34th Annual Scientific Meeting proceedings

Key Points

• Osteochondrosis and intra-articular trauma are the most common pathologic conditions of the canine tarsus
• Conservative treatments fail to consistently eliminate pain, restore full function, and provide long-term solutions
• Pantarsal arthrodesis is the current standard of care for intractable end-stage talocrural osteoarthritis
• The TATE ankle replacement is a resurfacing total joint replacement solution to these challenging problems
• Long-term follow-up on these initial cases using subjective assessment has been promising and readers are encouraged to consider TAR as a potential solution for patients with severe tarsal OA

Canine tarsal pathology
Although not as common as stifle, hip, and elbow problems, pathologic conditions of the canine tarsus typically lead to osteoarthritis (OA) and progression in chronic pain, exercise intolerance, and reduction in quality of life. The two most common injuries affecting the canine tarsus are osteochondrosis (OC)/osteochondritis dissicans (OCD) and traumatic injuries such as intra-articular fracture or luxation. OC/OCD of the canine tarsus is a frustrating disease for the small animal surgeon.

Current surgical treatment
Surgical treatment options that are considered standard of care include arthroscopic debridement of the OC lesion with curettage of the subchondral bone bed and pantarsal arthrodesis (PTA). These two treatments represent both ends of the surgical spectrum, with arthroscopy providing a minimally invasive approach and arthrodesis representing a salvage procedure capable of eliminating pain and providing acceptable function (but unable to restore normal kinetics). Furthermore, arthroscopic debridement does not uniformly provide long-term improvement in lameness for the majority of dog sand typically requires long-term additive medical management. While pantarsal arthrodesis can improve lameness and does not require long-term medical management, complications associated with pantarsal arthrodesis are not uncommon and include infection, implant failure, and peri-implant fracture. Thus, there is a clear need for an improved solution for dogs with debilitating intractable tarsal OA.

Total Ankle Replacement
Total Ankle Replacement (TAR) is a novel approach to the treatment moderate or severe osteoarthritis (OA) of the talocrural joint. The impetus to develop a Total Ankle Replacement was to improve function by eliminating pain and restoring at least some on the talocrural range of motion. Like with any surgical procedure, surgeons should anticipate complications and have exit strategies if these occur. The lack of such strategies has likely contributed to the limited popularity of total elbow and total knee replacement in dogs. Conversely, one of the potential benefits of a TAR approach is that PTA is considered standard of care for tarsal OA.

Like its predecessor, the TATE elbow, the TATE ankle uses a cartridge implant design where the two components, a titanium tibial component with a vitamin E infused/cross-linked UHMWPE bearing and a titanium talar component are inserted into the joint simultaneously. The titanium in contact with native bone has been coated with HA (hydroxyapatite) to improve bone ingrowth. In order to accomplish simultaneous tibial and talar insertion, the articular surfaces of the tibia and talus must be prepared concurrently. This is accomplished through a medial approach to the joint and medial malleolar osteotomy. The central axis of rotation is identified and established using a reference post termed the center of rotation (COR) post. The joint is fixed in place using a surgery-specific alignment plate with a series of Steinmann pins and sleeves. Next, a proprietary “Mill and Drill” technique allows the tibial and talar articular surfaces to be removed without having to luxate the joint and further compromise the surrounding soft tissues. Once the implant is inserted into the joint, expansion bolts are driven through horizontal implant posts expanding them into the surrounding subchondral bone, effectively locking each implant to its respective bone. The malleolus is then reattached using either a combination of screws and washers or a small bone plate, and the soft tissue is sutured. When compared to the TATE elbow surgery, the TATE TAR is simplified due to a relatively more straightforward approach, reduced soft tissue envelope, and the fact that the tarsocrural joint is a classic hinge-joint with two primary weight bearing surfaces as opposed to the complex bearing surfaces of the elbow. Importantly, there is currently only one TATE TAR implant size (17 mm) and as such, is an option for dogs approximately 30-45 kg body weight. Pre-operative templating is necessary to determine if individual patients meet current sizing requirements.

Clinical outcome
An initial series of 10 TATE TAR procedures was performed by Dr. Randy Acker at the Sun Valley Animal Center in Ketchum, Idaho beginning in 2014. Of those 10 cases, major complications occurred in 3 and minor complications in 4. The 3 major complications were the result of initial implant design that allowed the polyethylene bearing to dislodge from the tibial component when the tarsus was positioned in hyperflexion. Two of these cases were treated with explantation and PTA. One case was resolved by liner exchange and fixation of the new liner to the tibial component with PMMA. The implant design has subsequently been updated to prevent liner dislodgement.  

All of minor complications in the initial case series involved the tendon sheath of the deep digital flexor on the caudal aspect of the ankle (tendon sheath syndrome). Free bodies can develop within the tendon sheath contributing to lameness prior to surgery. These should be removed in conjunction with TAR. Additionally, abrasion of the sheath can occur during the milling process. The surgical technique has been updated to address this issue and the current technique requires identification and retraction of the flexor tendon during the milling process. Lastly, one patient in the original series had an osteophyte or bone fragment from the milling process dislodge and irritate the tendon sheath. A second surgery was required to identify and remove this structure 3-4 weeks post-op which resolved the lameness.

In 2018, The TATE TAR program was expanded to a group of 9 surgeons and associated joint replacement centers in North America and Europe. Although the exact number of surgical procedures performed to date is unknown, approximately 20-25 surgical procedures have been performed globally (in addition to the original 10 cases), with long-term follow-up of ~5 to 6 years. In the hands of a small group of experienced joint-replacement surgeons, initial results have been encouraging. Most dogs treated with this procedure exhibited marked improvement in lameness by 3-6 months post-operatively, with many dogs exhibiting a normal gait and no reported lameness even in the presence of heavy activity. In addition to the three liner dislodgement events discussed above, major complications in subsequent cases have included: bacteremia-induced implant-associated-infection (1), joint luxation (1), incomplete tibial ingrowth (1), undersized implant (1), tarsal sheath syndrome (1), and osteotomy screw loosening (1).

While these results are encouraging, it is important to remember that case numbers are small and results are somewhat subjective. Complications such as infection and polyethylene wear are possible. Infection can be treated with culture, debridement, lavage, and long-term antibiotics, however explantation with conversion to arthrodesis is a more likely outcome of implant-associated infection. Additionally, the polyethylene liner is somewhat thin (4mm) and long-term polyethylene wear is also possible. To date, a case of clinically significant poly wear and metal-on-metal contact has not been identified. If identified, explantation and arthrodesis or a technique to accomplish polyethylene exchange will be required.

In conclusion, the TATE TAR system is a promising solution for moderate to severe tarsal OA. As with any new procedure, major complications are possible and client-education must be a central component of decision-making. TAR should be considered a viable option for clients that desire long-term elimination of pain and restoration of tarsal function.