INTRODUCTION

Injuries of the Cranial Cruciate Ligament (CrCL) of the stifle are a common clinical finding in dogs of any breed, age and weight. CrCL injuries are consisting of partial tears and of complete rupture. Partial tears are associated to short term and self limiting acute lameness, evolving in chronic grade 1 lameness, usually underestimated and leading to complete rupture after a variable time. The cranial tibial thrust associated to the tibial slope and the weight bearing forces has been demonstrated to be responsible of the continuous stress on the damaged ligament leading to its complete rupture.

CrCL complete rupture is clinically associated to acute grade 3 lameness and can be a consequence of a chronic tear or it can be associated to a recent acute trauma, usually consisting of stifle hyperextension and internal rotation, the same condition leading to traumatic partial tears when of less amount.

When CrCL complete rupture is a evolution of partial tears the radiographic findings at time of diagnosis are consisting of chronic signs including osteophytes on the femoral trochlear ridges, on the distal poles of patella and on the tibial plateau, periarticular soft tissue swelling and infrapatellar increased density. When complete rupture is a consequence of acute trauma the immediate radiographic examination of the affected knee shows only periarticular soft tissue swelling and increased density of the infrapatellar triangle without any osteophyte. Clinical observation that several dogs have CrCL partial tears in both knees at the same time, even if with different degrees of severity, and that several dogs presented for acute CrCL complete rupture have partial tears in the opposite knee indicates that CrCL injuries are not always associated to a trauma and that in predisposed dogs they can be a consequence of a biomechanical imbalance of the forces acting on the knee joint causing a continuous stress on the CrCL. Traumatic and non-traumatic CrCL injuries can be differentiated by investigating the opposite knee too with physical and radiographic examination every time a diagnosis of CrCL lesion is performed. Traumatic injuries should show the opposite knee as normal, while the non-traumatic ones should show signs of chronic inflammation and degenerative joint signs.

CLINICAL STUDY

A prospective study of 251 consecutive dogs treated for CrCL failure was performed. Data collection included all relevant findings regarding patient signalment, history, radiographic aspects of both knee joint at the time of diagnosis and the surgical findings. Radiographic data included a thorough evaluation of the knee joint for acute and chronic inflammation signs, using low dose exposure to visualize periarticular soft tissues too, and a stressed position to evidence and measure the tibial compression test. The tibial slope was measured according to the indications described by Slocum. Pictures of dogs in standing position were taken, with adhesive marks on trochanter, lateral femoral condyle and lateral tibial malleolus to calculate the knee angle at rest in standing position. Dogs with both knee joints simultaneously affected, even with different degrees of severity, were listed as affected by non-traumatic CrCL injuries, unless a sure history of trauma was reported. Dogs with the opposite knee joint normal at the time of diagnosis were listed as affected by traumatic CrCL injuries. A correlation was studied between traumatic and non-traumatic CrCL injuries according to breed, sex, age, weight, tibial slope and knee angle. According to the surgical findings and to the amount of tibial compression test the partial tears or complete rupture were recorded.

DISCUSSION

This clinical study showed the evidence that in many dogs of several breeds CrCL injuries have a non-traumatic cause, being developed slowly and simultaneously in both knees, even with different degrees of severity. The median age of non-traumatic CrCL injuries is lower than in the traumatic group, indicating a breed and individual predisposition to early CrCL injury. Several breeds, like Boxer, English Bulldog, Dogue de Bordeaux, Newfoundland, Great Dane, Dobermann, Corso and Rottweiler show a higher incidence of non-traumatic CrCL injuries compared to other breeds; in these most affected breeds the tibial slope is not indicating an anatomical predisposition as their values are in the average of the dogs most affected by traumatic CrCL injuries and not by non-traumatic CrCL injuries. What appear to be a predisposing factor in the breeds with higher incidence of non-traumatic CrCL injuries is the angle of the knee joint in standing position, being wider that in the other breeds, leading to a more straight rear leg. The open angle of the knee enhances the inclined plane effect of the tibial slope and diminishes the opposing pull strength on the tibia by the hamstring muscles, leading to a continuous cranial tibial thrust and stress on the CrCL. The consequence of a wider knee angle with a more straight leg is therefore a biomechanical imbalance that could lead to a spontaneous CrCL injuries, particularly when associated to high physical activity or to high body weight. In contrary, in breeds without a significant incidence of non-traumatic CrCL injuries, the knee angle is closer, with a more angled rear leg, diminishing the effect of the inclined tibial plane which is almost parallel to the ground and enhancing the pull effect of the hamstring muscles, with a perfectly balance biomechanics.

Moreover, the incidence of non-traumatic CrCL injuries in breeds that usually have a normal knee angle, like German Shepherds and Labradors, was associated to increased age and body weight of the affected dogs at time of diagnosis; due to the increased weight these dogs showed a wider knee angle and more straight leg compared to the average knee angle of the breed, possibly because keeping a closer knee angle with an increased body weight would cause an excessive muscular tiring. Keeping the knee in a more straight position relieves the muscular effort and the dog is more comfortable, loading his body weigh through a bone to bone more vertical contact, but doing so the cranial tibial thrust is enhanced, favoured by the increased body weight too. Their rear legs assume the position of human legs in standing, walking and running, with a vertical loading through bones, but in man the tibial slope, which is about 5° to 7°, has no significant cranial tibial thrust, differently to the forces created by the inclined plane of the canine tibial slope. Increased body weight, particularly without a good muscular strength and exercise, is therefore a predisposing factor for non-traumatic CrCL injuries in dogs of all breeds. Other factors could play a role predisposing to CrCL injuries in some breeds, like the bowlegged conformation which causes stress on the CrCL; nevertheless this conformation does not appear to be determinant for non-traumatic CrCL injuries as it is observed in Rottweilers and Labradors both with traumatic and non-traumatic CrCL injuries. In these breeds, as in other large breeds, the occurrence of traumatic CrCL injuries is associated to very active dog with good muscular mass, enhanced by their bowlegged conformation, while the occurrence of non-traumatic CrCL injuries is associated to overweight and inactive dogs with poor muscular mass; this indicates that the muscular tone and strength are essential to balance the high forces acting in their knee, particularly when their conformation leads to straight rear legs.

More extensive studies should be conducted to investigate the biomechanics alterations in the knee joint due to the different morphology in canine breeds and a thorough morphological analysis in breeds predisposed to high incidence of non-traumatic CrCL injuries should be performed to evaluate the opportunity to influence the breed standards favouring a more physiological angulation of the rear legs in these breeds. According to Riser's studies, any breed selection going away from the ancestral dog is causing orthopaedic problems, because the anatomy of the dog can not match to the so wider variation we see in canine breeds without resulting in biomechanical and physiological imbalance.

Because CrCL injuries of non-traumatic origin have their ethiology in a biomechanical imbalance of the knee joint with enhancement of the cranial tibial thrust, and because the predisposing biomechanical alterations can not be modified, the best treatment in the affected dogs is the alteration of the knee biomechanics by neutralizing the cranial tibial thrust with the tibial plateau leveling osteotomy (TPLO) as proposed by Slocum. In traumatic CrCL injuries, traditional techniques of ligament reconstruction, extra or intra-capsular, can be successful providing the tibial slope is within a normal range.

CONCLUSIONS

In this study the evidence that several dogs exhibit CrCL injuries without a traumatic cause was demonstrated, locating predisposing factors in the wider knee angle and straight rear leg conformation of several breeds and in the overweight dogs of every breed. It appears that knee angle and overweight are more important than the anatomical tibial slope itself in causing an excessive cranial tibial thrust in dogs with non-traumatic CrCL injuries.

References

1.  Henderson R, Milton J: The tibial compression mechanism: A diagnostic aid in stifle injuries. J Am Anim Hosp Assoc 14:474, 1978

2.  Riser W.H., The dog as a model for the study of hip dysplasia: some aspects of growth, form and development of the normal and dysplastic hip joint. J. Vet. Pathol. 1975; 12 : 234-334.

3.  Slocum B Devine T: Cranial tibial thrust: A primary force in the canine stifle. J Am Vet Med Ass 183:456, 1983

4.  Slocum B, Devine T: Tibial Plateau Leveling Osteotomy for Repair od Cranial Cruciate Ligament Rupture in the Canine. Vet Clin North Am, Small Anim, vol 23, n. 4777-795, 1993