When orthopedic surgeons hear ankylosing spondylitis (AS), a few things immediately come to mind: bamboo spine, sacroiliac fusion, and patients who look like they’re perpetually examining the floor.
The natural assumption might be that an inflammatory condition known for stiffness and fibrosis might also produce stubborn postoperative knees AND spines — perhaps leading to higher rates of manipulation under anesthesia (MUA)?
A recent large database study set out to test exactly that assumption.
A National Look at AS and TKA
Using a national insurance claims database, investigators analyzed more than 2 million total knee arthroplasties (TKAs) performed between 2010 and 2020. Among them were 4,131 patients with a diagnosis of ankylosing spondylitis — a small but clinically relevant population.
To make the comparison fair, researchers performed propensity score matching at a 1:5 ratio, resulting in a cohort of 3,512 AS patients matched to 17,560 non-AS TKA patients.
The primary question was simple and clinically relevant: Do patients with ankylosing spondylitis require manipulation under anesthesia more often after TKA?
Secondary outcomes included common arthroplasty complications such as prosthetic joint infection, periprosthetic fracture, aseptic loosening, dislocation, and other postoperative medical issues.
The Big Surprise: No Extra Stiffness
Despite the theoretical concerns, the data told a reassuring story.
Manipulation rates were essentially identical between the groups:
- 90 days: 2.36% (AS) vs 2.90% (non-AS)
- 1 year: 3.42% vs 3.95%
- 2 years: 3.62% vs 4.08%
None of these differences were statistically significant.
In other words, AS patients did not demonstrate an increased risk of postoperative stiffness requiring MUA.
For surgeons who worry that inflammatory disease might doom the knee to arthrofibrosis, this finding is encouraging. The knee, it seems, does not necessarily inherit the spine’s tendency toward rigidity.
Implant-Related Complications? Also Similar
The study also examined major joint-related complications. Once again, the results were reassuring.
Rates of prosthetic joint infection, periprosthetic fracture, aseptic loosening, and prosthetic dislocation were not significantly different between AS and non-AS patients.
From a purely orthopedic standpoint, TKA appears just as safe and durable in patients with ankylosing spondylitis as it is in the general population.
But Not Entirely Risk-Free
While the joint-related outcomes were comparable, two postoperative medical complications stood out.
Patients with AS had slightly higher rates of postoperative anemia (6.44% vs 5.40%) and urinary tract infections (4.53% vs 3.47%).
The differences are modest but statistically significant.
Why might this occur? Several factors could be at play. AS patients often carry a higher inflammatory burden, may be on immunomodulatory medications, and frequently have comorbid systemic disease. All of these can influence perioperative physiology and infection risk.
Clinical Takeaways for Arthroplasty Surgeons
For orthopedic surgeons, the message is refreshingly straightforward.
First, don’t fear stiffness. Ankylosing spondylitis does not appear to increase the risk of postoperative knee stiffness or MUA after TKA.
Second, implant outcomes look solid. Major joint complications are comparable to those seen in standard TKA populations.
Third, keep an eye on medical issues. Mildly increased rates of anemia and UTIs suggest that perioperative optimization and medical vigilance still matter.
The Bottom Line
Ankylosing spondylitis may fuse the spine, but it doesn’t appear to sabotage knee arthroplasty outcomes.
So, the next time an AS patient rolls into your clinic with bone-on-bone knees and a stooped posture, you can reassure them — and yourself — that a total knee replacement should behave much like any other.
Their spine may be stiff. Fortunately, their new knee probably won’t be.
Origin Study Title: Effects of Ankylosing Spondylitis on Total Knee Arthroplasty Outcomes and Post-operative Manipulation Under Anesthesia Rates
Authors: Marc N. Gadda, M.D.; John Norton, M.D.; Mehul M. Mittal, BBA; Garen Collett, M.D.; Michael Huo, M.D.; Robert C. Weinschenk, M.D.