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Patellar Luxation: A Problem of Alignment By Barclay Slocum, DVM and Theresa Devine Slocum, MS Current treatment of patellar luxation is focused on the alignment of tissues immediately about the stifle. It is important to recognize that rotation of the stifle is the twisting motion around an axis through the center of the hock and lateral third of the medial tibial plateau articular surface. Torsion is the permanent twist of the bone around the same axis. The internally rotated stifle can be externally rotated by a lateral fabella to tibial tubercle suture. The medially luxating patella can be restrained in the trochlear groove by a lateral fabella to patellar suture. If the lateral retinacular tissues are lax, a lateral capsular imbrication, tensor fascia lata overlap, or vest over pants tightening of the lateral fabellopatellar ligament will stabilize the patella in the trochlear groove. If the medial retinaculum is tight, then a medial retinacular lengthening will allow the patella to be moved to the trochlea groove. Although the trochlear chondroplasties, i.e. trochlear sulcoplasty, subchondylar excavation or trochlear recession, are useful in deepening the trochlea, they should not be used to prevent patellar luxation. Trochlear sulcoplasty is removing the cartilage and subchondral bone of the trochlea sulcus to deepen the trochlear groove. Subchondral excavation after elevating the articular cartilage from the subchondral bone is useful in deepening the trochlear sulcus in puppies less than six months old with a medial patellar luxation. A trochlear recession creates a wedge containing the trochlear sulcus and its intact cartilage, and recesses the subchondral bone. This maintains the cartilage surface of the mature dog and is useful in deepening the trochlear sulcus. Each of these techniques purpose is to deepen a shallow trochlea, which has been realigned with the quadriceps mechanism. If the trochlear chondroplasty techniques are used to force the patella to stay in the trochlear sulcus without quadriceps alignment, lateral pressure becomes applied to the patella. The result is loss of patellar cartilage or luxation of the patella from the patellar groove. Therefore, patellar stability requires both trochlear depth and limb alignment. Tibial tubercle position is critically important to the alignment of the quadriceps mechanism and the alignment of the lower limb. If the tibial tubercle is normally positioned on the tibia and the patella is medially luxated, then relocating the patella in the trochlear groove will realign the quadriceps mechanism. If the tibial tubercle is medially positioned on the tibia, then a lateral tibial tubercle transposition will realign the quadriceps mechanism. If the tibial tubercle is normally positioned on the tibia and a lateral tibial tubercle transposition is performed, immediately afterward the quadriceps mechanism appears to be properly aligned but the distal end of the straight patellar tendon is lateral to the sagittal plane. Over time, the straight patella tendon will return to the sagittal plane. As this occurs, the limb becomes bowlegged as the stifle internally rotates to reestablish the patellar tendon in the sagittal plane. The medium term consequence of the iatrogenic bowlegged conformation is the predisposition to rupture of the cranial cruciate ligament. The long-term sequella of the bowlegged conformation is the degeneration of the medial compartment of the stifle as the chronic mechanical overload causes loss of the articular cartilage. There is a major principle of stifle mechanics to be learned from this behavior of the stifle. The straight patella tendon will return to the sagittal plane and any alteration of limb conformation will be manifested as limb malalignment. As the overall limb is aligned, particular attention needs to be directed to the position of the straight patellar tendon. The following considerations are maintained. The straight patellar tendon should be in the sagittal plane. It should be in line with an imaginary line drawn from the femoral head and foot. The straight patellar tendon should be perpendicular to the tibial plateau. The patella and tibial tubercle should be in the sagittal plane in both flexion and extension of the stifle without holding the stifle in either internal or external rotation. This is most easily done with the dog placed in dorsal recumbency and preventing the hip from abduction by placing a single fingertip on the lateral epicondyle of the stifle and flexing and extending the stifle using one fingertip of the opposite hand.
Non-quadriceps Mechanism Elements and Patellar Luxation Femoral Conformation and Alignment Retroversion of the Femoral Head Femoral head and neck retroversion causes the patient to externally rotate the hip in order to achieve a proprioceptively neutral joint. This necessitates the internal rotation of the stifle to allow the foot to be in the sagittal plane. The quadriceps mechanism is deviated at the patella by this action, causing a pull on the far side of the fabellopatellar ligament and retinaculum. These structures eventually give way and allow the patella to luxate. Distal Femoral Varus Distal femoral varus greater than 10 degrees creates an internal rotatory force which is inadequately opposed by the biceps femoris muscle. Consequently, repeated excesses of internal rotatory force at the stifle cause serial and catastrophic rupture of the caudal half of the cranial cruciate ligament followed by rupture of the cranial half of the cranial cruciate ligament. The same internal rotatory force deviates the quadriceps mechanism and stretches the lateral fabellopatellar ligament and joint capsule. Medial patellar luxation follows that capsular stretching. OCD of the Medial Femoral Condyle Osteochondritis dissecans of the medial femoral condyle creates a loss of femoral length on the medial femoral condyle. This causes a true genu varum due to the loss of bone substance. There is also a laxity of the cruciate ligaments created by this osseus deficiency which may be mistaken for a rupture of the cranial cruciate ligament. Both the genu varum and the ligamentous laxity allow an excessive internal rotation at the stifle. The consequence is deviation of the quadriceps mechanism at the patella and stretching of the lateral fabellopatellar ligament and joint capsule. Naturally, medial patellar luxation follows. Tibial Conformation and Alignment Valgus of the Proximal Tibia Valgus of the proximal tibia creates a knock-kneed appearance to the patient. This conformation causes the foot to be lateral to the stifle and produces an external rotatory force which produces external rotation at the stifle. If the proximal tibial valgus is excessive, the pes anserinus muscle group cannot counteract the external rotation with internal rotatory force, and the craniomedial joint capsule begins to stretch. This deviates the quadriceps mechanism laterally beginning at the patella which causes a lateral force at the patella and stretches the medial fabellopatellar ligament and joint capsule. This sequence of events that occurs with knock-kneed dogs results in craniomedial rotatory instability of the stifle and a lateral patellar luxation. Medial Displacement of the Tibial Tubercle The normal alignment of the hindlimb is present when the patella (not luxated), the tibial tubercle, the hock and the foot are all in the sagittal plane. If the tibial tubercle is pathologically positioned medial to the sagittal plane, the quadriceps mechanism will have a medial deviation beginning at the patella. The consequence of this conformation is stretching of the lateral fabellopatellar ligament and lateral joint capsule. Eventually the patella will luxate medially. If the medial patellar luxation is extreme, the internal rotation of the stifle will result in damage to the caudal band of the cranial cruciate ligament and possible rupture of the entire cranial cruciate ligament. Lateral transposition of the tibial tubercle is curative. Internal Tibial Torsion Internal tibial torsion is the permanent internal twisting of the tibia around its functional long axis. This causes the entire stifle to be lateral to a line between the femoral head and the foot. This results in an internal rotatory force that stresses the caudal half of the cranial cruciate ligament. Partial rupture or complete rupture of the cranial cruciate ligament results. Ruptures of the cranial cruciate ligament allow cranial migration of the tibia with respect to the femur which reduces the femoropatellar compression. This often results in medial patellar luxation and this luxation will often be uncontrolled until the cranial cruciate ligament rupture is treated. Functional Postures Affect on Alignment Crouch and External Rotation of the Hip Springer spaniel hunting crouch is a characteristic of the breed. The normal springer will stand with the joints of the hindlimbs in the sagittal plane. The same dog will assume a crouch while hunting and maintain that posture for many hours and many miles. The crouch is accomplished by externally rotating the hip, internally rotating the stifle, and lowering the center of gravity of the dog by flexing its joints. This seems to lower the dog closer to the ground and widen his base. This appears to be better for sniffing. The crouch position will develop extremely large biceps femoris muscles. The rectus femoris is the only head of the quadriceps which arises on the pelvis and inserts on the tibial tubercle. The other three heads arise on the femur. With the dog in the crouch position, the straight-line pull of the rectus femoris is deviated medially beginning at the patella. The consequence is a medially directed force on the patella that may stretch the lateral fabellopatellar ligament and joint capsule. The result is a medial patellar luxation that responds to lateral transposition of the rectus femoris to the cervical tubercle of the femur. Pathologic Conditions Causing Patellar Luxation Secondary to Rupture of the Cranial Cruciate Ligament If an internal rotatory force is applied to the tibia, the caudal half of the cranial cruciate ligament is stressed. Partial rupture or complete rupture of the cranial cruciate ligament may result. Ruptures of the cranial cruciate ligament allow cranial displacement of the tibia with respect to the femur and that translation reduces the femoropatellar compression. This often results in a medial patellar luxation which will continue until the cranial cruciate ligament rupture is treated. OCD of the Lateral Femoral Condyle Osteochondritis dissecans of the lateral femoral condyle creates a loss of femoral length on the lateral femoral condyle. This causes a true genu valgum due to the loss of bone substance. There is also a laxity of the cruciate ligaments created by this osseus deficiency which may be mistaken for a rupture of the cranial cruciate ligament. Both the genu valgum and the ligamentous laxity allow an excessive external rotation at the stifle. The consequence is deviation of the quadriceps mechanism at the patella and stretching of the medial fabellopatellar ligament and joint capsule. Naturally, lateral patellar luxation follows. This problem is often seen in giant breed dogs such as the Great Dane and Irish wolfhound. If the valgus is severe, the tibia may translate laterally with respect to the femur. Realignment of the hindlimb to the sagittal plane is mandatory. Excessive Tibial Plateau Slope If the slope of the tibial plateau is 30 degrees or greater, the cranial tibial thrust caused by axial compression of the tibia on that sloped tibial plateau is sufficient to stretch or rupture the cranial cruciate ligament. Partial rupture or complete rupture of the cranial cruciate ligament may result with explosive bursts such as chasing squirrels. Ruptures of the cranial cruciate ligament allow cranial displacement of the tibia with respect to the femur and that translation reduces the femoropatellar compression. This type of patient often lacks the cranial prominence created by the patella on the lateral silhouette of the hindlimb. The cranial translation of the tibia and the straight-line pull of the quadriceps mechanism to the tibial tubercle causes this peculiar long thigh appearance. In addition these patients are characteristically bandy-legged secondary to internal tibial torsion which creates internal rotation at the stifle. This often results in medial patellar luxation. This luxation will often be uncontrolled until the excessive slope of the tibial plateau and internal tibial torsion is addressed.
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