title: The Immediate Post-Operative Radiograph is an Unreliable Measure of Coronal Plane Alignment in Total Knee Replacement.
abstract: BACKGROUND
  Restoration of a neutral mechanical axis is a primary goal of total knee replacement (TKR). A mechanical axis within 3° of neutral has been correlated with improved implant longevity, function, and patient satisfaction. We hypothesize that the immediate post-operative radiograph is an unreliable method of measuring alignment following TKR surgery.
  
  
  METHODS
  Seventy-five consecutive patients had supine X-rays performed on day two post-operatively followed by standing long-leg radiographs (LLRs) 6 weeks post-operatively. Correlation was sought between the mechanical axis measured on the LLR and surrogate markers of alignment on the post-operative X-ray including component alignment and an estimation of anatomical alignment using the available length of femoral and tibial shafts. Inter- and intra-observer reliabilities were assessed.
  
  
  RESULTS
  The mean mechanical axis on the LLR was 180.5 (SD 3.0, range 175.1-187.1). Mean offset between anatomical axis and mechanical axis was 6.4°. The mean anatomical axis measured on the short-leg X-ray was 174.9 (SD 2.4, range 169.5-181.3). Mechanical axis on the LLR was compared to the anatomical axis measured on the short-leg radiograph (SLR) + 6° with an interclass correlation coefficient of 0.588 (p < 0.001). The level of disagreement between the short- and long-leg X-rays was assessed using the Bland-Altman method and demonstrated clinically important discrepancies of 5 or more degrees in 9% of cases. Inter- and intra-observer agreements were high on all measures (p < 0.001).
  
  
  CONCLUSIONS
  The long-leg weight bearing X-ray is an essential tool to accurately assess coronal plane alignment post TKR. While the immediate post-operative X-ray taken supine provides useful information to the surgeon on any immediate complications, our results indicate that it cannot be relied upon to determine correct restoration of the mechanical axis.
authors.affiliation: Department of Orthopaedics, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia ; Department of Surgery, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia.
authors.forename: Joshua
authors.initials: J
authors.lastname: Petterwood
authors.affiliation: Department of Orthopaedics, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia ; Department of Surgery, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia.
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Dowsey
authors.affiliation: Department of Orthopaedics, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia ; Department of Surgery, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia.
authors.forename: Daevyd
authors.initials: D
authors.lastname: Rodda
authors.affiliation: Department of Orthopaedics, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia ; Department of Surgery, St. Vincent's Hospital Melbourne, The University of Melbourne , Melbourne, VIC , Australia.
authors.forename: Peter F M
authors.initials: PF
authors.lastname: Choong
keywords: alignment; knee; knee replacement; long-leg radiographs; mechanical axis