VOL. 93-B, No. 5, MAY 2011 629  KNEE The position and orientation of total knee replacement components A COMPARISON OF CONVENTIONAL RADIOGRAPHS, TRANSVERSE 2D-CT SLICES AND 3D-CT RECONSTRUCTION M. T. Hirschmann, P. Konala, F. Amsler, F. Iranpour, N. F. Friederich, J. P. Cobb From Kantonsspital Bruderholz, Bruderholz, Switzerland  M. T. Hirschmann, MD, Orthopaedic Surgeon  N. F. Friederich, MD, Orthopaedic Surgeon, Professor Department of Orthopaedic Surgery and Traumatology Kantonsspital Bruderholz, CH- 4101 Bruderholz, Switzerland.  P. Konala, MRCS, MD, Radiologist  F. Iranpour, MD, PhD, Orthopaedic Surgeon  J. P. Cobb, MD, FRCS, MCh, Orthopaedic Surgeon, Professor Department of Musculoskeletal Surgery Imperial College, South Kensington Campus, London SW7 2AZ, UK.  F. Amsler, MSc, Statistician Amsler Consulting, Gundeldingerrain 111, CH-4059 Basel, Switzerland. Correspondence should be sent to Dr M. T. Hirschmann; e-mail: michael.hirschmann@ksbh.ch ©2011 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.93B5. 25893 $2.00 J Bone Joint Surg [Br] 2011;93-B:629-33. Received 12 October 2010; Accepted after revision 24 January 2011 We studied the intra- and interobserver reliability of measurements of the position of the components after total knee replacement (TKR) using a combination of radiographs and axial two-dimensional (2D) and three-dimensional (3D) reconstructed CT images to identify which method is best for this purpose. A total of 30 knees after primary TKR were assessed by two independent observers (an orthopaedic surgeon and a radiologist) using radiographs and CT scans. Plain radiographs were highly reliable at measuring the tibial slope, but showed wide variability for all other measurements; 2D-CT also showed wide variability. 3D-CT was highly reliable, even when measuring rotation of the femoral components, and significantly better than 2D-CT. Interobserver variability in the measurements on radiographs were good (intraclass correlation coefficient (ICC) 0.65 to 0.82), but rotational measurements on 2D-CT were poor (ICC 0.29). On 3D-CT they were near perfect (ICC 0.89 to 0.99), and significantly more reliable than 2D-CT (p < 0.001). 3D-reconstructed images are sufficiently reliable to enable reporting of the position and orientation of the components. Rotational measurements in particular should be performed on 3D-reconstructed CT images. When faced with a poorly functioning TKR with concerns over component positioning, we recommend 3D-CT as the investigation of choice. Malpositioning and malorientation are two major causes of pain following total knee replacement (TKR). 1-3 The assessment of the position of the components after TKR is gener- ally performed on radiographs. 4-12 Gross malposition, particularly in varus- valgus and flexion-extension, might be identi- fied on radiographs, but accurate measure- ment is prone to error owing to variation in rotation of the leg and magnification. 5,13 Mea- surement of rotation of the components is difficult. 14-16 Routine assessment of component position can also be performed on CT scans. 4,6 Depend- ing on the CT imaging protocol, the post- operative sagittal, coronal and rotational alignment of the components can be deter- mined. 17,18 Several different methods of mea- surement have been described. 4,19 For rotational measurement of the femoral compo- nent, most authors use transverse 2D-CT slices measuring the angle between the surgical epi- condylar axis and the posterior prosthesis axis. 4,6 Less frequently, CT images are recon- structed in 3D, with the images aligned to stan- dardised frames of reference, and rotational alignment is assessed on these images. 20 For this purpose, specific protocols have been described that include not only the knee but also the head of the femur and the mid- ankle. 21,22 This assessment of rotation may be more accurate and reproducible than measure- ments on 2D-CT slices. Only in 3D-CT can the measurements be adjusted for the variability of the position of the patient’s leg in the scanner, which may lead to a more reliable identifica- tion of the anatomical landmarks such as the medial and lateral femoral epicondyles. The primary purpose of this study was to evaluate which of these imaging modalities best documented the position and orienta- tion of the components of a TKR. We there- fore set out to establish the intra- and interobserver reliability of tibial and femoral component measurements in patients after TKR using radiographs, axial 2D-CT images and 3D-CT. Patients and Methods A total of 30 knees were investigated in 29 patients with a mean age of 71 years (39 to 89) who had undergone primary TKR. The study was approved by our institutional review board.