The accuracy of femoral component rotational measurements using computed tomography—a cadaveric study

Background CT scans can be used to assess the rotational alignment of the femoral component following total knee arthroplasty (TKA). This is done by calculating the posterior condylar angle (PCA). However, the methods used may not account for the biomechanical functionality of the TKA components. This cadaveric study aimed to determine whether the axis of scanning (mechanical or anatomical) alters the results of PCA calculations. Methods CT scans of 12 cadaveric adult femora were performed along the anatomical axis and the mechanical axis. The PCA was determined on each CT scan by measuring the relationship of the prosthetic posterior condyles to the surgical epicondylar axis of the femur. The mechanical and anatomical axis groups were further subdivided into best-fit and multi-slice subgroups. As a control, the posterior condylar angle was also calculated on photographic images of each femur. Bland-Altman plots were used to determine the correlation between the PCA values obtained from the different scanning axes and measurement techniques. Results There was no significant difference between the PCA measurements derived from anatomical and mechanical axis CT scans. The Pearson correlation co-efficient also indicated good correlation between the two scanning axes. Conclusion The axis of scanning does not significantly affect the PCA measurements. Therefore, the measurements may be reliably used for clinical decision-making, regardless of the axis of CT scanning.

Variation in the Posterior Condylar Angle

Background. Clinical success of TKR depends to a large extent on the correct positioning of the implants. The femur cut is usually made in 3° of external rotation relative to the posterior aspect of the femoral condyles, which is a typical value of the Posterior Condylar Angle (PCA). The aim of this study was to assess variation in the PCA and to identify possible correlations between the value of the PCA and the following parameters: gender, body side, body height and weight (BMI). Material and methods. The study group comprised 75 patients who underwent an Angio-CT study between 2012 and 2014 due to a suspected vascular disorder. For each knee, we measured the Posterior Condylar Angle and the Posterior Twist Angle. Results. The mean Posterior Condylar Angle was 2.7±2.1°. The differences between males and females were not statistically significant. The angles showed a high degree of symmetry between the contralateral knees. Conclusions. 1. Positioning of the femoral component of the knee joint endoprosthesis parallel to the transepicondylar line requires resecting the femur at approximately 3° of external rotation relative to the line tangential to the posterior aspect of the femoral condyles. This angle, however, may vary from 3.6° of internal rotation to 9.0° of external rotation which should be taken into consideration by the operating surgeon. 2. The values of the Posterior Condylar Angle do not correlate with gender, age, Body Mass Index and body side (with notable, near-perfect symmetry between the contralateral limbs). 3. In female patients, height and body weight may influence the Posterior Condylar Angle, but these correlations would require further studies of larger groups of patients. In male patients, we were unable to find such correlations.

Distal Femoral Rotational Axes in Indian Knees

Purpose. To measure the angular relationships of distal femoral rotational axes in 100 normal Indian knees. Methods. 42 men and 8 women aged 26 to 40 (mean, 31) years, with 100 normal non-arthritic knees were recruited. Anatomic landmarks were measured using computed tomography. They included the posterior condylar axis, the transepicondylar axis, the anteroposterior axis (Whiteside's line), the posterior condylar angle (PCA), the Whiteside-epicondylar angle (W-EP), and the Whiteside-posterior condylar angle (W-PC). Results. The mean PCA, W-EP, and W-PC were 5°, 90.8°, and 95.8°, respectively. The mean femorotibial alignment was 179.6°. The differences between the left and right sides were significant only for the W-EP and W-PC. Only the PCA and W-EP were weakly correlated ( r=0.338, p=0.001). Conclusion. There are differences in distal femoral rotational axes among Indian, Caucasian, and Japanese knees. Our data can be used to evaluate changes in those axes in ageing or arthritic patients.