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Judging the correct femoral component rotation in modern design knee replacement
ESSKA Academy. Harvey R. May 9, 2018; 213416; P09-1784 Topic: Biomechanics
Robert Harvey
Robert Harvey
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Abstract
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Objectives: Malrotation of the femoral component in TKR is a potent cause of instability and dissatisfaction in knee arthroplasty. In a measured resection technique, the rotational alignment was traditionally determined using anatomical landmarks and a jig to provide positioning relative to the posterior femoral condyles and trans epicondylar axis. In a Gap Balancing technique rotational alignment is driven by instruments measuring ligament tension in flexion, having balanced the extension gap first. In one Modern Knee arthroplasty system, there are instruments that allow both measured resection and balancing techniques to be used to generate femoral rotational alignment. We have used the soft tissue balancer to inform our measured resection technique.

Methods: 200 consecutives cruciate retaining Total Knee replacement utilising a modern knee system had femoral component rotation determined by using both balancing and measured resection instruments. Distal femoral and proximal tibial resections were undertaken and the extension gap balanced and checked with block spacers. The balancing instruments were used to mark the position for the femoral finishing block. The measured resection instrument was then placed on the distal femur with 3 degrees of rotation set to the posterior femoral condyles and the correlation of femoral rotation between the two systems measured.

Results: 34 cases (17%) had deviation of rotation between the two systems. 29 cases were Varus knees and 5 were valgus. All 29 Varus cases using the measured resection jig would have resulted in a least 2 degrees of internal rotation compared to the balanced position in flexion equalised to that in extension. There is a correlation between the severity of Varus deformity and the tendency to misjudge the position of the resection into internal rotation.

Conclusions: The balancing instrumentation can inform the femoral rotation position in a measured resection technique. A surgeon using a pure measured resection technique should be aware that there is a tendency to misjudge femoral rotation in severe Varus knees and should consider referencing 5 degrees of external rotation compared to posterior condyles.

Keywords:
Total Knee Replacement, Femoral component rotation, Measured resection technique, Gap Balancing technique
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