The overall goal of this protocol is to facilitate the consistent preparation of human cadaveric femora for computed tomography, or CT scanning and mechanical testing in a sideways fall on the hip configuration. This method can help answer questions in the field of osteoporosis research about bone imaging and bone biomechanical properties. The main advantage of this technique is that it is a comprehensive procedure for preparing proximal femurs in a reproducible manner for bone fracture research.
Begin by covering four or five pound rice bags with plastic bags to prevent their contamination and protect the DEXA surface with plastic lined paper. Transfer two of the plastic wrapped rice bags onto the scanner table. Then lay two 24 hour room temperature thawed femora on top of the rice bags.
Such that the proximal ends, including the femoral heads, are centered on the bags with the posterior sides facing down. Cover the exposed anterior approximal femur end with the other two rice bags, and position the machine head over the approximal femur. Now select the femur exam in the machine software interface, and position the arm on top of the cadaveric femurs.
After analyzing the bone marrow density, an automatic T-Score will be generated classifying the bone as normal, osteopenic or osteoporotic. Before beginning the bone modification sanitize the workspace with 70%isopropyl alcohol, and cover the table with absorbent paper pads with plastic film on one side. Then place the entire femur into a custom made cutting fixture with the head of the femur against the acrylic plate of the fixture.
Holding the diaphysis against the two pins on the cutting fixture, tighten the slotted plate on to the diaphysis. When the bone is secured, pass the cast cutter through the slotted plate as a guide, and incise the distal shaft of the femur. Now remove the bone from the fixture, and use a 25 millimeter deep curette to clean the marrow from the medullary cavity.
As needed use a sponge gauze to clean the interior surface of the bone. Then grip the bone with a dry cloth and drill a 10 millimeter hole through the distal end of the bone, approximately 25 millimeters from the proximal cut end of the specimen. Before beginning the bone potting procedure, label the potting container with the appropriate bone identification details, and adjust the embedding fixture to the proper orientation.
Place the potting container in the base of the embedding fixture inside a fume hood, and place the bone into the potting container. Then align the neck with the pointer in the fixture to adjust the internal rotation angle of the bone to the desired value. Next, mix 60 grams of PMMA powder with 30 grams of liquid resin, in a disposable cup compatible with PMMA until the powder has dissolved.
When the mixture is pourable, decant the PMMA solution into the potting container, until the container is about half full. And allow the PMMA to set for 10 to 15 minutes inside the fume hood. While the mixture is curing, carefully wrap the bone in saline saturated towels to prevent tissue dryness.
Periodically check the femur to ensure the bone remains aligned in the container during the curing process. After the resin has cured, completely fill the potting container with 90 grams of PMMA and liquid resin, and allow the PMMA to cure for another 10 to 15 minutes. When the additional resin has cured, rewrap the bone in saline soaked paper towels, and fully cover the bone with a plastic bag.
Before x-ray imaging, move the beam head to the desired height and place an unexposed cassette on the cart under the beam. Next place the femur on top of the cassette in the medial lateral position, and move behind the lead line portable wall for personal protection. Label the specimen images, and then use the trigger to expose the bones.
To scan the bones by computed tomography, secure a completely thawed plastic bag wrapped femur in the calibration phantom in the CT scanning fixture. Use the system laser guides to confirm that the middle of the fixture is properly aligned with the longitude in the laser of the CT scanner. Moving the table up or down until the horizontal laser rests between the head and trochanter as appropriate.
Then press the laser on/off button to recheck the fixture alignment with the CT lung access laser, and press the zero button to zero the table position. When the bone is in place use standard CT operating procedures to scan the bone with the appropriate scanning parameters, obtaining an image slice thickness of 0.4 millimeters and a pixel size of 0.30 to 0.45 millimeters depending on the size of the field of view. Bone mineral density scanning using a DEXA scanner as just demonstrated, allows measurement of the total hip and neck bone density.
As well as acquisition of the T-Score for each specimen. X-ray images obtained from this method can be used to discern the presence of prior fractures or diseases, such as cancer, that might affect femoral strength. CT images can be used to obtain three-dimensional bone geometry and volumetric bone mineral distribution to be used in quantitative CT-based finite element analysis.
While attempting this procedure it is important to remember to minimize the number of freeze style cycles, and to keep the femur moist during preparation steps. After watching this video, you should have a good understanding of how to prepare a femur for consistent imaging and testing of a sideways fall on the hip configuration.
