The landmarks of centers of the distal femur and the proximal tibia in sagittal plane for application in computer assisted total knee arthroplasty.

BACKGROUND: In the computer assisted total knee arthroplasty (CAS-TKA), the centers of the distal femur and the proximal tibia for the sagittal mechanical axis (SMA) of both femur and tibia have unclear references. Most CAS-TKA systems define both centers following the engineer's recommendation.

OBJECTIVE: To evaluate the centers of the distal femur and the proximal tibia in sagittal plane and to determine the sagittal mechanical axis (SMA) of the normal knee in relation to the Blumensaat's line and the tibial anteroposterior line.

MATERIAL AND METHOD: Fifty five normal healthy knees without flexion contracture or hyperextension were enrolled. The sagittal mechanical axis (SMA) was drawn from the center of the femoral head to the most prominent dome of the talus on the long standing hip-knee-ankle radiograph in lateral view. The point that the SMA passed the Blumensaat's line of distal femur and the tibial plateau at the proximal tibia were reported as the percentage of the total length of Blumensaat's line and of the anteroposterior width of tibial plateau r respectively.

RESULTS: Among 55 knees included in this study, 32 knees were right side and 23 knees were left sided. Of which, there were 14 females and 41 males. Their average age was 45 years. The average BMI was 25.67. The average knee alignment in sagittal plane was 1.7 degrees. At the distal femur, the point that SMA passed the Blumensaat's line was average 13% of the total length of Blumensaat's line. There were only 22% of the 55 knees that the SMA passed at the tip of Blumensaat's line. All of SMA in 55 knees passed within the first quarter of this line. At the proximal tibia, the point that SMA passed the tibial plateau average 45% of the anteroposterior width of tibial plateau.

CONCLUSION: The center of distal femur and proximal tibia on sagittal plane are not individual consistent. Therefore, they should be evaluated preoperatively before performing the CAS TKA to prevent the incorrect bone cutting, prosthesis malposition and poor soft tissue balance in sagittal plane.