Impaired contractile function of the supraspinatus in the acute period following a rotator cuff tear

Rotator cuff tears

Rotator cuff tears are the most common upper extremity condition seen by primary care and orthopaedic surgeons, with a spectrum ranging from tendinopathy to full-thickness tears with arthritic change. Some tears are traumatic, but most rotator cuff problems are degenerative. Not all tears are symptomatic and not all progress, and many patients in whom tears become more extensive do not experience symptom worsening. Hence, a standard algorithm for managing patients is challenging. The pathophysiology of rotator cuff tears is complex and encompasses an interplay between the tendon, bone and muscle. Rotator cuff tears begin as degenerative changes within the tendon, with matrix disorganization and inflammatory changes. Subsequently, tears progress to partial-thickness and then full-thickness tears. Muscle quality, as evidenced by the overall size of the muscle and intramuscular fatty infiltration, also influences symptoms, tear progression and the outcomes of surgery. Treatment depends primarily on symptoms, with non-operative management sufficient for most patients with rotator cuff problems. Modern arthroscopic repair techniques have improved recovery, but outcomes are still limited by a lack of understanding of how to improve tendon to bone healing in many patients.

Tenotomy-induced muscle atrophy is sex-specific and independent of NFκB

The nuclear factor-κB (NFκB) pathway is a major thoroughfare for skeletal muscle atrophy and is driven by diverse stimuli. Targeted inhibition of NFκB through its canonical mediator IKKβ effectively mitigates loss of muscle mass across many conditions, from denervation to unloading to cancer. In this study, we used gain- and loss-of-function mouse models to examine the role of NFκB in muscle atrophy following rotator cuff tenotomy - a model of chronic rotator cuff tear. IKKβ was knocked down or constitutively activated in muscle-specific inducible transgenic mice to elicit a twofold gain or loss of NFκB signaling. Surprisingly, neither knockdown of IKKβ nor overexpression of caIKKβ significantly altered the loss of muscle mass following tenotomy. This finding was consistent across measures of morphological adaptation (fiber cross-sectional area, fiber length, fiber number), tissue pathology (fibrosis and fatty infiltration), and intracellular signaling (ubiquitin-proteasome, autophagy). Intriguingly, late-stage tenotomy-induced atrophy was exacerbated in male mice compared with female mice. This sex specificity was driven by ongoing decreases in fiber cross-sectional area, which paralleled the accumulation of large autophagic vesicles in male, but not female muscle. These findings suggest that tenotomy-induced atrophy is not dependent on NFκB and instead may be regulated by autophagy in a sex-specific manner.

Influence of Infraspinatus and Subscapularis Pathologies on Supraspinatus Muscle Atrophy - A Retrospective Cohort Study

Introduction

Many factors could affect the supraspinatus (SSP) muscle after tendon rupture. We aimed to determine how infraspinatus and subscapularis tendon problems affect supraspinatus muscle atrophy associated with tears, in a retrospective cohort study conducted in a tertiary-level centre.

Material and Methods

Fifty-eight patients with a full-thickness SSP tendon tear who fulfilled the inclusion criteria were enrolled in the study. They were evaluated for tear retraction, fatty degeneration, and other rotator cuff tendon pathologies. Supraspinatus muscle was assessed using the Goutallier classification, and its average area was also measured. Accompanying lesions of the subscapularis and infraspinatus tendons and degree of supraspinatus muscle atrophy were evaluated using magnetic resonance imaging.

Results

Our results showed that supraspinatus tendon tears ranged between 3mm and 41mm, and the estimated average cross-sectional area of the SSP muscle was 247.6mm². Any degree of infraspinatus tendon pathology, ranging from tendinosis to full-thickness tears, was significantly correlated with the SSP muscle area (P < 0.05). The subscapularis tendon pathologies did not show a similar correlation. The interobserver and intraobserver reliabilities of the measurements were graded as excellent.

Conclusion

Impairment of any of the rotator cuff muscles may affect the other muscles inversely. Our study showed that all infraspinatus tendon pathologies and partial subscapularis tears affect and alter the SSP muscle belly. We suggest early intervention for supraspinatus tears to avoid further fatty degeneration, as muscle atrophy and fatty degeneration progress in combination with the accompanying lesions.

Infiltration of intramuscular adipose tissue impairs skeletal muscle contraction

KEY POINTS

Muscle infiltration with adipose tissue (IMAT) is common and associated with loss of skeletal muscle strength and physical function across a diverse set of pathologies. Whether the association between IMAT and muscle weakness is causative or simply correlative remains an open question that needs to be addressed to effectively guide muscle strengthening interventions in people with increased IMAT. In the present studies, we demonstrate that IMAT deposition causes decreased muscle strength using mouse models. These findings indicate IMAT is a novel therapeutic target for muscle dysfunction.

ABSTRACT

Intramuscular adipose tissue (IMAT) is associated with deficits in strength and physical function across a wide array of conditions, from injury to ageing to metabolic disease. Due to the diverse aetiologies of the primary disorders involving IMAT and the strength of the associations, it has long been proposed that IMAT directly contributes to this muscle dysfunction. However, infiltration of IMAT and reduced strength could both be driven by muscle disuse, injury and systemic disease, making IMAT simply an 'innocent bystander.' Here, we utilize novel mouse models to evaluate the direct effect of IMAT on muscle contraction. First, we utilize intramuscular glycerol injection in wild-type mice to evaluate IMAT in the absence of systemic disease. In this model we find that, in isolation from the neuromuscular and circulatory systems, there remains a muscle-intrinsic association between increased IMAT volume and decreased contractile tension (r2  > 0.5, P < 0.01) that cannot be explained by reduction in contractile material. Second, we utilize a lipodystrophic mouse model which cannot generate adipocytes to 'rescue' the deficits. We demonstrate that without IMAT infiltration, glycerol treatment does not reduce contractile force (P > 0.8). Taken together, this indicates that IMAT is not an inert feature of muscle pathology but rather has a direct impact on muscle contraction. This finding suggests that novel strategies targeting IMAT may improve muscle strength and function in a number of populations.

Extensor pollicis longus tendon rupture from Dorsal Nail Plate distal radius fixation with concomitant myostatic atrophy

Distal radius fractures are one of the most common upper extremity injuries seen by hand surgeons each year. Many of these fractures require reduction and surgical fixation because of displacement, comminution or inherent fracture instability. New hardware is designed and introduced each year to help surgeons manage these injuries. We report a case of a major complication from the use of the Dorsal Nail Plate hardware. The patient presented to our clinic with an extensor pollicis longus attrition rupture, which required tendon transfer for treatment. Intraoperatively, we also found ongoing extensor digitorum communis tendon attrition, myostatic atrophy of the extensor pollicus longus and ultimately a large bony defect following complex hardware removal. This case serves as a reminder to consider the potential complication profile of any hardware or technique you are considering using in your patients and to evaluate donor muscles and tendons when performing grafting or transfers.

Biotoxins in muscle regeneration research

Skeletal muscles are characterized by their unique regenerative capacity following injury due to the presence of muscle precursor cells, satellite cells. This characteristic allows researchers to study muscle regeneration using experimental injury models. These injury models should be stable and reproducible. Variety of injury models have been used, among which the intramuscular injection of myotoxic biotoxins is considered the most common and widespread method in muscle regeneration research. By using isolated biotoxins, researchers could induce acute muscle damage and regeneration in a controlled and reproducible manner. Therefore, it is considered an easy method for inducing muscle injury in order to understand the different mechanisms involved in muscle injuries and tissue response following injury. However, different toxins and venoms have different compositions and subsequently the possible effects of these toxins on skeletal muscle vary according to their composition. Moreover, regeneration of injured muscle by venoms and toxins varies according to the target of toxin or venom. Therefore, it is essential for researcher to be aware of the mechanism and possible target of toxin-induced injury. The current paper provides an overview of the biotoxins used in skeletal muscle research.