title: Poor reproducibility of the MRI measurement of distal femoral torsion.
abstract: Recent publications on patient-specific instrumentation for total knee arthroplasty have reported considerable variability in the axial positioning of the cutting guides for the femoral component. These personalized cutting guides are manufactured based on bone shape data, generated from magnetic resonance imaging (MRI) or computed tomography (CT). The goal of this study was to compare the reproducibility and accuracy of distal femoral torsion (DFT) values measured using these two imaging modalities. We hypothesized that MRI does not reproducibly and consistently measure DFT and is not as accurate as CT scan.
  
  
  METHODS
  Anonymized radiology records from 54 patients that included MRI and CT scans of the knee were read in random order by two observers, on two separate occasions. These records were from patients being considered for a meniscal or osteochondral graft and who had their knee explored, but who had not undergone femoral or tibial surgery and were free of osteoarthritis. The DFT was estimated using the posterior condylar angle (PCA), using both its anatomical and surgical definitions. The intra- and inter-observer reproducibility of the MRI and the differences relative to CT scan measurements were analysed.
  
  
  RESULTS
  The average intra-observer difference for the MRI evaluation of the anatomical PCA was 0.8±1.2°; it was 0.4±0.9° for the surgical PCA. More than 1° difference from the average was found in 8 cases (14%) using the anatomical PCA measurement and 4 cases (7.4%) when using the surgical PCA (P=0.4). The intraclass correlation coefficients (ICCs) were 0.67 (95% CI: 0.33-0.85) and 0.74 (95% CI: 0.47-0.89) for the anatomical and surgical PCA, respectively. The average inter-observer difference for the MRI evaluation of the anatomical PCA was 1.6±1.4°; it was 1.5±1.0° for the surgical PCA. More than 1° difference from the average was found in 27 cases (50%) using the anatomical PCA measurement and 22 cases (40%) when using the surgical PCA (P=0.4). The ICCs were 0.31 (95% CI: 0.14-0.65) and 0.48 (95% CI: 0.06-0.75) for the anatomical and surgical PCA, respectively. The average differences between the CT and MRI measurements were 1.4±1.1° (0.2-5°) and 1.1±0.8° (0-3.6°) for the anatomical and surgical PCA, respectively. Greater than 1° difference between CT and MRI was found in 29 records (54%) for the anatomical PCA and in 18 records (33%) for the surgical PCA (P=0.03).
  
  
  CONCLUSIONS
  DFT measurement on MRI is more reproducible and consistent when using the surgical PCA. MRI measurements differed by more than 1° relative to CT measurements in more than one-third of cases.
  Level III.
authors.affiliation: Département de chirurgie orthopédique, centre hospitalier de Versailles, 78150 Le Chesnay, France; Aix-Marseille université, CNRS, ISM UMR 7287, 13284 Marseille, France. Electronic address: ollivier.mt@gmail.com.
authors.forename: M
authors.initials: M
authors.lastname: Ollivier
authors.affiliation: Département de chirurgie orthopédique, centre hospitalier de Versailles, 78150 Le Chesnay, France.
authors.forename: C
authors.initials: C
authors.lastname: Stelzlen
authors.affiliation: Département de chirurgie orthopédique, centre hospitalier de Versailles, 78150 Le Chesnay, France.
authors.forename: P
authors.initials: P
authors.lastname: Boisrenoult
authors.affiliation: Département de chirurgie orthopédique, centre hospitalier de Versailles, 78150 Le Chesnay, France.
authors.forename: N
authors.initials: N
authors.lastname: Pujol
authors.affiliation: Département de chirurgie orthopédique, centre hospitalier de Versailles, 78150 Le Chesnay, France.
authors.forename: P
authors.initials: P
authors.lastname: Beaufils
mesh_terms: D000328:Adult; D000368:Aged; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects / Q000379:methods; D016022:Case-Control Studies; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging* / Q000601:surgery; D006801:Humans; D007719:Knee Joint / Q000601:surgery; D008279:Magnetic Resonance Imaging; D008297:Male; D015588:Observer Variation; D011897:Random Allocation; D015203:Reproducibility of Results; D013977:Tibia / Q000601:surgery; D014057:Tomography, X-Ray Computed; D054159:Torsion, Mechanical
keywords: CT scan; Distal femoral torsion; MRI; Patient-specific instrumentation; Total knee arthroplasty