Introduction ! The maintenance of joint function depends on an intact structure of its components, especially ar- ticular cartilage, but also on the functional and structural integrity of the muscles acting on and supporting a joint [6,13–15]. Weak muscles are assumed to reduce joint stability and mechanical shock absorption [14, 33, 34], and may predispose a joint to develop osteoarthritis (OA). However, in a classical point of view, strength deficits or atro- phic structure of these muscles are usually seen as a consequence of OA, i.e., an outcome of disuse induced by articular pain [6,14, 24, 25]. Which process the other one precedes has not yet been clarified. The interdependence of OA and altera- tions of periarticular muscles have especially been studied for the knee joint and the quadri- ceps muscle [2,14, 35]. It has been argued that a weak quadriceps muscle cannot produce enough strength to stabilize the knee and to prevent overload during the amortization of the mechan- ical impact during gait [14, 33,34]. The osteoarthritic hip joint has been less studied in this respect, but a reduced force of the gluteus medius muscle (GM) has already been associated with hip OA [2, 22,30, 31]. However, the biome- chanical situation around the hip appears more complicated than that around the knee. In fact, while the interaction of muscle and joint can be looked at unilaterally in the case of the knee, the hip joints and the abductor muscles interact bi- laterally during gait [8,17, 26,37]. When body weight gets supported by the hip joint of one side during gait, the ipsilateral abductors have to sta- bilize pelvic equilibrium in the frontal plane and ensure a smooth load transfer by their eccentric contraction in order to avoid excessive shock im- pact on the contralateral hip joint [16,17,19,38]. If the hip abductors, especially the GM, are not sufficiently strong, such will result in the typical gait pattern, the “Trendelenburg gait” [17,26]. This pattern, with a drop of the opposite hip due to the abductors’ deficit during unipodal walking, leads to adduction of the ipsilateral hip, moving the compressive force laterally to the acetabulum [28,36]. However, this pelvic drop is frequently avoided by the patients by bending the trunk, shifting the body gravity center closer to the load bearing hip (“Duchenne limping” or “abductor Abstract ! In order to understand the role of the gluteus medius muscle (GM) in hip joint osteoarthritis, the objective of this study was to analyze the cor- relation between morphometric data of GM sam- ples with osteoarthritis scores of ipsilateral and contralateral hips in 41 patients. GM samples ob- tained during unilateral hip replacement surgery were used to evaluate muscle fibers in the cross- sectional area (CSA) and other features indicative for muscle aging. Clinical symptoms were as- sessed by the Lequesne pain score. Hip osteo- arthritis was graded by the Kellgren score and by measuring the sum joint space width (sumJSW) at three different articular locations and minimal JSW in a. p. radiographs. Varying degrees of GM muscle atrophy correlated with the pain score; pain score also correlated with radiographic signs of osteoarthritis. GM CSA was significantly correlated with all radiographic signs of the con- tralateral hip, but only with the sumJSW in the ipsilateral hip. It can be concluded that a weak GM may be the result of ipsilateral osteoarthitis, but may especially predispose the contralateral hip to develop osteoarthritis. This can be associ- ated with an impaired GM capacity to avoid the shock impact in the load transfer during gait. Muscle strengthening is therefore recommended. Gluteus Medius Muscle Atrophy is Related to Contralateral and Ipsilateral Hip Joint Osteoarthritis Authors A. Amaro 1 , F. Amado 2 , J. A. Duarte 1,3 , H.-J. Appell 3,4 Affiliations 1 Health School, University of Aveiro, Aveiro, Portugal 2 Department of Chemistry, University of Aveiro, Aveiro, Portugal 3 CIAFEL, FADE, University of Porto, Porto, Portugal 4 Physiology and Anatomy, German Sport University, Cologne, Germany Key words l " hip osteoarthritis l " muscle histology l " muscle fiber area l " Kellgren index l " joint space width accepted after revision January 29, 2007 Bibliography DOI 10.1055/s-2007-965078 Published online May 29, 2007 Int J Sports Med 2007; 28: 1035 – 1039 © Georg Thieme Verlag KG Stuttgart • New York • ISSN 0172-4622 Correspondence Prof. Hans-Joachim Appell Physiology and Anatomy German Sport University Carl Diem Weg 6 50933 Cologne Germany Phone: + 49 221 49 82 54 30 appell@dshs-koeln.de 1035 Amaro A et al. Hip Osteoarthritis and … Int J Sports Med 2007; 28: 1035 – 1039 Orthopedics & Biomechanics