RESUMO
BACKGROUND: For patients with knee osteoarthritis, even slight anatomical variations in the femur or the tibia could affect total limb alignment during total knee replacement (TKR). Our hypothesis implies that the femoral valgus correction angle (VCA) in patients indicated for TKR, is variable and higher than the reported norm of 6° utilized in most intramedullary instrumentation systems, and that tibial bowing may result to a disparity of the tibial mechanical axis to the anatomical axis. METHODS: Our study is a retrospective review of 216 pre-operative arthritic knees, which investigated the lower limb axial alignment using digitally-stitched films. Patients excluded from the study are those with history of previous tibial or femoral osteotomy, secondary gonarthrosis, rheumatoid arthritis, previous femoral or tibial fracture, patients for bilateral TKR, or history of hip surgery. RESULTS: The mean age was 68-years old (range 39-86 years). The mean VCA was 7° (4.7-9.3) for men and 6.6° (4.9-9) for women. However, 71 patients (33%) had more than 7° VCA. Subsequently, 46 patients (21%) had tibial bowing producing an angle >1.5° between its mechanical and anatomic axis. CONCLUSIONS: The 6° standard when used as a guide may result in suboptimal prosthesis positioning during conventional TKR surgery. Therefore our findings suggest that the femoral valgus correction angle has a broad range, and using standard femoral intramedullary guides should not be overlooked.
RESUMO
OBJECTIVES: To develop three dimensional computer models of the anterior thoracolumbar spine implants or constructs (the novel single rod-screw implant and the standard implants) and to evaluate its biomechanical properties through a graphically reconstructed testing standard. METHODS: We developed a finite element modeling technique based on actual geometry of the implant constructs and mechanical property data from standard biomechanical studies on anterior thoracolumbar spinal instrumentation systems. Seven constructs were mounted on simulated vertebral bodies. Axial load sharing was measured through a range of applied axial loads from 100 N to 1600 N. The static destructive tests were conducted. The bending strength of each construct was calculated with a full length corpectomy graft in place, simulating reconstruction of the anterior column, and with no graft in place, simulating catastrophic graft failure. RESULTS: Static testing parameter demonstrated highly significant differences between devices. The plate construct formed the highest subset in bending strength of 1000-1100 N, whereas the single rod showed the lowest value of 300-400N. However, the bending strengths of single rod and dual rod both without bone grafts were not significantly different. With the graft in place, bending strength of the constructs significantly increased beyond the maximum set of load of 1600N, underlying the importance of the graft in overall construct strength. CONCLUSION: The 3-D finite element models for anterior thoracolumbar instrumentation system were designed with mechanical properties comparable to the actual biomechanical testing results. Although single rod construct has the lowest value, its bending strength is comparable to the standard dual rod system under static axial loading. Bone graft contributed to overall construct stiffness.
