Optimal for the relation between distal femur rotation

Optimal component alignment is crucialfor successful outcome of primary total knee arthroplasty (TKA). Femoralcomponent rotation is one of the most important factors in TKA, as rotationalmisalignment affects the flexion stability as well as tibiofemoral andpatellofemoral kinematics.

Flexion malalignment is a known cause of pain andstiffness 1-8.In the classical mechanical alignmentconcept, the femur must be implanted parallel to the surgical transepicondylaraxis 9. The surgical TEA is thought to best approximate the flexion/extensionaxis of the knee, however it can be difficult to palpate and referenceintraoperatively 10-14.

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The surgical epicondylar axis and posterior condylaraxis form the posterior condylar angle, which  is on average of 3 degrees of externalrotation. Although many factors such as gender, condylar hypoplasia, andcoronal alignment can disturb the rotation of the distal femur and change theangle between the posterior condylar line (PCL) and transepicondylar axis. Manystudies have demonstrated that the angle between PCL and surgical TEA may rangefrom 3° ofinternal rotation to 10° ofexternal rotation 13, 33, 34, 64.Aglietti et al.

studiedpreoperative knee CT scans, and developed a simplified formula for the relationbetween distal femur rotation and frontal alignment of the knee which increasesthe PCA by 1° per 10° of coronal deformityincrements from varus to valgus, resulting for instance in 2° external rotation for a20° varus knee and 5° external rotation for a 20° valgus knee. Based onthis concept and without the use of the preoperative CT scan, rotationalaccuracy is within ± 2° of TEA in 80% of the cases 65. We canaccept, that standard external rotation for about 80% of the knees are 3° for varus knees and 5° for valgus knees 21.The rationale for the TEA method isderived from the observation that the normal tibial joint line is between 3° and 5° of varus relative to the long axis ofthe tibia. If the tibial resection is made in 3° of varus, an equal symmetricalposterior condylar resection will result in a rectangular flexion gap. If thetibial resection is 90° tothe long axis of the tibia, 3° to 5° ofexternal rotation will be necessary in order to recreate a rectangular gap29.Determination of the surgical TEA isknown to be difficult; therefore most systems use the default posteriorreference and PCL landmark to determine the femoral component ER. Hungerford15 introduced the concept of 3 degrees of external rotation (ER) relativelyto the posterior condylar line (PCL) for the femoral component.

Femoral component rotation may bedetermined by several techniques; these include gap balancing techniques,dependent on ligament tension, and currently used by most surgeons variousmeasured resection techniques based on anatomical landmarks, including: theanatomical epicondylar axis (AEA), the surgical epicondylar axis (SEA), theposterior condylar line (PCL), the posterior condylar angle (PCA), theanteroposterior axis (APA, also known as Whiteside’s line) and the sulcus lineof the trochlear groove (SL) discussed in recent literature. Alternativemethods exist that involve patient-specific instrumentation (PSI) and computernavigation.

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