VEGF, VEGFR-1, and CTGF Cell Densities in Tendon Are Increased with Cyclical Loading: An In Vivo Tendinopathy Model Leena H. Nakama, 1 Karen B. King, 2 Sven Abrahamsson, 3 David M. Rempel 1,2 1 University of California, Berkeley and San Francisco, Joint Graduate Group in Bioengineering, 1301 South 46th Street, Bldg 163, Richmond, California 94804 2 University of California, San Francisco, Department of Medicine and University of California, Berkeley and San Francisco, Joint Graduate Group in Bioengineering 3 Malmo University Hospital, Malmo, Sweden, Department of Hand Surgery Received 7 June 2005; accepted 5 August 2005 Published online 6 January 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jor.20053 ABSTRACT: Tendon injuries can occur in athletes and workers whose tasks involve repetitive, high- force hand activities, but the early pathophysiologic processes of tendinopathy are not well known. The purpose of this animal study was to evaluate the effects of cyclical tendon loading on the densities of cells producing growth factors such as vascular endothelial growth factor (VEGF), its receptor, vascular endothelial growth factor receptor 1 (VEGFR-1), and connective tissue growth factor (CTGF) in the Flexor Digitorum Profundus (FDP) tendon at the epicondyle. The FDP muscles of nine New Zealand rabbits were electrically stimulated to contract repetitively for 80 h of cumulative loading over 14 weeks. The contralateral limbs served as controls. The tendons at the medial epicondyle insertion sites were harvested, and sections were immunostained with antibodies directed against VEGF, VEGFR-1, or CTGF. Positive-staining cells were counted in six regions of interest: three along the enthesis, and three corresponding regions 1500 microns distal to the enthesis. VEGF ( p ¼ 0.0001), VEGFR-1 ( p ¼ 0.046), and CTGF ( p ¼ 0.0001) cell densities were increased in the tendon of the loaded limb compared to the nonloaded limb. In addition, regional differences in VEGF, VEGFR-1, and CTGF cell densities were found. VEGF, VEGFR-1, and CTGF are increased in tendon experiencing cyclical loading and may play a role in the early vascular changes in the progression to tendinosis. ß 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:393– 400, 2006 Keywords: VEGF; CTGF; in vivo loaded tendon INTRODUCTION Tendon injuries due to overuse are a common problem in athletes and workers, and account for 30 to 50% of all sports-related injuries 1,2 and almost half of the occupational illnesses in the United States. 3 Epicondylitis, a tendinopathy at the elbow, is a common disorder in adults; the incidence in general practice is approximately 4 to 7 per 1000 patients per year, with an annual incidence of 1 to 3% in the general population. 4,5 Although epicondylitis is related to forceful and repetitive hand activities, little is known about the early mechanisms of injury that ultimately lead to tendinopathy. Identifying the initial biological changes in tendons exposed to cyclical loading may ultimately improve prevention and treatment options and further expand our under- standing of the etiology of tendinosis and its pathogenesis. Epicondylitis presents as localized pain, tender- ness, and occasionally swelling. 6 Biopsies of the tendon and surrounding scar tissue in patients with epicondylitis reveal fibrovascular and cellu- lar proliferation, intratendinous calcification and JOURNAL OF ORTHOPAEDIC RESEARCH MARCH 2006 393 Correspondence to: David Rempel (Telephone: 510 665- 3403; Fax: 510-665-3423 E-mail: drempel@itsa.ucsf.edu) ß 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.