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Patella Luxation

The patella (knee cap) is a small bone that sits within the tendon of the quadriceps femoral muscle. This bone is part of a system called the quadriceps mechanism. During the normal movement of the limb, contraction of the muscle and movements of the stifle (knee) result in the patella moving up and down in a straight line within the femoral groove (thigh bone).

The patella luxates laterally or medially when the quadriceps mechanism is not aligned or a trochlear groove is shallow. Pulling of the muscle on a bone that is not aligned (bone deformities of the hip, femur, or tibia) causes inappropriate movement of the patella out of the femoral groove (patella luxation).

Most of the time, the cause of patella luxation is genetic (e.g., bone deformities, flat femoral groove) (Hill's Pet Products, 2006).

(Hill's pet product 2006). Medial Luxation, Patella.

Grade of luxation

Grade 1

The patella could be luxated with manual pressure. In this case, there is no crepitus, and no signs of bone deformity are detected. This condition does not require surgical treatment.

Grade 2

The patella spontaneously luxates during normal movement, and no clinical signs, such as pain or crepitus, are detected. Skipping lameness may be observed.

Grade 3

The patella is luxated permanently but can be reduced with manual pressure. Pain could be detected when the patella is pushed within the groove.

Grade 4

The patella is luxated permanently and cannot be pushed within the femoral groove. The femoral groove can be palpated, and limb rotation may be detected.

A patella luxation grade 2 may progress to a grade 3 following cartilage and bone erosion secondary to the patella popping in and out of the femoral groove.

Clinical signs

The typical clinical sign is "skipping lameness." Patients lift the limb for a few steps when walking (especially when walking quickly) and then return to normal. The cause of this lameness is the patella moving in and out of the femoral groove. The popping out of the patella results in a locking of the stifle joint (knee joint) and "skipping lameness." Initially, the problem is only mechanical (no pain), but abnormal movements of the patella cause cartilage erosion, osteoarthritis degeneration, pain, and lameness.

In severe conditions (grade 4), the patella could be permanently luxated; in these cases, the tibia could be rotated from 60 to 90 degrees. If not corrected in early life, severe bone and ligament deformity develop.

The images describe evident joint cartilage erosion of the medial aspect of the femoral groove.


The aim of the surgery is to align the quadriceps muscle mechanism and stabilize the patella within the femoral groove. A variety of techniques could be used to achieve normal alignment.

Trochlear Wedge Resection:

The aim of the surgery is to make the femoral groove deeper. This should accommodate about 50% of the patella.

The bone is cut with a fine-toothed saw. Once a V-shaped bone fragment is removed, the bone is deepened, maintaining the same V-shape. The bone fragment is replaced into a larger and deeper defect.

The image below describes the surgical technique of a Trochlear Wedge Resection.

Patellar Groove Replacement (PGR)

The utilisation of a patellar groove replacement prosthesis reduces lameness linked to severe femoro-patellar arthritis, enhancing patellar stability, and rectifying the alignment of the extensor mechanism. The surgical procedure entails a femoral osteotomy followed by the placement of a prosthesis.

Manufactured by Kyon  the prosthesis boasts the advantage of low friction, ensured by a smooth Amorphous Diamond-Like Carbon (ADLC) coated groove surface.

Transposition of the Tibial Crest

The aim is to align the quadriceps mechanism by sitting the patella straight within the femoral groove.

This is achieved by making a vertical straight cut on the top and front of the tibia and moving the fragment laterally or medially according to the patella luxation. Finally, the fragment will be stabilized in a normal position with pins, or sometimes with wire, in a figure of 8. The bone will heal in 4-8 weeks.

Soft Tissue Reconstructive Technique

The aim is to stabilize the patella within the femoral groove.

Two techniques could be performed according to the degree of luxation

Patella luxation grade 2 or 3: an imbrication of the soft tissue (overlap of the fascia lata) on the opposite side of the luxation.

Severe degrees of luxation: a release of contracted soft tissue on the luxation side and imbrications (overlap of the fascia lata) on the opposite side would be performed.


Surgical complications are very rare. Infections will be treated with antibiotics.

Skin irritation due to pin migrations may be detected; in this case, pin removal may be required. Strict rest is crucial, and very active animals should be confined to crate rest. A sudden jump could lead to the failure of the surgery, which will be addressed with a review surgery.


Hill's Pet Products (2006). Hill's Atlas of Veterinary Clinical Anatomy. Topeka, USA: Veterinary Medicine Publishing Company.

Veterinary Instrumentation (2019). Patella Luxation - A Step By Step Guide. Available from: [Accessed: 15/06/19].

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