Title: Novel dissection approach of equine back muscles: new advances in anatomy and topography - and comparison to present literature.
Knowledge of the anatomy and topography of the equine back are essential for a correct diagnosis and treatment as well as communication among therapists, especially since different authors have not always agreed upon the anatomical topography of the epaxial back muscles. In this study, we performed a novel 3-D dissection procedure that focused on maintaining the integrity of the myofascial role in muscle topography. A total of 17 horses were carefully dissected, recorded and videotaped. The results revealed some interesting points. 1) The iliocostalis muscle (IL) was found to be clearly distinct from the longissimus dorsi muscle (LD) and positioned ventral to the lateral edge of LD. 2) Two distinct variations in the origin of the IL, i) from the Bogorozky tendon and the ventral epimysium of m. longissimus dorsi (LD) at the caudo-lateral region at L1 to L5, and ii) from the lumbar myofascia lateral to the lumbar transverse processes at the level of L2 to L4 have been found. 3) A fold in the LD from the thoracolumbar junction to the cervicothoracic junction was identified. It is concluded that: i) the IL muscle is the smallest of the 3 muscles in the erector spinae group with variations in the origo; ii) the LD fold may play a stabilizing role and the lateral section of the LD may act bilaterally in extension and unilaterally in lateral flexion; iii) the m. spinalis (SP), incorrectly labelled in previous studies, may be related to extension only in the cervicothoracic region, whilst the LD may be primarily responsible for extension in the thoracolumbar region.