Cranial cruciate ligament (CrCL) disease is one of the most common causes of lameness in dogs, and is particularly prevalent in common breeds such as the Rottweiler, Newfoundland and Labrador Retriever. Young dogs of these large breeds increasingly present with a history of chronic, intermittent low-grade hindlimb lameness, which responds to rest but recurs with the reintroduction of exercise. In some cases the lameness will be bilateral, and may be misdiagnosed as hip dysplasia or neurological disease if a thorough clinical examination is not performed. A complete rupture of the CrCL is usually easy to diagnose, by eliciting a cranial drawer in the unstable joint. With a partial rupture, however, the majority of the ligament remains intact, and the stifle is often stable on manipulation. These cases can be frustrating to diagnose, and owners are often reluctant to agree to an exploratory arthrotomy for diagnostic purposes. This lecture reviews the presentation and diagnostic evaluation of these cases.

Clinical Examination

Observation

Affected dogs will often sit asymmetrically, with the affected stifle rotated externally from the body, to avoid the discomfort of fully flexing the joint. The dogs will often have some difficulty rising, and will demonstrate a hindlimb lameness of varying degree, which will usually be more obvious if the dog is made to trot or to turn corners.

Palpation

Palpation of the affected limb will reveal a variable degree of muscle atrophy, depending upon the degree and duration of the lameness, and usually most noticeable in the quadriceps muscle group. Careful palpation of the stifle joint will typically reveal a joint effusion. In a normal stifle, the patellar ligament can be palpated as a tense, pencil-like band, and the joint capsule on either side 'dips' away from the ligament. When a joint effusion is present, the ligament is less distinct, as the joint capsule bulges out on either side, making it more difficult to palpate around the ligament. This is most obvious when the animal is weightbearing on the leg. In animals that have been affected for several weeks or longer, a smooth fibrous thickening may also be palpated on the medial aspect of the joint (medial buttress).

Manipulation

Manipulation of the limb will usually elicit discomfort, particularly on full extension of the joint. Two specific tests of stifle instability are performed; the cranial drawer test, and the test for cranial tibial thrust (CTT). Testing for cranial drawer can be painful and many dogs will tense the limb muscles to resist the force being applied, making it difficult to detect a subtle drawer motion. If an obvious cranial drawer is not detected, the test should always be repeated with the animal under sedation or anaesthesia. In contrast, testing for CTT does not appear to cause discomfort. However, the CTT is frequently subtle and can also be missed in the conscious dog; it is better assessed in the sedated or anaesthetised animal. It can also be helpful to compare the degree of internal rotation of the tibia that can be produced and compare this between sides.

Functional Anatomy

In order to understand the basis of the above tests and interpret them correctly, it is useful to review the basic functional anatomy and biomechanics of the canine stifle.

The CrCL is composed mainly of tension-carrying fibrous collagen, but also contains blood vessels and nerves. The CrCL primarily functions to limit cranial displacement and internal rotation of the tibia relative to the femur and to prevent stifle hyperextension, but it also provides some proprioceptive feedback to control stifle motion. Stifle joint stability is maintained by a balance between the actions of the:

 Passive restraints: cruciate and collateral ligaments, joint capsule and menisci

 Active restraints: stifle flexors (hamstrings and biceps femoris muscles) and extensors (quadriceps muscle group)

During normal weightbearing, the hock would flex were it not for the action of the gastrocnemius muscle, which also exerts a caudal pull through its origin on the caudal distal femur. By the law of equal and opposite reaction forces (Newton's Third Law), this would result in a cranial thrust of the tibia (CTT), due to the caudally sloping tibial plateau, unless resisted by an intact CrCL, caudal horn of meniscus and active forces generated by the stifle flexors (hamstrings and biceps femoris). This is the basis of the test for CTT.

Cranial Cruciate Ligament

The CrCL is considered to have two functional bands:

 The smaller craniomedial band (CMB) is permanently tense throughout the joint range of motion and is more commonly damaged, usually through twisting especially during flexion

 The larger caudolateral band (CLB) is taut in extension only, and is less commonly injured, usually through hyperextension

Thus partial tears, most commonly involving the craniomedial band, will only be evident if the stifle is tested in flexion. A study of 320 dogs with cranial cruciate rupture identified partial ruptures in 25 of the dogs (7.8%) and, of these, 20 (80%) had tears of the craniomedial band, and only one (4%) had a tear of the caudolateral band. Cranial drawer was present in only 13/25 dogs and, in these, was elicited only in flexion in nine of the dogs.

Assessing Cranial Drawer--Confusing Issues

A cranial drawer of more than 1-2 mm would in most cases be considered abnormal.

 Differentiating between partial and complete tears:

 Drawer motion must be assessed with the stifle joint in extension, at a normal standing angle, and in flexion. Care must be taken not to mistake internal rotation of the tibia for cranial drawer.

 Complete tears involve both bands resulting in instability in flexion and extension.

 Partial tears usually involve CMB, resulting in a small amount of drawer motion in flexion in the majority of cases (CLB relaxed), but no motion in extension (CLB taut).

 Chronic injuries: periarticular tissues become thickened and fibrotic (medial buttress), with only limited stretching possible. There is a gradual end to motion, due to tissue stretching.

 Skeletally immature dogs: have degree of normal joint laxity so there is some drawer motion but it comes to an abrupt end.

 Breed differences: increased craniocaudal laxity has been reported in Rottweilers compared to Greyhounds at certain joint angles.

 Caudal cruciate rupture: apparent 'cranial' drawer motion may be detected if the caudal cruciate ligament is ruptured. The 'starting' position of the tibia is abnormal; however, rupture of the caudal ligament allows the joint to subluxate caudally, and as the tibia is drawn forward and reaches its normal position, the cranial cruciate ligament becomes taut, abruptly stopping the movement.

Diagnostic Investigation

Radiography

In most cases, standard lateral and craniocaudal radiographic views of the stifle will demonstrate the presence of a joint effusion and a variable degree of osteoarthritis and medial buttress. A joint effusion will be most obvious on a lateral view. In a normal stifle, a triangle of radiolucent (black) fat occupies most of the cranial aspect of the joint (the 'fat pad sign') with a small region of radioopacity caudal to this due to the cruciate ligaments and menisci. If effusion is present, the radioopaque (white) fluid will compress and displace the fat pad cranially.

Aspiration of Joint Fluid

Aspiration from a normal stifle joint will usually produce 0.5-1 ml of colourless or yellow viscous fluid; however, three to five times this amount may be obtained from an effused joint. The fluid may appear a little less viscous, and the cytology is predominantly degenerative; however, occasional multinuclear cells may be seen, with slightly increased cell numbers (e.g., 3-5 cells x 10⁹/l), reflecting the inflammatory component of partial tears.

Other Imaging

The thin slices required to define the cruciate ligaments and menisci do not enable these structures to be easily differentiated on plain computed tomography (CT) scans. CT contrast arthrography can be used to detect partial cruciate ruptures and meniscal damage; examples of scans will be shown in the lecture. Magnetic resonance imaging (MRI) is the gold standard for examining intraarticular soft tissues in human medicine; however, the technique is expensive and consequently rarely used as part of the diagnostic workup for this condition in dogs.

Arthroscopy

Diagnostic arthroscopy allows visualisation of the cruciate ligaments and menisci, as well as the articular surfaces of the joint. Adequate visualisation of the intraarticular structures is only possible if most of the infrapatellar fat pad is removed. A large volume of sterile fluid is flushed through the joint during arthroscopy and the magnification produced by the scope enables clear visualisation of small tears and tags in the ligament and menisci, which may be missed on gross visual examination. Instruments can be introduced through other portals to palpate and manipulate these structures, probing for laxity and tears.

Arthrotomy

Diagnostic arthrotomy clearly enables inspection of both cruciate ligaments and menisci. It is also the only technique that allows simultaneous diagnosis and definitive treatment.

Conclusion

Most 'stable cruciates' occur as a result of partial tearing of the ligament. These can usually be diagnosed by careful manipulation of the stifle, under sedation or anaesthesia to eliminate muscle tone. The presence of repeatable stifle pain, with a marked joint effusion and radiographic evidence of degenerative joint disease, is sufficient evidence to justify an exploratory arthrotomy of a stable stifle joint.

References

1.  Duval JM, Budsberg SC, et al. Breed, sex and bodyweight as risk factors for rupture of the cranial cruciate ligament in young dogs. Journal of the American Veterinary Medical Association 1999; 215 (6): 811-814.

2.  Slocum B, Devine Slocum TD. Tibial plateau leveling osteotomy for repair of cranial cruciate ligament rupture in the canine. Veterinary Clinics of North America: Small Animal Practice 1993; 23: 777-795.

3.  Scavelli TD, Schrader SC, et al. Partial rupture of the cranial cruciate ligament of the stifle in dogs: 25 cases (1982-1988). Journal of the American Veterinary Medical Association 1990; 196(7): 1135-1138.

4.  Wingfield C, Amis AA, et al. Comparison of the biomechanical properties of Rottweiler and racing greyhound cranial cruciate ligaments. Journal of Small Animal Practice 2000; 41(7): 303-307.