18.1 Introduction

There is a medial and a lateral meniscus within each stifle joint. The menisci are fibrocartilaginous semilunar discs, covered in a synovial membrane [1]. Both menisci are thin and concave axially and thick and convex abaxially; the lateral meniscus is slightly thicker with a slightly greater arc than the medial meniscus [1]. Each meniscus has a cranial and caudal meniscotibial ligament, and there is a cranial intermeniscal ligament [1, 2]. The lateral meniscus also has a meniscofemoral ligament running from the caudal horn to the caudo‐lateral surface of the medial femoral condyle (Figure 18.1) [1]. The lateral meniscus is not attached to the joint capsule laterally due to the presence of the popliteal tendon of origin (Figures 18.1 and 18.2), in contrast to the medial meniscus which is attached to the joint capsule and medial collateral ligament via the coronary ligament [2]. The lateral meniscus is much more mobile due to these differences in ligamentous attachments (Figure 18.2) [2]. Vessels originate in the perimeniscal capsular and synovial tissues and penetrate the peripheral 25% of the menisci [3]. The functions of the menisci are complex and likely incompletely defined, but include load transmission, shock absorption, joint lubrication, and joint stability [4].

Meniscal pathology is most commonly seen secondary to cranial cruciate ligament (CCL) insufficiency [5]. The caudal pole of the medial meniscus is the most frequent location for damage because of its more intimate association with the tibia and joint capsule, which forces the medial meniscus to move with the tibia as it translates cranially and caudally in relation to the femur [5]. As the tibia is translating cranially and caudally, the caudal horn of the medial meniscus is crushed against the medial femoral condyle, leading to trauma and the development of tears [5]. The incidence of medial meniscal injury concurrent with CCL rupture is approximately 50% although the reported incidence ranges from 20% to 80% [6, 7]. In dogs with CCL insufficiency, risk factors for development of meniscal injury include those with complete or chronic CCL tears (versus partial or acute, respectively) and overweight dogs [7–9]. The added risk of a complete CCL tear is likely related to the increased instability that occurs with a complete versus a partial CCL tear [7].

Different types of meniscal tear morphology can develop, including vertical longitudinal (“bucket handle”), horizontal, radial, or degenerative tears (Figure 18.3), or the intact meniscus can become detached from the joint capsule and the entire horn can become mobile and displaced [5]. The bucket handle tear is the most common morphology of medial meniscal tears, and is characterized by a medial to lateral tear that allows cranial and caudal motion of the axial border [8, 10].

18.2 Preoperative Meniscal Evaluation

Although it is impossible to definitively determine meniscal status based on the gait examination alone, one study found that dogs with CCL rupture and concurrent meniscal injury were significantly lamer (as denoted by a lower peak vertical force and vertical impulse) than dogs with CCL rupture without meniscal pathology [11]. During stifle manipulation, displacement of mobile meniscal tears can create a palpable or audible click [8, 10, 12]. It is believed that cranial tibial subluxation in the CCL‐deficient stifle leads to impingement between the caudal pole of the meniscus and the femoral condyle, causing displacement of the torn portion of the meniscus and the resultant clicking that is heard and/or palpated.

When moving the stifle through a range of flexion‐extension motion, two studies reported that detection of a meniscal click was highly specific (~95%) but had poor sensitivity (38–58%) for the presence of a meniscal tear, with a click being most commonly associated with a bucket handle tear compared to other tear morphologies [8, 10]. Additionally, palpation during anesthesia improved the sensitivity of detection of meniscal click as a test for meniscal pathology [8, 10]. Valen et al. tested multiple physical examination tests and found that performing the tibial compression test under axial loading through a range of motion (so‐called “modified tibial compression test”) had the highest sensitivity (63%) and performing the cranial drawer in extension had the highest specificity (97%) for detection of a meniscal click as a diagnostic test for meniscal pathology [12]. Summarily, detection of a meniscal click is strongly indicative of the presence of a meniscal tear, specifically a bucket handle tear, but failure to detect a click does not necessarily mean that the meniscus is normal. This underpins the importance of a thorough meniscal evaluation at the time of surgical intervention, regardless of findings during the preoperative physical exam.

A number of diagnostic imaging modalities have been described for the assessment of menisci in dogs, including radiography, ultrasounography, computed tomography (CT), computed tomography arthrography (CTA), and magnetic resonance imaging (MRI). Although the status of the meniscus cannot be definitively determined by radiography, a study by Won et al. found a significant correlation between joint space narrowing on a standard mediolateral tibial plateau leveling osteotomy (TPLO) radiographic projection and the presence of a meniscal tear at surgery [13]. Ultrasound examination provides high sensitivity (90%) and specificity (93%) for diagnosis of medial meniscal tears [14]. Results, however, are highly dependent on the skill level of the ultrasonographer and the interpreter. CTA has shown a range of sensitivities (13–90%) and specificities (57–100%) for detecting simulated [15] and naturally occurring [16, 17] medial meniscal tears in dogs, highlighting the importance of the CTA protocol used and the skill level of the interpreter. In people, the use of dual‐detector helical CTA technology was found to be similarly sensitive and specific to MRI in detecting meniscal pathology [18], but a study reassessing the sensitivity and specificity of CTA for detection of meniscal pathology using similar newer CT technology has not been performed in dogs.

Magnetic resonance imaging examination is considered the most appropriate first‐line preoperative diagnostic for assessing meniscal pathology in people [19]. In veterinary studies using 1.5 T [20] or 3 T [21] MRI to detect meniscal pathology, sensitivity and specificity have ranged from 90% to 94% and 96 to 100%, respectively. Despite the utility of MRI for detecting meniscal pathology, it is not widely available in veterinary hospitals and its diagnostic utility in dogs is highly dependent on the magnetic strength, the size of the stifle joint, dog positioning, and the experience of the interpreter [12, 22].

Although MRI and CTA are noninvasive to minimally invasive methods of assessing the menisci preoperatively, both are associated with increased owner costs and require deep sedation or general anesthesia, with CTA also requiring an intraarticular injection, limiting their application for this purpose even in hospitals where the technology is available.

18.3 Exposure of the Meniscus and Diagnosis of Meniscal Tears

In animals with CCL disease undergoing surgical stifle stabilization, it is imperative that both the lateral and medial menisci are evaluated, as failure to identify a meniscal tear may contribute to a poorer outcome, resulting in a persistent or recurrent lameness, progressive osteoarthritis, and/or the need for additional surgical intervention. Although the caudal pole of the medial meniscus is the most commonly affected portion of the meniscus, the cranial pole, body, and caudal pole of both menisci should be examined. The joint examination can be performed via arthrotomy or arthroscopy.

Based on a cadaveric study using experimental models of various caudomedial meniscal lesions, it was found that the sensitivity and specificity of arthroscopy for detection of medial meniscal pathology were higher than those obtained using either a craniomedial or a caudomedial arthrotomy [23]. Importantly, the addition of a meniscal probe (Figure 18.4) improved diagnostic accuracy for detection of meniscal pathology, regardless of the approach used to examine the joint [23]. The odds ratio of correctly diagnosing the state of the medial meniscus when using a meniscal probe, when compared with observation alone, was 8.0 for arthroscopy, 2.1 for craniomedial arthrotomy, and 2.6 for caudomedial arthrotomy [23]. Another interesting finding of this study was the diagnostic superiority of a craniomedial arthrotomy over a caudomedial arthrotomy in CCL‐deficient joints; conversely a caudomedial arthrotomy was more accurate at correctly diagnosing meniscal pathology than a craniomedial arthrotomy in dogs with an intact CCL.

The diagnostic superiority of arthroscopy has been corroborated in clinical studies [24, 25]. In a prospective study of 163 dogs with CCL rupture, meniscal pathology was 1.9 times more likely to be detected in stifles examined by arthroscopy than by an arthrotomy [24]. Similarly, a retrospective study of 531 dogs found that the rate of meniscal tear detection was significantly higher with arthroscopy (odds ratio 1.5) compared to a craniomedial arthrotomy [25].

18.3.1 Meniscal Evaluation with an Arthrotomy

For the purposes of examining the stifle joint and performing meniscal evaluation, the joint is typically approached via a craniomedial, caudomedial or craniolateral arthrotomy. The surgical approach will be based on surgeon preference and which, if any, additional stifle stabilization procedures are being performed in that particular case.

The surgeon must exercise caution when performing the arthrotomy, as inadvertent cartilage, meniscal, and/or patellar ligament damage is possible when entering the joint. Typically, to best protect intraarticular structures and allow correct tissue apposition during closure, it is recommended to incise the periarticular fascia and elevate this from the underlying joint capsule before performing the arthrotomy. The surgeon should then tent up the joint capsule with thumb forceps and make a small incision in the joint capsule, making sure not to penetrate the joint too deeply (Figure 18.5). The arthrotomy is then extended in a manner that protects the underlying cartilage and meniscus and neighboring patellar ligament, such as using a pair of Mayo scissors or incising over a mosquito hemostat placed within the joint (Figure 18.6). The surgeon should be especially careful not to cause iatrogenic damage to the cranial pole of the meniscus and/or the intermeniscal ligament, as the joint capsule and fat pad are incised distally.

Improving exposure of the joint will improve diagnostic accuracy and reduce iatrogenic injury to intraarticular structures. Multiple techniques and instruments can aid in exposure of intraarticular structures. It is helpful to place a baby Gelpi to retract the infrapatellar fat pat abaxially (Figure 18.7). If an assistant is available, they can further retract the fat pad cranially using a sharp Senn/Volkmann retractor placed at the level of the intermeniscal ligament (Figure 18.7). The most challenging aspect of meniscal evaluation is the ability to visualize the caudal pole of the menisci. Many instruments have been used to help translate the tibial cranially, some developed specifically for this purpose, thus improving exposure of the caudal tibial plateau and caudal pole of the menisci. These include intraarticular devices such as levers (mosquito forceps, Hohmann retractor, various purpose‐designed stifle distractors, etc.) or self‐retaining devices (Gelpi retractor, Wallace stifle distractor, etc.), and extraarticular devices such as the Leipzig stifle distractor (extraarticular devices are discussed in the arthroscopy section of this chapter).

Intraarticularly placed lever devices are the simplest method of improving visualization of the caudal pole of the meniscus (Figure 18.7). The tip of the instrument is typically placed caudal to the tibial plateau and the instrument is levered against the trochlear groove of the femur (Figure 18.8). Although convenient, use of a lever instrument can result in multiple complications. The body of the lever can damage the cartilage of the trochlear groove (Figure 18.9). The tip of the lever instrument can be placed on (rather than caudal to) the tibial plateau, ideally between the caudal cruciate ligament and the caudal meniscotibial ligament of the medial meniscus. The advantage of placement at this location, over placing the tip caudal to the plateau, is that it may permit pushing the tibia distally in addition to generating cranial tibial subluxation. However, it is possible to create a small iatrogenic fracture when leverage is applied, if the instrument is not secured behind a ligamentous attachment, such as the caudal menisco‐tibial ligament (Figure 18.10). Additionally, the tip of the lever instrument can slip cranially and damage the cartilage of the tibial plateau (Figure 18.11). Regardless of whether the tip of the lever instrument is placed on or caudal to the tibial plateau, it is advisable to place the tip of the instrument medial to the caudal cruciate ligament. This helps retract the caudal cruciate ligament laterally and improves exposure of the caudal pole of the medial meniscus and the caudal meniscotibial ligament (Figure 18.12).

One challenge with use of a lever device is that it can impede the surgeon’s view of the lateral joint compartment when using a medial arthrotomy. Occasionally, the surgeon will have to remove the lever instrument and use a cranially positioned sharp Senn/Volkmann retractor or manually place the tibia in cranial drawer, to enable visualization of the caudal pole of the lateral meniscus. An additional limitation to the use of a lever instrument is the need for an assistant to maintain distraction as the surgeon examines the joint and treats any pathology. If sufficient assistance is not available, the use of a self‐retaining retractor is a viable option. A sharp self‐retaining retractor or distractor can be placed inside the joint, between the femur and tibia (Figure 18.13

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