Complex, nonreconstructable fractures of the femur are one of the most commonly encountered and challenging types of fractures in dogs.1 Currently, biological bridging osteosynthesis is the treatment of choice for many comminuted nonreconstructable fractures.2 Bridge plating preserves biological potential for healing while creating an internal splint and stable fixation. Spatial alignment of the bone is restored without precise anatomic reconstruction of bone fragments, thereby preserving blood supply from soft tissue attachments.2–6 Newer techniques of percutaneous plate application (termed MIPO) may further hasten bone healing as a result of improved periosteal perfusion.7, 8 From a mechanical perspective, bridge plating demands maximal performance from the bridging implants, which may be subject to plastic deformation or breakage early in the postoperative period or fatigue failure over time, depending on the healing potential of the patient. Addition of an IMR to the construct reduces stress applied to the plate; as much as a 10-fold extension of the fatigue life of the bone plate can be achieved.9 The IMR can also assist in reestablishment of the spatial alignment of the limb and main fracture segments.10 Requirements of the IMR and screws for available bone stock often dictate the need for unicortical placement of many of the plate screws.
The use of conventional bone plates in bridging osteosynthesis has several biological, mechanical, and surgical application limitations, especially when MIPO techniques are used. Construct stability is reliant on compression between the conventional plate and underlying bone that is generated by adequate screw fixation. When achieved, this compression of the plate against the bone disrupts periosteal vascular supply and can retard healing of the underlying bone.2,11,12 Where insufficient screw-to-bone fixation is available (because of poor bone quality, short bone fragment in juxta-articular fractures, or unicortical screw purchase), cyclic loading of conventional plate-bone constructs can lead to secondary loss of fracture alignment and stability. Additionally, conventional screws are primarily stabilized by their bony purchase. Bicortical purchase of screws imparts greater angular stability, compared with stability associated with unicortical application, wherein screws may toggle about their thin cortical purchase.3 Finally, precise anatomic contouring of conventional plates is required for proper spatial limb alignment and to prevent displacement of the fracture when the screws are tightened. This is not always feasible when MIPO techniques are used.3, 12 The ideal plating system would allow percutaneous application without requiring strict anatomic contouring of the bone plate and provide angular stability of screws, even when concurrent use of an IMR necessitates unicortical screw purchase.
The recently introduced LCPa uses special screws that lock (at a fixed angle) to the plate. In contrast to conventional plate fixation, the rigid link between the LCP and screwhead obviates the need for the bone plate to be compressed to the bone, minimizes disruption of periosteum and soft tissues, and prevents primary loss of fragment alignment caused by inexact plate contouring.3,4,13 The construct functions as an internal fixator, for which precise anatomic contouring of the plate is not mandatory to maintain fracture alignment and stability. Additionally, the angular stability of the locked screws allows superior fixation, compared with conventional plate constructs, when unicortical fixation is required.3,4,12 These attributes are ideal for bridging osteosynthesis and MIPO techniques, and LCPs have been successfully used in human medicine since their clinical release in 2000.3–5,14 In a study15 of experimentally induced gap fractures in canine femora, anatomically contoured LCP fixation was associated with greater gap stiffness (in lateral-medial bending) than anatomically contoured conventional fixation. However, in that study, the common clinical practice of combination fixation with an IMR and unicortical screw fixation was not simulated, and anatomic contouring of plates is not always feasible for newer MIPO strategies.
Although there are many potential merits to the use of LCPs in orthopedic procedures in dogs, it is evident that the relative mechanical advantage provided by these systems is influenced by the mechanical properties of the bone to which they are applied, especially when unicortical screw purchase is required.3, 4 As such, there is concern that plate constructs that use only uni-cortical screw fixation may undergo catastrophic failure when applied to the relatively thin cortical wall of bones in dogs. To the authors' knowledge, there are no published reports of the mechanical behavior of unicortical LCP constructs applied with or without an adjunctive IMR in dogs' bones. The purpose of the study reported here was to compare the mechanical behaviors of a semicontoured LCP-rod construct and an anatomically contoured LC–DCP-rod construct applied to experimentally induced gap fractures in canine femora. The mechanical behaviors were examined under the conditions of axial loading, prior to and during progressive cyclic loading. We hypothesized that, compared with anatomically contoured LC–DCP-rod constructs, semicontoured LCP-rod constructs would impart equivalent construct stiffness and fracture gap stability but would be more prone to catastrophic failure of the entire lateral cortical wall when loaded to failure.
Bone mineral density
Limited-contact dynamic compression plate
Locking compression plate
Minimally invasive plate osteosynthesis
LCP, Synthes Vet Inc, West Chester, Pa.
Delphi QDR Series, Hologic, Bedford, Mass.
LC-DCP, Synthes Vet Inc, West Chester, Pa.
Steinmann pin, IMEX Veterinary Inc, Longview, Tex.
MTS, model 809 axial/torsional test system, Eden Prairie, Minn.
Eagle digital system, Motion Analysis, Santa Rosa, Calif.
Matchad 2000, Mathsoft Engineering & Education, Cambridge, Mass.
SigmaStat, version 2.03, Systat Software Inc, San Jose, Calif.
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