Primary ligamentous support of the stifle joint is provided by collateral ligaments and the intra-articular cranial and caudal cruciate ligaments. Nutritional support for the intra-articular ligaments arises from small vessels traversing the ligament structure and, as important, through synovial fluid bathing the ligament. Mechanoreceptors and afferent nerve endings have been identified within the interfiber layers of the cranial cruciate ligament. Innervation of the ligament serves as a proprioceptive feedback mechanism to prevent excessive flexion or extension of the stifle joint. This protective action is through stimulation or relaxation of muscle groups which lend support to the joint. The geometry of the CCL femoral attachment is responsible for a reciprocal loosening and tightening of the ligament through a normal range of motion. The CCL arises within the intercondyloid fossa of the lateral condyle of the femur and extends diagonally across the joint space to insert onto the craniomedial tibial plateau. (Fig.1)

The ligament enters the joint space through the intercondylar notch (INC) and spirals approximately 90° as it crosses the joint between attachment sites. A intercondylar notch width index (ration of the width of the ICN to the width of the distal femur) has been established in the dog. The normal proximal notch index is .32 and normal distal notch width index .41.

The mechanism of injury of the cranial cruciate ligament directly reflects its function as a constraint to joint motion. Hyperextension of the stifle joint with forced external rotation of the femur are joint positions which favor injury to the CCL. Injury of the ligament can be purely traumatic; however, there are other factors which may be involved in the pathogenesis of cruciate disease. One of these is age related change in the structural and histologic properties of the ligament. Histologically, there is a loss of fiber bundle organization and metaplastic changes of the cellular elements. Biomechanically, there is a loss of structural strength and stiffness. Another factor which may contribute to CCL rupture is abnormal conformation. Certain breeds such as the Rottweiler, Mastiff, Labrador, and Chow Chow appear to stand "post legged" (have a greater standing angle of the stifle joint and/or tarsal joint). (Fig.2)

Individuals within these breeds may also be excessively "bow legged." Both conformational traits place strain on the ACL predisposing the dog to partial tearing of the ligament.

Dogs with ACL injury may present with an acute onset of lameness or with a lameness that gradually worsens with time and/or activity level. Dogs presenting with an acute lameness either have a complete traumatic rupture of the ACL or an acute exacerbation of an ongoing partial ACL tear. Dogs having an acute traumatic tear exhibit a non-weight bearing or minimally weight bearing lameness of the injured limb. With time (2-3 weeks) the lameness improves but most dogs (medium and large breeds) never return to pre-injury activity level. A dog may present with an acute lameness but have evidence of chronic arthritic changes radiographically and/or upon surgical intervention. These cases have an ongoing gradual tearing of ACL fibers. The acute onset of lameness is associated with a meniscal injury or complete rupture of the remaining ACL fibers. Careful questioning of the client will usually revel that the dog has shown a mild lameness associated with activity for three to six months. These cases are classified as having a partial tear of the ACL. .

Physical examination will show a decrease in thigh muscle mass when compared to the normal limb and crepitus may be evident through flexion and extension. When the joint is extended from a flexed position a clicking or popping may be heard and felt; this is commonly associated with a meniscal tear. However, it should be noted that the presence or absence of joint noise neither confirms or denies the presence of meniscal injury. Osteophytes are present along the medial and lateral trochlear ridges and a palpable enlargement of the medial surface of the joint will be evident. Instability can be difficult to elicit in this group of patients due to the proliferative response of the fibrous joint capsule in response to the gradual onset of instability. This is particularly true in large, apprehensive patients. Initially, there is no pain, detectable synovial effusion or crepitus but as time progresses, signs of instability and degenerative joint disease become evident. With the onset of joint effusion and the proliferative response of secondary joint restraints, the dog has difficulty flexing the stifle joint on sitting. Rather than folding the joint directly beneath the femur upon sitting, the dog will external rotate the stifle when sitting. This maneuver is referred to as the sit test. (Figs. 3&4)

Early diagnosis is dependent upon radiographic presence of joint effusion. A radiolucent line adjacent to the caudal joint capsule is representative of fatty tissue in the space between the joint capsule and popliteal muscle. Caudal displacement of this line is representative of joint effusion. (Fig.5) This is one of the earliest radiographic indications of partial anterior cruciate ligament injury. As changes progress, typical radiographic signs of DJD will be noted.

Once the diagnosis of ACL injury is established, the surgeon must then choose a method of treatment. There are three options for the surgeon intra-articular reconstruction, extra-articular reconstruction, and TPLO. The author prefers a TPLO for active breeds weighing more than 10klg. However, owner financial restraints, client preference or surgeon preference may dictate use of a reconstructive method. When using a traditional reconstruction, the surgery may be performed with the assistance of an arthroscope or through an open procedure. The same is true for a TPLO procedure, i.e., the intra-articular part may be done with a limited arthrotomy or through the arthroscope. Once the internal structures of the joint are treated, reconstruction of the CCL deficient joint is undertaken. Treatment of the CCL deficient joint is through surgical reconstruction of the cranial cruciate deficient stifle joint or through alteration of joint mechanics. Joint mechanics are altered by changing the tibial slope (Slocum or Montavon osteotomy). The Slocum tibial leveling osteotomy (TPLO) (Fig.6) is the most popular technique in the USA while both the Slocum and Montavon techniques are popular in Europe. The advantage of the Slocum TPLO is that this technique allows for correction of abnormal limb alignment at the same time as one is leveling the tibial plateau. In-Vitro studies of the Slocum technique suggest that the CCL deficient joint is stable under axial load following TPLO to 6 degrees. Craniocaudal translation remains present under passive manipulation (cranial drawer test) and is possible with sufficient anterior shear loading.

Stability is gained by re-direction of normal cranial tibial thrust to one of a caudal tibial thrust. When the joint is loaded after a TPLO, a caudal tibial thrust is generated. The caudal cruciate ligament stabilizes (prevents) the joint against caudal tibial translation. The advantage of TPLO techniques are their success in the large active dog relative to that seen with the traditional intra-articular or extra-articular techniques. The disadvantage of the TPLO techniques are their expense relative to traditional techniques. Also, the majority of dogs under 50-55 lb have classically performed very well with traditional extra-articular or intra-articular reconstruction of the CCL deficient joint. For these reasons, the author prefers to use a traditional reconstruction in dogs less than 55lbs and in those cases where the client cannot afford a TPLO. The choice of which traditional technique (intra-articular or extra-articular) to use has been a subject of debate for more than 30 years. Seemingly, dogs in this weight category (<55lbs) function equally well with an intra-articular or extra-articular reconstruction. Those favoring intra-articular reconstruction believe the technique more closely re-establishes the normal biomechanics of the stifle joint. Those favoring extra-articular techniques report on its simplicity relative to an intra-articular reconstruction. Which traditional technique to apply is at the discretion of the surgeon and most often is based upon personal experience with one or the other techniques.