Modified notching classification in inverted-bearing and conventional-bearing reverse total shoulder arthroplasty

Abstract

Background: Scapular notching is a common complication of reverse total shoulder arthroplasty (RTSA). Although the notching rate has reduced significantly thanks to modifications to the surgical technique and humeral and glenoid components, uncontrollable polyethylene (PE)-induced osteolysis can still occur. In contrast to conventional-bearing (CB-RTSA), inverted-bearing RTSA (IB-RTSA) systems, with PE glenospheres and metal or ceramic humeral liners, avoid PE abrasion through scapulohumeral contact. If present, the resulting notch has a different size and configuration than with CB-RTSA. Thus, a modified notching classification seems reasonable. Even in CB-RTSA, the early stages of notching show a different configuration than implied by the established Nerot-Sirveaux classification; therefore, a modification of the classification system is recommended.

Methods: In a prospective multicentre study, 250 cases underwent IB-RTSA. Of these, 39 died, and 28 were lost to follow-up for other reasons, leaving 183 shoulders for the final follow-up examination at a mean of 120.7 months (range 84.1-172.4 months). In the CB-RTSA group, we retrospectively evaluated radiographs of 59 consecutive patients with a follow-up ranging from 2 to 7 years. We analysed the appearance, evolution over time, and location of bone loss on the scapular neck according to the modified Nerot-Sirveaux classification.

Results: In IB-RTSA, notching resulted in a new morphological configuration: In contrast to the Nerot-Sirveaux classification, notching began far from the baseplate as an impression or abrasion of the humeral component in the inferior scapular rim. Due to simultaneous mechanical ablation, the defect gradually enlarged, but usually did not contact the baseplate or extend beyond the inferior peg. No signs of PE-induced osteolysis were found. The notching rate reached 19% after 2 years and approached 36% after 10 years. Most shoulders had grade 0 notching (64%), followed by grade 1 (29%); extensive erosion (grade 4) was not observed. In the CB-RTSA group, a small bony impression or erosion without signs of PE-induced osteolysis was observed medial to the scapular neck, without contact with the metaglene in the early phase, which was similar to the impression of the humeral component in IB-RTSA. The notching rate was 86% for CB-RTSA.

Conclusions: A new classification was developed for IB-RTSA, which corresponds to the pathophysiological processes and is compatible with the existing Nerot-Sirveaux classification. Bone erosion in CB-RTSA is not adequately represented by the established classification in the early stages. For this reason, a supplement to the existing classification is presented.

Level of evidence: Level 4, Case Series, Treatment Study.

Keywords: Complications; Inversed total shoulder arthroplasty; Inverted-bearing RTSA; Notching classification; PE-induced osteolysis; Reversed shoulder arthroplasty; Scapular notching.

Fig. 1

Notching classification by Nerot–Sirveaux. See text for explanation

Fig. 2

Typical case of scapular notching with inverted-bearing reverse total shoulder arthroplasty. Anteroposterior radiographs taken preoperatively and at 6, 12, 24, 48, and 120 months postoperatively. Scapular notching starts at 1 year postoperatively, with sclerotic lines developing at a later follow-up time point. In the immediate surrounding area of the notch, an increasing density of the bone structure can be recognised over time. The boundary line of the notch is smooth. In addition, it appears as if the trabeculae in the immediate vicinity are aligned in the direction of the notch, particularly clearly in the image after 7 years

Fig. 3

Typical course over of 10 Years with scapular notching Grad 1 in IB-RTSA (new classification see Fig. 6)

Fig. 4

a Example with extension of the notch to the inferior peg without metal contact in inverted-bearing reverse total shoulder arthroplasty. A sharp borderline and a densification of the bone in the surrounding area can also be seen. b Extension of the notch to the inferior peg with minor metal erosion in the form of the removal of the superficial structure (“threads”) can be observed in the case of a patient with a previous acromial fracture

Fig. 5

Contact of the humeral liner with the scapular rim (left image, red circle) leads to a lift-off of the joint components away from each other (right image, lateral arrow), creating a slight subluxation. Due to this decoupling, the contact zone dots to the new centre of rotation (red circle). Since a fixed centre of rotation is no longer given, translations between the humeral inlay and the scapular neck can occur with repeated movement (medial arrow)

Fig. 6

a–d Modified notching classification for inverted-bearing reverse total shoulder arthroplasty grades 1 to 3, see text for explanation (especially missing grade 4)

Fig. 7

a, b Two examples of initial notching in conventional-bearing reverse total shoulder arthroplasty. a 80-year-old female with cuff tear arthropathy, b 60-year-old female with rheumatoid arthritis. With this bearing couple, notching also begins in the area of contact of the humeral inlay with the scapular rim and not laterally under the baseplate

Fig. 8

a, b Examples of excessive grade 4 notching in in conventional-bearing reverse total shoulder arthroplasty with progression over 5 years. In addition, suspected polyethylene-induced osteolysis also around the humeral shaft in (a)

Fig. 9

Proposal of a modification of the notching classification according to Nerot and Sirveaux in conventional-bearing reverse total shoulder arthroplasty: grade 1A depicts the initial mechanically induced impression and/or erosion at the contact zone between the humeral liner and the scapular rim. The previous grade 1 would then have to be labelled grade 1B