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
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.
Donald Hulse, DVM, Dip ACVS
Texas A&M University
College Station, Texas, USA