Teaching Case

A 6-year-old female patient presented for evaluation of pain and swelling of the right thigh following entrapment of the affected leg in the springs of a trampoline. The patient reported severe pain with inability to bear weight on the affected right lower extremity.

Radiographic AP view of the right femur revealed increased distance at the distal lateral femoral physis without involvement of the metaphysis or epiphysis. (Fig. 1) Subsequently, T1 and T2-weighted Magnetic Resonance Imaging (MRI) and fat-suppressing sequences were performed. Further imaging demonstrated increased signal along the physis of the distal femur with surrounding oedema in the metaphysis and adjacent epiphysis in sagittal views. A fluid collection was also noted in the adjacent soft tissue, likely related to the periosteal/capsular injury. (Fig. 2) No focal articular cartilaginous or ligamentous defects were seen. Additional findings demonstrated a prominent low signal region interposed within the distal femoral physis posteriorly and laterally, as well as a focal region of discontinuity of the periosteum postero-laterally on coronal views (Fig. 3, 4). Axial MRI demonstrated posterolateral periarticular injury with additional confirmation of trapped periosteum within the femoral physis (Fig. 5, 6).

Background: Trapped periosteum refers to interposition of periosteum within the fracture line of a physis fracture. The mechanism of injury is thought to be a varus, valgus or hyperextension stress that results in tearing of periosteum on the side of bony distraction allowing a periosteal flap to become interposed within the fractured physis [1]. Trapped periosteum is a rare but serious complication of physis fracture as it can result in bony bridging and growth arrest [2]. Growth arrest is reported to be 40% in cases of distal femoral physeal fractures overall, with a trend of arrest more commonly seen in high energy injuries and in injuries with increasing Salter-Harris classification [3]. Magnetic Resonance Imaging (MRI) is recommended in instances where physis fracture detection may be difficult, particularly Salter-Harris types 1 and 5 [4], and/or when there is persistent physeal widening >3 mm indicating possible entrapped periosteum [5].
Clinical Perspective: Some authors indicate that because of the association of trapped periosteum with growth disturbance, open reduction is required in cases of trapped periosteum [6]. Other authors have reported subsequent distal femur growth arrest despite anatomic reduction through surgical removal of entrapped periosteum [7], highlighting the limitations of surgical reduction in the prevention of growth arrest.
Outcome: At the time of initial evaluation and diagnosis the decision was made to treat conservatively with long leg immobilization cast. The patient had follow-up with orthopaedic clinical staff at 2, 6 and 11 weeks. At each subsequent visit there was no radiographic or clinical evidence of growth arrest. The patient’s long leg cast was removed at 6 weeks and placed in a knee immobilizer with instructions to gradually increase range of motion and weight bearing as tolerated. At 11 weeks the patient was fully weaned from the knee immobilizer and displayed full pain-free range of motion with return to full weight bearing activities without complaint of pain or limitation. At 25 week primary care appointment the patient was noted to have continued normal growth and development, although this was not confirmed radiographically.
Take Home Message: MRI may be necessary in settings where risk of Salter type physeal fracture exists to confirm fracture and to detect other deleterious soft tissue complications such as periosteal entrapment. Cases of physeal fracture with trapped periosteum at the distal femur may be amenable to conservative management with close clinical follow-up to ensure normal healing and growth at the physis.