Isoheight points that measure constant femoral condyle heigh
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It is a challenge to evaluate the maintenance of medial and lateral soft tissue balance in total knee arthroplasty (TKA). This study was done to determine the “isoheight” points and the “isoheight” axis (IHA) that can measure constant medial/lateral condyle heights during flexion of the knee, and compare the IHA with two major anatomical axes, the transepicondylar axis (TEA) and the geometric center axis (GCA).

Twenty-two healthy human knees were imaged using a combined MRI and dual fuoroscopic imaging system while performing a single-legged lunge (0°–120°). The isoheight points of the medial and lateral femoral condyles were defined as the locations with the least amount of changes in heights during the knee flexion; an IHA is the line connecting the medial and lateral isoheight points. The measured changes of the condyle heights using the IHA were compared with those measured using the TEA and GCA. Results Overall, the IHA was posterior and distal to the TEA, and anterior to the GCA. The isoheight points measured condyle height changes within 1.2±2.3 mm at the medial and 0.7±3.3 mm at the lateral sides during the knee flexion. Between 0° and 45°, the condyle height changes measured using the GCA.

In conclusion there are isoheight points in the medial and lateral femoral condyles that can measure constant heights along with the full range of knee flexion and could be used to formulate an “isoheight” axis (IHA) of the femur. The condyle height changes measured by the TEA and GCA were greater than the IHA measurements along the flexion path. These data could be used as a valuable reference to evaluate the condyle height changes after TKA surgeries and help achieve soft-tissue balance and optimal knee kinematics along the flexion path.

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