Comparative study of femoral sizing between intraoperative measurement and CT-based PSI in total knee arthroplasty

Appropriate femoral sizing in total knee arthroplasty (TKA) is an important factor for flexion. An oversized femoral component will decrease flexion gap and postoperative flexion. By using custom cutting blocks technique with computed tomography (CT) imaging to design cutting blocks will help determine sizing of each patient. The differences of femoral sizing between intraoperative measurement and custom cutting blocks technique are still questionable.

To compare femoral component sizing between custom cutting blocks technique and conventional technique.

Retrospective cross-sectional study was collected from 2,321 patients underwent primary TKA with the same prosthesis design in Bangkok between January 2012 and December 2012. The population was separated into three groups, group A, 2,053 patients operated by orthopedic surgeons in Bangkok by conventional instrument femoral sizing, group B, 218 patients operated by senior experienced arthroplasty surgeon (Chotanaphuti T) at Phramongkutklao Hospital using the same measurement technique as group A, and group C, 50 patients operated by Chotanaphuti T using custom cutting blocks technique for femoral sizing. The femoral component sizes were assigned from small to large size as number 1 to 6. Chi-square test and Fisher's exact test was used to determine the significant differences between amount of patients of each group.

In group A, femoral components were selected in size 1 (n = 157; 8%), 2 (n = 576; 28%), 3 (n = 737; 36%), 4 (n = 431; 21%), 5 (n = 144; 7%), and 6 (n = 8; 0.4%) respectively. In group B, femoral components were selected in size 1 (n = 31; 14%), 2 (n = 55; 25%), 3 (n = 64; 29%), 4 (n = 31; 140%), 5 (n = 37; 180%), and 6 (n = 0; 0%) respectively. In group C, femoral components were selected in size 1 (n = 7; 14%), 2 (n = 19; 38%), 3 (n = 11; 220%), 4 (n = 12; 24%), 5 (n = 1; 20%), and 6 (n = 0; 0%) respectively. The most chosen size of group A and B were No. 3, but for group C was No. 2, which was smaller than the first two group by one size. There were statistical difference in group A versus group B (p < 0.0001), and group B versus group C (p = 0. 009), but not difference in group A versus group C (p = 0. 096).

Custom cutting blocks technique chooses a femoral component closer to knee anatomy of the patients and smaller than intraoperative technique. There are several possible causes, which include variable of level or sagittal angle error of distal femoral bone cut.