R. Palmer
Department of Clinical Sciences, Colorado State University, Fort Collins, CO, USA
Why Minimally Invasive Fracture Treatment?
The primary goal of all fracture treatment is restoration of normal limb function and we must never lose this focus. Surgical repair of fractures can be performed with open reduction and fixation (ORF) or minimally invasive osteosynthesis (MIO). When properly performed on carefully selected patients, MIO can restore normal limb function, but often with faster bone healing times and with less perioperative patient morbidity than associated with ORF. Morbidity concerns such as postoperative swelling, pain and incisional complications can often be reduced by virtue of the less aggressive surgical approaches.
Why Not Minimally Invasive Stabilization of All Fractures?
Just as MIO has inherent advantages, it also has some disadvantages that must be respected in case selection. First, there is a significant learning curve associated with the development of these skills. Most veterinarians are accustomed to visualization of the fractured bone for implant insertion, implant contouring, and restoration of bony alignment, etc; MIO often requires more training and advance planning. The inability to directly visualize the entire bone may result in fracture mal-alignment in some instances, especially for those inexperienced in MIO. Similarly, the inability to fully visualize the affected bone may result insufficient fixation. Intraoperative imaging using a fluoroscopic imaging (C-arm) unit is tremendously helpful for most (but not all) MIO fracture treatments. Such imaging represents a significant additional hospital expense and a source of radiation exposure to the patient and hospital staff. Such imaging may be less essential for some external skeletal fixation applications.
Sacroiliac Luxation Repair via MIO
Surgical fixation is indicated for SI luxation when one or more of the following clinical or radiographic signs are present: 1) significant instability and displacement of the hemi-pelvis, 2) neurologic deficits or pain attributable to the luxation or 3) obstruction/collapse of the pelvic canal.
Surgical fixation can be achieved by open reduction and internal fixation (ORIF) or minimally invasive osteosynthesis (MIO). ORF induces some morbidity associated with the surgical approach and traction for visualization of the articular surface of the sacral body.
MIO for SI luxations utilizes percutaneous reduction and fixation that drastically reduces the patient morbidity associated with open surgical exposure and reduction.
Studies by Bowlt, Shales and Langley-Hobbs have also made it clear that the “safe corridor” for lag screw fixation is relatively small and that there is considerable anatomic variation between patients that can challenge or preclude accurate and secure fixation when ORF is employed. While intraoperative imaging is required for MIO of SI luxation, this imaging simplifies the surgical procedure and improves fixation accuracy. Accurate application of fixation implants is important due to the proximity of important neural and vascular structures and it ensures optimal fixation strength.
Tomlinson et al. nicely described the technique for MIO of SI luxations under fluoroscopic guidance in the dog. Patients are positioned in lateral recumbency upon the surgical table and this position is adjusted until the fluoroscopic appearance of superimposition of the transverse processes of the lumbar vertebrae and a distinct triangular shape of the sacral body confirms perfect laterality. The luxation is reduced via percutaneous application of bone forceps on the ilial wing and/or ischium. Provisional fixation is achieved with a k-wire placed from the ilium into the sacral body using fluoroscopic guidance. Next, 1 or 2 screws are placed in lag fashion using the same intraoperative imaging technique. Because of the need for such imaging, the surgery table should have a radiolucent surface and operating room personnel should adhere to appropriate radiation safety guidelines.
Minimally Invasive Nail Fixation (MINO)
Interlocking nails (ILN) are specifically designed intramedullary devices with transverse cannulations that accept fixation bolts. The intramedullary nail imparts resistance to disruptive bending forces while the rigid interlock of the fixation bolts within the nail cannulations on each side of the fracture or osteotomy imparts stability against axial compression and rotational forces. Since these nail cannulations cannot be visualized after the nail is placed within the intramedullary canal, a specific aiming guide system is part of the instrumentation set to ensure accurate placement of the fixation bolts.
With technical training and acquired expertise, the nail and fixation bolts can often be placed percutaneously without disruptive of the fracture zone. Recent advances in veterinary ILN devices offer enhanced fixation strength imparted by angle-stable fixation bolts (I-Loc®, BioMedtrix, Whippany, NJ). Normograde insertion of ILN is the only option for MINO. MINO is an option for many highly comminuted (non-reducible) femoral, humeral and tibial fractures. Preoperative planning typically includes orthogonal radiographs of the intact contralateral bone using magnification calibration and templating methods to determine the optimal nail diameter and length.
Minimally Invasive Plate Osteosynthesis (MIPO)
MIPO is the application of bone plates, often in bridging fashion, without performing an open surgical approach to the fracture. MIPO is often used for highly comminuted (‘non-reducible’) diaphyseal fractures as anatomic reduction is not the goal for such fractures. Instead, “functional reduction” is the goal; also called “spatial alignment”, it restores bone length and proper alignment in the frontal, sagittal and transverse planes. Indirect reduction techniques are used to obtain this spatial alignment without opening fracture zone.
Preoperative planning for MIPO, as with MINO, typically requires orthogonal radiographs of the intact contralateral bone using magnification calibration and templating methods to determine the optimal plate width and length. Plates that span the full bone length are often used for MIPO to reduce fixation stress; longer plates with a limited number of screws at the ends of the plate are able to sustain greater loads to failure than shorter plates in which all holes are filled with screws. Plates are usually pre-contoured to the approximate bony contours by fitting to the bone surface on size-matched radiographs. As it is difficult to estimate the twisting contours of the bony surface from a 2-dimensional radiograph, fitting to size-matched skeleton can be very helpful. The precontoured plate is then sterilized and is ready for use at surgery.
Standard bone plates utilize screw fixation to pull the bone firmly against the underneath surface of the bone plate. Fixation, then, relies upon the strength of screw purchase and its ability to generate friction between the bone and the bone plate. Thus, conventional bone plating requires excellent screw purchase and precise anatomic contouring of the bone plate. Many locking plate/screw systems have been introduced to the veterinary market in recent years. In brief, these systems utilize a rigid interlock between the screw and bone plate. This interaction between the fixation element (screws) and bridging element (plate) resembles the mechanical principles of an external skeletal fixator in which fixation pins are rigidly linked to the connecting rod. Thus, many have referred to locking bone plates/screw systems as “internal fixators”, especially when used for bridging fixation. One advantage to locking plate/screw systems (compared to conventional plating) is that these plates do not require precise anatomic contouring (much like the connecting rod of external fixators is not contoured to the bone). Many of the MIPO implant systems and techniques were, consciously or not, developed to ascribe to internal fixation the many inherent advantages of external fixators.
MIO via External Skeletal Fixation
External skeletal fixation (ESF), though historically associated with high patient morbidity, was revolutionized by the development of advanced devices and surgical techniques in veterinary medicine. This resurgence of interest and innovation began in the 1990s and continues today. Current generation techniques and instrumentation permit user-friendly application of simpler, yet mechanically robust fixation frames. This concurrent optimization of methodology and instrumentation, which fostered a profound reduction in patient morbidity, revealed the inherent versatility and biological advantages of the system. Modern ESF includes linear, circular and hybrid external fixator devices. Linear ESF utilizes fixation pins and connecting rods, whereas circular ESF typically uses fine wires tensioned between clamps on rings placed around the limb. Hybrid ESF has become very popular for stabilization of juxta-articular fractures and osteotomies; the linear portion of the frame is secured to the longer bone segment while the fine-tensioned wires placed on a single ring are used for fixation of the short, juxta- articular bone segment.
Modern ESF is a very versatile system that is well suited to the ideals of MIO. It provides variable angle, locked fixation that can be applied with minimal/no disruption of the fracture zone. Rigid bilateral or multi-planar frames are relatively simple to apply in instances of nonload-sharing fixation of non-reducible fractures and simple, timely, progressive frame disassembly allows for gradual shift of loading from the fixation device to the healing bone. Understanding and strict adherence to the principles of ESF are essential in order to reduce complications and fully realize the advantages inherent to the system. Annual courses that provide comprehensive training opportunities are available in the US and elsewhere (www.imexvet.com/).
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