Diagnostic Imaging of the Physis

Shari T. Jawetz

Parina H. Shah

Hollis G. Potter

Physeal injuries are common in the pediatric population and injury can impart significant morbidity, with the potential for long-lasting complications. Trauma is the most frequent etiology of physeal damage, with approximately 18% to 30% of pediatric fractures involving the physis.¹,²,³ Injury to the physis is more common during times of rapid growth in both males and females.⁴ Traumatic physeal fractures are rare in children younger than 5 years of age, and the peak incidences are at ages 11 to 12 years in girls and 13 to 14 years in boys.⁵ Although less common, nontraumatic causes of physeal injury include infection, ischemia, neoplasm, radiation,⁶ metabolic derangement, thermal injury, sensory neuropathy, and iatrogenic injury.⁷

Physeal growth arrest occurs in 5% to 10% of all physeal fractures.⁴ Factors that may influence the risk of postinjury growth disturbance include the age of the child, the degree of skeletal maturity, amount of growth remaining, and degree and area of physeal involvement.⁴ Postinjury growth disturbances more commonly occur at the distal ends of the long bones compared with the proximal ends,³ and growth arrest more commonly occurs from physeal injuries to the lower rather than upper extremities.⁸

Up to 50% of distal femoral physeal fractures result in growth arrest.⁹ Growth of the proximal tibia and distal femur contribute to 55% to 70% of the growth of the lower extremity; thus, premature physeal closure about the knee joint can result in substantial limb length discrepancies.¹⁰

Physeal injuries may result in pathologic bars or bridges across the physis. The complications of physeal bars include limb length discrepancy, angular growth deformity, and altered joint biomechanics.⁶ The younger the child is at the time of physeal injury, the greater the risk of subsequent complications.⁷ The risk of physeal bar formation is also increased when a fracture is malreduced or when a fracture line extends through the osseous portions of the epiphysis.⁴

Ogden⁶ defined three types of physeal bridging in the setting of partial physeal arrest: peripheral (type 1), linear (type 2), and central (type 3). Peripheral bridges can vary in size, but when they include the peripheral zone of Ranvier, which is involved in physeal widening, severe angular deformities may occur over short periods of time. Linear bridges extend linearly across the physis and may also result in angular deformities. Central bridges form across the central portion of the physis, with uninjured growth plate completely surrounding the bridge; central bridges tend to result in longitudinal growth disturbance.

Physeal bridges tend to occur at the sites of physeal undulations, such that they are most common centrally in the distal femur and peripherally in the proximal tibia. The relative location of the physeal damage will impact the types of future growth disturbances. Premature eccentric physeal closure causes angular growth deformity, whereas early central physeal closure may lead to an overall limb length discrepancy without angular abnormality.¹⁰

Smaller physeal bridges may be of little to no clinical significance because they can be transient. Animal models have shown that physeal bars involving less than 7% of the cross-sectional area may be transient.¹¹ Growth of adjacent normal areas of the physis will cause traction on and ultimately fracture a small bridge, allowing for restoration of normal growth.¹² Growth disturbance has been shown to occur in animal models once damage has occurred to more than 7% of the cross-sectional area of the physis.¹¹

Physeal bars involving less than 25% of the cross-sectional area of the physis may be amenable to surgical resection with subsequent interposition of an inert material such as fat. Those patients with damage to less than 10% of the growth plate tend to have the best outcomes. Better outcome following physeal bar resection is also associated with younger age at the time of surgery, single site of bar formation, and a centrally located defect.⁵ Therefore, for preoperative planning, knowledge of a physeal bridge’s precise location and size is necessary.¹³

Knowledge of the normal patterns of physeal closure is vital in order to avoid incorrectly diagnosing a physeal bar in a patient who is actually maturing normally. Physiologic epiphysiodesis about the distal femur occurs in a predictable pattern, with the central physis maturing initially and subsequent physeal closure continuing in a centripetal fashion.¹⁴ Within the proximal tibia, the posterior aspect of the physis closes initially, with subsequent progression anteriorly⁴ (Fig. 36.1).

In 1963, Salter and Harris¹⁵ initially categorized fractures involving the physis (Fig. 36.2). This remains the most commonly used classification system guiding treatment of physeal injuries.¹⁵ Types I and II fractures are extra-articular, whereas types III to V fractures are intra-articular. Types III to V fractures may result in both joint incongruity and physeal damage.

Type I fractures are confined to the physis without extension into either the epiphysis or the metaphysis.

Type II fractures (Fig. 36.3) extend along the physis and exit through a portion of the metaphysis, often yielding a triangular-shaped metaphyseal fragment, sometimes referred to as a Thurston Holland fragment.

Figure 36.1. Sagittal 3-D images in a child at three time points ([A] age 12 years, [B] age 14 years, and [C] age 15 years) demonstrate closure of the femoral and tibial physes progressing centripetally in the distal femur and from posterior to anterior in the proximal tibia.

Type III fractures (Fig. 36.4) extend from the articular surface, through the epiphysis to the physis, through which the fracture exits.

Type IV fractures arise from the articular surface, extend through the epiphysis and physis, and exit through the metaphysis. Anatomic reduction of the physis is necessary to reduce the risk of bony bar formation.

Type V fractures are compression or crush injuries to the physis.

The Salter-Harris types III to V fractures are associated with the greatest risk of future growth disturbances.¹⁶

Figure 36.2. Artistic rendering of the types I to V Salter-Harris fractures. (Reprinted with permission from Mulholland MW, Lillemoe KD, Doherty GM, et al, eds. Greenfield’s Surgery: Scientific Principles and Practice. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.)

Of traumatic fractures involving the physis, the Salter-Harris type II fractures are the most common (60.9%) followed by type I (28.6%), type III (6.7%), and type IV (3.7%) fractures. No type V injuries were identified in the cohort studied by Kawamoto et al.²

Harris growth arrest lines, related to variations in the rate of bone growth, typically parallel the physis and can indicate physeal damage. These growth arrest lines may be seen on radiographs, computed tomography (CT), and magnetic resonance imaging (MRI) examinations. A decreased distance between the growth arrest line and the physis on the side of injury indicates a

deceleration of growth of the injured extremity. In contrast, complete absence of a growth arrest line in an injured extremity indicates complete cessation of growth. Growth arrest lines that are oblique rather than parallel to the physis suggest a tethering effect secondary to partial growth arrest. Other secondary signs of a partial growth arrest include progressive malalignment.⁵

Figure 36.3. A 13-year-old male with Salter-Harris type II fracture. The sagittal CT image (A) demonstrates widening of the proximal tibial physis anteriorly (arrowhead). Coronal CT image (B) demonstrates a tiny fracture through the metaphysis with a Thurston Holland fragment (arrow).

Figure 36.4. A 17-year-old male (bone age 15 years) with Salter-Harris type III fracture. Anteroposterior (AP) radiograph (A) demonstrates vertically oriented lucency through the lateral margin of the epiphysis extending to the physis (arrow). Coronal CT (B) image demonstrates additional vertical fracture line extending through the tibial epiphysis to the physis, and there is a subtle step-off of the articular surface (arrows).

RADIOGRAPHY

Radiographs are the first-line imaging approach,⁷ particularly in the setting of trauma, due to their cost-effectiveness and ease of accessibility.¹⁷ However, radiographs do not allow for direct evaluation of the cartilage, as noncalcified cartilage is lucent on radiographs and thus secondary signs of physeal injury must be relied upon to make a diagnosis.¹⁸ Secondary findings of physeal injury on radiographs include epiphyseal displacement; physeal widening; and indistinctness of the normal, sharply defined sclerotic margins on the opposing sides of the epiphysis and metaphysis.¹²

On radiographs, signs of a type I fracture include widening, haziness, sclerosis, or irregularity of the physis. These findings can be subtle, and radiographs of the contralateral, symptomfree side may be helpful for comparison. For the types II through IV fractures, the fracture lucency may be seen in the metaphysis and/or epiphysis, but the precise degree of involvement of the physis can often not be determined. Of note, the type V injuries are often radiographically occult at initial assessment, leading to a delay in diagnosis if radiographs are the only means of imaging used to evaluate the injured patient.

Stress radiographs may be useful in evaluating posttraumatic joint stability, but they are often not possible in the acute trauma setting because of pain inhibition. In addition, there is a risk that the applied stress could cause inadvertent injury to the physis, such that a subtle Salter-Harris type I fracture might be converted into a displaced Salter-Harris type III fracture.¹⁹

Figure 36.5. A 10-year-old female with tibial eminence avulsion seen on lateral radiograph (A) (black arrowhead). Sagittal inversion recovery (B) and fast spin echo (C) MR images demonstrate an obliquely oriented Salter-Harris type III fracture line extending from the articular surface through the epiphysis to the physis (arrow) with surrounding bone marrow edema pattern (asterisk), in addition to the tibial eminence avulsion (white arrowheads).

Radiographs are indicated for evaluation of fractures, but the use of radiography in isolation has significant limitations. Evaluation of the physis can be difficult when the x-ray beam is not parallel to the physis.²⁰,²¹ The irregular three-dimensional (3-D) nature of the physis, which includes anatomic undulations and ridges, limits the use of the two-dimensional imaging used in radiographs. Subtle nondisplaced fractures may be occult on radiographs (Fig. 36.5).¹²

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