Anatomical sector analysis of load-bearing tibial bone structure during 90-day bed rest and 1-year recovery Tomas Cervinka 1 , Jo ¨rn Rittweger 2,3 , Jari Hyttinen 1 , Dieter Felsenberg 4 and Harri Sieva¨nen 5,1 1 Department of Biomedical Engineering, Tampere University of Technology, Tampere, Finland, 2 Institute for Biomedical Research into Human Movement and Health, Manchester Metropolian University, Manchester, UK, 3 Institute of Aerospace Medicine, German Aerospace Center, Linder Hoehe, Cologne, Germany, 4 Charite ´ Universita ¨tsmedizin Berlin, Center for Muscle and Bone Research, Berlin, Germany and 5 Bone Research Group, UKK Institute, Tampere, Finland Correspondence Tomas Cervinka, Finn-medi 1, 4. Floor, Biokatu 6, 33520 Tampere, Finland E-mail: tomas.cervinka@tut.fi Accepted for publication Received 29 October 2010; accepted 3 January 2011 Key words bone loss; cortical bone; disuse; image preprocessing; trabecular bone Summary The aim of this study was to investigate whether the bone response to long bed rest– related immobility and during subsequent recovery differed at anatomically different sectors of tibial epiphysis and diaphysis. For this study, peripheral quantitative tomographic (pQCT) scans obtained from a previous 90-day ÔLong Term Bed RestÕ intervention were preprocessed with a new method based on statistical approach and re-analysed sector-wise. The pQCT was performed on 25 young healthy males twice before the bed rest, after the bed rest and after 1-year follow-up. All men underwent a strict bed rest intervention, and in addition, seven of them received pamidronate treatment and nine did flywheel exercises as countermeasures against disuse-related bone loss. Clearly, 3–9% sector-specific losses in trabecular density were observed at the tibial epiphysis on average. Similarly, cortical density decreased in a sector- specific way being the largest at the anterior sector of tibial diaphysis. During recovery, the bed rest–induced bone losses were practically restored and no consistent sector-specific modulation was observed in any subgroup. It is concluded that the sector-specific analysis of bone cross-sections has potential to reveal skeletal responses to various interventions that cannot be inferred from the average analysis of the whole bone cross-section. This approach is considered also useful for evaluating the bone responses from the biomechanical point of view. Introduction Bone structure can deteriorate in response to several factors such as disuse, immobilization, ageing, diseases and hormonal disturbances. Because the lower limb skeleton is primarily locomotive organ and capable to adapt to varying loading conditions (Frost, 2003; Ruff et al., 2006), disuse irrespective of its primary cause provides a useful model to investigate responses of bone structure to reduced loading. Experimental bed rest with )6° head down tilt is an established ground-based model to simulate the physiological effects of spaceflight (Pavy- Le Traon et al., 2007). Earlier bed rest studies have mostly relied on dual-energy X-ray absorptiometry (DXA) (LeBlanc et al., 1990; Zerwekh et al., 1998; Shackelford et al., 2004; Armbrecht et al., 2010) but with regard to bone structure these studies are limited by the inherent inability of DXA to yield tangible information on actual cross-sectional bone geometry, let alone separating the measured bone into trabecular and cortical compartments (Sieva ¨nen, 2000). Peripheral quantitative com- puted tomography (pQCT) offers a reasonable option to assess bone geometry and density without evident limitations of planar DXA (Sieva ¨nen et al., 1998; Sieva ¨nen, 2000). The few studies which have used pQCT or QCT technology indicated that bone loss is most prominent at endocortical bone regions both after bed rest (Rittweger et al., 2005, 2009, 2010) and space flight (Lang et al., 2006). It is obvious that different bone sectors experience specific loading environment in terms of biomechanics of the given site. A good example of sector-specific bone adaptation comes from patients with spinal cord injury, in whom electrical stimulation of the soleus muscle led to site-specific bone accrual in the posterior aspect of the distal tibia (Dudley-Javoroski & Shields, 2008). Apparently, normal locomotive muscle contractions mostly affect the anterior and posterior sectors of distal tibia via the ground reaction forces imposed on the feet in the front and the pulling forces mediated by the Achilles tendon in the back. Therefore, we hypothesized that the most substantial bone losses would occur at those sectors where the lack of locomotive loading is most evident because of immobility during bed rest and accordingly a faster recovery at the same sectors when the Clin Physiol Funct Imaging (2011) 31, pp249–257 doi: 10.1111/j.1475-097X.2011.01009.x Ó 2011 The Authors Clinical Physiology and Functional Imaging Ó 2011 Scandinavian Society of Clinical Physiology and Nuclear Medicine 31, 4, 249–257 249