PREDETERMINED ENTRY POINT AND VALGUS ANGLE FOR THE RESECTION OF DISTAL FEMUR IN TOTAL KNEE ARTHROPLASTY

Abstract

Introduction: Restoring normal mechanical axis is one of the key goals of the total knee arthroplasty (TKA). The majority of the surgeons resect the tibia perpendicular to its axis in the coronal plane, then use an intra-medullary jig inserted through the centre of the knee or slightly medial to centre of the knee to resect the distal femur at a 6 or 7degree valgus angle. The aim was to establish the safety of using a predetermined valgus angle (VA) and entry point (EP) in the primary TKA. We also studied the relationship between the VA and EP to the height, weight and BMI of the patient.

Materials and Methods: We studied 125 long leg radiographs of 125 patients who underwent TKA under the care of senior author. All the radiographs were taken in the preoperative clinic with knee in full extension and patella facing forward. The radiographs were used to measure the valgus angle and entry point of the femur. The patients with VA between 6–7 degrees and EP at the centre were defined as normal group and rest were defined as outliers.

Results: The VA ranged from 4 to 9.5 degrees (with a mean of 6.8 and SD 1.11). Only 66 (53%) knees had the VA between 6 and 7 degrees. The EP ranged from 30mm medial to 18mm lateral to the centre of the knee with a mean of 7.7mm medial to the centre of the knee (SD 6.1). The EP was at the centre of the knee in 31 (24.8%) knees and lateral to the centre in 19 (15.2%) knees. Only 14 (11.2%) knees were in the normal group. Overall there was no significant relationship between the EP and VA to the height, weight or BMI of the patient at p-value > 0.001.

Conclusions: The resection of distal femur using the predetermined valgus angle, the predetermined entry point is not a safe practice in TKA. The long leg radiographs of the knee should be studied to identify the outliers. In future computer-assisted surgery and digitalisation of the images may obviate the need for this. However, it may be prudent though to use pre-operative templating of long leg radiographs during the learning curve of computer assisted surgery as well.