Skeletal developmental patterns in the acromial process and distal clavicle as observed by MRI.

OBJECTIVE: To provide an MRI timeline of normal skeletal developmental patterns in the acromial process and distal clavicle in children up to 18 years of age. MATERIALS AND METHODS: Retrospective review of all shoulder MRIs obtained at our institution between January 2003 and March 2012, in children up to age 18, was performed. When available, radiographs and CT scans for these children were also reviewed. The following variables of the distal acromion and clavicle, with attention to morphology and MRI signal, were assessed: (1) Chondro-osseous junction and (2) Development and fusion of the secondary ossification centers. RESULTS: Ninety-eight children with 116 MR studies were identified from the data search. Of these, 13 patients were excluded and the final cohort included 85 children with 102 MRI studies. Forty-one of these patients also had shoulder radiographs. The cartilaginous precursors of the distal clavicle and acromion conformed to the final shape of these structures. The chondro-osseous interphases became progressively more lobulated and notched in the distal acromion and clavicle respectively. Appearance and fusion of the secondary ossification centers was significantly earlier in our study than previously reported. Acromial secondary ossification centers began forming at age 10 and clavicular ones, while uncommon, began forming at age 11. Fusion of acromial primary and secondary ossification centers began at age 14 and was generally complete after age 16. CONCLUSIONS: Based on MR imaging the development and fusion of the acromion and distal clavicle in children occur earlier than previously reported. They follow a sequential pattern and can serve as a blueprint for evaluating imaging studies of pediatric shoulders.

Shape of the acromion: congenital or acquired--a macroscopic, radiographic, and microscopic study of acromion.

Debate continues as to whether the differing shapes of the acromion are congenital or acquired. This has been investigated by neonatal cadaver study, adult cadaver study, radiographic study, magnetic resonance imaging study, or various other means. No one, to our knowledge, has investigated this by histologic study. A macroscopic and histologic study of 22 cadaveric shoulder joints was carried out to establish what, if any, developmental changes occur in the differing patterns of acromion. The cadaveric shoulders were dissected and examined macroscopically. All of the acromion processes were transected and photographed, and the histology of the anterior and inferior surfaces was studied. In all of the curved and hooked types of acromion, a common pattern of degeneration of collagen, fibrocartilage, and bone was observed, consistent with a traction phenomenon. None of the flat acromions exhibited these changes. Extensive histologic changes were noted on the anterior surface of acromion as compared to the inferior surface in curved or hooked acromion. We conclude that the different shapes of acromion are, therefore, acquired as a response to traction forces applied via the coracoacromial ligament and are not congenital in origin.

Radiology of postnatal skeletal development. VII. The scapula.

Twenty-four pairs of scapulae from fetal specimens and 35 pairs of scapulae from postnatal cadavers ranging in age from full-term neonates to 14 years, were studied morphologically and roentgenographically. Air-cartilage interfacing was used to demonstrate both the osseous and cartilaginous contours. When the entire chondro-osseous dimensions, rather than just the osseous dimensions, were measured, the scapula had a height-width ratio ranging from 1.36 to 1.52 (average 1.44) during most of fetal development. The exceptions were three stillborns with camptomelic, thanatophoric, and achondrogenic dwarfism in which the ratio averaged 0.6. At no time during fetal development was the glenoid cavity convex; it always had a concave articular surface. However, the osseous subchrondral countour was often flat or slightly convex. In the postnatal period the height-width ratio averaged 1.49. The ratio remained virtually unchanged throughout skeletal growth and maturation. In a patient with unilateral Sprengel's deformity the ratio for the normal side was 1.5, while the abnormal was 1.0. The cartilaginous glenoid cavity was always concave during postnatal development, even in the specimens with major structural deformities, although the subchondral osseous contour was usually flat or convex during the first few years of postnatal development. Ossification of the coracoid process began with the development of a primary center at three to four months. A bipolar physis was present between the primary coracoid center and the primary scapular center until late adolescence.