RADIOLOGY—PICTORIAL ESSAY Posterosuperior glenoid internal impingement of the shoulder in the overhead athlete: Pathogenesis, clinical features and MR imaging findings Chris Kon Fessa, 1 Anthony Peduto, 2 James Linklater 3 and Phillip Tirman 4 1 Prince of Wales Hospital, Sydney, New South Wales, Australia 2 Westmead Hospital, Sydney, New South Wales, Australia 3 Castlereagh Imaging, Sydney, New South Wales, Australia 4 Renaissance Imaging Medical Associates, Northridge, California, USA CK Fessa MBBS; A Peduto MBBS; J Linklater MBBS; P Tirman MD. Correspondence Dr Chris Kon Fessa, Department of Medical Imaging, Prince of Wales Hospital, Barker Street Randwick, Sydney, NSW 2031, Australia. Email: cfessa@yahoo.com.au Conflict of interest: No conflict of interest is declared. Submitted 30 April 2014; accepted 25 November 2014. doi:10.1111/1754-9485.12276 Summary Posterosuperior glenoid internal impingement (PGII) is an impingement syn- drome of the shoulder that is most commonly seen in the throwing or overhead athlete. The supraspinatus can be normally compressed or impinged between the greater tuberosity and the posterosuperior labrum in the abduc- tion and external rotation position. However, repetitive throwing and biomechanical abnormalities may lead to the intensification of this contact and to the clinical and pathological picture of PGII. The injured athlete usually complains of poor throwing performance and pain located in the posterosuperior aspect of the shoulder. Two main theories regarding the aetiology of PGII have been postulated with differing initial mechanisms. The MRI features of PGII have been described and include supraspinatus and anterior infraspinatus partial undersurface tears, bony changes at the humeral head and labral pathology, including a variation of the type II superior labrum from anterior to posterior lesion. This pictorial essay aims to present cases illustrating the pathophysiology, clinical features and recently described MRI findings, and discuss some of the MR protocol considerations. Key words: anterior instability; GIRD; glenohumeral internal rotation deficit; internal impingement; MRI; MRI arthrography. Introduction The act of throwing can be explained by the kinetic chain theory. 1 The production of energy by the lower limbs and trunk is directed through the shoulder and transferred to the object on release. The extreme forces funnelled through the shoulder in combination with biomechanical derangement and repetition leave the throwing shoulder susceptible to injury. One example of an injury that may result in significant disability for the throwing and overhead athlete is posterosuperior glenoid internal impingement (PGII). During the late cocking phase of throwing, the shoulder reaches a maximum external rotation of 170–180°, while abduction is maintained at 90–100°. In this ABER posi- tion, Walch et al. were the first to note that contact of the rotator cuff occurs between the greater tuberosity and the posterosuperior labrum (Fig. 1a). 2 This was termed posterosuperior glenoid impingement. Subse- quent studies showed that the same contact may occur in the shoulders of normal individuals placed in the ABER position using MRI and is a physiological phenomenon in this position. 3,4 However, as a result of the biomechanics of throwing or other overhead athletic activities, and certain structural adaptations which occur in the shoul- ders of these athletes, this contact is intensified and leads to pathological PGII. This clinical syndrome is characterised by posterosuperior pain and dysfunction, and a series of structural injuries to the shoulder. Ath- letes participating in sports, such as baseball, water polo, tennis and javelin, are particularly at risk of devel- oping PGII. Pathophysiology In these athletes, a pathological shift of the axis of glenohumeral joint contact/rotation occurs as the arm is brought into the ABER position during the throwing or Journal of Medical Imaging and Radiation Oncology 59 (2015) 182–187 © 2015 The Royal Australian and New Zealand College of Radiologists 182