3D Statistical Modeling Techniques to Investigate the Anatomy of the Sacrum, Its Bone Mass Distribution, and the Trans-Sacral Corridors Daniel Wagner, 1,2 Lukas Kamer, 1 Pol M. Rommens, 2 Takeshi Sawaguchi, 3 Robert Geoff Richards, 1 Hansrudi Noser 1 1 AO Research Institute Davos, Clavadelerstrasse 8, Davos 7270, Switzerland, 2 Department of Trauma Surgery, Centre for Orthopaedics and Traumatology, University Medical Centre Mainz, Langenbeckstr. 1, Mainz 55131, Germany, 3 Department of Orthopedics and Joint Reconstructive Surgery, Toyama Municipal Hospital, Toyama, Japan Received 2 February 2014; accepted 19 May 2014 Published online 24 June 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jor.22667 ABSTRACT: The complex anatomy of the sacrum makes surgical fracture fixation challenging. We developed statistical models to investigate sacral anatomy with special regard to trans-sacral implant fixation. We used computed tomographies of 20 intact adult pelves to establish 3D statistical models: a surface model of the sacrum and the trans-sacral corridor S1, including principal component analysis (PCA), and an averaged gray value model of the sacrum given in Hounsfield Units. PCA demonstrated large variability in sacral anatomy markedly affecting the diameters of the trans-sacral corridors. The configuration of the sacral alae and the vertical position of the auricular surfaces were important determinants of the trans-sacral corridor dimension on level S1. The statistical model of trans-sacral corridor S1 including the adjacent parts of the iliac bones showed main variation in length; however, the diameter was the main criterion for the surgically available corridor. The averaged gray value model revealed a distinct pattern of bone mass distribution with lower density particularly in the sacral alae. These advanced 3D statistical models provide a thorough anatomical understanding demonstrating the impact of sacral anatomy on positioning trans-sacral implants. ß 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1543–1548, 2014. Keywords: sacrum; statistical model; fracture; anatomy; corridor The sacrum exhibits a complex anatomy that is critical for treating sacral fractures. This is especially true in percutaneous minimally invasive osteosynthesis using sacroiliac screws 1 or trans-sacral implants, 2,3 the latter being increasingly used in the treatment of sacral insufficiency fractures. 4 These fractures, occur- ring predominantly in osteoporotic patients, are isolat- ed to the sacrum or a part of fragility fractures of the pelvic ring 5 and are typically located in the parafor- aminal lateral region of the sacral ala. 6 Complex anatomy, reduced bone mass, and limited intra- operative visibility make adequate fixation difficult to achieve. Trans-sacral implants must be placed through safe intraosseous pathways, also termed trans-sacral corri- dors. They extend laterally from the ilium, traversing the sacroiliac joint, passing through the vertebral body on level S1 or S2 to reach the contralateral side of the sacrum and the ilium. These pathways are bordered anteriorly by the cortex of the anterior sacrum, posteriorly by the vertebral canal, and superiorly and inferiorly by the adjacent neural foramen. In S1, the superior border is formed by the sacral ala. 7 The entrance and exit points are located on the outer surface of the iliac bone. In contrast to safe pathways for sacroiliac screws reaching the vertebral body, trans-sacral corridors are more limited in their critical diameter, 8,9 exhibiting an oval shape. 7 Their 3D volume was previously computed in an automatic process. 7 The upper sacral anatomy was highly vari- able with up to 35% of the sacra called “dysmorphic” 10 providing only limited space to position implants on level S1. Surgical fracture fixation is further compli- cated by areas of different bone mass, especially in the osteoporotic, where screw anchorage is reduced due to decreased bone mass. An area of decreased bone mass (the “alar void” 11 ) is located in the paraforaminal lateral region of the sacrum (the sacral ala), whereas in the vertebral bodies, bone mass is comparably higher. 12 Trans-sacral safe pathways cover distinct anatomi- cal volumes allowing dedicated implants to be placed safely. However, these volumes may display large inter-individual variation regarding size, shape, and available bone mass. These variables can significantly affect surgical decision making and hence the opera- tive procedure. We used innovative computed tomography (CT) based 3D statistical models to study the anatomy of the human sacrum, addressing the variability in size and shape and the impact on trans-sacral corridors. To assess bone mass distribution, we adopted meth- ods of averaging CT gray values given in Hounsfield Units (HU). METHODS CT Imaging A retrospective clinical series of 20 anonymized pelvic CT scans was used. Individuals with bony pathologies other than osteopenia or osteoporosis were excluded. The popula- tion consisted of 2 females and 18 males with a mean age of 57.2 years (28–78 years, std dev ¼12.5 years). Scans were performed with the multidetector CT scanners Siemens SOMATOM Sensation 64 and Siemens SOMATOM Defini- tion (Siemens AG, Erlangen, Germany) using Kernel B45f. Data were saved in DICOM-format, CT values were given in Grant sponsor: TK System of the AO Foundation, Davos Switzerland. Grant sponsor: DePuy Synthes, Solothurn, Switzerland. Correspondence to: Daniel Wagner (T: þ49 6131 177292; F: þ49 6131 174043; E-mail: wagner.daniel@gmx.ch) # 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. JOURNAL OF ORTHOPAEDIC RESEARCH NOVEMBER 2014 1543