Joint Kinematics The primary angular (or rotatory) motion of the tibiofemoral joint is flexion/extension, although both medial/lateral (internal/external) rotation and varus/ valgus (adduction/adduction) motions can also occur to a lesser extent. These motions occur about changing but definable axes.
In addition to the angular motions, translation in an anteroposterior direction is common on both the medial and lateral tibial plateaus; to a lesser extent, medial and lateral translations can occur in response to varus and valgus forces. The small amounts of anteroposterior and medial/lateral displacements that occur in the normal knee are the result of joint incongruence and variations in ligamentous elasticity.
Although these translations may be seen as undesirable, they are necessary for normal joint motions to occur. Excessive translational motions, however, should be considered abnormal and generally indicate some degree of ligamentous incompetence. We will focus on here on the normal knee joint motions, including both osteokinematics and arthrokinematics.
Flexion/Extension The axis for tibiofemoral flexion and extension can be simplified as a horizontal line passing through the femoral epicondyles. Although this transepicondylar axis represents an accurate estimate of the axis for flexion and extension, it should be appreciated that this axis is not truly fixed but rather shifts throughout the ROM. Much of the shift in the axis can be attributed to the incongruence of the joint surfaces.
The large articular surface of the femur and the relatively small tibial condyle create a potential problem as the femur begins to flex on the fixed tibia. If the femoral condyles were permitted to roll posteriorly on the tibial plateau, the femur would run out of tibia and limit the flexion excursion (Fig. 11- 25).
For the femoral condyles to continue to roll as flexion increases without leaving the tibial plateau, the femoral condyles must simultaneously glide anteriorly (Fig. 11-26A).
The initiation of knee flexion (0 to 25), therefore, occurs primarily as rolling of the femoral condyles on the tibia that brings the contact of the femoral condyles posteriorly on the tibial condyle. As flexion continues, the rolling of the femoral condyles is accompanied by a simultaneous anterior glide that is just sufficient to create a nearly pure spin of the femur on the posterior tibia with little linear displacement of the femoral condyles after 25 of flexion.
Extension of the knee from flexion is essentially a reversal of this motion. Tibiofemoral extension occurs initially as an anterior rolling of the femoral condyles on the tibial plateau, displacing the femoral condyles back to a neutral position on the tibial plateau.
After the initial forward rolling, the femoral condyles glide posteriorly just enough to continue extension of the femur as an almost pure spin of the femoral condyles on the tibial plateau (see Fig. 11-26B).
This description of the interdependent osteokinematics and arthrokinematics indicates that the femur was moving on a fixed tibia (e.g., during a squat). The tibia, of course, is also capable of moving on a fixed femur (e.g., during a seated knee extension or the swing phase of gait).
In this case, the movements would be somewhat different. When the tibia is flexing on a fixed femur, the tibia both rolls and glides posteriorly on the relatively fixed femoral condyles. Extension of the tibia on a fixed femur incorporates an anterior roll and glide of the tibial plateau on the fixed femur.
Role of the Cruciate Ligaments and Menisci in Flexion/Extension The arthrokinematics associated with tibiofemoral flex- ion and extension are somewhat dictated by the presence of the cruciate ligaments. If the cruciate ligaments are assumed to be rigid segments with a constant length, posterior rolling of the femur during knee flexion would cause the “rigid” ACL to tighten (or serve as a check rein). Continued rolling of the femur would result in the taut ACL’s simultaneously creating an anterior translational force on the femoral condyles (Fig. 11-27A).
During knee extension, the femoral condyles roll anteriorly on the tibial plateau until the “rigid” PCL checks further anterior progression of the femur, creating a posterior translational force on the femoral condyles (see Fig. 11-27B).
The anterior glide of the femur during flexion may be further facilitated by the shape of the menisci. The wedge shape of the menisci posteriorly forces the femoral condyle to roll “uphill” as the knee flexes. The oblique contact force of the menisci on the femur helps guide the femur anteriorly during flexion while the reaction force of the femur on the menisci deforms the menisci posteriorly on the tibial plateau (Fig. 11-28).
Posterior deformation occurs because the rigid attachments at the meniscal horns limit the ability of the menisci to move in its entirety. Posterior deformation also allows the menisci to remain beneath the rounded femoral condyles as the condyles move on the relatively flat tibial plateau.
As the knee joint begins to return to extension from full flexion, the posterior margins of the menisci return to their neutral position. As extension continues, the anterior margins of the menisci deform anteriorly with the femoral condyles.
The motion (or distortion) of the menisci is an important component of tibiofemoral flexion and extension. Given the need of the menisci to reduce friction and absorb the forces of the femoral condyles that are imposed on the relatively small tibial plateau, the menisci must remain beneath the femoral condyles to continue their function.
The posterior deformation of the menisci is assisted by muscular mechanisms to ensure that appropriate meniscal motion occurs. During knee flexion, for example, the semimembranosus exerts a posterior pull on the medial meniscus (Fig. 11-29), whereas the popliteus assists with deformation of the lateral meniscus.
Flexion/Extension Range of Motion Passive range of knee flexion is generally considered to be 130° to 140°. During an activity such as squatting, knee flexion may reach as much as 160° as the hip and knee are both flexed and the body weight is super-imposed on the joint. Normal gait on level ground requires approximately 60° to 70° of knee flexion, whereas ascending stairs requires about 80°, and sitting down into and arising from a chair requires 90° of flexion or more.
Knee joint extension (or hyperextension) up to 5° is considered within normal limits. Excessive knee hyperextension (i.e., beyond 5° of hyperextension) is termed genu recurvatum.
Many of the muscles acting at the knee are two-joint muscles crossing not only the knee but also the hip or ankle. Therefore, the hip joint’s position can influence the knee joint’s ROM. Passive insufficiency of the rectus femoris could limit knee flexion to 120° or less if the hip joint is simultaneously hyperextended. When the lower extremity is in weight-bearing, ROM limitations at other joints such as the ankle may cause restrictions in knee flexion or extension.
Case Application 11-3: Meniscal Entrapment Failure of the menisci to distort in the proper direction can result in limitations of joint motion and/or damage to the menisci. If the femur literally rolls up the wedge-shaped menisci in flexion (without either the anterior glide of the femur or the posterior distortion of the menisci), the increasing thickness of the menisci and the threat of rolling off the posterior margin will cause flexion to be limited.
Alternatively, the stress on the meniscus (especially the less mobile medial meniscus) may cause the meniscus to tear. Similarly, failures of the menisci to distort anteriorly during extension causes the thick anterior margins to become wedged between the femur and tibia as the segments are drawn together in the final stages of extension, thus limiting extension. The failure of the meniscus or femoral condyles to move appropriately on each other may be part of the explanation for Tina’s original injury to her medial meniscus, although it is likely that additional stresses to the meniscus contributed.
Example 11-1 Ski boots generally hold the ankle in dorsiflexion, preventing full knee extension when the foot is on the ground (see Fig. 11- 30A). The choice is either to walk with flexed knees or to walk on the heels. The same problem may be created by a fixed dorsiflexion deformity in the ankle/foot complex. The opposite situation happens with a limitation in dorsiflexion. A limitation to ankle dorsiflexion (e.g., caused by tight plantarflexors) may limit the amount of knee flexion that can be performed without lifting the heel off the ground.
If there is a fixed plantarflexion deformity at the ankle, the inability to bring the tibia forward in weight-bearing may result in a hyperextension deformity (genu recurvatum) at the knee (Fig. 11- 30B). The relationship between ankle and knee motions when the foot is on the ground can be exploited by intentionally altering ankle joint motion (e.g., through a heel lift or an ankle-foot orthosis) to prevent or control undesired knee motions.
Medial/Lateral Rotation Medial and lateral rotation of the knee joint are angular motions that are named for the motion (or relative motion) of the tibia on the femur. These axial rotations of the knee joint occur about a longitudinal axis that runs through or close to the medial tibial intercondylar tubercle. Consequently, the medial condyle acts as the pivot point while the lateral condyles move through a greater arc of motion, regardless of the direction of rotation (Fig. 11- 31).
As the tibia laterally rotates on the femur, the medial tibial condyle moves only slightly anteriorly on the relatively fixed medial femoral condyle, whereas the lateral tibial condyle moves a larger distance posteriorly on the relatively fixed lateral femoral condyle. During tibial medial rotation, the medial tibial condyle moves only slightly posteriorly, whereas the lateral condyle moves anteriorly through a larger arc of motion.
During both medial and lateral rotation, the knee joint’s menisci will distort in the direction of movement of the corresponding femoral condyle and, therefore, maintain their relationship to the femoral condyles just as they did in flexion and extension.
For example, as the tibia medially rotates (femur laterally rotates on the tibia), the medial meniscus will distort anteriorly on the tibial condyle to remain beneath the anteriorly moving medial femoral condyle, and the lateral meniscus will distort posteriorly to remain beneath the posteriorly moving lateral femoral condyle.
In this way, the menisci continue to reduce friction and distribute forces without restricting motion of the femur, as more solid or rigidly attached structures would do.
Axial rotation is permitted by articular incongruence and ligamentous laxity. Therefore, the range of knee joint rotation depends on the flexion/extension position of the knee. When the knee is in full extension, the ligaments are taut, the tibial tubercles are lodged in the intercondylar notch, and the menisci are tightly interposed between the articulating surfaces; consequently, very little axial rotation is possible.
As the knee flexes toward 90, capsular and ligamentous laxity increase, the tibial tubercles are no longer in the intercondylar notch, and the condyles of the tibia and femur are free to move on each other.
The maximum range of axial rotation is available at 90 of knee flexion. The magnitude of axial rotation diminishes as the knee approaches both full extension and full flexion. At 90°, the total medial/lateral rotation available is approximately 35°, with the range for lateral rotation being slightly greater (0° to 20°) than the range for medial rotation (0° to 15°).
Valgus (Abduction)/Varus (Adduction) Frontal plane motion at the knee, although minimal, does exist and can contribute to normal functioning of the tibiofemoral joint. Frontal plane ROM is typically only 8° at full extension, and 13° with 20° of knee flexion. Excessive frontal plane motion could indicate ligamentous insufficiency.
There is evidence that the muscles that cross the knee joint have the ability both to generate and control substantial valgus and varus torques. When there is ligamentous laxity, the excessive varus/valgus motion or increased dynamic activity of muscles attempting to control this excessive motion could precipitate greater peak stresses across the joint.
Coupled Motions Typical tibiofemoral motions are, unfortunately, not as straightforward as we have described. In fact, biplanar intra-articular motions can occur because of the oblique orientation of the axes of motion with respect to the bony levers. The true flexion/extension axis is not perpendicular to the shafts of the femur and tibia.
Therefore, flexion and extension do not occur as pure sagittal plane motions but include frontal plane components termed “coupled motions” (similar to coupling that occurs with lateral flexion and rotation in the vertebral column). As already noted, the medial femoral condyle lies slightly distal to the lateral femoral condyle, which results in a physiologic valgus angle in the extended knee that is similar to the physiologic valgus angle that exists at the elbow.
With knee flexion around the obliquely oriented axis, the tibia moves from a position oriented slightly lateral to the femur to a position slightly medial to the femur in full flexion; that is, the foot approaches the midline of the body with knee flexion just as the hand approaches the mid-line of the body with elbow flexion. Flexion is, therefore, considered to be coupled to a varus motion, while extension is coupled with valgus motion.
Automatic or Locking Mechanism of the Knee There is an obligatory lateral rotation of the tibia that accompanies the final stages of knee extension that is not voluntary or produced by muscular forces. This coupled motion (lateral rotation with extension) is referred to as automatic or terminal rotation. We have already noted that the medial articular surface of the knee is longer (has more articular surface) than does the lateral articular surface (see Fig. 11-3).
Consequently, during the last 30° of knee extension (30° to 0°), the shorter lateral tibial plateau/femoral condyle pair completes its rolling-gliding motion before the longer medial articular surfaces do. As extension continues (referencing non–weight- bearing motion of the tibia), the longer medial plateau continues to roll and to glide anteriorly after the lateral side of the plateau has halted.
This continued anterior motion of the medial tibial condyle results in lateral rotation of the tibia on the femur, with the motion most evident in the final 5 of extension. Increasing tension in the knee joint ligaments as the knee approaches full extension may also contribute to the obligatory rotational motion, bringing the knee joint into its close-packed or locked position.
The tibial tubercles become lodged in the intercondylar notch, the menisci are tightly interposed between the tibial and femoral condyles, and the ligaments are taut. Consequently, automatic rotation is also known as the locking or screw home mechanism of the knee. To initiate knee flexion from full extension, the knee must first be unlocked; that is, the laterally rotated tibia cannot simply flex but must medially rotate concomitantly as flexion is initiated.
A flexion force will automatically result in medial rotation of the tibia because the longer medial side will move before the shorter lateral compartment. If there is a lateral restraint to unlocking or derotation of the femur, the joint surfaces, ligaments, and menisci can become damaged as the tibia or femur is forced into flexion.
This automatic rotation or locking of the knee occurs in both weight-bearing and non–weight-bearing knee joint function. In weight-bearing, the freely moving femur medially rotates on the relatively fixed tibia during the last 30 of extension. Unlocking, consequently, is brought about by lateral rotation of the femur on the tibia before flexion can proceed.
The motions of the knee joint, exclusive of automatic rotation, are produced to a great extent by the muscles that cross the joint.
We will complete our examination of the tibiofemoral joint by first examining the individual contribution of the muscles, emphasizing their role in producing and controlling knee joint motion. We will then reexamine both the passive knee joint structures and the muscles in their combined role as stabilizers of this very complicated joint.