Effects of Functional Magnetic Stimulation on Pain, Function, and MRI-Derived Outcomes in Athletes with Tibial Bone Stress Injury: A Randomized Controlled Trial

Background: Tibial bone stress injury (TBSI) with associated bone marrow edema (BME) is a common and clinically challenging condition in athletes, often requiring prolonged load restriction and delayed return to sport. Evidence for adjunctive interventions that improve both symptoms and MRI-derived recovery remains limited. The aim of this study was to investigate whether adding Functional Magnetic Stimulation (FMS) to standardized rehabilitation improves pain, function, and MRI-derived outcomes over 16 weeks in athletes with MRI-confirmed TBSI. Materials and Methods: Forty athletes with Fredericson grade 2–3 TBSI were randomized to FMS plus rehabilitation (n = 20) or rehabilitation alone (n = 20). Both groups completed a 4-week load-based rehabilitation program, while the FMS group additionally received eight 30 min FMS sessions at 40 Hz. Outcomes were assessed at baseline, 4 weeks, and 16 weeks. The primary outcome was activity-related pain (NPRS), while secondary outcomes included lower-limb function (LEFS-GR), BME extent, and Fredericson grade. Continuous outcomes were analyzed using two-way mixed ANOVA, whereas Fredericson grade was analyzed using an ordinal generalized estimating equation model. The level of statistical significance was set at p < 0.05. Results: Groups were comparable at baseline. Significant group × time interactions favored FMS for NPRS, F(2, 76) = 12.46, p < 0.001, η2p = 0.247; LEFS-GR, F(1.08, 41.09) = 81.56, p < 0.001, η2p = 0.682; and BME extent, F(1.44, 54.85) = 49.33, p < 0.001, η2p = 0.565. At 16 weeks, the FMS group showed lower NPRS scores, higher LEFS-GR scores, and lower BME extent than controls. Fredericson grade also showed a significant group × time effect, Wald χ2(2) = 20.38, p < 0.001. The FMS group had significantly lower cumulative odds of classification in a higher Fredericson grade at 4 weeks (OR = 0.045, 95% CI: 0.012–0.163, p < 0.001), whereas the corresponding difference at 16 weeks remained directionally favorable but did not reach statistical significance (OR = 0.033, 95% CI: 0.001–1.029, p = 0.052). Conclusions: The addition of FMS to load-based rehabilitation was associated with greater clinical and functional improvements, greater reductions in BME extent, and a more favorable longitudinal trajectory in MRI severity classification than rehabilitation alone in athletes with TBSI. However, because the study did not include a sham-FMS condition and participants could not be blinded, the self-reported pain and functional outcomes should be interpreted cautiously. FMS may represent a useful adjunct to structured rehabilitation, although sham-controlled and longer-term studies should determine its effects on return-to-sport progression and recurrence risk.