title: Bone loss in the lower leg during 35 days of bed rest is predominantly from the cortical compartment.
abstract: Immobilization-induced bone loss is usually greater in the epiphyses than in the diaphyses. The larger fraction of trabecular bone in the epiphyses than in the diaphyses offers an intuitive explanation to account for this phenomenon. However, recent evidence contradicts this notion and suggests that immobilization-induced bone loss from the distal tibia epiphysis is mainly from the cortical compartment. The aim of this study was to establish whether this pattern of bone loss was a general rule during immobilization. We monitored various skeletal sites with different tissue composition during 5 weeks of immobilization. Ten healthy male volunteers with mean age of 24.3 years (SD 2.6 years) underwent strict horizontal bed rest. Bone scans were obtained during baseline data collection, at the end of bed rest and after 14 days of recovery by peripheral Quantitative Computed Tomography (pQCT). Sectional images were obtained from the distal tibia epiphysis (at 4% of the tibia's length), from the diaphysis (at 38%), from the proximal metaphysis (at 93%) and from the proximal epiphysis (at 98%), as well as from the distal femur epiphysis (at 4% of the femur's length) and from the patella. Relative bone losses were largest at the patella, where they amounted to -3.2% (SD 1.8%, p<0.001) of the baseline values, and smallest at the tibia diaphysis, where they amounted to -0.7% (SD 1.0%, p=0.019). The relative losses were generally larger from cortical than from trabecular compartments (p=0.004), and whilst all skeletal sites depicted such cortical losses, substantial trabecular losses were found only from the proximal tibia epiphysis. Results confirm that the differential losses from the various skeletal sites cannot be explained on the basis of trabecular vs. cortical tissue composition differences, but that endocortical circumference can account for the different amounts of bone loss in the tibia. The present study therefore supports the suggestion of the subendocortical layer as a transitional zone, which can readily be transformed into trabecular bone in response to immobilization. The latter will lead to cortical thinning, a factor that has been associated with the risk of fracture and with osteoarthritis.
authors.affiliation: Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Oxford Rd, Manchester, M1 5GD, UK. j.rittweger@mmu.ac.uk
authors.forename: Jörn
authors.initials: J
authors.lastname: Rittweger
authors.forename: Bostjan
authors.initials: B
authors.lastname: Simunic
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Bilancio
authors.forename: Natale Gaspare
authors.initials: NG
authors.lastname: De Santo
authors.forename: Massimo
authors.initials: M
authors.lastname: Cirillo
authors.forename: Gianni
authors.initials: G
authors.lastname: Biolo
authors.forename: Rado
authors.initials: R
authors.lastname: Pisot
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
authors.forename: Igor B
authors.initials: IB
authors.lastname: Mekjavic
authors.forename: Marco
authors.initials: M
authors.lastname: Narici
mesh_terms: D000328:Adult; D001510:Bed Rest / Q000009:adverse effects*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D035002:Lower Extremity / Q000000981:diagnostic imaging / Q000473:pathology; D008297:Male; D010329:Patella / Q000000981:diagnostic imaging / Q000473:pathology*; D013977:Tibia / Q000000981:diagnostic imaging / Q000473:pathology*; D014057:Tomography, X-Ray Computed