Luxating Patellas as a Cause of Ruptured Anterior Cruciate Ligaments

Several important structures contribute to stability of the knee joint (see Figure 1). The cruciate ligaments (both anterior and posterior) are central in their support of proper alignment. The patellar tendon, which extends from a large set of muscles on the femur, proceeds over the patella and then attaches to a point on the tibia bone, and stabilizes the knee in the same direction as the anterior cruciate ligament.

Figure 1 illustrates the anterior and posterior cruciate ligaments and trochlear groove.

The patellar tendon assists the anterior cruciate ligament in controlling abnormal motion in the joint. Together they help keep the femur and tibia properly aligned and prevent abnormal motion within the joint. When the tendon fails, the ACL is at much greater risk of suffering damage.
The patellas normally rest securely within a structure in the femur bone called the trochlear groove (see Figure 2). Many dogs are born with patellas that migrate out of this groove; this condition is called luxating patellas by vets in McKinney TX.

Figure 2 depicts normal position of the patellar (quadriceps) tendon, patella, and attachment point on the tibia.

The main problem with luxating patellas is that during development of the immature tibia, the attachment point of the patellar tendon arises too far to the medial side. In this situation, when the thigh muscles contract, the patellar tendon pulls the patella out of the groove to that side.
This abnormal position of the patellar tendon de-stabilizes the joint due to loss of the support that it normally provides. The anterior cruciate ligaments are then more prone to damage, because they must bear the entire load without help of the tendon.

It's not uncommon for vets in McKinney TX to see patients who have a previous history of luxating patellas suddenly experience an onset of severe lameness in that leg-usually because the anterior cruciate ligament has now ruptured.

Aside from preventing predisposure to cruciate ligament rupture, early correction of luxating patellas is important for another reason: abnormal motion of the patella-in and out of the proper position-stimulates early arthritic changes in the joint that worsen over time. Eventually these dogs can become chronically lame due to the abnormal motion and severe arthritis.

Surgery to repair both conditions is far more complicated than correction of the luxating patella alone. The procedure is more lengthy, and healing periods can be prolonged. In short, vets in McKinney TX agree that prompt repair of luxating patellas is the wise course of action, because it eliminates the pain of the luxating patellas, corrects the loss of joint stability, stops the early onset of arthritis, and protects the cruciate ligament from undue strain and the increased threat of rupture.

Stonebridge Animal Hospital
5913 Virginia Parkway, Ste 100
McKinney, TX 75071
(214) 856-7005