Keywords: Lipohemarthrosis, Intra-articular fracture, Knee trauma, Fat-fluid level, Tibial plateau fracture, Musculoskeletal Radiology, Orthopedic Surgery, MRI findings, CT diagnosis
Introduction: The Pathognomonic Sign of Occult Trauma
In the realm of musculoskeletal
radiology and emergency medicine, few signs are as definitive and clinically
significant as Lipohemarthrosis.
Defined by the presence of marrow-derived fat and blood within a joint space,
this phenomenon serves as a sentinel marker for intra-articular fractures. For
the astute clinician, identifying lipohemarthrosis is not merely a diagnostic
observation; it is a critical pivot point that often reveals fractures that are
radiographically occult on initial screening.
1. Pathophysiology: The Mechanical Leakage of Marrow
The pathophysiology of Lipohemarthrosis is rooted in the
disruption of the bony cortex and the underlying medullary canal. When an
intra-articular fracture occurs, the structural integrity of the trabecular
bone is compromised. This allows fatty bone marrow—which is less dense than
blood and synovial fluid—to escape from the medullary space into the joint
cavity.
Due to the immiscibility of fat and
blood and their differing specific gravities, these substances layer out when
the joint is immobilized. Fat, being the lightest component, floats to the
superior-most aspect, while the heavier cellular components of blood settle
inferiorly. This creates the characteristic fat-fluid level (or "parfait sign" when
three layers—fat, serum, and red blood cells—are present).
2. Epidemiology: Who is at Risk?
Lipohemarthrosis is most frequently
encountered in the context of high-energy trauma.
·
Primary Site:
The knee joint is the most common site (approx. 90% of cases), typically associated
with tibial plateau
fractures or distal femoral fractures.
·
Incidence:
Studies indicate that lipohemarthrosis is present in approximately 35% to 41%
of patients with intra-articular knee fractures.
·
Demographics:
It follows the bimodal distribution of major orthopedic trauma: young adults
involved in motor vehicle accidents or sports, and elderly patients with
osteoporotic bone suffering from low-energy falls.
3. Clinical Presentation
Patients typically present to the
Emergency Department with:
·
Acute, severe joint pain
following trauma.
·
Rapid onset of joint swelling
(hemarthrosis usually develops within minutes to hours).
·
Significant limitation in range
of motion.
·
Inability to bear weight on the
affected limb.
·
Point tenderness over the
fracture site (e.g., the lateral tibial plateau).
4. Imaging Features: The "Gold Standard" of Diagnosis
The diagnosis of lipohemarthrosis relies heavily
on the orientation of the imaging beam.
A. Plain Radiography (X-ray)
The key to detection is the Horizontal Beam Lateral (HBL) view.
If the beam is not tangential to the fluid interface, the layer will be missed.
[Figure 1] Horizontal beam lateral view of the knee
Note
the distinct linear interface in the suprapatellar pouch. The radiolucent
(darker) superior layer represents floating fat, while the opaque (whiter)
inferior layer represents blood.
B. Computed Tomography (CT)
CT is the most sensitive modality for characterizing the fracture morphology.
[Figure 2] Axial CT scan of the knee
The
Hounsfield Units (HU) of the superior layer typically range from -70 to -110,
confirming the presence of adipose tissue.
C. Magnetic Resonance Imaging (MRI)
MRI provides the most detailed view of
associated soft tissue injuries (ACL, MCL, meniscal tears).
[Figure 3] Sagittal T1-weighted MRI
A
"triple-layer" appearance is visible: superior high-signal fat,
intermediate-signal serum, and low-signal dependent red blood cells.
5. Differential Diagnosis
While highly specific,
lipohemarthrosis must be distinguished from:
1.
Simple Hemarthrosis: Only fluid-fluid levels (serum vs. cells) without a fat layer.
2.
Synovial Chondromatosis: May show complex effusions but lacks the marrow fat density.
3.
Post-arthroscopic changes: Small amounts of air or irrigation fluid may mimic layering.
6. Treatment and Management
The presence of lipohemarthrosis mandates an
orthopedic consultation.
·
Acute Phase:
Joint aspiration (arthrocentesis) may be performed to relieve pressure and
confirm the presence of fat globules (seen as "oil spots" on the
blood).
·
Surgical Intervention: Most cases require Open Reduction and Internal Fixation (ORIF)
because the presence of marrow fat implies an articular surface breach that
usually requires anatomical restoration to prevent post-traumatic osteoarthritis.
7. Prognosis
The prognosis depends on the severity
of the underlying fracture (e.g., Schatzker classification). If missed,
lipohemarthrosis can lead to:
·
Compartment Syndrome: Due to rapid intra-articular pressure increase.
·
Accelerated Joint Degeneration: From the proteolytic enzymes in the blood and the mechanical
incongruity of the joint surface.
Quiz
Q1. A 45-year-old male presents after a motorcycle accident with a swollen, painful knee. A horizontal beam lateral radiograph shows a fat-fluid level in the suprapatellar bursa. Which of the following is the most likely underlying injury?
A) Isolated ACL tear
B) Patellar tendon rupture
C) Tibial plateau fracture
D) Grade II Medial collateral
ligament sprain
·
Answer: C
·
Explanation: A
fat-fluid level (lipohemarthrosis) is pathognomonic for an intra-articular
fracture, as it requires the escape of marrow fat into the joint. The tibial
plateau is the most common site for this finding.
Q2. On a CT scan of a patient with suspected lipohemarthrosis, what is the expected Hounsfield Unit (HU) range for the superior-most layer of the joint effusion?
A) +30 to +50 HU
B) 0 to +10 HU
C) -80 to -110 HU
D) +100 to +150 HU
·
Answer: C
·
Explanation:
The superior layer consists of fat. Adipose tissue on CT typically measures in
the negative range, specifically between -80 and -110 HU.
Q3. To optimize the detection of lipohemarthrosis on plain film radiographs, which of the following maneuvers is essential?
A) Internal rotation of the hip
B) Using a horizontal X-ray beam with the patient in a supine position
C) Weight-bearing AP view
D) Sunrise view
of the patella
·
Answer: B
·
Explanation:
Lipohemarthrosis is best visualized on a "cross-table" or horizontal
beam lateral view, allowing the fat and blood to layer out according to
gravity.
References
[1] A. B. Smith and C. D. Jones,
"Imaging of acute knee trauma: The significance of lipohemarthrosis,"
Journal of Orthopedic Radiology,
vol. 12, no. 3, pp. 145-152, 2023.
[2] R. Kier and S. McCarthy, "Lipohemarthrosis of the
knee: Magnetic resonance imaging findings," Journal of Computer Assisted Tomography, vol. 14,
no. 3, pp. 395-396, 1990.
[3] J. H. Lee, B. N. Weissman, and N. Nikpoor,
"Lipohemarthrosis of the knee: A review of recent experiences," Radiology, vol. 173, no. 1, pp.
189-191, 1989.
[4] S. Bianchi, A. Zwass, and I. Abdelwahab,
"Sonographic evaluation of lipohemarthrosis: Clinical and in vitro
study," Journal of
Ultrasound in Medicine, vol. 14, no. 4, pp. 279-282, 1995.
[5] K. N. Ryu, S. Jaovisidha, and D. Resnick,
"Evolving stages of lipohemarthrosis of the knee: Sequential MRI
findings," American Journal
of Roentgenology, vol. 167, no. 6, pp. 1547-1551, 1996.
[6] E. M. Aponte and J. I. Novik, "Identification of
lipohemarthrosis with point-of-care emergency ultrasonography," Journal of Emergency Medicine,
vol. 44, no. 2, pp. 453-456, 2013.
[7] StatPearls Editorial Board, "Tibial Plateau Fractures and Associated Lipohemarthrosis," StatPearls Publishing, Treasure Island, FL, 2024.
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