Patterns of Age-Associated Degeneration Differ in Shoulder Muscles

Older age is associated with decreased overall shoulder strength but not direction-specific differences in the three-dimensional feasible torque space

Shoulder strength is reduced in older adults but has only been assessed in planar motions that do not reflect the diverse requirements of daily tasks. We quantified the impact of age on strength spanning the three degrees of freedom relevant to shoulder function, referred to as the feasible torque space. We hypothesized that the feasible torque space would differ with age and expected this age-effect to reflect direction-specific deficits. We measured strength in 32 directions to characterize the feasible torque space of the shoulder in participants without shoulder pain or tendinous pathology (n = 39, 19-86 years). We modeled the feasible torque space for each participant as an ellipsoid, computed the ellipsoid size and direction-specific metrics (ellipsoid position, orientation, and shape), and then tested the effect of age on each metric. Age was negatively associated with ellipsoid size (a measure of overall strength magnitude; -0.0033 ± 0.0007 (Nm/kg)/year, p < 0.0001). Contrary to our expectation, the effect of age on the direction-specific metrics did not reach statistical significance. The effect of age did not differ significantly between male and female participants. Three-dimensional strength measurements allowed us to constrain the direction of participants' maximum torque production and characterize the entire feasible torque space. Our findings support a generalized shoulder strengthening program to address age-related shoulder weakness in those without pain or pathology. Clinical exam findings of imbalanced weakness may suggest underlying pathology beyond an effect of age. Longitudinal studies are needed to determine the positive or negative impact of our results.

Risk factors for intracellular fatty accumulation in rotator cuff muscle: a histologic analysis

BACKGROUND

Fatty accumulation in rotator cuff muscles has been associated with shoulder dysfunction, risk of repair failure, and poor postoperative outcomes. This study sought to assess risk factors associated with true fatty accumulation based on histologic analysis and determine whether preoperative function directly correlated with this fatty rotator cuff accumulation.

METHODS

Supraspinatus muscle biopsy specimens obtained prospectively from patients undergoing arthroscopic rotator cuff repair were stained with LipidTOX to quantify lipid accumulation. Two-step cluster analysis with Goutallier classification was used to define the fatty and non-fatty rotator cuff groups. We further performed a receiver operating characteristic curve analysis to confirm the group cutoff values.

RESULTS

In total, 51 patients (aged 60.1 ± 10.5 years) were included. There were 19 high-grade partial tears, 10 small tears, 7 medium tears, 10 large tears, and 5 massive tears. Both cluster and receiver operating characteristic curve analyses yielded a cutoff value of 30% LipidTOX/4',6-diamidino-2-phenylindole (DAPI) separating the fatty vs. non-fatty groups. In the univariate analysis, patients with fatty rotator cuffs were aged 63.2 years on average compared with 59.7 years in the non-fatty group (P = .038). Female patients made up 57.1% of the fatty cohort, which was statistically higher than the non-fatty group (P = .042). Massive and large tears were more likely to occur in the fatty group (P = .005). In the multivariate analysis, full tendon tears had the largest predictive status of falling into the fatty group (odds ratio, 15.4; P = .008), followed by female sex (odds ratio, 4.9; P = .036). Patients in the fatty group had significantly higher American Shoulder and Elbow Surgeons scores (P = .048) and lower visual analog scale scores (P = .002).

DISCUSSION AND CONCLUSION

This prospective histologic assessment revealed that full-thickness rotator cuff tears and female sex were the largest risk factors for intracellular lipid accumulation. Although tear size correlated with fatty accumulation, the sex disparity is a noteworthy finding that warrants further research.

Aging-Related Rotator Cuff Tears: Molecular Mechanisms and Implications for Clinical Management

Shoulder pain and disabilities are prevalent issues among the elderly population, with rotator cuff tear (RCT) being one of the leading causes. Although surgical treatment has shown some success, high postoperative retear rates remain a great challenge, particularly in elderly patients. Aging-related degeneration of muscle, tendon, tendon-to-bone enthesis, and bone plays a critical role in the development and prognosis of RCT. Studies have demonstrated that aging worsens muscle atrophy and fatty infiltration, alters tendon structure and biomechanical properties, exacerbates enthesis degeneration, and reduces bone density. Although recent researches have contributed to understanding the pathophysiological mechanisms of aging-related RCT, a comprehensive systematic review of this topic is still lacking. Therefore, this article aims to present a review of the pathophysiological changes and their clinical significance, as well as the molecular mechanisms underlying aging-related RCT, with the goal of shedding light on new therapeutic approaches to reduce the occurrence of aging-related RCT and improve postoperative prognosis in elderly patients.

Serial Changes of Fatty Degeneration and Clinical Outcomes after Repair of Medium-Sized Rotator Cuff Tears

Background

This study was designed to longitudinally analyze quantitative intramuscular and perimuscular fat and evaluate clinical outcomes according to healing degree after rotator cuff repair.

Methods

From June 2013 through October 2018, patients who had undergone repair due to medium-sized rotator cuff tears and serial chest computed tomography (CT) preoperatively and at early (6-12 months) and late (at least 3 years) postoperative follow-ups were included. Supraspinatus (SST) intramuscular fat fraction ratio (IFFR) and perimuscular fat fraction ratio (PFFR) were calculated using chest CT. The rotator cuff integrity was categorized as healed, smaller retear (SRT), and larger retear (LRT) by comparing the preoperative tear size and retear size in shoulder CT arthrography at postoperative follow-ups. Clinical outcomes were evaluated using the American Shoulder and Elbow Surgeons (ASES) score, the University of California at Los Angeles (UCLA) shoulder rating scale, and the Constant score preoperatively and at early and late postoperative follow-ups.

Results

In the LRT group, compared with the preoperative values, there were increases in the SST IFFR and PFFR at the early (p = 0.002 and p = 0.006, respectively) and late (p < 0.001 and p < 0.001, respectively) postoperative time points. Late postoperative clinical scores (UCLA and Constant scores) were not improved compared to preoperative scores (p = 0.156 and p = 0.094, respectively). In the SRT group, there was no difference in the mean SST IFFR and PFFR between preoperative and early postoperative time points (p = 0.766 and p = 0.180, respectively), but the late postoperative values were higher than preoperative values (p = 0.009 and p = 0.049, respectively). Late postoperative clinical scores (ASES, UCLA, and Constant scores) in the SRT group improved compared to preoperative time (p < 0.001, p < 0.001, and p = 0.016, respectively). In the healed group, compared with the preoperative values, there was no difference in the mean SST IFFR and PFFR at postoperative time points; however, the late postoperative clinical scores (ASES, UCLA, and Constant scores) were improved (all p < 0.001).

Conclusions

In the SRT group, IFFR and PFFR progressed in the late postoperative period and clinical scores improved over time. However, in the LRT group, IFFR and PFFR progressed in the early and late postoperative periods and clinical scores did not improve at the late postoperative follow-up.

The prevalence and clinical course of shoulder injury related to vaccine administration (SIRVA) after COVID-19 vaccines in Dutch hospital workers

INTRODUCTION

Shoulder Injury Related to Vaccine Administration (SIRVA) is a rare disorder characterized by persistent shoulder pain and limited range of motion presenting within 48 h after vaccine administration. With the widespread distribution of the COVID-19 vaccine, the incidence of SIRVA is expected to rise. This sudden rise in vaccine administration presents an ideal opportunity to estimate the prevalence of SIRVA and to better characterize SIRVA.

OBJECTIVE

This study aims to investigate the prevalence of SIRVA following COVID-19 vaccine administration among hospital workers in the Netherlands.

METHODS

A questionnaire was sent to all hospital workers from a single non-academic hospital in the Netherlands. Respondents who had active SIRVA complaints were invited for an outpatient orthopaedic clinic assessment. Data was collected on participant characteristics and physical examination including assessment of active and passive range of motion (ROM). An ultrasound was performed to identify potential abnormalities.

RESULTS

32 out of 981 (3.3%) respondents reported shoulder pain with limited ROM occurring within 48 h after vaccine administration lasting for at least 7 days. Of these 32 respondents with SIRVA, 18 (56.2%) still reported active symptoms at the time of the survey. Clinical examination of 13 (72.2%) respondents with active SIRVA complaints showed limited glenohumeral ROM, limitations in activities of daily living and injection site pain. Twelve out of thirteen (92.3%) respondents with active SIRVA complaints showed abnormalities of the soft-tissue of the shoulder on ultrasound. Physiotherapy was the most common treatment modality for persistent SIRVA complaints (38.9%).

CONCLUSIONS

The prevalence of SIRVA is estimated at 3% in the adult working population. Signs and symptoms of SIRVA are variable in severity, localization and timing. Soft-tissue abnormalities is the most common clinical sign. This study contributes to clinician's knowledge on SIRVA, aiding in early recognition and treatment, which are imperative for prevention of persistent and severe shoulder pathology.

Emerging Role of Quantitative Ultrasound-Based Imaging Techniques for Characterizing Rotator Cuff Tears: A Scoping Review

Rotator cuff myosteatosis following cuff tears is very common and one of the most important prognostic factors in clinical management. Quantitative ultrasound-based imaging techniques (QUBIT) are frequently used along with magnetic resonance imaging (MRI) to evaluate rotator cuff fatty degeneration. However, the examination of rotator cuff tissue integrity by QUBIT is lacking a standardized imaging protocol and procedural methodologies. In this scoping review, we synthesized the current state of QUBIT against the reference imaging modalities in patients with rotator cuff tears. The literature search was extracted from 963 studies, with 22 studies included in the final review in accordance with the preferred reporting items for systematic reviews and meta-analyses extensions for scoping reviews. The selected studies included human participants and focused on measuring at least one prognostic or diagnostic factor using ultrasonography-based imaging with reference to MRI. The findings suggest both conventional B-mode ultrasound and shear wave elastography imaging were comparable to MRI-based imaging techniques for the evaluation of fatty infiltration and rotator cuff tear characterization. This review establishes guidelines for reporting shoulder-specific QUBIT aimed at developing a standardized imaging protocol. The objective was to enhance the diagnostic and prognostic capabilities of QUBIT in the clinical setting.

Supraspinatus muscle atrophy in relation to aging with or without shoulder pathology: A radiographic study

Introduction

Supraspinatus muscle atrophy is commonly associated with shoulder disease, but the effect of ageing on atrophy is not well understood. It was the aim of this study to investigate this effect using MRI scans in older patients.

Methods and materials

A retrospective review of MRI scans in patients aged >70 years was performed between Jan 2016-Dec 2018.Both normal and abnormal scans were included in the analysis which included quantifying muscle atrophy of the supraspinatus using Thomazeu's occupation ratio.

Results

There were 39 normal shoulder MRI scans with a mean age of 75 years (range: 70-88) and 163 abnormal scans with a mean age of 77 years (range: 70-93). The mean supraspinatus occupation ratio for normal MRI scans was 0.57 (range: 0.33-0.86) and abnormal scans 0.35 (range: 0.17-0.90). Occupation ratio was maintained with advancing until the age of 85 years before undergoing a significant declin following this.

Conclusion

This study has shown that the occupation ratio is significantly reduced with shoulder disease, but normal shoulders do not undergo significant atrophy of supraspinatus tendon with increasing age. An occupation ratio of <0.32 is unlikely to occur in normal shoulders and this awareness may be useful when planning shoulder surgery, specifically shoulder arthroplasty.

Ageing and joint position sense of the asymptomatic shoulder: An observational study

PURPOSE

This study aimed to quantify the extent to which age was associated with joint position sense (JPS) of the asymptomatic shoulder as measured by joint position reproduction (JPR) tasks and assess the reproducibility of these tasks.

METHODS

120 Asymptomatic participants aged 18-70 years each performed 10 JPR-tasks. Both contralateral and ipsilateral JPR-tasks were evaluated on accuracy of JPR under active- and passive conditions at two levels within the shoulder forward flexion trajectory. Each task was performed three times. In a subgroup of 40 participants, the reproducibility of JPR-tasks was assessed one week after initial measurement. Reproducibility of JPR-tasks was evaluated by both reliability (intra-class correlation coefficients (ICC's)) and agreement (standard error of measurement (SEM)) measures.

RESULTS

Age was not associated with increased JPR-errors for any of the contralateral or ipsilateral JPR-tasks. ICC's ranged between 0.63 and 0.80 for contralateral JPR-tasks, and from 0.32 to 0.48 for ipsilateral tasks, except for one ipsilateral task where the reliability was similar to contralateral tasks (0.79). The SEM was comparable and small for all JPR-tasks, ranging between 1.1 and 2.1.

CONCLUSION

No age-related decline in JPS of the asymptomatic shoulder was found, and good agreement between test and re-test measurements for all JPR-tasks as indicated by the small SEM.

Quantitative analysis of myofiber type composition in human and mouse skeletal muscles

Skeletal muscles are composed of different myofiber types characterized by the expression of myosin heavy chain isoforms, which can be affected by physical activity, aging, and pathological conditions. Here, we present a step-by-step high-throughput semi-automated approach for performing myofiber type quantification of entire human or mouse muscle tissue sections, including immunofluorescence staining, image acquisition, processing, and quantification. For complete details on the use and execution of this protocol, please refer to Abbassi-Daloii et al. (2022).1.

Are paraspinal muscle quantity, lumbar indentation value, and subcutaneous fat thickness related to disc degeneration? An MRI-based study

INTRODUCTION

The aim of the study was to investigate the relationship between lumbar disc herniation and Goutallier classification (GC), lumbar indentation value, and subcutaneous adipose tissue thickness.

METHODS

102 consecutive patients (59 female and 43 male) with lumbar back pain, numbness, tingling, or pain in the lower extremity indicating radiculopathy who had undergone lumbar magnetic resonance imaging (MRI) and had an intervertebral disc herniation in the L4-5 level, were included in the study. 102 patients who have undergone lumbar MRI in the same time period and have no disc herniation were chosen to be the control group and were selected so as to match the herniated group for sex and age. All these patients' scans were re-interpreted regarding paraspinal muscle atrophy (using the GC), lumbar indentation value, and subcutaneous adipose tissue thickness in the L4-5 level.

RESULTS

The Goutallier score was higher in the herniated group, compared with the non-herniated group (p < 0.001). There was no statistical difference between herniated and non-herniated groups regarding lumbar indentation value (LIV) and subcutaneous adipose tissue thickness (SATT). A Goutallier score of 1.5 provided the highest sensitivity x specificity value to indicate the disc herniation according to the statistical results. The individuals with a Goutallier score of 2, 3, and 4 have 2.87 times more likely to have disc herniation in their MRIs than the ones with a score of 0 and 1.

CONCLUSION

Paraspinal muscle atrophy seems to be related to the presence of disc herniations. The cut-off value of GC to indicate the disc herniation in this study might be useful to predict the risk for disc herniation regarding the Goutallier score. The LIV and SATT measured in magnetic resonance images were randomly distributed between individuals with herniated and non-herniated groups, and statistically, no relationship was observed between these groups regarding these parameters.

IMPLICATIONS FOR PRACTICE

The effect of the parameters studied in this research on disc herniations are expected to be an added value to the literature. The awareness of risk factors for intervertebral disc herniations might be used in preventive medicine to predict the risk and understand the tendency of an individual for disc herniations to occur in the future. Further investigations are needed to establish whether there is a causal relationship or correlation between these parameters and disc herniation.

A transcriptome atlas of leg muscles from healthy human volunteers reveals molecular and cellular signatures associated with muscle location

Skeletal muscles support the stability and mobility of the skeleton but differ in biomechanical properties and physiological functions. The intrinsic factors that regulate muscle-specific characteristics are poorly understood. To study these, we constructed a large atlas of RNA-seq profiles from six leg muscles and two locations from one muscle, using biopsies from 20 healthy young males. We identified differential expression patterns and cellular composition across the seven tissues using three bioinformatics approaches confirmed by large-scale newly developed quantitative immune-histology procedures. With all three procedures, the muscle samples clustered into three groups congruent with their anatomical location. Concomitant with genes marking oxidative metabolism, genes marking fast- or slow-twitch myofibers differed between the three groups. The groups of muscles with higher expression of slow-twitch genes were enriched in endothelial cells and showed higher capillary content. In addition, expression profiles of Homeobox (HOX) transcription factors differed between the three groups and were confirmed by spatial RNA hybridization. We created an open-source graphical interface to explore and visualize the leg muscle atlas (https://tabbassidaloii.shinyapps.io/muscleAtlasShinyApp/). Our study reveals the molecular specialization of human leg muscles, and provides a novel resource to study muscle-specific molecular features, which could be linked with (patho)physiological processes.

A unique sarcopenic progression in the mouse rotator cuff

BACKGROUND

In response to chronic injury, the muscles of the rotator cuff (RC) experience a unique degeneration characterized by extensive fatty infiltration and loss of contractile function. Human studies suggest this degeneration is also a feature of RC sarcopenia and may precede RC injury. In this study, we investigated whether RC muscles exhibit a similar unique sarcopenia in the mouse.

METHODS

Male and female mice were subdivided into four age groups: 3, 9, 18, and 24 months. The supraspinatus (SS) and infraspinatus muscles of the RC and the tibialis anterior (TA) muscle of the hindlimb were assessed. Muscle mass, contractile function, fibre cross-sectional areas and numbers, fatty infiltration, and fibrosis were assessed at each time point. Targeted transcriptional analyses were performed to assess the role of metabolic and inflammatory derangement in the pathology.

RESULTS

The 24-month-aged female mice exhibited decreased mass (25% lower than at 9 and 18 months, P < 0.01) in all muscles tested. However, only RC muscles also exhibited decreased contractile tension at this time point (20% lower than at 18 months, P < 0.005). Similarly, only female RC muscles exhibited increased fatty infiltration at 24 months (20% higher than 9 months, P < 0.05) and had elevated transcriptional markers of adipogenesis (2.4-fold higher Pparg and 3.8-fold higher Adipoq expression compared with 9 months, P < 0.001). Unbiased metabolic transcriptional profiling identified up-regulation of the antigen presentation (Z scores of 2.3 and 1.9 for SS and TA, respectively) and cytokine and chemokine signalling (Z scores of 3.1 and 2.4 for SS and TA, respectively) pathways in 24 month female muscle compared with 9. Further transcriptional analysis supported increased expression of pro-adipogenic inflammatory signals (6.3-fold increase in Il6 and 5.0-fold increase in Anxa2, P < 0.01) and increased presence of fibro-adipogenic progenitors (2.5-fold) in the 24-month-aged female RC compared with 9 months that together exacerbate fatty infiltration.

CONCLUSIONS

These data indicate that female mice replicate the unique sarcopenic pathology in the ageing human RC. Furthermore, they suggest that the exacerbated fatty infiltration is due to an interaction between higher resident fibro-adipogenic progenitor numbers and an elevated systemic inflammation in aged female mice. We conclude that female mouse RC muscle is a novel system to study both human RC degeneration and the signals that regulate sarcopenic fatty infiltration in general, which is prevalent in humans but largely absent from the rodent hindlimb.

The metabolic landscape in chronic rotator cuff tear reveals tissue-region-specific signatures

BACKGROUND

Degeneration of shoulder muscle tissues often result in tearing, causing pain, disability and loss of independence. Differential muscle involvement patterns have been reported in tears of shoulder muscles, yet the molecules involved in this pathology are poorly understood. The spatial distribution of biomolecules across the affected tissue can be accurately obtained with matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). The goal of this pilot study was to decipher the metabolic landscape across shoulder muscle tissues and to identify signatures of degenerated muscles in chronic conditions.

METHODS

Paired biopsies of two rotator cuff muscles, torn infraspinatus and intact teres minor, together with an intact shoulder muscle, the deltoid, were collected during an open tendon transfer surgery. Five patients, average age 65.2 ± 3.8 years, were selected for spatial metabolic profiling using high-spatial resolution (MALDI-TOF) and high-mass resolution (MALDI-FTICR) MSI in negative or positive ion mode. Metabolic signatures were identified using data-driven analysis. Verifications of spatial localization for selected metabolic signatures were carried out using antibody immunohistology.

RESULTS

Data-driven analysis revealed major metabolic differences between intact and degenerated regions across all muscles. The area of degenerated regions, encompassed of fat, inflammation and fibrosis, significantly increased in both rotator cuff muscles, teres minor (27.9%) and infraspinatus (22.8%), compared with the deltoid (8.7%). The intact regions were characterized by 49 features, among which lipids were recognized. Several of the identified lipids were specifically enriched in certain myofiber types. Degenerated regions were specifically marked by the presence of 37 features. Heme was the most abundant metabolite in degenerated regions, whereas Heme oxygenase-1 (HO-1), which catabolizes heme, was found in intact regions. Higher HO-1 levels correlated with lower heme accumulation.

CONCLUSIONS

Degenerated regions are distinguished from intact regions by their metabolome profile. A muscle-specific metabolome profile was not identified. The area of tissue degeneration significantly differs between the three examined muscles. Higher HO-1 levels in intact regions concurred with lower heme levels in degenerated regions. Moreover, HO-1 levels discriminated between dysfunctional and functional rotator cuff muscles. Additionally, the enrichment of specific lipids in certain myofiber types suggests that lipid metabolism differs between myofiber types. The signature metabolites can open options to develop personalized treatments for chronic shoulder muscles degeneration.

Shoulder movement complexity in the aging shoulder: A cross-sectional analysis and reliability assessment

Healthy individuals perform a task such as hitting the head of a nail with an infinite coordination spectrum. This motor redundancy is healthy and allows for learning through exploration and uniform load distribution across muscles. Assessing movement complexity within repetitive movement trajectories may provide insight into the available motor redundancy during aging. We quantified complexity of repetitive arm elevation trajectories in the aging shoulder and assessed test-retest reliability of this quantification. In a cross-sectional study using 3D-electromagnetic tracking, 120 asymptomatic subjects, aged between 18 and 70 years performed repetitive abduction and forward/anteflexion movements. Movement complexity was calculated using the Approximate Entropy (ApEn-value): [0,2], where lower values indicate reduced complexity. Thirty-three participants performed the protocol twice, to determine reliability (intraclass correlation coefficient [ICC]). The association between age and ApEn was corrected for task characteristics (e.g., sample length) with multiple linear regression analysis. Reproducibility was determined using scatter plots and ICC's. Higher age was associated with lower ApEn-values during abduction (unstandardized estimate: -0.003/year; 95% confidence interval: [-0.005; -0.002]; p < .001). ICC's revealed poor to good reliability depending on differences in sample length between repeated measurements. The results may imply more stereotype movement during abduction in the ageing shoulder, making this movement prone to the development of shoulder complaints. Future studies may investigate the pathophysiology and clinical course of shoulder complaints by assessment of movement complexity. To this end, the ApEn-value calculated over repetitive movement trajectories may be used, although biasing factors such as sample length should be taken into account.

Characterization of Infraspinatus Tendon Anatomy: The Soft-Tissue Portion of Remplissage

Purpose

To characterize the morphology of the infraspinatus (IS) tendon and evaluate the bony anatomy of the humeral head (HH) to determine if there is a correlation between HH measurements and the amount of available IS tendon.

Methods

The superior-inferior width as well as the medial-lateral (M-L) length of the inferior and superior portions of the IS tendon were measured in 15 human cadaveric shoulders. Three measurements were then obtained for each corresponding humeral head: (1) anterior to posterior (A-P) distance, (2) midcoronal humeral head distance (MCHH), and (3) M-L distance. Pearson correlation coefficients (R) of tendon measurements relative to HH measurements were determined.

Results

The mean ± SD HH measurements were 44.3 ± 3.3 mm for A-P, 49.3 ± 3.4 mm at the MCHH, and 52.2 ± 3.4 mm in the M-L plane. The mean M-L length of the superior portion of the IS tendon was significantly different from the inferior portion (42.4 vs 31.0 mm, P < .0001). The mean ± SD width of the IS tendon was 19.4 ± 3.0mm. There was a statistically significant correlation (R = 0.58) between the M-L length of the superior IS tendon relative to the M-L HH distance (P < .05) and the A-P HH distance (P < .05).

Conclusions

The superior M-L IS tendon length was significantly greater than the inferior M-L length. The M-L HH and the AP HH distances were significantly correlated to the M-L length of the superior portion of the IS tendon. These relationships may provide an estimation of the length of available IS tendon to help guide the management of Hill-Sachs lesions (HSLs).

Clinical Relevance

Knowledge of the available IS length can help optimize the management of HSLs following anterior shoulder dislocation. If IS tendon M-L length is less than HSL M-L length, then remplissage may result in capsulomyodesis rather than tenodesis. Placement of the superior anchor in a position that is as superior as possible within the HSL defect will maximize the opportunity for IS tenodesis.

Quantitative Ultrasonographic Analysis of Changes of the Suprascapular Nerve in the Aging Population With Shoulder Pain

Background

Older people are vulnerable to painful shoulder syndromes, the majority of which are derived from degenerative rotator cuff pathologies. The suprascapular nerve (SSN) is closely related to the rotator cuff complex, and its role in shoulder pain has recently been highlighted. This study aimed to explore the differences in SSN among older people with and without shoulder pain, and to investigate the potential factors influencing the nerve size using ultrasound (US) imaging.

Methods

Participants aged ≥60 years were enrolled in the study. A systematic and bilateral US examination of the rotator cuff tendons was performed. The SSN was examined from its origin in the brachial plexus to the spinoglenoid notch of the infraspinatus fossa. The association between the nerve’s cross-sectional area (CSA) and rotator cuff lesions was analyzed using the generalized estimation equation.

Results

Among the 94 participants, 45 (with bilaterally asymptomatic shoulders) were classified into the control group, whereas 49 (with at least one-sided shoulder pain) were classified into the group with shoulder pain. The average CSAs of the SSN at the level of the brachial plexus, supraspinatus fossa, and infraspinatus fossa were comparable between the patients in the control group and those with shoulder pain. There was a higher prevalence of rotator cuff lesions and enlarged distal SSNs in the painful shoulders than in the asymptomatic shoulders of patients with unilateral involvement. A full-thickness tear of the supraspinatus tendon was associated with swelling of the SSN in the supraspinatus fossa (β coefficient = 4.068 mm², p < 0.001).

Conclusion

In the older population, full-thickness tears of the supraspinatus tendon are independently associated with enlargement of the distal SSN. In cases with large rotator cuff tendon tears with poor response to conservative treatments, possible SSN entrapment should be considered and managed accordingly.

Age Differences in Motor Recruitment Patterns of the Shoulder in Dynamic and Isometric Contractions. A Cross-Sectional Study

Aging processes in the musculoskeletal system lead to functional impairments that restrict participation. Purpose: To assess differences in the force and motor recruitment patterns of shoulder muscles between age groups to understand functional disorders. A cross-sectional study comparing 30 adults (20–64) and 30 older adults (>65). Surface electromyography (sEMG) of the middle deltoid, upper and lower trapezius, infraspinatus, and serratus anterior muscles was recorded. Maximum isometric voluntary contraction (MIVC) was determined at 45° glenohumeral abduction. For the sEMG signal registration, concentric and eccentric contraction with and without 1 kg and isometric contraction were requested. Participants abducted the arm from 0° up to an abduction angle of 135° for concentric and eccentric contraction, and from 0° to 45°, and remained there at 80% of the MIVC level while isometrically pushing against a handheld dynamometer. Differences in sEMG amplitudes (root mean square, RMS) of all contractions, but also onset latencies during concentric contraction of each muscle between age groups, were analyzed. Statistical differences in strength (Adults > Older adults; 0.05) existed between groups. No significant differences in RMS values of dynamic contractions were detected, except for the serratus anterior, but there were for isometric contractions of all muscles analyzed (Adults > Older adults; 0.05). The recruitment order varied between age groups, showing a general tendency towards delayed onset times in older adults, except for the upper trapezius muscle. Age differences in muscle recruitment patterns were found, which underscores the importance of developing musculoskeletal data to prevent and guide geriatric shoulder pathologies.

Prognostic factors for the outcome of needle aspiration of calcific deposits for rotator cuff calcific tendinitis

Objective

To identify prognostic factors for the effectiveness of needle aspiration of calcific deposits (NACD) for rotator cuff calcific tendinitis (RCCT)

Methods

One hundred forty-nine patients with symptomatic RCCT were included in a prospective cohort study. Pain (VAS), shoulder function (SST and DASH), and quality of life (EQ-5D) were assessed at baseline and at 3, 6, and 12 months post-NACD. Univariate analyses (independent t tests or Mann-Whitney U tests depending on the distribution of data) were performed to build a multivariable linear regression model. Stepwise regression analysis through backward elimination was performed to evaluate the effect of predefined prognostic factors on the outcome.

Results

Patients who underwent multiple NACD procedures had less reduction of pain (p < 0.01). Furthermore, a larger reduction in VAS pain scores at 3 months post-NACD was associated with a larger reduction in VAS pain scores at 12 months (p < 0.01). More improvement of SST and DASH scores at 3 months was associated with better SST, DASH, and EQ-5D scores at 12 months (p < 0.01). Smaller-size calcific deposits were associated with less improvement of DASH (p = 0.03) and EQ-5D scores (p = 0.01). A longer duration of symptoms prior to NACD was associated with less improvement of EQ-5D scores (p = 0.01).

Conclusions

A good initial response after NACD is associated with better outcomes at 12 months. Patients with a longer duration of symptoms prior to NACD and patients who require multiple procedures showed inferior outcomes in terms of pain reduction and improvement of quality of life. Smaller-size calcific deposits are associated with a less favorable outcome of shoulder function and quality of life scores and might therefore be less susceptible for NACD.

Key Points

• A good initial response to NACD is associated with a better outcome in the longer term.

• A longer duration of symptoms and the need for multiple NACD procedures are associated with inferior outcomes.

• Smaller-size calcific deposits seem less susceptible for NACD.

Electronic supplementary material

The online version of this article (10.1007/s00330-020-06669-0) contains supplementary material, which is available to authorized users.

Middle-aged adults cocontract with arm ADductors during arm ABduction, while young adults do not. Adaptations to preserve pain-free function?

Middle-aged individuals cocontract with adductor muscles during abduction. This may be crucial for counteracting deltoid forces, depressing the humerus and ensuring free passage of subacromial tissues underneath the acromion during abduction. We questioned whether adductor co-contraction is always present, or develops during ageing, in which case it may explain the age-related character of common shoulder conditions such as Subacromial Pain Syndrome. In a cross-sectional analysis with electromyography (EMG), activation patterns of the latissimus dorsi, teres major, pectoralis major and deltoid muscle were assessed during isometric force tasks in 60 asymptomatic individuals between 21 and 60 years old. Cocontraction was expressed as the degree of antagonistic activation relative to the same muscle's degree of agonistic activation, resulting in an activation ratio between -1 and 1, where lower values indicate more cocontraction. Using linear regression analyses, we found age-related decreases in the activation ratio of the latissimus dorsi (regression estimate: -0.004, 95% CI: -0.007 to 0.0, p-value: 0.042) and teres major (regression estimate: -0.013, 95% CI: -0.019 to -0.008, p-value: <0.001). In contrast to young individuals, middle-aged individuals showed a high degree of adductor cocontraction during abduction. This may indicate that during ageing, alterations in activation patterns are required for preserving pain-free shoulder function.

Optimizing methods to quantify intramuscular fat in rotator cuff tears with normalization

OBJECTIVE

To determine which normalization method may best account for confounding individual factors, such as age or BMI, when quantifying fat infiltration on MRI in patients with rotator cuff tears, the effects of normalization using three different muscles (teres major; triceps brachii; teres minor) were compared.

METHODS

Thirty-seven consecutive patients diagnosed with rotator cuff pathology were included. MRI fat-water sequences were used to quantify rotator cuff intramuscular fat (%fat). Three reference muscles (teres major, triceps, teres minor) were used to derive normalized %fat. Relationships between intramuscular %fat and tear size, age, and BMI in each rotator cuff muscle, before and after normalization, were compared with Fisher transformations (α = 0.05).

RESULTS

Normalization with teres major ameliorated confounding relationships of age and BMI on rotator cuff %fat. In contrast, normalization with triceps maintained the confounding relationships between %fat and age in supraspinatus (p = 0.03) and infraspinatus/teres minor (p = 0.028). Normalization with teres minor maintained the confounding relationship between %fat and BMI in subscapularis (p = 0.039). Normalization with teres major best-maintained relationships between tear size and infraspinatus/teres minor %fat (p = 0.021). In contrast, normalization with triceps or teres minor eliminated all significant relationships with tear size.

CONCLUSIONS

Results of this pilot study suggest normalization to teres major using MRI-based %fat quantification methods can effectively control for individual factors, such as BMI or age, and may have utility in evaluating and monitoring rotator cuff fat infiltration attributed specifically to a tendon tear.

The effect of obesity on fatty infiltration of the rotator cuff musculature in patients without rotator cuff tears

BACKGROUND

The modified Goutallier classification system describes the fatty infiltration of rotator cuff musculature (RCM) seen on magnetic resonance imaging (MRI) to assist with surgical decision-making for patients with rotator cuff tears (RCT). We describe the relationship between body mass index (BMI) and fatty infiltration in patients without RCT.

METHODS

Twenty-five patients from each of four different BMI ranges (< 25 kg/m²; 25 kg/m² to 30 kg/m²; 30 kg/m² to 35 kg/m²; > 35 kg/m²) were randomly selected from 1088 consecutive shoulder MRI scans (T1 parasagittal series). Four physician-readers evaluated MRI scans and assigned modified Goutallier grades (0 to 4) in each of the four rotator cuff muscles, as well as two adjacent muscles.

RESULTS

Grade distributions varied significantly based on BMI category for infraspinatus (p = 0.001), teres minor (p < 0.001), subscapularis (p = 0.025), teres major (p < 0.001) and deltoid (p < 0.001). Higher grades were evident with a diagnosis of diabetes mellitus in three of six muscles (p < 0.05), hyperlipidaemia in one muscle (p = 0.021) and greater patient age in three muscles (p < 0.05).

CONCLUSIONS

Obese and severely obese patients without RCT have more fatty infiltration seen on MRI. Patient factors (older age and diagnosis of diabetes mellitus) can be predictive of fatty infiltration in RCM. Fatty infiltration of RCM is not solely attributable to the presence of a RCT.

Rotator cuff muscles fatty infiltration increases with age: retrospective review of 210 patients with intact cuff on computed tomography arthrography

HYPOTHESIS

We aimed to demonstrate that rotator cuff fatty infiltration (FI) increases physiologically with age and that a FI degree of 2 might be encountered in patients with an intact cuff.

METHODS

We retrospectively evaluated the FI of 210 patients (classified in 7 age groups: <20, 20-30, 30-40, 40-50, 50-60, 60-70, and >70 years) who had an intact cuff on computed tomography (CT) arthrography. The Goutallier grading system was used to assess FI of rotator cuff muscles and of the deltoid muscle. FI was then compared between groups for each muscle, and determinants (age, sex, and body mass index) of FI were assessed.

RESULTS

FI of all evaluated muscles progressively increased with age, but a statistically significant increase between 2 consecutive age groups was observed for all muscles only after age 40 years. For each group, intermuscle comparison showed that the FI distribution was not significantly different between each rotator cuff muscle at any age. Age was a determinant for FI for all muscles, but sex and body mass index also influenced deltoid FI. A median FI value of 2 was found in the "over 70 group" for the supraspinatus, but in the 60 to 70 group for infraspinatus and subscapularis and in the 50 to 60 group for the deltoid.

CONCLUSION

FI of rotator cuff muscles increased with age, with a significant acceleration after 40 years. Moreover, a FI graded 2, a common threshold for management decision, was commonly found in those aging patients with intact cuff on CT arthrography.

Early Arthroscopic Repair of Acute Traumatic Massive Rotator Cuff Tears Leads to Reliable Reversal of Pseudoparesis: Clinical and Radiographic Outcome

PURPOSE

It was the aim of this study to analyze the clinical and radiographic outcome after early arthroscopic repair regardless of the age of patients.

METHODS

Patients with massive traumatic cuff tear and clinical pseudoparesis for forward elevation treated by subsequent early arthroscopic repair from 2011 until 2014 were included in this retrospective study. Exclusion criteria were Goutallier grade ≥3 fatty infiltration and prior shoulder problems or surgery. Magnetic resonance imaging (MRI), radiographs, and functional assessments were performed preoperatively and at follow-up.

RESULTS

A total of 21 patients (male/female 15/6; age range: 30-83) were included. Preoperative MRI showed complete 2 tendon tears in 7 patients, 3 tendon tears in 13 patients and all tendons ruptured in one patient. All patients had full passive range of motion and the mean active elevation was 35.7° (range: 0°-60°). Nine patients also had a pseudoparesis for external rotation (mean: 10°, range: -30° to 40°). The mean delay until surgery was 33 days (range: 13-60). At follow-up (mean: 39 months, range: 24-60) all patients showed reversal of pseudoparesis, mean elevation of 165.2° (range: 110°-180°) and mean external rotation of 49.3° (range: -20° to 80°). The mean Constant score was 82 points (range: 56-95), and the mean subjective shoulder value was 93% (range: 50-100). The overall retear rate was 20% (n = 4). Fatty infiltration increased at least 1 grade in patients who had a retear and in 56% of patients (n = 9) without retear. Age was not a predictor for retear.

CONCLUSIONS

This study shows that early arthroscopic repair of traumatic massive RCT with pseudoparesis may lead to successful results regardless of patients' age. A complete restoration of the function can be expected even in patients with retear. The retear rate is low and the increase of fatty infiltration minimal.

LEVEL OF EVIDENCE

Level IV, case series.

The presence of a rotator cuff tear interferes with age-dependent muscle atrophy of intact shoulder muscles. An MRI study with 3 years' follow-up

Rotator cuff muscle atrophy is frequently studied, but it is unknown whether redistribution of mechanical load in the presence of a rotator cuff tear influence muscle atrophy that is observed in patients. We hypothesized that in the presence of a supraspinatus tear, redistribution of mechanical load towards teres minor and deltoid slows down atrophy of these muscles over time. In this retrospective observational study of 129 patients, we measured the cross-sectional surface-areas on MRI of shoulder muscles in an intact rotator cuff (n = 92) and in a supraspinatus-tear group (n = 37) with a mean follow-up of 3 ± 1.8 years. Mixed models were applied to evaluate changes in surface-area of the rotator cuff and deltoid with adjustments for age, sex and follow-up time. In patients with an intact rotator cuff, the mean surface-area of the teres minor decreased 6 mm²/year (95% CI 0.7-11.1, P = 0.026) and the mean deltoid surface-area decreased 75 mm²/year (95% CI 24.5-124.8, P = 0.004). The presence of a rotator cuff tear was associated with less reduction of teres minor and deltoid surface-area in patients <50 years, with an effect of a tear of 22 mm²/year (95% CI 1.7-41.7, P = 0.034) and 250 mm²/year (95% CI 75.8-424.3, P = 0.006), respectively. Whereas the surface-area of teres minor and deltoid decrease over time in patient with an intact rotator cuff, the decline in surface-area of these muscles was substantially less in the presence of a rotator cuff tear. Our findings indicate that atrophy may be reduced if an increase in mechanical load is exerted onto the muscle.

Shoulder muscle volume and fat content in healthy adult volunteers: quantification with DIXON MRI to determine the influence of demographics and handedness

OBJECTIVE

We aimed to provide mean values for fat-fraction and volume for full-length bilateral rotator cuff and deltoid muscles in asymptomatic adults selected on the basis of their good musculoskeletal and systemic health, and to understand the influence of gender, age, and arm dominance.

MATERIALS AND METHODS

Seventy-six volunteers aged 20 to 60 years who were screened for normal BMI and high general health were included in the study. MRI was performed at 3 Tesla using three-point DIXON sequences. Volume and fat-signal fraction of the rotator cuff muscles and the deltoid muscle were determined with semi-automated segmentation of entire muscle lengths. Differences according to age, gender, and handedness per muscle were evaluated.

RESULTS

Fat-signal fractions were comparable between genders (mean ± 2 SD, 95% CI, women 7.0 ± 3.0; 6.8-7.2%, men 6.8 ± 2.7; 6.7-7.0%) but did not show convincing changes with age. Higher shoulder muscle volume and lower fat-signal fraction in the dominant arm were shown for teres minor and deltoid (p < 0.01) with similar trends shown for the other rotator cuff muscles.

CONCLUSIONS

Bilateral fat-signal fractions and volumes based on entire length shoulder muscles in asymptomatic 20-60 year old adults may provide reference for clinicians. Differences shown according to arm dominance should be considered and may rationalize the need for bilateral imaging in determining appropriate management.

Implications on older women of age- and sex-related differences in activation patterns of shoulder muscles: A cross-sectional study

We conducted a cross-sectional study to assess differences in neuromotor attributes of shoulder muscles between age groups in both sexes and to better understand functional disorders in older women. Twenty young (20-42 years old), 20 middle-aged (43-67), and 20 older (> 68) adults participated in a comparative surface-electromyography study of five muscles. We identified age-related differences in women, especially in scapula stabilizer muscles. There was a tendency for both sexes of delayed onset times with increasing age, excepting the upper trapezius muscle in females. The results highlight the importance of understanding musculoskeletal aging in women to adequately guide physical therapeutic approaches.

Quantitative Measurement of Muscle Atrophy and Fat Infiltration of the Supraspinatus Muscle Using Ultrasonography After Arthroscopic Rotator Cuff Repair

Objective

To investigate the utility of ultrasonography to objectively examine morphological changes (i.e., muscle atrophy and fatty infiltration) of the supraspinatus muscle.

Methods

Thirty-four patients were prospectively enrolled in this study. The degrees of muscle atrophy and fat infiltration were measured using ultrasonography 3–4 months after arthroscopic supraspinatus tendon repair. Shoulder function (i.e., shoulder active range of motion, visual analogue scale, and constant score) was examined. Using the symmetricity of the muscles in the human body, the degrees of morphological changes of the supraspinatus muscle were quantitatively measured. The associations between the morphological changes of the supraspinatus muscle and shoulder function were identified.

Results

There were statistically significant differences in the cross-sectional area (CSA) and echogenicity between the surgery and non-surgery sides (p<0.001). The CSA ratio, which represents the degree of muscle atrophy, was associated with shoulder forward flexion, external rotation, and constant score; however, the echogenicity ratio, which represents the degree of fat infiltration, was not associated with shoulder function after surgery.

Conclusion

This study demonstrated that shoulder function could be predicted by evaluating the morphological changes of the supraspinatus muscle using ultrasonography and that objective evaluation is possible through quantitative measurement using the symmetricity of the human body.

Effects of age and pathology on shear wave speed of the human rotator cuff

Rotator cuff tears are common and often repaired surgically, but post-operative repair tissue healing, and shoulder function can be unpredictable. Tear chronicity is believed to influence clinical outcomes, but conventional clinical approaches for assessing tear chronicity are subjective. Shear wave elastography (SWE) is a promising technique for assessing soft tissue via estimates of shear wave speed (SWS), but this technique has not been used extensively on the rotator cuff. Specifically, the effects of age and pathology on rotator cuff SWS are not well known. The objectives of this study were to assess the association between SWS and age in healthy, asymptomatic subjects, and to compare measures of SWS between patients with a rotator cuff tear and healthy, asymptomatic subjects. SWE images of the supraspinatus muscle and intramuscular tendon were acquired from 19 asymptomatic subjects and 11 patients with a rotator cuff tear. Images were acquired with the supraspinatus under passive and active (i.e., minimal activation) conditions. Mean SWS was positively associated with age in the supraspinatus muscle and tendon under passive and active conditions (p ≤ 0.049). Compared to asymptomatic subjects, patients had a lower mean SWS in their muscle and tendon under active conditions (p ≤ 0.024), but no differences were detected under passive conditions (p ≥ 0.783). These findings identify the influences of age and pathology on SWS in the rotator cuff. These preliminary findings are an important step toward evaluating the clinical utility of SWE for assessing rotator cuff pathology. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:282-288, 2018.

Fatty infiltration of stage 1 or higher significantly compromises long-term healing of supraspinatus repairs

BACKGROUND

Fatty infiltration (FI) compromises outcomes of rotator cuff repairs. Most clinicians consider FI of the infraspinatus, whether it is torn or intact, because it is most rapidly affected. The purpose of this study was to report long-term outcomes of isolated supraspinatus repairs and to determine their associations with FI of the infraspinatus and supraspinatus.

METHODS

The records of 182 patients who underwent repair of isolated supraspinatus tears and had preoperative magnetic resonance imaging were retrieved. Of these, 147 patients were evaluated at 10 years' follow-up using the Constant score and magnetic resonance imaging scans.

RESULTS

Preoperative FI was greater in the supraspinatus (52% stage ≥1) than in the infraspinatus (29% stage ≥1). The 10-year Constant scores were influenced by FI of the supraspinatus (P = .006) but not of the infraspinatus (P = .422). Multivariable regression confirmed that Constant scores were significantly lower for female patients, repetitive work, and stage 1 and stage 2 FI of the supraspinatus in addition to open surgery. Retear rates (Sugaya types IV-V) were also influenced by FI of the supraspinatus (P = .001) but not of the infraspinatus (P = .979). Shoulders with supraspinatus FI at stages 0, 1, and 2 had retear rates of 10%, 22%, and 31%, respectively. Multivariable regression affirmed that the odds of retears are significantly increased by both stage 1 and stage 2 FI of the supraspinatus.

CONCLUSIONS

The Constant scores and retear rates were significantly associated with FI of the torn supraspinatus (stage ≥1) but not of the intact infraspinatus. The authors recommend that rapid surgical intervention be considered, before accumulation of fat, especially for young, active patients.

Quantitative assessment of fatty infiltration and muscle volume of the rotator cuff muscles using 3-dimensional 2-point Dixon magnetic resonance imaging

BACKGROUND

In patients with rotator cuff tears, muscle degeneration is known to be a predictor of irreparable tears and poor outcomes after surgical repair. Fatty infiltration and volume of the whole muscles constituting the rotator cuff were quantitatively assessed using 3-dimensional 2-point Dixon magnetic resonance imaging.

METHODS

Ten shoulders with a partial-thickness tear, 10 shoulders with an isolated supraspinatus tear, and 10 shoulders with a massive tear involving supraspinatus and infraspinatus were compared with 10 control shoulders after matching age and sex. With segmentation of muscle boundaries, the fat fraction value and the volume of the whole rotator cuff muscles were computed. After reliabilities were determined, differences in fat fraction, muscle volume, and fat-free muscle volume were evaluated.

RESULTS

Intra-rater and inter-rater reliabilities were regarded as excellent for fat fraction and muscle volume. Tendon rupture adversely increased the fat fraction value of the respective rotator cuff muscle (P < .002). In the massive tear group, muscle volume was significantly decreased in the infraspinatus (P = .035) and increased in the teres minor (P = .039). With subtraction of fat volume, a significant decrease of fat-free volume of the supraspinatus muscle became apparent with a massive tear (P = .003).

CONCLUSION

Three-dimensional measurement could evaluate fatty infiltration and muscular volume with excellent reliabilities. The present study showed that chronic rupture of the tendon adversely increases the fat fraction of the respective muscle and indicates that the residual capacity of the rotator cuff muscles might be overestimated in patients with severe fatty infiltration.

Contribution of Lumbar Spine Pathology and Age to Paraspinal Muscle Size and Fatty Infiltration

STUDY DESIGN

Retrospective chart analysis of 199 individuals aged 18 to 80 years scheduled for lumbar spine surgery.

OBJECTIVE

The purpose of this study was to quantify changes in muscle cross-sectional area (CSA) and fat signal fraction (FSF) with age in men and women with lumbar spine pathology and compare them to published normative data.

SUMMARY OF BACKGROUND DATA

Pathological changes in lumbar paraspinal muscle are often confounded by age-related decline in muscle size (CSA) and quality (fatty infiltration). Individuals with pathology have been shown to have decreased CSA and fatty infiltration of both the multifidus and erector spinae muscles, but the magnitude of these changes in the context of normal aging is unknown.

METHODS

Individuals aged 18 to 80 years who were scheduled for lumbar surgery for diagnoses associated with lumbar spine pain or pathology were included. Muscle CSA and FSF of the multifidus and erector spinae were measured from preoperative T2-weighted magnetic resonance images at the L4 level. Univariate and multiple linear regression analyses were performed for each outcome using age and sex as predictor variables. Statistical comparisons of univariate regression parameters (slope and intercept) to published normative data were also performed.

RESULTS

There was no change in CSA with age in either sex (P > 0.05), but women had lower CSAs than men in both muscles (P < 0.0001). There was an increase in FSF with age in erector spinae and multifidus muscles in both sexes (P < 0.0001). Multifidus FSF values were higher in women with lumbar spine pathology than published values for healthy controls (P = 0.03), and slopes tended to be steeper with pathology for both muscles in women (P < 0.08) but not in men (P > 0.31).

CONCLUSION

Lumbar muscle fat content, but not CSA, changes with age in individuals with pathology. In women, this increase is more profound than age-related increases in healthy individuals.

LEVEL OF EVIDENCE

3.

Molecular signatures of age-associated chronic degeneration of shoulder muscles

Chronic muscle diseases are highly prevalent in the elderly causing severe mobility limitations, pain and frailty. The intrinsic molecular mechanisms are poorly understood due to multifactorial causes, slow progression with age and variations between individuals. Understanding the underlying molecular mechanisms could lead to new treatment options which are currently limited. Shoulder complaints are highly common in the elderly, and therefore, muscles of the shoulder's rotator cuff could be considered as a model for chronic age-associated muscle degeneration. Diseased shoulder muscles were characterized by muscle atrophy and fatty infiltration compared with unaffected shoulder muscles. We confirmed fatty infiltration using histochemical analysis. Additionally, fibrosis and loss of contractile myosin expression were found in diseased muscles. Most cellular features, including proliferation rate, apoptosis and cell senescence, remained unchanged and genome-wide molecular signatures were predominantly similar between diseased and intact muscles. However, we found down-regulation of a small subset of muscle function genes, and up-regulation of extracellular region genes. Myogenesis was defected in muscle cell culture from diseased muscles but was restored by elevating MyoD levels. We suggest that impaired muscle functionality in a specific environment of thickened extra-cellular matrix is crucial for the development of chronic age-associated muscle degeneration.

PABPN1-Dependent mRNA Processing Induces Muscle Wasting

Poly(A) Binding Protein Nuclear 1 (PABPN1) is a multifunctional regulator of mRNA processing, and its expression levels specifically decline in aging muscles. An expansion mutation in PABPN1 is the genetic cause of oculopharyngeal muscle dystrophy (OPMD), a late onset and rare myopathy. Moreover, reduced PABPN1 expression correlates with symptom manifestation in OPMD. PABPN1 regulates alternative polyadenylation site (PAS) utilization. However, the impact of PAS utilization on cell and tissue function is poorly understood. We hypothesized that altered PABPN1 expression levels is an underlying cause of muscle wasting. To test this, we stably down-regulated PABPN1 in mouse tibialis anterior (TA) muscles by localized injection of adeno-associated viruses expressing shRNA to PABPN1 (shPab). We found that a mild reduction in PABPN1 levels causes muscle pathology including myofiber atrophy, thickening of extracellular matrix and myofiber-type transition. Moreover, reduced PABPN1 levels caused a consistent decline in distal PAS utilization in the 3’-UTR of a subset of OPMD-dysregulated genes. This alternative PAS utilization led to up-regulation of Atrogin-1, a key muscle atrophy regulator, but down regulation of proteasomal genes. Additionally reduced PABPN1 levels caused a reduction in proteasomal activity, and transition in MyHC isotope expression pattern in myofibers. We suggest that PABPN1-mediated alternative PAS utilization plays a central role in aging-associated muscle wasting.

PABPN1 is a multifunctional regulator of mRNA processing and its levels are reduced in skeletal muscles from midlife onwards. Reduced PABPN1 levels in a mouse model causes muscle atrophy and muscle fiber switches. We show that PABPN1-regulated muscle atrophy is regulated, in part, by up regulation of Atrogin1 and reduced expression of proteasome genes via an alternative polyadenylation site utilization. This study reveals a functional role for alternative polyadenylation site utilization in muscle atrophy and suggests a role for RNA processing in muscle aging.