Correction of the CSA does not decrease the re-tear rate after SSP repair

The re-tear rate after supraspinatus tendon (SSP) repair is high despite optimization of surgical techniques. The critical shoulder angle (CSA) > 35° has been identified as a risk for re-tear. Surgical reduction of the CSA performing additional lateral acromioplasty (ALAP) is intended to reduce the risk. We aimed to evaluate the effect of ALAP on the re-tear rate in patients with a CSA > 35° after SSP repair. The study included 49 shoulders fulfilling predefined inclusion criteria. The patients were assigned to two groups; group I (25 SSP) was additionally treated with an ALAP. In group II (24 SSP) standard acromioplasty was performed during reconstruction. Magnetic resonance imaging and a clinical examination (Constant Score; CS) were performed 12 months after surgery. After 15 months group I showed a re-tear rate of 20%, a decreased CSA (mean 38.0° preoperatively vs. 34.5° postoperatively) and an increase in CS (mean 53.9 vs. 84.6). After 16 months group II showed a re-tear rate of 16.7%, no difference between pre- and postoperative CSA (37.8° vs. 37.6°), and an increase in CS (48.9 vs. 84.5). There was no difference between the groups in terms of re-tears and postoperative CS (p = 0.4557), but there was a significant difference in postoperative CSA (p < 0.0001). We did not find a decrease in the failure rate after ALAP. Unlike other studies, patients with a postoperative CSA > 35° did not have a worse clinical outcome and no increased rate of re-tears compared with patients with a mechanically reduced CSA. Additive lateral acromioplasty does not decrease the re-tear rate after arthroscopic SSP repair. However, ALAP significantly decreases the CSA to normal values.


Background
The re-tear rate after arthroscopic rotator cuff repair is high despite advances in surgical techniques [2,19,23]. In particular, age, tear size, retraction of the tendon, and fatty infiltration of the muscle determine the outcome after arthroscopic rotator cuff repair [5,13,24]. Recently, the critical shoulder angle (CSA) was described as a risk factor for both a rotator cuff tear as well as a re-tear after arthroscopic rotator cuff repair [7,10,16,22].
The CSA was initially characterized by Moor et al. [16]. It describes the angle between the line of the lateral superior and inferior bony landmarks of the glenoid and the line between the lateral margin of the acromion and the inferior rim of the glenoid in a true anteroposterior radiograph (. Fig. 1). Moor et al. showed that patients with a CSA > 35°h ave a greater risk for a supraspinatus tendon (SSP) tear.
A lower CSA (<30°) was associated with a greater risk for osteoarthritis [16].
Further research has found a connection between the CSA and rotator cuff tears, in particular with an SSP tear [4,17,18]. Biomechanical research indicates that a higher CSA could lead to destabilizing joint forces during thoracohumeral abduction.
An increased SSP load may compensate the destabilization and the higher shear forces [9], which can result in more stress for the SSP.
There is debate regarding the optimal treatment of the CSA [6,8,11,22]. Questions are how to treat a high CSA, whether the CSA needs to be surgically corrected, if this correction changes the biomechanics for the benefit of the SSP, and, most importantly from a patient's point of view, whether the clinical and radiological outcome can be improved by changing the CSA.
The purpose of this study was to evaluate the effect of an additional lateral acromioplasty (ALAP) on postoperative CSA, tendon integrity, and clinical outcome after arthroscopic rotator cuff repair.
Our hypotheses were that: (1) ALAP decreases the CSA, (2) ALAP increases tear integrity, and (3) ALAP leads to better clinical outcome measured by the Constant Score (CS).

Study design and patients
A total of 48 patients with 49 treated SSPs were included after approval of the institutional ethics committee (No. 02/17). One patient had both shoulders treated for an SSP tear within 5 months. Patients with a CSA > 35°, a mechanical outlet impingement, and a rotator cuff tear were divided into two groups depending on their treating surgeon (S.L., M.L.). Inclusion criteria were age < 70 years, no comorbidities (diabetes, rheumatoid predisorder, neurological disorders), no previous shoulder surgery on the same side, no osteoarthritis of the shoulder, atrophy of the SSP muscle < stage III, and written informed consent [26]. All patients had a lesion of the SSP. In total, 15 patients had an additional partial tear of the infraspinatus tendon and seven patients had an additional partial tear of the subscapularis tendon. The preoperative diagnostics included magnetic resonance imaging (MRI), evaluation of the CS, strength measurement in 90°abduction and 30°horizontal flexion, and a true anteroposterior radiograph.
The CSA was calculated from a true anteroposterior radiograph using the technique described by Moor et al. [16].

Surgical technique
Arthroscopic rotator cuff repair with subacromial decompression was performed y one of the two senior surgeons. The 25 patients of group I received an ALAP to reduce the CSA, while the 23 patients (24 shoulders) of group II received standard acromioplasty. An ALAP was used to reduce the CSA by a 10-mm arthroscopic resection of the lateral acromion. Other pathologies such as tendinitis of the long biceps tendon, SLAP lesion, subscapularis partial tear, or infraspinatus partial tear were repaired if required.
All patients received standardized physiotherapy. There were two rehabilitation protocols. The shoulder was immobilized in an abduction cushion (30°abduction) for 3 weeks in the case of a single tendon reconstruction. If more than a single tendon repair was performed, immobilization duration was 6 weeks. For the first 6 weeks, only passive motion was allowed. Afterwards, active exercising was possible.

Postoperative evaluation
Within 2 days of surgery, a true anteroposterior radiograph was obtained to evaluate the CSA and the positions of the anchors. The follow-up to evaluate the effects of the ALAP took place at the earliest after 12 months. The CS, strength measurement, and the range of motion were used for clinical evaluation. Tendon integrity was evaluated by MRI using the classification by Sugaya [23]. Three patients were evaluated with ultrasound due to claustrophobia. The MRI and ultrasound examinations were assessed by the three senior surgeons (S.L., M.L., M.S.).

Statistical analysis
Prism 8 for macOS (Version 8.4.3, Graph-Pad Software, San Diego, CA USA) was used for statistical analysis. Mean and standard deviation (SD) were used to summarize the data. To evaluate the preand postoperative differences (CSA, CS) in the same group, a Wilcoxon matchedpairs signed rank test was used. A Mann-Whitney U test was performed to evaluate differences in age, follow-up time, CSA, and CS between the groups and for the comparison in postoperative strength, pain, CS, and CSA between patients with healed and non-healed tendons. A two-tailed p value of < 0.05 was used to assess a possible significant difference. To evaluate the difference in healing rates of the two groups, Fischer's exact test was performed. Multiple logistic regression analysis was used to determine whether there are associations between tendon integrity and age, tear size, postoperative CSA, follow-up time, or group assignment. The significance level was set at p < 0.05.

Abstract
Background. The re-tear rate after supraspinatus tendon (SSP) repair is high despite optimization of surgical techniques. The critical shoulder angle (CSA) > 35°has been identified as a risk for re-tear. Surgical reduction of the CSA performing additional lateral acromioplasty (ALAP) is intended to reduce the risk. Objective. We aimed to evaluate the effect of ALAP on the re-tear rate in patients with a CSA > 35°after SSP repair. Patients and methods. The study included 49 shoulders fulfilling predefined inclusion criteria. The patients were assigned to two groups; group I (25 SSP) was additionally treated with an ALAP. In group II (24 SSP) standard acromioplasty was performed during reconstruction. Magnetic resonance imaging and a clinical examination (Constant Score; CS) were performed 12 months after surgery. Results. After 15 months group I showed a re-tear rate of 20%, a decreased CSA (mean 38.0°preoperatively vs. 34.5°postoperatively) and an increase in CS (mean 53.9 vs. 84.6). After 16 months group II showed a re-tear rate of 16.7%, no difference between preand postoperative CSA (37.8°vs. 37.6°), and an increase in CS (48.9 vs. 84.5). There was no difference between the groups in terms of re-tears and postoperative CS (p = 0.4557), but there was a significant difference in postoperative CSA (p < 0.0001). We did not find a decrease in the failure rate after ALAP. Unlike other studies, patients with a postoperative CSA > 35°did not have a worse clinical outcome and no increased rate of re-tears compared with patients with a mechanically reduced CSA. Conclusion. Additive lateral acromioplasty does not decrease the re-tear rate after arthroscopic SSP repair. However, ALAP significantly decreases the CSA to normal values.

Keywords
Rotator cuff · Supraspinatus · Critical shoulder angle · Re-tear · Additional lateral acromioplasty  Fig. 2). The postoperative CSA of patients in group I was significantly lower than that in group II (p < 0.0001, . Table 1, . Fig. 2).
Multiple logistic regression analysis to analyze the effect of various variables (explanatory variables) on the probability of a re-tear (tendon integrity) showed no significant effects for age, adjacent tear of infraspinatus and/or subscapularis tendon, postoperative CSA, follow-up time, or group assignment (p > 0.05).

Discussion
The results suggest that an ALAP has no effect on the re-tear rate of the SSP after Recent studies reported re-tear rates between 15.3% and 28.5% [2,3,7,19,21]. Two of these studies evaluated the CSA and found an absence of tendon healing in 25% and 28.5% of their patients after rotator cuff repair and showed that patients with healed tendons had a lower CSA than patients with non-healed tendons. Subsequently, a higher CSA was categorized as a risk factor for re-tears [7,21]. Further, a CSA > 35°was described as a risk factor for SSP tears [4,16]. Since the CSA has been identified as a risk factor for tears and re-tears of the SSP, there are considerations to reduce the CSA by using ALAP [1,8,12]. Altintas et al. described no discontinuity in deltoid origin when performing the ALAP in a cadaveric in vitro study [1]. Further, Gerber et al. and Tauber et al. undertook a treatment study to show that ALAP does no harm to the deltoid origin or the deltoid muscle and does not alter its function [8,25]. Gerber et al. performed an arthroscopic rotator cuff repair and ALAP on patients with degenerative, full-thickness rotator cuff tears and a CSA > 33°. No dehiscence or increase in fatty infiltration of the deltoid muscle was observed during the follow-up with MRI [8]. Tauber et al. performed a lateral acromioplasty together with a rotator cuff repair on 20 patients. They reported good clinical results, no re-tear, and a reduced postoperative CSA [25]. These findings support the assumption that ALAP does not have a negative effect on the deltoid origin and it reduces the CSA. Further, Gerber et al. described a significantly larger CSA (36°v s. 34°) among patients with non-healed tendons.
This is the first study to evaluate the ALAP in patients with a CSA > 35°and an SSP tear. An overall failure rate of 18.4% is comparable to similar studies. Following the results, there is no recommendation for ALAP when performing an arthroscopic SSP repair on patients with a CSA > 35°.
Overall, 20% of the patients treated with ALAP and 16.7% of the patients without surgical treatment of the CSA suffered a re-tear. In this collective, a higher postoperative CSA was not a risk factor for a re-tear. The control group in this study had a mean postoperative CSA slightly lower than 38°. Multiple logistic regression analysis showed no association between a higher postoperative CSA and tendon integrity. Patients with re-tears had equal results in terms of pain, strength, and CS in comparison with those with healed tendons at the follow-up. Besides not showing an effect on tendon integrity, ALAP did not improve the postoperative clinical outcome. These results are in contrast to the findings of previous studies on CSA [7,8]. Garcia et al. reported that patients with a higher CSA had a worse clinical outcome.
The strength in the group of patients with intact tendons was larger but there was no significant difference between the groups. This is in line with the findings of other studies on re-tear rates and risk factors [19,24]. However, the present results are contradictory to other studies, which support the theory that ALAP has a positive effect on tendon integrity. A possible explanation for this might be the relatively short follow-up time used in the present study. The studies by Gerber et al. and by Garcia et al. had a mean follow-up of 30 and 26.1 months, respectively [7,8]. The mean overall followup of our study was only 15.9 months. Within this short time frame, ALAP did not show a positive effect. This is in line with the findings of Tashjian et al., who described a negative correlation between duration of follow-up with healed tendons at follow-up [24].
Due to the complex pathogenesis of a re-tear and various risk factors that influence the healing, the CSA might not have a strong impact on re-tears. Including more patients with radiological and clinical examinations will follow to

Limitations
One limitation of this study is the overall study size. Group samples of 25 and 24 patients are acceptable but there is a lack of power. The tear sizes were not matched, which can lead to bias in a small group. A larger sample size might have led to more significant effects between both the groups and the healing rates. As data collection is ongoing, further results will be published.
Furthermore, patients in group I (ALAP) were significantly older than those in group II. Patient age has also been described as a risk factor for a retear [5,14,15,20]. Multiple logistic re-gression analyzing a possible effect of age on the re-tear rate showed no association between age and tendon integrity.
The allocation of patients to the groups depended on the treating surgeon and was thereby not randomized. This results in a level of evidence of III.
There was only one independent surgeon who measured the CSA in the radiographs, which leads to potential subjective bias.
Another limitation is the short duration of follow-up, which might be correlated with a higher rate of healed tendons. This could be reversed by a longer follow-up [24]. Further studies will evaluate whether ALAP has a positive longterm effect on tendon healing in patients with a CSA > 35°after an arthroscopic SSP repair. Finally, the difference in tear size should also be mentioned. In group I, 15 patients had an isolated SSP tear and in group II, there were 12 isolated SSP tears. A multiple regression analysis that was performed to observe a possible effect of the tear size on the re-tear rate showed no significant association between tear size and tendon integrity.

Practical conclusion
4 Additional lateral acromioplasty decreases the critical shoulder angle to standard values. 4 No positive effect of additional lateral acromioplasty on the tendon integrity could be found. 4 Additional lateral acromioplasty does not increase the clinical outcome as measured by the Constant Score.
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