Atypical ulnar fracture induced by chronic mechanical overload in the absence of antiresorptive therapy: A case report

INTRODUCTION

Osteoporosis is a major risk factor for fragility fractures, and antiresorptive therapies such as bisphosphonates and denosumab are widely used to reduce this risk. Although effective, prolonged exposure to these agents has been associated with atypical fractures, most commonly in the femoral diaphysis, as characterized by the American Society for Bone and Mineral Research criteria.^[1] These fractures are thought to result from impaired microdamage repair due to excessive suppression of bone turnover.^[1]

While atypical fractures at non-femoral sites are uncommon, atypical ulnar fractures represent a rare entity. In a large multicenter cohort study, atypical ulnar fractures accounted for only 0.59% (11 of 1,873) of all ulnar fractures.^[2] A 2024 systematic review identified that only 40 documented cases existed in the global literature up to 2021.^[3] Most published reports of atypical ulnar fractures have been associated with substantial antiresorptive exposure.^[4,5] We present a case of an atypical proximal ulnar fracture in a patient with severe osteoporosis, occurring despite the absence of prolonged antiresorptive therapy and indicating chronic mechanical loading.

CASE REPORT

An 84-year-old right-handed female patient presented with persistent left forearm pain following a minor contact sustained when the patient accidentally brushed her arm against a desk. Medical history included severe osteoporosis (t-scores of dual-energy X-ray absorptiometry: −3.2 at L2–3, −3.7 at the femoral neck) and a lumbar compression fracture treated with vertebroplasty 3 years earlier, resulting in chronic low back pain. The patient had received a single denosumab injection 3 years earlier and had no history of long-term antiresorptive therapy. The patient was taking telmisartan, amlodipine, bisoprolol, furosemide, hydralazine, rosuvastatin, and edoxaban. For the management of chronic knee osteoarthritis and low back pain, the patient was taking celecoxib, gabapentin, afloqualone, and limaprost. Concurrent use of a proton pump inhibitor alongside nonsteroidal anti-inflammatory drugs was suspected but could not be confirmed through the available medical records. The patient denied any current or prior history of corticosteroid therapy.

After a compression fracture 3 years earlier, the patient experienced persistent back pain and habitually used her non-dominant left forearm as a weight-bearing pillar against furniture for postural support, which allowed her to use her dominant right hand for daily tasks, and at times moved by crawling at home, bearing weight predominantly through the forearms. The patient reported no pain in the proximal forearm before the inciting event.

Physical examination revealed hyperkeratotic changes (“kitchen elbow sign”) over the posterior proximal ulna bilaterally, consistent with chronic pressure and friction.^[6] Radiographs demonstrated a transverse fracture of the proximal ulnar shaft, specifically occurring at the site of its maximal posterior apex angulation [Figure 1a]. Computed tomography showed marked cortical thickening and prominent sclerosis along the fracture margins, fulfilling the criteria for a Stage III fracture according to the Kinoshita classification [Figure 1b].^[2] Radiographs showed no evidence of cortical hypertrophy or a non-displaced fracture in the contralateral proximal ulna or in the bilateral femur, and bone scintigraphy demonstrated no abnormal uptake in the contralateral elbow or in either femur.

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Figure 1: (a) Anteroposterior radiograph shows a transverse fracture at the proximal ulnar diaphysis with minimal comminution; (b) Computed tomography image demonstrates sclerotic fracture margins and cortical thickening at the dorsal and volar cortices.

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Surgical exploration confirmed an atrophic nonunion, with sclerotic fracture edges, absence of callus formation, and granulation tissue at the fracture site [Figure 2]. The patient underwent curettage, open reduction, and internal fixation with a plate and screws and allogeneic cancellous bone grafting 16 days after the injury [Figure 3]. Histopathological analysis of the ulnar shaft revealed dead bone fragments and degenerative changes, consistent with the locally suppressed bone turnover [Figure 4]. The surrounding granulation tissue showed degenerated material with chronic inflammatory infiltration, corroborating the clinical finding of an atrophic non-union. Furthermore, the subcutaneous tissue exhibited dense fibrosis, providing a pathological correlation for the localized hyperkeratosis and the history of chronic mechanical pressure. The post-operative course was favorable. Pain resolved within 4 weeks, and radiographic union was achieved by 7 weeks. At 2 years, the patient demonstrated a solid union and a nearly full range of motion, maintaining functional recovery [Figure 5].

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Figure 2: Posterior forearm skin was hyperkeratotic (arrows), and an atrophic non-union with sclerotic fracture edges (arrowhead) was observed intraoperatively.

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Figure 3: Immediate post-operative radiographs show open reduction and internal fixation with a plate and screws after curettage and allogeneic bone grafting at the fracture site (arrows).

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Figure 4: Histopathological findings of the ulnar fracture site (hematoxylin and eosin stain, ×40) demonstrate dead bone fragments (arrows) and degenerative changes, characterized by the absence of osteocytes within the lacunae.

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Figure 5: Radiograph at 2 years postoperatively demonstrates solid union and maintained alignment at the fracture site (arrows).

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DISCUSSION

The present case highlights the importance of distinguishing atypical fractures from stress fractures. Stress fractures result from repetitive loading that exceeds the bone’s repair capacity and are classified as fatigue or insufficiency fractures.^[7,8] Fatigue fractures typically occur in normal bone under excessive stress and usually present as linear or oblique fractures, show callus formation during healing, and resolve with rest. In contrast, our patient was 84 years old with osteoporotic bone, and the fracture was transverse. Insufficiency fractures occur in weakened bone under physiological loading and are common in the pelvis or vertebrae of elderly patients. In contrast, atypical fractures demonstrate characteristic features, including a transverse configuration, minimal comminution, cortical thickening, and sclerosis. In this case, the presence of posterior cortical hypertrophy, marked sclerosis, and histopathological findings of dead bone fragments with the absence of osteocytes support the diagnosis of an atypical fracture rather than a simple stress fracture. The diagnosis of an atypical ulnar fracture is based on the extrapolation of the American Society for Bone and Mineral Research Task Force criteria established for the femur.^[1] In this case, the fracture exhibited five major features: Minimal trauma, transverse configuration, non-comminution, involvement of both cortices, and diaphyseal location analogous to that of the femoral shaft. Among minor features, generalized cortical thickening was observed, whereas prodromal pain and bilateral involvement were absent. Although there may be overlap between atypical and stress fractures, particularly in osteoporotic bone subjected to chronic loading, the presence of a non-comminuted transverse pattern, combined with localized cortical thickening and sclerosis, supports an atypical fracture phenotype rather than a fatigue or insufficiency fracture.

Atypical fractures are most associated with long-term antiresorptive therapy.^[1,4,5] Previous studies have shown that most atypical ulnar fractures occur in patients with prolonged bisphosphonate exposure.^[2-4] In contrast, our patient had no history of long-term antiresorptive therapy. Although a single denosumab injection had been administered 3 years prior, denosumab does not accumulate in bone and its effects diminish within months after discontinuation.^[9] Therefore, persistent systemic suppression of bone turnover is unlikely. This may explain the relatively rapid fracture healing (7 weeks) compared to previously reported drug-associated cases.^[2]

Biomechanically, the ulna has a physiological bow that generates tensile stress on the dorsal cortex during axial loading. Repetitive loading at the apex of this curvature can lead to localized accumulation of microcracks. In this case, the fracture occurred near the apex of ulnar bowing, consistent with previous reports demonstrating a correlation between fracture location and ulnar curvature.^[4]

The patient’s functional limitation provides a loading mechanism. Due to chronic low back pain, the patient frequently bore weight through the forearms while leaning on desks and, at times, crawling at home. Such repetitive forearm loading likely increased focal tensile stress on the dorsal cortex near the apex of bowing. Bilateral hyperkeratotic skin changes over the posterior proximal ulna further support chronic mechanical loading and are consistent with the “kitchen elbow sign” described in previous reports.^[6]

Management of atypical ulnar fractures is challenging due to their unfavorable biological environment, with non-union rates reported as high as 30% despite surgical fixation.^[5,9] In this case, fracture union was achieved within 7 weeks, which is faster than typically reported in drug-associated cases. This favorable outcome is likely related to the absence of prolonged systemic suppression of bone turnover, combined with thorough surgical debridement and stable internal fixation.

This study has limitations. Systemic bone turnover markers, such as C- or N-telopeptide or osteocalcin, were not measured at presentation, limiting comprehensive evaluation of systemic metabolic bone activity and differentiation between localized and systemic suppression of bone turnover. However, histopathological findings showing dead bone fragments and the absence of osteocytes strongly support locally suppressed bone turnover. Furthermore, the rapid radiographic union suggests that overall bone healing potential was relatively preserved compared with cases with prolonged antiresorptive exposure.

CONCLUSION

Chronic abnormal mechanical loading on osteoporotic bone can contribute to the development of atypical ulnar fractures, even in the absence of prolonged antiresorptive therapy. Recognition of abnormal loading patterns, including characteristic changes in forearm skin, may aid in early diagnosis. Early surgical management with thorough debridement, bone grafting, and stable fixation appears important for achieving favorable outcomes.