In vivo hip and lumbar spine implant loads during activities in forward bent postures

Long-term measurements on the lumbar spinal alignment during daily life revealed that humans spent 90% of the day in a forward bent posture. Compared to standing, this posture leads to a substantial increase in spinal loading. The lumbar spine and pelvis, however, contribute differently to the total amount of flexion, which could possibly indicate a different timing of maximum loads in both structures during flexion. This study aimed to evaluate the in vivo implant forces in the hip and lumbar spine during activities in forward bent postures. This work utilized data collected in earlier in vivo measurements on patients either with telemeterized hip endoprostheses (HE) or vertebral body replacements (VBR). The following activities were investigated: standing, upper body flexion with and without weights in the hands using different lifting techniques (straight and bent knees). The maximum resultant forces in VBR were considerably lower than in HE. Increases in flexion inclinations caused direct increases of the resultant forces within VBR, followed by a plateau or even a decrease of the force until maximum inclination. The resultant force in HE displayed an almost continuous increase until the maximum inclination. This general curve behavior resulted in different HE-VBR load ratios, which were affected by lifting additional weights or different lifting techniques. The results emphasize that maximum loads in the spine, in contrast to the hip, do not necessarily occur at maximum upper body flexion as normally expected, rather already at intermediate flexion angles in VBR patients. The results form the basis for more detailed insilico analyzes.