| POSTDOCTORAL FELLOWSHIP
(1) Original Aims
Aim 1: Further develop the image analysis technology for assessing changes to mouse knee joint soft tissue with microCT, including cartilage, meniscus, ligaments, and tendons.
Aim 2: Assess whole-joint changes in the knee, including bone and soft tissues, from both unloading and reloading using the established HLU model. This will be accomplished with two studies:
Aim 2a: Study the effects of hindlimb unloading on integrated joint properties, mimicking the STS-135 Space Shuttle flight profile. Hypothesis: Degradation of bone strength, as assessed by computational FEA, will be similar to that observed in mice flown on STS-135 (13-days of unloading). Similarly, degradation of meniscus volume and density will also be observed with few changes in tendon and ligaments.
Aim 2b: Study the effects of longer-term unloading followed by reloading on whole-joint structural and functional properties. Hypothesis: Longer periods of unloading cause greater degradation in bone volume and strength, as well as larger changes in the connective soft tissues. There will be limited recovery after 4-weeks of reloading.
(2) Key Findings
The development of the computational methods were performed on the bone tissues from mice flown on STS-135 and to investigate changes in the knee joint of hemophilic mice. FEA of the Proximal Femur of the STS-135 mice found significant reduction in femoral neck stiffness (-12%), which was prevented from sclerostin anti-body treatment. FEA of the Lumbar Vertebrae (L5) of the STS-135 mice found a significant reduction in L5 compressive stiffness (-20%) and a -4% reduction in bone structural efficiency (stiffness/amount of bone). In addition, in the non-loading L5 vertebrae, bone volume was an excellent indicator of bone stiffness using linear regression (P<0.01, R2=0.992) across all specimens. Imaging analysis of the knee joint in hemophilic mice found significant loss of bone in the proximal tibia and mineralization of the joint soft tissues (tendons, ligaments, menisci, cartilage) 2-weeks after induced joint bleeding.
(3) Impact of Key Findings
The findings from FEA of the proximal femur suggest that we reconsider the boundary conditions used in the mechanical testing of femoral neck strength for future studies. Mechanical loading of the femoral head must consider the lower density bone regions of the femoral head when attempting to characterize the strength of the femoral neck. The findings from FEA of the Lumbar vertebrae shed some insight to the difference gravitational unloading has between weight bearing (femur, tibia) and the non-weight bearing L5 vertebrae. Further investigation should consider the differences in bone morphology and how that affects the individual bone's relationship between bone volume, structure, and bone stiffness. The finding of bone loss and identification of rapid joint soft-tissue mineralization has implications to osteoarthritic degradation following joint injury and inflammation.
(4) Proposed research plan
In the coming year, we propose to use the analytical methods developed over the past year to analyze tissue samples from a study using hind limb unloading (HLU), modeling the STS-135 experiment profile of disuse, in mice already performed by Mary Bouxien's group at Harvard. In addition, the information gained from the Harvard study will help guide our 13-day HLU and 4-week HLU with reloading studies. In collaboration with CASIS and the pharmaceutical company Novartis, our lab plans to launch mice on SpaceX-4, which will be an excellent opportunity to use all the developed assays to look at skeletal and joint degeneration from longer term spaceflight. Mice will be exposed to at least 21 days of microgravity, with planning in progress for 60-day exposure on SpaceX-6. The microCT analysis to obtain bone morphometry, bone strength through FEA, and soft tissue properties will continually be refined as we apply them to the studies.