We recruited all consecutive patients with severe varus osteoarthritis (OA), valgus OA, rheumatoid arthritis (RA) with medial instability or osteoarthritis after high tibial osteotomy (HTO) who underwent primary TKA using a CCK implant; specifically, the NexGen Legacy Constrained Condylar Knee (LCCK; Zimmer Biomet Inc., Warsaw, IN, USA); between April 2014 and August 2018. All patients provided informed consent and the study and all its protocols were approved by the local institutional review board of the authors’ affiliated institutions.
Preoperative transverse computed tomography (CT) images (Aquilion ONE; Canon Medical Systems Corporation Japan, Tochigi, Japan) were obtained at levels from the hip to ankle joints at 1.25-mm intervals and 1.25 mm thickness with a field of view of 400 and pitch of 1.375. Patients were placed in the supine position for CT examination, and the affected knee was naturally extended, monitoring for any feeling of internal or external rotation. Images were acquired in Digital Imaging and Communications in Medicine format (DICOM) from the software of the CT scanner.
1. Three-dimensional templating of the femoral component and the straight extension stem
We imported DICOM data sets into 3D pre-operative planning software (ZedKnee; Lexi, Tokyo, Japan), and 3D femoral bone models were reconstructed using the software with the 3D coordinate system embedded into them. The femoral mechanical axis was defined as the line connecting the center of the femoral head and the midpoint of the SEA. The coronal plane was defined as the plane of the femoral mechanical axis and the SEA. The anatomical axis of the distal femur was determined automatically using the software.
The femoral component was templated by referring to the alignments described below. The coronal alignment was set at 6° valgus relative to the anatomical axis of the distal femur considering the specific valgus angle of the extension stem of the CCK implant. Sagittal alignment was set parallel to the anatomical axis of the distal femur to avoid notching [10, 11], and rotational alignment was parallel to the SEA. The size of the femoral component was chosen as the best match for the AP dimension of the native femoral lateral condyle . The ML position was set to minimize overhang and underhang as much as possible .
To investigate the suitability of inserting a straight extension stem into the intramedullary canal of the distal femur, we used a 100-mm extension stem as per the manufacturer’s recommendation. The extension stem was set to avoid invasion of the intramedullary wall of the femoral cortex (Fig. 3). We measured the maximum diameter of the straight extension stem in cases where it was able to be used. Next, we measured the acceptable varus-valgus and extension-flexion angle deviations of the extension stem when the stem diameter was down-sized by 1 or 2 mm, respectively, relative to the maximum diameter (Fig. 4).
2. Actual primary TKA using a CCK implant
Primary TKA was performed using a CCK implant in these all knees. The distal femur was prepared so as to reproduce the simulation described above using the intramedullary rod. Whether or not the straight extension stem could be used was investigated retrospectively, and stem diameter was evaluated in cases which used a straight extension stem.
3. Short-term evaluation of clinical and radiographical outcomes
All patients were followed regularly after surgery. Range of motion (ROM), radiographs and The Knee Society Score 2011 , were assessed at the latest follow-up. The coronal alignments of whole-leg, femoral and tibial components are measured and the existence of component loosening and cortical bone hypertrophy around the stem were evaluated.
4. Statistical analysis
Maximum diameter of the straight extension stem was compared between computer simulation and actual surgery using Wilcoxon signed-rank sum test. In addition, its rate of concordance between each measurement was evaluated as intraclass correlation coefficient (ICC). All 3D templating and measurements were carried out twice by one examiner with an interval of at least 1 month and once by another examiner on the study group. Intra- and inter-observer differences in templated femoral component size and maximum stem diameter were evaluated as ICC. JMP 14.3 (SAS Institute Inc., Cary, NC, USA) was used to analyze the data. Significance was set at a p-value of <0.05.