Patella Luxation
The patella (knee cap) is a small bone, which 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 being moved 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. A pulling of the muscle on a bone that is not aligned (bone deformities of hip, femur or tibia) causes an inappropriate movement of the patella out of the femoral groove (patella luxation).
Most of the time the cause of patella luxation is genetic (eg. bone deformities, flat femoral grove)
(Hill's pet products 2006)
Grade of luxation:
Grade 1: patella could be luxated with manual pressure. In this case, there is no crepitus and no signs of bone deformity. This condition does not require surgical treatment
Grade 2: patella spontaneously luxates during the normal movement, no clinical signs, such as pain or crepitus, are detected. The skipping lameness may be detected
Grade 3: patella is luxated permanently but it can be reduced with manual pressure. Pain could be detected when the patella is pushed within the groove
Grade 4: patella is luxated permanently and can not be pushed within the femoral groove. The femoral groove can be palpated. Rotation of the limb 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.
Normal stifle
(Hill's pet products 2006)
Patella luxation
(Hill's pet products 2006)
Clinical signs
The typical clinical sign is a “skipping lameness”. Patients lift the limb up for a few steps when they are 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”. At the beginning of the pathology, the problem is only mechanic (no pain), however, the 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 develops (see below).
The images describe evident joint cartilage erosion of the medial aspect of the femoral groove.
Treatment
The aim of the surgery is to align the quadricep muscle mechanism and stabilise the patella within the femoral groove. A variety of techniques could be used to have normal alignment.
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Trochlear Wedge Resection
The aim of the surgery is to make the femoral grove 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 describes the surgical technique of a Trochlear Wedge Resection (Veterinary Istrumentation, 2019)
Intraoperative image before performing a Trochlear Wedge Resection.
Intraoperative image of a Trochlear Wedge Resection. A deepening of the femoral groove was performed and the bone-cartilage fragment was replaced in a deeper groove.
Intraoperative image of a Trochlear Wedge Resection. A deepening of the femoral groove was performed and the bone-cartilage fragment was replaced in a deeper groove.
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Transposition of the Tibial Crest
The aim is to align the quadricep 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 stabilised in a normal position with pins, or sometimes with wire, in a figure of 8.
The bone will heal in 4-8 weeks.
Postoperative radiograph mediolateral projection of the left stifle (knee). The tibial crest was fixed with pins and wire.
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Soft Tissue Reconstructive Technique
The aim is to stabilise 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) of the opposite side of the luxation;
Severe degrees of luxation, a release of contract soft tissue on the luxation side and imbrications (overlap of the fascia lata) on the opposite side would be performed.
Complications
Surgical complications are very rare. Infections will be treated with an antibiotic.
Skin irritation because of pin migrations may be detected; in this case, pin removal may be required. Strict rest is very important, very active animals should be confined to crate rest. A sudden jump could lead to the failure of the surgery. This will be addressed with a review surgery.
References
Hill’s Pet Products (2006). Hill’s Atlas of Veterinary Clinical Anatomy. Topeka, USA: Veterinary Medicine Publishing Company.14
Veterinary Istrumentation (2019). Patella Luxation - A Step By Step Guide. Available from: https://veterinary-instrumentation.co.uk/vi-academy?procedure=10 [Accessed; 15/06/19]
Case studies
Before
The video shows a medial patella luxation grade 2. it is possible to luxate the patella medially after a gentle pressure on the patella bone. The patella comes back in a normal position following a full extension of the leg.
After
After a surgical repair, it is not possible to luxate the patella anymore. The patient was treated with a combination of surgical techniques: Transposition of the Tibial Crest, Sulcoplasty and Soft Tissue reconstruction (overlap of the fascia lata).
