Meniscal Ramp Lesions

Introduction

Meniscal ramp lesions have a reported incidence ranging from 16% to 24% in primary and revision anterior cruciate ligament (ACL) reconstructions. There is no consensus on the exact definition of a meniscal ramp lesion. A meniscal ramp lesion was first described in 1983 by Hamberg et al., who reported this type of lesion during open surgical repair. In 1988 Strobel characterised a particular type of meniscal injury associated with ACL rupture involving the peripheral attachment of the posterior horn of the medial meniscus (PHMM). This injury was a meniscocapsular separation at the PHMM but was of special interest because of its potential hidden location within the posterior septum, especially when the knee is near full extension ( Fig. 18.1 ). Since its original description, different anatomical locations have been proposed as the site of injury. The definition has been expanded in the literature as a longitudinal tear of the peripheral attachment of the PHMM at the meniscocapsular junction, with a length of 2.5 cm. Others have suggested that a ramp lesion involves the meniscotibial attachment of the posterior attachment of the medial meniscus and also vertical tears of the posterior medial meniscus itself, rather than only involving tears at the meniscocapsular junction. ^,

The name ramp derives from its arthroscopic appearance of a downwards ‘ramp’ when viewing the meniscocapsular junction posteromedially. This area of the posterior medial knee has a potential fold in the capsule as it attaches posteriorly to the medial meniscus, and thus forms a ‘blind spot’ during arthroscopic surgery when viewing from anterior portals ( Fig. 18.2 ). Tears may be often underrecognised if surgeons fail to assess this zone with the capsule under tension (retracting the capsule posteriorly with the use of a probe).

Thaunat et al. proposed a classification system for meniscal ramp lesions in 2016. This system was the first to allow for a comprehensive assessment of ramp lesions arthroscopically according to subjective, expert opinion. Authors described five meniscal ramp tear types: (1) meniscocapsular lesion, (2) partial superior lesions of the posterior medial meniscus, (3) partial inferior (‘hidden’) lesions of the posterior medial meniscus with meniscotibial ligament disruption, (4) complete tear at the red-red zone of the posterior aspect of the medial meniscus with meniscotibial ligament disruption and (5) double (vertical) tear of the posterior medial meniscus with meniscotibial ligament disruption.

Anatomy

The medial meniscus is a semilunar fibrocartilage structure that covers approximately 50% of the medial tibial plateau. It is broader posteriorly, measuring approximately 11 mm in width and becoming narrower anteriorly towards its anterior meniscal root attachment. ^, The meniscocapsular attachment of the PHMM has an average length of 20 mm, which corresponds with the entire length of the posterior horn, which measures on average 21 mm in length ( Fig. 18.3 ). The posteromedial capsule does not attach directly to the superior portion of the medial meniscus. It attaches approximately one-third below the superior margin of the PHMM. This anatomical description is different from the lateral meniscus and relates directly to the reported hidden location of ramp lesions. Specifically, this hidden area may be responsible for missed diagnoses of ramp tears during preoperative MRI scans and it further supports the utility of viewing the PHMM posteromedially during arthroscopy to confirm or deny the presence of a ramp lesion at the time of ACL surgery.

The posterior meniscotibial ligament attachment of the PHMM has an average length of 14 mm at its insertion of the posterior tibia. On average, the most lateral point of the meniscotibial ligament attachment on the posterior medial meniscus is 16.5 mm posterior and 7.7 mm medial to the centre of the posterior medial meniscus root attachment ( Fig. 18.4 ). Previous authors have reported that the meniscotibial ligament attachment merges with the posterior meniscocapsular attachment to form a common PHMM attachment at the most posterior point of the meniscocapsular junction ( Fig. 18.5 ). This shared attachment has been validated with histological analysis and may have direct implications regarding treatment strategies ( Fig. 18.6 ).

Pertaining to vascularity, the meniscus can be divided into three zones: red-red, red-white and white-white, designated from the outer periphery to the inner margin, respectively. These zones are often used for classifying the location of meniscal lesions according to its proposed blood supply and for decision-making treatment options (e.g., meniscus repair versus meniscectomy). The area of where ramp lesions occur is highly vascular; therefore some authors theorise that these lesions have an inherent capacity to heal without surgical repair.

Biomechanics

The menisci have a number of reported functions within the knee, including load transmission and distribution of forces, joint lubrication, cartilage nutrition, proprioception and acting as secondary stabilising structures. The medial meniscus has been reported to have an essential role in stabilising the knee in chronic ACL-deficient knees. Biomechanical studies have demonstrated the importance of the menisci for the longevity of the knee joint and the interdependence between the medial meniscus and the ACL, specifically the PHMM as a secondary stabiliser to anterior tibial translation (ATT).

Muriuki et al. described changes in tibiofemoral contact pressures after vertical tears of the PHMM compared with radial split tears. The authors concluded that vertical tears of the PHMM increased contact pressure and reduced contact area in the medial and lateral compartments, with no differences compared with a total medial meniscectomy. In 2001, Papageorgiou et al. demonstrated the biomechanical interdependence between an ACL-reconstructed graft and the medial meniscus. These authors reported increased force up to 54% in the ACL-reconstructed graft after a medial meniscectomy, further advocating the potential for increased ACL reconstruction graft failure with medial meniscal deficiency. Data suggest that medial meniscocapsular tears, when left untreated, may predispose the ACL-reconstructed knee to increased ATT and potential increased strain in the ACL-reconstructed graft, which correlate to ACL reconstruction graft failure.

The biomechanical functions of the PHMM attachments are essential because investigations have reported that meniscal deficiency is a major clinical factor to predict ACL reconstruction graft failure. DePhillipo et al. reported significant increases in ATT at 30 and 90 degrees in an ACL-deficient knee with the presence of both meniscocapsular and meniscotibial ramp lesions. These authors also reported significant increases in knee internal/ external rotation and pivot shift with ramp lesions that was not restored with an isolated ACL reconstruction but was restored with combined ACL reconstruction and meniscal ramp repair. Furthermore, these increases in knee kinematics have been corroborated by previous authors; ^{, , ,} Table 18.1 reports the maximal residual differences in knee kinematics among prior biomechanical studies assessing the effects of meniscal ramp lesions.

The intuitive theories behind inherent knee instability and meniscal ramp lesions are becoming more recognised. Biomechanical data have provided insight to the roles of both the meniscocapsular attachment and the meniscotibial attachment of the posterior medial meniscus. ^{, ,} If the superior meniscocapsular joint capsule or the inferior meniscotibial ligament is torn, this may create further knee instability with increased ATT and knee rotation. ^{, , ,} However, anatomical and histological analyses have reported that these two structures share a common PHMM attachment, and thus it is theorised that both the meniscocapsular and meniscotibial attachments may function together as an anatomical unit rather than two independent structures. A biomechanical study supports the aforementioned theory because these authors reported no significant differences in knee kinematics between a meniscocapsular-based tear and a meniscotibial-based tear in ACL-deficient and ACL-reconstructed knees. This suggest that although ramp lesions may occur in two separate locations, instead of only at the meniscocapsular junction as previously described, an inside-out repair of the PHMM may be adequate to address lesions of both structures and restore knee stability. ^,

Diagnosis