Early effects of muscle atrophy on shoulder joint development in infants with unilateral birth brachial plexus injury.
OBJECTIVE
Shoulder deformities in children with a birth brachial plexus injury (BBPI) are caused by muscle imbalances; however, the underlying mechanisms are unclear. The aim of this study was to assess the early interactions between shoulder muscles and shoulder joint development.


METHODS
In a retrospective magnetic resonance imaging (MRI) study of 36 infants (21 males, 15 females) younger than 12 months (mean 4.8 mo) with unilateral BBPI, volumes and thicknesses of standardized segments of the infraspinatus, subscapularis, and deltoid muscles were measured in both shoulders and expressed as ratios of pathological/unaffected side. The relation between muscle ratios and humeral head subluxation, passive external rotation, glenoid version, and deformity was analysed.


RESULTS
Compared with the unaffected side, the muscles of the affected side were of significantly smaller volume and thickness. The subscapularis was the most severely affected muscle, its volume being only 64% (SD 21%) and its thickness only 79% (SD 23%) of the corresponding values on the unaffected side (p < 0.001). Severe subluxation was predicted by the combination of low infraspinatus and subscapularis volume ratios (r(2) = 0.223; p = 0.014), but not by muscle thickness ratios. Subluxation was related to passive external rotation (p < 0.05), glenoid version (p < 0.01), and deformity (p < 0.01).


CONCLUSIONS
In infants with BBPI, muscle size is decreased during in the first months of life by both atrophy and, possibly, by a reduction in the number of sarcomeres in series. These effects are strongly related to shoulder joint subluxation.
Van Gelein Vitringa
Valerie M
VM
Department of Orthopaedic Surgery, VU Medical Centre, Amsterdam, the Netherlands.
Jaspers
Richard
R
Mullender
Margriet
M
Ouwerkerk
Willem J
WJ
Van Der Sluijs
Johannes A
JA
D001720:Birth Injuries / Q000175:diagnosis / Q000503:physiopathology*; D001917:Brachial Plexus / Q000293:injuries* / Q000473:pathology / Q000503:physiopathology; D020516:Brachial Plexus Neuropathies / Q000503:physiopathology*; D003286:Contracture / Q000175:diagnosis / Q000503:physiopathology; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008279:Magnetic Resonance Imaging; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation / Q000473:pathology; D009133:Muscular Atrophy / Q000175:diagnosis / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology; D012189:Retrospective Studies; D012518:Sarcomeres / Q000473:pathology / Q000502:physiology; D012783:Shoulder Dislocation / Q000175:diagnosis / Q000503:physiopathology; D012785:Shoulder Joint / Q000294:innervation* / Q000473:pathology / Q000503:physiopathology*