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 immobilizationinduced
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%, pb0.001) 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.
