Knee Posture Predicted from Subchondral Apparent Density in the Distal Femur: An Experimental Validation

Abstract

Spatial patterning in the apparent density of subchondral bone can be used to discriminate between species that differ in their joint loading conditions. This study provides an experimental test of two hypotheses that relate aspects of subchondral apparent density patterns to joint loading conditions. First, the region of maximum subchondral apparent density (RMD) will correspond to differences in joint posture at the time of peak locomotor loads; and second, differences in maximum density between individuals will correspond to differences in exercise level. These hypotheses were tested using three age-matched samples of juvenile sheep. Two groups of five sheep were exercised, at moderate walking speeds, twice daily for 45 days on a treadmill with either a 0% or 15% grade. The remaining sheep were not exercised. Sheep walking on the inclined treadmill used more flexed knee postures than those in the level walking group at the time of peak vertical ground reaction forces. Kinematic measurements of knee posture were compared with knee postures estimated from the spatial position of the RMD on the medial femoral condyle. Our results show that the difference in the position of the RMD between the incline and level walking groups corresponded to the difference in knee postures obtained kinematically; however, exercised and nonexercised sheep did not differ in the magnitude of apparent density. These results suggest that patterns of subchondral apparent density are good indicators of the experimental modifications in joint posture during locomotion and may, therefore, be used to investigate differences between species in habitual joint loading.

Subchondral Bone Apparent Density and Locomotor Behavior in Extant Primates and Subfossil Lemurs Hadropithecus and Pachylemur

“…1, inset point C) and the anterior extent of the RMD (Fig. 1, inset point E), as this angle (θ) most closely matched kinematically measured joint angles in the experimental study of Polk et al (2008). We exported coordinate measurements from tpsdig2 and calculated angles using the law of cosines.…”

“…These researchers used differences in substrate orientation to alter the locomotor postures of 2 groups of sheep during twice-daily bouts of exercise for a period of 45 d. They obtained kinematic measurements for the knee postures during locomotion and compared these measurements to joint posture estimates obtained from bone apparent density patterns. They found that the position of the region of maximum subchondral bone apparent density could be used to reconstruct joint postures that closely matched kinematically measured joint postures (Polk et al 2008). Further, because this study did not find differences in peak apparent density magnitudes between the experimental groups, they inferred that the observed patterns of apparent density resulted from the differences in habitual loading conditions instead of differences in loading magnitude.…”

“…Following Polk et al (2008Polk et al ( , 2009), we quantified the regions of maximum density (RMD) on a median slice through the medial femoral condyle (MFC). To obtain these measurements, we obtained a single, oblique, 2-dimensional slice through the long axis of the MFC for each specimen (slice thickness=0.234 mm).…”

“…Protocols for assessing bone density follow Polk et al (2008Polk et al ( , 2009). We measured bone apparent density in all of the specimens using the same protocol on the same computed tomography (CT) scanner (GE Lightspeed Plus) at the Mount Auburn Hospital in Cambridge, MA.…”

“…Polk et al (2008) experimentally investigated the relationship between apparent density variation and joint posture. These researchers used differences in substrate orientation to alter the locomotor postures of 2 groups of sheep during twice-daily bouts of exercise for a period of 45 d. They obtained kinematic measurements for the knee postures during locomotion and compared these measurements to joint posture estimates obtained from bone apparent density patterns.…”