Occult Distal Femoral Physeal Injury With Disruption of the Perichondrium Nancy Chauvin, MD and Diego Jaramillo, MD, MPH Abstract: The authors describe a case of distal femoral physeal injury with disruption of the perichondrium in a 9-year-old girl after a sledding accident. The patient presented with knee pain, limited range of motion, and inability to bear weight. Initial radiographs were normal. A magnetic resonance imaging of the knee demonstrated abnormal signal and wid- ening of the distal femoral physis with elevation of the posterior distal femoral periosteum. This case illustrates the main magnetic resonance imaging findings in an occult Salter Harris type I injury: increased phy- seal thickness and signal intensity on water-sensitive sequences, peri- chondrial disruption, and intracartilaginous fracture. Key Words: physeal injury, perichondrial disruption, pediatric knee, intracartilaginous fracture (J Comput Assist Tomogr 2012;36: 310Y312) P hyseal fractures are common in children. Approximately 15% to 30% of all long bone fractures during childhood in- volve the physis. 1 It is important to recognize physeal compo- nents of fractures, as injury to the physis in a growing child can lead to the development of osseous bridging and growth dis- turbances of the bone. 2 While most physeal fractures can be obvious on plain radiography, physeal fractures of the knee, particularly the distal femur, can be very subtle or even radio- graphically occult. Magnetic resonance imaging (MRI) is often done in children with severe injury to the joints to evaluate the status of the ligaments, menisci, and articular cartilage; in any of these cases, there may be unsuspected fractures involving the physis. Occult physeal injuries are typically Salter-Harris type 1 fractures without displacement. We present a case of a 9-year-old girl who sustained an occult distal femoral physeal injury with disruption of the peri- chondrium after a sledding accident. CASE REPORT A previously well 9-year-old girl had persistent left knee pain and limited range of motion after a sledding accident. She was seated in front of a sled, which collided with tree. The patient was unsure of the position of her legs; however, she immediately noted diffuse left knee pain and inability to bear weight. Physi- cal examination revealed very limited range of knee motion with significant guarding. Pain was localized mostly over the pos- terolateral join line. She denied medial knee tenderness. There was no patellar sensitivity, and her neurovascular system was intact. She was unable to bear weight on her left leg. Initial anteroposterior and lateral radiographs of the left knee were normal. A magnetic resonance imaging (MRI) of the knee was performed to evaluate for internal derangement. The MRI revealed no evidence of osteochondral, meniscal, or liga- mentous injury. A coronal water-sensitive intermediate-weighted fat-suppressed image (Fig. 1) demonstrated that the distal fem- oral physis was widened compared to the proximal tibial physis and that the physis showed high signal intensity. Within the sub- stance of the lateral physis was a line suggestive of a horizontal fracture through the cartilage (Fig. 2), compatible with a Salter Harris I fracture. Adjacent metaphyseal edema was also present. The sagittal intermediate-weighted image at the level of the an- terior cruciate ligament (Fig. 3) demonstrated a tear of the pos- terior distal femoral perichondrium. The patient was placed in a long leg cast. Follow-up ra- diographs at 6 weeks demonstrated interval healing with normal alignment. The distal femoral physis remained opened. The cast was subsequently removed, and she was placed in a knee im- mobilizer and began physical therapy. Radiographs obtained at 6 months after the injury demonstrated normal alignment with a normal-appearing distal femoral physis, without evidence of growth arrest or bony bar development. She was asymptomatic at that time. DISCUSSION The pediatric skeleton responds to trauma differently from adults. The weakest constituent in pediatric bones is the zone of provisional calcification, which is the transition zone between physeal cartilage and bone. Physeal cartilage is weaker than both bone and ligaments, often rupturing in the setting of trauma. The physis is the weakest when it is at its thickest, which is during periods of active growth, such as during puberty. 3 The perios- teum consists of an outer fibrous layer of collagen and fibro- blasts and an inner cellular layer of osteoprogenitor cells. The periosteum in children is thicker compared with that in adults and more loosely attached along the shaft but is tightly tethered at the level of the physis. 4 The perichondrial ring of LaCroix (perichondrium) consists of a thin ring of intramembranous bone and an outer fibrous layer, which supports the weak chondro- osseous junction. The perichondrium surrounds the main phy- seal cartilage, and it is necessary for the circumferential growth of the bone. It is continuous with the periosteum and is tightly tethered to the physis at the groove of Ranvier. Owing to the loose attachment, the periosteum is often stripped in the setting of trauma but is seldom disrupted. Large traumatic subperios- teal collections are common but typically stop at the perichon- drium. It is important to note that 35% of all physeal bridges that require resection occur at the distal femur. 3 The analysis of the fracture line within the cartilage can also have prognostic importance. In undulating physes like the distal femur, even a Salter-Harris type I fracture can involve the ger- minal zone. Experimental studies suggest that MRI can detect whether the fracture line remains within the hypertrophic zone CASE REPORT 310 www.jcat.org J Comput Assist Tomogr & Volume 36, Number 3, May/June 2012 From the Department of Radiology, Children’s Hospital of Philadelphia, Philadelphia, PA. Received for publication February 6, 2012; accepted February 14, 2012. Reprints: NancyChauvin, MD, Department of Radiology, The Children’s Hospital of Philadelphia, 34th Street and Civic Center Boulevard, Philadelphia, PA 19104-4399 (e-mail: chauvinn@email.chop.edu). There are no grants or financial support to acknowledge. The authors have no conflicts of interest to disclose. Copyright * 2012 by Lippincott Williams & Wilkins Copyright © 2012 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.