Update in Patellar Luxation in Dogs--Indications for Arthroscopically-Assisted Surgery
Arthroscopy of the stifle has revolutionized the treatment of knee injury in man. The use of arthroscopic-assisted surgical techniques for patellar luxation has benefits compared to arthrotomy. If arthrotomy can be avoided, lower patient morbidity is expected. Arthroscopy allows evaluation of the trochlear groove and patellar articular surface. Arthroscopy also allows assessment of the menisci and cruciate ligaments, both of which may have concomitant injury in patients with patellar luxation. A release incision of the medial retinaculum can also be performed under arthroscopic guidance.
Benefits of Arthroscopy
Important advantages of arthroscopy compared to arthrotomy include decreased pain, earlier return to function, improved visualization and more precise and accurate treatment. Other potential advantages include reduced scarring of the skin, decreased periarticular fibrosis and improved long term function.
Pain following surgery of the stifle can be substantial. Disruption of tissues leads to pain. Pain is generated locally by cellular mechanisms and activation of pain receptors. The perception of pain is dependent on transmission of impulses through the peripheral and central neural pathways. The source of pain may include skin, subcutaneous tissues, muscle, ligaments, tendons, synovial membrane, and subchondral bone. Inflammatory mediators within the synovial fluid also cause pain. Surgical pain can be decreased by appropriate preemptive analgesia, adjunctive NSAID therapy, reducing the number and extent of tissues invaded, and by meticulous handling of tissues. Arthroscopic-assisted surgery is minimally-invasive, sparing soft tissues around the joint, thereby reducing painful stimuli.
Return to Function
Early return to function is desirable to reduce muscle atrophy and preserve joint motion following surgery. Limb disuse quickly leads to muscle atrophy. The loss of muscle mass results in increased force on the joint, which may predispose to osteoarthritis and additional injury to ligamentous structures. Pain, tissue swelling, activity restriction and bandaging contribute to postoperative loss of joint range of motion. Early range of motion exercise is advantageous due to the tendency for joints to become stiff following surgery. Arthroscopic-assisted techniques also help to preserve joint range of motion due to its effect on decreasing postoperative pain and swelling.
Visualization of Joint Structures
Arthroscopic evaluation is superior to open surgical evaluation for 3 reasons:
1. Magnification of joint structures
2. Greater access to joint structures
3. Assessment of joint structures in a fluid medium
Magnification of intraarticular structures allows for more accurate identification of pathological change. Early osteoarthritic changes to articular cartilage not visible to the naked eye are clearly seen arthroscopically. Fine and course fibrillation, superficial erosions and neovascularization of the cartilage are readily evaluated and documented. Small radial and axial tears of the menisci often become evident only after magnification. Arthroscopic evaluation of the menisci is also improved due to the ability to position the scope directly adjacent to meniscus in both the cranial and caudal joint compartment. The menisci can be evaluated arthroscopically despite the presence of an intact cranial and caudal cruciate ligament. The scope can also be positioned in all 4 compartments of the joint (proximal, medial, lateral, distal), allowing a thorough evaluation of the synovium, patella, femoral trochlea, femoral condyles, tibial plateau, cruciate ligaments and menisci. These structures can be assessed for hyperplastic change, inflammation, erosion, osteophyte production, cartilage defects, tears and dysfunction.
Assessment of the joint in a fluid medium is optimal because synovial changes, cartilage surface morphology, and meniscal pathology become more evident. The end result of the enhanced visualization provided by arthroscopy is an improved ability to identify and document the presence and severity of pathological change; which allows for more accurate grading and classification of lesions.
Arthroscopic-assisted Medial Release Incision
A lateral scope portal and a medial instrument portal are used. The tip of the scope is positioned in the medial compartment of the stifle. The stifle is held in extension. The position of the patella is noted in relationship to the trochlear sulcus. The medial joint capsule is identified. The joint capsule and retinaculum are incised with a radiofrequency probe or by sharp dissection using arthroscopic scissors or a meniscal knife. Bleeding is controlled by increasing fluid flow and electrocoagulation. The retinaculum is incised until the medial tension on the patella is released, allowing the patella to return to a position within the trochlear groove. The patella should return to this normal position spontaneously and not by applying digital pressure. It should be noted that this technique only addresses one facet of the pathophysiology of patellar luxation. Alignment of the quadriceps mechanism should also be performed as needed. This can be performed through a minimally-invasive surgical approach in combination with arthroscopic-assisted medial release incision.
Cartilage erosion of the patella (p) and trochlear groove (tr) can be assessed arthroscopically.
A RF probe is used to incise the medial retinaculum
The medial tension on the patella is released allowing it to return to its proper position in the trochlear groove.