Samples selection
After approved by the regional ethics committee of our institution, we retrospectively reviewed 963 CT images of the knee between January 2015 and August 2017 in our institution. Indications for CT were different from the aim of this study. Inclusion criteria: (1). patients accepted ultrahigh resolution CT examination of the knee; (2). the age range of patients is 18 years to 60 years; (3). scanning direction of CT was paralleled to anterior tibial crest, the selected sagittal section contained PCL tibial attachment and highest vertex of the tibial marrow cavity. Exclusion criteria: patients with displaced fractures involving knee, congenital skeletal dysplasia, previous knee surgery, inflammation or tuberculosis of bone and joint, tumor around the knee joint, and any knee abnormalities caused by disease.
Computed tomography imaging
All included patients accepted standard clinical knee CT performed on a 64-multi-detector-row CT (SOMATOM Sensation, Siemens AG, Wittelsbacherplatz 2, Muenchen, Germany). Scanning parameters included a gantry rotation speed of 1.00 s/rotation, 0.3 mm collimation width × 12 detectors, a CT pitch factor of 0.90 and a field of view of 25–30 cm. CT dose index (CTDI) volume was 20.9 mGy. Each patient was fixed in a supine position with the knee extended naturally.
Measurements of the tibial anatomic characteristics
The measurements of the tibial anatomic characteristics were performed with the ST-PACS CDMedical software Vision 3.1 (Crealife, Beijing, China) by two independent, blinded observers. All CT images were evaluated by observer 1, and observer 2 measured 100 cases randomly selected from all specimens and blinded to results from observer 1. After 1 month, observer 1 measured 50 cases again which were randomly selected from all images. In this way, the intra- and inter-observer reliability were determined.
The measurements were taken on sagittal section that provided the most inclusive and wide PCL tibial attachment, including the thickness of cancellous bone (L1), the theoretical optimal angle of the tibial tunnel relative to plateau (TOA, producing the minimal “killer turn” effect on the premise of the satisfactory tunnel bone quality), L2 - the distance from anterior tunnel aperture to anterior end of tibial plateau, L3 - the distance from anterior tunnel aperture to tibial tuberosity (lowest edge of patellar ligament attachment). Surgeons could easily locate the tibial tunnel position during transtibial PCL reconstruction by measuring L2 and L3.
L1 was measured from the proximal vertex of the tibial marrow cavity to tibial plateau along the anatomic axis of marrow cavity, the proximal vertex was observed on sagittal section (Fig. 1). Subsequently, TOA was measured between tibial plateau and the extension cord that connecting the center of PCL insertion site with a point 5 mm superior from the marrow cavity vertex (Fig. 1).As described by Lee et al. [14], we chose the point 5 mm superior from the marrow cavity vertex because a 10-mm-diameter tibial tunnel was usually used in PCL reconstruction. L2 was measured from anterior end of tibial plateau to anterior tunnel aperture (Fig. 1). L3 was measured from the lowest edge of patellar ligament attachment site on tibia to anterior tunnel aperture (Fig. 2).
Statistical analysis
All statistical analysis were performed using SPSS software (version 22.0, Inc., Chicago, IL, USA). The measurement results were shown as arithmetic mean ± standard deviation, to check up the data of all groups with normal test. All data were analyzed for the cohort as a whole, including age, height, and sex cohorts. One-way analysis of variance (ANOVA) was used to assess parameters among age cohorts and height cohorts, while independent t test between males and females. P < 0.05 was considered statistically significant. The intraclass correlation coefficients (ICCs) were used to analyze the calculation of intra- and inter-observer reliability. The ICCs ranges from 0.00 (no agreement) to 1.00 (perfect agreement).