There are isoheight points that measure constant femoral condyle heights along the knee flexion path

Abstract

Purpose: It is a challenge to evaluate the maintenance of medial and lateral soft tissue balance in total knee arthroplasty (TKA). This study aimed 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).

Methods: Twenty-two healthy human knees were imaged using a combined MRI and dual fluoroscopic 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 (medial: 3.0 ± 1.8 mm, lateral: 2.3 ± 2.0 mm) were significantly larger than those of the IHA and the TEA (p < 0.05). Between 90° and 120°, the changes of the condyle heights measured using the TEA (medial: 5.3 ± 1.8 mm, lateral: 3.3 ± 1.8 mm) were significantly larger than those of the IHA and GCA (p < 0.05).

Conclusion: There are isoheight points in the medial and lateral femoral condyles that can measure constant heights along 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.

Level of evidence: IV.

Keywords: Femoral condyle heights; Gap balance; In vivo knee flexion; Isoheight characteristics; Soft tissue balance; Total knee arthroplasty.

Fig. 1.

(A) The 3D knee joint model with tibial Cartesian coordinate system, the transepicondylar axis (TEA) and the geometrical center axis (GCA), the sagittal plane circular sections of the medial and lateral femoral condyles (including local coordinate systems on the circular sections). Measurements of femoral condyle heights with respect to the tibial cutting plane were also shown. (B) A subject performing a quasi-static single-legged lunge, which was captured the dual fluoroscopic system. (C) A virtual dual fluoroscopic system used for reproduction of the in vivo knee positions along the flexion path.

Fig. 2.

(A) Heat maps of the changes (mm) of the medial and lateral femoral condyle heights along the flexion path of the knee to illustrate the uniqueness of the IHA. The positions of the TEA, GCA and IHA on the condyle circular sections in full extension were marked. X and Y axes point to the posterior and proximal directions, respectively. (B) Diagrams showing the condyle height changes along the knee flexion path.

Fig. 3.

Medial and lateral femoral condyle heights along the flexion path of the knee measured using (A) TEA, (B) GCA, and (C) IHA. “*” indicates significant differences between the medial and lateral condyle heights (p<0.05). The shaded area represents ± one standard deviation.