Trapped Periosteum

Clinical History: A 14 year-old male complains of knee pain after landing awkwardly while playing basketball. A radiograph (not shown) shows mild widening of the medial femoral physis. A fat-suppressed Proton density-weighted coronal image (1a) of the knee is provided. What are the findings? What is your diagnosis?

1a

Figure 1:

A 14 year-old male complains of knee pain after landing awkwardly while playing basketball. A radiograph (not shown) shows mild widening of the medial femoral physis. A fat-suppressed Proton density-weighted coronal image (A) of the knee is provided.

Findings

2a

Figure 2:

The fat-suppressed proton density-weighted coronal image demonstrates a Salter Harris I fracture of the medial distal femoral physis (arrows) with mild widening and fluid signal at the physis. A linear low signal intensity structure is in the medial physis (red arrow), representing periosteum that has displaced from the medial femoral metaphysis (arrowheads).

Diagnosis

Salter I fracture of the distal femur with trapped periosteum.

Introduction

One of the complications of a physeal fracture is trapped periosteum. If the segment of trapped periosteum is of sufficient size, then adequate fracture reduction cannot be achieved and this in turn can cause premature physeal closure (PPC)¹. Most commonly, the fractures are Salter-Harris (SH) I and II types. The most commonly involved sites are the distal femur, proximal² (3) and distal tibia³, proximal humerus⁴ (4) and distal radius⁵, but trapped periosteum can theoretically occur at any physeal site.

3a

Figure 3:

The T2-weighted coronal image demonstrates trapped periosteum in the proximal medial tibial physis (arrow).

4a

Figure 4:

A proton density-weighted fat suppressed coronal oblique image demonstrates trapped periosteum in the lateral aspect of the proximal humeral physis (arrow).

If a physeal injury involves fixed or transient widening of the physis and periosteal disruption, the periarticular structures can become entrapped. In the knee, structures that may become entrapped with the periosteum include the medial collateral ligament⁶ and pes anserinus⁷. In the proximal humerus, possible structures entrapped with the periosteum include the deltoid muscle and biceps tendon⁴. In the wrist, entrapment of the pronator quadratus muscle has been reported⁸. In the ankle, the posterior tibialis tendon has been reported as a periarticular structure that can be entrapped³.

Although PPC is the major concern when structures are entrapped with a physeal fracture, in rare circumstances the entrapment is a surgical emergency. Grace reported anterior tibial neurovascular entrapment in three patients after SH II fractures of the distal tibia. In two of these patients, vascular compromise of the foot occurred which was successfully treated with open reduction and internal fixation⁹.

Mechanism of Injury

Most SH II fractures are secondary to hyperextension and varus or valgus angular stresses. When the epiphyseal-metaphyseal unit is disrupted, the periosteum is typically stripped from the underlying bony cortex on the distraction side and remains intact on the compression side¹⁰ (5, 6). PPC is common at certain anatomic sites and with certain mechanisms of injury. With fractures of the distal tibial physis, PPC after SH I and II fractures occurred less commonly with supination-external-rotation-type injuries (35%) compared to pronation-abduction-type injuries (54%)¹¹.

5a

Figure 5:

The proton density-weighted fat suppressed image demonstrates trapped periosteum on the distraction side of the injury (arrow).

6a

Figure 6:

In the same patient the slightly more posterior coronal proton density-weighted fat suppressed image demonstrates a sprain injury of the medial collateral ligament (arrows) indicative of a distraction injury.

Imaging Findings

Radiographic Evaluation: Radiography is the mainstay of imaging for physeal injuries. Although the periosteum is not directly visualized with radiography, inadequate reduction can suggest the presence of trapped periosteum. One study concluded that SH I or II fractures of the distal tibia with a residual physeal gap of > 3 mm after closed reduction is considered suggestive of trapped periosteum. A residual physeal gap > 3 mm after closed reduction was associated with PPC in 60% of patients versus 17% when the gap was < 3 mm¹.

CT Imaging: CT with multiplanar reconstructions offers improved delineation of osseous structures compared to radiography. Physeal widening and compression and metaphyseal and epiphyseal fractures are better visualized with CT compared to radiography. Additionally, late complications of bony bridging are better seen and quantified with CT compared to radiography. In practice, CT is used selectively for problem solving. Like radiography, CT cannot visualize periosteum.

MR Imaging: MR imaging is used in selected cases of physeal injuries. MR imaging can detect displaced and nondisplaced physeal injuries with greater confidence than radiography (7,8). In a study of pediatric knee examinations for traumatic injuries, MR imaging was superior to radiography in the delineation of the fractures. The diagnosis or confirmation of fractures with MR imaging altered clinical management in seven of eight patients. Additionally, four of these patients had ligamentous and meniscal injuries only seen with MR imaging¹².

7a

Figure 7:

A proton density-weighted fat suppressed sagittal image demonstrates trapped periosteum in the posterior physis (arrows) and a non-displaced fracture of the distal femoral metaphysis (arrowheads).

8a

Figure 8:

In another patient, a coronal proton density-weighted fat suppressed image demonstrates a SH III fracture with trapped periosteum in the medial distal femoral physis (arrow) and a parasagittal fracture plane through the intercondylar notch of the distal femoral epiphysis (arrowheads).

In addition to visualizing menisci and ligaments, MR imaging allows assessment of articular cartilage and non-ossified epiphyses. MR, like CT, provides multiplanar imaging and is done without ionizing radiation. Other complications such as osteonecrosis and bony bridging can be diagnosed with MR imaging.

In patients where satisfactory reduction cannot be obtained, MR imaging can be used to look for a mechanical cause. A recent report indicates that MR imaging can detect trapped periosteum¹³. In select situations where satisfactory reduction cannot be obtained, MR imaging can be used not only to detect trapped periosteum but other periarticular structures such as ligaments, neurovascular structures, tendons and muscles can also be evaluated.

Using radiography alone to make treatment decisions means that trapped periosteum will be underdiagnosed. In some patients, trapped periosteum is not associated with physeal widening and therefore will not be diagnosed with radiography. Randomized longitudinal studies would have to be performed to determine the significance of trapped periosteum when no physeal displacement is present.

Treatment

The treatment for trapped periosteum is open reduction and internal fixation. The patients diagnosed with trapped periosteum are those with fractures that cannot be reduced. This cohort of patients likely has relatively large segments of trapped periosteum. Presumably another cohort exists that have smaller entrapped segments of periosteum that go undiagnosed (9). The current indication for treatment is based on reduction as seen on radiography and not the presence or absence of trapped periosteum seen with MR imaging. No randomized trials exist that validate any therapeutic approach to patients with trapped periosteum.

9a

Figure 9:

A sagittal proton density-weighted fat suppressed image demonstrates a small slip of trapped periosteum (arrow) in the anterior distal femoral physis. The clinical significance of small trapped slips of trapped periosteum is uncertain.

Conclusion

In patients with Salter-Harris I and II fractures that have inadequate closed reduction, MR imaging can be used to diagnose entrapment of periarticular soft tissue structures. Trapped periosteum, in isolation or in combination with other trapped soft tissue structures, can be diagnosed with MR imaging. Open reduction and internal fixation in these circumstances may reduce the incidence of premature physeal closure.