Biomechanics of theKnee Complex : 4DR. DIBYENDUNARAYAN BID [PT]THE SARVAJANIK COLLEGE OF PHYSIOTHERAPY, RAMPURA, SURAT
Ligaments The roles of the various ligaments of the knee have received extensive attention, which reflects their importance for knee joint stability and the frequency with which function is disrupted through injury.
Given the lack of bony restraint to virtually any of the knee motions, the knee joint ligaments are variously credited with resisting or controlling: 1. excessive knee extension 2. varus and valgus stresses at the knee (attempted adduction or abduction of the tibia, respectively) 3. anterior or posterior displacement of the tibia beneath the femur 4. medial or lateral rotation of the tibia beneath the femur 5. combinations of anteroposterior displacements and rotations of the tibia, together known as rotatory sta- bilization of the tibia
The large body of literature available on ligamentous function of the knee joint can be confusing and appears contradictory. This may be due to some confusion in terms as to whether the tibia or the femur is being referenced, but it is more likely due to complex and variable functioning and to dissimilar testing conditions.
It is clear that ligamentous function can change, depending on the position of the knee joint, on how the stresses are applied, and on what active or passive structures are concomitantly intact.
Medial Collateral Ligament The MCL can be divided into a superficial portion and a deep portion that are separated by a bursa. The superficial portion of the MCL arises proximally from the medial femoral epicondyle and travels distally to insert into the medial aspect of the proximal tibia distal to the pes anserinus (Fig. 11-15).
The deep portion of the MCL is continuous with the joint capsule, originates from the inferior aspect of the medial femoral condyle, and inserts on the proximal aspect of the medial tibial plateau. Throughout its course of travel, the deep portion of the MCL is rigidly affixed to the medial border of the medial meniscus (see Fig. 11-10).
The MCL, specifically the superficial portion, is the primary restraint to excessive abduction (valgus) and lateral rotation stresses at the knee. The knee joint is best able to resist a valgus stress at full extension because the MCL is taut in this position. As joint flexion is increased, the MCL becomes more lax and greater joint space opening is allowed (medially gapping).47
With the knee flexed, the MCL plays a more critical role in resisting valgus stress despite the permitted joint gapping. Grood et al. determined that at close to full extension, the MCL accounted for 57% of the restraining force against valgus opening, but at 25° of knee flexion, the MCL accounted for 78% of the load.
This difference is likely due to the greater bony congruence and inclusion of other soft tissue structures (e.g., posteromedial capsule, ACL) that at full extension can more effectively assist with checking a valgus stress. The MCL also plays a supportive role in resisting anterior translation of the tibia on the femur in the absence of the primary restraints against anterior tibial translation.
The MCL has the capacity to heal when ruptured or damaged, because of its rich blood supply. An isolated injury, therefore, does not often necessitate surgical stabilization but is often left to heal on its own, although this remodeling process can take up to a year.
Lateral Collateral Ligament The lateral collateral ligament (LCL) is located on the lateral side of the tibiofemoral joint, beginning proximally from the lateral femoral condyle. The LCL then travels distally to the fibular head (Fig. 11-16), where it joins with the tendon of the biceps femoris muscle to form the conjoined tendon.
Unlike the MCL, the LCL is not a thickening of the capsule but is separate throughout much of its length and is thereby considered to be an extracapsular ligament. The LCL is primarily responsible for checking varus stresses, and like the MCL, limits varus motion most successfully at full extension.
Grood et al. reported that at 5° of knee flexion, the LCL accounted for 55% of the restraining force against varus stress. This capacity increased to 69% with the knee flexed to 25°. Although the LCL’s primary role is to resist varus stresses, its orientation enables the LCL to limit excessive lateral rotation of the tibia as well.
Anterior Cruciate Ligament The relatively high rate of injury of the ACL by athletes and other active individuals has resulted in the ACL’s being one of the most highly researched ligaments in the human body. The ACL is attached to the anterior tibial spine (see Fig. 11-9), where it extends superiorly and posteriorly to attach to the posteromedial aspect of the lateral femoral condyle (Fig. 11-17).
The ACL courses posteriorly, laterally, and superiorly from tibia to femur. In addition, the ACL twists inwardly (medially) as it travels proximally. The ACL may also be considered to consist of two separate bands that wrap around each other.
Each of these bands is thought to have a different role in controlling tibiofemoral motion. The anteromedial band (AMB) and the posterolateral band (PLB) are each named for their origins on the tibia. The major blood supply to the ACL arises primarily from the middle genicular artery.
The ACL functions as the primary restraint against anterior translation (anterior shear) of the tibia on the femur. This role, however, belongs to either the AMB or the PLB, depending on the knee flexion angle. With the knee in full extension, the PLB is taut; as knee flexion increases, the PLB loosens and the AMB becomes tight, as demonstrated by the data plotted in Figure 11-18.
This shift in tension between the bands allows some portion of the ACL to remain tight at all times. In the intact joint, forces producing an anterior translation of the tibia will result in maximal excursion of the tibia at about 30° of flexion when neither of the ACL bands are particularly tensed. The ACL is also responsible for resisting hyperextension of the knee.
There appears to be essentially no anterior translation of the tibia possible in full extension when many of the supporting passive structures of the knee are taut (including the PLB of the ACL).
In addition to its primary restraint against anterior shear, the ACL can act as a secondary restraint against either varus or valgus motions (adduction and abduction rotations respectively) at the knee. With valgus loading, the lengths of both bands of the ACL increase as knee flexion increases. After injury to the MCL, a valgus moment will increase the strain on the ACL throughout the flexion range.
Although the ACL may not make an important contribution to limiting medial rotation of the tibia, medial rotation of the tibia on the femur increases the strain on the AMB of the ACL, with the peak strain occurring between 10° and 15°. This is most likely due to the orientation of the ACL, inasmuch as it winds its way medially around the PCL, becoming tighter with medial rotation.
Regardless of the rotational effect on the ACL’s loading pattern, injury to the ACL appears to occur most commonly when the knee is slightly flexed and the tibia is rotated in either direction in weight- bearing. In flexion and medial rotation, the ACL is tensed as it winds around the PCL. In flexion and lateral rotation, the ACL is tensed as it is stretched over the lateral femoral condyle.
The muscles surrounding the knee joint are capable of either inducing or minimizing strain in the ACL. With the tibiofemoral joint in nearly full extension, a quadriceps muscle contraction is capable of genera- ting an anterior shear force on the tibia, thereby increasing stress on the ACL.
Fleming et al. reported that the gastrocnemius muscle similarly has the potential to translate the tibia anteriorly and strain the ACL because the proximal tendon of the gastrocnemius wraps around the posterior tibia, effectively pushing the tibia forward when the muscle becomes tense through active contraction or passive stretch.
The hamstring muscles are capable of inducing a posterior shear force on the tibia throughout the range of knee flexion, becoming more effective in this role at greater knee flexion angles.
The hamstrings, therefore, have the potential to relieve the ACL of some of the stress of checking anterior shear of the tibia on the femur. With the foot on the ground, the soleus muscle may also have the ability to posteriorly translate the tibia and assist the ACL in restraining anterior tibial translation (Fig. 11-19).
Given the potential of individual muscles to either increase or decrease loads on the ACL, it is not surprising that co-contraction of multiple muscles across the knee can influence the strain on the ACL.
For example, co-contraction of the hamstrings and quadriceps muscles will allow the hamstrings to counter the anterior translatory effect of the quadriceps and reduce the strain on the ACL.
In contrast, activation of both the gastrocnemius and the quadriceps muscles results in greater strain on the ACL than either muscle alone would produce, unless the hamstrings also co-contract to mitigate the anterior translation imposed by the gastrocnemius.
Although muscular co-contraction will limit the strain imposed on the ligaments of the knee, it comes at a price. Co-contraction will reduce the anterior shear force on the tibia, but it increases joint compressive loads.
Posterior Cruciate Ligament The PCL attaches distally to the posterior tibial spine (see Fig. 11-9) and travels superiorly and somewhat anteriorly to attach to the lateral aspect of the medial femoral condyle (see Fig. 11-17). Like the ACL, the PCL is intracapsular but extrasynovial. The PCL is a shorter and less oblique structure than the ACL, with a cross-sectional area 120% to 150% greater than that of the ACL.
The PCL blends with the posterior capsule and periosteum as it crosses to its tibial attachment. The PCL, again like the ACL, is typically divided into an AMB and a PLB that are each named for their tibial origins. When the knee is close to full extension, the larger and stronger AMB is lax, whereas the PLB becomes taut. At 80° to 90° of flexion, the AMB is maximally taut and the PLB is relaxed.
The PCL serves as the primary restraint to posterior displacement, or posterior shear, of the tibia beneath the femur. In the fully extended knee, the PCL will absorb 93% of a posteriorly directed load applied to the tibia. This ability of the PCL to assume such a large load in full extension restricts posterior displacement to very minimal amounts.
Unlike the ACL, which resists force better at full extension, the PCL is more adept at restraining motion with the knee flexed. Maximal posterior displacement of the tibia occurs at 75° to 90° of flexion, however, because with greater knee flexion, the secondary restraints against posterior translation become ineffective. Sectioning of the PCL, therefore, increases posterior translation at all angles of knee flexion.
Like the ACL, the PCL has a role in restraining varus and valgus stresses at the knee and appears to play a role in both restraining and producing rotation of the tibia. The orientation of the PCL may result in a concomitant lateral rotation of the tibia when posterior translational forces are applied to the tibia. The PCL resists tibial medial rotation at 90° but less so in full extension. The PCL does not resist lateral rotation very well.
In the absence of the PCL, muscles must be recruited to actively stabilize against excessive posterior tibial translation. The popliteus muscle shares the role of the PCL in resisting posteriorly directed forces on the tibia and can contribute to knee stability when the PCL is absent. In contrast, an isolated hamstring con-traction might destabilize the knee joint in the absence of the PCL because of its posterior shear on the tibia in the flexed knee.
Contraction of the gastrocnemius muscle also significantly strains the PCL at flexion angles greater than 40° , whereas quadriceps contraction reduces the strain in the PCL at knee flexion angles between 20° and 60°.
Ligaments of the Posterior Capsule Several structures reinforce the “corners” of the posterior knee joint capsule (Fig. 11-21). The posteromedial corner of the capsule is reinforced by the semimembranosus muscle, by its tendinous expansion called the oblique popliteal ligament, and by the stronger and more superficial POL. The posterolateral corner of the capsule is reinforced by the arcuate ligament, the LCL, and the popliteus muscle and tendon. The arcuate ligament is a Y-shaped capsular thickening found in nearly 70% of knees. (Attachments of these ligaments are given in Table 11-1.)
Both the POL and the arcuate ligaments are taut in full extension and assist in checking hyperextension of the knee; the POL and arcuate ligaments also check valgus and varus forces, respectively. The orientation of the lateral branch of the arcuate ligament allows it to become tight in tibial lateral rotation.
Iliotibial Band The IT band (or ITB) or IT tract is formed proximally from the fascia investing the tensor fascia lata, the glu-teus maximus, and the gluteus medius muscles. The IT band continues distally to attach to the lateral inter-muscular septum and inserts into the anterolateral tibia (Gerdy’s tubercle), reinforcing the anterolateral aspect of the knee joint (see Fig. 11-16).
Despite the muscular attachments to the IT band, it remains an essentially passive structure at the knee joint; a contraction of the tensor fascia lata (TFL) or the gluteus maximus muscles that attach to the IT band proximally produce only minimal longitudinal excursion of the band distally. The IT band moves anterior to the knee joint axis as the knee is extended, and posteriorly over the lateral femoral condyle as the knee is flexed (Fig. 11-22).
The IT band, therefore, remains consistently taut, regardless of the hip or knee’s position. The fibrous connections of the IT band to the biceps femoris and vastus lateralis muscles form a sling behind the lateral femoral condyle, assisting the ACL in checking posterior femoral (or anterior tibial) translation when the knee joint is nearly full extension.
With the knee in flexion, the combination of the IT band, the LCL, and the popliteal tendon crossing over each other increases the stability of the lateral side of the joint and even more effectively assists the ACL in resisting anterior displacement of the tibia on the femur (see Fig. 11-22).
Despite its lateral location, the IT band alone provides only minimal resistance to lateral joint space opening. The IT band also attaches to the patella via the lateral patellofemoral ligament of the lateral retinaculum. As we shall see, this attachment of the IT band to the lateral border of the patella may affect patellofemoral function.