Reverse total shoulder arthroplasty (RTSA) has become the treatment of choice for chronic pseudoparalysis of the shoulder caused by irreparable rotator cuff tears; osteoarthritis with glenoid retroversion greater than 20°; complex humeral fractures in the elderly; and unstable, hemi-, or anatomical total shoulder replacements.1–7 Complications are more common after RTSAs than after anatomical shoulder arthroplasties.6,8–10 A postoperative fracture of the acromion or the scapular spine is a complication and major concern specific to RTSA,9,11–19 having a reported incidence of 0.9% to 7.2%.17 Conservative treatment is preferred for acromial fractures.14,16,17,20 These patients generally have sustained improvement compared with their preoperative state,20 and the results with this complication have been considered comparable to those of patients with a preoperative fracture of the acromion.21 However, more recent literature indicates decreased ranges of abduction and flexion and a higher rate of dissatisfaction among patients with acromial or scapular spine fractures vs without.15
What causes acromial fractures has not been elucidated. Osteoporosis has been reported to be a risk factor.17 The posterosuperior screw fixing the base-plate has been reported to cause stress rising leading to spinal fractures.17 Lädermann et al22,23 hypothesized, but could not prove, that excessive arm lengthening after RTSA could be a risk factor. Anecdotally, however, the current authors have observed that if a RTSA is implanted in the presence of an os acromiale, the os acromiale is displaced downward in the postoperative course, whereas an inferiorly displaced os acromiale or acromial fracture is reduced when the arm is passively abducted. The same could be seen in non-operated on shoulders with an os acromiale (Figure 1). These observations are compatible with a lengthened deltoid causing increased displacing forces.
Shoulder with an os acromiale in a neutral position. Acromion–humeral distance of 4 mm (A). Shoulder with abducted arm. Acromion–humeral distance of 0 mm (B).
The purpose of this study was to identify risk factors associated with the occurrence of acromial fractures following RTSA, with special emphasis on the biomechanical alterations of the deltoid muscle. Only fractures of the acromion were studied.
Materials and Methods
The ethical committee at the authors' institution approved the acquisition of the data for this study. A total of 1146 RTSAs performed between 1999 and 2016 at a single center were retrospectively analyzed. Inclusion criteria were complete clinical follow-up and availability of a preoperative radiograph and/or computed tomography (CT) scan. Exclusion criteria were preoperative os acromiale or fracture of the acromion, a scapular spine fracture and previous fractures of the humerus, and RTSA for fracture or prosthetic revision surgery.
A symptomatic acromial or scapular spine fracture was identified in 28 patients (2.4%) during the postoperative course. There were 21 acromial (1.8%) and 7 (0.6%) scapular spine fractures. The acromial fractures were classified according to Crosby et al11 as types I to III (Figure 2). Only type I and type II fractures were included. Regarding type III, specifically scapular spine fractures were excluded because they may have had an etiology different from that of the acromial fractures. The presence of an acromial fracture was suspected when any shoulder pain occurred after RTSA, especially if it had a sudden onset after postoperative improvement and/or if the pain was localized in the region of the acromion, acromio-clavicular joint, or scapular spine. The presence of a fracture was ultimately determined through the use of conventional radiography (n=11), CT scan (n=3), or both (n=7). Computed tomography scans were performed only if the fracture could not be confirmed by conventional radiography alone.
Classification of Crosby et al11 of acromial and scapular spine fractures.
Patients with a fracture (group 1) were compared with patients without a fracture (group 2), matched for age and sex in a 1:4 ratio. In group 1, 18 shoulder anatomical inverse/reverse (Zimmer Biomet, Warsaw, Indiana) and 3 Delta III (DePuy International, Leeds, United Kingdom) systems were used. The shoulder anatomical inverse/reverse system (Zimmer Biomet) was exclusively implanted in group 2.
The mean clinical follow-up for group 1 and group 2 was 50 months (range, 4–184 months for group 1; range, 4–189 months for group 2). Clinical records were used for collecting data on demographics including age, sex, history of previous surgery, diabetes, smoking, reported osteoporosis, arm dominance, indication, surgeon, type of prosthesis, and date of surgery. Pre- and postoperative radiographs and preoperative CT scans were used for radiographic analysis. The measurements on preoperative conventional radiographs included acromial thickness in an anteroposterior and a true lateral Neer view, anterior and sagittal acromial slope, glenoid inclination (beta angle according to Maurer et al24), length of the acromion, critical shoulder angle, acromion–humeral distance (ACHD), and Hamada classification. Comparing pre- and postoperative radiographs, pre- to postoperative arm length (“distalization”), medialization of the center of rotation, lateralization of the humerus, and the “delta angle” (Figures 3–4) were measured. The delta angle is defined by a line connecting the superior and the inferior border of the glenoid fossa and a second line from the lateral acromial border through the center of rotation of the glenohumeral joint measured on anteroposterior radiographs. The preoperative angle is called delta 1 and the postoperative angle is called delta 2. The difference between these 2 angles from pre- to postoperatively was defined as the delta angle.
Pre- and postoperative measurements on plain radiographs and computed tomography scans.
Delta angle is defined by a line connecting the superior and the inferior border of the glenoid fossa and a second line from the lateral acromial border through the center of rotation of the glenohumeral joint measured on anteroposterior radiographs. The preoperative angle was called delta 1 and the postoperative angle was called delta 2. The difference between these 2 angles from pre- to postoperatively was defined as the delta angle.
Osteopenia was identified by measuring average Hounsfield units 3 times in different cross-sections of the acromion (anterior, middle, and posterior in coronal planes, only cancellous bone without cortical bone) (Figure 3) on a preoperative CT scan, similar to previous studies25–29 of the hand and the spine. Shoulders with an acetabularization (Hamada 3, 4b, and 5) were combined as the wear group. The acromio-clavicular joint was assessed for arthritis, width of the joint space, and alignment on CT scans. The acromio-clavicular joint was considered stable if the joint space was 5 mm or less and if the clavicle was not displaced more than half of the clavicular width.
Categorical variables were reported as frequencies with the corresponding percentages, whereas continuous variables were reported as mean±SD. The Kolmogorov–Smirnov test was used to test for a normal distribution of data. In the setting of a normal distribution, continuous variables were compared via independent-samples t test. A nonparametric Mann–Whitney U test was used in the case of a non-normal distribution. Categorical variables were compared via chi-square test or Fisher's exact test, when appropriate. Statistical significance was defined by P<.05. Data analysis was performed using SPSS Statistics version 21.0 (IBM Corp, Armonk, New York).
Twenty-one (1.8%) acromial fractures were identified among 1146 RTSAs. There were 4 type I and 17 type II fractures. One fracture was traumatic. Twenty were not associated with any major traumatic event and were considered chronic (insufficiency) fractures. Preoperative CT scans were unavailable for 4 patients. In 2 patients, the humeral head was completely necrotic, and medialization, distalization, and the delta angle could not be measured. A summary of the results is provided in Table 1.
The mean age of group 1 was 74 years (range, 64–83 years), with 17 (81%) women and 4 (19%) men. In group 1, the right side was affected in 12 (57%) and the left in 9 (43%) cases. The dominant limb was affected in 12 (57%) cases. In group 2, the right and the left arms were operated on in 53 (63%) and 31 (37%) cases, respectively. This corresponded to 53 (63%) dominant limbs. No significant difference was observed regarding arm dominance between group 1 and group 2 (57% vs 63%; P=.615). Acromial fractures occurred at a mean of 9 months postoperatively (range, 1–65 months). The 21 RTSAs in group 1 were performed by 9 different surgeons, with 9 (43%) cases being implanted by the senior surgeon. The RTSAs of group 2 were implanted by 12 different surgeons, with 38 (45%) cases being implanted by the senior surgeon. There was no significant difference in the occurrence of acromial fractures for the 2 types of prostheses used (anatomical reverse, 18 of 924, 1.9%; Delta III, 3 of 222, 1.4%; P=.8).
Regarding demographic data, a significantly higher rate of osteoporosis was reported at the time of implantation of the prosthesis in the fracture group compared with the control group (33% vs 13%; P=.047). Conversely, there was no preoperative difference in the bone density of the acromia between the 2 groups as measured with CT scan. The radiographic measurements showed a significant difference in the delta angle in group 1 vs group 2 (29.4°±8.1° vs 19.5°±9.7°; P<.001). Delta 1 (preoperative) was not different in the 2 groups. The interclass correlation coefficient for the delta angle was 0.839. There was a significant difference between group 1 and group 2 regarding lateralization of the humerus (4.1±7.1 mm vs 8.4±6.1 mm; P=.006), preoperative glenoid inclination (beta angle) (72.0°±5.5° vs 76.5°±6.8°; P=.005), lateral acromial slope (117.3°±11.2° vs 121.7°±17.0°; P=.044), and the pre- to postoperative difference in the glenoid inclination (beta angle) (9.2°±8.0° vs 4.4°±9.4°; P=.022). Most of the acromial fractures occurred in the prolongation of the posterior border of the clavicle (16 of 21, 76.2%) (Figure 5). On the basis of this observation, the authors focused on the stability of the acromio-clavicular joint. However, neither instability nor arthritis of this joint seem to be a risk factor for acromial fractures or specific fracture location. Shoulders with Hamada 3, 4b, and 5 were not associated with acromial fractures. Nineteen percent in group 1 and 26% in group 2 showed an acetabularization. Thus, medialization without distalization of the glenoid component as well as high preoperative upward tilt (despite intraoperative correction) were associated with acromial fractures.
The most frequent localization of the anterior extension of the acromial fracture was directly in line with the posterior border of the clavicle (16 of 21 cases).
Acetabularization (Hamada 3, 4b, and 5) and acromial thickness were not risk factors for the development of acromial fractures. A higher delta angle was associated with an increase of acromial fractures. This angle is a reflection of high medialization and little distalization of the center of rotation. The distalization of the humerus and the medialization of the center of rotation alone were not significant risk factors for acromial fractures. The authors believe that the combination of a high degree of medialization and little distalization of the center of rotation creates a higher force level in the middle part of the deltoid to elevate the arm. Because the middle part of the deltoid arises at the lateral acromion, the tension force of the acromion may increase and induce an acromial fracture. The authors are currently studying the exact biomechanics of these configurations for further analysis.
Lateralization of the humerus after RTSA seems to have a protective effect against an acromion fracture. This could be explained by a better lever arm for the deltoid and a biomechanical advantage when the humerus is lateralized during the elevation or abduction (Figure 6). That inclination of the glenoid has an influence on the biomechanics of the shoulder has previously been investigated in several studies30–33 and is reflected in the critical shoulder angle. It is unclear why patients with a higher inclination of the glenoid preoperatively (lower beta angle) and a lower inclination of the glenoid (larger beta angle) during RTSA have an increased risk for fracture.
The lever arm of the deltoid muscle is increased when the humerus, respectively the greater tuberosity, is lateralized. With an increased lever arm, less muscle force is required to achieve the same movement.
Osteoporosis has been described as a risk factor for acromial fractures by Otto et al.17 The current data confirm this finding. If patients have osteoporosis, there is a higher risk for acromial fractures. Contrary to this, there was no difference between the groups when measuring the Hounsfield units on the preoperative CT scans of the acromion. Sex is most likely connected to the higher prevalence of osteoporosis in females.
This study had several limitations. Only preoperative CT scans were available. Postoperative CT scans were not available for all of the acromial fractures. This was a retrospective study. Therefore, most of the radiological measurements were performed on plain radiographs. The data of an in-house study reveal the limitations of plain radiography in this context.34 Another limitation was the heterogeneity of the 2 groups. While all patients were included in the fracture group, independent of previous shoulder implants or humeral fractures, patients with these findings were excluded from the control group.
The risk of acromial fracture was not influenced by the distalization of the humerus or the glenoidal acetabularization. There was an increase of acromial fracture with a high delta angle, less lateralization of the humerus by the RTSA, the inclination of the glenoid component after RTSA, and a lower anteroposterior slope of the acromion preoperatively. Also, documented preoperative osteoporosis was a risk factor.
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|Variable||Patients With Fractures (n=21)||Control Patients (n=84)||P|
|Age, mean±SD (range), y||73.8±6.0 (64.0–83.0)||73.8±5.9 (64.0–83.0)||.994|
|Acromion, mean±SD (range), mm|
| Thickness||8.9±2.2 (5.0–14.0)||9.5±1.8 (5.0–14.0)||.216|
| Sagittal thickness||8.3±1.8 (5.5–12.0)||8.8±1.8 (5.5–14.0)||.272|
| Length base to tip||55.9±5.0 (48.0–66.0)||54.6±5.1 (42.0–70.0)||.316|
|Beta angle, mean±SD (range)|
| Preoperative||72.0°±5.5° (58.0°–81.0°)||76.5°±6.8° (54.0°–90.0°)||.005|
| Postoperative||81.2°±8.2° (63.0°–93.0°)||80.9°±9.2° (60.0°–111.5°)||.875|
| Difference||9.2°±8.0° (−3.0°−22.0°)||4.4°±9.4° (−23.0°−31.5°)||.022|
|Slope, mean±SD (range)|
| Coronal||−0.4°±6.2° (−11.0°−11.0°)||−4.1°±7.1° (−29.0°−8.0°)||.052|
| Sagittal||117.3°±11.2° (93.0°–135.0°)||121.7°±17.0° (54.0°–170.7°)||.044|
|ACHD, mean±SD (range), mm||6.4±3.3 (0.0–12.0)||7.0±4.1 (0.0–24.0)||.490|
|Hounsfield units, mean±SD (range)||139.4±61.2 (41.0–262.0)||186.4±123.7 (17.0–660.0)||.250|
|Lateralization, mean±SD (range), mm|
| Preoperative||47.4±6.0 (35.5–59.5)||46.9±5.8 (32.0–63.0)||.677|
| Postoperative||51.1±5.3 (43.0–62.5)||55.2±5.8 (40.0–68.0)||.005|
| Difference||4.1±7.1 (−8.0–17.0)||8.4±6.1 (−8.0–27.0)||.006|
|Distalization, mean±SD (range), mm||24.1±8.0 (13.0–37.5)||21.3±7.9 (2.0–38.0)||.171|
|Medialization, mean±SD (range), mm||17.6±4.2 (10.0–26.0)||18.8±6.2 (1.0–34.0)||.236|
|Delta 1 preoperative observer 1, mean±SD (range)||19.6°±10.9° (4.0°–44.0°)||20.7°±13.8° (1.0°–88.0°)||.842|
|Delta 2 postoperative observer 1, mean±SD (range)||49°±8.9° (22.0°–64.0°)||40.2°±10.8° (11.5°–78.0°)||.059|
|Delta difference observer 1 (delta angle), mean±SD (range)||29.4°±8.1° (14.0°–42.5°)||19.5°±9.7° (−8°–37.5°)||<.001|
|Delta 1 preoperative observer 2, mean±SD (range)||20.4°±12.7° (2.0°–42.0°)||23.5°±14.1° (2.0°–78.0°)||.549|
|Delta 2 postoperative observer 2, mean±SD (range)||46.3°±8.4° (20.0°–55.0°)||40.8°±11.0° (15.0°–89.0°)||.446|
|Delta difference observer 2 (delta angle), mean±SD (range)||25.9°±9.4° (9.0°–45.0°)||17.3°±11.5° (−19.0°–37.5°)||.006|
|CSA, mean±SD (range)||33.5°±4.6° (25.0°–44.0°)||33.6°±6.0° (20.0°–63.0°)||.942|
|AC width, mean±SD (range), mm||3.3±3.4 (0.0–12.0)||2.5±2.5 (0.0–19.0)||.643|
| Cuff tear||90%||76%|
|History of prior surgery||29%||29%||1.000|