Edge loading wear and impingement has been a cause for concern in many different forms of total joint replacement and may lead to rim fracture and the need for revision 1,12–14,28
. Although impingement has been observed in conventional total shoulder replacements, there are only sporadic reports of evidence of impingement of the polyethylene glenoid of reverse shoulders with the exception of studies on scapular notching. Nam and colleagues recently reported on 14 retrieval specimens and noted inferior quadrant polyethylene wear was the most common wear pattern18
. Although there was a strong correlation between scapular notching and inferior component deformation, there was no correlation between scapular notching and inferior component abrasive wear. They did not make a specific distinction between intended articulating surface and rim wear and did not note any association between inferior screw contact and severity of polyethylene wear. Their retrieved implants had been functioning for only 0.46 +/− .5 years. Additionally, their components were revised mainly for instability or infection, while the retrievals in our collection were revised primarily for loosening.
The predominant form of wear in our specimens was rim wear from impingement with the scapula and the inferior locking screw. There was almost no detectable wear on the bearing surface, even though the implants had been functioning for 1.9 years, on average (Range: 1.3–3.3 y). Therefore, our hypothesis that rim wear would be greater than articulating wear was correct. All of the polyethylene inserts in this study showed evidence of bony impingement at the rim. However the severity varied greatly and did not correlate with the scapular notching scores, contrary to our second hypothesis. This is likely because of the small number of cases and the relative lack of severe notching scores. However, we did note that the polyethylene rim wear was most severe in cases where the notch contacted the inferior screw. Rim damage has been mentioned in a handful of previous case reports, which mention that the polyethylene component of the retrieval displayed wear22
. However, the majority of the studies do not report on the extent of rim wear in order to compare with our results 3,7,22
. Nam and colleagues recently reported on 14 retrieved specimens in which inferior quadrant polyethylene wear was most common. However, wear on the intended articulating surface was grouped with rim wear when calculating the damage score, and therefore, separate analysis of rim and articulating surface wear was not included. Nyffeler et al. reported severe wear of the rim covering an arc of 120° and as deep as the metal tray 21
in a case study. Of particular concern are reported cases, as seen in our study, where there is evidence of wear between the inferior screw and the metal shell 15
. This wear mechanism has the potential to release metal debris, which could have a negative clinical effect, especially in patients with metal hypersensitivity.
In addition to undesired metal on metal wear, the contact of the of the humeral component on the inferior locking screw may induce a moment on the glenoid component leading to distraction of the component at the superior glenoid as most inferior screws are locked to the baseplate. Assuming that the locking mechanism with the baseplate does not fail, contact between the inferior screw and the humerus could lead to premature mechanical loosening of the glenoid baseplate from a rocking mechanism or to a fatigue fracture of the inferior locking screw. The latter mechanism was observed in only one retrieval in our collection. The broken screw in this case subsequently created abrasive wear on the articulating surface of the humeral inlay ().
There is currently debate in the literature about altering the glenoid component design and surgical technique to reduce scapular notching (and rim impingement). Factors that could potentially decrease impingement and notching include inferior placement of the glenoid sphere, decreased depth of the humeral cup, and lateralization of the glenoid center of rotation. Frankle et al., evaluated the Reverse Shoulder Prosthesis (Encore, DJO Surgical, Austin, Texas) and found zero incidence of scapular notching 8
. However, Levy and Blum have recently described a case report where they observed scapular notching in the same design16
. This design laterilizes the center of rotation 6 to 10 mm from the glenosphere. To date, our collection does not include any retrievals of this design to investigate the incidence of impingement. However, Nam and colleagues noted inferior quadrant wear in 5 implants of this type but a low incidence of radiographic notching. Despite the potential for decreased notching, some researchers are hesitant to adopt lateralization because of increases in the shear forces seen in the glenoid 3,6,17
In designs where the center of rotation is medialized to the native glenoid surface (e.g. Delta III, Delta xtend, Tornier, etc.), scapular notching has been reported as occurring in anywhere from 25% to 96% of patients 2,3,9,11,15,23–26,30,31
. The temporal progression of the severity of scapular notching still remains unclear. Previously, it was thought to occur rapidly and then become stable showing no progression 23–26,30
. Recently, it has been suggested that the severity of the notch progresses with implantation time9,15
. The clinical significance of scapular notching is also currently unknown, as studies have reported conflicting results with respect to correlating scapular notching scores with functional or pain scores 2,15,23,25,26,30
While we observed severe wear at the rim of some of our components, we did not observe an appreciable amount of wear on the intended articulating surface of the component. This was evident in the micro-CT analysis, as well as, in the observation that machining marks from manufacturing were still present on many of the articulating surfaces. In a finite element study by Terrier et al., it was predicted that reverse shoulders could generate as much as 5 times more wear than their anatomical shoulder counterparts27
. Since we did not see significant wear on the intended articulating surface of our retrievals, it is possible that the loading paradigms used in these models do not apply to patients with reverse total shoulders.
We recognize that this study has several limitations, not the least of which is our small sample size. Although this limits the power for statistical testing, we were able to document evidence of impingement on all of our retrievals. Additionally, in two of our cases, the presence of contact between the inferior screw and the humeral component was either unrecognized or under appreciated on preoperative radiographs. This highlights the need for adequate, standardized pre-operative radiographs. Our observations are also limited to a population that was revised after a short term of implantation. Since reverse total shoulder arthroplasty has only been approved for use by the FDA since 2004, longer-term retrievals are, as of yet, unavailable at our institution. Future studies will allow for the assessment of the progression of impingement in mid to long-term implantation of reverse total shoulder replacement.