Adjustable Stiffness, External Fixator for the Rat Femur Osteotomy and Segmental Bone Defect Models

The overall goal of this procedure is to perform a large bone defect surgery to study novel treatments for bone regeneration and repair. This is accomplished by first implanting the external fixator on the femur of a rat. Next, a jiggly wire is used to create a five millimeter large segmental bone defect.

Then a treatment such as bone morphogenetic protein two is delivered on a scaffold to enhance bone healing. Finally, the in vivo stiffness at the selected time point is changed. Ultimately, results can be obtained that show the progress of bone regeneration with drug therapies and the influence of the local mechanical environment on the healing outcome through in vivo, x-ray, and micro ct, and using histology, biomechanical testing, and molecular evaluation at the end of the study.

The main advantage of this technique of existing external fixation devices is that it's commercially available and is a standardized self-locking angle or stable system where screws are embedded in a plate, whereas other designs found in the literature have a plate that is secured with the screws to have a tight group with RSNA wires, which makes this type of design prone to loosening and requires screws to be retied biweekly. In addition, it is lightweight, small size, and made of peak, which allows the use of radiological imaging in vivo. And last but not least, the design allows in vivo adjustment of fixation stiffness After anesthetizing a rat with iso fluorine and checking the level of sedation, administering antibiotic and analgesic, and shaving and cleaning the entire right hind leg.

According to the text protocol, use a scalpel to make an approximate incision on the surface of the right femur running cranial lateral through the skin three to four centimeters from the greater trocanter to the supracondylar region of the knee with gentle dissection. Separate the fascia lata, and without cutting the muscle tissue, expose the shaft of the femur, then spread apart the MVAs laterals and MBIs fmo, and lift the M tensor fasci to expose the full length of the femur to release the muscle tissue from the femur prior to the osteotomy. Place a henahan elevator perpendicular to the exposed surface of the femur, and then using a scalpel release the muscle in the adjacent area continue to advance around the femur, staying close to the bone until all the surrounding muscle tissue is released from the entire middle section of the bone.

To create a five millimeter large bone defect loop, two pieces of jiggly wire saw around the bone in a media lateral orientation. Next position, one piece on the distal side of the femur close to the knee joint, and a second piece on the proximal side close to the hip joint. Then using S-shaped curve, dissecting and ligature forceps clamp.

The jiggly wire saw piece on each side so that they stay still after externally rotating the femur position the external fixator plate on the anterolateral surface of the bone. In this position, the soft tissue layer is at its thinnest, which prevents excessive soft tissue tension under the fixator plate. After the wound is closed, then slightly lift the external fixator plate off the bone surface to make sure that the holes of the plate are centered to the bone surface.

In order to prevent the tip of the drill bit from slipping, use the 1.00 millimeter counter sinker to center the position of the first hole by creating a small indentation. Use a small clamp to hold the external fixator parallel to the longitudinal axis of the bone, and using a power drill with a 0.79 millimeter drill bit with a tip centered in the previously created indentation on the bone surface. Pre-drill, the first hole on the proximal side of the femur, insert the tip of the counter sinker into one of the holes on the fixator plate.

If the tip of the counter sinker easily fits into the hole, then this is the top side of the fixator. If it doesn't fit, then flip the fixator plate over to make sure that the larger holes of the fixator plate are oriented on the top. After the orientation is confirmed, clip the plate on the saw guide and then clip the unit on the bone so that the first pre drilled hole is aligned with the first hole on the plate.

Use the 0.70 millimeter square box wrench inserted into the hand drill to drive the first mounting pin into the hole. Doing this will permit reproducible positioning for the remaining mounting pins as soon as the tip of the mounting pin is in contact with the bone. Start turning the wrench under continuous axial loading applied to the proximal end of the hand drill.

After about five full turns, make sure that the thread at the proximal end of the mounting pin catches the external fixator plate's body. This thread locks the system. Stop turning when the end of the bone thread is closed to the top surface of the bone.

After the first mounting pin is in place, insert the 1.0 millimeter counter sinker into the hand drill and insert it through the furthest hole until the tip is in contact with the bone. Then start turning the counter sinker under continuous axial loading applied to the proximal end of the hand drill stop After two to three full turns. This freshly created groove prevents the 0.79 millimeter drill bit from slipping off.

Then drill the furthest hole from the first mounting pin on the distal side. Insert the 0.70 millimeter square box wrench in the hand drill, and then insert the mounting pin into the tip. Carefully insert the tip into the plate of the external fixator without losing the alignment of the second pre-drilled hole.

After the pins on either end are in place, counter sink and pre-drill the remaining two middle holes. The implantation order of the two middle pins is not important. After the external fixator is in place past 0.22 millimeters.

Jiggly wire saws one at a time through the two grooves underneath the femur, and while dispensing saline onto the bone, create a five millimeter segmental defect by reciprocal back and forth motion. Then remove the saw guide After the defect or osteotomy is made. Apply a treatment such as recombinant human bone morphogenetic protein two on an absorbable collagen sponge.

Then use Hybo Vicryl suture for zero to close the muscular layer and the fascia lata and Ethicon Monocryl three zero suture to close the skin to change the external fixator stiffness in vivo. After sedating the rat and administering analgesia according to the text protocol, insert the tip of the 0.50 millimeter square box wrench into the interlocking screw attached to the side of the assembled fixator. And carefully turn it counterclockwise until the pin is halfway out.

Repeat the procedure for the second pin on the same side of the external fixator plate. When both pins on the same side are halfway out, use forceps or a clamp and a gentle motion to remove the connection element on the opposite side. If it doesn't come off easily, then turn the screws a couple more times each.

After the connection element is removed, slide the desired stiffness connection element in place and from the opposite side. Using the square box wrench, turn until the interlocking screw is halfway out on the opposite side. Repeat for the second screw.

Remove the second connection element and replace it with the same stiffness connection element as the one replaced on the opposite side. Then drive in the interlocking screw until it sticks out a similar distance on each side. Repeat for the second screw.

Radiological examination confirmed that fixators of all stiffnesses maintained a one millimeter or a five millimeter from oral defect during the entire eight weeks of the experiment as demonstrated here. This was especially important for the five millimeter critical size defects where spontaneous healing does not occur. No, or infections, including pin infections were observed and pin loosening was absent if the instructions of the application were followed.

If the weight of the rat at the time of surgery exceeded 250 grams and a smaller size plate was used, the loading on the mounting pins increased to a critical level so that one to two weeks after surgery pin pullouts were occurring on the distal side of the femur. As shown here, larger animals also have thick muscle tissue that creates tension on the skin around the implant after it closes. This will cause itching and has resulted in rats chewing through the fixator, confirming the importance of choosing an animal of the proper weight for the chosen external fixator.

Following this procedure, other methods like different size of osteotomies between 0.3 to three millimeters in mouse and up to six millimeters in the rat can be performed in order to answer additional questions like investigating novel drug therapies to enhance bone healing.