Of successful mammoth hunting and glenoid component failure - modeling causation

 

The goal of the Shoulder Arthroplasty Failure Research initiative is to reduce the risk of future patients having a surgical revision for failure of their shoulder arthroplasty.

It is apparent that this goal cannot be accomplished simply by classifying the failure (glenoid component failure, instability, infection, etc).  Such a classification does not generate actionable intelligence that can be used to reduce the rate of failure. Rather we need to identify and seek to address the factors contributing to each type of failure.

In his "Book of Why"

Pearl has set out a compelling approach to the study of causation based on a model that identifies the factors that may affect the outcome of interest. 

He gives the example of a mammoth hunt taking place 10,000 years ago. Here's my modification of the figure in his book.

Note that the design of the model does not attempt to predict the relative importance of each of the factors on a given day, (1) because cumulative data (i.e. experience) from past hunts progressively modifies the weight attached to each factor and (2) because these factors vary from hunt to hunt (e.g. rain vs sunshine).

Note also that the items listed on the left are not under the hunters' control, but they do affect the chances of a successful hunt and may modify the hunter-controlled factors on the right (a bigger mammoth may indicate the need for more and more experienced hunters). In other words, this causal diagram helps the hunters imagine a plan for a hunt that has yet to take place.

Let's now do some time travel to today where we confront the fact that about one in ten shoulder arthroplasties requires a surgical revision. Published articles indicate that a leading indication for surgical revision is glenoid component failure  Reducing the risk of future glenoid failures requires a causal model analogous to that for the mammoth hunt, which identifies factors that could contribute to revision for a failed glenoid component.

This causal model provides a framework enabling the investigating surgeon to (1) evaluate the role that each factor (both modifiable and non-modifiable) may have played in each case of glenoid component failure and (2) asses the counterfactual argument: "if this factor had not been present, would the chances of glenoid component failure have been reduced?" As in the case of the mammoth hunt, cumulative data from past failures progressively modifies the weight that can be attached to each factor.

The bad news is that there are a lot of factors to consider in each case (my current count is 25, but I'd appreciate the reader adding to the list). The good (?) news is that there are lots of cases of glenoid failure out there from which we can collect the data that will inform future practice.

While the opportunity to improve the chances of a successful mammoth hunt has passed

The opportunity to reduce the risk of patients having surgical revision for a failed glenoid component is clear and present.

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Follow on facebook: https://www.facebook.com/shoulder.arthritis
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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).

Recent JBJS article on failures (51%) of anatomic total shoulder arthroplasties

Patients and surgeons are interested in the causes and prevention of shoulder arthroplasty failures. Failures can be viewed in terms of the characteristics of the surgeon, the patient, the shoulder, and type of failure. Regardless of the cause of the failure, it is the patient (not the surgeon or the implant company) that bears the consequences. Thus, in each case we should do our best to determine "what could have been done to prevent the failure experienced by the patient?" In this way, each failure - whether ours or someone else's - becomes a learning opportunity. At an early morning meeting in August 2025, a small group of shoulder surgeons took the first steps in developing a Shoulder Arthroplasty Failure Research (SAFR) program with the goal of learning from individual cases of arthroplasty failure.

The June, 2025 JBJS article, High Failure Rates of Polyethylene Glenoid Components in Stemless Anatomic Total Shoulder Arthroplasty for Primary and Secondary OA, is interesting to consider in this regard.  These components were used in this series

At a mean followup of 72 months, out of 197 patients, over half (101) had failures necessitating surgical revision, 86 because of glenoid component loosening. What might be done to prevent these failures? 

Left:  Intraoperative view following explantation of the polyethylene glenoid with a 4 x 2-cm bone defect. Right: Explanted components with severe glenoid wear and superior glenoid abrasion, with the pegs completely separated from the body of the glenoid component.

Is the problem (as the title might suggest) the use of a stemless humeral component with a polyethylene glenoid component (in which case failures could be avoided by using a stemmed humeral component)? 

Or is it the type of polyethylene glenoid component being used (in which case a different glenoid component design could be used)? 

Or is it the technique by which the glenoid component was inserted (in which case greater attention could be directed at the quality of glenoid component preparation, cementing and seating?). 

These three elements are surgeon-controlled variables (in contrast to patient age, sex, diagnosis, BMI, critical shoulder angle and lateral acromial angle, which were measured in this study but which are of lesser interest in that they are not modifiable by the surgeon). 

So...is the stemless humeral component or the glenoid component the problem? Prior studies of this glenoid component (with either stemless or stemmed humeral components) reported glenoid component loosening rates between 25% and 100% after 5 years when used with either stemless or stemmed humeral components: Univers II shoulder prosthesis: a multicenter, prospective randomized controlled trial and Radiologic midterm results of cemented and uncemented glenoid components in primary osteoarthritis of the shoulder: a matched pair analysis. Perhaps failure could be avoided with a different glenoid component.

Or...might the technique of glenoid component insertion be an issue?

In this 6 week postoperative x-ray from the article there is cement between the back of the glenoid component and the glenoid bone (yellow arrows).  Cement in this location can crack and displace leaving the component unsupported. 

This problem can be avoided by placing the component directly on the properly reamed glenoid bone without interposed cement as shown below.

The other issue shown on this x-ray is the large amount of bone in the glenoid vault surrounding the pegs (red arrow) that prevents bone ingrowth around them.

This problem can be avoided by careful preparation of the glenoid bone with good carpentry so that bone ingrowth is enabled.

Perhaps the risk of failure could be reduced using a different insertion technique

As shown below, this patient experienced glenoid component failure

How could this failure have been prevented?

Comment: This is an example of the type of analysis that we hope to carry out in the Shoulder Arthroplasty Failure Research program. Stay tuned!

Looking to make wise choices

Barred Owl

Union Bay Natural Area

May 2025

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).

Managing the loose glenoid component after anatomic total shoulder arthroplasty.

While total shoulder arthroplasty provides excellent long-term clinical outcomes in the treatment of glenohumeral arthritis with an intact rotator cuff, symptomatic loosening of the glenoid component is a recognized complication (see Glenoid component failure in total shoulder arthroplasty). However, as emphasized in Glenohumeral osteoarthritis - what type of arthroplasty has the lowest 10 year revision rate?, the cumulative revision rate for glenoid loosening is actually quite low (<2% at 12 years after arthroplasty) when a glenoid component with cross linked polyethylene is used.




Optimizing the technique of glenoid arthroplasty may further reduce the rate of glenoid component loosening (see The glenoid component in total shoulder arthroplasty: getting it done right).

When glenoid loosening does occur, What is the Optimal Management of a Loose Glenoid Component after Anatomic Total Shoulder Arthroplasty.? These authors conducted a systematic review of the available literature to determine the published outcomes for different management strategies addressing symptomatic glenoid loosening.

14 studies met their inclusion criteria with 483 patients having a mean age of 67. The times of followup and the rates of re-operation for four different surgical approaches are shown below. Note that these re-revision rates are ten times higher than the glenoid revision rate for primary total shoulder arthroplasty.

Glenoid bone grafting during revision arthroplasty was performed in 335 of the 483 patients (69%).

The humeral component was explanted in 291 instances (64%); 161 out of 164 (98%) patients having  conversion to an RSA required removal of the humeral component. Six studies reported at least 1 intraoperative complication, with the most common complication being humeral fracture (shaft or greater tuberosity) during stem removal (40 of the 44 total intraoperative complications reported).

The most common repeat surgery following conversion to hemiarthroplasty was conversion back to a total shoulder arthroplasty with glenoid reimplantation.

The most common repeat surgery following conversion to 1-stage TSA was conversion to a reverse shoulder arthroplasty.

The most common repeat surgery following conversion to RSA was conversion to a hemiarthroplasty, typically because of baseplate failure.

Among the 14 studies included in this systematic review, only 4 studies mention obtaining cultures during revision arthroplasty. Positive cultures were noted in 15% of the patients; Cutibacterium accounted for 72% of the positive cultures.

Comment: The management of a patient with a loose anatomic glenoid needs to be tailored to the characteristics of the patient and the shoulder. 

Some patients with a loose glenoid component are insufficiently symptomatic to justify a revision surgery, especially in light of the one in five chances of a re-revision.

In others, a conservative stem-preserving procedure (avoiding the risk of humeral fracture) may be indicated consisting of glenoid component removal with exchange of the humeral head to a size and eccentricity that conforms to the residual glenoid bone as shown in the examples below.

One or two stage re-implantation of a glenoid component or revision to a reverse total shoulder can be considered in certain cases considering the associated risks and benefits.

Hopefully the need for revision for glenoid component loosing will continue to decrease with attention to the details of component selection, positioning and fixation  (see this link)

You can support cutting edge shoulder research that is leading to better care for patients with shoulder problems, click on this link.

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Follow on twitter: https://twitter.com/shoulderarth
Follow on facebook: click on this link
Follow on facebook: https://www.facebook.com/frederick.matsen
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).

Glenoid component failure after total shoulder arthroplasty

Glenoid component failure remains a most important cause of failure after total shoulder arthroplasty (see Glenoid component failure in total shoulder arthroplasty).  Glenoid component failure may be related to 

(1) poor surgical technique, such as inadequate seating of the component (see The radiographic evaluation of keeled and pegged glenoid component insertion), the use of back-side cement to compensate for inadequate glenoid bone preparation 

or poor cement technique

(2) inferior polyethylene leading to wear and particulate debris (see this link), 

(3) metal backed components (see Metal-Backed Glenoid Components Have a Higher Rate of Failure and Fail by Different Modes in Comparison with All-Polyethylene Components: A Systematic Review

(4) poor humeral component placement

(5) instability or cuff failure giving rise to rocking horse loosening from eccentric loading of the glenoid component (see Glenoid loosening in total shoulder arthroplasty. Association with rotator cuff deficiency)

and (6) infection (see Loose glenoid components in revision shoulder arthroplasty: is there an association with positive cultures?)

Glenoid component failure may become clinically evident years after the index procedure as in this case that came to revision 24 years after the index procedure.

 

An interesting question is "how should we monitor our patients for the possibility of glenoid component loosening?" which, as the example above shows, may become manifest years or decades after the arthroplasty.  Obviously having patients come back annually or periodically for x-rays is impractical for them and for us. Since clinical failure is more relevant than radiographic lucent lines, one approach is to send short questionnaires such as the Simple Shoulder Test annually to each patient and to be on the watch for deterioration in comfort and function as discussed here, Patient Functional Self-assessment in Late Glenoid Component Failure at three to eleven years after Total Shoulder Arthroplasty.

The surgical management of a loose glenoid component requires a thorough evaluation of the shoulder pathoanatomy, shoulder function, possibility of infection, and wishes of the patient.

In some cases, arthroscopic glenoid component removal with concurrent culturing of five tissue specimens for Cutibacterium and other organisms can be considered as described here Arthroscopic Glenoid Removal for Symptomatic Component Loosening in Anatomic

Total Shoulder Arthroplasty: Can it Work?

However, this approach does not allow for revision of the humeral head component to compensate for the loss of the glenoid component, which can be accomplished at open revision as shown below.

Finally, consideration can be given to conversion to a reverse total shoulder recognizing the technical difficulty and complication rate as described here, 

Glenoid loosening and failure in anatomical total shoulder arthroplasty: is revision with a reverse shoulder arthroplasty a reliable option?

You can support cutting edge shoulder research that is leading to better care for patients with shoulder problems, click on this link.

Follow on twitter: https://twitter.com/shoulderarth

Follow on facebook: click on this link

Follow on facebook: https://www.facebook.com/frederick.matsen

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).

Treatment of glenoid component loosening in anatomic shoulder arthroplasty

Revision Reverse Shoulder Arthroplasty forAnatomical Glenoid Component Loosening Was Not Universally Successful A Detailed Analysis of 127 Consecutive Shoulders

These authors point out that glenoid component loosening is a primary cause of failure of anatomical total shoulder arthroplasty (aTSA) and is commonly associated with glenoid bone loss. 

They evaluated the outcome and survival following revision to a reverse total shoulder arthroplasty (RSA) for the treatment of loosening of a polyethylene cemented glenoid component in the setting of failed aTSA in 127 shoulders.

The mean age at the time of surgery was 70 years (range, 41 to 93 years). In all cases, the humeral component was revised and a standard glenoid baseplate was utilized. Bone graft was used at the discretion of the treating surgeon. The mean duration of follow-up was 35 months (range, 24 to 84 months).

Overall, revision to RSA resulted in significant improvements in terms of pain and motion, however on average patients did not achieve over 90 degrees of elevation.

Sixteen shoulders (13%) underwent re-revision surgery for the treatment of baseplate loosening. 

Radiographic baseplate loosening was present in 6 additional shoulders (overall rate of baseplate loosening, 17%). 

Intraoperative fracture or fragmentation of the greater tuberosity occurred in 30 shoulders (24%). 

Other reoperations included resection for deep infection (3 shoulders), arthroscopic biopsies for unexplained persistent pain (2 shoulders), humeral tray exchange for dislocation (2 shoulders), revision for humeral loosening (1 shoulder), irrigation and debridement for hematoma (1 shoulder), and internal fixation of periprosthetic fracture (1 shoulder) (overall reoperation rate, 20%). 

Among shoulders with surviving implants at the time of the most recent follow-up, pain was rated as none or mild in 83 shoulders (65.4%) and the average active elevation and external rotation were 132 and 38, respectively. 

The authors conclude that loosening of glenoid components following aTSA remains a challenging problem. While revision to RSA is a viable treatment option, great care must be taken to ensure primary stability of the reverse baseplate. As a result, they have become more cautious when considering revision to RSA in shoulders with severe glenoid bone loss and consider arthroscopic glenoid removal or revision to an appropriately sized hemiarthroplasty as alternative options for the treatment of this challenging clinical problem in patients with severe glenoid bone loss.

Another recent article, The effect of glenoid bone loss and rotator cuff status in failed anatomic shoulder arthroplasty after revision to reverse shoulder arthroplasty also evaluated outcomes and the risk of re-revision in patients with a failed anatomic total shoulder arthroplasty (aTSA) that were revised to a reverse shoulder arthroplasty (RSA) based on rotator cuff deficiency and glenoid bone loss. They found that the overall re-revision rate was 11.4%, with a mean time to re-revision of 22 months (range, 0-89 months). The odds ratio was 1.786 for subsequent revision in patients with glenoid loosening compared with those without loose glenoids on preoperative radiographs.

Comment: We have found that many patients with glenoid component failure can be safely and effectively managed by glenoid component removal, smoothing of the residual glenoid bone and inserting a humeral head component with a large diameter of curvature. Here are three example cases.

and a fourth

So, as emphasized by these authors, a patient with a failed glenoid aTSA glenoid component should not reflexively be treated by revision to a reverse. In selected individuals, safer alternatives may exist.

How you can support research in shoulder surgery Click on this link.

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).

Total shoulder arthroplasty - failure of the glenoid component used with a stemless humeral component

 Glenoid Component Loosening in Anatomic Total Shoulder Arthroplasty: Association between Radiological Predictors and Clinical Parameters—An Observational Study

These authors studied 147 patients with primary osteoarthritis who underwent aTSA with a stemless humeral implant and a pegged glenoid between January 2011 and December 2016.

  

Anteroposterior radiographs were evaluated at six, 12, 24 months, and five years post-TSA for lateral humeral offset (LHO), joint gap (JG), acromiohumeral distance (AHD), andradiolucency (modified Franklin score); patients were included. 

36% of patients had "severe systemic disease".

Over half of the glenoids were type A

All pegs were cemented. Over half of the glenoid components were the small size

The five x-rays below show progressive loss of joint gap and osteolysis around the humeral and glenoid component. 

The average changers in the radiographic parameters is shown below.

The changes in clinical scores over time are shown below with a drop off at 5 years. Both constant score (CS) and subjective shoulder value (SSV) markedly decreased at five years follow-up compared to one year (p < 0.001 for both). 

AHD, LHO, and JG all showed a consistent and statistically significant decline over time, with the joint gap decreasing by half.  Consistently, smaller joint gap and acromiohumeral distance were correlated with lower subjective shoulder values (p = 0.03 and p = 0.07, respectively).The relationship between the AHD, offset and joint gap to the degree of glenoid loosening and clinical scores are shown below. Massive loosening was associated with a 14.5 points lower SSV (p < 0.01). Narrowing of the joint gap was significantly correlated with increased radiolucency (p < 0.001) and tended toward worse SSV (p = 0.06).

The authors concluded that radiographic parameters displaying medialization and cranialization after aTSA with a cemented pegged glenoid are useful predictors of impaired shoulder function.

Comment: In this study there are many factors that could have influenced the radiographic and clinical outcomes, including serve systemic disease in over one third, the status of the rotator cuff, the degree of cross linking of the polyethylene, the glenohumeral pathoanatomy, the degree of glenoid reaming, the use of a small glenoid component, the cement technique and the placement of the humeral component. Note that in the example given in the article, the humeral component was placed superiorly in relation to the "perfect circle". Superior positioning of the humeral component could lead to eccentric wear of the superior glenoid component (A recent article, Radiographic humeral head restoration after total shoulder arthroplasty: does the stem make a difference? reported, "Restoration of humeral anatomic parameters occurred significantly less with stemless implants than with short- and standard-stem implants. The stem of a shoulder arthroplasty implant aids surgeons in accurately restoring patient-specific anatomy.")

Thus it would be of interest to know which pf these eight factors were related to loss of the joint gap and to superior migration of the humeral head. It would also be of interest to know what percentage of the 211 shoulders were surgically revised.

How you can support research in shoulder surgery Click on this link.

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).