A 74-year-old woman was found alone at home by her son after an accidental fall from the stairs. She complained left shoulder, hip and knee pain. She was admitted to our ER and she underwent a clinical examination: the orthopaedic surgeon requested radiographic evaluation of the involved segments.
Plain films of the left shoulder and hip were normal. The antero-posterior (AP) X-ray of the left knee showed a cortical disruption on the medial femoral condyle (Fig. 1a); the latero-lateral (LL) projection revealed a fat-blood interface (FBI) sign in the suprapatellar pouch (Fig. 1B). A CT examination was performed, in order to confirm the diagnosis of femoral fracture: it depicted a cortical interruption on the medial side of femoral condyle (Figs. 2a, b) and confirmed the presence of a trilaminar fat/serum/fluid level in the suprapatellar pouch (Fig. 2c).
A lipohaemarthrosis results from an intra-articular fracture with leakage of fat and blood from the bone marrow into the joint; it is most frequently observed in the knee, associated with a fracture of tibial plateau or distal femur, but it has also been described in the hip, shoulder and elbow. FBI is an acronym referring to the components that form a lipohaemarthrosis, and it stands for Fat/Blood Interface: it can be seen on plain films as well as on CT, MRI and ultrasound images, and it is the expression of the known phenomenon of fat (the fatty marrow) floating on water (blood) [1].
The fat-fluid level may be seen on any horizontal beam radiograph. In the knee this can be achieved with a cross-table horizontal lateral view, where a long horizontal line is seen in the suprapatellar pouch. Patients with a prominent suprapatellar plica may present a double fat-fluid level (Fig. 3). It is important to remember that the absence of FBI sign on plain radiographs does not exclude the presence of an intra-articular fracture. It is also important to know that a simple haemarthrosis can create a subtle fluid/fluid level (expression of separation of serum and red-cells), known as haematocrit effect: this should not be confused with a lipohaemarthrosis, in which all three layers can be seen in some cases. CT and MRI have a much higher sensitivity to depict differences of density, to identify intra-articular fat and to prove a haematocrit effect, with three layers visible (fat/serum or synovial fluid/red blood cells) [2]. Ultrasound is not very widely used in the assessment of post traumatic knee, but the fat/fluid level can be identified as an echogenic layer above the hypoechoic blood [3].
Lipohaemarthrosis of the knee
The patient is a 74-year-old female who was injured due to an accidental fall down the stairs, complaining of pain in the left shoulder, left hip, and left knee. X-ray and CT examination of the left knee show:
• A clear fat-blood interface (FBI) within the knee joint space, visible on horizontal radiographs or CT scans, where a low-density fatty layer is seen above a high-density blood layer in the joint effusion.
• No obvious deformity or displacement of the knee joint, but soft tissue swelling around the knee is noted.
• The bone quality appears relatively poor (likely due to advanced age and possible osteoporosis), with decreased clarity of local trabecular structures.
• Correlating with clinical symptoms and imaging findings, there is a high suspicion of an intra-articular fracture.
Notably, the appearance of an FBI sign in the knee joint is often associated with intra-articular fractures. A simple hemarthrosis or synovial injury might result in fluid-fluid levels (separation of serum and red blood cells), but the fat layer predominantly originates from bone marrow or fat release at the fracture site.
Considering the history of a fall, advanced age, imaging findings (prominent fat-blood interface), and symptoms, the most likely diagnosis is: “Intra-articular fracture of the left knee (tibial plateau fracture highly suspected) with accompanying fat-blood effusion.” For further clarification of fracture line extent and detail, CT multiplanar reconstructions or MRI may be considered.
Rehabilitation should be individualized, gradual, and initiated early but with care to avoid re-injury (FITT-VP: Frequency, Intensity, Time, Type, Progression, and Volume).
Monitor for pain, swelling, or other adverse symptoms throughout the rehabilitation process. Seek medical evaluation if symptoms recur or new complications arise.
This report is based on the current medical history and imaging data provided and is for reference only. It does not replace a face-to-face consultation or professional medical opinion. If further treatment or diagnostic evaluation is necessary, please seek timely care from a qualified medical institution and specialist.
Lipohaemarthrosis of the knee
