Significance of asymmetrical posteromedial and posterolateral femoral condylar chamfer cuts in total knee arthroplasty

Sagittal magnetic resonance imaging-scan orientation significantly influences accuracy of femoral posterior condylar offset measurement

BACKGROUND

In total knee arthroplasty the femoral posterior condylar offset (PCO) may serve as a potential branch for correct femoral component positioning. The technique of adjusting the sagittal magnetic resonance imaging (MRI)-scan on which it is measured has not been investigated in previous literature, but may be subject to variances due to knee joint positioning or axial localizer scan angulation. The purpose of this study was to investigate the effect of simulated femur rotation on the accuracy of PCO measurement.

MATERIALS AND METHODS

Ten asymptomatic knee joints underwent MRI investigations. A sagittal plane perpendicular to the transepicondylar axis was defined as the true-sagittal plane (tsP). Sagittal images were reformatted in the tsP and angulated by 5° and - 5° in medial and lateral direction. In total each knee received three scans in 0°, 5° and - 5° axial localizer scan angulation. Medial and lateral PCO measurement was performed in each MRI-scan angulation.

RESULTS

Simulated external rotation decreased medial PCO size by 1.7 mm (95% CI 0.5994-3.127) (p = 0.012), and simulated internal rotation increased medial PCO size by 2.1 mm (95% CI 1.142-2.994) (p = 0.001). Lateral PCO size increased by 1.9 mm (95% CI 0.5660-3.412) and decreased by 2.1 mm (95% CI 1.142-2.994) with simulated external and internal rotation, respectively (p = 0.011; p = 0.0007).

CONCLUSION

This study shows the high sensitivity of medial and lateral PCO measurements to small changes of MRI axial localizer scan angulations simulating minor degrees of internal or external femur rotation. Thus, absolute PCO values should be interpreted with caution if the sagittal image acquisition is not standardized.

Posterior reference guides do not always maintain the size of posterior femoral condyles in TKA

PURPOSE

Posterior reference guides are provided by many manufacturers. However, the true posterior femoral condyle bone resection thicknesses using posterior reference guides are not stated by the manufacturers. The purpose of this study was to analyse the influence of the posterior reference guide designs on the posterior femoral condyle bone resection thickness.

METHODS

Thickness of posterior femoral condyle bone resection and thickness of prostheses were investigated in 8 types of total knee prostheses using the production drawings provided by the manufacturers.

RESULTS

Posterior femoral condyle bone resection thickness differed between prostheses. Change in size of the medial posterior condyle ranged from -0.5 to 1.4 mm at 0° external rotation, from -2.9 to 1.4 mm at 3° external rotation, and from -5.3 to 1.4 mm at 6° external rotation. Change in size of lateral posterior condyle ranged from -0.5 to 1.4 mm at 0° external rotation, from -0.4 to 3.4 mm at 3° external rotation, and from -0.4 to 5.3 mm at 6° external rotation.

CONCLUSIONS

This study showed that posterior femoral condyle bone resection thickness was different for each posterior reference guide. The rotation centre of posterior reference guides influenced the bone resection thickness in the posterior femoral condyle. The size of the posterior femoral condyles increased in some guides but decreased in other guides. The maximum differences in size changes of the posterior femoral condyles between the guides were 1.9 mm at 0° external rotation, 4.1 mm at 3° external rotation, and 6.3 mm at 6° external rotation. To control the size of posterior femoral condyles, the posterior reference guide design should be checked before use.