Intramedullary versus extramedullary femoral alignment systems in total knee replacement

The accuracy of intramedullary (IM) versus extramedullary (EM) distal femoral alignment systems was compared in 200 consecutive total knee replacements (TKRs) on standing full-length lower-extremity roentgenographs. Intramedullary femoral alignment systems were used in 125 TKRs and an EM system was used in 75 TKRs. All tibial cuts were made using an EM tibial cutting guide. Roentgenographic measurements of (1) femoral-tibial angle, (2) distal-femoral resection angle, (3) proximal-tibial resection angle, (4) joint line orientation, (5) physiologic femoral valgus, and (6) distance of the lower-extremity mechanical axis from the center of the knee were made. No significant intragroup differences were seen in the average values obtained for each of the six roentgenographic measurements. However, the percentage of distal-femoral resections outside the accepted normal range (94 degrees-100 degrees) was higher in the EM group (28%) versus the IM group (14.4%) at p = 0.019. Likewise, the percentage of joint line orientations outside the desired normal range was higher in the EM groups when compared with the IM group (21.3% versus 11.2%, p = 0.052). A disturbing number of proximal-tibial resection angles were inaccurate in both groups. Improper tibial cuts were seen in 20.6% of Group 1 and in 24% of Group 2 when an EM tibial cutting guide was used. A range of distal-femoral cuts exists when using either an IM or an EM femoral alignment guide. This study demonstrated a statistically significant improvement in distal-femoral resection accuracy when using an IM femoral alignment system. Methods of improving proximal-tibial bone resections are needed.(ABSTRACT TRUNCATED AT 250 WORDS)