Subacromial Space Width: Does Overuse or Genetics Play a Greater Role in Determining It? An MRI Study on Elderly Twins.
BACKGROUND
Age and peripheral microcirculation disorders are the main causes of rotator cuff degeneration. Acromion variants may affect subacromial space width, causing a pathological narrowing of the space that may compromise the cuff integrity. However, it is not clear if the subacromial space width is genetically determined or if it changes according to loading conditions. To clarify this unresolved question, we performed an MRI (magnetic resonance imaging) study with the aim of evaluating the acromiohumeral distance in a group of elderly monozygotic and dizygotic twins, and we analyzed the obtained data using the twin design to separate the contributions of shared and unique environments.


METHODS
We identified twenty-nine pairs of elderly twins. On MRI scans, we evaluated the acromiohumeral distance and health status of the rotator cuff tendons. Heritability, defined as the proportion of total variance of a specific characteristic in a particular population due to a genetic cause, was estimated as twice the difference between the intraclass correlation coefficients for monozygotic and dizygotic pairs. The influence of shared environment, due to environmental factors that contribute to twin and sibling similarity, was calculated as the difference between the monozygotic correlation coefficient and the heritability index. One-way ANOVA (analysis of variance) was used to estimate the differences among job categories, both in the total cohort and within zygosity groups.


RESULTS
The intraclass correlation coefficient was substantially higher for monozygotic than for dizygotic twins, indicating a high degree of concordance of the acromiohumeral distance in pairs of individuals who shared 100% of their genes. The heritability index was 0.82, and shared and unique environmental contributions were both 0.09. There were no significant differences among subjects in different job categories, either in the total cohort (p = 0.685) or within the monozygotic (p = 0.719) and dizygotic groups (p = 0.957).


CONCLUSIONS
The acromiohumeral distance is mainly genetically determined and only marginally influenced by external factors.
Departments of Orthopaedics and Traumatology (S.G., V.A., and V.C.), Rehabilitation (T.V.), and Radiologic Sciences (C.C.), University of Rome Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail address for S. Gumina: s.gumina@tiscali.it.
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Gumina
Departments of Orthopaedics and Traumatology (S.G., V.A., and V.C.), Rehabilitation (T.V.), and Radiologic Sciences (C.C.), University of Rome Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail address for S. Gumina: s.gumina@tiscali.it.
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Arceri
National Centre for Epidemiology, Surveillance and Health Promotion, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy. E-mail address: protocollo-centrale@iss.mailcert.it.
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Fagnani
Departments of Orthopaedics and Traumatology (S.G., V.A., and V.C.), Rehabilitation (T.V.), and Radiologic Sciences (C.C.), University of Rome Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail address for S. Gumina: s.gumina@tiscali.it.
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Venditto
Departments of Orthopaedics and Traumatology (S.G., V.A., and V.C.), Rehabilitation (T.V.), and Radiologic Sciences (C.C.), University of Rome Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail address for S. Gumina: s.gumina@tiscali.it.
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Catalano
Departments of Orthopaedics and Traumatology (S.G., V.A., and V.C.), Rehabilitation (T.V.), and Radiologic Sciences (C.C.), University of Rome Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail address for S. Gumina: s.gumina@tiscali.it.
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Candela
National Centre for Epidemiology, Surveillance and Health Promotion, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy. E-mail address: protocollo-centrale@iss.mailcert.it.
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Nisticò
D000368:Aged; D016208:Databases, Factual; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D017006:Rotator Cuff / Q000473:pathology*; D000070636:Rotator Cuff Injuries; D012782:Shoulder / Q000473:pathology*; D013708:Tendon Injuries / Q000209:etiology* / Q000235:genetics / Q000473:pathology*; D014429:Twins, Dizygotic