Abstract
Introduction
Rotational or axial alignment is an important concept in total knee surgery. Malrotation of the femoral component can lead to patellofemoral maltracking, pain and stiffness. In reconstruction surgery of the knee, achievement of correct rotation is even more difficult because of the lack of anatomical landmarks. The linea aspera is often the only remaining landmark, but its reliability is questionable.
Goal of research
Can custom-made 3D-guides help with rotational alignment of the knee after a wide resection of the distal femur?
Material and methods
Custom-made 3D-guides were designed from CT-scans, with the help of the commercially available Mimics software (Materialise NV, Leuven, Belgium) and SolidWorks (SolidWorks Corp., MA, USA). Anterior was defined as 90° relative to the PCL, with the center of the best-fitting inner cylinder, inside the femoral diaphysis, as rotation point.
Firstly, the accuracy of the 3D-guides was tested. Twelve 3D-guides, on different heights, were made for 3 cadaveric femora. Anterior was marked with a pin and the position was evaluated with CT-scan.
Secondly, to mimic surgery, seven reconstruction prostheses were placed in 4 cadavers, using the 3D-guide to indicate anterior and cutting surface. Resection height was aimed at 13cm. The position of the prostheses was also evaluated using CT-scan.
Results
First test: The pins deviated on average 0.65° (SE: 0.75°) from anterior. Eighty-three percent deviated less then 1° from anterior, and only 2 pins deviated more than 1° (1.5° and 2.6°). The resection height indicated by the 3D-guide was on average 2.4mm (SE: 0.7mm) to high.
Second test: The 7 reconstruction prostheses deviated on average 3.1° (SE: 2,18°) from anterior, with 4 prostheses deviating more than 1°. The 2 prostheses in endorotation were placed more lateral then was planned, while the 2 in exorotation were placed more medial.
Deviation in the coronal and sagittal plane was respectively 1.56° (SE: 1.64°) and 1.84° (SE: 1.04). The mean height was 12.9cm.
Discussion
The 3D-guides were accurate in indicating a previously established ‘anterior’ point on the femur and the resection height, but when used to position the femoral component during surgery they inadequately controlled rotation. The 3D-guides did not take into account that centering of the prosthesis could be a problem. When the prosthesis was place more medial or more lateral than anticipated the rotation point of the component was changed and when then aligned with the previously indicated anterior mark, it was placed respectively in exorotation and endorotation.
Future research
Will aim to develop custom-made 3D-guides that also guide centering of the femoral component. Repercussion on function and kinematics of improved axial alignment will be evaluated with knee simulator testing and a control group.
