa line that intersects the glenoid face at the insertion point (yellow line on left image); this is the trajectory of the drill. Measure the angle between this line and the face of the upper glenoid (yellow line on right image) to guide the tilt of the drill at surgery.

Step 4 On the axillary view, draw the desired trajectory of the central screw so that it will just penetrate the anterior cortex of the subscapularis fossa when fully inserted (central image). Note the angle of this line with the gleniod face (right image) to guide version of drill at surgery.

The resuling arthroplasty closely approximates the plan.

Another example

And another

CT-free planning is consisted with the philosophy expressed in the preface of Practical Evaluation and Management of the Shoulder (1994): "This book is directed at the type of practice we see evolving for the coming decades, when resources will not be as plentiful and increasing premiums will be placed on economy and effectiveness. In this spirit, we emphasize what can be accomplished with the basics: the clinical history, the physical examination, a few plain radiographs, simple patient-conducted rehabilitation programs, and well-characterized surgical procedures. "

Thanks to our two shoulder fellows, Jake Checketts and Dave Daniels, for their help with this post. They easily mastered this method and routinely apply it to all but the unusual cases of reverse shoulder arthroplasty (i.e. those with severe bone loss, complex revision cases, significant deformity).

Making a Plan

Osprey 

Union Bay Natural Area

2024

Follow on twitter/X: https://x.com/RickMatsen
Follow on facebook: https://www.facebook.com/shoulder.arthritis
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/

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

Preventing Scapular Spine Fractures: The Superior Baseplate Screw Question

My friend, Ed McFarland, keeps an ever watchful eye on this blog. He pointed out that in yesterday's post I'd failed to mention another surgeon-controlled variable bearing on risk of scapular spine fractures after reverse total shoulder: the superior baseplate screw. So here's what I think we know about that.

The Clinical and Laboratory Evidence

Increased scapular spine fractures after reverse shoulder arthroplasty with a humeral onlay short stem: an analysis of 485 consecutive cases reported: "we found that 57.1% of the scapular spine fractures (12 of 21) occurred at the distal tip of the superior screw"

Scapular fractures after reverse shoulder arthroplasty: evaluation of risk factors and the reliability of a proposed classification found "Of 16 scapular spine fractures, 14 occurred from a screw tip."

Scapula fractures after reverse total shoulder arthroplasty: classification and treatment: "We no longer recommend placing the most superior screw for metaglene fixation because this appears to lead to a potential stress riser that may contribute to this fracture of the scapular spine"

Scapular spine base fracture with long outside-in superior or posterior screws with reverse shoulder arthroplasty: an in vitro study reported "The main finding of the study was that when the superior metaglene screw (with or without a posterior or anterior screw) enters into the base of the spine with an outside-in configuration, it leads to a fracture at the base of the scapular spine compared to the short screw "not touching the scapula spine" group."

This leads to some interesting questions:

(1) Is it the screw or the hole?

The evidence is that putting a short screw into a too-long hole doesn't reduce the weakening of the bone. The hole is the problem. Leaving the drilled hole incompletely filled may actually worsen the weakening of the scapular spine. Even "appropriate length" screws that don't breach the cortex may still weaken the spine if the drill hole penetrates into the spine base.

(2) Which screw hole is the one to worry about?

We often say "go short on drilling the superior hole". I took a look at the scapular model that lives on my desk. As we see, the spine is not directly superior, but rather posterior-superior.

From this we recognize that the "worry hole" may not be the superior hole (12 o'clock) but rather a posterior-superior hole (10:30 for right shoulder, 1:30 for left shoulder) depending on how the baseplate is rotationally oriented on the prepared glenoid (see diagram of a left shoulder below in which the red trapezoid represents the position of the scapular spine and, for the baseplate orientation in the middle figure, the "worry hole" is at 1:30). 

Of note, with a monoblock baseplate with a central screw, the rotational orientation of the holes is determined by when the central screw is tightened and is often not straight up and down as shown in the left image.

(3) How important are the superior screws?

We took a look at this question in the post "Learning from baseplate failure", noting that most failed baseplates fail by rocking into superior tilt.

This is because the rocking moment is applied to the inferior aspect of the glenosphere.

This rocking moment (red) is resisted by compression of the upper baseplate against the reamed bone (yellow) and resistance to pullout by the central and inferior screws (green).

When this stabilization mechanism fails, the gleosphere rocks up and out. Note that the superior screw is not in a position to resist this rocking.

So how important is the superior screw?

Central Fixation Dominates (60-70% of Total Stability)

The biomechanical literature consistently shows that central fixation quality matters far more than peripheral screw configuration. Studies examining central screw versus central peg fixation demonstrate massive differences in micromotion - on the order of 300-400% - while peripheral screw number shows much smaller effects. Central peg fixation was associated with 358% greater micromotion at all peripheral screw positions compared with central screw fixation.  See Central fixation element type and length affect glenoid baseplate micromotion in reverse shoulder arthroplasty, Factors affecting fixation of the glenoid component of a reverse total shoulder prothesis, and Glenoid baseplate screw fixation in reverse shoulder arthroplasty: does locking screw position and orientation matter?

Scapula fractures after reverse total shoulder arthroplasty: classification and treatment."We no longer recommend placing the most superior screw for metaglene fixation."

Peripheral Screw Number: Often Doesn't Matter

Reverse shoulder arthroplasty glenoid fixation: is there a benefit in using four instead of two screws? found no significant variations in micromotion and displacement between 2 screws and 4 screws

Factors contributing to glenoid baseplate micromotion in reverse shoulder arthroplasty: a biomechanical study. found that in homogeneous bone density models, increasing screws from 2 to 4 did not enhance initial baseplate stability

Glenoid baseplate fixation using hybrid configurations of locked and unlocked peripheral screws. found no differences in baseplate micromotion for 1, 2, 3, or 4 peripheral locked screws. Maximum difference between 1-locked and 4-locked screw groups was only 29 µm.

Of course these are lab studies in which the model cannot duplicate the issues surgeons face with glenoid bone of varying quality. 

It is also worth noting that a lateralized glenosphere has a greater rocking moment so may need fixation with more than two screws. See Quantifying the competing relationship between adduction range of motion and baseplate micromotion with lateralization of reverse total shoulder arthroplasty. Furthermore, most biomechanical studies report "micromotion" but don't specify whether they tested rocking/eccentric loading that causes the clinical failure mode we see clinically.


Putting It All Together: A Strategic Approach

The Fixation Hierarchy

Tier 1 - Essential (Contribute Most to Preventing Clinical Failure)

Central screw: 60-70% of total stability; must be long and engage far cortex
Inferior screws: Resist the rocking moment that causes baseplate failure; I go long on these.

Tier 2 - Important (When Safe Trajectories Available)

Anterior screws: Contribute meaningfully to fixation with low fracture risk- usually can safely maximize length of bone hole and screw length.

Tier 3 - Optional/Risk-Benefit Consideration

Superior/posterior-superior screws contribute only 8-12% to total fixation
Positioned near center of rocking loosening, contribute minimally to preventing superior rocking (the dominant failure mode)
Are associated with 40-70% of scapular spine fractures when trajectory enters spine base
I "short" the drill hole in the "worry holes", or in small patients leave them out all together.

Conclusion:

Central fixation dominates - A long central screw engaging far cortex provides 60-70% of baseplate stability.

Inferior screws prevent the failure mode that most commonly occurs - Since baseplates usually fail by rocking superiorly with inferior screw pullout, I maximize inferior screw purchase to directly address the clinical failure mechanism.

Posterior/Superior screws create risk without proportional benefit - They contribute 8-12% to total fixation but minimal resistance to superior tilt, while contributing to 40-70% of spine fractures when drill trajectories penetrate the spine base.

Maximize fixation where it prevents clinical failure (central, inferior), minimize risk where it doesn't (spine-directed trajectories). This requires intraoperative anatomic judgment based on actual baseplate position.

As we noted in our recent post on preventing acromial/scapular spine fractures, this represents another surgeon-controlled variable in a multifactorial problem. Combined with appropriate humeral positioning, CAL preservation when possible, and judicious glenosphere selection, selective screw strategy can help minimize these challenging complications.

It's about holes

Red-Breasted Sapsucker

Washington Park Arboretum

2020

Follow on twitter/X: https://x.com/RickMatsen
Follow on facebook: https://www.facebook.com/shoulder.arthritis
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/

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

Preventing Acromial/Scapular Spine Fractures in Reverse Shoulder Arthroplasty: Defining what the surgeon can control.

I'll start out with a few conclusions:

(1) Acromial and scapular spine stress fractures are clinically important complications of reverse total shoulder arthroplasty (RSA), occuring in 3-11% of cases and often resulting in poor functional outcomes such as persistent pain and limited active motion. 

(2) The principal risk factors - female sex, advanced age, poor bone quality, inflammatory arthropathy, cuff deficiency, corticosteroid use, thinned acromion from prior surgery or erosion, proximal humeral migration - are not under the control of the surgeon, except as they affect the decision to proceed with RSA surgery. 

(3) Surgeons do control humeral and glenosphere component selection and positioning. However the evidence guiding practice is not robust due to the lack of standardized nomenclature and what parameters should be measured in future clinical research. 

Here are a set of four easy to make measurements the importance of which is supported by the review below. These address the problem of uncertainty and inconsistency found in published studies regarding RSA geometry. Such measurements will be important in answering the key questions surgeons have: what component positions provide the best function and which minimize the risk of complications such as scapular stress fractures for my patients?

(1) Acromio-humeral distance measured from the acromion to the greater tuberosity along a line parallel to the bony glenoid face (both post-op and pre-op to post-op change).

(2) The perpedicular distance between the glenosphere center of rotation (COR yellow dot) and the glenoid bony face (yellow line)

(3) The perpendicular distance between the lateral extent of the glenosphere and the glenoid bony face (blue line)

(4) The perpendicular distance between the tuberosity and the glenoid bony face (long black line).

Consistent use of these measurements would address much of the current ambiguity in the literature, as illustrated by the following review.

Glenosphere lateralization 

One of the issues in reviewing the literature on "glenosphere lateralization" is a failure of many articles to define the term. Are the authors talking about lateralization of the center of rotation in relation to the glenoid bone (yellow line) or lateralization of the lateral aspect of the glenosphere in relation to the glenoid bone (blue line)? The former affects the deltoid moment arm and the range of impingement-free range of motion, while the latter contributes to the global lateralization of the humeral tuberosity (black line) which affects the soft tissue tension that is important for stabilizing the articulation through concavity compression. As seen in Know Your Glenospheres these two dimensions can be varied independently by changing the diameter of curvature of the glenosphere. The effect of the humerus on the global lateralization is the difference between the black and blue lines. 

Biomechanical studies

Implant positioning in reverse shoulder arthroplasty has an impact on acromial stresses and The effect of load and plane of elevation on acromial stress after reverse shoulder arthroplasty found that glenosphere lateralization, but not humeral lateralization, increased acromial stress.

Factors Influencing Acromial and Scapular Spine Strain after Reverse Total Shoulder Arthroplasty: A Systematic Review of Biomechanical Studies found glenoid lateralization was consistently associated with increased acromial and scapular spine strain. 

In addition, transection of the coracoacromial ligament resulted in significantly increased strains. Although preserving the integrity of the CAL is not an implant-related factor, it is a surgeon-controlled variable. Its importance is demonstrated in two basic science papers: Scapular Ring Preservation: Coracoacromial Ligament Transection Increases Scapular Spine Strains Following Reverse Total Shoulder Arthroplasty and Coracoacromial ligament integrity influences scapular spine strain after reverse shoulder arthroplasty and finally the clinical study Does Preservation of Coracoacromial Ligament Reduce the Acromial Stress Pathology Following Reverse Total Shoulder Arthroplasty? Transection of the coracoacromial ligament consistently increased scapular spine strain in biomechanical studies and was associated with higher clinical fracture rates in the 265-patient study (29.4% vs 13.2% with CAL section vs. preservation).

Clinical evidence

There is a lack of clinical studies that have actually measured the radiographic glenosphere COR lateralization and correlated it with acromial fracture risk.  

Up to 8 mm of glenoid-sided lateralization does not increase the risk of acromial or scapular spine stress fracture following reverse shoulder arthroplasty with a 135° inlay humeral component examined RSA patients categorized based on implant specifications (metallic offset from baseplate and glenosphere selection). The amount of glenoid-sided lateralization varied from 0 to 8 mm in 2-mm increments. The actual glenosphere COR lateralization with respect to the glenoid bone was not measured radiographically. 

Does isolated glenosphere lateralization affect outcomes in reverse shoulder arthroplasty? compared shoulders with the COR 2 mm lateral to the glenoid bone to those with the COR 6 mm lateral to the glenoid bone. Acromion and spine fractures were found it 3% of the 2 mm group and in 1% in the 6 mm group.

Lateralized versus nonlateralized glenospheres in reverse shoulder arthroplasty: a systematic review with meta-analysis found no difference in acromion/ spine fracture rates between RSAs catagorized as "lateralized" and "nonlateralized". Data on the difference in COR to glenoid bone distance for the two groups are not presented.

The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem did not find a fracture rate difference between lateralized and non-lateralized glenospheres. Data on the difference in COR to glenoid bone distance for the two groups are not presented

Implant-Positioning and Patient Factors Associated with Acromial and Scapular Spine Fractures After Reverse Shoulder Arthroplasty found that "excessive" glenoid-sided and global lateralization were associated with higher fracture rates; "excessive" is not defined. "Total glenoid lateral offset" was defined as the sum of lateralization contributed by the glenosphere, baseplate, and bone graft if present. Data on the relation of the glenosphere COR to the humeral bone are not presented.

Humeral position

The humerus can be moved distally by the glenosphere (inferior positioning on the glenoid bone, inferior tilt, inferior offset) and by the humerus (using an onlay component, high positioning of an inlay component). Humerus distalization can be documented in terms of postoperative position or as the change in preoperative to postoperative position. 

Different methods have been used to characterize humeral distalization making it difficult to compare studies. It seems most intuitive to directly measure acromiohumeral distance: the distance from the lateral acromion to the lateral prominence of the tuberosity along a line parallel to the glenoid face. This approach can be used both before and after RSA.

Influence of implant design and parasagittal acromial morphology on acromial and scapular spine strain after reverse total shoulder arthroplasty: a cadaveric and computer-based biomechanical analysis found that lengthening above 25 mm produced large strains in the acromion and scapular spine.

Up to 8 mm of glenoid-sided lateralization does not increase the risk of acromial or scapular spine stress fracture following reverse shoulder arthroplasty with a 135° inlay humeral component found that the change in acromiohumeral distance (delta AHD) was significantly higher in the stress fracture group. For every 1cm increase in delta AHD, there was a 121% increased risk for fracture. For every 1mm increase in inferior glenosphere overhang, there was a 19% increase in fracture risk.

The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem found "Increased postoperative distalization is associated with an increased risk of SSF after RSA." While the authors also concluded that  "An onlay stem resulted in a 10 mm increase in distalization compared with an inlay stem, and a 2.5 times increased risk of SSF. " it is apparent that what's important is not only the component design (inlay vs onlay) but also on the amount of distalization, which is influenced by both design and implant position. An onlay component can be inset in the humerus while an inlay component can be placed high with respect to the tuberosity.

Acromial Fractures in Reverse Shoulder Arthroplasty: A Clinical and Radiographic Analysis  greater arm lengthening was more common in the fracture group 

Predictive factors of acromial fractures following reverse total shoulder arthroplasty: a subgroup analysis of 860 shoulders  showed that a significant association of higher postoperative lateralization (by lateralization shoulder angle), lower distalization (by distalization shoulder angle), a lower acromiohumeral distance, and higher age were predictive only for Levy type III fractures. These result are  contradictory to other data, possibly because of small numbers (only 16 Levy III fractures) and the confounder of older age. Notably among the 860 shoulders the fracture types most clearly related to deltoid tension (Levy I and II) showed no association with any measured parameter.

The data on the effect of humeral component lateralization on acromial/spine fractures is inconclusive.

Summary: The current data are incomplete and, in many cases, inconclusive. Humeral distalization beyond 20-25mm appears to increase fracture risk (biomechanical threshold ~25mm; clinical data showing 121% increased risk per 10mm increase in delta acromiohumeral distance). Transection of the coracoacromial ligament consistently increased scapular spine strain in biomechanical studies and was associated with higher clinical fracture rates in the 265-patient study (29.4% vs 13.2% acromial pathology with CAL section vs. preservation).

Basic science data suggest that glenosphere COR lateralization may increase fracture risk, but clinical studies using categorical classifications ('lateralized' vs 'non-lateralized') without actual measurements are inconclusive. 

The effect of humeral lateralization remains unclear. 

Thus for high-risk patients (elderly women, inflammatory arthropathy, prior acromioplasty, thin acromion), limiting the change in acromiohumeral distance (delta AHD) to <20mm,  preserving the CAL, and avoiding excessive glenoid lateralization may be prudent pending better evidence—which will require standardized measurements such as those proposed above.

See also Preventing Scapular Spine Fractures: The Superior Baseplate Screw Question

Fractures

Mt. Rainier National Park

July 2024

Follow on twitter/X: https://x.com/RickMatsen
Follow on facebook: https://www.facebook.com/shoulder.arthritis
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/

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

Know your glenospheres

The geometry of reverse total shoulder (RSA) implants along with the soft tissue tension and balance are important determinants of the outcome for patients having this procedure. Surgeons need to understand the characteristics of the implants they use so they can made good decisions for each of their patients. While humeral component design and placement, soft tissue management, and the preoperative to postoperative change in acromio/humeral distance are critical variables in RSA, here we'll zone in on the glenosphere.

The nomenclature used to describe the geometry of the glenosphere can be confusing and differs among the manufacturing companies. 

Here's a simplified approach to characterizing three key elements of glenosphere anatomy that can be applied to any make of glenosphere

(1) Glenosphere diameter of curvature (blue line) 

2) Distance from center of rotation (COR) to glenoid bone (yellow line)

Note that this distance needs to include the thickness of the baseplate lying between the glenosphere and the bone, which varies by manufacterer and with the use of augments.

The "names" of the glenospheres may not be accurate descriptions their geometry. For example for the 32 mm diameter implants in one company's system, there are 32 "neutral", 32 -4, and 32 -6. Including a 3 mm thick baseplate, the distances from the COR to the glenoid bone surface are, respectively 13 mm, 9 mm, and 7 mm (not 0, -4 and -6).

The COR lateralization from bone for the 32, 36 and 40 diameter of curvature glenospheres in this system are shown below.

Note that the COR to bone distance is the same for the 32 -6 and the 40 "neutral"

3) The glenosphere's contribution to the lateralization of the humerus. This is equal to the COR lateralization from the glenoid bone + the radius of the glenosphere (shown by the blue line in the figure below). 

These values for the 32, 36, and 40 mm glenospheres are shown in the right column of the chart below.

Note that the distance from glenoid bone to the lateral aspect of the glenosphere is the same for the 32 -6, the 36 -4 and the 40 -4. 

Since even the most sophisticated preoperative planning software cannot predict which glenosphere will provide the best mobility and stability, we have to rely on intraoperative trialing to determine the best glenoid component geometry. A chart like the above is handy to guide our component trialing by pointing out the COR lateralization (which affects deltoid moment arm and range of motion) and total lateralization (which affects soft tissue tension and stability thorough concavity compression) for the different options. 

During trialing the surgeon can assess shuck when the humerus is pulled laterally, unwanted bone contact, range of motion (including extension), and stability using a variety of tests including the two hand lever test.  

A 32 -6 may be a reasonable starting point in a small patient with a tight shoulder. An average sized patient with cuff tear arthropathy might merit starting with a 32 -4. A large patent needing stability may need a 36 neutral or 40 -4.

There are many important elements of a good reverse total shoulder arthroplasty. Selecting the best glenosphere for the patient is one of them.

Making good choices

Lewis' Woodpecker

Tualatin

2020

Follow on twitter/X: https://x.com/RickMatsen
Follow on facebook: https://www.facebook.com/shoulder.arthritis
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/

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