Tibial Plateaus and Cruciate Disease in Dogs Explained
Updated: Mar 12, 2018
It appears that cranial cruciate ligament disease (CCLD) is becoming increasingly prevalent, however, is this because of increased owner awareness, the development of new surgical techniques or is it becoming an increasing problem in dogs? Todays blog is going to explain the various jargon that is often referred to when talking about CCLD, how CCLD develops and leads to rupture, the possible causes of CCLD and the tibial plateau levelling osteotomy (TPLO) surgery.
First of all what is the cranial cruciate ligament?
The cranial cruciate ligament runs from the bottom of the femur, on the outside surface, to the top of the tibia, on the inside surface, passing through the middle of the stifle joint in a fan-like manner. The cranial cruciate ligament functions to prevent the tibia moving forward in relation to the femur, to prevent stifle hyperextension and prevent excessive internal tibial rotation during flexion.
(Source: Animal Medical Clinic, 2013)
And what is a tibial plateau?
The tibia bone is the canine equivalent to our shin bone, joining the stifle and hock joints on the front aspect of the hind limb. The tibial plateau is the articular surface of the tibia within the stifle joint and is reported in ‘normal’ individuals to have a 23 degree caudal slope (1).
(Source: adapted from Elizabeth Street Pet Hospital, not dated)
A higher tibial plateau angle, exceeding 30 degrees, has been reported to increase the risk of CCLD in dogs (2; 3). The Quadricep muscles, the thigh muscles on the front portion of the hind limbs, function to extend (straighten) the stifle joint and resist ground reaction force, the force exerted by the ground on an object in contact with it. The combined effect of Quadricep muscle contraction and ground reaction force compresses the femur against the tibial plateau. In the healthy stifle, the femur is prevented from sliding down the tibial plateau by a combination of the cranial cruciate ligament and the hamstring muscles. However, a greater tibial plateau angle requires more resistance, therefore placing the cranial cruciate ligament under greater strain and causing an accumulation of micro-trauma, the micro-tearing of ligament fibres, which leads to ligament degeneration.
The core of the cruciate ligaments have a particularly sparse blood supply. However, the caudal cruciate ligament attaches to highly vascularised soft tissue and therefore has a much richer blood supply than the cranial cruciate ligament, which is completely sealed within the stifle joint membrane. The cranial cruciate ligaments ability to repair following micro-trauma is therefore greatly diminished, leading to gradual degeneration and partial or complete cranial cruciate ligament rupture (4; 5; 6). Cranial cruciate ligament ruptures may appear spontaneous, however, in most cases are due to this accumulation of wear and one slip or turn can push the ligament past its threshold.
Once the cranial cruciate ligament has partially or fully ruptured it can no longer resist the compression between the femur and tibia and it allows the femur to slip down the tibial plateau during stifle extension. This is referred to as ‘cranial tibial thrust’ because the veterinarian will feel the tibia slip forward in comparison to the femur (7).
So what causes an elevated tibial plateau angle?
The answer is, no one really knows.
Su et al. (2015) reported higher tibial plateau angles in small dog breeds, neutered dogs, dogs with cranial cruciate ligament disease and dogs with ‘normal’ (non-luxating) patellas. However, this study analysed medical records from a referral practice and therefore was skewed towards dogs experiencing stifle disease and did not accurately represent the general population.
Mixed breeds represented 31.8% of the post-tibial plateau levelling osteotomy (TPLO) dogs in Coletti et al.'s (2014) study, while Labrador Retrievers represented 20.3% in a population size of 1231 dogs. These results correlate with a retrospective study of 27,254 cases performed by Bellumori et al. (2013) who reported a higher prevalence of CCLR in mixed breed dogs than pure bred dogs, with a 1.3 fold increase in prevalence. This could, however, be a result of the large number of mixed breed dogs within the UK population rather than a predisposition for mixed breeds to develop CCLD.
Vedrine et al. (2013) reported conflicting results of healthy Labrador Retrievers having significantly larger tibial plateau angles compared to healthy Yorkshire Terriers. However, more breeds need to be studied before inferring these results on to large and small dog breeds.
Duerr et al. (2007) reported that CCLD patients with a tibial plateau angle exceeding 35 degrees in one stifle are three times more likely to have been neutered before six months of age compared to CCLD patients with tibial plateau angles below 30 degrees in both limbs. They also went on to report that CCLD patients with tibial plateau angles exceeding 35 degrees in both stifles were 13.6 times more likely to have been neutered before six months of age, compared to CCLD patents with tibial plateau angles below 30 degrees in both limbs. The dogs with tibial plateau angles exceeding 35 degrees were also significantly younger at onset of hindlimb lameness attributed to CCLD. However, this study only looked at dogs exceeding 18.1kg at time of CCLD diagnosis.
On the other hand, in human studies the contact between the intercondylar fossa and the cranial cruciate ligament at various degrees of stifle flexion and extension has been associated with anterior cruciate ligament rupture, with these contact points most commonly affected. Additionally, congenital stenosis of the intercondylar fossa has been linked to anterior cruciate ligament rupture in humans (10).
Comerford et al. (2011) performed a literature review on the causes of CCLD in dogs, titled ‘Update on the aetiopathogenesis of canine cranial cruciate ligament disease’ and summarised that genetics and breed type is likely to contribute to abnormal stifle conformation, such as an elevated tibial plateau angle, which in turn leads to an abnormal gait pattern and abnormal stresses and strains on the components of the joint, consequently causing inflammation and cranial cruciate ligament degeneration. However, also identified that the abnormal conformation may physically compress the cranial cruciate ligament, leading to micro-trauma, inflammation, degeneration and disease, with obesity exacerbating the condition.
So in summary, an elevated tibial plateau angle has been attributed to early neutering, genetics, bony pathology within the stifle, obesity and inconsistently to dog size and elevated tibial plateau angles predispose to CCLD and rupture.
The Tibial Plateau Levelling Osteotomy (TPLO) -
The TPLO surgery aims to alter the tibial plateau angle and render the cranial cruciate ligament redundant. A circular cut is made in to the top of the tibia surrounding the tibial plateau. The top portion of the tibia, and with it the tibial plateau, are draw down the curved incision until the tibial plateau angle is at or near zero, at which point it is fixed in place with a metal plate and screws. The compressive force between the femur and tibia no longer translate in to forward and backwards movements and the cranial cruciate ligament is no longer required to restrain this motion.
(Source: Elizabeth Street Pet Hospital, not dated)
I hope that you have found this blog post interesting and that I have clarified some of the confusing veterinary jargon surrounding canine cruciate disease. Please share with anyone that might be interested and please get in touch if there are any areas that you would like me to expand on.