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What is the optimal valgus pre-set for intramedullary femoral alignment rods in total knee arthroplasty?

By G. Maderbacher, A. Keshmiri, J. Schaumburger, F. Zeman, A. M. Birkenbach, B. Craiovan, J. Grifka and C. Baier

Abstract

In total knee arthroplasty (TKA), intramedullary guides are often used for aligning the distal femoral cutting block. Because of the highly varying angles between the mechanical axis and the anatomical femoral axis (AMA), different valgus pre-sets have been recommended. The present study investigated the optimal valgus pre-set (measuring the AMA in long-leg radiographs or at 5A degrees, 6A degrees, 7A degrees or 8A degrees valgus) to align the cutting block perpendicularly to the mechanical axis. The AMA was preoperatively measured in weight-bearing long-leg radiographs. After alignment of the cutting block by means of an intramedullary rod, deviation of the block from the mechanical femoral axis was measured with a pinless navigation device. The true AMA (tAMA) was calculated by adding the valgus pre-set of the alignment rod to the deviation measured with the navigation device. Mean deviations between the tAMA and (a) the AMA measured by the surgeon, (b) the AMA calculated with the computer software, (c) 5A degrees, (d) 6A degrees, (e) 7A degrees and (f) 8A degrees valgus pre-sets were measured for each patient. The lowest mean differences were determined. The 40 knees measured showed a mean tAMA of 7.2A degrees valgus (1.7 SD) (range 4A degrees-11.5A degrees). The following mean differences and 95 % limits of agreement were calculated: 2.2 (-1.2, 5.5) to the tAMA for the 5A degrees valgus pre-set, 1.2 (-2.2, 4.5) for 6A degrees, 0.2 (-3.2, 3.5) for 7A degrees and -0.8 (-4.2, 2.5) for 8A degrees. AMA measurements by the surgeon and with the digital medical planning software yielded mean differences of 0.6 (-3.1, 4.3) and 0.4 (-4.1, 4.8), respectively. In the present setting, the best mean distal femoral cutting block alignment perpendicular to the mechanical femoral axis could be achieved with a valgus pre-set of 7A degrees and not by measuring the AMA. Nevertheless, we recommend conducting weight-bearing radiographs of the entire leg prior to TKA for easy detection of any anatomical varieties, old fractures, long stems of total hip arthroplasties or cement. However, surgeons must be aware that exact coronal component alignment can only be achieved by navigational devices. Diagnostic study, Level II

Topics: 610 Medizin, ddc:610
Publisher: 'Springer Science and Business Media LLC'
Year: 2017
DOI identifier: 10.1007/s00167-016-4141-y
OAI identifier: oai:epub.uni-regensburg.de:39641
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