Vitamin D deficiency is particularly prevalent among elderly individuals. It is estimated that vitamin D deficiency affects approximately one billion individuals -nearly 50% of the global population- across diverse ethnic and age groups [1]. Vitamin D deficiency impacts multiple organ systems beyond the musculoskeletal system and contributes to a range of conditions, including cardiovascular disease, immune dysfunction, and metabolic disorders [2,3]. Previous studies have demonstrated that vitamin D deficiency plays a significant role in insufficiency fractures secondary to osteoporosis in elderly patients, primarily through mechanisms such as impaired bone mineralization and muscle weakness [4-7]. Furthermore, the prevalence of vitamin D deficiency has been steadily rising, with a marked increase observed in younger populations, suggesting a growing risk of osteoporotic fractures in this age group [8]. Despite these implications, few studies have investigated fractures associated with vitamin D deficiency in young adults.

Here, we report the case of a 39-year-old male diagnosed with a subchondral insufficiency fracture of the medial tibial condyle following minor trauma with a history of multiple rib fractures. This case report was approved by the Institutional Review Board (IRB) of the Jeju National University Hospital (IRB No. JEJUNUH 2025-06-024).

A 39-year-old male patient presented to our hospital with a 3-day history of right knee pain that began after running on a sports field. He reported no significant trauma at the time of onset but noted progressive discomfort in the medial aspect of the knee. He had previously been evaluated at a local orthopedic clinic, where ultrasonography suggested a cystic lesion near the medial meniscus, prompting a referral for further evaluation. He was employed as a restaurant worker, spending more than 10 hours a day cooking and serving. He smoked and consumed alcohol in moderation. He had no recent history of trauma, steroid use, or illness and his family medical history was unremarkable. There was no history of excessive training, malignancy, or infection. The patient’s height, weight, and body mass index were 162.5 cm, 72 kg, and 27.2 kg/m², respectively. Plain radiographs of the knee revealed no abnormalities (Fig. 1).

Physical examination revealed tenderness over the medial aspect of the right knee without obvious deformity, swelling, or limited range of motion. There were no other clinical signs or symptoms suggestive of a systemic disease, and the patient’s presentation was limited to localized right knee pain. Based on the prior ultrasonography findings, we suspected a medial meniscal lesion and performed magnetic resonance imaging (MRI). The MRI revealed bone marrow edema, low signal intensity on T1-weighted images, and patchy increased signal intensity on T2-weighted images of the medial tibial condyle, suggesting a subchondral insufficiency fracture (Fig. 2).

After a detailed medical history was obtained, the patient’s background became notable. He had a history of multiple rib fractures, each occurring after minor trauma, such as a sudden force during physical activity or an accidental impact while playing with his child. These episodes raised concerns about the underlying bone fragility. Low bone mineral density (BMD) was found in the lumbar spine (BMD, 0.63 g/cm², T score, -3.0; Z score, -2.3) on dual-energy X-ray absorptiometry (DXA). To evaluate for additional fractures at other skeletal sites, assess bone turnover, and rule out other metabolic bone diseases, bone scintigraphy was performed, which demonstrated increased uptake localized to the medial aspect of both knees (Fig. 3). Laboratory tests revealed a 25-hydroxyvitamin D level of 13.0 ng/mL, indicating severe vitamin D deficiency (<20 ng/mL). Other laboratory findings, including thyroid function tests and parathyroid hormone (PTH) levels, were within normal ranges. The results of the laboratory tests are summarized in Table 1.

Although a comprehensive hormonal evaluation, including follicle-stimulating hormone, luteinizing hormone, and testosterone levels, was not performed, the patient denied any symptoms typically associated with hypogonadism or other endocrine disorders. Given the absence of other clinical symptoms aside from the fractures, the patient was diagnosed with secondary osteoporosis due to vitamin D deficiency, which contributed to both the current subchondral insufficiency fracture and his previous multiple rib fractures. Management included supplementation with vitamin D and calcium, along with PTH analog therapy. During subsequent follow-ups, the patient reported significant symptomatic improvement, and no additional fractures occurred. At the 2-year follow-up, the patient remained asymptomatic with no new fractures, maintained excellent functional status, and demonstrated sustained improvement on repeat DXA and bone scintigraphy.

Vitamin D plays a crucial role in various physiological systems through vitamin D receptors, which are activated by their metabolically active form 1,25-dihydroxyvitamin D₃ (1,25[OH]₂D₃) [9]. Vitamin D deficiency adversely affects calcium metabolism, osteoblastic activity, and bone remodeling, with low serum 25(OH)D levels associated with secondary hyperparathyroidism and increased bone turnover [10-14]. Since most circulating vitamin D3 is synthesized in the skin from 7-dehydrocholesterol via ultraviolet B (UVB) radiation rather than dietary intake, adequate sun exposure is essential for maintaining vitamin D sufficiency [15-17].

The prevalence of vitamin D deficiency has become a significant global health concern, affecting approximately half of the world’s population and, contrary to previous assumptions, showing particularly high rates among younger individuals, owing to modern lifestyle changes and occupational factors [8,18,19]. Occupational environments with limited sunlight exposure, such as indoor work settings, significantly contribute to this growing epidemic. Shift and indoor workers are particularly susceptible due to limited UVB exposure during peak hours [18,20]. Our patient’s occupation as an indoor restaurant worker for more than 5 years exemplifies this high-risk occupational category. Recent studies have highlighted a strong correlation between occupational settings and vitamin D deficiency. A large-scale Korean study revealed that shift and office workers were particularly vulnerable due to limited sunlight exposure during peak UVB hours [8,21]. Similarly, an Indonesian study found a significant association between vitamin D deficiency and women employed in indoor occupations [22]. This suggests that younger patients, particularly those with such risk factors, may be at increased risk of vitamin D deficiency.

The relationship between vitamin D deficiency and bone fragility has been studied extensively. Ruohola et al. [23] conducted a landmark prospective study of 756 finnish military recruits, demonstrating that those with serum 25(OH)D concentrations below 75.8 nmol/L (30.3 ng/mL) had a 3.6-fold increased risk of stress fractures during basic military training. This study established a clear dose-response relationship between vitamin D status and fracture risk in physically active young men. Similarly, Lappe et al. [24] conducted a randomized controlled trial involving 5,201 female navy recruits and found that calcium and vitamin D supplementation (2,000 mg calcium and 800 IU vitamin D daily) reduced the incidence of stress fractures by 20%. More recently, Williams et al. [25] reported on vitamin D3 supplementation in 118 National Collegiate Athletic Association (NCAA) Division I athletes from high-risk sports, demonstrating a dramatic reduction in stress fractures from 7.51% in historical controls to 1.69% in the supplemented group.

Our case shows several distinct features compared with previously documented cases. First, the patient developed a subchondral insufficiency fracture despite being sedentary and primarily working indoors as a restaurant worker. Second, unlike stress fractures that typically occur in weight-bearing bones due to repetitive loading, our patient presented with an insufficiency fracture, indicating that severe vitamin D deficiency can compromise bone integrity even without excessive physical loading.

Our case highlights the increasing relevance of vitamin D deficiency in younger populations, which has traditionally been overlooked in osteoporosis research. Unlike previous studies that primarily focused on elderly populations, this case demonstrates the potential for osteoporotic fractures in young adults, emphasizing the importance of systematic evaluation when young adults present with unexplained fractures or bone pain.

Based on our experience, clinicians should consider a systematic evaluation that includes the assessment of occupational and lifestyle risk factors, bone mineral density, and targeted imaging such as bone scintigraphy. In patients without known predisposing conditions, such as malignancy, inflammatory diseases, or long-term glucocorticoid use, vitamin D deficiency could be an important consideration. High-dose vitamin D combined with calcium supplementation provided a therapeutic foundation for our patient, while severe cases with established osteoporosis and fragility fractures may benefit from anabolic agents such as teriparatide to accelerate fracture healing.

From a broader perspective, this case highlights the value of preventive strategies targeting high-risk occupational groups. Routine screening of vitamin D levels in young adults, particularly those with occupational or lifestyle risk factors, could be instrumental in preventing long-term complications [19]. Public health campaigns focusing on occupational health awareness, dietary supplementation, and lifestyle modifications could help mitigate the increasing prevalence of vitamin D deficiency. Moreover, given that supplementation is more cost-effective than managing vitamin D deficiency-related complications, tailored interventions for these vulnerable groups represent an important public health priority [26-28].

In summary, our report describes the case of an insufficiency fracture in the right knee of a young adult. Insufficiency fractures related to vitamin D deficiency should be considered even in young adults presenting with atypical pain not attributable to significant physical activity, particularly in at-risk groups. We recommend performing a BMD test and obtaining a thorough history of the patient’s occupation, living environment, and lifestyle factors as soon as possible.