The Joint Line Trap in Total Ankle Arthroplasty

Reading studies on this topic call for heightened scrutiny; here's one way to look at them

The Joint Line Trap in Total Ankle Arthroplasty
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Editor’s Note: Sometimes at CORRelations, we’ll cover an article we think is likely to cross your desk, but is one whose message we have reservations about. Here’s one. — SSL

What’s the Claim?

A retrospective study of patients undergoing total ankle arthroplasty (TAA) found a modest correlation between an elevated postoperative joint line and the following parameters 1 year after TAA:

  • Decreased dorsiflexion
  • Reduced total ROM
  • Poorer outcomes scores for pain and function

How’s It Stack Up?

It’s clear enough from prior studies, everyone’s experience, and simple common sense that more-severe ankle arthritis and a higher preoperative joint line go together. While it’s possible to force it back down, this comes with serious tradeoffs, and one has to be very, very careful not to mistake correlation with causation — as this study does, starting in its title — the word “influence” implies something that this study (and no study yet) has proven.

The concerning issue here, of course, is that of confounding variables. To take just one possible example, patients with more-severe arthritis and higher pre-op joint lines also often have poorer pre-op scores reflecting greater disability before surgery. If we don’t get their joint lines back down to normal, it’s true that they won’t score as well after surgery as might patients with milder pre-op arthritis, but that may be more a function of less-severe pre-op impairment than anything to do with the joint line itself. An analogy might be that patients who smoke often have yellow fingers, but treating the nicotine staining of the skin of patients’ hands will do nothing to prevent lung cancer.

What’s Our Take?

The fact that the amount of rainfall in San Francisco rose and fell with the number of printing-press operators working in Rhode Island doesn’t mean that if we lay off printers in Providence the weather in SF will improve. The rooster crowing doesn’t bring the day, either.

These are examples of the post hoc ergo propter hoc fallacy, as is the main finding of this study.

Last words on that topic go to CORRelations’ advisor in Foot & Ankle, Greg Guyton, MD, who nails it (as does the accompanying commentary in the journal where the article was published):

The joint line height ratio the authors cite almost never gets back to normal. Intraoperative tensioning is done through soft tissue releases as necessary and trialing larger spacers; the known tradeoff is increasing stability versus decreasing motion. Although the data of the study are correct, the implication that one should focus on restoring the native joint line is a bad suggestion. Patients with higher joint lines likely had worse arthritis and worse soft tissue contractures to begin with. Because of the contractures, they were stable at a smaller poly size during the surgery. Using a larger poly to try to restore the joint line may result in overtensioning those ankles, and probably will not improve outcomes as the authors speculate. 

Source

Palma J, Shaffrey I, Kim J, et al. Influence of Joint Line Level on Clinical Outcomes and Range of Motion in Total Ankle Arthroplasty. Foot Ankle Int. 2024;45:10-19.

Further Commentary

Easley ME, Anastasio AT. Range of Motion After Total Ankle Replacement: Are We Just Stuck? Foot Ankle Int. 2024;45:30-32.