Measuring the motion of the knee joint has proved to be a challenging endeavor. Our measurement tools have advanced over the years. Beginning measurement techniques focused on goniometers; however, this technique could only be applied to the static knee, was inaccurate, and had no varus/valgus measures.¹¹ In order to capture angular movement, goniometers were attached to pins driven into bones and combined with reflective markers.¹²,¹³,¹⁴,¹⁵ These results were more reliable; however, ethical issues led to fewer subjects in studies. Angular motion has been successfully measured in cadavers using electromagnetic sensors fixed to bones about the knee.¹⁶ Advancement in imaging modalities has allowed researchers to study the relative motion of the articular surface. Displacement during motion can now be imaged by cine-CT,¹⁷ static CT combined with computerized image matching,¹⁸ CT combined with fluoroscopy, X-rays combined with fluoroscopy,¹⁹,²⁰ or radiosteriometric analysis (RSA) plus CT.²¹ To avoid radiation, MRI has been used; however, it has only been found to track static or quasi-static knee joint motion.¹¹ Today, 3D imaging and motion may be measured using three techniques: RSA in conjunction with CT or MRI; fluoroscopy in conjunction with CT, MRI, or radiographs; or MRI by itself.¹¹

In order to make measurements of the knee in motion, a coordinate system had to be established. A coordinate system based on posterior femoral circles was proposed by McPherson et al in 2004.²¹ This system places an origin at the center of the posterior spherical portion of the medial femoral condyle so that the origin of the axis system approximately coincides with the center of rotation of the knee.¹¹ Flexion facet centers (FFCs) were defined by Iwaki and Pinskerova as the center of the posterior articular surfaces of both femoral condyles in a sagittal view²²,²³ (Fig. 36C-1). A transverse line through the femoral flexion facet centers (medial and lateral condyles) marks the first axis of McPherson’s coordinate system. Iwaki and Pinskerova also described tibial flexion facets, which are the posterior border of the tibial articular surface.²²,²³ The second axis of the coordinate system is perpendicular to the first and perpendicular to the tibial flexion facet. The second axis will penetrate the medial femoral flexion facet center and run distally to the posterior tibial cortex. The third axis in this coordinate system is 90° to both the first and second axes in the anteroposterior direction. It also penetrates the femoral flexion facet center. The proposed coordinate system allows for measurements of the FFCs relative to the articular surface of the tibia at varying degrees of flexion (Fig. 36C-2). Hill et al and Johal et al also displayed similar information with motion of the femoral condyle at their FFCs in each compartment (Fig. 36C-3).²⁴,²⁵

Many have studied the movement of the femoral condyles throughout knee range of motion. These studies depend upon tracking the movement of the individual femoral condyles by measuring the distances between the center of their posterior circular surfaces and the posterior border of the tibia.¹¹ The arc of active function is defined by some researchers as 10° to 30° of flexion to 110° to 120° of flexion.¹¹ The medial femoral condyle, during this arc, may be viewed as a sphere which rotates to produce a variable combination of flexion, longitudinal rotation (and minimal varus if liftoff occurs laterally). Translation in the anteroposterior direction in this arc of the medial femoral condyle is minimal and has been measured to be ±1.5 mm²⁵ (Figs. 36C-2 and 36C-3). The lateral femoral condyle also rotates, but in contrast to the medial side it tends to translate posteriorly about 15 mm by a mixture of rolling and sliding¹⁸,²⁵,²⁶,²⁷ (Figs. 36C-2 and 36C-3). As a result, during the arc of active function, the femur tends to externally rotate as the tibia internally rotates. This motion has been shown in the living non-weight-bearing knee.²⁴,²⁵ In the living weight-bearing knee during a squat the general pattern of motion is again the same although backward movement of the lateral femoral condyle may occur earlier in flexion.²⁴,²⁵

Contact area of the tibiofemoral joint should not be confused with the movement of the condyles. In imaging studies “contact area” is defined as the point in the tibiofemoral joint where the subchondral plates of the tibia and femur are closest.¹¹ In 1990, O’Connor et al reviewed the subject of contact area and cadaveric studies and concluded that the areas moved posteriorly with flexion on both medially and laterally, but more so laterally²⁷ (see Fig. 36C-4). In a cadaveric MRI study, Pinskerova et al found contact area displacements both medially and laterally to be up to 8 mm from 0° to 30°, but at greater than 30° of flexion, found the medial contact area stationary and lateral contact area moving posteriorly up to 15 mm.²³ Dynamic studies, using fluoroscopy, of the contact areas in various activities, such as stairs and using a chair, were performed by Komistek et al and Kanisawa et al Both groups showed posterior motion of medial and lateral contact areas past the first 10° of flexion.⁸,¹⁹ Komistek et al found only 2 mm of posteriorly shifted contact area in all activities but variably posterior shift from 4 to 14 mm depending on the activity performed.⁸ Kanisawa et al found no medial contact area change from 0° to 60° but up to 4 mm of posterior movement from 60° to 80°. Laterally, Kanisawa found from 10° to 30° the contact area was shifted posteriorly 5 mm. Surprisingly, the lateral contact area moved anteriorly by 5 mm during 30° to 80° of flexion.¹⁹ These results show the variability of contact area during knee range of motion and the differences between contact area and movement of the condyles. Fig. 36C-4 displays the concept of minimal movement of the FFCs and posterior movement of the contact area during flexion.²⁸