ankylosis of the SIJ and ossification of STL, SSL and transverse acetabular ligament (TAL). Case Report During routine osteology teaching program for undergraduate medical students of Narayana Medical College, Nellore,

Sacro-iliac joint (SIJ) is the largest axial joint and is considered as a bottom joint of spinal column relating to the hip bones. It is a diarthroidial joint consisting of two types of articulation: (1) an antero-inferior synovial joint between the C-shaped auricular surfaces of the sacrum and ilium and (2) a postero-superior syndesmosis situated between the interosseous surfaces of the ilium and sacrum. The joint surfaces are irregular with ridges and depressions that are reciprocal and are greater in males. These irregular articular surfaces restrict the movements and contribute to the strength of the joint and enhance the stability rather mobility during the transmission of the weight from the vertebral column to the lower limbs. Eventhough the irregular surface of the joint help to maintain more in the stability, slight antero-posterior rotation occurs around a transverse axis of the joint. These movements are increased in pregnant women than in men and normal women [1]. Several muscles such as gluteus maximus, piriformis, biceps femoris and ligaments such as anterior and posterior sacroiliac, sacrotuberous and sacrospinous ligaments surrounding the joint influence the movement and stability of this joint [2]. Ankylosis or fusion of the joint and ossification of the adjacent ligaments can decrease the mobility of the joint. The muscles and ligaments acting on this joint, traverses the SIJ both in front and back and can be the causes of pain and inflammation if these joints are in dysfunction [3]. Sacrotuberous (STL) and sacrospinous (SSL) ligaments are the two important structural components of the pelvic cavity because they connect the pelvic bones to vertebral column (sacrum). STL is a strong ligament provides support and stabilizes the sacroiliac region by compensating its rotation, therefore limiting the movement of the lower portion of sacrum [4]. The pudendal neurovascular bundle passes around the SSL close to its attachment to the ischial spine [5]. An ossified STL or SSL may crush or lesion the pudendal nerve when passing through the intraligamentous space of these ligaments resulting in the pudendal nerve entrapment syndrome [6]. In the present case, we describe a pelvic bone with bilateral ankylosis of the SIJ and ossification of STL, SSL and transverse acetabular ligament (TAL).