Trabecular bone pattern and mechanical load in the juvenile knee joint

The relationship between trabecular bone pattern and mechanical load in specific parts of the body had been proposed for over a century. It is now widely accepted that differences in trabecular bone patterns can be attributed to magnitude, direction and distribution of mechanical force associated with different activities. In the juvenile, several key developmental milestones including weaning, crawling, sitting and the development of a bipedal stance around 1 year of age all have a bearing on the forces passing through the skeleton and ultimately the resultant trabecular bone pattern. Bipedal locomotion continues to be practiced for several years before a mature walking gait is adopted. Therefore, changes in trabecular bone pattern related to differing mechanical loads throughout skeletal development are expected, especially in the bones of the lower limb and particularly the knee joint. Ground reaction force, joint reaction force and muscle force are applied to the bones during locomotion, contributing to mechanical load which can result in a specific trabecular bone patterns developing. The distal femur and the proximal tibia were investigated in this study to find the effects of mechanical load from bipedal locomotion on trabecular bone pattern, as these bones are the major components associated with knee flexion and extension. Specimens from Scheuer juvenile bone collection were used for the investigation. Plain radiographs of prenatal to late adolescent specimens were taken for a preliminary study to observe qualitative features of the trabecular bone at specific developmental milestones. Gradient colour-mapping was applied to the radiographs to distinguish areas of different radiographic intensity. Vertical trabeculae were found in both the distal femur and proximal tibia, and were present throughout the observed age range. This trabecular bone alignment is thought to be related to the direction of bone deposition associated with normal endochondral ossification in subjects under 1 year old, while the direction of ground and joint reaction force might be factors in older age groups. Horizontal, oblique, and curved alignments to the trabecular architecture were found only in subjects over 1 year of age. Relationship with muscle attachment and the position of the bones during knee flexion were proposed as explanations for these trabecular orientations. The findings from the preliminary study informed the subsequent three-dimensional analysis and quantification of trabecular patterns within specific areas of interest within these bones. Micro-computed tomography was performed on specimens within the prenatal to 7-year-old age range. Six trabecular bone parameters were analysed from the reconstructed images: bone volume fraction; trabecular bone thickness, separation and number; and degree of anisotropy by mean intercept length and star volume distribution. Statistical analysis was performed with each bone parameter to indicate significant differences between different regions of the bone, and between different age groups. More homogeneous trabecular bone was found in subjects under 1 year old in the femoral and tibial metaphyses and epiphyses, despite a few characteristic trabecular features found in each bone type. In 1-7 year old subjects, the trabecular bone adjacent to the cortex tended to be thicker and denser than the deeper trabecular bone. The trabecular pattern in subjects over 1 year of age corresponded with mechanical load distribution during walking gait, with predilection to transfer forces though the cortical bone. Characteristic trabecular bone patterns in each bone were found to correspond with anterior and posterior cruciate ligament attached to the tibial epiphysis; patellar tendon attached to the tibial tuberosity; and articular surfaces between the femur, tibia and patella. Both imaging techniques highlighted a reduction of trabecular bone structure during the first two years after birth, with the weakest trabecular bone signature found between 1 and 2 years old. This resorptive change could be related to reduced mechanical stimulation, compared to the intra-uterine environment, and lack of direct nutritional supply after birth. Recovery of trabecular bone integrity was observed after 2 years old with more remarkable trabecular patterns, in which effective mechanical stimulation from bipedal locomotion was suspected.