Introduction:
The hock, medically known as tarsus, is a common location for pain and lameness in the back legs of the horse. The lower hock joints (distal intertarsal (DIT) and tarsometatarsal (TMT) joints) are the most common source of lameness, with symptoms derived from arthritis (osteoarthritis (OA), degenerative joint disease (DJD)). Conformation and accumulation of repetitive forces play a role in disease development. Horses with lower hock pain can often be managed medically, but in select cases further intervention is necessary. Ankylosis, or joint fusion, of the lower joints can develop over time following significant cartilage damage, which permits bone to heal together. It is also not uncommon to find a single joint out of the lower joints totally or partially fused while the other is look normal on radiographs. Naturally occurring joint fusion might take months to years to occur, without any intervention, and rarely result in complete bony union and return to soundness.
Diagnosis:
Proper diagnosis starts with a complete physical examination, along with a passive and active lameness exam. Definitive diagnosis of the lower hock joints as the cause of lameness requires a joint block of both lower joints with local anesthetic, simultaneously or at different occasions. It should be reminded that periarticular injection or leakage of local anesthetic solution, while blocking the joint, may temporarily block the pain associated with the suspensory ligament and other structures down the leg.
Nuclear scintigraphy (bone scan) can also be utilized to help isolate the lower hock joints. Once the lameness is localized, digital radiographs are recommended. However, the severity of the bony change has poor association between the duration and degree of lameness. Other techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI) are more reliable but are less available, often need general anesthesia and are a more expensive.
Treatments:
A. Non-surgical approach
The most common treatment to distal hock arthritis is intraarticular injection of corticosteroids. Side effects of corticosteroid use are rare, but can include infection and laminitis, both of which can be very serious and possibly fatal. The most common corticosteroids utilized are methylprednisolone acetate (Depo Medrol), triamcinolone acetonide (Vetalog), and betamethasone (Betavet).
Historically, both lower hock joints were always injected individually. Since 2006, the necessity of injecting both joints individually has been questioned. It is the authors’ collective opinion that individual administration of both the lower joints is most appropriate.
In conjunction with corticosteroid, hyaluronate (HA) joint supplement is commonly injected to further exert an anti-inflammatory effect. It helps to reestablish synovial fluid viscosity and potentially increase the duration of the injection. However, the necessity of adding HA has recently been called into question.
Systemic non-steroidal anti-inflammatories, although often forgotten, still have an important role in managing lower tarsal arthritis. By far, Bute (phenylbutazone) has been the most commonly used drug, followed by flunixin meglumine (Banamine), meloxicam and most recently firocoxib (Equioxx). A prospective study has shown that firocoxib significantly improve lameness score throughout a 14-day period with few adverse effects, which included edema of the lips, mouth ulcers, colic, lethargy and sedation. Local application of diclofenac liposomal cream (Surpass) over the distal hock joints has been reported, with an inconsistent response noted. Supplementation with systemic polysulphated glycosaminoglycan (Adequan), glucosamine/chondroitin, or hyaluronate sodium (Legend) are also beneficial adjunctive treatments, which are best utilized alongside local therapies.
B. Facilitated ankylosis (joint fusion)
The primary indication for lower hock joint fusion is arthritis that is unresponsive to medical therapy. As the severity of arthritis progresses, new bone formation contributes to joint stability and facilitates joint fusion. Several techniques have been used to promote joint fusion of the lower hock joints, which are sub-classified as surgical or chemical approaches. Chemical joint fusion can be performed either standing or under general anesthesia. Several chemical agents have been used successfully, with reported complications and limitations. Complications such as severe pain, chronic lameness and marked peri-articular swelling are well reported post treatment.
C. Surgical Arthrodesis
Surgical placement of implants to promote bone fusion and increase joint stability has been used successfully on the lower hock joints. Implants used include plates and stainless-steel basket. The disadvantage of this techniques is that there is little soft tissue coverage present over the implant which increase the risk of infection post surgery.
Facilitated joint fusion by drilling of the articular surface is one of the most commonly used surgical procedures to treat distal hock joint arthritis. The horse is placed under general anesthesia and three diverging drill holes are created through the lower hock joints. The goal is to create the most amount of cartilage damage as possible. The prognosis following surgery is considered good, with success rates reported from 47 to 85% for return to intended use.
Laser-facilitated joint fusion has been used only experimentally. Both Nd:YAG and diode lasers have been used to destroy articular cartilage by superheating and vaporizing joint fluid. In one study using sound horses, the use of a laser promoted less joint fusion than joint drilling. However, it is worth mentioning that clinically affected horses may respond differently than healthy horses.
When surgical drilling, chemical injections, and laser surgery were compared in normal horses, surgical drilling and chemical injections resulted in enhanced bone bridging, while laser caused less pain and discomfort immediately post-surgery. Further research on horses with arthritis of the lower hock joints is needed to determine whether laser surgery can create lameness resolution without facilitating bony fusion as a by-product of surgery.
Outcome/prognosis
The complications of lower hock joint fusion are similar to other joints. The most commonly reported complications in the literature is post-operative infection, implant failure, laminitis in the opposite limb, long-term lameness and development of angular limb deformities. In general, for complicated cases of lower hock joint arthritis, surgical arthrodesis techniques can offer a good prognosis for resolution of lameness symptoms and should be considered as an option for cases that are refractory to medical management.
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Authors:
Rolf B. Modesto, VMD, DACVS-LA
Dane M. Tatarniuk, DVM, MS, DACVS-LA