Meniscus root tears biomechanically disrupt normal joint loading and lead to joint overload with the possible development of spontaneous osteonecrosis of the knee and early-onset osteoarthritis. Proper identification and treatment of meniscal root tears has been proven to restore joint loading and improve patient outcomes.
Key points
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Medial meniscus root tears are biomechanically equivalent to a subtotal medial meniscectomy.
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Lateral meniscus root tears provide both an important joint loading function and also result in increased anterior tibial translation and internal rotation in the anterior cruciate ligament–deficient knee.
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Anatomic root repairs restore joint loading and knee stability.
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Meniscal root repairs in patients with no to mild osteoarthritis yield significantly improved patient outcomes and stall the progression of osteoarthritis.
Introduction
Within the knee joint, the meniscus provides shock absorption and stability by generating circumferential stresses as load bearing occurs. This is enabled by the root attachments of the meniscus to the tibia, preventing meniscus extrusion and a subsequent alteration of the transmitted hoop stresses. Meniscus root tears lead to an increase of peak tibiofemoral contact pressure and tibiofemoral contact area, which has been shown to lead to altered biomechanics and an acceleration of degenerative changes of the knee joint. The treatment method for meniscus injuries now primarily focuses on preservation and anatomic restoration, because nonoperative and meniscectomy treatments are associated with poorer clinical outcomes and a higher rate of conversion to total knee arthroplasty. Not only have root repairs demonstrated a restoration of tibiofemoral joint mechanics, but comparison studies have ultimately shown the outperformance of root repair procedures relative to nonoperative and meniscectomy procedures in delaying progression of osteoarthritis. In this review, we detail the function and etiology of meniscus root injuries by the anatomic and biomechanical features of the root attachments, along with the validated treatment procedures currently detailed in the literature.
Meniscal root features
Anatomy of the Meniscal Roots
Generally, the medial meniscal root attachments are greater in area than those of the lateral meniscus, with the anterior horn of the medial meniscus being the largest and strongest footprint of all. The fan-shaped anterior horn of the medial meniscus inserts broadly onto the anterior intercondylar crest, with its posterior aspect measuring roughly 11.5 mm anterior to the center of the anterior cruciate ligament (ACL) tibial insertion and a reported area of 61.4 mm 2 , 139.0 mm 2 , and 101.7 mm 2 . Berlet and Fowler reported various insertions within close proximity of each other in this area, while supporting a potential fibrous link with the ACL.
The anterior horn attachment of the lateral meniscus is smaller than the anterior root attachment of the medial meniscus, with early estimates reporting an attachment area of 44.5 mm 2 and 93 mm 2 , and more recent data reporting 99.5 mm 2 . This attachment appears directly anterior to the lateral tibial eminence and laterally adjacent to the tibial insertion of the ACL. Ziegler and colleagues reported a lateral distance of 7.5 mm to the center of the ACL tibial attachment and supported earlier reports of attachment between the anterior horn of the lateral meniscus to the bundles of the ACL.
The posterior root attachment of the medial meniscus is known to insert anteromedial to the posterior cruciate ligament (PCL) tibial attachment, appearing 9.6 mm posterior and 0.7 mm lateral to the medial tibial eminence, while being 3.5 mm lateral from the articular cartilage inflection point of the medial tibial plateau ( Fig. 1 ). Early approximations reported attachment areas of 47.2 mm 2 and 80 mm 2 , but more recent studies report areas of 30.4 mm 2 and 68.0 mm 2 . Johannsen and colleagues attribute this reduction in approximated area to excluding the transverse shiny white fibers when measuring the medial root attachment; on inclusion, the total root attachment area increased to 69.6 mm 2 .
Early in the literature, the posterior horn of the lateral meniscus was typically known as the smallest of the root attachments, with early studies reporting 28.5 mm 2 and 115 mm 2 , and more recent studies reporting 39.2 mm 2 and 83.1 mm 2 ; recent studies suggest the posterior root of the medial meniscus may be the smallest on exclusion of the transverse shiny white fibers. This posterior lateral meniscus insertion appears just anterior to the insertion of the posterior attachment of the medial meniscus. Directly 12.7 mm anterior from the PCL tibial attachment and 10.8 mm posterior to the posteromedial bundle of the ACL, the posterior root of the lateral meniscus inserts 1.5 mm posterior and 4.2 mm medial to the lateral tibial eminence apex, while being 4.3 mm medial from the lateral articular cartilage edge ( Fig. 2 ).
Structural Properties of the Root Attachments
During ultimate failure testing, Ellman and colleagues reported the failure mechanism of meniscus root injuries to exclusively consist of small bony avulsions of the meniscal root. The anterior roots sustain greater force before failure relative to the posterior roots, with the anterior medial and anterior lateral roots failing at strengths of 655.5 N and 652.8 N, respectively and the posterior medial and posterior lateral roots falling at 513.8 N and 509.0 N, respectively. Literature suggests the ability of the anterior roots to sustain greater force may be derived from the features of greater thickness and uncalcified fibrocartilage relative to the posterior roots.
As previously stated, the inclusion or exclusion of the shiny white fibers within the posterior medial meniscal root attachment can affect the properties of the attachment. Once sectioned, ultimate failure strength lessened to 245.9 N from 513.8 N, suggesting the shiny white fibers account for 47.8% of the native root strength. A similar decrease, but less profound, was reported with sectioning of the surrounding fibers of the posterior lateral and anterior medial roots.
Etiology of meniscus root tears
The meniscus is responsible for distributing tibiofemoral loads, joint stabilization, and congruency. These abilities stem from both the intrinsic material properties and gross anatomic structure and attachments, with the root insertions enabling transformation of axial tibiofemoral stress into circumferential hoop stresses. Root tears are reported to have been encountered/diagnosed in 4.3% of all arthroscopic surgeries, with posterior medial and posterior lateral root tears accounting for 52% and 41% of all meniscal root tears, respectively. Increased mobility of the anterior attachments is thought to account for the higher failure strength and, ultimately, fewer failures, relative to the posterior roots. Disruption of these root attachments often leads to extrusion and a subsequent failure of load distribution accompanied by increased contact pressures and stress concentration, resulting in greater vulnerability of underlying cartilage degeneration. This resultant biomechanical condition mirrors that of an absent meniscus.
Long-term outcomes of lateral root disruption has been shown to significantly affect the lateral compartment, due to the function of the lateral meniscus in transmitting 70% of lateral compartment load, and the posterior root transmitting 50% to 85% between extension and 90° of flexion, respectively. Disruption also may affect the conditions of neighboring structures within the knee, because the lateral meniscus posterior root contributes to stabilization of anterior tibial translation at lower flexion angles and internal rotation at higher flexion angles ; absence of the posterior lateral meniscus root attachment results in greater strain placed on the ACL, which also supports anterior tibial translation and internal rotation.
The posterior root of the medial meniscus carries a greater load than its anterior root, making medial menisucs posterior root injuries more likely to induce disruptions to contact pressures and load distribution. Significant correlations exist between posterior root tears of the medial meniscus and spontaneous osteonecrosis of the knee, likely resulting from reduced medial meniscus mobility from its robust attachment to the medial tibial plateau coupled with the medial compartment enduring significantly greater loads during weightbearing.
Diagnosis of meniscal root tears
Root tears can be challenging to diagnose with simple clinical evaluation because no clinical test or deliberate sign for a definitive diagnosis exists. Patients may describe a sudden onset of pain after a small twisting injury or deep knee flexion, resulting in a recurrent effusion, joint line pain, loss of flexion, or deep knee flexion pain. Other times, the patient may experience popping or locking of the knee. Meniscal tests, like the McMurray and Apley tests, may be positive but not necessary with a mechanical click. Various techniques for evaluation have been described, one being the valgus/varus test in the fully extended knee. This test will be positive due to extrusion or lack of residual hoop strain of the meniscus when compressing the ipsilateral joint space. Important risk factors to identify include concomitant injuries, osteoarthritis (OA), alignment, and high body mass index (BMI).
A thorough history, physical examination, and MRI will need to be performed to assess potential root pathology. Choi and colleagues reported a positive predictive value of 100% for detection of medial root tears and a 93% sensitivity and 100% specificity on a 3T MRI scanner. De Smet and Mukherjee reported an MRI specificity of 88% for lateral meniscus tears and 86% for medial meniscus tears and a sensitivity of 83% for lateral meniscus root tears and 97% for medial meniscus tears using a 1.5T MRI scanner. Lee and colleagues reported that the use of T2-weighted sequences increases diagnostic specificity and sensitivity when diagnosing meniscus root tears. Other signs of meniscus root tear to look for on the MRI are meniscus extrusion, subchondral bone marrow edema, and the “ghost” sign. It has been reported that medial meniscus extrusion 3 mm or greater at the level of the medial collateral ligament shows a strong correlation with a meniscus root tear, but should not be used as the only diagnostic criterion. Subchondral edema can be an indirect expression for a dysfunctional meniscus and is often seen when a root tear is present.
The “ghost sign” (shown in Fig. 3 ) is a characteristic finding for meniscus root tear and is radiologically defined as a sudden absence of a well-defined meniscus structure in the sagittal plane on the MRI.
Outcomes of meniscus root repairs
Treatment of meniscal root tears has transitioned into the favoring of preservation of the meniscus by various root repair techniques, as opposed to treatment by meniscectomy. Although meniscectomy procedures have been shown to significantly improve from preoperative conditions in both the International Knee Documentation Committee (IKDC) and Lysholm, these subjective measures were superior with repairs, in addition to lesser joint space narrowing and progression of OA. Furthermore, although root repair surgeries do not avert OA progression entirely, similar OA grades compared with preoperative conditions have been reported following repair techniques, in addition to slower progression of OA determined by decelerative OA changes and lesser joint space narrowing in relation to meniscectomy procedures. Lateral posterior root techniques vary from no treatment, all-inside repair, and transtibial pull-out repair, whereas medial posterior root techniques mainly consist of various transtibial pull-out and all-inside techniques. This review highlights the anatomic transtibial pull-out repair technique, which is supported by restoration of tibiofemoral joint mechanics in biomechanical models, in addition to displaying significant improvements in outcomes of both lateral and medial posterior root repairs, with age not factoring into differential outcomes.
On comparison of the transtibial pull-out repair technique versus nonoperative treatment of the lateral meniscus posterior root repair, performed in concomitance with ACL reconstruction, Pan and colleagues found that both patient groups obtained significant improvement in Lysholm and IKDC scores, but the repair group reported a reduced occurrence of the development of OA. Given the common occurrence of lateral meniscus posterior avulsions with ACL injuries, a modification of this technique to account for potential tunnel convergence was performed in a case series, in which positive outcomes were similarly reported.
To better generalize treatment protocols for posterior meniscal root tears, LaPrade and colleagues performed the anatomic transtibial pull-out technique on patients ranging from 18 to 65 years of age, who either sustained a medial or lateral posterior root tear. A significant improvement was reported for postoperative outcomes and patient satisfaction, regardless of the age or laterality of meniscal injury. The decision of meniscal root repair had previously been considered an age-related factor, although now it seems other factors should be more heavily considered, including OA grade, high BMI, or postoperative rehabilitation protocol compliance.
Detailed anatomic transtibial pull-out repair surgical technique
A radial root tear of the posterior horn of the medial meniscus (shown in Fig. 4 ) is the most common indication for a root repair, ideally within the first centimeter of the attachment of the meniscus to ensure the correct restoration of biomechanics. Also, restoration of joint loading capacity of the meniscus is essential, by ensuring the meniscus is pulled back into the joint as part of the surgical repair, recreating the cushioning effect of the medial meniscus.
