Discussion

Osteoarthritis (OA) of the distal tarsal joints is one of the most prevalent causes of hindlimb lameness in the horse. Diagnosis is made by a combination of procedures which include physical and lameness examinations, diagnostic analgesia and imaging modalities. Nuclear scintigraphic evaluation may be useful in certain cases.
In spite of significant diagnostic advances, treatment options available to the general practitioner are still limited. Those available can be divided into nonsurgical and surgical. Nonsurgical treatments involve systemic and local medication which, combined with exercise management, attempt to reduce inflammation and pain (Baxter et al. 2003). Surgical options include procedures aiming to stimulate ankylosis of the small tarsal joints (Zubrod et al. 2005), to desensitise the area by tibial neurectomy (Imschoot et al. 1995) or by reducing pressure on the tarsus by cunean tenectomy (Baxter et al. 2003). Several methods of stimulating ankylosis by physical or chemical cartilage destruction have been described. These include drilling across the joint surfaces, with or without additional internal fixation, the use of a laser and controlled injection of monoiodoacetate (MIA) (Bohanon et al.
1991). Results from the use of MIA vary greatly between studies (Dowling et al. 2004; Zubrod et al. 2005) and may require up to 3 injections to achieve ankylosis (Bohanon et al. 1991).
Monoiodoacetate causes rapid cell death and although one study has found that its use did not cause significant pain in the post treatment period (Dowling et al. 2004) others, (Bohanon et al. 1991; Sammut and Kannegieter 1995; Zubrod et al. 2005) as well as the author, have seen significant discomfort following its use. Monoiodoacetate however does seem to induce significant new bone formation and a stable ankylosis. In view of other options for chemical arthrodesis, the author considers MIA not to be an acceptable method any longer.
Recently, Shoemaker et al. 2006, described the effects of
intra-articular infiltration of ethanol into the tarsometatarsal joints of sound horses. Importantly, this report described histological evidence of ankylosis 12 months post injection and reported no associated significant complications or discomfort post injection.
Since then, 2 publications (Lamas et al. 2012; Carmalt et al.
2012) have evaluated the effects of this approach in clinical cases. The results of these studies have shown that facilitated ankylosis with ethanol is a valid treatment option for pain caused by osteoarthritis of the distal tarsal joints.
Although the success rates of this treatment in these 2 studies differ by a significant degree, they have common findings in few points: 1) The treatment with ethanol appears generally safe with signifcant complications only occurring in 2 cases in one study (Lamas et al. 2012); 2) A radiographic contrast study of the injected tarsal joints is highly recommended in order to prevent injection into the proximal intertarsal-tibiotarsal joint; 3) Radiographic evidence of ankylosis is either minimal or difficult to assess; 4) Authors of both studies recommended caution in the use ethanol as first line treatment in cases of OA of the distal tarsal joints.
Results also appear to be more encouraging in the early stages of the process compared with horses where the condition was considered chronic and unresponsive to intra-articular steroids (Lamas et al. 2012). The explanations for this are speculative but are likely to be associated with the presence or development of multiple pathologies involving other structures in the region.
Regarding the technique, it involves a single needle approach with a 2 or 3 way tap attached to the needle. The radiographic contrast (iohexol) is injected first and radiographs evaluated for presence of contrast within the PITJ and TBT joint. The contrast is allowed to drip out of the joint and then 70% ethanol is injected until there is resistance (2 - 3 ml) (Lamas et al. 2012).
No discomfort is expected post injection but a mild subcutaneous swelling at the injection site might occur due to leakage around the needle or the joint entry site, which should disappear within days. To minimise this, a minimal number of attempts should be made to enter the joint.
Of greater concern is the fact that a small percentage of horses developed PITJ OA and associated significant lameness long-term (approximately 6 months post treatment) in one study (Lamas et al. 2012). This complication has also been reported after MIA treatment (Bohanon et al. 1991; Sammut and Kannegieter 1995) and could be due to: a change in tarsal biomechanics, with distal ankylosis increasing stress in the proximal joints; or possible contamination of the proximal joint with ethanol at the time of treatment. Regardless of its cause, this should be seen as a potential complication and explained to owners.
The author supports the use of 70% ethanol solution for treatment of distal tarsal joint OA in cases which do not respond to corticosteroid medication. Careful case selection and accurate contrast-facilitated injection technique are mandatory to minimise potential complications of this treatment.