The eccentric mechanotransduction, neuro-muscular transmission, and structural reversibility of muscle fatty infiltration. An experimental advanced disuse muscle-wasting model of rabbit supraspinatus
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Department of Arthroscopy, Minimally Invasive Surgery and Sports Traumatology Medical University of Lodz, Lodz, Poland
Morphometry Division, Department of Pathology, Medical University of Lodz, Lodz, Poland
Clinic of Orthopaedic and Traumatology Polish Mother’s Memorial Hospital Research Institute, Lodz, Poland
Department of Extrapyramidal Diseases, Medical University of Lodz, Lodz, Poland
Jarosław Fabiś   

Department of Arthroscopy, Minimally Invasive Surgery and Sports Traumatology Medical University of Lodz, Kosciuszki 4, 90-419 Lodz, Poland, Poland
Submission date: 2020-09-13
Final revision date: 2020-12-14
Acceptance date: 2020-12-26
Online publication date: 2021-03-05
Arch Med Sci 2021;17(5)
Full-thickness rotator cuff tear is present in almost 50% of patients over age 65 years, and its degree is known to be a good predictor of the severity of muscle-wasting (MW) sarcopaenia, also known as fatty degeneration (FD). A FD CT grade > 2° is recognized as a borderline of its reversibility. A disuse model of supraspinatus FD (grade 2) in rabbits provides clinically relevant data. Therefore, the present study evaluates the correlation between eccentric mechanotransduction, neuromuscular transmission (NT), and reversibility of muscle fatty infiltration (MFI) in rabbit supraspinatus FD > 2°.

Material and methods:
The supraspinatus tendon was detached from the greater tubercle, infraspinatus, and subscapularis in 16 rabbits. The tendon was reinserted after 12 weeks, and the animals were euthanized 24 weeks after reconstruction. MFI was measured in the middle part of the supraspinatus. Single-fibre EMG (SFEMG) examination of the supraspinatus NT was performed on 4 animals.

The power of analysis was 99%. Significant differences in MFI volume were found between the operated (4.6 ±1.1%) and the opposite control sides (2.91 ±0.61%) (p < 0.001). SFEMG revealed no significant differences between the disuse and the control supraspinatus muscles (p > 0.05); however, 6.5% of the examined muscle fibres exhibited NT disorders combined with blockade of conduction in 2.5% of muscle fibres.

Critical MFI in a disuse model of rabbit supraspinatus FD, CT grade > 2°, is substantially reversible by eccentric training despite subclinical impairment of neuromuscular transmission. In addition, 0.63% reversal of MFI is correlated with 1% hypertrophy of type I and II muscle fibre diameter.