Ligament Injuries to the Knee



Ligament Injuries to the Knee


Jacqueline M. Brady, MD, FAAOS, FAOA


Dr. Brady or an immediate family member serves as a paid consultant to or is an employee of Miach.





ABSTRACT

Ligament injuries in the knee are common and can be caused by low-energy trauma in patients with predisposing factors or high-energy injuries in trau-matic settings. When knee ligament injuries render the tibiofemoral joint unstable, the chondral surfaces and menisci are at risk of ongoing injury from excessive shear forces. Likewise, recurrent patellofemoral instability can lead to cumulative chondral injury concerning for later posttraumatic arthritis. Surgical intervention is warranted when shown to prevent ongoing instability. Isolated medial collateral ligament and posterior collateral ligament tears and many first-time patellofemoral dislocations may be managed nonsurgically, with physical therapy directed at strength and periarticular mechanics. Anterior cruciate ligament tears in athletes pursuing sports involving change of direction, medial patellofemoral ligament injuries in the setting of recurrent patellar instability and/or a loose chondral/osteochondral injury, and multiligamentous knee injuries should be managed surgically. Primary repairs of the medial patellofemoral ligament and posterolateral corner injuries have been shown to have a high rate of failure; therefore reconstruction or augmentation should be used. Grafts used for collateral and cruciate ligament reconstruction are placed under resting tension to gain maximal stability, but medial patellofemoral ligament grafts are used as a checkrein only, so they are set to a length that minimizes tension and allows some patellar translation to avoid medial patellofemoral joint overload. Surgical intervention for any knee ligament injury should be guided by the native anatomy, and a careful approach to allow knee motion by optimizing graft isometry will improve outcomes.

Keywords: knee dislocation; ligament reconstruction; patellar instability


Introduction

Although full extension is a stable position for the knee in terms of bony anatomy, the knee joint is not a pure hinge. With range of motion, the joint surfaces begin to roll and glide. The knee ligaments are key to maintaining stability of the knee during this process.


Anatomy


Anterior Cruciate Ligament

The anterior cruciate ligament (ACL) has rotational and translational roles in stabilizing the knee. It has two bundles named for their position within the tibial insertion. The anteromedial bundle is the more vertical bundle, serving to restrain anterior translation of the tibia relative to the femur. The femoral footprint of the native ACL is posteriorly located on the lateral wall of the intercondylar notch, centered on the bifurcate bony ridge. The tibial footprint is broad, but centered between the tibial spines, generally in line with an anatomically normal posterior border of the anterior horn of the lateral meniscus. The Lachman examination is the most sensitive physical examination for ACL integrity, and it primarily tests the anteromedial bundle. The posterolateral bundle serves as a rotational restraint and is linked to the pivot shift test in the knee. Patient outcomes after ACL reconstruction surgery have been demonstrated to correlate with the presence or absence of a pivot shift on examination.1,2


Posterior Cruciate Ligament

The posterior cruciate ligament (PCL) is broad and stout and also plays translational and rotational roles in stabilization: posterior translation of the tibia and anteromedial rotation of the tibia relative to the femur. The two bundles in the PCL are the anterolateral and posteromedial bundles. The femoral footprint is very anterior and vertical within the intercondylar notch, abutting the articular cartilage. The tibial footprint is very posterior, originating behind the tibial plateau at the so-called champagne glass drop-off of the tibia. The PCL rarely sustains a complete tear in isolation, so a
grade 3 PCL injury (full-thickness injury resulting in more than 10 mm posterior translation of the tibia relative to the femur on posterior drawer testing) should alert the examiner to the likely presence of concomitant ligamentous injuries.


Posterolateral Corner

In evolutionary history, the fibula was part of the knee articulation. It is now separate from the tibiofemoral articulation, but the lateral tibial plateau remains convex, leading to a need for an intricate combination of active and static stabilizers in the lateral knee. The posterolateral corner of the knee includes the lateral collateral ligament, popliteus tendon and popliteofibular ligament, iliotibial band, and biceps femoris. Other structures, such as the arcuate ligament and the posterolateral joint capsule, are described as part of the posterolateral corner of the knee, but these are not necessarily included in the surgical reconstruction. This combination of structures stabilizes the knee against varus load and external tibial rotation. Injury to this complex can be subtle, and the dial test is helpful to evaluate the extent of injury when it can be compared with an uninjured contralateral knee (Figure 1).







Medial Collateral Ligament

The medial collateral ligament (MCL) has two components: superficial and deep. The deep MCL has attachments to the medial meniscus of the knee. The superficial MCL is the primary restraint to valgus force, originating from the posterior and superior aspects of the medial femoral epicondyle and inserting 6 cm distal to the tibiofemoral joint line. MCL injuries are common in isolation, and the prognosis for recovery without surgical intervention is known to be better when the ligamentous injury is proximal. This is thought to relate to the anatomy of the distal insertion: tibial-side injuries sometimes result in a situation akin to the Stener lesion in ulnar collateral ligament injuries of the thumb: the MCL tears from its tibial insertion and retracts enough to pull out from under the pes tendon insertion. Although no conclusive data prove this theory, the inability of the tibial-side MCL to approximate its native insertion is thought to hamper its potential to scar with an acceptable degree of stability.



Posterior Oblique Ligament

In recent years, the understanding of the rotational stability of the medial side of the knee has improved. The posterior oblique ligament originates from the adductor tubercle of the knee and attaches to the posterior aspect of the tibia and joint capsule. It controls anteromedial rotation and aids in stabilizing against valgus forces. In addition, it plays a minor role in protecting against posterior tibial translation.


Medial Patellofemoral Ligament

The patella is a sesamoid bone within the extensor mechanism, and it experiences significant shearing loads in comparison with the compressive loads experienced by the tibiofemoral joint. With wider hips than knees, especially in women, humans are predisposed to lateral patellar tracking, and sometimes frank instability. The medial patellofemoral ligament (MPFL) is the primary restraint to lateral patellar translation.3 It is a thickening of the joint capsule that originates in the saddle region between the adductor tubercle and medial epicondyle of the knee and inserts primarily above the equator of the patella. A reflection to the quadriceps tendon has been described, and this structure is thus perhaps more accurately described as the medial patellofemoral complex.4 When patients have predisposing anatomy such as trochlear dysplasia, patella alta, a high tibial tuberosity to trochlear groove distance, significant genu valgum, or excessive femoral anteversion, patients can experience a patellar subluxation or dislocation, almost universally tearing the MPFL.5


Imaging

When possible, weight-bearing radiographs are preferred to evaluate the integrity of the joint with dynamic loads. A comparison view of the uninjured knee can be useful if findings are subtle or if suspicion remains regarding joint space in the setting of chondromalacia or meniscal pathology. In the case of cruciate ligament injury, a true lateral radiograph can be useful to determine the position of the tibia relative to the femur under loads.


Anterior Cruciate Ligament

A Segond fracture on an AP knee radiograph is pathognomonic for an ACL tear. This finding represents avulsion of the anterolateral ligament from the tibia (Figure 2). It is not always present in the setting of an ACL tear, but when found on plain radiographs should prompt MRI evaluation of soft-tissue integrity in the knee.6 Because ACL tears are commonly associated with meniscal and chondral injuries in the knee, a weight-bearing tunnel view can also be helpful in evaluating the integrity of the posterior portion of the tibiofibular joint.

Although more common in patients with skeletal immaturity, ACL injuries can involve tibial spine avulsions in any age range. If displaced, the tibial spine can be blocked from anatomic reduction by either the intermeniscal ligament or the anterior horn of the medial meniscus. Even with anatomic reduction and fixation, studies have shown residual laxity in the ACL, indicative of initial intrasubstance injury.

Radiographs also are important to determine alignment when planning surgical intervention. The posterior tibial slope has increasingly been recognized as a risk factor for failure of ACL reconstruction.7,8 Significant coronal plane malalignment, particularly genu varum, also can cause overload thought to be responsible for excessive strains on native and reconstructed ligaments. Particularly in revision situations, standing bilateral limb length and alignment radiographs, a lateral view of the tibia and fibula, and CT to evaluate the size and location of the previous tunnels are important parts of the workup for ACL injuries in the knee.







MRI is the gold standard for evaluating the ACL. In many cases bony contusions consistent with a pivot-shift injury mechanism are visible on the weight-bearing surface of the lateral femoral condyle and posterolateral tibial plateau (Figure 3). These bony contusions are predictive of later posttraumatic arthritis.9 The ACL itself is generally readily visible on MRI. Complete rupture is generally plainly evident. Partial injury can be subtle, and the presence of typical bone bruises helps the evaluator confirm the incompetence of the ligament in these cases.

The location of the tibia relative to the femur on any lateral or sagittal imaging study can hint at chronicity of any ACL tear: when the tibia is observed to subluxate slightly anteriorly, a more chronic injury and underlying anatomic factors such as increased posterior tibial slope and/or posterior horn meniscus pathology are suspected.


Posterior Cruciate Ligament

Avulsions of the PCL can occur from the femur or from the tibia. Depending on the size and location of the fragment, CT is helpful to confirm the nature of the injury when an avulsion is present because the bony overlap seen on radiography can be confounding. Whether the PCL is injured in its midsubstance or from an avulsion, the lack of integrity of the ligament can result in posterior sag of the tibia relative to the femur, which often is visible on a lateral radiograph if carefully scrutinized.

The evaluation of PCL injury on MRI can be nuanced in the setting of acute injury. Often, an acutely injured ligament will be read as grade 3 by a radiologist based on full-thickness edematous signal seen across its midsubstance. The radiographic grade 3 does not necessarily correlate with a grade 3 posterior drawer test finding on examination, however, and it is important to realize that a PCL in continuity may have the potential to scar sufficiently to render the knee relatively stable. Surgical intervention should be based on physical examination, although a completely disrupted PCL seen on MRI can help direct the physical examination. If the ankle-brachial index is less than 0.9, or the pulses are asymmetric, contrast-enhanced CT is indicated to evaluate for vascular injury. Vascular injury is more common in posterior knee dislocations, likely because of the higher degree of force required to produce the injury. It also can occur in anterior knee dislocations, however, via intimal stretch and resultant thrombosis.





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May 1, 2023 | Posted by in ORTHOPEDIC | Comments Off on Ligament Injuries to the Knee

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