Introduction

Cranial cruciate ligament (CrCL) rupture is the most common orthopaedic condition in dogs and usually occurs secondary to a degenerative disease process rather than as an acute traumatic injury. CrCL rupture is not as common in cats and this may be because the CrCL in the dog is smaller than the caudal cruciate ligament whereas the reverse is true in cats. CrCL rupture is also probably under-reported in cats. In cats, CrCL rupture is often assumed to be traumatic but the traumatic event itself is rarely observed and therefore this is only an assumption. There is evidence that at least in a subset of cats, the condition is degenerative, as is the case in dogs.¹ Concurrent meniscal damage is common in canine CrCL deficient stifles and a similarly high incidence has been reported in cats.²

Patellar luxation (PL) is less common in cats than in dogs; however, the incidence may be underestimated. Both traumatic and developmental PL occur with developmental being more common. PL can be uni- or bilateral and is more commonly medial than lateral. In developmental cases, it is likely that there is a hereditary component, but the exact mode of inheritance is unclear. Certain breeds are predisposed to development of PL including the Abyssinian and the Devon Rex. As is the case in dogs, the most common changes seen with PL are a shallow trochlear groove and medial displacement of the tibial tuberosity.

History

The history for cats with CrCL rupture is generally of an acute onset marked lameness. A traumatic inciting cause is reported in some cases although this may be less than expected. One study reported only 4% of feline CrCL rupture cases had acute traumatic events witnessed.² Cats with degenerative CrCL rupture tend to be overweight.

Most cats presenting for developmental PL are relatively young but the history can be variable. Lameness may be of either acute/gradual onset, permanent/intermittent and unilateral/bilateral. A shuffling or crouched pelvic limb gait and a reduced inclination or ability to jump are commonly reported. Intermittent locking of the stifle joint has also been reported and this can be associated with sudden- onset distress and vocalisation.

Orthopaedic Examination

Diagnosis of CrCL rupture is generally straightforward based on clinical signs including lameness, stifle pain, tibial thrust and laxity in cranial drawer. Stifle effusion is normally palpable and crepitus may be evident upon joint manipulation. Partial tears may be more challenging to diagnose, although pain on stifle manipulation, particularly extension, and moderate-marked lameness are generally evident.

Diagnosis of PL in cats differs to that in dogs. Clinically normal cats have some degree of laxity of the stifle joint with subluxation of the patella considered a normal finding. The standard Putnam grading scheme of I-IV as used in dogs can be used but an alternative scheme of A-D has also been proposed for use in cats (Table 1), which takes this increased physiological laxity into consideration.³ In clinically affected cats, grades II and III (Putnam scale) are the most common.

Table 1. Grading system for patellar luxation

In a compliant cat, PL can be detected with the patient either standing or in lateral recumbency. Luxation is generally easiest to achieve with the stifle in extension and the tibia internally rotated. In a less compliant patient, examination under sedation can be performed.

Imaging

Normal radiographic findings to be aware of include the sesamoid bones in the medial and lateral heads of the gastrocnemius muscle and the sesamoid bone in the tendon of origin of the popliteal muscle. One or more of these sesamoid bones may not be evident radiographically as in cats these can be fibrocartilaginous. The normal shape of the patella is sharply pointed on a lateral view and triangular on a caudocranial view.

It is common to see mineralization within the articular space on the mediolateral view. This often appears to be within the cranial pole of the medial meniscus and may represent degenerative calcification. Although meniscal mineralisation has not been associated with increased pain scores in cats, the presence of mineralisation has been associated with articular cartilage degeneration.⁴ However, in some cases this mineralisation appears to be clinically insignificant and is thought to represent a meniscal sesamoid bone, the lunula.⁵

Pathological changes are most evident in the medial compartment radiographically, followed by the patellofemoral joint. Osteoarthritis of the stifle joint is characterised by osteophyte formation on the patella, along the trochlear margins and on the caudal edge of the tibia.

Radiographs of cats with CrCL rupture typically show joint effusion. Cranial displacement of the tibia may be apparent and in degenerative or chronic cases, osteoarthritic changes may be evident. Distal displacement of the popliteal sesamoid bone may also be seen.

Radiographs of cats with PL typically show mild changes only. Cats with grade IV PL may show more severe osteoarthritic changes. The position of the patella on the radiograph will depend on the grade of the luxation and cannot be relied upon for diagnosis.

Treatment

Both medical and surgical treatments are appropriate for treatment of CrCL rupture when no other ligaments are damaged. Until recently, the general recommendation was to manage cats with CrCL rupture medically using a combination of a short course of NSAIDs, exercise restriction for a period of several weeks and weight loss. This recommendation stems primarily from one study where 18 cats with CrCL rupture were treated conservatively and complete restoration of a normal gait was subjectively observed five weeks post injury.⁶ However, studies using force plate analysis have demonstrated that it takes 3–4 months for limb loading to return to near-normal post injury with medical treatment^7,8 and that even after one year, forces remained slightly lower than normal.⁸ In the majority of cats following medical therapy, laxity in cranial drawer persists.

Surgical and medical methods of treatment for CrCL rupture have not been directly compared. The evidence base suggests that surgical stabilisation is beneficial but not absolutely necessary. Cats may return to normal function more quickly after surgical stabilisation¹ and stabilisation may reduce or slow down osteoarthritis progression.³ The author advises surgical management in obese cats, young and very active cats, cases with multiple orthopedic injuries and cases where lameness has not significantly improved after 6 weeks of conservative management.

While numerous surgical procedures have been described to stabilise CrCL deficient stifles in dogs, extracapsular procedures are generally favoured in cats. In dogs, techniques aimed at achieving dynamic stabilisation have been suggested to offer more favourable outcomes than techniques providing passive stability and it has been suggested that corrective osteotomy techniques may be superior to extracapsular stabilisation in obese cats, cats with chronic CrCL rupture and cats with concurrent stifle osteoarthritis.

Cranial closing wedge ostectomy, tibial plateau levelling osteotomy, tibial tuberosity advancement and the modified Maquet technique have all been performed in cats. Prognosis for return to normal function after CrCL rupture is good–excellent although some degree of osteoarthritis is likely to develop which may warrant treatment at a later stage.

PL is sometimes diagnosed incidentally and in these cases treatment may not be warranted. Treatment is only recommended in high-grade cases or in low-grade cases with associated clinical signs. Persistent or recurrent PL will inevitably lead to cartilage erosions and early surgery may assist in minimizing these. Surgical repair has been reported to provide more favourable results when compared to conservative management.

The main aims of surgery are to deepen the trochlear groove and restore normal alignment between the quadriceps muscle, patella and tibial tuberosity. Surgical techniques in cats include femoral trochlear sulcoplasty, tibial tuberosity transposition, soft tissue imbrications and/or release and partial parasagital patellectomy with a combination of techniques being employed in a single stifle as required.

The outcome following surgical stabilisation of PL is excellent in the majority of cases. However, complications do occur including recurrence of PL, K-wire migration, tibial fracture, seroma formation and infection.

Complications have been reported to be more likely in cases that have had previous femoral fractures or stifle surgery, grade IV luxations and following tibial tuberosity transposition.⁹

References

1.  Harasen GL. Feline cranial cruciate rupture: 17 cases and a review of the literature. Vet Comp Orthop Traumatol. 2005;18:254–257

2.  Ruthrauff CM, Glerum LE, Gottfried S. Incidence of meniscal injury in cats with cranial cruciate ligament ruptures. Can Vet J. 2011;52:1106–1110

3.  Voss K, Langley-Hobbs SJ, Montavon PM. Stifle joint. In: Feline Orthopaedic Surgery and Musculoskeletal Disease. London: Elsevier; 2009:475–490

4.  Freire M, Brown J, Robertson ID. Meniscal mineralization in domestic cats. Vet Surg. 2010;39:545–552.

5.  Whiting PG, Pool RR. Intrameniscal calcification and ossification in the stifle joints of three domestic cats. J Am Anim Hosp Assoc. 1984;21:579–584.

6.  Scavelli TD, Schrader SC. Nonsurgical management of rupture of the cranial cruciate ligament in 18 cats. J Am Anim Hosp Assoc. 1987;23:337–340.

7.  Herzog W, Adams ME, Matyas JR, et al. Hind limb loading, morphology and biochemistry of articular cartilage in the ACL-deficient cat knee. Osteoarthritis Cartilage. 1993;1:243–251.

8.  Suter E, Herzog W, Leonard TR, et al. One-year changes in hind limb kinematics. Ground reaction forces and knee stability in an experimental model of osteoarthritis. J Biomechanics. 1998;31:511–517.

9.  Rutherford L, Langley-Hobbs SJ, Whitelock R, et al. Complications associated with corrective surgery for patellar luxation in 85 feline surgical cases. J Fel Med Surg. 2015;17:312–317.