Valgus Knee Instability in an Adolescent

Ligament Sprain or Physeal Fracture?

Kenneth R. Veenema, MD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 27 - NO. 8 - AUGUST 99

In Brief: A 15-year-old boy was hit on the lateral aspect of his left knee while playing football. The injury was initially diagnosed as a medial collateral ligament sprain, and radiographs were negative. Stress views, however, demonstrated medial widening of the physis consistent with a Salter-Harris type 1 injury to the distal femur, and magnetic resonance imaging (MRI) demonstrated a type 3 injury extending through the epiphysis. Stress radiographic views or MRI is diagnostic of distal femoral physeal fracture, and a positive diagnosis should prompt referral.

Valgus loading from a lateral blow is a common mechanism of knee injury in sports activity and often results in an isolated injury to the medial collateral ligament (MCL), but, among children and adolescents, fracture of the distal femoral physis is also a possibility. These fractures, though uncommon, are frequently associated with significant morbidity, including fracture displacement, joint motion loss, and growth-plate arrest with subsequent angular deformity and limb-length discrepancy. Complications can occur even with nondisplaced fractures.

The typical treatment for an MCL injury—aggressive functional rehabilitation emphasizing early motion, strength maintenance, and early return to activity with protective bracing—could cause displacement or impair healing if a distal femoral physeal fracture has occurred. Thus it is important to exclude this fracture before starting treatment. The following case report emphasizes the importance of considering distal femoral physeal fracture in any skeletally immature athlete who presents with posttraumatic valgus knee instability and tenderness at the distal femoral growth plate.

Case Report

A 15-year-old high school football player sustained a valgus injury to his left knee when, with his leg extended, he was struck laterally by another player during a game. He described immediate disability and swelling but was able to limp off the field. He denied hearing a pop and had no history of earlier knee injury. He was evaluated on the sidelines and held out for the remainder of the game.

Subsequent training-room evaluation demonstrated an effusion, medial tenderness, and valgus instability. He was placed in a knee immobilizer, given crutches, and instructed to see his physician the next day for treatment of an MCL injury.

The following day he was evaluated by his primary care physician, who referred him to our sports medicine clinic for management and functional rehabilitation of his presumed MCL sprain.

Physical exam. On examination, the patient had a 3+ effusion of the left knee, a flexion range of 10° to 90°, and tenderness of the distal femur at the origin of the MCL. This pain increased with valgus stress. A 6- to 8-mm increase in medial joint-line opening was present with valgus stress at both the limit of extension and at 30° of flexion, but a firm end point was felt. Varus stress caused no lateral tenderness, pain, or instability.

Lachman and anterior drawer tests demonstrated mildly increased translation, but a ligamentous end point was difficult to assess because of guarding by the patient. A posterior drawer test was negative. No patellar tenderness, patellar instability, or apprehension was present. The patient was able to do a straight-leg raise from the supine position without difficulty. His neurovascular exam was normal.

Imaging studies. Initial anteroposterior (AP) and lateral plain radiographs were unremarkable (figure 1). The distal femoral physis appeared close to maturity but was still open. A subsequent AP valgus stress view demonstrated widening of the medial aspect of the distal femoral physis, but no epiphyseal or metaphyseal extension of the fracture line was evident (figure 2).

The patient was presumptively diagnosed as having a Salter-Harris type 1 fracture of the distal femoral physis (a type 1 fracture follows the physeal line). However, magnetic resonance imaging (MRI) was obtained to exclude an accompanying anterior cruciate ligament (ACL) injury. The MRI demonstrated an intact ACL, but there was clear evidence of fracture-line extension through the epiphysis to the intra-articular surface of the distal femur, indicating a type 3 injury (figure 3). No joint surface incongruity or MCL injury was evident.

Treatment. A long leg cast was applied and strict non-weight-bearing with crutches for 4 weeks was prescribed. The patient's progress was followed with weekly plain radiographs to assess for any fracture displacement.

After 4 weeks there was good radiographic evidence of periosteal bone formation along the distal femoral metaphysis and physis. The cast was subsequently removed, and no pain was present at the fracture site. The patient was placed in an adjustable hinged brace allowing protected range of motion and was told to start joint motion exercises, but to avoid weight-bearing for an additional 4 weeks.

At the 8-week point he had regained full motion. He gradually began to resume bearing weight and also started a quadriceps and hamstring restrengthening program.

After 12 weeks he demonstrated 80% of full strength in the quadriceps and hamstrings as compared with the opposite side and was bearing his full weight without pain. A functional progression of impact and pivot activities was begun, but he was withheld from any contact activity.

At 16 weeks, he demonstrated full strength in the quadriceps and hamstring muscles as compared with the uninjured side and was participating in noncontact activities without difficulty. He was therefore allowed to return to unrestricted activities. Plain radiographs at the 16-week visit demonstrated closure of the medial aspect of the distal femoral physis and near-closure of the lateral physis. Limb lengths and valgus carrying angles of the femur relative to the tibia and pelvic bones (Q-angles) were equal bilaterally.

Follow-up clinical and radiographic examinations were planned for 6-month and 1-year intervals to assess for growth-plate closure and evidence of limb-length discrepancy or angular deformity.

Discussion

Any time valgus instability is present in a skeletally immature individual, a distal femoral physeal fracture should be considered. As with this patient, this injury may not be suspected, because ligament injuries are the more common result of lateral knee trauma and because fractures of this physis are relatively uncommon. In a recent report of a series of pediatric patients (1), fractures of the distal femoral physis accounted for 7% of lower-extremity physeal fractures, while those to the distal tibial physis made up 72%.

Early detection of distal femoral physeal fractures is important because this physis accounts for 70% of the femur's longitudinal growth and 40% of the lower extremity's (2). Fractures of the distal femoral physis have been shown to result in a significant incidence of limb-length discrepancy and angular deformity, either of which may be more severe than predicted by the initial Salter-Harris classification (3). It should be noted, however, that although distal femoral physeal fractures are more frequent in adolescents than in younger children, complications in adolescents are less common because growth-plate closure is imminent (4).

Besides age, factors that affect growth deformity following distal femoral physeal fractures are initial displacement of the fracture and the ability to maintain an anatomic reduction (3,5).

Anatomic factors. The placement of ligamentous attachments about the distal femoral physis makes it vulnerable to injury (figure 4). The posterior capsule, MCL, and cruciate ligaments all attach to the distal femoral epiphysis, leaving the physis fully exposed to valgus loads applied to the extended knee. In contrast, the MCL attaches at a site distal to the proximal tibial metaphysis, making the proximal tibial physis less vulnerable to damage from valgus loads.

Because of growth-related anatomic factors, distal femoral physeal fractures are more frequent in adolescents than in younger children and more likely to result from relatively minor trauma, such as from sports activities. During adolescence, the periosteum overlying the physis is thin and relatively weak compared with the strong metaphyseal bone. Also, at this time the MCL remains stronger than the cartilaginous physis. This makes the distal femoral physis particularly prone to injury. Furthermore, the knee is subjected to increasing forces during athletic activities in adolescence. In younger children, fractures of the distal femoral physis are often a result of more severe trauma, such as car-pedestrian accidents.

Appropriate imaging. This case also illustrates the importance of appropriate imaging studies. Displaced physeal fractures are obvious both clinically and radiographically, but nondisplaced physeal injuries may look normal on initial radiographs. Further imaging studies are essential if this fracture is suspected.

Stress views. Before the advent of MRI, stress views were traditionally recommended if a skeletally immature individual's initial films were negative but the clinical exam suggested valgus instability with distal femoral tenderness. This recommendation was supported by reports (6,7) of adolescents who had sports-related Salter-Harris type 1 and type 3 distal femoral physeal fractures that were not evident on initial radiographs but were demonstrated by stress films.

To avoid further injury when obtaining stress views, the knee should be carefully flexed to 30° and subjected to gentle valgus stress while slight traction is applied to the leg. Conscious sedation with intravenous narcotics and benzodiazepines may facilitate the exam and prevent further physeal injury if muscle spasm and/or pain prevents adequate relaxation.

Magnetic resonance imaging. The advent of MRI is redefining diagnosis of distal femoral physeal fractures. In two recent series (8,9), early MRI raised the Salter-Harris classification to a higher grade than was initially apparent on plain radiographs in more than 50% of cases.

In a skeletally immature patient, MRI should be considered in lieu of stress films if plain radiographs are negative, a nondisplaced distal femoral physeal fracture is suspected, and the patient has an acute hemarthrosis. Use of MRI in this situation protects the physis from further injury and potential fracture displacement.

MRI is also helpful in diagnosing ligamentous injury (such as to the ACL), which, in one study (10), occured in 38% of patients (6 of 16) who had femoral physeal fractures. MRI provides excellent visualization of the knee ligaments and may help confirm the extent of these associated injuries when physical exam findings are equivocal.

Treatment. Treatment for a nondisplaced distal femoral physeal fracture should include immobilizing the knee in a long leg cast, prescribing non-weight-bearing use of crutches, and following up with weekly radiographs to assess for fracture displacement. If displacement is suspected or diagnosed, the patient should be referred to an orthopedic specialist.

References

1. Mann DC, Rajmaira S: Distribution of physeal and nonphyseal fractures in 2,650 long-bone fractures in children aged 0-16 years. J Pediatr Orthop 1990;10(6):713-716
2. Pritchett JW: Longitudinal growth and growth-plate activity in the lower extremity. Clin Orthop 1992;Feb(275):274-279
3. Lombardo SJ, Harvey JP Jr: Fractures of the distal femoral epiphyses. Factors influencing prognosis: a review of 34 cases. J Bone Joint Surg (Am) 1977;59(6):742-751
4. Beaty JH, Kumar A: Fractures about the knee in children. J Bone Joint Surg (Am) 1994;76(12):1870-1880
5. Thomson JD, Stricker SJ, Williams MM: Fractures of the distal femoral epiphyseal plate. J Pediatr Orthop 1995;15(4):474-478
6. Simpson WC Jr, Fardon DF: Obscure distal femoral epiphyseal injury. South Med J 1976;69(10):1338-1340
7. Torg JS, Pavlov H, Morris VB: Salter-Harris type-III fracture of the medial femoral condyle occurring in the adolescent athlete. J Bone Joint Surg (Am) 1981;63(4):586-591
8. Smith BG, Rand F, Jaramillo D, et al: Early MR imaging of lower-extremity physeal fracture-separations: a preliminary report. J Pediatr Orthop 1994;14(4):526-533
9. Jaramillo D, Hoffer FA, Shapiro F, et al: MR imaging of fractures of the growth plate. AJR Am J Roentgenol 1990;155(6):1261-1265
10. Bertin KC, Goble EM: Ligament injuries associated with physeal fractures about the knee. Clin Orthop 1983;Jul(177):188-195

Dr Veenema is an assistant professor of emergency medicine and orthopedics in the department of orthopedics, division of athletic medicine, at the University of Rochester School of Medicine in Rochester, New York. He is a member of the American Medical Society for Sports Medicine and the American Board of Emergency Medicine and holds a certificate of added qualifications in sports medicine. Address correspondence to Kenneth R. Veenema, MD, University Sports Medicine, 2180 South Clinton Avenue, Rochester, New York 14618.