Clinical diagnosis
Case 223
【Discussion】
Patella is the largest sesamoid bone in human. It begins to ossify at near five years-old (1). It functions to extend lower extremity along with quadriceps muscle, quadriceps tendon, patella tendon. The connection and function strengthen with age: ossification of patella, growth plate fixation, robust development of quadriceps tendon and patella tendon.
Patella tends to be susceptible to be injured by various loads. Patella fractures are categorized avulsion fracture, horizontal fracture, vertical fracture, complex fracture and smash pieces fracture (1). Horizontal fracture is the most common followed by vertical fracture. Horizontal fracture is often required by surgical fixation because knee flexion makes horizontal fracture line separate and widen by tension of both quadrate tendon and patella tendon, resulting in non-fixation without treatment, while vertical fracture can be treated by mere cast due.
Patella is fixed between upward quadriceps tendon and downward patella ligament attached to tibia. Adolescence athletes of soccer and running injure the tendon and ligament. The injury of quadriceps tendon is called Sinding-Larsen-Johansson (SLJ) disease. The injury of patella tendon and tibia tuberosity is called Osgood-Schlatter (OS) disease. These diseases are relatively chronic while sleeve (avulsion) fracture is accidental (2, 3). Sleeve fracture is categorized by avulsion fracture of patella bone at the lower part of patella. Knee radiograph and CT depict a bone piece separated by lower part of tibia. Sleeve fracture arises from extraordinarily strong contraction of quadriceps tendon (extension) in time of adolescence when patella is unstable and not so robust as patella tendon (2, 3). Duplicate patella can occur following sleeving fracture because avulsion bone fragment rapidly grows. Sleeve fracture occurs following an evident episode of trauma or sport injury. Meanwhile, SLJ disease and OS disease are chronic micro-damages to arise from repeated athletic movement.
There is soft tissue below patella and anterior to femur which called Hoffa fat pad (HFD). The posterior aspect of HFD is adjacent to femur synovium, indicating it situates in an articular capsule and extra knee joint space. As a result, it protects knee joint and plays a role of cushion. Meanwhile, HFD can impinge between patella and femur when extraanatomical movement is required (4-7). It injures HFD, called Hoffa’s disease.
Contusion implies parenchymal injuries including micro-vessel rupture, causing edema and hemorrhage. Muscle contusion, brain contusion, pulmonary contusion, and internal organ contusion as well as bone contusion are listed. In our case, knee joint MRI showed bone edema of lower end of patella, implying patella contusion but no evidence of avulsion fracture or sleeve fracture.
【Summary】
We present a fifteen-year-old boy suspicious of right patella injury. MRI showed bone edema of lower end of patella, implying patella contusion. It is borne in mind that patella fracture can occur irrespective of in adolescence or in adult, while Sinding Larsen Johansson (SLJ) disease, Osgood Schlatter (OS) disease and sleeve fracture occurs merely in adolescence. SLJ disease implies injury of proximal end of patella tendon, OS disease, injury of distal end of patella tendon and tibia tuberosity, and sleeve fracture, avulsion fracture of distal end of patella. Hoffa disease occurs most in adults and arises from impinge between patella and distal end of femur. Hoffa fat pad situate intra articular space and extra articular space. Then, when Hoffa fat pad damage and micro-damage of articular capsule induce formation of ganglion occupying Hoffa fat pad. In our case, bone marrow edema at distal end of patella and Hoffa fat pad with no fragment of patella bone were shown on MRI, indicating contusion of distal end of patella and Hoffa fat pad.
【References】
1.Dupuis CS, et al. Injuries and conditions of the extensor mechanism of the pediatric knee. Radiographics. 2009; 29: 877-86
2.Hunt DM, et al. A review of sleeve fractures of the patella in children. The Knee. 2005;12 (1): 3-7.
3.Bates D,et al. Patellar sleeve fracture: demonstration with MR imaging. (1994) Radiology. 1994;193: 825-
4.Hoffa A. Influence of adipose tissue with regard to the pathology of the knee joint. JAMA. 1904;43:795–796.
5.Jacobson JA, et al. MR imaging of the infrapatellar fat pad of Hoffa. Radiographics. 1997;17(3):675–691.
6.McCarthy CL, et al. The MRI appearance of cystic lesions around the knee. Skeletal Radiol., 2004; 33: 187-209
7.Beall DP, et al.Cystic masses of the knee: magnetic resonance imaging findings. Curr. Probl. Diagn. Radiol. 2005; 34: 143-159
2021.3.10