This protocol describes a minimally invasive osteosynthesis technique using an intramedullary screw for standardized stabilization of femur fractures, which can be used to analyze endochondral bone healing in mice.
Bone healing models are necessary to analyze the complex mechanisms of fracture healing to improve clinical fracture treatment. During the last decade, an increased use of mouse models in orthopedic research was noted, most probably because mouse models offer a large number of genetically-modified strains and special antibodies for the analysis of molecular mechanisms of fracture healing. To control the biomechanical conditions, well-characterized osteosynthesis techniques are mandatory, also in mice. Here, we report on the design and use of a closed bone healing model to stabilize femur fractures in mice. The intramedullary screw, made of medical-grade stainless steel, provides through fracture compression an axial and rotational stability compared to the mostly used simple intramedullary pins, which show a complete lack of axial and rotational stability. The stability achieved by the intramedullary screw allows the analysis of endochondral healing. A large amount of callus tissue, received after stabilization with the screw, offers ideal conditions to harvest tissue for biochemical and molecular analyses. A further advantage of the use of the screw is the fact that the screw can be inserted into the femur with a minimally invasive technique without inducing damage to the soft tissue. In conclusion, the screw is a unique implant that can ideally be used in closed fracture healing models offering standardized biomechanical conditions.
Bone healing studies in mice are in great demand because of a broad spectrum of antibodies and genetically-modified animals. These facts allow to study the molecular mechanisms of bone healing1. In the past few years, different bone healing models for mice have been developed2. These models can be divided into open models, in which the bone is osteotomized using an open lateral surgical approach and in closed models, in which the bone is fractured based on the fracture model introduced by Bonnares and Einhorn3. Using this technique, a standardized transverse fracture can be produced by a 3-point bending device and intramedullary implants can be inserted through a small medial parapatellar incision in a minimally invasive technique avoiding a major soft tissue trauma.
The intramedullary screw can be applied for closed fracture stabilization in mice. The screw offers rotational and axial stability. This is achieved by fracture compression through a proximal thread and a distal head4. Further advantages of the screw are the simple surgical technique, the low grade of invasivity, the low implant weight and, most notably, a higher stability providing standardized and controlled biomechanical conditions compared to other intramedullary implants5. In fact, in the most closed fracture models, the fragments are stabilized only by simple pins, which is associated with a complete lack of rotational and axial stability and a high risk of pin and also fracture dislocation. This can markedly influence the healing process, which may result in delayed healing or non-union formation.
It is well known that the stability of the fracture fixation has a tremendous impact on the healing process6,7. A high rigid fixation results in intramembranous healing, while a less rigid fixation, which may allow micromovements in the fracture gap, results in endochondral healing. Stabilization of the fracture with the intramedullary screw shows predominantly an endochondral healing with a large amount of callus tissue, particularly after 2 weeks of fracture healing. The possibility to harvest a large amount of callus tissue enables the analysis of multiple parameters by different techniques.
Here, we report on the design and application of the intramedullary screw in mice, as well as on its advantages and disadvantages in experimental studies on normal endochondral bone healing.
Critical steps of the surgical procedure are to find the correct entry point for screw implantation in the middle of the femur condyles at the intercondylar notch as well as the optimal orientation of the needle parallel to the bone axis for reaming of the intramedullary cavity. To avoid an incorrect entry position, the surgeon should prepare the notch until an optimal view is achieved. To control the orientation during reaming, the femur of the mice should be held with the fingers in a stable position. A further critica…
The authors have nothing to disclose.
This work was supported by RISystem AG, Davos, Switzerland.
Mouse Screw | RISystem AG | 221,100 | |
Guide wire | RISystem AG | 521,100 | |
Centering bit | RISystem AG | 590,205 | |
Hand drill | RISystem AG | 390,130 | |
Cotton-Swab (150 mm, small head) | Fink Walter GmbH | 8822428 | |
Suture (5-0 Prolene) | Ethicon | 8614H | |
Forceps | Braun Aesculap AG &CoKG | BD520R | |
Scissors | Braun Aesculap AG &CoKG | BC100R | |
Needle holder | Braun Aesculap AG &CoKG | BM024R | |
27 G needle | Braun Melsungen AG | 9186182 | |
Scalpel blade size 15 | Braun Aesculap AG &CoKG | 16600525 | |
Heat radiator | Sanitas | 605.25 | |
Depilatory cream | Asid bonz GmbH | NDXZ10 | |
Eye lubricant | Bayer Vital GmbH | 2182442 | |
Xylazine | Bayer Vital GmbH | 1320422 | |
Ketamine | Serumwerke Bernburg | 7005294 | |
Tramadol | Grünenthal GmbH | 2256241 | |
Disinfection solution (SoftaseptN) | Braun Melsungen AG | 8505018 | |
CD-1 mice | Charles River | 22 | |
X-ray Device | Faxitron MX-20, Faxitron X-ray Corporation | 2321A0988 | |
Fracture device small | RISystem AG | 891,100 |