Meniscal Lesions: A Global and Latin American Perspective on Knee Biomechanics, Pathophysiology, and the Quest for Total Meniscal Replacement

Abstract:

Meniscal lesions are a pervasive cause of knee dysfunction, significantly affecting global health, with a pronounced impact in Latin America due to rising sports injuries and an aging population(Chambers & Chambers, 2019). This paper provides an in-depth review of the knee’s physiology and biomechanics, explores the different types of meniscal lesions in relation to knee biomechanics, and discusses the meniscus’s anatomy, histology, and embryology. The current options for total meniscal replacement are critically evaluated, focusing on their limitations and the reasons behind the lack of a fully viable meniscal prosthesis. The paper concludes with a call for heightened research and innovation to meet the pressing clinical need for a durable meniscal replacement.

Introduction:

Global Burden of Meniscal Lesions

Meniscal injuries represent one of the most frequent causes of knee pain and disability worldwide, contributing to an increased risk of osteoarthritis, decreased mobility, and reduced quality of life (Englund et al., 2008). The prevalence of meniscal tears is particularly high among athletes and the elderly, with estimates suggesting that over one-third of the population over 50 may have some form of meniscal pathology, often asymptomatic until an acute injury exacerbates the condition (Roos et al., 1998). In Latin America, the incidence of meniscal lesions has been on the rise, driven by increased participation in high-impact sports and a growing elderly population. The region also faces unique healthcare challenges, including disparities in access to advanced surgical interventions and the availability of rehabilitation services (Vega et al., 2016).

Focus on Latin America

In Latin America (LATAM), the burden of meniscal injuries is compounded by socioeconomic factors that limit access to advanced medical care. For instance, the availability of arthroscopic surgery, which is the gold standard for meniscal repair, varies widely across the region. In rural areas, patients may have to travel long distances to reach specialized centers, delaying treatment and potentially worsening outcomes (Vega et al., 2016). Additionally, there are several limitations for patients to access one adequate healthcare system that can transfer for an orthopedic evaluation patients from rural zones in many LATAM countries.

The reality of many rural clinics and small hospitals in Latin America is the lack of adequate resources to help detect an early lesion in patients.  There is strong evidence of sports induced lesions of the meniscus in literature, but there are low references in working related meniscal lesions in our countries. Also, cultural factors such as a reluctance to undergo surgery or engage in prolonged rehabilitation, can also impact treatment outcomes. This highlights the need for tailored approaches to meniscal injury management in our region, requires unique vision, management to improve the challenges we face.

Certain clinical, morphologic, and topographic characteristics of meniscal lesions are associated with an increased risk of difficult or erroneous diagnosis. The factors contributing to missed meniscal tear in the setting of ACL injury are the performance of the MRI assessment shortly after the traumatic event, small ofthe tear(Fernandez Alvarez, n.d.; Lecouvet et al., 2018)

Physiology and Biomechanics of the Knee:

Knee Joint Structure

The knee joint is a pivotal structure in the human body, providing support, stability, and mobility. It comprises three main components: the femur, tibia, and patella, all connected by ligaments and cushioned by cartilage.

The menisci are a pair of nonsymmetric semicircular (crescent moon–shaped) fibrocartilaginous structures attached to the tibial plateau at their anterior and posterior ends. There is an attachment to the tibia through the menisco-tibial ligament. They are composed mostly of type I collagen bundles with a complex network of fibers varying in orientation to provide structure and support. Most of the fibers are circumferential and provide resistance to compression. Radially oriented fibers resist longitudinal tearing between the circumferential fibers.  This structure develops over the first 10 years of life and at that point is similar in content and structure to adult menisci. During development, blood and nutrients are supplied from the periphery through the entire width of the meniscus until the 9 month postpartum, at which point the inner third will be avascular. Through the rest of development this avascular zone will increase until only the peripheral 10% to 30% is vascular. The argument to repair more central tears in the pediatric meniscus stems directly from this idea that it is relatively (Chambers & Chambers, 2019)

Meniscus Anatomy

The medial meniscus measures approximately 45.7 mm in length and 27.4 mm in width [1, 6]. Its width decreases from posterior to anterior and has been reported to range from 12.6 to 17.4mmin the posterior third, 9.3 to 12.2mmin the middle third, and 7.6 to 9.0 mm in the anterior third of the medial meniscus [7, 8, 9••, 10]. The thickness of the medial meniscus is relatively consistent from anterior to posterior and ranges from 5.2 to 6.9 mmin thickness over the entire meniscus [4, 7, 10]. The medial meniscus can also be divided into five anteroposterior zones (Fig. 1) [11–13]. This includes the anterior

root attachment (zone 1), the anteromedial zone between the posterior border of the anterior root and the anterior border of the superficial medial collateral ligament (zone 2A and 2B), the portion of the meniscus that is adjacent to the superficial medial collateral ligament (zone 3), the posterior horn (zone 4), and the posterior root (zone 5). Of clinical note, zone 4 is the most common location for meniscal tears and is the location where meniscal repair is most commonly performed [4, 9••, 14, 15]. Anatomic studies have demonstrated that the medial meniscus covers between 51 and 74% of the medial tibial plateau surface area [2, 16, 17]. A study by Bloecker et al. attempted to examine this property using MRI and found that the medial meniscus covered 50% of the medial tibial plateau [18].

The lateral meniscus measures approximately 35.7 mm in length and 29.3 mm in width [6]. Unlike the medial meniscus, the width of the lateral meniscus is relatively consistent across the entire structure for a given specimen [4]. It has been reported to range from 9.8 to 12.0 mm in the posterior third, 10.0 to 12.5 mm in the middle third, and 10.0 to 11.9 mm in the anterior third of the meniscus [7, 10, 19]. The lateral meniscus is thinner on the anterior third, where it ranges from 3.8 to 4.73 mm in thickness [7, 10, 19]. The thicker middle third and posterior third range from 5.9 to 6.5 mm and 5.3 to 6.2 mm in thickness, respectively [7, 10, 19]. The lateral meniscus can be classified into six zones based on anteroposterior location (Fig. 1) [13]. These include the anterior root (zone 1), the anterolateral zone between the anterior root and the anterior border of the popliteal hiatus (zones 2A and 2B), the popliteal hiatus (zone 3), the posteroinferior popliteomeniscal fascicle (zone 4), the ligamentous zone (zone 5), and the posterior root (zone 6). Anatomic studies have demonstrated that the lateral meniscus covers between 75 and 93% of the lateral tibial plateau surface area [2, 16, 17]. The MRI study by Bloecker et al. found that the lateral meniscus covered 59% of the lateral tibial plateau [18].

Biomechanical Forces and Meniscal Function

The knee joint is subjected to complex biomechanical forces during various activities, including walking, running, jumping, and pivoting. The menisci play a vital role in modulating these forces, particularly during weight-bearing activities. They increase the contact area between the femur and tibia, reducing the stress on the articular cartilage (Ghadially, 2010). Meniscal injuries often occur due to excessive rotational forces or direct trauma, leading to tears that can vary in type and severity. Radial tears, for instance, disrupt the circumferential collagen fibers that provide tensile strength, significantly compromising the meniscus’s ability to absorb shock (McDermott & Amis, 2006).

Biomechanics

The load distribution properties of the meniscus allow for greater contact area and decreased contact pressures within the tibiofemoral joint [67]. This is particularly important in the lateral compartment, where 70% of the load is transmitted to the lateral meniscus, compared to 50% in the medial compartment, due to the larger role of the lateral meniscus in joint congruity [67]. In addition to Ahmed and Burke’s study, which demonstrated a 50– 70% decrease in contact area and subsequent increase in contact pressures following medial meniscectomy, numerous recent laboratory studies have further demonstrated the effects of various meniscal pathologies on knee joint biomechanics, kinetics, and kinematics [68].

Biomechanical Effect of Meniscus Pathology Vertical meniscal tears run parallel to the circumferential ECM fibers and are less likely to disrupt the meniscus’ biomechanical function as these tears typically do not compromise the meniscus’ ability to convert axial loads into hoop stresses. For example, a cadaveric study from Goyal et al. found no difference in contact pressures between specimens with an intact lateral meniscus compared to specimens with an artificially created vertical tear [69].

While this may be true in the body of the meniscus, vertical tears in the horns of the meniscus may be more problematic. A recent finite element analysis study by Zhang et al. demonstrated that vertical tears at the horns of the menisci increase peak compressive and shear stresses on the menisci, cartilage, and subchondral bone in both static and dynamic-flexion simulations [70]. The authors reported more meaningful biomechanical alterations following tears in the medial meniscus and tears in the posterior horn. This is further corroborated by Chen et al.’s cadaveric study, which demonstrated impaired contact pressure after longitudinal tearing of the medial meniscus [71].

Like vertical tears, horizontal cleavage tears do not disrupt the circumferential collagen fibers. However, horizontal cleavage tears do have a higher correlation with altered biomechanics. In a 2017 cadaveric study, Beamer et al. reported a 70% increase in contact pressures across all flexion angles [72]. Furthermore, when managing this tear pattern with partial meniscectomy, prior studies have demonstrated that resection of one medial meniscal leaflet increased contact pressures by 33–46%, while resection of both leaflets increased pressure by 75–79% [73, 74].

Radial tears are tears that extend perpendicularly across the circumferential collagen fibers and can disrupt the meniscus ability to convert loads into hoop stresses.

Large radial tears and root tears can be functionally equivalent to a total meniscectomy because they fully disrupt the meniscus’ circumferential collagen fibers leading to functional failure of the meniscus [75]. In the setting of partial radial tears, the meniscus retains a degree of its inherent biomechanical function. Cadaveric studies have demonstrated that partial tears up to 60–66% of the meniscal width have little/no impact on the meniscus load dissipating properties [76, 77].

Like radial tears, meniscal root tears are also functionally equivalent to a total meniscectomy. The meniscal roots anchor the menisci to the tibia in order to prevent

extrusion and facilitate meniscal function [4]. Similar to radial tears, root tears also leave the meniscus unable to convert axial loads into hoop stresses [75]. This tear pattern leads to total functional failure of the meniscus and places the knee joint at high risk for accelerated degenerative changes and impaired biomechanics/kinematics. A controlled cadaveric study by Allaire et al. demonstrated that meniscal root tears allow for increased lateral tibial translation (LTT) and increased knee adduction angles (KAA) [26••]. These findings appear to translate to in vivo kinematics. A study by Marsh et al. demonstrated that medial meniscus root dysfunction significantly increased LTT during level walking, decline walking, and squatting [78]. Additionally, a study by Ishii et al. used inertial motion sensors to assess gait in patients with meniscal root dysfunction, reporting a positive correlation between the magnitude of increment in meniscal extrusion during weight-bearing and increased knee lateral thrust [79]. Varus thrust has long been recognized as risk factor for progression of medial compartment cartilage lesions and progression of knee osteoarthritis [80, 81]. A varus thrust gait pattern is the prime feature of one of four distinct gait patterns for severe knee osteoarthritis as described by Leporace et al., which the authors deem as a more significant feature than peak joint angles [82].

Impact on In Vivo Knee Kinetics

The knee adduction moment (KAM) is a well-established kineticsmeasure that correlates withmedial compartment loading of the knee during weight-bearing activities [83, 84]. KAM is a product of both the ground force acting on the knee joint during the stance phase and the perpendicular distance that this force acts from the center of the joint. A larger KAM leads to increased varus thrust and results in increased medial joint loads, which are directly correlated withmedial compartment articular cartilage thickness and progression of knee osteoarthritis [85].

The meniscus plays an important role in normalizing the KAM experienced within the knee joint. Thorlund et al. assessed 3-D gait analysis of 23 patients with a medial meniscus tear without radiographic knee osteoarthritis, before and 1 year after partial meniscectomy. Despite a significant improvement in Knee Injury and Osteoarthritis Outcome Scores (KOOS), there was a consistent increase in peak KAM after meniscectomy in comparison to the contralateral limb [86•]. Furthermore, Hall et al. reported an increase in the peak knee flexion moment (KFM) in a 2-year follow-up of partial meniscectomies [87]. The combination of these studies further implicates meniscectomies in the progression of osteoarthritis, as an increased KFM has been linked to cartilage wear in early osteoarthritis, while an increased KAM is strongly associated with more severe osteoarthritis [85].

The same pattern of abnormal kinetics was reported in different studies comparing partial meniscectomy and meniscal repair in the setting of an ACL reconstruction.

Capin et al. assessed an athletic cohort of patients after completing full rehabilitation of an ACL reconstruction [88•]. The authors used a validated electromyography-driven musculoskeletal model and classified groups according to concomitant medial meniscus treatment. Subjects in the partial meniscectomy group demonstrated a higher peak KAM in the surgical limb relative to the contralateral limb; however, this increased KAM was not observed in the surgical limb of the intact meniscus and the meniscal repair groups. Estimated medial tibiofemoral compartment contact forces were also increased in the meniscectomy group in comparison with the other two groups at 2 years of follow-up [89].

Impact on In Vivo Knee Kinematics The meniscus plays a critical role in physiologic in vivo knee kinematics. There is ample evidence demonstrating the alteration in knee kinematics when there is suboptimal meniscal function. A study by Zhang et al. examined gait kinematics in ACL deficient patients with or without meniscal injures [90].

The authors demonstrated that meniscal injuries impaired physiological kinematics and this alteration of normal knee function was dependent on location of the meniscal tear.

Patients with concomitant medial and lateral meniscus tears had abnormal sagittal excursion, particularly anterior tibial translation (ATT), while patients with an isolated medial meniscus tear showed a significant increase in lateral tibial translation (LTT). Hosseini et al. reported similar findings during stair-climbing through a dynamic fluoroscopy-based assessment [91]. A subsequent study examining 3-D gait analysis demonstrated significantly increased axial plane rotation angles during the entire gait cycle in patients with concomitant unstable meniscus tears versus isolated ACL tears [92].

Similarly, a separate study by Ren et al. demonstrated significantly increased external tibial rotation during the pre-swing phase in ACL-deficient patients with concomitant medial meniscus posterior horn tears [93].

There is already convincing data indicating that the benefits of meniscus repair also translate to the in vivo setting and mitigate abnormal kinematics following a knee injury [89, 94]. For example, Wang et al. assessed the kinematics of a total of 32 patients who underwent ACL reconstruction and concomitant medial meniscus treatment at 2 years of followup [94]. The authors divided their patient cohort into repair and partial meniscectomy groups and compared these groups to a control group composed of 20 healthy participants.

Patients in the partial medial meniscectomy group walked with an increased knee adduction angle (KAA) during early and mid-stance phases of gait and had an increased tibial external rotation during early stance. This difference was not observed betweenmeniscus repair and healthy control groups. Discoid lateral menisci have also been shown to result in altered kinematics. Gait analyses by Li et al. demonstrate significantly lower peak knee flexion angles (KFA) during stance and swing phases and reduced adduction-abduction angles during gait in patients with a discoid meniscus [95]. A similar pattern of limited knee excursion has been further reported in the literature when comparing discoid meniscus groups versus healthy controls as well as when comparing symptomatic discoid menisci versus asymptomatic discoid menisci [92, 96].

Healthy knees experience external rotation during stance and internal rotation during the swing phase, while the discoid meniscus groups demonstrated decreased internal rotation during the swing phase. Lin et al. hypothesized that the resulting non-physiological horizontal shear stress initiated the destruction of the meniscus [96].

Types of Meniscal Lesions

Meniscal lesions can be classified based on their location, pattern, and extent of the tear. Common types include radial tears, horizontal cleavage tears, flap tears, complex tears, and bucket-handle tears. Each type has distinct biomechanical implications. For example, radial tears disrupt the hoop stress that helps distribute load, leading to increased focal stress on the cartilage and potentially accelerating degenerative changes (Bonnin et al., 2012). Horizontal cleavage tears, often degenerative, split the meniscus into upper and lower segments, causing pain and mechanical symptoms (Metcalf, 2004). Understanding the biomechanics behind these tears is crucial for determining the appropriate treatment strategy.

Anatomy, Histology, and Embryology of the Meniscus:

Anatomical Features of the Meniscus

The meniscus is a fibrocartilaginous structure with a distinct regional anatomy. It consists of an outer vascular zone (red zone), which has a rich blood supply, and an inner avascular zone (white zone), where blood supply is limited (Arnoczky, 1999). This vascular distribution has significant implications for healing potential. Tears in the red zone are more likely to heal spontaneously or respond well to surgical repair due to the robust blood supply, while tears in the white zone often require more aggressive interventions, such as meniscectomy, due to the limited healing potential (Mow & Huiskes, 2005).

Histological Composition

Histologically, the meniscus is composed primarily of type I collagen, which provides tensile strength, along with proteoglycans, glycoproteins, and other matrix proteins that contribute to its viscoelastic properties (Fithian et al., 1990). The collagen fibers are arranged in a circumferential pattern to resist hoop stresses and radially oriented fibers to resist shear forces. This complex architecture allows the meniscus to withstand the mechanical demands placed on the knee joint during various activities (Makris et al., 2011). The extracellular matrix of the meniscus is also rich in water, which helps to absorb shock and distribute load across the knee joint.

Embryological Development

Embryologically, the menisci develop from the interzone mesenchyme of the fetal knee, with differentiation into fibrocartilage occurring by the 8th week of gestation (Sweigart & Athanasiou, 2001). The developmental pathways involved in meniscal formation are crucial for understanding the tissue’s complex structure and function. Insights into these processes have informed tissue engineering approaches, aiming to replicate the meniscus’s unique properties. However, replicating the exact biomechanical and biological functions of the native meniscus in a synthetic or engineered construct remains a significant challenge (Ghadially, 2010).

Meniscal Replacement Options:

Current Surgical Techniques

The primary goal of meniscal surgery is to preserve as much of the meniscus as possible while restoring its function. Techniques such as meniscectomy (partial or total), meniscal repair, and meniscal transplantation are commonly employed depending on the type and severity of the lesion (Englund et al., 2008). Partial meniscectomy, where only the damaged portion of the meniscus is removed, is the most common procedure but can lead to early-onset osteoarthritis due to altered joint mechanics (Roos et al., 1998). Meniscal repair is preferred when possible, particularly for tears in the vascular zone, as it preserves meniscal tissue and maintains its biomechanical functions (Petersen & Laprell, 2000).

Meniscal Allografts

Meniscal allograft transplantation is a treatment option for patients with symptomatic meniscal deficiency. The procedure involves the implantation of a donor meniscus into the recipient’s knee, aiming to restore joint function and alleviate pain (Verdonk et al., 2006). Although clinical outcomes have been promising, with improvements in pain and function reported in most patients, the procedure is not without risks. Complications include graft shrinkage, immune rejection, and limited long-term durability (Verdonk et al., 2012). Moreover, the availability of donor tissue and the cost of the procedure limit its widespread application, particularly in resource-constrained settings like Latin America (Vega et al., 2016).

Synthetic Meniscal Implants

In recent years, synthetic meniscal implants, such as the Collagen Meniscus Implant (CMI) and the Actifit scaffold, have been developed as alternatives to allografts. These devices aim to provide a scaffold that supports tissue ingrowth and gradually integrates with the host tissue (Verdonk et al., 2012). However, while these implants have shown some success in preclinical and early clinical trials, they often fail to replicate the biomechanical properties of the native meniscus, leading to suboptimal long-term outcomes (Fischenich et al., 2015). Issues such as poor integration, implant degradation, and insufficient load-bearing capacity remain significant challenges.

Why a Complete Viable Meniscal Replacement Has Not Yet Been Developed:

Technical Challenges

Developing a complete viable meniscal replacement involves overcoming several technical challenges. The meniscus’s complex architecture, including its regional variation in collagen fiber orientation, vascular supply, and cellular composition, is difficult to replicate in a synthetic or tissue-engineered construct (Makris et al., 2011). Current approaches have struggled to match the native meniscus’s biomechanical performance, particularly in terms of load distribution, shock absorption, and long-term durability (Fithian et al., 1990). Additionally, the meniscus’s ability to integrate with the surrounding joint tissues, essential for preventing implant failure, has been difficult to achieve with existing materials and techniques (Mow & Huiskes, 2005).

Regulatory and Economic Barriers

In addition to technical hurdles, regulatory and economic barriers have slowed the development of a viable meniscal replacement. Regulatory agencies, such as the FDA in the United States and similar bodies

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