Subacromial Surgery for Irreparable Posterosuperior Rotator Cuff Tears.

The irreparable posterosuperior rotator cuff tear describes a tear of the supraspinatus and/or infraspinatus tendon that is massive, contracted, and immobile in both the anterior-posterior and medial-lateral directions. Patients with an intact subscapularis and preserved forward elevation are challenging to treat because there is not a consensus treatment algorithm. For low-demand, elderly patients, several subacromial surgical options are available that can provide pain relief without the risks or burden of rehabilitation posed by reverse total shoulder arthroplasty or a complex soft-tissue reconstruction (e.g., superior capsular reconstruction, tendon transfer, bridging grafts). Debridement, more specifically the "smooth-and-move" procedure, offers a reliable outcome with documented improvements in pain and function at long-term follow-up. Similarly, the biodegradable subacromial balloon spacer (InSpace; Stryker, Kalamazoo, MI) has been shown to significantly improve pain and function in patients who are not responsive to nonoperative treatment. Disease progression with these options is possible, with a small percentage of patients progressing to rotator cuff arthropathy. Biologic tuberoplasty and bursal acromial reconstruction are conceptually similar to the balloon spacer but instead use biologic grafts to prevent bone-to-bone contact between the humeral head and the acromion. Although there is no single gold standard treatment, the variety of surgical techniques allows patients and surgeons to effectively manage these challenging situations.

Criteria, Timing, and Factors Associated With Return to Competitive Sport After Rotator Cuff Surgery.

Rotator cuff pathology in competitive athletes is common and may produce chronic symptoms and joint disability, impairing sports participation and leading to premature retirement. Athletes are a high-functioning patient population with unique characteristics. Decision-making for return to sport is a complex and multifactorial process. Literature is sparse and does not provide precise guidelines to assist physicians to make the right decision. This review aimed to highlight factors affecting outcome, timing, and criteria for return to competitive sport after rotator cuff surgery to help physicians to clearly counsel athletes and make high-quality decisions for return to sport.

Rotator Cuff Tendinopathy: Pathways of Apoptosis.

Rotator cuff repair is usually successful, but retear is not uncommon. It has been previously identified that there is a higher incidence of apoptosis in the edges of the torn supraspinatus tendon. A prospective cohort study was conducted with 28 patients-14 rotator cuff tear patients, 5 instability patients, and 9 Anterior cruciate ligament reconstruction patients to determine whether there was any increase in several genes implicated in apoptosis, including Fas receptor (FasR), Fas ligand, Aifm-1, Bcl-2, Fadd, Bax, and caspase-3. There was a significant expression of Bax (P=0.2) and FasR (P=0.005) in the edges of torn supraspinatus tendons, and in intact subscapularis tendons, there was a significant expression of caspase-3 (P=0.02) compared with samples from the torn supraspinatus tendon (P=0.04). The cytochrome c pathway, with its subsequent activation of caspase-3, as well as the TRAIL-receptor signaling pathway involving FasR have both been implicated. The elevated expression of Bax supported the model that the Bax to Bcl-2 expression ratio represents a cell death switch. The elevated expression of Bax in the intact subscapularis tissue from rotator cuff tear patients also may confirm that tendinopathy is an ongoing molecular process.

The Fate of the Shoulder Post Rotator Cuff Repair: Biomechanical Properties of the Supraspinatus Tendon and Surrounding Structures.

The study aimed to describe the changes in biomechanical properties of the supraspinatus tendon, deltoid muscle, and humeral head post arthroscopic rotator cuff repair using shear wave elastography. Shear wave velocity of the tendon, deltoid, and humeral head of 48 patients was measured at predetermined sites at 1 week, 6 weeks, 12 weeks, 6 months, and 12 months post repair. One-way ANOVA with Tukey's correction and Spearman's correlation were performed. Mean±SEM healing tendon stiffness, adjacent to tendon footprint, increased from 1 week (6.2±0.2 m/s) to 6 months (7.5±0.3 m/s) and 12 months (7.8±0.3 m/s) (P<0.001). Mean±SEM deltoid muscle stiffness was higher at 12 months (4.1±0.2 m/s) compared to 1 week (3.4±0.1 m/s) and 12 weeks (3.5±0.1 m/s) (P<0.05). Humeral head stiffness did not change. Following arthroscopic rotator cuff repair, supraspinatus tendon stiffness increased in a curvilinear fashion over 6 months. From 6 months, deltoid muscle stiffness increased, corresponding to when patients were instructed to return to normal activities.

Arthroscopic rotator cuff repair combined with platelet-rich plasma products can reduce the rate of retearing and improve clinical outcomes: A meta-analysis of randomized controlled trials.

BACKGROUND: Although several studies on the potential benefits of protein-rich plasma (PRP) therapy for rotator cuff injuries have been published, the results have been conflicting. Therefore, this study aimed to determine whether PRP is beneficial for the prevention of retears after arthroscopic rotator cuff repair (ARCR). METHODS: Two reviewers conducted independent literature searches based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Randomized controlled trials (RCTs) comparing a PRP treatment group with a control group were included. The quality of evidence was assessed using the Cochrane Collaboration Risk of Bias Tool. Clinical outcomes were compared using the risk ratio (RR) for dichotomous variables and weighted mean difference (WMD) for continuous variables. Statistical significance was set at P < .05. RESULTS: This review included 21 RCTs (1359 patients). Significant results were noted in favor of PRP treatment compared with controls based on retearing rates (16.5% vs 23.6%, respectively; P = .002) and the Constant score in the short term (WMD: 1.98; 95% confidence interval [CI], 0.27-3.70; I2 = 0%; P = .02), medium term and long term (WMD: 2.56 [95% CI: 1.57-3.55]; I2 = 2%; P < .001); the University of California, Los Angeles score in the short term (WMD: 1.14 [95% CI: 0.43-1.85]; I2 = 25%; P = .002) but not in the medium and long term (WMD: 0.66 [95% CI: -0.16 to 1.48]; I2 = 57%; P = .11); and the visual analog scale score in the short term (WMD: -0.63 [95% CI: -0.83 to-0.43]; I2 = 41%; P < .001), medium and long term (WMD: -0.12 [95% CI: -0.19 to-0.05]; I2 = 0%; P = .008). There was no significant difference in American Shoulder and Elbow Surgeons scores between the treatment and control groups in the short term (WMD: -0.48 [95% CI: -2.80 to 1.85]; I2 = 22%; P = .69) or medium and long term (WMD: 0.92 [95% CI: -1.56 to 3.39]; I2 = 40%; P = .47). CONCLUSION: Intraoperative use of PRP reduces the risk of rotator cuff repair failure, improves clinical outcomes, and reduces recurrence rates.

Translational Research on Orthobiologics in the Treatment of Rotator Cuff Disease: From the Laboratory to the Operating Room.

Rotator cuff disease is one of the most common human tendinopathies and can lead to significant shoulder dysfunction. Despite efforts to improve symptoms in patients with rotator cuff tears and healing rates after rotator cuff repair, high rates of failed healing and persistent shoulder morbidity exist. Increasing interest has been placed on the utilization of orthobiologics-scaffolds, cell-based augmentation, platelet right plasma (platelet-rich plasma), and small molecule-based strategies-in the management of rotator cuff disease and the augmentation of rotator cuff repairs. This is a complex topic that involves novel treatment strategies, including patches/scaffolds, small molecule-based, cellular-based, and tissue-derived augmentation techniques. Ultimately, translational research, with a particular focus on preclinical models, has allowed us to gain some insights into the utility of orthobiologics in the treatment of rotator cuff disease and will continue to be critical to our further understanding of the underlying cellular mechanisms moving forward.

When is a Reverse Shoulder Arthroplasty Indicated for a Rotator Cuff Tear?

Rotator cuff tears are the most frequent shoulder injury in patients over 50 years old. Treatment of rotator cuff tears is individualized based on the age, demands, and symptoms of the patient. Nonoperative treatment may often be effective, especially in lower-demand or elderly patients. Surgical repair is indicated after failure of nonoperative treatment or in patients whose high-demand lifestyle requires full strength and function of the shoulder. In older patients, larger tears are less likely to heal and rotator cuff repair may not reliable. In cases of failed previous rotator cuff repair, there may not be enough viable tissue for complete repair. If there is underlying arthritis, a rotator cuff repair is unlikely to resolve all symptoms, and treatment to care for the arthritis and rotator cuff tear is indicated. Reverse shoulder arthroplasty provides the surgical answer for these scenarios and is now being used for numerous indications of shoulder pathology.

Factors affecting medium-term patient satisfaction after arthroscopic repair of small to medium-sized rotator cuff tears: An observational study.

This study aimed to assess the effect of the status of the tendon and patient factors on patient satisfaction after rotator cuff repair. Forty-six patients treated for tears with a minimum of 5-year follow-up were included. Gender, age, and active smoking status were recorded. Pain visual analogue scale, American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form, Simple Shoulder Test, and Single Assessment Numeric Evaluation were recorded preoperatively and at the last follow-up. Patients were divided into groups of highly satisfied (HS) and vaguely satisfied (VS) patients. Patients were evaluated with MRI both preoperatively and at their last follow-up. Of the 46 patients, 17 were HS and 29 were VS. The HS group had 7 re-ruptures, 4 of which were progressed tears, whereas the VS group had 15 re-ruptures, 4 of which were progressed tears. There was no difference in the rate of re-ruptures or progressed tears between groups. The HS group had a higher frequency of males. However, frequencies of active smoking or osteoarthritis of grade 2 or higher were lower in the HS group. It was shown that patient satisfaction after repair depends on patient-related factors like gender and smoking rather than tendon healing or degeneration.

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis.

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

Platelet-rich plasma in the management of rotator cuff tendinopathy.

Platelet-rich plasma (PRP) is an autologous blood-derived product that has gained significant attention in the treatment of tendinopathies as one of many treatment modalities in the field of regenerative medicine. Rotator cuff pathologies are the leading cause of shoulder pain. Despite the high success rate with conservative treatment and arthroscopic surgical repair, the search for a nonsurgical treatment has made PRP a trending topic recently. The purpose of this article is to review the recent literature data on the possible role of PRP in different rotator cuff pathologies. The limited data in the literature about isolated tendinopathies without a tear (tendonitis and tendinosis) suggests potential benefit from conservative treatment with PRP. Many studies exist for the treatment of rotator cuff tears solely using PRP or as an augmentation to arthroscopic repair. Despite the large number of randomized clinical trials and meta-analyses about the topic, there is no consensus in favor of routine application of PRP in this patient population. The existing reliable evidence of benefit seems to be restricted to its use in only inflammatory tendinopathies with preserved tendon integrity.

The subacromial bursa modulates tendon healing after rotator cuff injury in rats.

Rotator cuff injuries result in more than 500,000 surgeries annually in the United States, many of which fail. These surgeries typically involve repair of the injured tendon and removal of the subacromial bursa, a synovial-like tissue that sits between the rotator cuff and the acromion. The subacromial bursa has been implicated in rotator cuff pathogenesis and healing. Using proteomic profiling of bursa samples from nine patients with rotator cuff injury, we show that the bursa responds to injury in the underlying tendon. In a rat model of supraspinatus tenotomy, we evaluated the bursa's effect on the injured supraspinatus tendon, the uninjured infraspinatus tendon, and the underlying humeral head. The bursa protected the intact infraspinatus tendon adjacent to the injured supraspinatus tendon by maintaining its mechanical properties and protected the underlying humeral head by maintaining bone morphometry. The bursa promoted an inflammatory response in injured rat tendon, initiating expression of genes associated with wound healing, including Cox2 and Il6. These results were confirmed in rat bursa organ cultures. To evaluate the potential of the bursa as a therapeutic target, polymer microspheres loaded with dexamethasone were delivered to the intact bursae of rats after tenotomy. Dexamethasone released from the bursa reduced Il1b expression in injured rat supraspinatus tendon, suggesting that the bursa could be used for drug delivery to reduce inflammation in the healing tendon. Our findings indicate that the subacromial bursa contributes to healing in underlying tissues of the shoulder joint, suggesting that its removal during rotator cuff surgery should be reconsidered.

Clinical and cost-effectiveness of individualised (early) patient-directed rehabilitation versus standard rehabilitation after surgical repair of the rotator cuff of the shoulder: protocol for a multicentre, randomised controlled trial with integrated Quintet Recruitment Intervention (RaCeR 2).

INTRODUCTION: Despite the high number of operations and surgical advancement, rehabilitation after rotator cuff repair has not progressed for over 20 years. The traditional cautious approach might be contributing to suboptimal outcomes. Our aim is to assess whether individualised (early) patient-directed rehabilitation results in less shoulder pain and disability at 12 weeks after surgical repair of full-thickness tears of the rotator cuff compared with current standard (delayed) rehabilitation. METHODS AND ANALYSIS: The rehabilitation after rotator cuff repair (RaCeR 2) study is a pragmatic multicentre, open-label, randomised controlled trial with internal pilot phase. It has a parallel group design with 1:1 allocation ratio, full health economic evaluation and quintet recruitment intervention. Adults awaiting arthroscopic surgical repair of a full-thickness tear are eligible to participate. On completion of surgery, 638 participants will be randomised. The intervention (individualised early patient-directed rehabilitation) includes advice to the patient to remove their sling as soon as they feel able, gradually begin using their arm as they feel able and a specific exercise programme. Sling removal and movement is progressed by the patient over time according to agreed goals and within their own pain and tolerance. The comparator (standard rehabilitation) includes advice to the patient to wear the sling for at least 4 weeks and only to remove while eating, washing, dressing or performing specific exercises. Progression is according to specific timeframes rather than as the patient feels able. The primary outcome measure is the Shoulder Pain and Disability Index total score at 12-week postrandomisation. The trial timeline is 56 months in total, from September 2022. TRIAL REGISTRATION NUMBER: ISRCTN11499185.

What are the Factors that Impact the Outcomes of Arthroscopic Rotator Cuff Repair?

BACKGROUND: The cuff is a complex structure with many factors affecting it. How much it is affected after repair is still being investigated. AIM: The aim of this study was to analyze the functional results of individuals who received arthroscopic rotator cuff repair and assess the various factors that could impact these outcomes. METHODS: The study included 57 patients with a mean age of 58.8 years (range: 39-71) who underwent arthroscopic rotator cuff repair between 2013 and 2020, with a minimum of 6 months after the operation. Functional and clinical outcomes of the patients were evaluated using preoperative and postoperative scores (QDASH, ASES, and VAS). The study also analyzed how demographic factors, type and duration of the tear, comorbidities such as diabetes mellitus (DM) and hypertension (HT), and other pathologies may have affected the patients' scores. RESULTS: The effect of the operation on the scores in rotator cuff tears was highly significant (P < 0.05). Age, gender, tear size, fatty degeneration, anchor configuration, comorbidities, and additional procedures presented variable values on postoperative scores, but were not statistically significant. The effect of DM and HT on VAS scores was statistically significant (P < 0.05). CONCLUSION: The results of rotator cuff repair appear to be influenced by several factors, including the sex of the patient, type and duration of tear, comorbidities, and surgical procedures used. Although these factors had an effect on the scores, they were not statistically significant.

Socioeconomic and Other Risk Factors for Retear after Arthroscopic Surgery for Nontraumatic Rotator Cuff Tear.

Background and Objectives: Few studies have investigated the socioeconomic factors associated with retear after rotator cuff repair. This study aimed to identify the risk factors, including socioeconomic factors, for rotator cuff retear in patients who underwent arthroscopic rotator cuff repair. Materials and Methods: This retrospective study included 723 patients diagnosed with full-thickness rotator cuff tears who underwent arthroscopic rotator cuff repair from March 2010 to March 2021. The outcome variable was rotator cuff retear observed on postoperative magnetic resonance imaging or ultrasonography. Sex, age, obesity, diabetes, symptom duration, and tear size were the independent variables. Socioeconomic variables included occupation, educational level, type of medical insurance, and area of residence. We compared patients with and without retear and estimated the effects of the independent factors on retear risk. Results: The mean age of the patients, symptom duration, and tear size were 62.4 ± 8.0 years, 1.8 ± 1.7 years, and 21.8 ± 12.5 mm, respectively. The age, type of medical insurance, diabetes, tear size, and symptom duration differed significantly between patients with and without retearing (p < 0.05). Age, occupation, type of medical insurance, diabetes, initial tear size, and symptom duration significantly affected the risk of retear. Patients who performed manual labor had a significantly higher retear rate (p = 0.005; OR, 1.95; 95% CI, 1.23-3.11). The highest retear risk was seen in patients with Medicaid insurance (p < 0.001; OR, 4.34; 95% CI, 2.09-9.02). Conclusions: Age, initial tear size, and symptom duration significantly affect retear risk after arthroscopic rotator cuff repair. Occupation and type of medical insurance were also risk factors for retear. Socioeconomically vulnerable patients may be at a greater risk of retear. Proactive efforts are required to expand early access to medical care.

Leukocyte-Poor Platelet-Rich Plasma as an Adjuvant to Arthroscopic Rotator Cuff Repair Reduces the Retear Rate But Does Not Improve Functional Outcomes: A Double-Blind Randomized Controlled Trial.

BACKGROUND: Whether the use of PRP as an adjuvant of rotator cuff repairs leads to improved tendon healing and better functional outcomes remains unclear in clinical evidence. PURPOSE: The main purpose of this study was to assess whether the use of leukocyte-poor platelet-rich plasma (LP-PRP) as an adjuvant to arthroscopic rotator cuff repair (ARCR) decreases the rate of retears compared with a control group. The secondary objective was to analyze whether LP-PRP improves patient-reported outcomes. STUDY DESIGN: Randomized controlled trial; Level of evidence, 1. METHODS: This was a double-blind randomized controlled trial at a single center. A consecutive series of 96 patients with rotator cuff tears <3 cm were enrolled and randomly allocated to the control group (double-row suture-bridge ARCR alone [n = 48]) and the PRP group (double-row suture-bridge repair, followed by 1 LP-PRP injection during surgery [n = 48]). The visual analog scale (VAS) for pain, the American Shoulder and Elbow Surgeons (ASES) score, the Single Assessment Numeric Evaluation (SANE), and the Pittsburgh Sleep Quality Index (PSQI) were administered preoperatively and at 6- and 12-month follow-up. Magnetic resonance imaging (MRI) was performed to evaluate tendon integrity at 6-month follow-up. Both patients and assessors were blinded to the intervention received during surgery. RESULTS: The mean patient age was 56.1 ± 2.98 years. Of the 96 patients, 90 had MRI performed at 6 months after surgery (94% radiological follow-up). The retear rate in the PRP group was 15.2% (7/46 [95% CI, 6%-28%]), which was lower than that in the control group of 34.1% (15/44 [95% CI, 20%-49%]) (P = .037). Therefore, the risk ratio of ruptures in patients exposed to LP-PRP was 0.44 (95% CI, 0.2-0.9; P = .037). Overall, the ASES, VAS, SANE, and PSQI scores showed a statistical improvement after surgery (P < .001). There were no significant differences in functional scores between the groups. Most of the patients exceeded the minimal clinically important difference for the ASES, SANE, and VAS without significant differences between the groups. CONCLUSION: In patients with rotator cuff tears <3 cm undergoing double-row suture-bridge repair, a 5-mL dose of LP-PRP injected at the tendon-bone interface significantly reduced the retear rate. However, the use of LP-PRP in terms of postoperative pain and patient-reported outcomes failed to show clinically meaningful effects. REGISTRATION: NCT04703998 (ClinicalTrials.gov identifier).

Arthroscopic cuff repair: footprint remnant preserving versus debriding rotator cuff repair of transtendinous rotator cuff tears with remnant cuff.

BACKGROUND: In transtendinous full thickness rotator cuff tears (FTRCT) with remnant cuff, conventionally, cuff remnant of the greater tuberosity (GT) is debrided for better tendon to bone healing. However, larger cuff defect caused overtension on the repaired tendon. The purpose of this study was to compare the clinical outcomes and tendon integrity between remnant preserving and remnant debriding cuff repairs in the transtendinous FTRCT with remnant cuff. METHODS: From March, 2012 to October, 2017, a total of 127 patients who had the transtendinous FTRCT with remnant cuff were enrolled in this study. Rotator cuff tears were repaired arthroscopically, with patients divided into two groups: group I (n = 63), where rotator cuff remnants were preserved during the repair, and group II (n = 64), where the remnants were debrided during the repair. Clinical outcomes were assessed at the last follow-up (minimum 2 years) using the UCLA score, ASES score, SST score, Constant Shoulder score, and range of motion (ROM). The analysis of structural integrity and tendon quality was performed using the Sugaya classification on postoperative MRI scans at 8 months after surgery. RESULTS: At the final follow-up, UCLA, ASES, SST, and CS scores significantly improved from preoperative values to postoperative (all p < 0.05): UCLA (I: 19.6 ± 6.0 to 31.7 ± 3.2, II: 18.0 ± 5.7 to 31.5 ± 3.2), ASES (I: 54.3 ± 10.7 to 86.5 ± 12.5, II: 18.0 ± 5.7 to 85.8 ± 12.4), SST (I: 5.6 ± 2.8 to 10.2 ± 2.0, II: 5.0 ± 2.9 to 10.1 ± 2.5), CS (I: 74.0 ± 17.2 to 87.8 ± 9.7, II: 62.0 ± 19.2 to 88.3 ± 6.2). However, there were no significant differences between the two groups (p > 0.05). Also, remnant preserving cuff repair yielded significantly better tendon quality on postoperative MRI (p < 0.05). The incidence of re-tear (Sugaya's Type IV and V) was not significantly different between the two groups (I:17% vs. II:19%; p = 0.053). CONCLUSIONS: Remnant preserving rotator cuff repairs, which facilitate tendon-to-tendon healing, are superior in terms of tendon quality and are the preferred option for transtendinous FTRCT. TRIAL REGISTRATION: Retrospectively registered.

Surgeon Factors and Trends Associated With the Use of Subacromial Decompression at the Time of Rotator Cuff Repair.

PURPOSE: Subacromial decompression (SAD) has historically been described as an essential part of the surgical treatment of rotator cuff disorders. However, investigations throughout the 21st century have increasingly questioned the need for routine SAD during rotator cuff repair (RCR). Our purpose was to assess for changes in the incidence of SAD performed during RCR over a 12-year period. In addition, we aimed to characterize surgeon and practice factors associated with SAD use. METHODS: Records from two large tertiary referral systems in the United States from 2010 to 2021 were reviewed. All cases of RCR with and without SAD were identified. The outcome of interest was the proportion of SAD performed during RCR across years and by surgeon. Surgeon-specific characteristics included institution, fellowship training, surgical volume, academic practice, and years in practice. Yearly trends were assessed using binomial logistic regression modeling, with a random effect accounting for surgeon-specific variability. RESULTS: During the study period, 37,165 RCR surgeries were performed by 104 surgeons. Of these cases, 71% underwent SAD during RCR. SAD use decreased by 11%. The multivariable model found that surgeons in academic practice, those with lower surgical volume, and those with increasing years in practice were significantly associated with increased odds of performing SAD. Surgeons with fellowship training were significantly more likely to use SAD over time, with the greatest odds of SAD noted for sports medicine surgeons (odds ratio = 3.04). CONCLUSIONS: Although SAD use during RCR appears to be decreasing, multiple surgeon and practice factors (years in practice, fellowship training, volume, and academic practice) are associated with a change in SAD use. CLINICAL RELEVANCE: These data suggest that early-career surgeons entering practice are likely driving the trend of declining SAD. Despite evidence suggesting limited clinical benefits, SAD remains commonly performed; future studies should endeavor to determine factors associated with practice changes among surgeons.