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A woman in her 30 s had complaints of pain over her right distal thigh and knee for 3 months. The pain was diffuse, dull aching and non-radiating. Rest pain was present. It was aggravated with activity and decreased partially with analgesics. She had no other joint pains. No history of trauma, fever, loss of weight or appetite was noted. She had no previous medical conditions. On examination, she was found to have medial joint tenderness. Her knee range of movement was upto 110° of flexion. No other significant finding was noted. Standing radiographs of the knee (figure 1) at first glance showed medial joint space narrowing. On careful inspection, abnormal areas of lucency were noted in the medullary cavity of the distal end of femur, surrounded by inconspicuous sclerosis. No periosteal reaction or soft tissue changes were noted. This radiograph was seen earlier by two orthopaedic surgeons who the patient had consulted, and she was told to have arthritis changes only. We performed an MRI (figure 2), which showed a serpiginous lesion involving the medullary cavity of the distal end of femur and proximal end of tibia. It was noted to involve the metaphyseal and epiphyseal regions, extending till the articular surface. The lesion was hypointense on T1 and hyperintense on T2 and STIR sequences. The classical ‘smoke up the chimney’ appearance was present. All her blood investigations were found to be within normal limits. She was diagnosed to have bone infarct of the distal femur and proximal tibia. Concomitant arthritis was medial tibiofemoral joint was present.
Radiographs of knee showing areas of lucency (red arrow), surrounded by serpiginous sclerosis (yellow arrow) in the medullary cavity of the distal metaphysis of femur. Decreased medial tibiofemoral joint space is also noted.
MRI of the knee. (A) T1 weighted sagittal section showing hypointense lesions (red arrows) in the distal end of femur and proximal end of tibia. Classical ‘smoke up the chimney’ appearance of the lesion. Lesion in the proximal tibia seen extending till articular surface. (B) T2 weighted sagittal section showing periphery of the lesion representing the regions of sclerosis (yellow arrow) to be hyperintense. (C) T1 weighted coronal section showing the central portion of the lesions (blue arrow) to have signal similar to that of normal marrow, while the periphery of the lesion is hypointense (green arrow). (D) T2 weight coronal section showing the sclerotic region to be hyperintense (orange arrow). The distal femoral lesion is seen extending till the articular surface. (E and F) Axial sections showing hypointense lesions (pink arrows) in proximal tibia and distal femur, respectively.
Bone infarct is a rare conditioning characterised by osteonecrosis within the medullary cavity of the bone. It occurs due to ischaemia, resulting in destruction of bone architecture.1 The possible aetiology suggested include trauma, caisson disease, sickle cell disease, long-term steroid use, alcoholism and dyslipidaemia.2 In our case, no risk factor could be identified. The association between arthritis of the knee joint and bone infarct has not been studied earlier. It usually involves the metaphyseal region and is located around the knee joint. It is often asymptomatic. It can be diagnosed on radiographs by the presence of metaphyseal, medullary lesion with a serpiginous border.3 MRI shows a hypointense lesion with a rim of hyperintensity on T2 weighted sequences. An important finding to note in MRI is that the central signal remains that of normal marrow.4 This helps differentiate it from its closest differential diagnosis of enchondroma, in which the central marrow signal is absent.5 Though predominantly benign, it is important for this condition to be identified and followed up, as be associated with secondary malignancy.6 7 In our case, the lesion was atypically involving the epiphysis as well, extending until the articular surface. No such cases have been reported in literature. The association with arthritis of the joint also remains unexplored. Its occurrence in a young adult with no other risk factors suggests its aetiology is still to be fully understood. The fact that the radiograph findings were inconspicuous and missed by two other surgeons, while the MRI had such large lesions suggests that the lesion could very often pass undiagnosed.
Patient’s perspective
I have been having this knee pain since 3 months. I consulted two orthopaedic doctors previously and they both asked for X rays. They had said X rays showed some age-related changes. Only after coming to this hospital, I was told that there is some destruction in my bone and I will need to follow-up regularly with X rays.
Learning points
Bone infarct can be a rare cause of pain around the knee joint.
Smoke up the chimney appearance on imaging is characteristic.
On MRI, central signal intensity similar to marrow is an important finding to differentiate it from enchondroma.
Ethics statements
Patient consent for publication
Footnotes
Twitter @drsathishmuthu
Contributors SP obtained the digital images of the investigations and was responsible for writing of the draft of the manuscript. SM and NJ performed analysis and highlighting of radiographs and MRI images, as well as reviewed the manuscript. MJ is the chief orthopaedic surgeon under whom the patient was evaluated and he obtained the informed consent from the patient for this publication.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Case reports provide a valuable learning resource for the scientific community and can indicate areas of interest for future research. They should not be used in isolation to guide treatment choices or public health policy.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.