Teaching Case

A 9-year-old girl was referred to the emergency department after falling from an electric scooter. The emergency physician noticed a swollen right knee and patella alta. During the clinical examination, the patient was unable to perform active knee extension. No previous medical history was known.

A lateral radiograph of the right knee demonstrated a patella alta, an irregular delineated bone fragment anterior to the femoral condyles, and a small amount of fluid in the suprapatellar recess (Figure 1).

The T1 and T2 FS weighted MR images showed periosteal stripping at the apex of the patella combined with a slack patellar tendon and a small bone fragment at the base of the patella (Figures 2 and 3). The bone fragment consists of a hypointense cortex and isointense to slightly hypointense trabecular bone (Figure 3) and is connected to the tibia by a small strand of T1 and T2 hypointense periosteum (Figures 2 and 3).

The patellar tendon is buckled and oedematous, hence the slight T2 hyperintense signal (Figure 2).

These findings confirm a patellar periosteal stripping proximally and a tibial periosteal sleeve avulsion fracture distally.

Additionally, there was a rupture of both the medial and lateral patellofemoral retinacula (Figure 4).

Sleeve fractures are characterised by the avulsion of a strand of cartilage or periosteum, with or without pieces of cortical and sometimes trabecular bone, and occur in skeletally immature patients [1,2].

Sleeve fractures from the inferior patellar pole are rare, but they are a well-recognized entity in children after trauma [1]. Even though the incidence of patellar fractures in children is only 1%, the patellar sleeve fracture is the most common type of patellar fracture in patients younger than 16, at 57% of all patellar fractures. The peak incidence is at 12.7 years old, with a male-to-female ratio of 3:1 [3].

Much more rare is the tibial periosteal sleeve fracture. It is a rare variant of the tibial tuberosity avulsion fracture and wasn’t originally included in the Watson-Jones or Ogden classifications of tibial tuberosity avulsion fractures. Several authors have now proposed to include these fractures in a modified classification [1,2].

Bifocal patellar tendon avulsion lesions are extremely rare [4]. This case depicts a patient with a combined patellar periosteal stripping and a tibial periosteal sleeve fracture.

Forced knee flexion with simultaneous forceful quadriceps muscle contraction causes a high tensile force at the patellar apex and the tibial apophysis. In children, these forces can cause avulsion fractures [1,4].

A suggestive history, combined with a joint effusion, a high-riding patella and loss of active extension, is very suggestive of a patellar tendon injury [1,4). MR imaging is required for the evaluation of the affected parts of the tendon, i.e., origin, tendon or insertion [4].

Treatment of these avulsion fractures consists of open reduction and internal fixation by an orthopaedic surgeon and suturing of the periosteum. The patient-reported outcome is excellent [1,4].

Take home message

Children with a swollen knee, a patella alta and a history of forced knee flexion who can’t perform active knee extension should be examined for a patellar tendon injury. These injuries can be suspected on plain radiography but are better recognized on MRI as bone fragments with a connected strand of periosteum. An orthopaedic surgeon can treat these fractures with an excellent patient-reported outcome.

All patient data have been completely anonymised throughout the entire manuscript and related files.