16.3.1 Etiopathogenesis

There are thought to be two main forms of cruciate ligament rupture in the cat: traumatic and degenerative. Seven of 50 cats developed bilateral rupture in an epidemiological study [2]. In another series of cases, the cats that had suffered cruciate ligament rupture were heavier than the general population of cats and it was purported that this injury parallels the degenerative CCL ruptures seen in overweight small‐breed dogs [3]. Periarticular osteophytosis (Figure 16.1) may be present when the cat presents with an apparent acute onset of lameness, similar to the situation in dogs. Osteoarthritis was graded as moderate to severe in 10 of 11 stifle joints in cats with CCL rupture, a finding that does not support the assumption of acute traumatic CCL rupture and making a degenerative etiology more likely [5].

In the traumatic form, there may be an isolated CCL rupture but there can also be damage to other periarticular structures such as the collateral ligaments, menisci and caudal cruciate ligament, usually known as multiligamentous, disrupted or deranged injuries. In the most severely affected stifles, the femorotibial joint may be dislocated. It is very important to distinguish between an isolated CCL rupture and a deranged stifle with rupture of other ligaments as the treatment and prognosis are dramatically different between the two. Misdiagnosing a multiligamentous injured stifle as an isolated CCL rupture can have serious consequences for the outcome in the cat.

16.3.2 Clinical Signs

Cats with complete CCL ruptures will have hindlimb lameness and stifle joint swelling, and the cranial drawer test and tibial thrust test will be positive.

Partial cranial cruciate rupture has been seen in cats and in this situation the drawer test may be negative. A retrospective study evaluating 98 feline stifle joints with naturally occurring CCL disease identified 6% partial ruptures [6]. Only one partial CCL rupture was detected in another study [7] which might be due to a low incidence in cats, low case numbers, or the fact that the minimal clinical signs are not observed by owners.

Traumatic ruptures may be more sudden in onset and other conditions may be present such as rupture of other ligaments (medial or lateral collaterals and caudal cruciate), patella luxation or fractures and concurrent injuries elsewhere in the body. In a cadaveric study utilizing normal cats, CCL sectioning resulted in a cranial tibial translation of nearly 9 mm [8]. Instability in cats with traumatic rupture can be pronounced and the cranial drawer can be dramatic.

16.3.3 Diagnosis

The diagnosis of CCL rupture is made by a combination of the history and physical examination and radiographic changes (Algorithm 16.1). Radiographs are taken to confirm the suspected diagnosis and rule out other potential causes or comorbidities (Table 16.1). Affected stifles will show compression of the infrapatellar fat pad associated with a joint effusion. It can be more difficult to detect stifle joint effusion on a radiograph in a cat compared to a dog; good‐quality radiographs and careful interpretation are required so compression of the intraarticular fat pad is not missed [9] (Figure 16.2).

Intraarticular mineralization may be present and this has commonly been associated with medial compartment degenerative joint disease [10], particularly if lesions are large [11]. Small discrete lesions are common, and these may be normal or incidental findings [12].

The cat should be sedated or anesthetized prior to performing testing for cruciate ligament integrity. Both the cranial drawer and tibial compression test should be conducted. Instability will be palpable with complete ruptures; this may be pronounced if the cat has a traumatic CCL rupture. The results of the tests may be inconclusive in the presence of partial ruptures.

It is important to diagnose the cat with a deranged, disrupted or multiligamentous rupture where in addition to a CCL rupture, there will be other ligamentous structures injured, most commonly the medial or lateral collateral ligament, caudal cruciate ligament, and medial meniscus. In multiligamentous injuries, there is usually significant displacement of the tibiofemoral joint or complete luxation. With the cat sedated or anesthetized, in addition to the cranial drawer and tibial compression tests, varus and valgus maneuvers should be performed to detect concurrent collateral ligament injury. However, it is worth noting that some cats with traumatic rupture of the CCL can have marked subluxation of the stifle without disruption of other ligaments (Figure 16.3).

16.6.1 Conservative Management

There is limited published evidence about the effectiveness and outcome of conservative management of CCL rupture in cats. In one study from the 1980s where 18 cats were treated conservatively, they took an average of 5 weeks to regain gait that was considered normal by veterinarian observation [19]. The conservative management consisted of keeping the cats indoors in a room or cage for 4 weeks and then restricting activity indoors for a further 4–6 weeks, together with instigating weight loss when appropriate. A satisfactory outcome was defined as a clinically normal gait with little or no apparent muscle atrophy at the time of follow‐up; this was considered to have been achieved in all cats in a mean duration of 4.8 weeks, with a range of 1–16 weeks.

Experimental transection of the CCL in cats [5] caused a distinct unloading of the deficient hindlimb when compared with the nonoperated hindlimb, which persisted for about 16–18 weeks. A decrease in muscle mass was found from muscles of the experimental hindlimb compared to the mass of muscles from the contralateral hindlimb, which was largest at 4 weeks and smallest at 35 weeks post CCL transection [5]. There was a delayed appearance of osteophytes in this model compared with the dog, suggesting that cats may be better able to compensate for CCL deficiency. This may be associated with factors of loading, the smaller size of cats compared with dogs, or a better developed neuromuscular control system in the cat [5].

Complications with conservative management include persistent lameness [9]. Eighty percent of the cats in one study showed persistent cranial drawer motion and evidence of degenerative joint disease upon physical examination and radiography despite apparent resolution of the lameness [19]. In a study of 50 cats with CCL rupture, 28 were treated conservatively and they were found to experience less chronic pain at long‐term follow‐up, and better quality of life (QoL) on the feline musculoskeletal pain index (FMPI). However, one of the limitations of this study was a possible selection bias due to a clinical decision to treat less lame cats with less severe joint disease conservatively [2].

If lameness persists following conservative management then there are various options, including continuing conservative management for a longer period of time or opting for surgical management (Algorithm 16.2). The cat should be reassessed in case a multiligamentous injury has been missed or there is another cause for the lameness such as a meniscal tear which may require treatment.

16.6.2 Surgical Stabilization

Surgery is purported to give a quicker return to function and given the high rate of meniscal pathology in cats with CCL ruptures, exploratory surgery for meniscal assessment and concurrent stifle stabilization should be considered in feline patients [6].

In an experimental model of CCL transection, the medial meniscus was often reported as ragged at 12 weeks and in one cat it was visibly torn [5]. Treatment of meniscal injury (Figure 16.6) is by partial meniscectomy and careful removal of the damaged portion, preserving the intact remainder of the meniscus.

Treatment or removal of the small meniscal mineralizations is generally not recommended as they may be incidental findings and not pathological. Attempts at removal will cause damage to the meniscus. Nevertheless, due to the possibility of cartilage lesions secondary to or concurrent with the large intraarticular mineralizations in the joint capsule or fat pad, excision can be performed if needed but taking care not to damage the meniscus or intermeniscal ligaments [10, 11].