BACKGROUND: The symmetry and equality of the flexion and extension gap are essential for successful endoprosthetic knee arthroplasty. Cruciate ligament sparing endoprosthetic designs are implanted with a measured resection technique, so that the posterior bone resection corresponds to the posterior condyle thickness. However, this correlation only applies if the sagittal alignment is set at 0°. The aim of the present study was therefore to investigate the extent to which the flexion gap is influenced by a flexed implantation of the femoral component. METHODS: The implant geometry of all available sizes of the knee systems Columbus, e.motion (Aesculap), PFC Sigma (DePuy), Natural Knee II, Innex, Nexgen LPS Flex and Gender (Zimmer), and TC Plus (Smith & Nephew) was recorded. Based on this data, a virtual implantation of the femoral component with a sagittal alignment between 0° and 5° of flexion was simulated. The resulting flexion gaps were calculated depending on the component alignment. The relationships between component alignment (in degrees) and flexion gap (in mm) were documented for every implant. RESULTS: The narrowing of the flexion gap with increasing flexion was more or less linear in the range investigated and was dependent on the system used and the implant size. A narrowing of the flexion gap by 1 mm resulted from 2° (1.9°-2.3°) flexion in the e.motion prosthesis, 1.9° (1.6°-2.4°) in the Columbus, 1.6° (1.5°-1.8°) in the PFC Sigma, 2.0° (1.7°-2.4°) in the Nexgen LPS Flex and Gender, 1.7° (1.6°-1.8°) in the Innex, 2.2° (1.5°-2.6°) in the TC Plus and 2.0° (2.0°-2.1°) in the Natural Knee. CONCLUSIONS: Even a small flexion of the femoral component leads to a reduction of the flexion gap and thus potentially to limited mobility in the measured resection technique. On the other hand, in navigation-assisted implantation, slight flexion of the component can possibly be used to adjust the flexion gap smoothly. LEVEL OF EVIDENCE: II.
BACKGROUND: The symmetry and equality of the flexion and extension gap are essential for successful endoprosthetic knee arthroplasty. Cruciate ligament sparing endoprosthetic designs are implanted with a measured resection technique, so that the posterior bone resection corresponds to the posterior condyle thickness. However, this correlation only applies if the sagittal alignment is set at 0°. The aim of the present study was therefore to investigate the extent to which the flexion gap is influenced by a flexed implantation of the femoral component. METHODS: The implant geometry of all available sizes of the knee systems Columbus, e.motion (Aesculap), PFC Sigma (DePuy), Natural Knee II, Innex, Nexgen LPS Flex and Gender (Zimmer), and TC Plus (Smith & Nephew) was recorded. Based on this data, a virtual implantation of the femoral component with a sagittal alignment between 0° and 5° of flexion was simulated. The resulting flexion gaps were calculated depending on the component alignment. The relationships between component alignment (in degrees) and flexion gap (in mm) were documented for every implant. RESULTS: The narrowing of the flexion gap with increasing flexion was more or less linear in the range investigated and was dependent on the system used and the implant size. A narrowing of the flexion gap by 1 mm resulted from 2° (1.9°-2.3°) flexion in the e.motion prosthesis, 1.9° (1.6°-2.4°) in the Columbus, 1.6° (1.5°-1.8°) in the PFC Sigma, 2.0° (1.7°-2.4°) in the Nexgen LPS Flex and Gender, 1.7° (1.6°-1.8°) in the Innex, 2.2° (1.5°-2.6°) in the TC Plus and 2.0° (2.0°-2.1°) in the Natural Knee. CONCLUSIONS: Even a small flexion of the femoral component leads to a reduction of the flexion gap and thus potentially to limited mobility in the measured resection technique. On the other hand, in navigation-assisted implantation, slight flexion of the component can possibly be used to adjust the flexion gap smoothly. LEVEL OF EVIDENCE: II.