flexion and extension of both the affected and unaffected limbs not only at the initial consultation but at all follow‐up visits. Additional information on measurement of joint motion can be found in Chapter 15.
Figure 1.5 An example of a positive sit test in a patient with a right CCL tear. Notice how the right hindlimb is projected outward and the patient is not sitting squarely.
One question that every clinician should ask themselves is: how do you know your patients are truly improving and how can you detect subtle complications? Gathering as much objective information as possible is vital; muscle mass measurements as well as measurement of joint motion of both the affected and unaffected limbs should be part of the objective information gathered (more information on this can be found in Chapter 15). With CCL pathology, there may be an abnormal sit, sometimes referred to as a “positive sit test.” This is characterized by patients sitting with the affected limb projecting out to the side or tucked under them (Figure 1.5) rather than sitting square (Figure 1.6). Alternatively, patients may sit with weight shifted off the affected limb (Figure 1.5). The abnormal sitting posture is thought to be due to discomfort associated with hyperflexion of the stifle when forced to sit squarely. Unfortunately, some dogs may still exhibit an abnormal sit test following CCL stabilization. The reason for this is unknown but it could be due to continued stifle discomfort upon full flexion.
Figure 1.6 An example of a square sit in a patient with no CCL pathology. Notice how both stifles are fully flexed and the patient is sitting square.
Instability of the stifle is commonly demonstrated through the cranial drawer test and tibial compression test. The cranial drawer test (Figure 1.7) is performed most commonly and tends to be the mainstay of testing for stifle instability by general veterinarians. It is performed by applying a force to the tibia while holding the femur stable, thereby creating craniocaudal translation of the tibia. The operator may stand either behind (caudal) the patient or behind and slightly to the side (caudal and lateral). One hand is placed on the distal femur with the thumb on the lateral fabella and the index finger on the patella. The other hand is placed on the proximal tibia with the thumb on the fibular head and the index finger on the tibial tuberosity. The goal is to move the hand on the proximal tibia cranially while holding the hand on the distal femur stable. Commonly, mistakes made by the inexperienced operator stem from trying to move both hands simultaneously or trying to grab the tissues (both soft and bony) too firmly. Forcing cranial drawer or grasping the tissues too firmly will cause the patient's muscles to tense, making interpretation difficult. Cranial drawer should first be checked in extension, and if positive is likely indicative of a complete tear (typically greater than 75% tearing of the CCL as subjectively noted by one author, DD). If negative, it should then be checked in flexion. A positive cranial drawer in flexion but negative in extension typically indicates an incompetent (unstable) partial CCL tear (usually 50–75% tearing of the CCL as subjectively noted by one author, DD). If cranial drawer is negative in both extension and flexion, then the stifle should be placed in hyperextension to evaluate for discomfort. Discomfort with joint effusion and negative cranial drawer may indicate a competent (stable) partial CCL tear (usually less than 50% tearing of the CCL as subjectively noted by one author, DD).
Figure 1.7 Demonstration of the cranial drawer test. One hand is placed on the distal femur with the thumb on the lateral fabella and the index finger on the patella. The other hand is placed on the proximal tibia with the thumb on the fibular head and the index finger on the tibial tuberosity. The goal is to move the hand on the proximal tibia cranially while holding the hand on the distal femur stable.
The make‐up of the craniomedial and caudolateral bands of the CCL can explain why it is possible for the cranial drawer test to be positive in flexion even if it is negative in extension. The craniomedial band is the primary supporter of tibial translation and tends to degenerate first. During range of motion, it is taut in both flexion and extension. The caudolateral band is a secondary supporter of tibial translation and is taut in extension but lax in flexion. Therefore, if the craniomedial band is torn, cranial drawer will be absent in extension but present in flexion. Lack of cranial drawer may indicate tearing of the caudolateral band with an intact craniomedial band or subtle tearing of the craniomedial band or both the craniomedial and caudolateral band. In anxious or nervous patients or those with negative cranial drawer, the authors recommend performing a sedated examination to ensure there is no instability. Unfortunately, when chronic periarticular fibrosis or advanced OA is present, cranial drawer may be negative due to the presence of significant fibrous tissue or permeant translation of the tibia in relation to the femur. Skeletally immature patients often exhibit some physiological cranial drawer (“puppy drawer”) of up to about 3–5 mm. However, there should be an abrupt stop point at the end of cranial drawer to differentiate this from pathological cranial drawer.
The tibial compression test (Figure 1.8), while still a passive test, aims to mimic load bearing of the stifle. The operator may stand either behind (caudal) the patient or behind and slightly to the side (caudal and lateral). One hand is placed on the distal femur with the thumb on the lateral fabella and the index finger on the patella. The other hand is used to hold the metatarsals and tarsocrural joint. With the stifle held stable, the tarsocrural joint is flexed and extended. The clinician observes for a cranial‐to‐caudal motion of the tibial tuberosity indicating pathology of the CCL. Mistakes made by the inexperienced operator stem from trying to flex and extend both the stifle and tarsocrural joint simultaneously. In addition, the stifle should be held in a sagittal plane with no excessive internal or external rotation. Holding the stifle internally rotated can lead to a false positive of tibial thrust.
Diagnostic imaging of the stifle in patients with CCL pathology is usually centered around radiographic evaluation. Radiographic examination is warranted in every case of hindlimb lameness even if surgical intervention is not an option. It should be suggested that good‐quality orthogonal views of both the affected and unaffected stifles should be performed (Figures 1.9 and 1.10). While the CCL itself is not visible on radiographs (unless associated with an avulsion of the CCL), there are two key findings that can be appreciated: osteoarthritic changes and intraarticular changes (meniscal edema, synovial hyperplasia, and joint effusion).
