Atraumatic iliopsoas heterotopic ossification with postoperative atypical haemolytic uraemic syndrome

Summary

We describe a case of a middle-aged woman presenting with a 2-year history of progressive, painless restriction of left hip movement without preceding trauma or surgery. Imaging demonstrated a well-defined ossified mass within the iliopsoas muscle exhibiting a classical zonal maturation pattern. Following unsuccessful conservative therapy, delayed surgical excision after radiological maturation was undertaken and histopathology confirmed heterotopic ossification. On postoperative day 3, she developed acute anaemia, thrombocytopenia and severe renal impairment. Laboratory evaluation supported thrombotic microangiopathy, with atypical haemolytic uraemic syndrome considered the most probable diagnosis. The patient was managed with therapeutic plasma exchange and temporary haemodialysis, resulting in full haematological and renal recovery. At follow-up, hip function improved substantially with no evidence of recurrence. This case emphasises the need to recognise uncommon atraumatic iliopsoas heterotopic ossification and to maintain vigilance for postoperative thrombotic microangiopathy to ensure early multidisciplinary intervention and favourable outcomes.

Background

Heterotopic ossification refers to the formation of mature lamellar bone within extraskeletal soft tissues and is most commonly associated with trauma, surgery or neurological injury.^{1 2} Around the hip, heterotopic ossification typically affects the abductor musculature, whereas isolated involvement of the iliopsoas muscle is infrequently reported, particularly in the absence of identifiable precipitating factors.¹ Few cases of atraumatic iliopsoas heterotopic ossification have been reported.³ Surgical excision is reserved for patients with significant functional impairment and is generally associated with favourable outcomes.¹

Atypical haemolytic uraemic syndrome (aHUS) is a rare, life-threatening thrombotic microangiopathy characterised by haemolytic anaemia, thrombocytopenia and acute kidney injury, most often triggered by complement dysregulation following systemic stressors.^{4 5} Although postoperative aHUS has been described after major surgical procedures, its occurrence following elective orthopaedic surgery is uncommon.⁶

We report a case of a woman in her late 40s with atraumatic iliopsoas heterotopic ossification who developed postoperative aHUS following surgical excision, underscoring the need for vigilant postoperative surveillance and early multidisciplinary management.

Case presentation

A woman in her late 40s presented to our outpatient department with a 2-year history of progressively worsening restriction of movement of the left hip. The onset was insidious and initially painless, with gradual progression to functional limitation affecting activities such as squatting, sitting cross-legged and prolonged walking. There was no history of trauma, intramuscular injections, previous surgery, anticoagulant use, neurological injury or bleeding disorders. She denied constitutional symptoms, including fever, weight loss or night sweats. There was no relevant family history of musculoskeletal disease or thrombotic microangiopathy.

On examination, there was fullness of the left femoral triangle and a palpable, firm, non-tender, immobile, hard, bony mass. Mild wasting of the left hip and thigh musculature was noted. Hip examination revealed marked restriction of flexion, internal rotation and adduction, while extension and abduction were relatively preserved. There were no signs of local inflammation, and distal neurovascular examination was normal.

Investigations

Plain radiographs of the pelvis and left hip demonstrated an irregular ossified mass lying anterior to the hip joint, extending from the region of the superior pubic ramus to the lesser trochanter (figure 1).

Figure 1. (A) Anteroposterior and (B) lateral radiographs of the left hip demonstrating an ossified mass (arrows) extending from the level of the superior pubic ramus to the lesser trochanter and lying anterior to the hip joint.

CT showed a well-defined trabeculated lesion within the iliopsoas muscle, characterised by an ill-defined exophytic osseous outgrowth with heterogeneous mineralisation is seen arising from the anterior aspect of the proximal diaphysis of the left femur and the left iliac blade. The underlying cortex shows mild cortical erosion without evidence of medullary invasion. The lesion involves the iliopsoas muscle and demonstrates peripheral mature lamellar bone with a relatively immature central component (figure 2).

Figure 2. Preoperative CT images showing a well-defined mass with peripheral mature lamellar bone and an immature central portion (arrows) along the course of the iliopsoas muscle, in axial (A,B), coronal (C,D) and three-dimensional volume-rendered reconstructions (E,F).

Differential diagnosis

Based on clinical and radiological findings, the differential diagnoses included parosteal osteosarcoma, chronic calcified haematoma, infective pathology such as a calcified psoas abscess and localisation heterotopic ossification. The absence of aggressive imaging features, lack of systemic symptoms favoured a diagnosis of atraumatic localisation heterotopic ossification of the iliopsoas muscle.

Treatment

Initial conservative management with sustained-release indomethacin 75 mg one time per day was administered for 4 months without improvement in symptoms or hip range of motion. Given persistent functional impairment, surgical excision was planned after confirmation of radiological maturation of the lesion and was performed approximately 24 months after symptom onset. The procedure was performed with the patient in the supine position, using a modified Hardinge approach to the hip, providing adequate exposure to the iliopsoas compartment. In the iliopsoas muscle, a well-defined, mature, ossified mass measuring about 12 cm was found. It was carefully guarded from the adjacent femoral neurovascular components, circumferentially separated, osteotomised and excised in toto. Full passive hip range of motion was verified intraoperatively. The histopathological examination of the excised sample diagnosed heterotopic ossification of mature lamellar bone, clearly zonal with a differentiated appearance (figure 3).

Figure 3. H&E, 20×: The innermost region of the bone (blue arrow) displays immature fibroblastic/myofibroblastic proliferation in a vascular, collagenous background. The intermediate zone (red arrow) transitions to osteoblastic stroma with immature woven bone trabeculae, while the outer layer (black arrow) shows mineralised osteoid maturing into lamellar bone with large osteocytes.

The closed suction drain collected about 360 mL and was emptied on postoperative day 2, with subsequent output of less than 20 mL in 24 hours, with no indication of wound complications or infection. From postoperative day 2, a comprehensive rehab programme of active-assisted and active hip mobilisation was initiated followed by weight-bearing as tolerated.

Postoperatively on day 3, pallor and decreased urine output were experienced, without fever, hypotension or evidence of active bleeding. No clinical signs of sepsis or wound complications were apparent. Laboratory work demonstrated microangiopathic haemolytic anaemia as well as a haemoglobin concentration of 8.3 g/dL and severe thrombocytopenia with platelet count of 41×10⁹/L. Renal function was critically impaired with a serum creatinine of 12.6 mg/dL and serum urea of 204 mg/dL. Peripheral blood smear examination revealed anisopoikilocytosis with fragmented red blood cells, including schistocytes, consistent with haemolysis. Lactate dehydrogenase was increased at 714 U/L and total serum bilirubin levels were 4.4 mg/dL, with a predominantly conjugated fraction.

Assessment of complement showed decreased serum complement C3 (40 mg/dL) and complement C4 levels within normal limits. On urinalysis, there was significant proteinuria, with a 17.3 urine protein–creatinine ratio. Coagulation profile was not consistent with disseminated intravascular coagulation, presenting mildly prolonged activated partial thromboplastin time of 34.2 s and near-normal prothrombin time of 12.0 s with normal international normalised ratio (1.0). Blood cultures were negative. Laboratory testing demonstrated ADAMTS13 (A Disintegrin And Metalloproteinase with a Thrombospondin type 1 motif, member 13) activity of 62% with no detectable inhibitor, supporting a diagnosis of aHUS.

Outcome and follow-up

Therapeutic plasma exchange was halted after six sessions once haemoglobin elevated to 97 g/L, the platelet count increased to approximately 103×10⁹/L and haemolysis markers returned to normal, including a decrease in total bilirubin from 4.4 mg/dL to 0.7 mg/dL.

Renal replacement therapy was stopped after three haemodialysis sessions following recovery of urine output and consistent stepwise improvement, with serum creatinine decreasing from a peak of 12.6 mg/dL to a final value of 1.7 mg/dL, while serum urea levels also decreased concurrently. No further plasma exchange or haemodialysis was required, as the patient continued to show haematological and renal improvement.

The preoperative Modified Harris Hip Score of 46 improved to 58 at 6 weeks and 68 at 3 months, corresponding with early gains in hip mobility and daily activities. Continued improvement was observed at 6 months (score 76) and 12 months (score 84), with pain-free ambulation and unrestricted activities of daily living. Serial anteroposterior and lateral radiographs obtained immediately postoperatively and at 6 weeks, 12 months demonstrated no evidence of recurrent heterotopic ossification (figure 4).

Figure 4. Serial follow-up radiographs of the left hip. (A,B) Immediate postoperative anteroposterior and lateral radiographs demonstrating complete excision of the heterotopic ossification. (C,D) Six-week follow-up radiographs. (E,F) Twelve-month follow-up radiographs demonstrating maintained radiological stability with no evidence of recurrent heterotopic ossification.

At final follow-up, haematological and renal parameters remained stable, with no recurrence of thrombotic microangiopathy. There was no clinical or radiological evidence of recurrence of heterotopic ossification.

Discussion

Atraumatic heterotopic ossification of the iliopsoas muscle is an uncommon clinical entity. Heterotopic ossification around the hip most commonly involves the abductor musculature and is usually associated with trauma, surgery, neurological injury or metabolic disturbances.^{1 4} Isolated involvement of the iliopsoas muscle in the absence of identifiable precipitating factors is infrequently encountered and poses diagnostic challenges due to its resemblance to malignant or infective conditions.

Chouhan et al³ have reported a case of atraumatic heterotopic ossification involving the iliopsoas muscle. The patient had a gradual, painless restriction of hip mobility without any past history of trauma or surgery, precisely resembling the gradual onset seen in the current instance. Imaging in both cases showed a mature ossified mass in the iliopsoas muscle, with peripheral ossification and a comparatively immature core, consistent with the zonal pattern typical of heterotopic ossification.² Surgical excision after radiological maturation had favourable functional results in both instances, establishing delayed excision as an efficacious therapeutic approach for symptomatic patients.¹

However, the majority of reported cases of iliopsoas or psoas muscle ossification have been attributed to discernible causes, such as trauma, spinal surgery, intramuscular haemorrhage, neurological injury or the use of bone morphogenetic proteins.^{1 4} These secondary forms are fundamentally distinct from the current case, in which no triggering factors were discovered despite thorough evaluation. This distinction is critical when addressing rarity, as secondary iliopsoas heterotopic ossification, while uncommon, is recognised, whereas idiopathic involvement remains limited to isolated case reports.

The present case is further distinguished by the development of postoperative aHUS, a rare but potentially life-threatening systemic complication. aHUS is a complement-mediated thrombotic microangiopathy caused by dysregulation of the alternative complement pathway.⁵ Large clinical series have demonstrated that infections, pregnancy, transplantation, autoimmune disease and certain drugs are the most common triggers, while postoperative cases are infrequently reported.^{5 7}

Matsukuma et al⁸ reported postoperative atypical haemolytic syndrome associated with complement C3 mutation, highlighting surgery as a potent complement-amplifying trigger. In the present patient, major surgery likely acted as a complement-amplifying trigger in a genetically susceptible host.⁵ The diagnosis was supported by the presence of microangiopathic haemolytic anaemia, thrombocytopenia, acute kidney injury, reduced complement C3 levels, ADAMTS13 activity more than 10% and exclusion of secondary causes. Contemporary reviews emphasise the importance of distinguishing aHUS from thrombotic thrombocytopenic purpura and other thrombotic microangiopathies in the postoperative setting.^{3 6} Negative blood cultures, absence of hypotension and coagulation parameters not consistent with disseminated intravascular coagulation made sepsis-related microangiopathy unlikely.

Limitations of this report are the unavailability of complement genetic analysis and antifactor H antibody testing due to the resource-limited setting. However, the clinical presentation and laboratory profile were strongly suggestive of complement-mediated thrombotic microangiopathy.⁵ Eculizumab was not administered due to limited availability and cost constraints, and the patient demonstrated sustained clinical and biochemical recovery with plasma exchange and supportive renal replacement therapy, consistent with outcomes reported in other aHUS cases.^{9 10}

Clinically, this case highlights the need for heightened vigilance for uncommon systemic complications following orthopaedic surgery and reinforces the importance of early multidisciplinary management. Complete excision of atraumatic iliopsoas heterotopic ossification resulted in sustained functional recovery, while timely recognition and treatment of aHUS led to full renal and haematological recovery.

In summary, this case report describes an uncommon association of atraumatic iliopsoas heterotopic ossification with postoperative aHUS, expanding the orthopaedic and haematological literature and underscoring the need for multidisciplinary vigilance in complex postoperative care.

Patient’s perspective.

Over a period of two years, I slowly noticed increasing stiffness in my left hip. Initially, there was no pain, but routine activities such as sitting cross-legged and walking gradually became difficult. When the doctors informed me that an abnormal bone growth had developed within a muscle around my hip, I was relieved to learn it was not cancer, though I felt anxious about undergoing surgery. Following the surgery, I unexpectedly became very weak and was told that my kidneys were not working properly. This was extremely frightening. I needed dialysis and plasma therapy, which was both physically exhausting and emotionally overwhelming. With timely treatment, my condition steadily improved. My kidney function recovered, and my hip movement improved significantly. I am now able to resume my normal daily activities. This experience taught me the importance of early diagnosis and seeking medical care without delay.

Learning points.

• Atraumatic heterotopic ossification of the iliopsoas muscle is an exceptionally rare condition, but it should be considered in progressive, painless hip stiffness without trauma.

• Recognition of the classical zonal maturation pattern on imaging is key to distinguishing heterotopic ossification from malignancy and guiding the timing of surgical excision.

• Unexplained postoperative anaemia, thrombocytopenia and acute kidney injury should prompt urgent evaluation for thrombotic microangiopathy, even after elective orthopaedic procedures.