When Joints Take a Beating: Mechanical Stress Accelerates Ankylosing Spondylitis in Mice

Ankylosing spondylitis has long been framed as a disease of genes and immune misfiring, but a new experimental study adds a stubbornly physical culprit to the mix: microtrauma. In research published in PLOS One, investigators report that a week of controlled mechanical stress applied to the ankle joints of mice can dramatically worsen the inflammatory and structural damage that follows in a well-established mouse model of the disease. The findings suggest that everyday mechanical overload—small, repetitive injuries that most people would shrug off—may act as a genuine accelerant for the pathological processes that fuse joints and destroy bone in this debilitating form of inflammatory arthritis.

The research team, led by Seung Hoon Lee and Tae-Hwan Kim along with colleagues, set out to address a striking gap in the experimental literature. Although clinicians have long suspected that physical trauma contributes to the onset or worsening of ankylosing spondylitis—patients frequently report that symptoms begin in regions subjected to mechanical strain—rigorous laboratory evidence for this connection has been scarce. Human studies, by their nature, struggle to separate mechanical stress from the many confounding factors that accompany it, from occupation and sport to infection and genetic background. The Korean group therefore turned to a carefully controlled animal model in which mechanical load could be manipulated with precision and its downstream consequences measured at the tissue level.

Their experimental design hinged on the SKG mouse, a strain carrying a mutation in the ZAP-70 gene that predisposes it to a spondyloarthritis-like syndrome closely mirroring human ankylosing spondylitis. When these mice receive an injection of curdlan, a gel-forming polysaccharide that triggers innate immune activation through inflammasome pathways, they develop progressive peripheral arthritis as well as inflammation at the entheses—the critical junctions where tendons and ligaments anchor into bone. Because the disease unfolds in a synchronized, predictable fashion after injection, the model offers a clean experimental window in which to ask whether an added mechanical insult changes the course of pathology.

To induce physiological microtrauma, the researchers immobilized the right ankle joints of the mice for one week using surgical plaster combined with a pipette tip as a rigid splint. The logic is counterintuitive but physiologically sound: when a limb is immobilized, the contralateral limb must compensate, absorbing greater mechanical load during normal movement. In this arrangement, the splinted right ankle served as an internal control, shielded from loading, while the freely moving left ankle became the microtraumatized limb, subjected to compensatory overload. After the plaster was removed, the mice received an intraperitoneal injection of 3 milligrams of curdlan to ignite the inflammatory cascade, and the investigators then tracked both ankles over time under identical systemic conditions.

Clinical arthritis scores, assessed weekly for five weeks, told the first part of the story. The microtraumatized left ankles of the curdlan-injected mice consistently exhibited more severe arthritis than their protected right counterparts, despite both joints sharing the same bloodstream, immune system, and genetic makeup. The only meaningful difference between them was mechanical history. That asymmetry, replicated across the experimental cohort, provides some of the most direct evidence to date that sub-traumatic mechanical stress—far below the threshold of a fracture or dislocation—can amplify inflammatory disease in a susceptible host.

Under the microscope, the differences became even more pronounced. Histological analysis at six weeks revealed aggravated synovitis, the inflammation of the joint lining that drives much of the pain and destruction in spondyloarthritis, and exacerbated enthesitis, the inflammation at tendon and ligament insertion sites that is considered a hallmark lesion of the disease family. Immunohistochemistry then pinpointed the molecular fingerprint of this aggravation: the synovium and enthesis of the overloaded ankles expressed markedly higher levels of the pro-inflammatory cytokines tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6. These signaling molecules are central players in inflammatory arthritis—biologic drugs targeting tumor necrosis factor in particular have transformed the treatment of ankylosing spondylitis in humans—so their local upregulation in mechanically stressed joints provides a plausible mechanistic bridge between tissue strain and immune escalation.

The structural consequences were equally striking when viewed in three dimensions. Using micro-computed tomography, the team quantified bone changes with high resolution and found a significantly elevated volume of inflamed bone in the microtraumatized ankles, indicating that mechanical stress had not merely intensified the soft-tissue inflammation but had also driven pathological remodeling of the skeleton itself. In ankylosing spondylitis, such remodeling is doubly destructive: chronic inflammation erodes bone while abnormal repair processes produce new bone that progressively fuses joints, robbing them of mobility. Any factor that accelerates this process is therefore of substantial clinical concern.

Perhaps the most unexpected finding emerged far from the site of the original stress. The left hip joints—the contralateral hip, located on the same side as the overloaded ankle—showed proteoglycan loss and narrowing of the joint space, classic radiological signs of cartilage degeneration. This distant pathology implies that the effects of compensatory microtrauma are not confined to the stressed joint itself. Altered gait and load distribution likely propagated abnormal forces up the limb to the hip, where the combination of mechanical derangement and systemic inflammation conspired to degrade cartilage. Alternatively, inflammatory mediators generated in the traumatized ankle may have contributed to distant tissue damage. Either way, the observation underscores how a focal mechanical insult can ripple through the musculoskeletal system when the body is primed for autoinflammation.

Taken together, the results position mechanical stress as a genuine aggravating factor in ankylosing spondylitis progression rather than a passive bystander. The compensatory overload of a single ankle was enough to worsen clinical scores, intensify cytokine expression in synovium and enthesis, expand the burden of inflamed bone, and instigate cartilage breakdown in a distant joint. For patients and clinicians, the message is provocative: protecting joints from repetitive strain, particularly during active or incipient phases of inflammatory disease, might meaningfully slow structural damage. The study also raises questions that the authors’ model is well suited to explore in future work—whether the same holds true for other joints, whether timing matters, and whether interventions that redistribute mechanical load could blunt the effect. For now, the experiment offers a compelling laboratory demonstration of what rheumatologists have long suspected: in a susceptible body, the slow accumulation of small injuries can help turn an inflammatory predisposition into a disease that locks joints in place.

Subject of Research: The effect of mechanical microtrauma on joint inflammation and structural damage in a mouse model of ankylosing spondylitis

Article Title: Effects of microtrauma on joint structural changes in the Curdlan-injected SKG mouse model of ankylosing spondylitis

Article References: Lee, S. H., Kim, D., Jeon, C., Jo, H.-R., Youn, J., Jo, S., & Kim, T.-H. (2026). Effects of microtrauma on joint structural changes in the Curdlan-injected SKG mouse model of ankylosing spondylitis. PLOS One, 21(10), e0360912. https://doi.org/10.1371/journal.pone.0360912

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360912

Keywords: ankylosing spondylitis, microtrauma, SKG mouse model, curdlan, synovitis, enthesitis, TNF-alpha, micro-CT, bone remodeling, spondyloarthritis, mechanical stress, joint pathology