Improving Sagittal Alignment Of Tibial Component In TKA With Extramedullary Instrumentation

OBJECTIVE The sagittal slope of the tibial component may be implicated in the clinical performances and longevity of total knee arthroplasty (TKA). Extramedullary instrumentations are widely used in TKA. A few investigations suggested that the proximal portion of tibial crest and fibular diaphyseal axis are closely parallel to the tibial mechanical axis in the sagittal plane. However, to what extent these anatomical references may guide the surgeon to achieve a proper sagittal alignment of tibial component has yet to be established. The object of this study was to analyse the relationship between an extramedullary instrumentation, positioned parallel to the sagittal mechanical axis, and the anterior tibial profile and to establish whether two or more portions in the anterior tibial profile may be identified to place the extramedullary rod parallel to the sagittal mechanical axis. MATERIALS AND METHODS Fifty cadaveric dried tibiae of Caucasian individuals were analyzed. A standard intramedullary rod used to perform the tibial cut in TKA was inserted into the medullary canal as reference for the mecanichal axis of the tibia in the sagittal plane. A standard tibial cutting block used for TKA was connected to the intramedullary rod and set at 0° varus/valgus and 0° sagittal slope. The tibial cutting block was secured with 2 pins to the proximal part of the tibia 10 mm from the tibial plateau, to simulate a standard tibial cut. An extramedullary rod was then inserted into the tibial cutting block to evaluate its relationship with the entire anterior tibial profile. A standard lateral radiograph was taken having placed each tibia with its antero-posterior axis parallel to the horizontal plane to obtain a correct lateral view radiograph. When the extramedullary rod was not parallel to the intramedullary one, it was adjusted on the cutting block until the two rods were parallel each other. The distance between the anterior profile of the tibia and the extramedullary rod was assesses using AUTOCAD software at points located every 10 mm. A mathematical model was then applied to take into account the different tibial length and to measure the distance between the anterior tibial border and the extramedullary rod at interval of 2% of the entire tibial length. This generated 38 measurements along the anterior border of for each tibia, included between 20% and 95% of the whole tibial length from the tibial plateau to the distal metaphysis. A paired t-test (significance p<0.05) was used to assess the difference in the distance from the anterior tibial border and the extramedullary rod between different points of the anterior tibial border. RESULTS Three tibiae showing a sagittal deformity which caused a malalignment of the intramedullary rod with respect to the sagittal mechanical axis were excluded, leaving 47 tibiae eligible for the study. The average tibial length was 31.98 mm (range 28.28-37.5mm, SD 2.3). The distance from the anterior tibial border and the extramedullary rod showed a non-linear distribution, the lower values being found at 20% of the tibial length and the higher between 70% and 80% of the tibial length. Between 80% and 90% of the tibial length, the distance between the anterior tibial border and the extramedullary rod decreased but it was still significantly greater than in the proximal tibia. The anterior tibial border showed much less variability among individuals in the proximal diaphysis than in the middle-distal diaphysis and distal metaphysis. CONCLUSIONS Although the anterior profile of tibial border is commonly used as intraoperative reference for the sagittal alignment of the tibial component, to our knowledge no study has analysed its relationship with the mechanical axis, along the whole tibial length. This study has shown that when the extramedullary rod is placed at the same distance from the anterior tibial border at 2 points located at 56% and 90% of the tibial length , the extramedullary rod is likely to be aligned with the sagittal mechanical axis.