Lipohaemarthrosis: Causes, Pathophysiology, Diagnosis, and Management

What is the cause of Lipohaemarthrosis?

Lipohaemarthrosis is a specific type of joint effusion characterized by the presence of both fat and blood within the joint cavity, almost exclusively caused by an intra-articular fracture.

Understanding Lipohaemarthrosis

Lipohaemarthrosis is a medical term derived from Greek roots: "lipo" (fat), "haem" (blood), and "arthrosis" (joint condition). It describes the unique phenomenon where fat globules mix with blood inside a joint space. This distinct combination is not merely a random occurrence but a critical diagnostic sign, particularly in the context of musculoskeletal trauma.

The Primary Cause: Intra-Articular Fracture

The overwhelmingly predominant cause of lipohaemarthrosis is an intra-articular fracture. This refers to a bone fracture that extends into, or directly involves, the joint surface. For lipohaemarthrosis to occur, the fracture must disrupt the cancellous (spongy) bone within the joint, allowing its contents to escape into the synovial fluid.

How Intra-Articular Fractures Lead to Lipohaemarthrosis (Pathophysiology)

The mechanism by which an intra-articular fracture causes lipohaemarthrosis is a direct consequence of the anatomy and physiology of bone and joints:

• Bone Marrow Release: Cancellous bone, found at the ends of long bones and within flat bones, contains bone marrow. Bone marrow is rich in adipose (fat) cells. When a fracture occurs within this cancellous bone, the integrity of the bone is compromised, leading to the release of these fat cells and their associated fat globules.
• Blood Vessel Rupture: Any fracture, especially one involving the bone marrow, will inevitably rupture local blood vessels. This results in bleeding (haemorrhage) into the surrounding tissues, including the joint space if the fracture extends intra-articularly.
• Entry into Joint Space: For the fat and blood to enter the joint, there must be a breach in the articular cartilage and/or the joint capsule. The fracture itself typically provides this pathway, allowing the contents of the fractured bone marrow to extravasate directly into the synovial cavity.
• Fat-Fluid Level Formation: Once inside the joint, the fat, being less dense than blood and synovial fluid, will layer on top of the blood. This creates a characteristic "fat-fluid level" or "lipohemarthrosis level," which is a hallmark sign seen on imaging studies, particularly on horizontal beam X-rays, CT scans, or MRI.

Common Joints Affected

While lipohaemarthrosis can occur in any joint where an intra-articular fracture takes place, some joints are more commonly affected due to their anatomical structure and susceptibility to certain types of fractures:

• Knee: This is the most frequently affected joint. Fractures of the tibial plateau, distal femur, and patella are common causes. The knee joint has a significant amount of intra-articular fat (e.g., Hoffa's fat pad), and its large size makes effusions more noticeable.
• Shoulder: Fractures of the humeral head or glenoid.
• Ankle: Fractures of the distal tibia (pilon fractures) or talus.
• Elbow: Fractures of the distal humerus or radial head.

Clinical Presentation and Diagnosis

Patients presenting with lipohaemarthrosis will typically have a history of acute trauma to the affected joint. Clinical signs include:

• Acute Joint Pain: Often severe and localized to the injured joint.
• Swelling (Effusion): Rapid accumulation of fluid within the joint, leading to visible swelling and tension.
• Limited Range of Motion: Pain and swelling restrict the joint's movement.

Diagnosis is primarily made through imaging:

• X-rays: While a fracture line may be visible, the classic fat-fluid level is best appreciated on a horizontal beam lateral X-ray (e.g., cross-table lateral knee view), where the less dense fat floats above the denser blood and synovial fluid.
• CT Scan (Computed Tomography): Highly sensitive for detecting the fat-fluid level and excellent for visualizing complex fractures, especially in the knee.
• MRI (Magnetic Resonance Imaging): Can confirm the presence of fat and blood and provides detailed imaging of soft tissue and bone marrow edema, helping to fully characterize the fracture and any associated injuries.
• Arthrocentesis (Joint Aspiration): If performed, aspirating the joint fluid would reveal a bloody effusion with visible fat globules, which may appear as oily droplets on the surface of the aspirated fluid.

Importance of Prompt Diagnosis and Management

Recognizing lipohaemarthrosis is crucial because it serves as an unequivocal sign of an intra-articular fracture, even if the fracture line itself is subtle or not clearly visible on initial plain radiographs. Undiagnosed or inadequately managed intra-articular fractures can lead to significant long-term complications, including:

• Post-traumatic Osteoarthritis: Damage to the articular cartilage.
• Joint Instability: Due to ligamentous or capsular injury.
• Chronic Pain and Dysfunction: Resulting from improper healing or joint surface irregularity.

Therefore, the presence of lipohaemarthrosis necessitates a thorough orthopedic evaluation and appropriate management, which often involves surgical intervention to restore joint congruence and stability.

Conclusion

Lipohaemarthrosis is a distinct and highly significant clinical finding. Its cause is almost exclusively rooted in intra-articular fractures, where a breach in the bone and joint capsule allows bone marrow fat and blood to mix within the synovial space. As an "Expert Fitness Educator," it's vital to understand that this condition is a red flag indicating a serious underlying bone injury, demanding prompt and accurate diagnosis for optimal patient outcomes and to prevent long-term joint complications.