The FS is classified into primary and secondary types [4]. Primary FS shows idiopathic onset and is defined as an idiopathic global limitation of humeroscapular motion resulting from contracture and loss of compliance of the glenohumeral joint capsule. Secondary FS is associated with known pathology and is further classified into intrinsic, extrinsic, and systemic subtypes [5]. Conditions such as rotator cuff tendinosis and tears, and calcific tendinitis precede intrinsic secondary FS. Secondary extrinsic FS may be associated with post-trauma, iatrogenic (postsurgical), high impact force or low-level repetitive activity. Systemic secondary FS occurs in patients with co-morbidities such as diabetes, thyroid abnormalities or cardiac disease.

The pathological changes associated with FS include chronic synovial inflammation followed by fibroticcontracture of the rotator interval, the glenohumeral capsule, and ligaments. Therefore, FS can be considered is a soft tissue affliction, not a bone disorder, which limits glenohumeral joint motion secondary to shortening and thickening of soft tissues [3].

The natural course of FS characteristically shows the following 3 stages of evolution: the painful, stiff, and resolution phase [6]. Hannafin and Chiaia [7] described 4 stages including a preadhesive, freezing, frozen, and thawing phase based on duration of symptoms, pain and limitation of motion, physical examination under anesthesia, and arthroscopic findings. The mean duration of symptoms in patients with FS is 30.1 months (range 12~42 months). Some degree of motion limitation, particularly in internal rotation, persists in patients during the thawing phase; however, minimal functional limitations are observed. Determination of these stages is essential to administer appropriate treatment, predict the prognosis, and establish appropriate patient and physician expectations. The staging of FS is determined clinically based on symptoms and physical examination. Intra-articular injection of local anesthetic is useful to distinguish between the painful and the stiff stages. Early diagnosis of FS shortens the duration of shoulder stiffness, thereby significantly reducing the patient's morbidity.

FS can usually be diagnosed on the basis of accurate history taking and clinical findings. However, FS may mimic other causes of shoulder pain including arthritis, rotator cuff disorders, calcific tendinitis, and impingement syndrome. Cyriax [8] described atypical pattern of progressively restricted shoulder joint movement attributed to capsular restriction that was termed a capsular pattern of range-of-motion restriction. The capsular pattern of limitation is described as maximal limitation of external rotation, followed by limitation of abduction, which is more significantly limited than internal rotation.

Normal radiographs are needed to exclude other painful shoulder disorders that may also present with painful restriction of shoulder motion. Therefore, in addition to radiographs, imaging modalities such as arthrography, ultrasonography, and magnetic resonance imaging (MRI) are required to exclude other painful concurrent disorders and to confirm FS. Previously, conventional arthrography was used as a standard diagnostic modality for FS. It reveals a small volume capacity of the glenohumeral joint (7~10 mL), synovial irregularities and serrations of the joint capsule, non-filling of long head of the bicipital tendon sheath, and obliteration of the subcapsular or axillary recess (AR) [910].

Recently, the following characteristic MRI findings are used to successfully diagnose FS: complete obliteration of the fat triangle under the coracoid process, thickened coracohumeral ligament, increased signal intensity of the joint capsule and synovial membrane in the rotator cuff interval and AR with gadolium-chelate enhancement using indirect (intravenous) MR arthrography [1112].

Ultrasonography can accurately identify soft tissue changes such as coracohumeral ligament and AR thickening, and the presence of fibro-inflammatory soft tissue in the rotator cuff interval in patients with FS. Ultrasonography measurement of coracohumeral ligament thickness is useful to diagnose FS [1314] (Figure 1). Previous studies reported that power Doppler ultrasonography showed hypoechogenicity surrounding the long head of the biceps tendon with increased vascularity at the rotator cuff interval in all patients with FS [1516] (Figure 2). However, another study reported that power Doppler ultrasonography showed increased vascularity in the rotator cuff interval in only 29% of patients with FS [17]. In a recent study increased vascularity was observed in the rotator cuff interval in only 10 of 75 patients (13.3%) during the freezing or frozen stage of FS. Therefore, the validity of Doppler ultrasonography in the diagnosis of FS remains controversial [18]. Sonoelastography is a recently developed technique based on real-time ultrasonography for evaluation of tissue elasticity. This technique is based on the principle that pressure on tissues produces changes in those tissues depending upon their elasticity and that tissue compression produces a certain degree of strain (displacement), which is lesser in hard tissues and greater in softer tissues [19]. Pervious report investigating patients with FS have described that shear wave sonoelastography revealed increased stiffness of the thickened coracohumeral ligament in the affected shoulder compared with that in the unaffected shoulder [20]. This finding suggests that increased stiffness of the coracohumeral ligament may contribute to the limited range of shoulder external rotation. Ultrasonography measurements of the AR thickness in asymptomatic shoulders showed high intra- and inter-rater reliability; thus ultrasonography is a useful imaging modality to measure the AR thickness (Figure 3). Moreover, the AR thickness was shown to vary with sex, height, and weight of subjects [21]. Therefore, measurement of the AR thickness in the affected shoulder is not recommended as a good diagnostic indicator to diagnose FS. The ratio of the AR thickness between the affected and the unaffected shoulders could serve as a reliable diagnostic standard in the patients with unilateral FS. Comparison of imaging findings using arthrography, ultrasonography, and MRI with clinical impairment in patients with the freezing or frozen stage of FS showed that ultrasonography and arthrographic findings were correlated with clinical assessment scores. However, not all MRI parameters including the thickness of the coracohumeral ligament, AR, and enhanced portion of the rotator cuff interval showed such a correlation [18]. Although arthrography and ultrasonography are complementary, ultrasonography measurements of AR and coracohumeral ligament thickness are recommended to diagnose FS for the following reasons; ultrasonography is a noninvasive modality, it is easy to compare ultrasonography parameters between the affected and the unaffected sides, it reflects the clinical characteristics of FS, and ultrasonography parameters serve as reliable anatomical indicators of FS.

The goal of treatment in patients with FS is pain reduction and restoration of normal function and motion of the shoulder. Conservative treatments for FS include patient education, oral administration of nonsteroidal anti-inflammatory drugs and opioids, physical therapy, therapeutic exercise, joint mobilization and manipulation, intra-articular corticosteroid and hyaluronic acid injections, suprascapular nerve block, glenohumeral distention arthrography, manipulation under general anesthesia, interventional microadhesiolysis, and translational mobilization with serial capsular distention. Decision-making regarding treatment of FS warrants careful attention to the patient's symptoms and the stage of the disease because patients with FS require an individualized therapeutic strategy according to patient's needs. Prior to treatment initiation, patients should be categorized into the ‘more pain than limitation of shoulder mobility’ or ‘more limitation than pain’ groups.