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The lever arm ratio of the rotator cuff to deltoid muscle explains and predicts pseudoparalysis of the shoulder: the Shoulder Abduction Moment index
- Source :
- The bonejoint journal. (12)
- Publication Year :
- 2018
-
Abstract
- Aims In patients with a rotator cuff tear, tear pattern and tendon involvement are known risk factors for the development of pseudoparalysis of the shoulder. It remains unclear, however, why similar tears often have very different functional consequences. The present study hypothesizes that individual shoulder anatomy, specifically the moment arms (MAs) of the rotator cuff (RC) and the deltoid muscle, as well as their relative recruitment during shoulder abduction, plays a central role in pseudoparalysis. Materials and Methods Biomechanical and clinical analyses of the pseudoparalytic shoulder were conducted based on the ratio of the RC/deltoid MAs, which were used to define a novel anatomical descriptor called the Shoulder Abduction Moment (SAM) index. The SAM index is the ratio of the radii of two concentric spheres based on the centre of rotation of the joint. One sphere captures the humeral head (numerator) and the other the deltoid origin of the acromion (denominator). A computational rigid body simulation was used to establish the functional link between the SAM index and a potential predisposition for pseudoparalysis. A retrospective radiological validation study based on these measures was also undertaken using two cohorts with and without pseudoparalysis and massive RC tears. Results Decreased RC activity and improved glenohumeral stability was predicted by simulations of SAM indices with larger diameters of the humeral head, being consequently beneficial for joint stability. Clinical investigation of the SAM index showed significant risk of pseudoparalysis in patients with massive tears and a SAM < 0.77 (odds ratio (OR) 11). Conclusion The SAM index, which represents individual biomechanical characteristics of shoulder morphology, plays a determinant role in the presence or absence of pseudoparalysis in shoulders with massive RC tears.
- Subjects :
- Male
medicine.medical_specialty
Rotator Cuff Injuries
03 medical and health sciences
Rotator Cuff
0302 clinical medicine
Physical medicine and rehabilitation
Deltoid muscle
medicine
Humans
Orthopedics and Sports Medicine
Rotator cuff
In patient
Range of Motion, Articular
Aged
Retrospective Studies
Rupture
030222 orthopedics
Muscle Weakness
business.industry
Shoulder Joint
Biomechanics
030229 sport sciences
Deltoid Muscle
Tendon
Radiography
medicine.anatomical_structure
Surgery
Female
business
Subjects
Details
- ISSN :
- 20494408
- Issue :
- 12
- Database :
- OpenAIRE
- Journal :
- The bonejoint journal
- Accession number :
- edsair.doi.dedup.....eebeb3de2df6ab48e6ccc066332b2be5