title: What is the optimal valgus pre-set for intramedullary femoral alignment rods in total knee arthroplasty?
abstract: OBJECTIVE
  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 5°, 6°, 7° or 8° valgus) to align the cutting block perpendicularly to the mechanical axis.
  
  
  METHODS
  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) 5°, (d) 6°, (e) 7° and (f) 8° valgus pre-sets were measured for each patient. The lowest mean differences were determined.
  
  
  RESULTS
  The 40 knees measured showed a mean tAMA of 7.2° valgus (1.7 SD) (range 4°-11.5°). The following mean differences and 95 % limits of agreement were calculated: 2.2 (-1.2, 5.5) to the tAMA for the 5° valgus pre-set, 1.2 (-2.2, 4.5) for 6°, 0.2 (-3.2, 3.5) for 7° and -0.8 (-4.2, 2.5) for 8°. 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.
  
  
  CONCLUSIONS
  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 7° 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.
  
  
  METHODS
  Diagnostic study, Level II.
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany. Guenther.Maderbacher@klinik.uni-regensburg.de.
authors.forename: G
authors.initials: G
authors.lastname: Maderbacher
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Keshmiri
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Schaumburger
authors.affiliation: Center of Clinical Studies, University Medical Center Regensburg, Franz-Josef-Strauß-Allee 11, 93053, Regensburg, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Zeman
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany.
authors.forename: A M
authors.initials: AM
authors.lastname: Birkenbach
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Craiovan
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Grifka
authors.affiliation: Department of Orthopedic Surgery, University Medical Center Regensburg, Kaiser-Karl-V Allee 3, 93077, Bad Abbach, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Baier
mesh_terms: D000368:Aged; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D017760:Bone Malalignment / Q000000981:diagnostic imaging / Q000209:etiology; D001858:Bone Nails; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging / Q000601:surgery*; D006801:Humans; D008297:Male; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000601:surgery*
keywords: Alignment; Cutting block; Femur; Intramedullary alignment; Navigation; Pinless; Rod; TKA; Total knee arthroplasty