footEducation

Ankle replacement is occasionally performed as a treatment for end-stage ankle arthritis (See Figure #1).  It is typically indicated in older, lower demand individuals, as the lifespan of the ankle replacement is uncertain.  Patients that have arthritis (or fusions) involving the joints below the ankle (subtlar joint and talonavicular joint -See Figure #2), may benefit from an ankle replacement as it will help to preserve some hindfoot motion.  Relatively young, active patients typically do not do well with ankle replacements in the long run due to an unacceptably high failure rate.  These younger patients are usually best served with a well performed ankle fusion.  The results of ankle replacement, unfortunately, have not matched the results we see in knee and hip replacements.  This is a product of many factors including the fact that the ankle joint bears about twice the force that the knee joint does and yet it has approximately half the surface area. 

Figure 1A:  Normal Ankle Joint on X-ray

Figure 1B:  Ankle Arthritis on X-ray

(Loss of ankle joint space)

The procedure depends on the type of prosthesis that is used.  In all ankle replacements the arthritic surface of the distal tibia is removed, as is the arthritic surface of the top surface talus.  In certain prosthesis, such as the Agility from DePuy (See Figure #3), the tibia and the fibula are fused in order the increase the weightbearing surface area.  In other prosthesis, such as the Tornier Salto Talaris (See Figure #4), only the tibial surface and some of the talar dome are removed.  The resected areas of bone are then replaced with a prosthesis.  The prosthesis typically, has a metal surface on the tibial and talar sides with a polyethylene surface attached to the tibial component (See Figure #5).

Figure 3: Depuy Agility Total Ankle Arthroplasty

Figure 4:  Tornier Salos Talaris Total Ankle Replacement

These include the potential complications that can occur with any surgery such as:

Infection

Wound healing problems

Deep Vein Thrombosis (DVT)

Pulmonary Embolism (PE)

Nerve Injury

There are some specific complications that occur with total ankle replacement including:

Deep Infection of the Prosthesis.  The deep wound infection rate in patients undergoing Total Ankle Replacement is not necessarily any higher than with other major ankle surgery, however, the significance of a deep infection is profound.  A deep infection often requires removal of the prosthesis in order to eradicate the infection.  Needless to say, this is a major complication when it occurs.  The “deep infection rate” is on the order of 1-3%.

Major Wound Breakdown.  Most total ankle replacements are inserted through an incision in the front of the ankle.  This area of tissue has a somewhat tenuous blood supply.  It is supplied by one main artery, whereas most other areas of the body are supplied by two or more major arteries.  For this reason difficulty with wound healing occurs at a much higher rate in patients undergoing total ankle replacement.  Often, this requires immobilizing the ankle for a few weeks to improve the chances of successful wound healing.  However, in some patients an area of wound breakdown, or failure to heal will occur.  This can be a difficult and potentially devastating complication if the failure to obtain adequate wound healing leads to a deep infection involving the prosthesis.

Tibial Nerve Injury.  There is a chance of an injury to the surrounding nerves including the tibial nerve when the ankle joint is prepared to receive the ankle prosthesis.

Failure of the Ankle Replacement over time.  All joint replacements will eventually fail if the patient uses the joint enough, and lives long enough.  There are a variety of ways that joint replacements can fail leading to pain and dysfunction.  Perhaps the most common mode of failure of a joint replacement is from shifting of the prosthesis when the supporting bone becomes weak from repetitive loading or osteolysis. 
Total ankle replacements historically have failed earlier, and at a substantially higher rate when compared to knee and hip replacements.  For examples studies have suggested that certain hip replacements have good or excellent results in over 90% of patients after 18 years.  In knee replacements the figures are close to 90% success after an average of 13 years.  However, in ankle replacements the best prospective study suggests that 85% of patients have a successful result after 5 years -not bad, but no where near as successful as knee and hip replacement surgery.

Ankle replacements fail at a higher rate because of a variety of factors related to the ankle joint itself including:

• the small joint surface area (half the size of the knee joint)
  high joint reactive forces during walking (2-4 x body weight, almost twice that of the knee joint)
• Uneven distribution of force across the tibial prosthesis (see Figure #6)
• The lower bone of the ankle (Talus) is relatively small and has a poor blood supply providing a less than ideal base of support for the prosthesis.
• There are limits to how much bone can be removed from the ankle joint and this limits the size of polyethylene that can be used.  The smaller the polyethylene the poorer the wear characteristics
• The relatively confined nature (many important structures nearby) of the ankle joint makes placing an ankle replacement technically challenging for the surgeon.

When an ankle joint fails a revision surgery is necessary.  Often the prosthesis can be replaced.  However, there is much less bone stock available around the ankle so revision surgery is often substantially more difficult with results that are less predictable that the original operation.