Bone plating has been used as a method of fracture management since the late 1800's. The principles for fracture management developed by the AO/ASIF group emphasized a rapid return to pain-free function following fracture repair. Initially the AO/ASIF principles recommended that precise anatomic fracture reconstruction be performed prior to plating. Anatomic reduction generally required extensive exposure and manipulation of the fractured bone to facilitate anatomic reconstruction of the fracture fragments. Continued research in the area of fracture healing led to a change in the philosophies and goals of fracture osteosynthesis, focusing on minimally invasive fracture stabilization techniques. The basic principles of biological osteosynthesis include: 1. Minimal iatrogenic soft tissue disruption. 2. Indirect fracture reduction 3. Appropriate stable fixation. 4. An early return to function of the limb. These principles are based on preserving blood supply by minimizing exposure and disruption of the fracture site. Fracture union occurs by indirect bone healing with formation of a bridging callus followed by bony remodeling.
Definition of MIPO
Percutaneous plating involves the application of a bone plate without making an extensive surgical approach to expose the fracture site. The bone segments are reduced using indirect reduction techniques. Small plate insertion incisions are made at each end of the fractured bone and an epi-periosteal tunnel is made connecting those incisions. The plate is inserted through one of the insertion incisions and tunneled along the periosteal surface of the bone, spanning the fracture site. Screws are applied at the proximal and distal ends of the plate through the insertion incisions or if necessary, through additional stab incisions. Screws are not placed in the holes located in the central portion of the plate, which is often positioned over the fracture.
As with most techniques, there are both advantages and disadvantages associated with MIPO. Operative time is reduced compared to anatomic reconstruction once familiarity with the procedure is developed. Minimally invasive procedures carry less of a risk of bacterial infection in comparison to open reconstruction procedures due to shorter duration of surgery, less soft tissue trauma, and decreased potential for intra-operative contamination of the fracture site. The fracture hematoma is not removed at surgery and may contribute to increased rate of callus formation. Fractures stabilized with MIPO should heal in a similar manner to fractures stabilized with external skeletal fixation applied in a closed fashion, but would require less patient and fixator care in the post-operative convalescence period. There are some obvious disadvantages associated with MIPO. The technique can be technically challenging to learn and apply. MIPO may not be suitable for simple fractures and articular fractures which require precise anatomic reduction and compression. MIPO does not allow direct visualization of the fracture site, therefore, access to intra-operative fluoroscopy or radiography greatly facilitates the surgical procedure.
Practical Approach to MIPO
Appropriate case selection is crucial to the success of MIPO. As with any technique, not all fractures are amenable to percutaneous plate stabilization. Although MIPO is most applicable to comminuted diaphyseal or metaphyseal fractures which may not be amenable to anatomic reduction, the technique can be utilized in some simple transverse fractures. Although the MIPO technique can be applied to proximal limb fractures, we have found that femoral and humeral fractures are typically more challenging to reduce using indirect techniques than antebrachial and crural fractures. Femoral and humeral fractures may be amenable to MIPO after using an intra-medullary pin, femoral distractor or traction table to achieve reduction and alignment of the fracture.
Appropriate pre-operative planning is an essential component of the MIPO technique. Well-positioned, orthogonal view radiographs of both the fractured and the contra-lateral limb segment (if intact) are required to properly plan the procedure. Pre-contouring of an appropriate length plate can be performed utilizing images of the contralateral limb if these images are available. Implant selection should be based on fracture pattern and location as well as the animal's size and weight. Longer plates with fewer screws have been shown to be stronger than shorter plates with the maximal number of screws. The use of longer plates with fewer screws has also been reported for elastic plating in young dogs. We have had success utilizing DCP, LC-DCP, or LCP systems for MIPO procedures. Newer plating systems utilizing a locking plate/screw interface such as the LCP lend themselves particularly well to MIPO due to the limited amount of contouring necessary prior to application. Unlike traditional plating systems utilizing bicortical screws, locked screw-plating systems have angular stability, increasing their load-carrying capacity. The angular stability results from the threaded screw head locked into the threaded plate hole, forming a fixed-angle construct. For MIPO application another important advantage of locking plates is the minimal contouring required for application of the plate. In contrast to traditional plating which requires optimal contouring to maintain reduction of the fracture, locking plates act as internal fixators, and therefore do not displace the fragment during screw tightening regardless of the precision of contouring.
Indirect reduction techniques are generally utilized when performing MIPO fracture stabilization. The fractured limb segment is aligned and original length is restored. The intermediate fracture fragments are left undisturbed in the soft tissue envelope. In biological terms, indirect reduction techniques confer an enormous advantage by minimizing the iatrogenic damage incurred during surgery. If correctly applied, it will add minimal iatrogenic damage to tissues already traumatized by the fracture. A circular external fixator with a wire engaging both the proximal and distal fracture segment can be used to apply distraction to the fracture, restoring length and alignment. We routinely use a two ring construct to distract the major fracture segments and obtain functional alignment prior to plate insertion. The femoral distractor is a specific instrument designed to distract fracture segments. Being a unilateral fixator, the femoral distractor is particularly useful for humeral and femoral fractures. An intramedullary Steinmann pin can be used to assist with reduction and alignment of the fracture. The tip of the pin is blunted before the pin is introduced into the distal fracture segment, allowing enough force to be applied to achieve distraction of the proximal and distal fracture segments. Distraction by this method is very effective at stretching out contracted muscles and returning the fractured bone to original length. The pin can be left in place if desired, or can be removed once appropriate reduction is achieved.
MIPO technique consists of inserting the plate through one of the incisions, sliding it through the soft tissue tunnel along the surface of the bone, over the fracture site, until the end of the plate is visualized in the second incision. If available fluoroscopy should be used to visualize that the plate is properly contoured and positioned on the bone. If necessary the plate can be removed and re-contoured. Precise contouring and positioning of the plate becomes less critical if a locking plate is used. Once the plate is fitted to the bone, screws are placed. Typically one or more screws are placed to secure either the proximal or distal segment and then the alignment of the limb segment is re-assessed. Screws can then be placed through the remaining accessible holes via both the proximal and distal insertion incisions. Filling all the holes in the plate with screws is not necessary when applying long plates. Gautier and Sommer recommended the use of 2 or 3 bicortical screws per major fracture segment with a total plate-screw density (quotient of the number of plate holes over the number of screws utilized) of between 0.5 to 0.4 for MIPO applications using locking plates in human patients. In addition they recommended that peripheral screws be inserted at plate ends and centrally located screws be inserted close to the fracture site to maximize working leverage and minimize pull-out forces acting on the screws.
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