Management of articular cartilage lesions is based on the concept that providing blood with mesenchymal stem cell precursors access to the lesion encourages healing by formation of fibrocartilage. Several marrow stimulating techniques have been described to achieve this. Abrasion arthroplasty involves uniform removal of subchondral bone until bleeding is achieved. This can be accomplished in the canine elbow by use of either a curette or burr attachment on a small joint shaver. The shaver is usually more rapid and efficient and generally just as accurate. Another marrow stimulating technique is microfracture. In this technique numerous microcracks are created in the subchondral bone plate with a specialized micropick to allow bleeding at the lesion surface.
Arthroplasty and Microfracture
Indications for abrasion arthroplasty or microfracture vary with the size and degree of cartilage loss. In general, lesions small to moderate size (1–2 cm in humans) can be treated with resurfacing techniques. Debridement of Grade I lesions with chondromalacia is up to the discretion of the surgeon. Small areas of grade II fibrillation in the absence of other lesions also may be left undisturbed. Larger areas of Grade II cartilage disease can be treated with abrasion arthroplasty or microfracture. Grade III lesions are areas of full thickness fibrillation. Use a curette or burr to remove the diseased cartilage while being careful not to damage any of the surrounding more normal cartilage. Grade IV cartilage damage is full thickness loss of cartilage and exposure and, in some cases, eburnation of the subchondral bone. They are treated with abrasion or microfracture until adequate bleeding occurs. Producing diffuse effective bleeding varies in difficulty between joints. Combining abrasion and microfracture may help increase subchondral bleeding. In cases of eburnation it may be difficult or impossible to get significant bleeding with these techniques.
To perform abrasion arthroplasty, insert a hand burr or preferentially a power shaver burr through the instrument portal. Either method will produce significant bone debris that can clog the egress portal and impede visualization, therefore it is important to monitor and maintain the flow of fluid through the joint during this procedure. Spin the burr to remove subchondral bone over the area of the lesion. Check for resulting bleeding frequently by stopping inflow of fluid and ensuring adequate outflow to decrease the pressure in the joint. When bleeding is observed diffusely from the lesion bed, lavage the joint to remove the remaining bone debris and close routinely.
To perform microfracture, insert an appropriately angled micropick into the joint and press the tip against the subchondral bone surface. Have an assistant tap the pick handle once or twice. The pick should be held securely to avoid gouging the surface and adjacent healthy cartilage. Apply the micropick diffusely across the diseased area and check for resulting bleeding frequently by stopping inflow of fluid and ensuring adequate outflow. The penetrations should be 2–3 mm apart and 1–2 mm in depth. When bleeding is observed diffusely from the lesion bed, lavage the joint to remove the remaining bone debris and close routinely.
Objective evidence documenting the efficiency of abrasion arthroplasty or microfracture is not available in the dog. In humans, microfracture appears to be more effective than abrasion arthroplasty and is the marrow stimulating technique of choice. The technique is highly dependent on appropriate post-operative rehabilitation. In humans, 4–6 weeks of non-weight bearing activity coupled with active or passive range of motion is necessary for an ideal outcome. Overall, the results of abrasion arthroplasty have been unpredictable and symptoms often recur 2–3 years after surgery. Nevertheless, good to excellent results are reported in 50–60% of patients. Microfracture has shown promising results as first line treatment in smaller lesions. Seventy four percent of patients reported significant reduction in pain and swelling and improved function. In another series of patients, the Lysholm and Tegner Scores improved significantly.
Removal of a section of the medial coronoid is a treatment modality which has been described and has good results. Studies have shown that microfractures are present in the subchondral bone of the medial coronoid distant to a visible fragment. This fact supports the rationale for subtotal choronoidectomy. The procedure is performed through a mini-arthrotomy or arthroscopically guided. The recommended amount of coronoid removed varies but, in general, removing the coronoid devoid of articular cartilage is a good strategy. Removing excessive coronoid may present long term complications by losing the medial buttress weight bearing area of the elbow. Owner-based evaluation studies have demonstrated a significant increase in limb use/function.
Biceps Ulnar Release Procedure
The biceps/brachialis muscles constitute a large muscular complex. The anatomic origin and insertion of the biceps and brachialis muscles are such that the muscular complex exerts considerable force on the medial compartment of the elbow. The force exerted by the biceps is continuous since it is a pennate muscle with a central tendon. More importantly, because the insertion of the biceps/brachialis complex is at the ulnar tuberosity, a large polar (rotational) moment is exerted at the cranial segment of the medial coronoid. The magnitude of the polar moment is a product of the moment arm (distance from the ulnar tuberosity to the tip of the coronoid) multiplied by the force created by the biceps/brachialis muscular complex. The polar moment rotates and compresses the craniolateral segment of the medial coronoid against the radial head. The compressive force is medial to lateral transverse to the long axis of the coronoid. A compressive force generates internal shear stress at an oblique angle to the applied compressive force. In this situation, maximal internal shear stress would be oblique to the long axis of the coronoid. Under the right circumstances, the polar moment and resultant compressive force produced by the biceps/brachialis complex may produce sufficient internal shear stress to exceed the material strength of the cancellous bone in the craniolateral segment of the medial coronoid. The result would be microfracture/fragmentation adjacent to the radial head at an oblique angle to the long axis of the medial coronoid. The surgical technique involves releasing the ulnar insertion of the biceps to unload the medial compartment and prevent the rotational moment rotating the coronoid into the radial head.
Techniques to Lateralize the Weight Bearing Axis in the Elbow
The cause of mechanical overload can be associated with progressive humeral varus associated with mechanical weight bearing mechanical axis. This process occurs in the human knee and is the most common cause of knee medial compartment OA in humans. The normal mechanical axis in the frontal plane in the canine forelimb is a line from the center of the shoulder joint to the center of the radiocarpal joint. The mechanical axis courses through the medial compartment across the medial humeral condyle and medial coronoid. The anatomic axis in the frontal plane demonstrates the normal varus angulation of the distal humerus and weight bearing axis through the medial compartment of the elbow. The deviation of the mechanical axis medially and the humeral varus become more apparent with the progression of medial compartment osteoarthritis similar to the process in the human knee. The result of overload of the medial compartment is collapse of the medial compartment and OA. Future treatment strategies are based on lateralization of the mechanical axis.
Sliding Humeral Osteotomy (SHO)
Sliding humeral osteotomy involves creating a midshaft transverse humeral osteotomy and translating (sliding) the diaphysis distal to the osteotomy medially. Doing so shifts the weight bearing axis through the elbow joint from the medial compartment to the lateral compartment. Owner and veterinarian Visual Assessment Scores (VAS) have improved in all cases with a notable decrease in pain upon elbow manipulation.
Elbow replacement is an option in dogs that have end stage elbow osteoarthritis and conservative/less invasive surgical modalities have not resolved clinical pain. A number of prostheses are available but the most popular one today is the TATE elbow. Clinical outcome studies indicate that a mechanical lameness may persist but that the dogs appear to be less painful. An elbow prosthesis presently in clinical trial is the CUE (canine unicompartmental elbow). The concept is simple and carries little morbidity. Information concerning this technique will be forthcoming in the near future.