Intrinsic Risk Factors for the Development of Patellar Tendinitis in an Athletic Population A Two-Year Prospective Study* Erik Witvrouw,²‡§ PT, PhD, Johan Bellemans,‡ MD, PhD, Roeland Lysens,‡ MD, PhD, Lieven Danneels,‡ PT, and Dirk Cambier,² PT, PhD From the ²Department of Physical Therapy, Faculty of Medicine, University of Gent, Gent, Belgium, and ‡Faculty of Physical Therapy & Rehabilitation Sciences, Catholic University of Leuven, Leuven, Belgium ABSTRACT Retrospective studies have suggested various factors that might cause a disposition to develop patellar ten- dinitis, but no prospective data exist to determine any relationships. The purpose of this study was to deter- mine the intrinsic risk factors for the development of patellar tendinitis in an athletic population. Before the study, 138 male and female students of physical edu- cation were evaluated for anthropometric variables, leg alignment characteristics, and muscle tightness and strength parameters. During the 2-year study, 19 of the 138 students developed patellar tendinitis. In all cases the diagnosis was confirmed by the presence of a hypoechogenic nodular lesion in the proximal region of the patellar tendon. Univariate and stepwise discrimi- nant function analyses were performed comparing the various measurements. These analyses revealed that the only significant determining factor was muscular flexibility, with the patellar tendinitis patients being less flexible in the quadriceps and hamstring muscles (P  0.05). The results of this study demonstrate that lower flexibility of the quadriceps and hamstring muscles may contribute to the development of patellar tendinitis in an athletic population. Therefore, the prevention of this condition in athletes should be focused on screen- ing for and treating poor quadriceps and hamstring muscle flexibility. Musculoskeletal overuse injuries are common, and the number of injuries related to physical activity and sport is increasing. 8, 25, 56 Tendon injuries are common in sports because much stress and force are focused to the tendon part of the muscle-tendon unit during physical activity, increasing the risk of injury to tendon tissue. 20, 25, 51 The term “jumper’s knee,” or patellar tendinitis, is used to describe an overuse injury that involves pain at either the attachment of the quadriceps tendon to the superior pole of the patella, at the distal insertion of the extensor mechanism at the tibial tuberosity, or (most commonly) at the proximal attachment of the patellar tendon at the inferior pole of the patella. 2, 13, 45 The onset of this pain is often insidious. Pain may be related to microtearing of the collagen and elastin fibers, inflammation, edema, mucoid degeneration, fibroid necroses, erosion of the inferior pole of the patella, or rupture of the patellar ten- don. 1, 3, 14, 17, 49, 58 Histologically, the lesions of jumper’s knee have been localized to the bone-tendon junction. There is no particular age predilection, although patellar tendinitis is most common in skeletally mature patients. The cause of jumper’s knee is, as of most injuries, con- sidered to be multifactoral, 6 including both intrinsic and extrinsic factors. 7 The term “extrinsic risk factor” refers to the amount of training, the intensity of training, the train- ing surfaces, the environmental conditions, the footwear and equipment, and training errors. 7, 42 The importance of these in the development of patellar tendinitis is well known. Studies have shown that as the intensity or fre- quency of training increases, the incidence of patellar tendinitis rises. 52, 53 The fact that not every athlete who is exposed to intense and repetitive training develops jumper’s knee suggests that there may be intrinsic risk factors for this condition as well. Several authors hypothesize that malalignment, *Presented at the 26th annual meeting of the AOSSM, Sun Valley, Idaho, June 2000. § Address correspondence and reprint requests to Erik Witvrouw, PT, PhD, Gent University Hospital, Faculty of Medicine, Department of Rehabilitation Sciences and Physiotherapy, De Pintelaan 185, 9000 Gent, Belgium. No author or related institution has received any financial benefit from research in this study. 0363-5465/101/2929-0190$02.00/0 THE AMERICAN JOURNAL OF SPORTS MEDICINE, Vol. 29, No. 2 © 2001 American Orthopaedic Society for Sports Medicine 190