The Meniscus and Meniscal Scaffolds for Partial Meniscal Replacements

The meniscus is the most common damaged structure of the knee, accounting for almost one million cases of knee surgeries performed annually in the United States alone. A complete meniscectomy (complete meniscus removal) was the most common procedure performed in 1889 and was the standard procedure in the next 80 years. However, follow-up radiographic studies from the late 1960s to 1980s reported a high frequency of post-meniscectomy osteoarthritis of the knee. The meniscus functions to transmit load, absorb shock, stabilize the knee joint and nourish the joint. A complete integrity of the meniscus is crucial in maintaining the normal biomechanics of the knee and preventing the onset of premature or traumatic osteoarthritis. 3D Printing of silicone allows arthroscopic replacement of damaged menisci, either totally or partially, enabling the patient to return to work and sports almost instantaneously after surgery. This review summarizes the meniscal structure, biomechanical properties, meniscal lesions, the characteristics and clinical outcomes of various biodegradable synthetic and biological meniscal scaffolds. DOI: 10.29011/2576-9596.100037 Meniscal Structure and Biomechanical Properties Meniscal Anatomy The menisci are a pair of fibrocartilaginous cushions which sits on the tibial plateau in the knee joint. They act as knee cushions which transmit body weight evenly across the knee joints, thus minimizing contact stresses between femur and tibia and damages to the articular surfaces. Meniscal injuries predisposed the knees to developing premature osteoarthritis (Figure 1). Figure 1: Anatomy of the meniscus. The meniscus is divided into 3 zones, the outermost vascular red-red zone, middle red-white zone and the innermost avascular white-white zone. Cells are spindled-shaped in the outermost redred zone while chondrocyte-like in the innermost white-white region. The meniscus obtains its limited blood supply from the perimeniscal capillary plexus within the synovial and capsular tissues of knee. These plexus, extending for one to three millimeters over the articular surfaces of menisci, are branches of the inferior and superior branches of the lateral and medial geniculate arteries. The vascular supply to meniscus is age dependent. In adult, tears which occur at the most vascularized, peripheral 3 mm of the menisci are most amenable to repair and cellular regeneration, as opposed to the generally avascular tears, greater than 5 mm from the menisci-synovial junction, which are not reparable. For both the medial and lateral menisci, the vascular penetration is about 10-30% (Figure 2). Citation: Luis E, Song J, Yong WY (2018) The Meniscus and Meniscal Scaffolds for Partial Meniscal Replacements. Sports Injr Med: JSIMD-137. DOI: 10.29011/25769596.100037 2 Volume 2018; Issue 03 Sports Injr Med, an open access journal ISSN: 2576-9596 Figure 2: Regional variations in vascularisation and cell population of the meniscus. Meniscal Composition and Cell Characteristics The meniscus has a highly heterogenous mix of ECM and cellular distribution. Meniscal ECM is categorized by region. More than 80% of the red-red region is composed of type I collagen by dry weight and the remaining comprises collagen types II, III, IV, VI and XVIII. In the white-white region, total collagen comprises 70% of dry weight, with collagen types II and Types I accounting for 60% and 40%, respectively (Figure 2). Meniscal Lesions and Development of Knee OA Meniscal injuries eventually can lead to knee OA and knee OA induces further meniscal tears, thus propagating the vicious cycle. An injured meniscus triggers the synovium to release various inflammatory cytokines, which further induce degenerative changes within the matrix body and cause meniscal extrusion from the knee joint. These extrusions increase the stress on the tibial cartilage and further aggravate the injury [1]. Similarly, the collagen fibers are arranged randomly in the most superficial region, radially in the middle layer and circumferentially in the innermost layer. The circumferential fibers provide hoop-stress against the compressive loads exerted across the knee-joint (Figure 3). The circumferentially arranged fibers have a tensile strength of 50 to 300 MPa, while the radially arranged fibers have a tensile strength of 3 to 70 MPa. Figure 3: Ultrastructure of collagen fibers within meniscus. Meniscal Injury Patterns All meniscal lesions can be classified into eight categories according to the Casscells classification, namely i) Vertical longitudinal (bucket handle, ii) Vertical transverse (radial), iii) Horizontal tear (cleavage), iv) Oblique tear (flap), v) detachment of meniscal horns, vi) complex tear, vii) Degenerative and viii) miscellaneous (discoid), However, for therapeutic purposes, the meniscal injuries can simply be classified clinically into peripheral meniscal lesions and central avascular lesions. The pattern of meniscal lesions is also age-dependent. Traumatic injuries in the young and athletes usually result in longitudinal tear patterns or vertical radial full thickness tears pattern. These tears usually occur in the vascular red-red zones and are therefore more amenable to repair. On the contrary, degenerative tears in the elderly are usually horizontal, intra-substance and complex in nature. These lesions are less amenable to repair [2]. Meniscal Treatment Options Conventional treatments include meniscal repairs, partial meniscectomies, total meniscectomies, partial meniscal substitute replacements, porous meniscal implants or total artificial meniscal replacements, allograft and autograft meniscal transplantations [3-8]. A partial meniscectomy is usually performed for irreparable or degenerative meniscal lesions. However, the procedure reduces the contact area between the femoral condyle and tibial platform, thus predisposing the knee to osteoarthritis [9]. Consequently, more emphasis is placed on meniscal repair and reconstruction techniques. Repair procedures range from inside-out, outside-in and all inside techniques [10]. Meanwhile, reconstructive strategies restoring meniscal functions such as meniscal allografts, Small Intestinal Submucosa (SIS) implants and autogenous tendon grafts have also been experimented [11-13]. The first free meniscal allograft transplantation, performed by MIlachowski and Wirth in 1984, reduces pain and improves knee functions in relatively young patients after a short followup. However, its chondroprotective effects have not been proven. Concerns of disease transmission, graft shrinkage and deteriorating material properties have hindered its widespread use [14-16]. Similarly, the SIS and autogenous tendon grafts have not obtained satisfactory results [17,18]. Mechanical Properties and Force Transduction of Meniscus From table 1, the posteromedial region of the meniscus has Citation: Luis E, Song J, Yong WY (2018) The Meniscus and Meniscal Scaffolds for Partial Meniscal Replacements. Sports Injr Med: JSIMD-137. DOI: 10.29011/25769596.100037 3 Volume 2018; Issue 03 Sports Injr Med, an open access journal ISSN: 2576-9596 the lowest compressive properties and tensile modulus. These results, together with the relative immobility of the posterior horns of the meniscus as shown above, would explain the frequent occurrences of most clinical traumatic tears in these regions. Study Compressive Aggregate Modulus MPa Tensile Properties Stiffness MPa Medial Superior Human Meniscus Sweigart et al. Circumferential fibers (lateral meniscus) Anterior 0.15 +/0.03 124.58+/-39.51 Central 0.10 +/0.03 91.31 +/23.04 Posterior 0.11 +/0.02 143.73+/-38.91 Medial Inferior (Medial meniscus) Anterior 0.16 +/0.05 106.21+/-77.95 Central 0.11 +/0.04 77.95+/-25.09 Posterior 0.09 +/0.03 82.36+/-22.23 Table 1: Posteromedial region of the meniscus has the lowest compressive properties and tensile modulus. The Effect of Knee Flexion on Knee Contact Area and Joint Forces By occupying 60% of the total contact areas between femur and tibia, the menisci distribute forces evenly the underlying articular cartilage, thus minimizing point contact. The contact area decreases 4%, with simultaneous increase in contact forces, for every 30 degrees of knee flexion. The menisci bear 40 to 50% of the total transmitted load across the knee joint in extension and 85% of the total transmitted load across the knee joint at 90 degrees flexion. In full knee flexion, the lateral meniscus and medial menisci transmit 100% and 50% of the load respectively. The meniscal motion allows maximal congruency during knee flexion and helps to protect the menisci from injury. In the study by Vedi V et al. [19], meniscal movement was studied using a dynamic MRI: With weight bearing, the anterior horn of medial meniscus moves through a mean of 7.1 mm, the posterior horn moves 3.9 mm and 3.6 mm of mediolateral radial displacement. With weight bearing, the anterior horn of the lateral meniscus moves 9.5 mm, the posterior horn moves 5.6 mm and there was 3.7 mm of radial displacement. This relative immobility of the posterior horn of the medial meniscus may account for its susceptibility to injuries and tears. With sufficient stress, usually rotatory nature in a weight bearing, flexed knee, either meniscus may be torn insubstance or from its peripheral attachment [20,21]. Being a secondary knee stabilizer, the menisci confer some stability to the normal knees and especially the ligament-deficient knees. The menisci are connected anteriorly by the transverse ligament and attached peripherally to the capsular ligament on the medial and lateral side of the knee joint and its horns to the interarea of the tibia. The Effect of Meniscectomy (Removal of Meniscus) on Contact Forces Both Paletta and Kurosawa et al. reported a 50% decrease in total contact area and corresponding 200 % to 300% increase in peak local contact load, following a total meniscectomy. Correspondingly, partial (16to34%) meniscectomy lead