Reverse Shoulder Biomechanics and Scapular Notching

Gregory Gilot, MD

The FDA approved the use of the reverse shoulder prosthesis in the United States in 2004, nearly 25 years following its re-debut in France. Combined, there are 40 years of experience addressing varying shoulder conditions once treated with non-prosthetic or unconstrained shoulder arthroplasty solutions. Surgical techniques and prosthetic designs have evolved over the years, leading to improvements in clinical outcomes, implant longevity and lower rates of complications. The earliest design, the Grammont-style reverse shoulder arthroplasty (RSA), was built on the engineering premise of a medialized center of rotation, as measured at the center of the glenosphere, in reference to the native glenoid face. While a medialized center of rotation increases the mechanical efficiency of the deltoid, it tends to result in mechanical impingement between the humeral polyethylene insert and the scapula neck. Cadaveric and biomechanical studies as well as computer simulation models have demonstrated the increased risk of the following complications: scapular notching, decreased rotation and prosthetic instability; whereas a lateralized center of rotation has demonstrated superiority in maximizing overall passive arc of motion and preventing scapular notching. In addition, the tensioned soft tissues (remaining rotator cuff and deltoid) effectually improve implant stability.^1,2

Surgical lateralization can be achieved via metal or bone. Using a thicker baseplate and/or glenosphere achieves glenoid sided lateralization while increasing joint loading. The increased compressive and shearing forces increases the potential for glenoid loosening.³ To offset the torque forces seen in metallic increase offset (MIO-RSA), bone graft can be placed under the baseplate; and once healed, theoretically, the center of rotation is lateralized and remains at the bone-implant interface.

In a study by Boileau et al, a retrospective case series of 143 shoulders were treated with a BIO-RSA for rotator cuff deficiency.⁴ It was a multicenter retrospective study with a minimum of five years’ follow-up. All patients in the study had a Grammont-style RSA with the use of autograft harvested from the humerus for bony lateralization. The mean follow-up period was 75 months, the preoperative diagnosis varied, the average age was 72 years, and 72% were women. Patients undergoing revisions or those with insufficient humeral head autograft were excluded. From a technical standpoint, a cylinder of cancellous bone was harvested from the humeral head of 7mm and 10mm thickness, for 42mm sphere and 36mm sphere, respectively, and placed on the baseplate. All baseplates were implanted with a lengthened central peg in 10° of inferior tilt.

Acromial Fracture: An Uncommon but Increasingly Preventable Complication After Reverse Shoulder Replacement

Thomas Obermeyer, MD

Read complete study: Acromial and Scapular Fractures After Reverse Total Shoulder Arthroplasty with a Medialized Glenoid and Lateralized Humeral Implant: An Analysis of Outcomes and Risk Factors

The reverse shoulder prosthesis was introduced in France in the 1980s and was FDA-approved for use in the United States in 2004. Since that time, it has revolutionized the field of shoulder replacement. The reverse shoulder prosthesis has a proven track record of providing predictable and sustainable pain relief and functional improvement for patients with a host of shoulder pathologies, ranging from osteoarthritis to rotator cuff arthropathy. The stability and functional improvements afforded by the reverse shoulder prosthesis are achieved, in part, by transitioning the large outer deltoid muscle of the shoulder into a mechanically advantaged motor of the arm. As a result of the forces generated by the deltoid muscle, high loads are transmitted to the site of origin of that muscle on the scapula leading to fractures in a rare cohort of patients. These so-called acromial fractures are not widely studied due to their infrequency and the relative heterogeneity of reverse shoulder prosthesis designs on the market. Improvements in understanding acromial fractures is important as their occurrence can impair the clinical outcome of the reverse shoulder prosthesis. A recent study by Routman et al. is, to date, the largest cohort of patients reported, which sheds light on the patient risk factors and prosthesis-related factors leading to this unfortunate and uncommon complication of reverse shoulder replacement.

“Reversomania” – Are We Operating Too Soon?

Kaveh R. Sajadi, MD

Read complete study: A 10-year experience with reverse shoulder arthroplasty: are we operating earlier?

When Dr. Paul Grammont introduced the reverse total shoulder arthroplasty (rTSA) in the late 1980s/early 1990s, it was a revolutionary treatment for a previously unsolved clinical condition, rotator cuff tear arthropathy. The original design had an unfortunately high complication rate which limited its use. Modern designs have significantly improved the complication rate as well as the success of the operation, and indications for rTSA have significantly increased. This increasing utilization has led some to coin the phrase “reversomania”, suggesting it is being done too often or perhaps too soon.

Clearly, complication rates for rTSA have decreased,¹ and the rate of rTSAs performed have significantly increased with more rTSAs now being performed than anatomic shoulder arthroplasty (aTSA).² The authors of the study, “A 10-year experience with reverse shoulder arthroplasty: are we operating earlier?”, sought to determine if, due to increased comfort and success with rTSA, surgeons are performing rTSA earlier in the disease process. In other words, has the “tipping point”, the point at which the patient’s symptoms are severe enough that the patient and surgeon elect to undergo rTSA changed, or has the threshold for performing surgery moved?

The Technique for Assessing and Correcting Glenoid Inclination May Be Implant-Specific

Moby Parsons, MD

Read complete study: The reverse shoulder arthroplasty angle: a new measurement of glenoid inclination for reverse shoulder arthroplasty

There is ample support in the literature that superior inclination of the glenoid baseplate in reverse total shoulder arthroplasty (rTSA) can lead to a higher risk of instability and premature implant failure. This is because a superiorly inclined baseplate experiences a greater shear vector imparted by the action of the deltoid muscle. Because cuff deficient arthritic shoulders often develop superior glenoid erosion, shoulder surgeons must carefully assess preoperative glenoid inclination when planning rTSA to avoid implant malposition. Preoperative planning platforms now allow surgeons virtually correct glenoid deformity while also optimizing implant fixation, backside contact with host bone, and avoidance of bone impingement such as scapular notching.

Boileau et al recently described a new measurement of glenoid inclination called the Reverse Shoulder Angle (RSA). Their contention is that referencing the more traditional ß-angle (the angle formed by a line connecting the superior and inferior glenoid face with a perpendicular to a line along the floor of the supraspinatus fossa) may inadvertently cause surgeons to superiorly incline small, flat-backed reverse baseplates when placed on the inferior half of the glenoid. Therefore, the RSA angle measures only the inferior half of the glenoid rather than the full glenoid face. This results in an average measured inclination of 25° + 8°, which is a 10° + 5° compared to the ß-angle. The authors note that Favard E1 glenoids with central erosion are at risk for baseplate malposition.

Clinical Studies Validate the Accuracy of GPS in Targeting Implant Placement in Anatomic and Reverse Shoulder Arthroplasty

Moby Parsons, MD

Read complete study: Computer navigation re-creates planned glenoid placement and reduces correction variability in total shoulder arthroplasty: an in vivo case-control study

The incidence of shoulder arthroplasty continues to increase, and this has brought continued innovation in preoperative planning and implant design. Concurrently, with improved imaging techniques, our understanding of pathologic changes that affect the joint have begun to elucidate strategies to address glenoid wear. Durability of the glenoid implant remains the weak link of shoulder arthroplasty,^2,10 and both clinical and biomechanical studies have demonstrated that excessive residual retroversion or inclination,^7,9 excessive corrective reaming,^4,5 recurrent humeral subluxation⁶ and insufficient implant contact¹ with bone are all risk factors for component loosening or failure. While augmented anatomic and reverse glenoid implants are now widely available, their use remains fairly limited relative to the frequency of pathologic wear. Guidelines on their use have also yet to be established, and a surgeon’s ability to accurately place such implants in cases with significant to extreme glenoid wear are uncertain. Prior cadaveric studies examined surgeon accuracy in recreating a preoperative plan using conventional instruments with free-hand techniques. The studies demonstrated significant variability and an average error of + 6-10° relative to the planned correction.¹¹ This indicates that “eyeballing” glenoid implant placement may lead to significant variation from the plan. Particularly in cases of moderate to severe pathologic wear, such as the Walch B2 or B3 glenoid. This inaccuracy can be the difference between long-term durability and mid-term failure.

CT-based preoperative planning is now widely available and increasingly popular as it allows surgeons to virtually plan implant selection and placement and optimize parameters such as correction of inclination and version, peg placement in the glenoid vault, backside contact and amount of corrective reaming. While this can provide a very quantitative method of glenoid reconstruction, planning alone does not guarantee proper bone preparation or implant placement without additional technology to assist surgeons in replicating the plan. While patient-specific instrumentation (PSI) can improve the accuracy of implant placement over free-hand techniques, it still has a margin of error of + 4° on average.^3,12 As studies have shown increased stresses in the cement mantle over 10° of residual version, this amount of error could potentially lead to pathologic implant malposition. Furthermore, PSI does not allow surgeons to adjust the plan intraoperatively if needed.

Stress Fractures After rTSA. Not Every Onlay is Created Equal.

Ian Byram, MD

Read complete study: Increased scapular spine fractures after reverse shoulder arthroplasty with a humeral onlay short stem: an analysis of 485 consecutive cases

Since its FDA clearance in 2003, reverse shoulder arthroplasty (RSA) has become increasingly utilized for the treatment of arthritis, fractures, and rotator cuff failures in the United States. Initial reports demonstrated a high rate of complications, including instability, impingement, scapular notching, and subsequent implant failure.¹  Many implant designs have been successfully modified to minimize these problems. While scapular notching seen with the initial Grammont style prosthesis has decreased with modern lateralized implant designs, scapular spine fractures have become more prevalent.

The authors of this study analyzed a series of RSA cases utilizing a short stem, 145° neck-shaft inclination prosthesis hoping to shed light on implant features that may lead to scapular spine fractures. In a retrospective review of 485 consecutive arthroplasties, they reported 21 (4.3%) scapular spine fractures that were diagnosed at a mean of 8.6 months after surgery. Comparing those patients with matched controls, patients with fractures demonstrated significantly worse Constant scores and forward flexion. They concluded that utilization of a lateralized “onlay” convertible humeral design creates increased stress on the deltoid and acromion, resulting in an undesirable rate of scapular spine and acromial fractures.

Scapular Fractures After RSA – The Bane of Our Existence

Stephanie Muh, MD

Read complete study: Scapular fracture in reverse shoulder arthroplasty (Grammont Style): prevalence, functional, and radiographic results with minimum 5-year follow up

The development of postoperative scapular spine or acromial base fractures remains a difficult problem to treat. With a reported incidence of 1-10% in the literature, there has been no consensus on the best treatment, and most authors report poor post-fracture functional outcomes.^1-3 Both operative and nonoperative management of these fractures have been advocated with variable outcomes.^2,3

This retrospective study presents data from a large cohort of patients with a single prosthetic design (Grammont style with medialized center of rotation and medialized humerus with inlay prosthesis). The authors found an overall prevalence of 1.3% scapular fractures in 1,953 implants. This seems to correlate with other literature where the incidence ranged from 1-10%.^1-4 When evaluating the functional results with a minimum of five years follow-up, 19 fractures were identified with an overall improved range of motion and Constant score. It is important to note, however, the patients with fractures did not improve as greatly as those without fractures. While the postoperative Constant score improved from 25.6 to 47 postoperatively, the authors note the average postoperative Constant score in a non-fracture group is 70. They did not directly compare the fracture group range of motion to non-fracture group which I think would have added valuable information. It would have been interesting if the authors presented initial improvement postoperatively and compared this to post-fracture outcomes and commented if there was function.

To Repair, or Not to Repair, That is the Question…

Kaveh Sajadi, MD

Read complete study: The effect of subscapularis repair on dislocation rates in reverse shoulder arthroplasty: a meta-analysis and systematic review

Whether ‘tis nobler in the mind to bear the slings and arrows of our colleagues…

The role of the subscapularis in reverse total shoulder arthroplasty (rTSA) continues to be debated. Reverse TSA is most commonly performed via a deltopectoral approach, which often requires takedown of an intact, though possibly degenerated or atrophic, subscapularis tendon. Surgeons debate the importance of its repair at the conclusion of the surgery. This debate centers on the importance and role of the subscapularis in stability of the prosthesis, range of motion (both internal and external rotation), and outcomes. Those in favor of repair cite studies indicating lower dislocation rates with repair and better internal rotation; those opposed express concern about it potentially limiting external rotation and possibly opposing the deltoid in elevating the arm.¹ This meta-analysis and systematic review sought to compare the dislocation rates and outcomes in rTSA with and without subscapularis tendon repair using the highest level of data available.

This meta-analysis and systematic review sought to compare the dislocation rates and outcomes in rTSA with and without subscapularis tendon repair using the highest level of data available.

It is important to clarify some descriptions used in the study. Since the introduction of the original Grammont style reverse prosthesis in the late 1980s and early 1990s, many different design modifications have been introduced.² The primary differentiating characteristic between implant designs is the location of the center of rotation (COR) of the new glenohumeral articulation. By their very nature, all reverse designs medialize the COR compared with the native shoulder. However, systems with the COR at the glenoid face, as the original Grammont design, are considered medialized and those with the COR lateral to the glenoid are referred to in this paper as lateralized designs. Furthermore, the implant can have a medialized humerus (Grammont) or a lateralized humerus. Routman HD, et al, proposed a classification to standardize the nomenclature, dividing implants into the medial glenoid/medial humerus (Grammont), lateral glenoid/medial humerus, and medial glenoid/lateral humerus.^2,3 Some of the implants in this study, classified as lateralized glenoids by the authors, are classified as medial glenoid/lateral humerus by Routman.

The Effect of Lateralization on the Rotator Cuff Following Reverse Shoulder Arthroplasty

Stephanie Muh, MD

Read complete study: Clinical Outcomes after Reverse Shoulder Arthroplasty with and without subscapularis repair: The Importance of considering glenosphere lateralization

There continues to be significant debate on whether subscapularis repair is necessary during reverse total shoulder arthroplasty. Historically subscapularis repair was felt to be necessary to provide anterior soft tissue stability and help prevent post-operative dislocation. There have been multiple articles written on both the advantages and disadvantages of subscapularis repair. It should be noted that most previous articles that strongly advocated subscapularis repair included the traditional Grammont-style prosthesis with a medialized glenosphere and valgus medialized humeral stem (155 degrees).

This article retrospectively reviews patients who underwent reverse total shoulder arthroplasty (RTSA). They first stratified the subscapularis repair group versus no repair. The authors then looked at the effects of glenosphere lateralization (0mm lateralization vs +3 or +6mm) in both groups to determine if this played a significant role. The humeral stem used in the study has a neck shaft angle of 147 degrees but a humeral stem with no lateralization. ASES score from baseline was the primary outcome measured with a minimum of two-year follow-up.

The study demonstrated patients with no subscapularis repair and lateralization had the most overall improvement in ASES scores. This supports the design rationale of the Equinoxe^® reverse shoulder system.

Overall, the authors found no difference in ASES scores when comparing subscapularis repair versus no repair. The authors also reported that subscapularis management and lateralized glenosphere individually did not have significant effect on ASES scores. However, analysis did find that patients with subscapularis repair with a lateralized glenosphere did worse compared to subscapularis repair with medialized glenosphere and no repair with lateralized glenosphere.

This article demonstrates the impact of the combined effect of subscapularis management with glenosphere lateralization on clinical outcomes. The study demonstrated patients with no subscapularis repair and lateralization had the most overall improvement in ASES scores. This supports the design rationale of the Equinoxe^® reverse shoulder system.

Revision with a Platform System

Alberto Rivera, MD

Read complete studies:

Conversion to reverse shoulder arthroplasty: humeral stem retention versus revision

Platform shoulder arthroplasty: a systematic review

The number of shoulder arthroplasty procedures is rapidly increasing. Therefore, shoulder revision is becoming a commonly performed procedure. Historically revising a hemiarthroplasty or total shoulder replacement ended up in a hemiarthoplasty, resection arthroplasty, arthrodesis or more recently reverse arthroplasty. This type of revision usually required stem removal, which could potentially lead to humeral fracture with or without the need of an osteotomy, increasing surgical time, bleeding and neural damage.  Also, late complications, such as osteotomy nonunion and malunion could develop. Another important factor to take into consideration is the added cost of using additional implants such as a new stem, cement, cables or allograft in the setting of humeral stem revision. Modular implants using a platform system allows for a faster revision with fewer complications and potentially less cost.

“Recent publication by Williams and Colleagues (1) reported on 17 patients who underwent modular conversion and nine who had revision of humeral stem. Pain, stability and ASES scores improved significantly.”

I believe the use of modular platform in primary shoulder arthroplasty either hemi or total should be the standard of care.

In my experience, revising TSA to RSA has evolved to a more straightforward procedure with the use of modular components of the platform shoulder type. I believe the use of modular platform in primary shoulder arthroplasty either hemi or total should be the standard of care.