Femoral curvature influence for above knee bone anchored prostheses. A cadaveric study

BACKGROUND. The bone-anchored prosthetic devices have been investigated in the present study because of certain disadvantages of socket suspension prosthesis worsening the quality of life of amputees. There is a standard alignment for socket suspension prosthesis. Osseointegrated prosthetic alignment has to be released at the same principles, taking into account some features regarding intramedullary stem fixation and femoral shift anatomy.
AIM:
1. Study of the femoral shift canal axis and mechanical axis orientation.
2. Experimental research of intramedullary stem position for the bone-anchored prosthesis.
3. Study result evaluation for the bone-anchored prosthesis alignment.
METHOD. Thirty-one Caucasian cadaver femurs were used (13 females, 18 males, 11 left, 20 right). The X-ray was performed for all steps of standard anterior and lateral view.
1. Femur main anatomical points were marked by 3 mm metal loads to improve measurements. 2. Femur was amputated in the middle shift; custom made stem was implanted. 3. Same Femur was amputated 7 cm below trochanter minor, custom made stem was implanted. All X-ray images were analysed graphically. The femur canal axis was divided for 20 segments. Angles between the femoral mechanical axis and femoral canal axis and between the femoral stem axis and mechanical axis were found.
RESULTS. Analysis of a native femur revealed the following: 1. According to the anterior X-ray view, the femoral canal has an almost constant angle between the femoral canal axis and the femur mechanical axis 5-7 degrees. 2. In lateral images, the angle is variable. In proximal 1/3 of the femur, it is almost constant 7 – 9 degrees, then it decreases, becomes neutral in 55 – 60% of the femoral length and distally turns negative - minus 3 – 4 degrees. Implanted femoral stem orientation showed a correlation with native femur measurements. The angle after the first and second amputation in frontal view was constant 5 – 7 degrees. The lateral view angle was variable as for a native femoral 8-9 degrees flexion angle proximally and 6-8 degrees in the middle part.
DISCUSSION AND CONCLUSION. The study shows that bone-anchored prosthesis alignment features for a transfemoral amputee are inducted by intramedullary fixation and femur shaft curvature. The residual femur in the neutral position has angles with load lines in the frontal and sagittal planes. Straight femoral stem anchored prosthesis needs posterior placement of the knee joint. Posterior placement for bone-anchored prosthesis is necessary as for a socket one. The posterior position of the knee joint could provide posteriorly curved femoral stem or/and specific prosthetic components.