The role of the meniscus in knee stability and tibiofemoral alignment and its unique ability to turn axial joint loads into hoop stresses have become increasingly understood. With the integral role the meniscus plays in joint preservation, it is easy to surmise why there is an established association among injury and subsequent loss of meniscal tissue and progression of osteoarthritis. Meniscal root tears specifically have been reported to be of particular importance. Biomechanical studies have demonstrated root tears have similar biomechanical characteristics as meniscus-deficient knees. As our understanding of the meniscal roots and their complex relationship with tibiofemoral health has evolved, there has been a push towards developing surgical techniques to more fully reproduce native anatomy and biomechanics. This chapter is meant to be a comprehensive review on meniscal root tears and will be a valuable tool for anyone who treats these injuries.
The focus of this chapter is on both meniscus zones 1 and 5 or the anterior and posterior roots, respectively ( Fig. 16.1 ). The anterior root has a very broad attachment that is quite variable in its location. Multiple insertional patterns for the insertion have been described, including those by Berlet and Fowler, who described four distinct insertional patterns of the anterior root. Type I, the most common, is located in the flat intercondylar region of tibial plateau; type II is more medial, close to the articular tibial surface; type III, more anterior on the tibial downslope; and type IV showed no solid fixation.
LaPrade et al. performed a quantitative and qualitative analysis of the anterior root attachments that helped further define the dimensions of these attachments and their relationship to landmarks. The anterior root of the medial meniscus had a mean area of 110.4 mm 2 . This area was larger than previously reported, which was due to further characterisation of the attachment and showed that it is more complex than initially appreciated. The root had a main central dense attachment, so defined as the central attachment, which accounted for 56.3 mm 2 of the root attachment; however, it also had additional supplemental attachments. The centre of the root was anterior to the apex of the medial tibial eminence (27.5 mm), anterolateral to the closest edge of the articular cartilage, the medial tibial plateau (7.6 mm), and anteromedial to nearest edge of the anterior cruciate ligament (ACL). The so-called supplemental fibres of the anteromedial root comprised 44.7% of the root attachment.
The posterior root of the medial meniscus was qualitatively described as being posterior from the medial tibial eminence apex (11.5 mm), lateral from the articular cartilage inflection point of the medial tibial plateau (3.5 mm) and anteromedial to the PCL tibial attachment point (8.2 mm). The last relation, its proximity to the PCL tibial insertion, is of particular interest because of reported iatrogenic injuries to the posteromedial meniscal root during posterior cruciate ligament reconstructions, specifically injury to the supplemental fibres of the posteromedial root, so named the shiny white fibres. , The footprint of the posteromedial root in its entirety has been reported to range from 47.3 mm 2 to 80 mm 2 . The shiny white fibres (SWFs) are a significant portion of that insertion, with one anatomical study by LaPrade et al. characterising them as comprising 60.8% of the attachment. Other studies have described the SWFs having a more modest attachment, 38.8% of the attachment. Although the anatomical studies differ in their characterisation of the percentage of root attachments the SWFs form, it is clear that they form a large portion of the overall posterior root attachment ( Fig. 16.2 ).
These SWFs in particular are in close proximity to the posterior cruciate ligament tibial attachment ( Fig. 16.3 ). The centre of the PCL tibial attachment was a mean 7.8 mm from the SWF point (the posterolateral-most point of the SWFs). Given that the tibial tunnel in PCL reconstructions is usually 10 to 12 mm in diameter, this provides only a 2-mm cushion anteriorly for placement of the tunnel. A thorough understanding of these root attachments is integral to a safe and successful surgical procedure.
The lateral meniscus shows a much greater variety in size, shape, and thickness than the medial meniscus. On average, it is much more ‘circular’ than the medial meniscus, with its anterior root attachment being posterior to the anterior medial meniscal root and its posterior root being anterior to the posterior medial meniscal root. Because of its shape it occupies a larger portion of the articular surface than the medial meniscus (70% versus 50%).
It has long been understood that there is an intimate association between the anterolateral meniscal root attachment and the tibial attachment of the ACL. However, studies have now further allowed us to understand this relationship. LaPrade et al. found that in its entirety the anterolateral meniscal root had the largest tibial footprint, approximately 141 mm 2 , and 63.2% of the anterolateral (AL) meniscal root overlapped the ACL attachment, 88.9 mm 2 ; this was also the equivalent of 40.7% of the ACL tibial insertion. Other pertinent arthroscopic landmarks of the anterior lateral meniscal root were 14.4 mm anteromedial to the lateral tibial eminence, 7.1 mm anteromedial to the nearest edge of lateral articular cartilage, and 5.0 mm anterolateral to the centre of the ACL tibial attachment.
The posterolateral meniscal root is the most complex and varied of the four meniscal root attachments, in part because of the meniscofemoral ligaments. The posterior meniscofemoral ligament, or the ligament of Wrisberg, when present (approximately 69% of knees), originates from the posterior horn of the lateral meniscus or from its periphery and passes posterior to the posterior cruciate ligament (PCL) to reach its femoral attachment; the anterior meniscofemoral ligament, when present (approximately 74% of knees), originates from the posterior horn of the lateral meniscus and passes anterior and distal to the PCL to reach its femoral attachment. Gupte et al. showed us that the anterior meniscofemoral ligament attachment was on the inner aspect of the medial condyle of the femur, between the distal margin of the femoral attachment of the PCL and the edge of the condylar articular cartilage. This was compared with the posterior meniscofemoral ligament, which was found to attach more posteriorly, at the proximal margin of the attachment of the PCL.
Similar to the other root attachments, the posterolateral root attachment had supplemental fibres, which coursed to the posterior aspect of the lateral margin of the medial tibial eminence. The centre of the root was located 4.3 mm medial from the nearest lateral tibial plateau articular cartilage margin; 12.7 mm anterior superior and medial from the most superior margin of the PCL tibial attachment; and 10.1 mm posterior, superior and medial from the anterolateral root attachment. The overall area of the posterolateral root is by far the smallest of the four root attachments, approximately 39.2 mm 2 ( Fig. 16.4 ).
Biomechanics and Pathology
What has become increasingly clear in biomechanical studies is the role the menisci, and in particular the roots, play in decreasing axial load forces across the tibial plateau and in tibiofemoral stability. The menisci are composed of an intricate network of collagen fibres, proteoglycans and glycoproteins that allow shock absorption and the conversion of tibiofemoral axial loads into hoop stresses throughout flexion and extension. , Approximately 50% to 70% of the total weight transmitted through the knee is transmitted by either the medial or lateral meniscus. Biomechanical studies have demonstrated that loss of meniscus, via degenerative changes or meniscectomy, leads to substantially increased contact pressures. Root tears have been shown to immensely affect joint biomechanics and have been reported to lead to contact forces similar to a complete meniscectomy. This often leads to significant joint space narrowing and rapid progression of tibiofemoral osteoarthritis.
Ellman et al. were able to describe the strength of the meniscal root attachments by recreating a shear type mechanism and creating root tears. They found ultimate failure strengths for the four meniscal roots ranged from 509 N to 655.5 N, with the anteromedial (AM) root having the highest reported ultimate failure strength (UFS) and the posterolateral (PL) root having the lowest. This somewhat conflicts with Kopf et al., who had a similar experimental design and found the AL root was the strongest and the AM root was the weakest. This may be slightly because of method differences, or perhaps because of the significant variability present in AM root attachments discussed earlier in the anatomy section. However, what the two studies did concur on was that the majority of all root failure occurred directly at the bone–meniscus interface.
Ellman et al. also sought to describe the importance of the ‘supplemental attachments of each respective root because of their failure to be addressed in many operative techniques; they found that the supplemental fibres accounted for 17.6% to 47.8% of the native root attachments’ UFS. The most important of the supplemental fibres was the SWF of the medial meniscus (47.8%). Stiffness was also assessed in their study, with values ranging from 122.7 to 151.1 (AL root highest, PM root lowest). Additionally, the supplemental attachments did have an effect, with the SWF again being most important, accounting for 34.2% of the total native stiffness of the PM root.
The high UFS and stiffness values of the root attachments help to explain why root tears are a relatively uncommon occurrence and usually are associated with high-energy injury; PM root tears specifically are usually chronic in nature, with some reports stating that as many as 70% of PM root tears are chronic.
Regardless of mechanism of injury, what has become evident, as alluded to previously, is the integral role the root attachments play in dissipating tibiofemoral axial load. Marzo et al. simulated PM root tears in 11 cadaveric specimens and found that peak contact pressure in the medial compartment increased by approximately 32.3% and average contact area decreased by approximately 20%. Allaire et al. similarly assessed the peak contact pressure and contact area of the medial compartment after PM root injury and found PM root injuries to be comparable to a complete medial meniscectomy in regards to both increased peak contact pressure and decreased contact area.
Similar findings have been reported in regards to PL root tears. LaPrade et al. demonstrated in a cadaveric study that posterior horn root avulsions and adjacent radial tears led to significantly increased contact pressures in the lateral compartment compared with an intact root state. Schillhammer et al. similarly found contact pressure increased by approximately 50% in the lateral compartment after simulated PL root tears. Furthermore, similar to PM root tears, maximum contact areas were significantly decreased by approximately 32.5%.
Given that the major function of the meniscus is to dissipate axial load, these increases in contact pressure and reciprocal decreases in contact area are somewhat inherent. However, kinematic studies have also characterised the meniscal roots’ importance regarding tibiofemoral stability. Frank et al. assessed PL meniscal root tears and the associated meniscofemoral ligaments’ effect on anterior tibial translation and internal rotational stability. They found that sectioning of the PL meniscal root was found to lead to significantly increased anterior tibial translation at 30 degrees of flexion, while significantly increasing internal rotation at 75 and 90 degrees of flexion. This led them to conclude that the PL meniscal root is an important stabiliser to anterior tibial translation at lower flexion angles and internal rotation at higher flexion angles.
Allaire et al. in their landmark study also characterised more kinematic consequences of PM root tears. They found that both external rotation and lateral tibial translation were greatly increased in a PM root tear state compared with native intact PM root state. Hein et al. also helped to further explain kinematically the cause of the observed increase in tibiofemoral contact pressures in root-deficient states. They showed that medial displacement or medial meniscal root extrusion and gap formation between the root attachment site and the medial meniscal body are both significantly increased in a PM root tear state. They also demonstrated that the meniscal repair state did not fully correct medial meniscal extrusion, with resultant persistent extrusion compared with the intact state; this finding has been replicated in many studies.
Given the significant effects on contact forces and contact areas, it stands to reason that meniscal root tears can lead to significant and accelerated progression of osteoarthritis. Krych et al. examined initial diagnostic MRIs of PM root tears and compared these with follow-up MRIs at one of two time points, within 12 months (subacute) or at a time later than 12 months (chronic group). They found that although classification of the tear (based on LaPrade criteria) did not change, meniscal extrusion degree, femoral modified Outerbridge grade and tibial modified Outerbridge grade all worsened significantly in both the subacute and chronic groups. This suggests that there is radiographic evidence of worsening osteoarthritis within a year of an untreated root injury. Further, a 2019 systematic review by Hussain et al. found several studies reported a correlation between PM root tears and development of subchondral inefficiency fractures or spontaneous osteonecrosis of the knee that were previously thought to be caused by vascular insufficiency. These findings bolster the argument that these root tears can have detrimental effects to the health of the tibiofemoral joint in relatively short order.
Therefore a misdiagnosis or failure to address these injuries can lead to advanced knee osteoarthritis and the potential for earlier surgical management in the form of total knee arthroplasty (TKA). A study of 197 TKAs found that 92.8% of TKAs in patients younger than age 60 had a meniscal tear as a leading cause for the development of OA and the need for a TKA.
These studies have given us a very thorough and comprehensive understanding of the meniscal roots and their role in the kinematics and biomechanics of the native knee. Both the PL and PM root help dissipate axial loads and act as a restraint to excessive rotation and translation about the joint. Their kinematic function has helped also to further our understanding regarding the pathology of root tears.
As previously alluded to, root tears can occur in both the chronic and the acute setting. Tears in the acute setting are commonly associated with high-energy trauma; they have been reported concurrently with ACL tears and with multiligamentous injuries. The mechanism commonly described is hyperflexion and squatting, positions that athletes, plumbers, gardeners and others may have to assume on a daily basis. The posterior roots transmit more load, especially in deep flexion, than the anterior roots, and this is likely part of the explanation for posterior root tears being more common than anterior root tears.
The most common root tear is the posterior medial root tear. Literature has cited their incidence as 10% to 21% of arthroscopic meniscal repair or meniscectomies. The actual incidence is likely much higher than reported because many of these tears occur in a chronic insidious setting, and MRI has been shown to be somewhat unreliable in diagnosing root tears, with an inability to accurately identify them up to 18% of the time on the medial side and 40% of the time for the lateral root. Missing these injuries can be extremely detrimental because of the described kinematic and biomechanical effects on the knee. Risk factors can be helpful for assessing for PM root tears; some such risk factors have been described in studies and include female sex, increased age and body mass index (BMI) and decreased sports activity level.
The epidemiology and pathology of the PL meniscal root are quite different than the PM root, and this has been thought to be due to anatomy as previously discussed. The PL root is much more mobile than the PM root because of fewer meniscocapsular attachments. The hypothesis is that this mobility leads to less stress encountered on the PL root than the PM root. Chronic instability, such as that associated with ACL insufficiency, has been found to affect PL roots far less than PM roots. Although chronic instability has been found to be less associated with PL root injury, acute ACL injury has been found to have a very close relationship with PL root injury. The mechanism is thought to be due to the excessive anterior tibial translation associated with the injury, which concurs with biomechanical studies that show that the PL root acts to restrict anterior translation at lower levels of knee flexion. Given this is the likely mechanism associated with PL root tears, participation in sports has also been found to be a common risk factor, with studies reporting 70% to 87% of PL root tears being associated with sports and specifically pivot-contact sports. A 2019 study by Praz et al. assessing the epidemiology of PL root tears in 3956 patients undergoing ACL reconstruction found that 6.6% of patients had a concomitant PL root tear; independent risk factors for PL root tear were sports injury mechanism and associated medial meniscal tear.
Much less is known regarding anterior root tears, and there are very limited data regarding their injury patterns or prevalence. Mobility of the roots again seems to be related because they have been reported by Thompson et al. to be much more mobile than the posterior roots. Some studies have discussed prevalence of anterior horn tears; however, the few that discuss anterior root tears specifically describe these injuries as largely as iatrogenic in nature or caused by variant anterior root attachments such as those discussed in the anatomy subsection. ,
As previously noted, root tears are described as avulsion injuries of the bony meniscal attachments or radial tears within 1 cm of those bony attachments. A number of classification systems have been proposed and used in diagnosing these injuries. West et al. described an arthroscopic classification system for PL root tears based on three typical tear patterns; type I was a root avulsion, type II tears were isolated radial split tears within 1 cm from the root insertion and type III injuries were complex root tears with radial and longitudinal components. Ahn et al. described PL root tears by tear morphology: type I, oblique flap; type II, T shape; type III, longitudinal cleavage; or type IV, chronic inner loss. Petersen and Forkel described PL root tears based on location and involvement of the meniscofemoral ligaments: type I, an avulsion injury of the PL root with intact meniscofemoral ligaments; type II, a radial tear between the root and the origin of the meniscofemoral ligaments with the meniscofemoral ligaments being intact; and type III, a complete detachment of the PL horn with associated meniscofemoral injury.
The most accepted classification scheme was described by LaPrade et al. and largely focused on location of the lesion and tear morphology. They assessed 71 meniscal root tears in 67 patients at the time of arthroscopy and found five distinct tear types. Type 1 are partially stable root tears. Type 2, the most common, are radial tears within 10 mm of the bony attachment, subdivided into 2A, 0 to <3 mm; 2B, 3 to <6 mm; and 2C, 6 to <9 mm. Type 3 tears are bucket handle tears with a complete root detachment, type 4 are complex oblique tears with a complete root detachment extending into the root, and type 5 are bony avulsion fractures ( Fig. 16.5 ).
Most of these classification systems were devised via arthroscopic analysis, which helps to explain the complexity involved with diagnosing these injuries clinically and radiographically. These tears can present subtly and be quite difficult to diagnose accurately before surgery. However, a thorough physical examination and set radiographic pearls can aid in diagnosis to help avoid missing these injuries.
Early diagnosis of meniscal root injuries is integral to optimal long-term outcome after meniscal root injury. However, clinical diagnosis has been shown to be very difficult because of the variety of signs and symptoms patients may present with. Diagnosis of PM root tears specifically can be difficult given the chronic nature of many of these injuries. Commonly, patients with root tears complain of joint-line pain; however, mechanical symptoms are much less common. In a review of 21 patients with PM root tears, only 14.3% complained of knee locking and 9.3% complained of knee giving way. Furthermore, patients’ histories are at times compromising because they often cannot recall any inciting traumatic event leading to their injury.
Physical examination is still imperative because there are still some common clinical findings, most notably posterior knee pain with deep flexion and joint line tenderness. Kim et al. described these two findings as being present 66.7% (pain with deep flexion) and 61.9% (joint-line tenderness) of the time in their cohort; they also found a positive McMurray test in 57.1%. However, joint effusion was far less sensitive in this cohort, with only 14.3% of patients presenting with an effusion. Seil et al. described a novel clinical manoeuvre in which they placed patients in full knee extension and applied a varus load, and this manoeuvre reproduced meniscal extrusion that was palpable on examination.
PL root tears are also somewhat difficult to assess in clinic, however, for a vastly different reason. Because a majority of these injuries are traumatic and associated with other ligamentous injuries, it is difficult to isolate and ascertain the root integrity from the history and physical examination alone. On physical examination, patients with anterior translation excessive to what would be considered normal with a suspected isolated ACL injury should warrant further investigation regarding the integrity of the PL root.
Plain films are less revealing in work-up for root injuries; however, they are not without merit. Standing AP views and PA flexion views can give some clues as to a root injury. Significant unilateral joint space narrowing, especially in younger patients or patients without other tell-tale radiographic signs of osteoarthritis, could be a sign of meniscal extrusion and acute injury because of a root tear. Plain films can also be extremely useful in work-up for meniscal root injury in regards to assessing the overall health of the tibiofemoral joint. The extent of osteoarthritic changes will affect treatment options and the expected outcome for patients with meniscal root injuries.
MRI remains the gold standard imaging modality for diagnosis of root injuries. However, even MRI historically has been inconsistent. One study with a cohort of 67 patients with arthroscopically confirmed PM root tears found that those root tears were only demonstrated in 72.9% of the MRIs.
Visualisation of the root attachments themselves can be difficult via MRI. T2-weighted imaging is usually best for viewing the menisci; however, there is variability regarding which cuts are optimal ( Fig. 16.6 ). The PM root has been described to be most easily seen on two consecutive coronal cuts. , Others have found axial imaging to be the most sensitive and specific. The PL root and PL root tears on the other hand have been described as being best seen on a combination of sagittal and coronal images. De Smet et al. described the use of three distinct coronal and three distinct sagittal images for diagnosis of PL root tears and found a sensitivity of 93% and a sensitivity of 89%. They also stated presence or absence of an associated ACL tear did not affect diagnostic accuracy. MRI studies have also found PL root tears are present in only 2.9% of knee injuries; however, PL root tears were present in 8% of ACL tears. Another study found lateral root tears present in only 1% of noninjured ACLs; however, 9.8% of ACL-injured knees had PL root injuries. This should be understood in the context of the difficulty of lateral meniscal root tear diagnosis on MRI as described by Krych et al.