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Ioanna K Bolia, Kevin Mertz, Ethan Faye, Justin Sheppard, Sagar Telang, Jacob Bogdanov, Laith K Hasan, Aryan Haratian, Denis Evseenko, Alexander E Weber, Frank A Petrigliano USC Epstein Family Center for Sports Medicine at Keck Medicine of USC, Los Angeles, CA, USACorrespondence: Ioanna K Bolia, 1520 San Pablo Street Suite 2000, Los Angeles, CA, 90033, USA, Tel +1 9703432813, Fax +1 818-658-5925, Email frank.petrigliano@med.usc.eduAbstract: Knee fibrosis is characterized by the presence of excessive connective tissue due to dysregulated fibroblast activation following local or systemic tissue damage. Knee fibrosis constitutes a major clinical problem in orthopaedics due to the severe limitation in the knee range of motion that leads to compromised function and patient disability. Knee osteoarthritis is an extremely common orthopedic condition that is associated with patient disability and major costs to the health-care systems worldwide. Although knee fibrosis and osteoarthritis (OA) have traditionally been perceived as two separate pathologic entities, recent research has shown common ground between the pathophysiologic processes that lead to the development of these two conditions. The purpose of this review was to identify the pathophysiologic pathways as well as key molecules that are implicated in the development of both knee OA and knee fibrosis in order to understand the relationship between the two diagnoses and potentially identify novel therapeutic targets.Keywords: fibrosis, molecules, knee, osteoarthritis

Ioanna K Bolia, Kevin Collon, Jacob Bogdanov, Rae Lan, Frank A Petrigliano USC Epstein Family Center for Sports Medicine at Keck Medicine of USC, Los Angeles, CA, USACorrespondence: Frank A PetriglianoUSC Epstein Family Center for Sports Medicine at Keck Medicine of USC, 1520 San Pablo Street Suite 2000, Los Angeles, CA, 90033, USATel +1 9703432813Fax +1 818-658-5925Email frank.petrigliano@med.usc.eduAbstract: Athletes participating in overhead sports are at particularly high risk of shoulder impingement syndrome. Subcoracoid impingement is defined as impingement of the anterior soft tissues of the shoulder between the coracoid process and the lesser tuberosity. Subacromial impingement syndrome (SIS) occurs due to extrinsic compression of the rotator cuff between the humeral head and coracoacromial structures or intrinsic degeneration of the supraspinatus tendon and subsequent superior migration of the humerus. Internal impingement is a major cause of shoulder pain in overhead athletes, and it occurs due to repetitive impingement of the articular surface of the rotator cuff with the glenoid during maximum abduction and external rotation of the arm. When examining athletes with suspected impingement of the shoulder, it is important to discuss the sport-specific motion that regenerates the symptoms and perform a combination of physical examination tests to improve the diagnostic accuracy. Radiographic evaluation is recommended, and the extent of soft tissue abnormalities can be assessed on ultrasound or magnetic resonance imaging of the shoulder. Management of shoulder impingement syndrome can be conservative or operative, based on the severity and chronicity of symptoms and the associated structural abnormalities. This review provides an update on the management of SIS, subcoracoid impingement, and internal impingement in the athletic population.Keywords: impingement syndromes, shoulder, subacromial impingement syndrome, joint, subcoracoid, internal, athletes