AS Led to Rare Ankle, Foot Bone Fusion for Man, 69: Case Study

Researchers suggest "boot sign" terminology for X-ray findings

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by Steve Bryson PhD |

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Ankylosing spondylitis (AS) led to bone formation that resulted in the rare complete fusion of ankle and foot bones in a 69-year-old man, a case study reported.

His AS was discovered only after hospitalization for kidney problems and an examination for gout.

The researchers suggested the terminology “boot sign” for X-ray findings of the complete ankle and foot fusion because they resemble the appearance of a boot.

The case study, “Complete Bilateral Ankle Fusion: A Rare Complication of Ankylosing Spondylitis,” was published in the journal Cureus.

AS is a type of arthritis marked by long-term inflammation in the spine’s joints, especially where the spine joins the pelvis — known as the sacroiliac joints. Although less common, the condition also has been associated with eye inflammation (uveitis), inflammatory bowel disease, the skin condition psoriasis, as well as arthritis in the arm and leg joints.

The combination of inflammation and bone changes leads to the formation of boney growths. These growths fuse adjoining vertebrae and result in the classic sign of a “bamboo” spine in images.

Such changes also trigger chronic low back pain, mainly occurring in most men before age 40. Because AS progresses slowly, diagnostic delays of up to 10 years can occur.

In this report, investigators based at the Einstein Medical Center Philadelphia, in Pennsylvania, described the case of an older man whose ankle joints had fused due to undiagnosed or untreated AS.

“We present a rare case of AS with the fusion of both ankles that was either unrecognized, untreated, or a combination of both for multiple decades,” the investigators wrote.

The patient was treated now, at the clinic, for pain in his sides, along with nausea and vomiting. An African American male, the patient had a history of chronic low back pain, swelling in the lower extremities, walking problems for nearly 20 years, gout, and chronic kidney disease.

Based on blood tests, he was sent to the emergency room to be assessed for acute worsening of kidney function. Upon examination, there were no signs of rash, joint pain, swelling, or other uncommon signs of AS. He was admitted to the hospital to undergo dialysis to support kidney function.

Because urine analysis showed moderate blood, the team conducted a CT scan of his abdomen and pelvis for possible kidney stones. Although the CT was negative for kidney involvement, it revealed advanced AS of his spine, or bamboo spine.

After three days in the hospital, he complained of hand and foot pain typical of a gout flareup. On physical examination, he had tender swelling of the hand joints, the knees, and the joints of all toes. Notably, his ankles had no range of motion. In addition, he had a large growth under the skin on the right elbow.

Based on these findings, he was diagnosed with acute gout and first treated with allopurinol, a standard gout therapy, and then an anti-inflammatory steroid.

X-rays of the hands and feet were conducted to assess joint damage, which showed the complete fusion of the bones of both ankle joints and the hindfoot (heel) and midfoot, consistent with AS affecting the ankle.

His acute arthritis, including foot pain, resolved on the fourth day of treatment. Despite improvements in all other joints, his ankles still had no range of motion.

His past walking difficulties were confirmed by a previous X-ray from 16 years before that showed significant swelling around the ankle joint and partial fusion of the ankle joints. Because additional medical records were unavailable, it was unknown whether diagnosis and treatment were previously offered.

“There are limited reports of ankylosis of the foot in patients with AS,” the researchers concluded. “In this case, we report a rare presentation of bilateral [two] ankle joint ankylosis in AS as an incidental finding while searching for gouty erosions during an acute gout flare.”

Given the novel findings, the team made recommendations for appropriate new terminology for any future cases.

“We suggest the terminology ‘boot sign’ for radiographic findings of complete ankle fusion and hindfoot fusion akin to the appearance of a boot,” the researchers wrote. “It is important to note that the multiple superimposed issues can potentially complicate the clinical picture.”

Overall, this case highlights the need for better diagnosis of AS, the team said.

“Early detection of and intervention against ankylosing spondylitis are necessary for better patient outcomes and quality of life,” they wrote.