Meniscal tears are a common clinical finding in dogs. Injuries usually involve the medial meniscus and typically occur following cranial cruciate rupture. Treatment options include primary repair, total meniscectomy, partial meniscectomy, and hemimeniscectomy. The treatment of choice for meniscal tears is primary repair or partial meniscectomy, if possible, at the time of stifle stabilization. Meniscectomy inevitably leads to osteoarthritis and a perpetual cycle of articular cartilage degeneration. Ongoing research is focused on increasing the healing potential of the meniscus via improved blood supply and matrix scaffolds. In cases of severe damage, meniscal transplants or prostheses may be available in the future.
Meniscal injuries are common in cranial cruciate-deficient stifles. This article reviews the anatomy, function, pathophysiology of injury, diagnosis, treatment, and areas of current research pertaining to meniscal disease.
The medial and lateral canine menisci are C-shaped, fibrocartilaginous disks found in the stifle between the articulating surfaces of the femur and tibia. The menisci are anchored to the tibia and femur by five meniscal ligaments and to each other by a single intermeniscal ligament. The five meniscal ligaments are the cranial tibial ligaments of the lateral and medial meniscus, caudal tibial ligaments of the lateral and medial meniscus, and femoral ligament of the lateral meniscus (FIGURE 1). In addition to the ligamentous attachments, the medial meniscus is firmly attached to both the joint capsule and the medial collateral ligament. These additional attachments make the medial meniscus less mobile than the lateral meniscus. The lateral meniscus is not attached to the lateral collateral ligament and has fewer attachments to the joint capsule, particularly around the popliteal tendon.1,2
Menisci are composed of dense fibrocartilage containing collagen (type I),a proteoglycans, elastin, and chondrocytes.1,2 The meniscal horns are richly innervated with mechanoreceptors and free nerve endings compared with the meniscal body.3 The body of the meniscus has minimal blood supply, whereas the meniscal horns have an abundant blood supply. A vascular plexus within the joint capsule derived from the medial and lateral genicular arteries supplies only the peripheral 25% of the meniscus.4 The remainder of the meniscal body relies on diffusion of synovial fluid for nourishment. Because of this sparse blood supply, meniscal injuries occurring axial to the peripheral rim rarely heal and thus have great clinical significance.
The menisci serve many functions within the stifle joint. They perform a load-transmitting function by distributing approximately 65% of the compressive load delivered during weight bearing.1,4 By distributing this compressive force, the menisci spare the articular cartilages of the femur and tibia. This load-distributing function is made possible by the five meniscal ligaments. Axial joint loading tensions these insertional ligaments and the circumferential collagen fibers of the meniscal body. As a result, a portion of the compressive load is transformed into hoop stressesb at the meniscal periphery, thus sparing the articular cartilage.3 The flexible menisci also improve joint stability by improving incongruity between the femur and tibia.2 The nerve supply to the menisci is thought to provide a sensory function, allowing precise muscle actions to occur in response to pressure changes in the joint.2 They are also thought to contribute to the hydrostatic lubrication of articular cartilage and prevent synovial entrapment between weight-bearing surfaces.1,4
In dogs, meniscal injury is almost always associated with rupture, either partial or complete, of the cranial cruciate ligament. When meniscal injury is associated with acute or chronic cruciate disease, the medial meniscus is damaged in 96% to 100% of cases.2,4,5 This correlation is directly attributed to the anatomic difference between the two menisci. The medial meniscus is more firmly attached to the tibia, joint capsule, and medial collateral ligament than the more mobile lateral meniscus. Therefore, when the cranial cruciate ligament fails, the cranial displacement of the tibia, when weight bearing, wedges the medial meniscus between the femoral condyle and the tibial plateau. Excessive internal rotation of the tibia during stifle flexion following cruciate rupture may also contribute to medial meniscal damage from the medial femoral condyle as part of the "screw home" mechanism.c,5 The lateral meniscus is spared this fate because of the presence of the caudal femoral attachment, which allows increased mobility.1 However, several reports have identified lateral meniscal injuries, including a recent publication describing radial tears in the cranial horn of the lateral meniscus in 77% of cranial cruciate deficient stifles undergoing arthroscopy.6,7 Although Hulse and Johnson7 described lameness associated with a lateral caudal pole tear, the clinical significance of most lateral meniscal tears is unknown.
When associated with cranial cruciate rupture, injury to the medial meniscus may occur during the rupture or as a function of chronic joint instability.8 Forty-eight percent to 70% of patients presented for cranial cruciate rupture have concurrent medial meniscus damage.2,8,9 The damage to the medial meniscus can be classified based on anatomic orientation and the type of tear (FIGURE 2). Different lesions reported include a folded caudal horn, longitudinal tears, fibrillation of the surface, transverse tears, "bucket-handle" tears, and compression injuries.2,8 The bucket-handle tear and folded caudal horn (pole) are the most commonly reported medial meniscal tears.2,9
Although most meniscal injuries in dogs result from cranial cruciate ruptures, other reported abnormalities include meniscal calcification and ossification, lateral meniscal damage secondary to osteochondritis dissecans lesions,10 and meniscal injuries secondary to multiple ligamentous injuries from automobile trauma (deranged stifles). Clinical signs of meniscal injuries without concurrent cruciate tears include mild to severe hindlimb lameness, joint effusion, pain on flexion and extension of the stifle, and an audible meniscal click. Meniscal ossification (formation of bone within meniscal tissue) and mineralization (deposition of minerals, mainly calcium) may be primary or secondary conditions. The primary form is considered idiopathic, and the presence of ossicles (small pieces of bone that form within soft tissues) is considered normal in reptiles, amphibians, birds, and some rodents. In higher mammals, they are rare but may be vestigial structures.10 Ossification or mineralization may also occur secondary to either metabolic abnormalities or trauma such as cranial cruciate rupture. Metabolic abnormalities include pseudogout in dogs and hyperparathyroidism and hypothyroidism in humans.10,11
Although cranial cruciate tears have traditionally been thought to occur in large-breed dogs, small breeds and cats can also be affected. Any animal with a cranial cruciate tear is at risk of a meniscal injury. An in-depth discussion on cruciate disease is beyond the scope of this article.
Physical examination and historic findings can lend suspicion to meniscal injury, but definitive diagnosis is typically made during surgery with either arthroscopy or arthrotomy. Clients may report an audible "clicking" sound when the patient ambulates, or a click may be producible on flexion and extension of the stifle, often indicating meniscal damage. A lack of a meniscal click does not rule out the presence of meniscal pathology. Patients may also have acute exacerbation of lameness when a meniscal injury occurs. Eliciting a cranial drawer sign may be hindered if a folded or torn caudal pole is wedged between the femoral condyle and the tibia, thus fixing the tibia in a cranial drawer position.
Other modalities that have been used to identify meniscal disease noninvasively in humans include ultrasonography and magnetic resonance imaging (MRI). In humans, the posterior half of the medial meniscus can be visualized consistently using ultrasonography, whereas the posterior half of the lateral meniscus is inconsistently seen. In dogs, neither caudal pole can reportedly be visualized.12,13 However, one study used ultrasonography when evaluating meniscal regeneration in the caudal pole of the medial meniscus.14 MRI is a common modality used in humans to diagnose stifle abnormalities, including meniscal tears, ligamentous injuries, and cartilage lesions. When using fat-saturated T2-weighted or proton density-weighted sequences, the sensitivity of diagnosing radial meniscal tears was 89% as confirmed by arthroscopy; the other 11% was false-negative findings.15 Other reports have published sensitivities and specificities for meniscal tears ranging from 90% to 95%.16 Normal canine stifle structures have been described with low-field MRI, but it is seldom used for diagnosing meniscal injury in veterinary medicine.17
Because of lack of adequate blood supply to the meniscus and poor healing potential, surgery is the treatment of choice for meniscal injuries. Visualization is paramount in evaluating the integrity of the menisci and initiating definitive treatment. This can be accomplished with a medial or lateral parapatellar arthrotomy or arthroscopy. To view the menisci with a parapatellar arthrotomy, the tibial plateau should be levered cranially with a Hohmann retractor or mosquito hemostat placed at the caudal aspect of the plateau or hooked under the intermeniscal ligament. By placing the tibial plateau in a cranial drawer position and using suction to remove excess joint fluid, the menisci can be fully viewed and probed for tears. Stifle retractors are also available to help improve visualization of the menisci. Arthroscopy can also be performed to visualize the components of the joint, debride remaining cruciate fibers, and address meniscal tears. The caudal pole of the medial meniscus can also be accessed through arthrotomy just caudal to the medial collateral ligament. This alternative approach is very limited and does not allow visualization of the cranial cruciate ligament. However, caudal pole meniscal tear procedures and meniscal releases can be performed with this technique, which is most commonly used during tibial plateau leveling osteotomy (TPLO).
Treatment options for damaged menisci include total meniscectomy (removing the entire meniscus), partial meniscectomy (removing only the damaged portion), hemimeniscectomy (removing the entire caudal pole), or primary repair. Proponents of total meniscectomy contend that it eliminates potential for continued clinical lameness due to lesions left behind from a partial meniscectomy and is technically easier to perform than some other techniques8 (FIGURE 3). Total meniscectomy also allows partial regeneration by fibrocartilaginous material from the well-vascularized joint capsule.18 However, this regenerate material likely cannot distribute load by creating hoop stresses. Twenty-eight experimental canine total meniscectomies performed by Flo8 had regenerative fibrocartilage. However, articular cartilage changes, including softening and fibrillation, were noted 12 months after surgery.
Nevertheless, partial meniscectomy has become the technique of choice for many surgeons because it has been shown to result in fewer degenerative changes than does total meniscectomy4,19,20 (FIGURE 4). Partial meniscectomy also allows a portion of the meniscus to remain in situ and provide lubrication and protection to the articular cartilage.1 Malcolm and Daniel21 evaluated the change in pressure transmitted to the articular cartilage following removal of portions of the lateral meniscus and documented that an intact meniscus transmits 29% of the load. When one-quarter of the meniscus is removed, the load transmitted increases to 45%; when a total meniscectomy is performed, the load transmitted increases to 313%. The conclusion was that the peripheral rim of meniscus remaining intact following a partial meniscectomy provides protective effects for the articular cartilage and decreased degenerative change.21 Data for changes in load transmission following partial meniscectomy of the medial meniscus were not described.
Hemimeniscectomy is removal of the caudal pole of the meniscus while the cranial pole remains intact (FIGURE 5). This procedure may be indicated when there is extensive injury to and fibrillation of the caudal pole, preventing the surgeon from sparing a normal rim of meniscus with a partial meniscectomy. However, hemimeniscectomy has been shown to have no advantages over total meniscectomy in terms of degenerative changes noted, despite the caudal pole being replaced by fibrocartilage.22-24
Primary repair involves direct suturing of the torn meniscus or suturing the meniscus to its peripheral joint capsule attachments. This technique has the advantage of leaving the meniscus in situ, thus providing continued protection of the articular cartilage. Disadvantages include technical difficulty and potential for poor healing, given the lack of adequate blood supply to 75% of the axial region. Historically, primary repair was performed only in cases of peripheral capsular tears.1 However, a recent report has documented primary repair in 92 meniscal injuries—either caudal peripheral detachment from the joint capsule or full-thickness longitudinal tears in the abaxial periphery—with good results.25 Despite this encouraging report, primary repair of the meniscus is not commonly performed in veterinary medicine.
A meniscal release involves either a mid-body medial meniscus transection or transection of the caudal tibial ligament (FIGURE 6). These maneuvers are intended to allow the caudal horn more mobility in the presence of cranial tibial thrust.26 The maneuver is part of TPLO for cranial cruciate repair. Either method of release can be performed through a parapatellar arthrotomy. The mid-body transection can also be performed through a small arthrotomy caudal to the medial collateral ligament. In a clinical evaluation of 212 stifles over 4 years, no cases of injury to the medial meniscus were noted following meniscal release in conjunction with TPLO.26 Another study evaluated the effects of medial meniscal releases in rabbits via transection of either the cranial or caudal tibial ligament. Histologic evaluation at 6 and 12 weeks after surgery revealed healing of the ligaments in elongated positions, increased articular cartilage degeneration at the medial tibial plateau, and synovitis within the treated joints.27 Transecting the medial meniscus at either the tibial ligament or mid-body would theoretically eliminate formation of hoop stresses during axial loading and decrease load distribution. However, given the promising clinical results for preventing meniscal injury after cruciate rupture, further studies are warranted to define the effects of meniscal release in dogs.
Because meniscal injuries are nearly always associated with cranial cruciate rupture and the progression of osteoarthritis, pain management is an important aspect of treatment. In the perioperative period, pain can be addressed with a combination of local anesthetics such as epidural or intraarticular bupivacaine, epidural or systemic opioids, and NSAIDs. For chronic lameness associated with osteoarthritis, management is multimodal, including weight loss for obese animals. Consistent low-impact exercise modification and physical therapy along with oral disease-modifying osteoarthritic drugs (e.g., glucosamine, chondroitin sulfate) can be combined with weight management to minimize the necessity of daily NSAID use. Readers should review more comprehensive publications on long-term management of osteoarthritis.
Because of the inevitable progression of osteoarthritis following meniscectomy (partial, total, or hemimeniscectomy), research has focused on improving the healing potential of the meniscus via improved blood supply and matrix scaffolds. Hulse et al28 investigated the use of vascular access channels or incisions made through remaining meniscal tissue to the peripheral synovium after partial meniscectomy. This exposes the nonvascular axial portion of the meniscus to the blood supply from the synovial vascular plexus and promotes healing and regeneration. Other methods include using free synovium or synovial pedicle flaps.29,30 A recent attempt to increase vascular supply involved using bioengineered meniscal tissue from cows treated with an adenovirus vector encoding the hepatocyte growth factor gene to induce blood vessel formation.31
Matrix biomaterials can also be used as a scaffold for meniscal regeneration with varying success. Some examples include using fibrin clots, porcine small intestinal submucosa, and collagen implants.14,32-35 Welch et al33 investigated placing small intestinal submucosa in 4-mm defects in an avascular portion of the medial meniscus in 16 dogs. Fifty percent of the defects were filled with small intestinal submucosa, and 50% served as controls. Healing was compared between the two groups, and no increased tissue regeneration was noted in the dogs with small intestinal submucosa implants.33 However, Cook et al14 demonstrated meniscal regeneration in four of five dogs that received small intestinal submucosa implants in surgically created meniscal defects (caudal pole hemimeniscectomy) compared with control dogs. Because hemimeniscectomy involves both the vascular and avascular portion of the meniscus, access to a blood supply may explain the more positive results from the study by Cook et al.14 Encouraging results have also been reported with collagen implants formulated from type I collagen from bovine Achilles tendons. In a phase 2 clinical study, eight humans received these meniscal implants and were followed for at least 24 months. All eight patients improved clinically based on pain and activity scores and had no progression of osteoarthritis.36
Another avenue of research has sought to replace severely damaged menisci with either biologic transplants (autogenous or allogenic) or permanent prostheses. Autogenous sources of tissue have included the quadriceps and Achilles tendons.37,38 Allogenic implants from cadaveric sources have been shown to heal to the joint capsule following transplantation in approximately 80% of cases, and promising short-term clinical improvements have been well documented.39-41 Attempts have also been made to develop an artificial meniscal prosthesis. Although numerous materials have been tested, a recent study using polyvinyl alcohol hydrogel in rabbits yielded promising results.42
Meniscal injuries are common in dogs and typically occur following cranial cruciate rupture. The medial meniscus is usually involved because of its unique anatomic structure. The treatment of choice is primary repair or partial meniscectomy during stifle stabilization. However, even partial meniscectomy inevitably leads to osteoarthritis and a perpetuating cycle of articular cartilage degeneration. Therefore, continued research is warranted to develop methods of improving meniscal blood supply, increasing healing potential through matrix scaffolds, or providing meniscal transplants or prostheses.