DR. DIBYENDUNARAYAN BID [PT]T H E S A R VA J A N I K C O L L E G E O F P H Y S I O T H E R A P Y,

R A M P U R A , S U R AT

Biomechanics of the

Knee Complex : 8

Patellofemoral Joint

Embedded within the quadriceps muscle, the flat, triangularly shaped patella is the largest sesamoid bone in the body.

The patella is an inverted triangle with its apex directed inferiorly. The posterior surface is divided by a vertical ridge and covered by articular cartilage (Fig. 11-39).

This ridge is situated approximately in the center of the patella, dividing the articular surface into approximately equally sized medial and lateral facets.

Both the medial and lateral facets are flat to slightly convex side to side and top to bottom.

Most patellae also have a second vertical ridge toward the medial border that separates the medial facet from an extreme medial edge, known as the odd facet of the patella (see Fig. 11-39).

The posterior surface of the patella in the extended knee sits on the femoral sulcus (or patellar surface) of the anterior aspect of the distal femur (Fig. 11-40).

The femoral sulcus has a groove that corresponds to the ridge on the posterior patella and divides the sulcus into medial and lateral facets.

The lateral facet of the femoral sulcus is slightly more convex than the medial facet and has a more highly developed lip than does the medial surface (see Fig. 11-2).

The patella is attached to the tibial tuberosity by the patellar tendon.

Given the shape of the articular surfaces and the fact that the patella has a much smaller articular surface area than its femoral counterpart, the patellofemoral joint is one of the most incongruent joints in the body.

The patella functions primarily as an anatomic pulley for the quadriceps muscle.

Interposing the patella between the quadriceps tendon and the femoral condyles also reduces friction as the femoral condyles contact the hyaline cartilage-covered posterior surface of the patella rather than the quadriceps tendon.

The ability of the patella to perform its functions without restricting knee motion depends on its mobility.

Because of the incongruence of the patellofemoral joint, however, the patella is dependent on static and dynamic structures for its stability.

We must closely examine the oddly shaped patella and the uneven surface on which it sits in order to understand the normal motions of the patella that accompany knee joint motion and the tremendous forces to which the patella and patellofemoral surfaces are susceptible.

The goal of such examination is to understand the many potential problems encountered by the patella in performing what appears to be a relatively simple function.

A comprehension of the structures and forces that influence patellofemoral function leads readily to an understanding of the common clinical problems found at the patellofemoral joint as it attempts to meet its contradictory demands for both mobility and stability.

Patellofemoral Articular Surfacesand Joint Congruence

In the fully extended knee, the patella lies on the femoral sulcus.

Because the patella has not yet entered the intercondylar groove, joint congruency in this position is minimal, which suggests that there is a great potential for patellar instability.

Stability of the patella is affected by the vertical position of the patella in the femoral sulcus, because the superior aspect of the femoral sulcus is less developed than the inferior aspect.

The vertical position of the patella, in turn, is related to the length of the patellar tendon.

Ordinarily, the ratio of the length of the patellar tendon to the length of the patella is approximately 1:1 and is referred to as the Insall-Salvati index.

A markedly long tendon produces an abnormally high position of the patella on the femoral sulcus known as patella alta, which increases the risk for patellar instability.

The interaction of the height of the lateral lip of the femoral sulcus with patella alta may also be a factor in patellar instability.

In this condition, the lateral lip is not necessarily underdeveloped (although it may be), but the high position of the patella places the patella proximal to the high lateral wall, rendering the patella less stable and easier to sublux.

In patients with patella alta, the tibiofemoral joint must be flexed more before the patella translates inferiorly enough to engage the intercondylar groove.

This leaves a larger knee ROM within which the patella is relatively unstable.

Given the incongruence of the patella, the contact between the patella and the femur changes throughout the knee ROM (Fig. 11-41).

When the patella sits in the femoral sulcus in the extended knee, only the inferior pole of the patella is making contact with the femur.114

As the knee begins to flex, the patella slides down the femur, increasing the surface contact area.

In this manner, the first consistent contact between the patella and the femur occurs along the inferior margin of both the medial and lateral facets of the patella at 10° to 20° of knee flexion.

As tibiofemoral flexion progresses, the contact area increases and shifts from the initial inferior location on the patella to a more superior position.

As the contact area shifts superiorly along the posterior aspect of the patella, it also spreads outward to cover the medial and lateral facet.

By 90° of knee flexion, all portions of the patella have experienced some (although inconsistent) contact, with the exception of the odd facet.

As flexion continues beyond 90°, the area of contact begins to migrate inferiorly once again as the smaller odd facet makes contact with the medial femoral condyle for the first time.

At full flexion, the patella is lodged in the intercondylar groove, and contact is on the lateral and odd facets, with the medial facet completely out of contact.

Motions of the Patella

As the contact between the patella and the femur changes with knee joint motion, the patella simultaneously translates and rotates on the femoral condyles.

These movements are influenced by and reflect the patella’s relationship to both the femur and the tibia.

Consequently, the description of motions can appear quite complicated.

When the femur is fixed and the tibia is flexing, the patella (fixed to the tibial tuberosity via the patellar tendon) is pulled down and under the femoral condyles, ending with the apex of the patella pointing posteriorly in full knee flexion.

This sagittal plane rotation of the patella as the patella travels (or “tracks”) down the intercondylar groove of the femur is termed patellar flexion.

Knee extension brings the patella back to its original position in the femoral sulcus, with the apex of the patella pointing inferiorly at the end of the normal ROM.

This patellar motion is referred to as patellar extension.

In addition to patellar flexion and extension, the patella rotates around a longitudinal (or nearly vertical) axis and tilts around an anteroposterior axis.

Rotation about the longitudinal axis is termed medial or lateral patellar tilt and is named for the direction in which the anterior surface of the patella is moving (Fig. 11-42).

When the tibia medially rotates beneath the femur during axial rotation, the patella must remain in the intercondylar groove during the relative lateral rotation of the femur.

This relative motion of the femur forces the patella to face more laterally; this is termed lateral rotation.

Patellar tilt is also dictated somewhat by the asymmetrical nature of the femoral condyles.

For instance, the more anteriorly protruding lateral femoral condyle forces the anterior surface of the patella to tilt medially during much of knee flexion.

Rotation of the patella about an anteroposterior axis (termed medial or lateral rotation of the patella) is, like patellar tilt, necessary in order for the patella to remain seated between the femoral condyles as the femur undergoes axial rotation on the tibia.

Because the inferior aspect of the patella is “tied” to the tibia via the patellar tendon, the inferior patella continually points toward the tibial tuberosity while moving with the femur (Fig. 11-43).

Figure 11-43

■ A. Medial rotation of the patella.

The inferior pole of the patella follows the tibial tuberosity during medial rotation of the tibia.

B. Lateral rotation of the patella.

The inferior pole of the patella follows the tibial tuberosity during lateral rotation of the tibia.

Therefore, when the knee is in some flexion and there is medial rotation of the tibia on the fixed femur, the inferior pole of the patella will point medially;this is termed medial rotation of the patella.

In lateral rotation of the patella, the inferior patellar pole follows the laterally rotated tibia.

The patella laterally rotates approximately 5° as the knee flexes from 20° to 90°, given the asymmetrical configuration of the femoral condyles.

The patella, although firmly attached to soft tissue stabilizers (for example, the extensor retinaculum), undergoes translational motions that are dependant on the point in the tibiofemoral ROM.

The patella translates superiorly and inferiorly with knee extension and flexion, respectively.

During active extension, the patella glides superiorly.

If this glide is restricted, quadriceps function is compromised, and passive knee extension may be lost.

During active tibiofemoral flexion, the patella glides inferiorly.

A restricted inferior glide could therefore limit knee flexion.

There is a simultaneous medial-lateral translation of the patella that accompanies the superior-inferior glide that is referred to as patellar shift (see Fig. 11-42).

The patella is typically situated slightly laterally in the femoral sulcus with the knee in full extension.

As knee flexion is initiated, the patella shifts medially as it is pushed by the larger lateral femoral condyle and as the tibial medially rotates with unlocking of the knee.

As knee flexion proceeds past 30°, the patella may shift slightly laterally or remain fairly stable, inasmuch as the patella is now firmly engaged within the femoral condyles (Fig. 11-44).

Consequently, the patella shifts as the knee moves from full extension into flexion.

Failure of the patella to slide, tilt, rotate, or shift appropriately can lead to restrictions in knee joint ROM, to instability of the patellofemoral joint, or to pain caused by erosion of the patellofemoral articular surfaces.

Therefore, passive mobility of the patella is often assessed clinically to determine the presence of hypermobility or hypomobility of the patella with respect to the femur.

Patellofemoral Joint Stress

The patellofemoral joint can undergo very high stresses during typical activities of daily living.

Joint stress (force per unit area) can be influenced by any combination of large joint forces or small contact areas, both of which are present during routine flexion and extension of the tibiofemoral joint.

The patellofemoral joint reaction (contact) force is influenced by both the quadriceps force and the knee angle.

As the knee flexes and extends, the patella is pulled by the quadriceps tendon superiorly and simultaneously by the patella tendon inferiorly.

The combination of these pulls produces a posterior compressive force of the patella on the femur that varies with knee flexion.

At full extension, the quadriceps posterior compressive force on the patella is minimized and due exclusively to the origin of the vastus medialis and vastus lateralis muscles on the posterior femur.

Despite the small contact area that the patella has with the femur in full extension, the minimal posterior compressive vector of the vastus lateralis and vastus medialis muscles maintains low joint stress at full extension.

This is the rationale for the use of straight-leg raising exercises as a way of improving quadriceps muscle strength without creating or exacerbating patellofemoral pain.

As knee flexion progresses from full extension, the angle of pull between the quadriceps tendon and the patellar tendon decreases, creating greater joint compression (Fig. 11-45).

This increased compression occurs whether the muscle is active or passive.

If the quadriceps muscle is inactive, then elastic tension alone increases with increased knee joint flexion.

If the quadriceps muscle is active, then both the active tension and passive elastic tension will contribute to increasing the joint compression.

This compression, of course, creates a joint reaction force across the patellofemoral joint.

The total joint reaction force is therefore influenced by the magnitude of active and passive pull of the quadriceps, as well as by the angle of knee flexion.

Although the compressive force arising from the quadriceps increases as the knee flexes from 0° to 90°, the patellar contact area also increases.

The increase in contact area with increased compressive force functions to minimize patellofemoral joint stress until approximately 90° of flexion.

As knee flexion continues beyond 90°, the contact area once again diminishes and patellofemoral stress increases as only the lateral and odd facets make contact with the femoral condyles.

Patellofemoral joint reaction forces can become very high during routine daily activities.

During the stance phase of walking, when peak knee flexion is only approximately 20° , the patellofemoral compressive force is approximately 25% to 50% of body weight.

With greater knee flexion and greater quadriceps activity, as during running, patellofemoral compressive forces have been estimated to reach between five and six times body weight.

Deep knee flexion exercises that require large magnitudes of quadriceps activity can increase this compressive force further.

Although reaction forces at other lower extremity joints may reach these same magnitudes, they do so over much more congruent joints; that is, the compressive forces are distributed over larger areas.

At the normal patellofemoral joint, the medial facet bears the brunt of the compressive force.

Several mechanisms help minimize or dissipate the patellofemoral joint compression on the patella in general and on the medial facet specifically.

In full extension, there is minimal compressive force on the patella; therefore, no compensatory mechanisms are necessary.

As knee joint flexion proceeds, the area of patella contact gradually increases, spreading out the increased compressive force.

From 30° to 70° of flexion, the magnitude of contact force is higher at the thick cartilage of the medial facet near the central ridge.

This articular cartilage is among the thickest hyaline cartilage in the human body.

The presence of this thick cartilage is better able to withstand the substantial compressive forces transmitted across the medial facet of the patella.

Within this same ROM, the patella has its greatest effect as a pulley, maximizing the MA of the quadriceps.

With a larger MA, less quadriceps muscle force is needed to produce the same torque, minimizing patellofemoral joint compression.

As flexion proceeds, the MA diminishes, which necessitates an increase in force production by the quadriceps.

Beyond 90°, however, the patella is no longer the only structure contacting the femoral condyles.

At this point in the flexion range, the quadriceps tendon contacts the femoral condyles, helping to dissipate more of the compressive force on the patella.

The vertical position of the patella can also significantly influence patellofemoral stress.

Singerman and colleagues demonstrated that in the presence of patella alta, the onset of contact between the quadriceps tendon and femoral condyles is delayed.

As flexion increases, patellofemoral compressive forces will therefore continue to rise.

In contrast to patella alta, the patella can also sit lower than normal.

If the patella is positioned more inferiorly, it is termed patella baja and may be due to a shortened patellar tendon.

With patella baja, the contact between the quadriceps tendon and the femoral condyles occurs earlier in the range, resulting in a concomitant reduction in the magnitude of the patellofemoral contact force.

End of Part - 8