U. Matis, Prof. Dr. med. vet., Dr. med. vet. habil., DECVS
The creation of a harmonious partnership between the fractured bone and its repair is of paramount importance. The first decision to be made is whether the fracture requires immediate fixation or whether repair could be delayed to allow the patient to recover from shock, or from blunt thoracic or abdominal trauma. Prompt intervention is necessary in open fractures and fractures with associated neurological signs resulting from compression of the spinal cord or peripheral nerves. Likewise, joint fractures and growth plate injuries need timely repair to minimize secondary trauma to the articular surfaces or to reduce the risk of growth abnormalities. Time is also a critical factor in the repair of severely dislocated bone fragments and in regions with vulnerable vascularization, such as the femoral head. Diaphyseal and pelvic fractures are much more forgiving as long as the surgeon is able to master the challenge of contracted musculature through skilful reposition maneuvers.
The second decision entails the choice of repair method: conservative treatment or osteosynthesis. Regardless of the method chosen, every fracture must be adequately stabilized until clinical healing is complete, and if possible, the fixation method should allow functional use of the limb throughout the healing period. Thus, the surgeon must choose a fixation method (partner 1) that is individually tailored to the patient and its fracture (partner 2). When conservative treatment is chosen, the owner must be reliable and both patient and owner must be cooperative. When internal fixation is chosen, the implant system must be of sufficient size and strength to sustain cyclical load. However, to prevent bone atrophy caused by stress protection, the entire load should not be carried by the implant alone. The length of time an implant is functional depends on the degree of strain (motion) present at the implant-bone interface. With a high degree of movement between bone and implant, bone resorption and implant loosening occur. Interface stress is dependent upon three factors: 1) the surface area between the bone and implant; 2) the ability of the implant to resist motion within the bone; and 3) the amount of weight-bearing load carried by the implant.
Threaded implants (e.g., screws to secure a plate or interlocking nails, threaded transfixation pins) distribute the load over a larger surface area than implants with smooth surfaces (smooth pins, cerclage wires), which hold the bone through friction. Threaded implants are better able to sustain micro-movements within the bone than are smooth pins and wire, which concentrate strain over a smaller implant-bone interface and tend to have more problems with bone resorption and early implant loosening.
Implants intended to hold bone fragments at a distance, such as buttress plates, bear more load than plates that are designed for neutralization or compression. An implant that is required to provide buttress function over a long period of time must be sufficiently rigid so that it will not break or bend during the healing phase. No fixation system can withstand repeated loading indefinitely. The fixation will eventually fail if fracture union does not occur to protect it from repetitive loading. Thus, the surgeon must not only consider mechanical factors but must ensure optimal preservation of the biological reaction capacity of the bone. Fracture treatment should always be regarded as a "race" between fracture healing and fixation failure. To win this race, the orthopaedic surgeon must apply a balanced concept of fracture treatment. Extensive efforts to reconstruct comminuted fractures may compromise the vascular supply of intermediate fragments and diminish their contribution to fracture healing. On the other hand, surgical techniques that focus too much on preservation of soft tissues may be unsuccessful if the mechanical stabilization required for fracture repair is underestimated.
The mechanical requirements for fracture fixation depend on the size and activity level of the patient, the number of injured limbs and the extent to which the fractured bone, after reduction, will support the implant. The degree to which the bone shaft can be reconstructed determines the required implant stiffness. An oblique diaphyseal fracture that can be stabilized with interfragmentary screws and a neutralization plate tolerates a more flexible implant than a comminuted fracture, which must be supported until healing by callus formation has occurred. Bending of the implant with subsequent implant fatigue and breakage constitutes a particular risk with the requirements of fracture healing of the eccentrically loaded femur. The risk of implant failure is increased when biological factors in the area of the fracture are unfavourable.
Open fractures or fractures caused by high-energy trauma, in which there is not only shattering of the bone but also extensive separation from soft tissue, tend to heal slowly. The situation becomes even worse when the patient is geriatric or suffers from systemic disease, such as hyperadrenocorticism. In these patients, the mechanical requirements for fixation are much higher. In some cases, the surgeon encounters numerous denuded and separated bone fragments while accessing the fracture site, which is an indication that soft tissue healing is likely to be compromised. In the majority of cases with shattered or comminuted fractures, some soft tissue-bone attachment is still intact and must be preserved using an indirect reduction technique. Without opening the soft tissue envelope over the fracture site, the main fragments are reduced and stabilized with a long implant anchored only on the bone ends. Minimal impairment of circulation in the fracture area results in a more rapid callus formation and thus, load-bearing by the healing bone.
Compared to the cortical diaphyseal bone, cancellous bone, such as the epiphysis and metaphysis, has an advantage from a biological point of view because the blood supply is better and therefore fractures heal more quickly. However, reduction must be precise in or near a joint to ensure anatomic healing and thus, unimpaired joint mobility.
Clinical factors include the disposition of the patient and the compliance of the owner. In quiet co-operative patients, fractures distal to the elbow or stifle can be treated conservatively with external coaptation provided that the owner is willing and able to return for regular reassessments and bandage changes. In contrast, the surgeon should choose a fixation method that requires as little maintenance as possible in patients that require sedation or anaesthesia even for minor manipulations. Particularly for diaphyseal fractures distal to the stifle or elbow, the surgeon has a wide range of choices between conservative or surgical treatments as well as among different types of implants. For fractures that can be successfully stabilized using any of the various evidence-based methods, it is appropriate for the surgeon to choose a technique according to his/her own preference and experience. However, optimal fracture fixation must be the main goal.
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2. Matis U. Die Biologische Osteosynthese. In: Kleintierkrankheiten Band 3. Bonath KH, Prieur WD. Orthopädische Chirurgie und Traumatologie. Ulmer Stuttgart 1998:230-8