In vitro biomechanical comparison of a plate-rod combination-construct and an interlocking nail-construct for experimentally induced gap fractures in canine tibiae

OBJECTIVE: To compare structural properties of a plate-rod combination-bone construct (PRCbc) and interlocking nail-bone construct (ILNbc) by use of an experimentally induced gap fracture in canine tibiae. SAMPLE POPULATION: 12 paired canine tibiae. PROCEDURE: Specimens were implanted with a plate-rod combination consisting of a 3.5-mm, limited-contact, dynamic-compression plate combined with an intramedullary rod or 6-mm interlocking nail. Ostectomy (removal of 10-mm segment) was performed. Paired constructs were loaded for bending, compression, or torsion measurements (4 constructs/group). Compliance was determined by fitting regression lines to the load-position curves at low (initial compliance) and high (terminal compliance) loads. RESULTS: Bending compliances did not differ significantly between constructs. For the ILNbc, initial compliance was greater than terminal compliance in compression and torsion. Initial compliance and terminal compliance for the PRCbc were similar in compression and torsion. Initial compliance in compression and torsion was greater for the ILNbc, compared with initial compliance for the PRCbc. Maximum deformations in bending and compression were similar between constructs; however, maximum torsional angle was significantly greater for the ILNbc, compared with values for the PRCbc. CONCLUSIONS AND CLINICAL RELEVANCE: The study documented that for an experimentally induced gap fracture in canine tibiae, a plate-rod combination is a significantly less compliant fixation method in torsion and compression, compared with an interlocking nail. Considering the deleterious effects of torsional deformation on bone healing, a plate-rod combination may represent a biomechanically superior fixation method, compared with an interlocking nail, for the treatment of dogs with comminuted tibial diaphyseal fractures.     

Influence of limb positioning and measurement method on the magnitude of the tibial plateau angle

OBJECTIVE: To evaluate the effect of limb positioning and measurement technique on the magnitude of the radiographically determined tibial plateau angle (R-TPA). STUDY DESIGN: In vitro study, R-TPA was determined by 6 blinded observers and image measurement software. ANIMALS: Five canine cadaver hind limbs. METHODS: The legs were positioned on a custom-made positioning device simulating a radiographic tabletop technique in lateral recumbency. True lateral positioning was defined by superimposition of femoral and tibial condyles on the radiographic projection. Radiographs were taken while the specimens were relocated in a proximal, distal, caudal, and cranial direction with respect to the radiographic beam. For each specimen, 25 different radiographic views were obtained and 6 blinded observers determined the radiographic TPA using 2 different methods. The conventional method used precise anatomic landmarks to determine the tibial plateau. To simulate osteoarthritic changes complicating identification of these landmarks, the tangential method estimated the tibial plateau as the tangent to the central portion of the tibial plateau. After periarticular soft tissue dissection the anatomic tibial plateau angle (A-TPA) was determined. The A-TPA and the R-TPA were compared. RESULTS: The R-TPA significantly decreased as limb position with respect to the X-ray beam changed from cranial proximal to caudal distal. The maximal mean radiographic R-TPA difference was 3.6 degrees with the first and 5.7 degrees with the second method. Regardless of the method used there was no significant difference between A-TPA and R-TPA in the true lateral position. In the peripheral positions, however, significant differences between anatomic and radiographic TPA were seen. CONCLUSIONS: Limb positioning influenced the radiographic appearance of the tibial plateau and the magnitude of the measured TPA. Cranial and proximal positioning of the limb relative to the X-ray beam leads to overestimation whereas caudal and distal positioning leads to underestimation of the TPA. CLINICAL RELEVANCE: True lateral positioning of the tibia defined by superimposition of the femoral and tibial condyles should be used for accurate TPA determination before tibial plateau leveling osteotomy.