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The acromioclavicular joint (ACJ) is a widely used, versatile and complex gliding synovial joint that provides stability and increases function; the ability to raise the arm above the head with a greater degree of arm rotation. The ACJ is vulnerable to trauma, instability, dislocation, osteoarthritis and synovitis.1 The ACJ is assessed for dislocation, fluid collection, cysts and bone erosions,2 3 but measurement of normal ACJ parameters is still unclear. That makes it difficult to interpret ACJ pathology.4 Ultrasound (US) enables direct imaging of the ACJ and has been proven to be as accurate as x ray assessment.4
The aim of the present study was to obtain US dimensions of the ACJ in healthy volunteers. US assessment (SonoSite Titan with 5–10 MHz L38 linear transducer; SonoSite, Bothell, Washington, USA) of 50 ACJs was performed on 12 males and 13 females, mean (SD) age 46 (12) years with a body mass index (BMI) of 28.5 (5.4) kg/m2. On the longitudinal standard view,3 depicting the “mushroom” form of the ACJ, the joint longitudinal (c–d) (maximal distance between internal and external margins of synovial cavity) and the joint dome (e–f) (perpendicular from the joint nadir to the joint length line) were measured (fig 1A). On the transverse view, revealing the oval shape of the joint cavity section, ACJ height (h–n) (craniacaudal size) and width (w–t) (anterior–posterior size) were measured (fig 1B). Using properties of the Sonosite Titan to measure spatial objects, the ACJ volume and the ACJ volume/BMI ratio were measured as well; the results are shown in table 1. According to the data of the table and 2 SD deviation for the borderline limit of statistical range, an ACJ longitudinal of 2 cm, width 2.7 cm, height 2 cm, dome 0.46 cm, volume 4 cm3 and volume/BMI 0.2 might be ascribed to synovial enlargement. A subgroup of 13 persons over the age of 50 had ACJs higher (1.47 (0.31) vs 1.18 (0.2); p<0.001) and of larger volume (2.24 (0.98) vs 1.6 (0.54); p = 0.006) compared to the subgroup of 12 persons younger than 50. No difference in BMI was found between the two groups (p = 0.44). No difference in ACJ volume was found between the male and female groups (p = 0.67).
The US biometric previously published anatomical studies of the ACJ proposed several parameters, including superficial joint space distance (7.7 (2.2) mm), deep joint space distance, the joint capsule–clavicular and acromial bone rim distances, maximal distance between the joint capsule and superficial and deep joint space,5 the width of the joint space.6 However, this seems difficult to define in a reproducible way,6 in part due to the very variable and unclear points of the distance scores. Furthermore, these investigators did not use a transversal view, showing the real height and anterior–posterior width of the ACJ (fig 1B). In other studies standard reference values for ACJ were measured using bone–capsule distance from the medial end of acromion and lateral end of clavicula,7 which proved to be close to the dome value (2.5 (1.7) vs 2.7 (0.8) mm) in our trial. However, variability of bone (osteophytes, different forms) as a point of the score might deviate natural joint dimensions.
Acromioclavicular joint space measurement (4.1 mm in the Alasaarela study6 and 5.2 mm in the Schmidt study)7 has some problems due variability and poor identification of “the joint floor”, partially due to intra-articular meniscoids.5 By analysing the US results of the ACJ in longitudinal and transverse positions in 25 healthy people, normal simply defined parameters independent of another structures of the joint were found that could enable the delineation of pathological deviations in patients with diseases of the ACJ. Healthy persons older than 50 years of age had larger ACJs.
Competing interests: None declared.
Ethics approval: Ethics approval was obtained.
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