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ОглавлениеBasic principles of fracture treatment
David M. Nunamaker
1.3 Precise anatomic reconstruction
1.5 Soft tissue considerations
1.1 Introduction
Goal of AO fracture treatment: Functional fracture treatment with early joint mobility and gradually increasing weight bearing.
A full fifteen years after the publication of the Manual of Internal Fixation in the Horse by Springer Verlag, the principles of internal fixation using AO techniques remain intact. New techniques have expanded the capabilities of the surgeon, and experience with established techniques has added a perspective that was not present a decade ago. Immediate full weight bearing following fracture fixation remains a goal that cannot always be achieved in the horse. Functional fracture treatment with early joint mobility and gradually increasing weight bearing as tissue healing progresses are admirable goals. In the horse, early or immediate full weight bearing is a necessity difficult or impossible to side-step. The use of casts and splints to protect internal fixation devices from failure, and techniques such as plate luting that increase the fatigue life of those implants can combine to significantly improve results [1, 2].
Successful internal fixation allows sharing of loads between bone and implants.
Successful internal fixation starts with the anatomic reconstruction of bone and joint surfaces that allows the sharing of loads between the reconstructed bone and the implants. Anatomic reconstruction can be accomplished by screws alone or screws combined with a plate. Interfragmentary compression is absolutely essential for maintaining bone contact between fragments to protect the relatively weak implants. Orthopedic implants by themselves are not able to withstand the full force of weight-bearing without failure.
1.2 Surgical approaches
Accurate alignment of fracture fragments is made possible by surgical approaches that allow adequate visualization.
Incisions into badly bruised skin with subcutaneous hemorrage carry a high risk of infection.
The accurate alignment of fracture fragments and the perfect reconstruction of joint surfaces is made possible by surgical approaches that allow adequate visualization. Perfect reduction cannot be ensured if the joint surfaces are reduced without direct vision, and inadequate exposure of shaft fractures may not allow reduction of overriding fracture fragments or proper placement of plates or screws. Surgical approaches should also be designed to maintain vascular integrity and to avoid areas of compromised soft tissues. Evaluation of compromised skin may be difficult, and decisions need to be modified based upon the amount of time which has elapsed since the injury. In general, incisions into badly bruised skin with subcutaneous hemorrhage carry a high risk of subsequent infection. Devitalized skin may mean delaying open reduction and internal fixation. External skeletal fixation alone may be considered or in combination with minimal internal fixation using screws placed through stab incisions [3, 4]. Casts or splints are not usually helpful in preserving unstable fractures with compromised skin, but bulky bandages such as the Robert Jones Dressing can be useful. When planning an open reduction and internal fixation of equine fractures, the skin incision must always be far enough away from the proposed placement of the plate(s) to ensure soft tissue coverage. In general, the incision line should not be directly over the implants.
Keep skin incisions away from the intended location(s) of plate(s).
1.3 Precise anatomic reconstruction
Interfragmentary compression creates large “normal” (rectangular) forces.
Axial and rotational alignment must be preserved at the time of fracture reduction.
Interfragmentary compression occurs whenever two fracture surfaces are pressed tightly together.
Use bone grafts as a replacement for early bridging callus.
Normal function in the horse depends upon anatomic reconstruction of fractures and joint surfaces. Slight malalignments in the reconstruction of a fractured bone can lead to significant deviations in foot position and leg conformation in this long-legged species. It is important that axial and rotational alignment are preserved at the time of fracture reduction.
Comminuted fractures may make anatomic reconstruction more difficult since many small fragments may no longer be salvageable. Length, axial, and rotational alignment can be maintained using interfragmentary screws. Voids in the bony cortex are filled with a cancellous bone graft. The grafts are used wherever possible because they cause formation of an early structural bridge. This can be important in preserving the integrity of the internal fixation.
Cancellous bone grafts can be used as a kind of callus replacement over potentially weak areas of the reconstruction; stable internal fixation may limit natural callus formation.
Nowhere is anatomic reconstruction more important than in the case of a fractured joint. Here, even a small step or incongruity in the surface may lead to degenerative joint disease and associated loss of function. When dealing with displaced fractures, direct visualization is essential for adequate reduction. Intraoperative image intensification or radiographic monitoring can help ensure reduction but the images obtained can be misleading.
1.4 Stable fixation
Interfragmentary compression is at the heart of internal fixation using screws and plates.
Fig. F1A: Creating large “normal forces” upon the fracture plane is the main objective of interfragmentary compression.
Interfragmentary compression creates large normal forces (forces perpendicular to the fracture planes) that prevent movement of the individual bone fragments (Fig. F1A). These large normal forces in turn create large frictional forces that prevent sliding of the fracture fragments over each other. Although usually thought of as being achieved only with the use of lag screws, interfragmentary compression occurs whenever two fracture surfaces are pressed tightly together. For instance, it occurs when plates are used to compress the surfaces of a transverse fracture or osteotomy (Fig. F1B). This axial compression is often combined with interfragmentary compression produced by individual lag screws in fractures that have transverse and oblique components (Fig. F1C). When fractures are treated with casts or splints, healing occurs with motion and callus formation. Relative motion between individual fragments fixed with screws or with screws and a plate may be detrimental to fracture healing.
Fig. F1B: A plate applied to the reduced fracture provides axial compression.
Fig. F1C: A combination of plate axial compression and interfragmentary screw compression in a comminuted fracture.
Large gaps seem less sensitive to small amounts of motion than small gaps. This observation can be explained by the fact that equal amounts of motion in small gaps and large gaps represent a different percentage of that gap. Healing tissues can only stretch so far before they rupture. As healing progresses and motion decreases the tissue's ability to stretch diminishes as well, i.e. from granulation tissue to cartilage to bone. Theoretical and experimental studies have explored this phenomenon which has been termed interfragmentary strain [5, 6].
Since interfragmentary strain may influence fracture healing, relative motion should be controlled by the internal fixation and special attention must be given to small gaps that may be subject to delayed healing or non union due to micromotion. These small gaps may also increase the risk of implant failure through the cyclic loading that occurs during weight bearing.
Combine axial compression produced by a plate with interfragmentary compression produced by a lag screw.
Interfragmentary strain may influence fracture healing.
Small gaps may also increase the risk of implant failure through cyclic loading.
1.5 Soft tissue considerations
Adequate vascularity of soft tissues and bone is very important.
Nutrient vessels and periosteal soft tissues provide a blood supply to the bone.
Adequate vascularity of soft tissues and bone is important if fracture healing is to occur. Most equine fractures are high energy events with bone literally exploding into the surrounding soft tissues. This initial trauma may devitalize the soft tissues as well as the bone. Bone receives its blood supply by way of its nutrient vessel and periosteal soft tissue attachments. Much of this blood supply may be interrupted at the time of the fracture. While the nutrient vessel is almost always compromised, the integrity of the periosteal blood supply can be difficult to assess prior to surgical intervention. This makes open reduction and internal fixation a risky technique since avascular tissue will be at a higher risk for necrosis and infection. Further loss of blood supply to the bone may occur due to the exposure necessary for open reduction and implant placement. Proper evaluation of soft tissue viability will influence the outcome of postoperative complications, such as infection and wound dehiscence. Adequate first aid and preoperative care are essential for the preservation of the remaining blood supply following injury.
Whenever possible, internal fixation using lag screws should be accomplished under radiographic control through stab incisions to minimize additional soft tissue compromise. This is usually performed for nondisplaced fractures. Sometimes additional stab incisions can be used for the insertion of screws even when open approaches are used for visualization and reduction. As an alternative to expansion of the primary incision, this technique serves to limit the necessary exposure. Soft tissues should always be protected during drilling and tapping by the use of drill guides and tap sleeves.
The need for open reduction and internal fixation must be balanced by its risks. Experience with this paradox in the horse will help define each surgeon's abilities and limitations.
Avascular tissue will be at a higher risk for necrosis and infection.
Anesthetic recovery can be a critical time.
The need for open reduction and internal fixation must be balanced by its risks.
1.6 Functional rehabilitation
As stressed in this chapter, anatomic reconstruction, stable internal fixation with good soft tissue evaluation, and careful surgical handling should permit early weight bearing with pre-
servation of joint function. To accomplish these goals, external casts or splints must sometimes be used in the postoperative period or at least during recovery from anesthesia. Anesthetic recovery can be a critical time in the early treatment of a horse with a fracture, and protection of the animal and the reconstructed fracture is of paramount importance. Casts are often used to help ensure a safe recovery. Concern for preservation of the animal and its operated extremity has resulted in specialized recovery techniques such as the raft/pool recovery system.
Healing of bone itself does not ensure full functional rehabilitation. Failure of bone healing, however, does ensure failure of functional rehabilitation. Therefore, healing of bone is the first criterion for the rehabilitation of an afflicted animal.
Success in fracture treatment is measured according to expectations. Some injuries are so severe that they are lifethreatening. In such cases, just saving an animal's life to allow it to be pasture sound may be a success. In other cases, the animal will be expected to perform at levels equal to or surpassing those of its former status. Here, fracture fixation and bone healing constitute only a small part of the total rehabilitation process. Further efforts will be required to bring the horse back to its former athletic prowess. Truly successful fracture treatment must involve the whole animal and reach beyond the gains made in the operating theatre.
Success in fracture treatment is measured against preoperative expectations.
1.7 References
1. Nunamaker DM, Richardson DW, Butterweck DM (1991) Mechanical and biological effects of plate luting. J Orthop Trauma; 5:138–145.
2. Young DR, Richardson DW, Nunamaker DM, et al. (1989) Use of dynamic compression plates for treatment of tibia diaphyseal fractures in foals: Nine cases (1980–1987). J Am Vet Med Assoc; 194:1755.
3. Nunamaker DM, Richardson DW, Butterweck DM, et al. (1986) A new external skeletal fixation device that allows immediate full weightbearing: Application in the horse. Vet Surg; 15:345.
4. Nunamaker DM, Richardson DW (1992) External skeletal fixation in the horse. Proc 37th Annual Convention of Am Assoc Equine Pract; 549.
5. Perren SM (1979) Physical and biological aspects of fracture healing with special reference to internal fixation. Clinical Orthop; 138:175.
6. Cheal EJ, Mansmann KA, DiGioia III AM, et al. (1991) Role of interfragmentary strain in fracture healing: ovine model of a healing osteotomy. J Orthop Res; 9:131–142.
1.7.1 Online references
See online references on the PEOS internet home page for this chapter: http://www.aopublishing.org/PEOS/01.htm
