The knee of a dog is just like a human knee in that it consists of three main bones: The thigh bone or femur, The shin bone or tibia and the knee cap or patella. The patella nestles within a groove in the end of the thigh bone and is attached to the shin bone by the patellar tendon. The knee cap is also attached to the thigh bone and the pelvis by the large and powerful group of muscles at the front of the thigh called the quadriceps. This combination of quadriceps-patella-tendon-tibia creates a pulley system that produces and maintains extension of the knee. Without this mechanism neither humans nor dogs or cats would be able to maintain a standing position. This would be very disabling. In the most severe forms of patellar luxation (slipping or dislocating knee cap), this is exactly what happens with dogs unable to maintain an upright hindlimb stance and instead they walk with very crouched hindquarters. The patella can slip towards the inner side of the thigh bone as shown in the image or to the outside. Slipping towards the inside is most common and is called Medial Patellar Luxation (MPL). Whilst most of the information on this page relates to Medial Patellar Luxation, it is relevant to dogs with Lateral or outward patellar luxation.
The signs associated with patellar luxation can vary depending on the grade of patellar luxation. Generally we grade this problem on a four point scale. The higher the grade the more permanent the luxation is. In dogs with Grade 1 patellar luxation, the patella may quickly jump in and out of the groove. This may cause a skipping lameness of varying frequency. In between skips, your dog may walk perfectly normally. Most dogs will not squeak or yelp when the skip occurs and they will not seem to notice that it has happened. In Grade 2, the patella may jump out of the groove and stick in this position for a number of steps before popping back into the groove. Again the frquency of this can vary as can the duration of non weight bearing lameness. Also the steps between these epiosodes will likely appear normal. In Grade 3 patients, the patella is almost permanently positioned outside of the groove and there is often no skipping. Instead, the dog will walk with a crouched gait and often be seen to shuffle. The shape of the leg is usually abnormal in appearance with a bowing of the limb. In Grade 4 patents, an upright stance cannot be achieved, a poor shuffling gait and limb deformity are seen. Some dogs (especially Bull type dogs) may show little skipping but appear lame at all times and stiff on rising. These dogs have a patella that sits permanently on the inner ridge of the groove and have rubbed away the cartilage on the back of the patella and on the ridge. This problem will be covered separately.
Some dogs with Patellar Luxation may also have Cranial Cruciate Ligament pathology and this may need management at the same time as the surgery for Patellar Luxation. In some cases, the state of the Cruciate may not be known until the time of surgery. These dogs will generally appear lame at all times and stiff on rising from rest. X ray findings along with palpation of the joint under anaesthesia may help to identify the presence of this addiitonal pathology before surgery.
Certainly in grades 2 and over and in those patients with cartilage loss, surgery is the optimal form of management. It is not possible to use Rehabilitation or medical therapy instead of surgery in these patients. Some patients with grade 1 patella are not managed surgically especially if the problem is very intermittent and does not appear to be having a significant impact on quality of life. This is not always the case however and as untreated Grade 1 Patellar Luxation may become worse or put the Cruciate ligament at risk, surgery may still be advisable.
Tibial Tuberosity Transposition
This is indicated in almost every patient with Patellar Luxation. Recurrence of patellar luxation after surgery can often be attributed to failure to perform this procedure. In this procedure the tip of the shin bone is separated from the shin bone and moved to the side opposite the luxation. It is held in position by two small pins. The tip of the shin bone will heal in this new position over 4-6 weeks after the surgery.
Deepening the Groove (Trochleoplasty or Sulcoplasty)
This is often performed too as the absence of the pressure from the patella combined with its movement over the ridge can result in a shallow groove. Rarely will we perform this without Tibial Tuberosity Transposition as failure to combine these is associated with a higher rate of recurrence of Patellar Luxation.
Tightening the soft tissues opposite the side of luxation
The joint capsule becomes stretched on the side away from the dislocation. This extra tissue is reduced by creating a “tuck” in the joint capsule. This procedure is never (except where patellar luxation is definitely due to trauma to these tissues) performed alone. If the other procedures are not performed, the “tuck” will stretch and the luxation will recur.
Releasing the soft tissues on the side of the dislocation
This is required in most grade 3 patients and some grade 2 patients. If the patella is in an abnormal position for long periods the tissue to the side of the patella will shorten and this may need to be release. The quadriceps muscle group is often released at the same time.
Femoral and/or Tibial Osteotomy
In the higher grades (3 and 4), the femur and tibia are usually bowed or bent. In this procedure the Femur and sometimes the Tibia are fractured and straightened. Plates are used to allow the bones to heal with this new shape.
Patella Groove Replacement (PGR)
This is discussed in more detail below. It was devised and designed by Kyon AG It is particularly indicated for dogs that have lost cartilage on the underside of the patella. As mentioned previously this is often seen in Bull type dogs. Andy Torrington has performed the most of these surgeries in the UK and lectures on this surgery, helping to train other surgeons to perform this procedure.
As with other Orthopaedic surgeries a period of rest is required after Patellar surgery. This allows any fractures to heal and the soft tissues to become strong. The period of rest will rarely exceed 12 weeks. In some patients, cage confinement may be necessary but this is not common.
Weeks 1 and 2
House and Garden only permitted. Whilst in the garden lead restraint should be used at all times. Periods in the garden may be frequent but should not exceed 5 minutes. Stairs and getting on and off furniture must be avoided.
Weeks 3 and 4
In addition to the above, dogs may have up to three, five minute on lead walks per day. Stairs and furniture must be avoided.
Weeks 5 and 6
The walks are extended to 15 minutes three times daily on lead. No stairs and no furniture access.
Walks are extended gradually back to normal during this period. Lead restraint is still used and whilst stairs can be accessed this should be infrequent.
Normal lifestyle in all respects.
In this procedure we replace the groove with a geometrically perfect, diamond coated Titanium artificial groove. This procedure has revolutionised the way we manage Patellar Luxation seen in association with loss of cartilage on the underside of the patella.
In patients with cartilage loss, there is continuous friction between the patella and the ridge of the femur. This friction causes heat and pain. The diamond coating on the groove results in an almost frictionless gliding between the patella and the artificial groove. This eliminates heating and pain. The perfect geometry of the groove holds the patella perfectly in the groove throughout flexion and extension of the joint.
This procedure can also be helpful to revise previous unsuccessful surgery for patellar luxation and to avoid performing the more complex procedures such as Femoral and Tibial osteotomies as the groove can be positioned (in most dogs) perfectly in line with the patella without changing the shape of these large bones.
This shows the positioning of the Prosthesis in the knee, with in this case fours screws anchoring the base of the prosthesis to the femur. Bone will grow into the perforated base plate and ultimately it will be this that holds the groove in place. This process of integration takes around 8-10 weeks in most patients.