Chapter 59 Anterior Cruciate Ligament Reconstruction in Skeletally Immature Patients

Allen F. Anderson, Christian N. Anderson

Intrasubstance tears of the anterior cruciate ligament (ACL), a common injury in adults, are relatively rare in children and adolescents.¹ Presumably, this difference in prevalence of ACL tears is due to anatomical and biomechanical differences that predispose skeletally immature knees to physeal and bone injury rather than ACL tears. Despite being an uncommon occurrence, ACL tears in children and adolescents have recently been reported with increasing frequency.²^–²⁰ The increased recognition of this injury may be attributed to an increase in sports participation combined with improved examination and diagnostic methods. When a skeletally immature patient presents with a torn ACL, the physician is confronted with a difficult decision because nonoperative treatment may result in instability with subsequent meniscal tears and early degenerative changes,^2,^6,⁹^–¹¹ and surgery may cause iatrogenic leg length discrepancy or angular deformities.^{5–7,12–15}

Management of ACL tears in skeletally immature patients remains controversial because of a deficiency in the basic science literature on physeal growth and response to injury. Clinical studies published on the treatment of this condition have contributed to the confusion by having poor methodology with low levels of evidence and combining patients with different levels of maturation and methods of treatment.*

Although a fundamental lack of knowledge is the cause of the controversy surrounding treatment of ACL injuries in children and adolescents, a rational approach to management of this problem can still be developed based on current understanding of the natural history of this injury, normal growth and development, response of the physis to injury, and treatment options. This approach enables the surgeon to estimate whether the patient is at high, intermediate, or low risk of iatrogenic growth disturbance and to choose the method of treatment depending on the level of risk.

Natural History

The natural history of ACL tears in children and adolescents has not been clearly documented, but it can be extrapolated from the results of studies published on nonoperative treatment. The unique challenge of treating intrasubstance ACL tears in skeletally immature patients combined with the absence of an efficacious surgical procedure resulted in a historical approach of nonoperative treatment, consisting of bracing, quadriceps and hamstring strengthening, counseling, and activity modification. A growing body of evidence from studies of nonoperative treatment proves that the natural history of ACL tears in children and adolescents is generally poor for behavioral or other reasons. ACL deficient patients in this age group are noncompliant with activity modification, and consequently they often experience recurrent instability, meniscal damage, and sports-related disability. Kannus and Jarvinen ¹⁰ treated 25 patients with grade II partial ACL tears and seven patients with grade III complete ACL tears. Eight years after the initial injury, the results were excellent or good for the patients with Grade II ACL tears. The long-term results of grade III ACL injuries were poor because these patients developed chronic instability and posttraumatic arthritis. Kannus and Jarvinen reported that the results of nonoperative treatment for complete ACL tears in this age group were not acceptable. Angel and Hall³ evaluated 27 children and adolescents who had a torn ACL. At the time of follow-up, the majority had pain and limitations of activity. Eleven of 12 patients in their series of children younger than age 14 were disabled with knee function. Graf et al⁹ found that seven of eight children treated conservatively sustained new meniscal tears within 15 months. McCarroll et al¹¹ found that 37 of 38 adolescent patients had episodes of instability, and 27 of 38 had symptomatic meniscal tears. Eleven of 18 patients in the series of Mizuta et al²⁰ developed degenerative changes within 51 months, and the researchers stated the results were “poor and unacceptable.” Millet et al²¹ also found that the incidence of meniscal injuries increased significantly in chronic cases.

Ideally, operative treatment of ACL injuries in skeletally immature patients could be postponed until physeal closure. The results of these studies, however, indicate that nonoperative treatment may actually result in a greater risk to the knee than surgery.

Assessing Skeletal Maturity

The central issue in treatment of ACL tears in the pediatric age group is the patient’s skeletal age, which determines the relative risk and potential consequences of iatrogenic physeal injury. Some skeletally immature patients will have a great deal of growth remaining, whereas others will have minimal growth of the distal femur and proximal tibial physes. The consequences of growth disturbance may be severe in the former patients and insignificant in the latter patients. The lack of specific documentation of skeletal maturity in clinical studies (i.e., wide-open physis) published on the treatment of ACL tears in skeletally immature patients is a source of the controversy surrounding the treatment of the condition. A rational approach to management of this problem can be based on estimation of the relative risk (high, intermediate, or low) by determining the patient’s chronological age, skeletal age, and physiological age.

For large populations, chronological age is an excellent predictor of skeletal maturity; however, patients may show a significant variance from the average. Consequently, it is important to determine the skeletal age with radiographs. The most common method of estimating skeletal age is by comparing an anteroposterior radiograph of the patient’s left hand and wrist with the age-specific radiographs in the Greulich and Pyle atlas.²²

Physiological age can be determined with Tanner staging of sexual maturation.²³ Patients are preliminarily staged prior to surgery by questioning them about the onset of menarche or growth of axillary hair. After induction of anesthesia and prior to surgery, Tanner staging is determined by examining the patient’s secondary sexual development, including the growth of pubic and axillary hair, breasts, and genitalia. Prepubescent patients are categorized in Tanner stage I and II of development, pubescent patients are in stage III, and postpubescent patients are in Tanner stage V (Table 59-1).

Table 59-1 Tanner Staging Classification of Secondary Sexual Characteristics

Tanner Stage	Male Sexual Characteristics	Female Sexual Characteristics
Stage I (prepubescent)	Testes <4 mL or <2.5 cm No pubic hair	No breast development No pubic hair
Stage II	Testes 4 mL or 2.5–3.2 cm Minimal pubic hair at base of penis	Breast buds Minimal pubic hair on labia
Stage III (pubescent)	Testes 12 mL or 3.6 cm Pubic hair over pubis Voice changes Muscle mass increases	Elevation of breast; areolae enlarge Pubic hair of mons pubis Axillary hair Acne
Stage IV	Testes 4.1–4.5 cm Pubic hair as adult Axillary hair Acne	Areolae enlarge Pubic hair as adult
Stage V (postpubescent)	No growth Testes as adult Pubic hair as adult Facial hair as adult Mature physique	No growth Adult breast contour Pubic hair as adult
Other	Peak height velocity: 13.5 years	Peak height velocity: 11.5 years Menarche: 12.7 years

Normal Growth and Development

The physes of the distal femur and proximal tibia are the most rapidly growing in the body. Anderson et al ²⁴ estimated that the distal femoral physis contributes 40% and the proximal tibia physis contributes 27% of the overall lower extremity length. More recently, Pritchett²⁵ reported that the distal femur grows at 1.3 cm per year until the last 2 years of maturity, when the growth rate drops to 0.65 cm per year. The rates of the proximal tibial growth are 0.9 cm per year and 0.5 cm per year in the last 2 years. The peak height velocity for males is 13 to 15 years of age (average 13.5 years), and it rarely occurs before Tanner stage IV. Twenty percent of males do not hit peak height velocity until Tanner stage V. For females, the peak height velocity occurs in Tanner stage III between 11 and 13 years of age (average 11.5 years). Peak height velocity in females precedes menarche by approximately 1 year.

The severity of iatrogenic growth deformity is determined in part by the patient’s skeletal maturity at the time of injury. It has been estimated that complete closure of the proximal tibial physis in the average 12-year-old boy, complete closure of the distal femoral in a 13-year-old boy, or complete closure of the femoral and tibial physes of a 14-year-old boy will result in a 3-cm leg length discrepancy. A leg length discrepancy of 1.2 cm is considered within normal variance.

The greatest concern, however, is not leg length discrepancy but angular deformity. An over-the-top femoral groove may result in a valgus/flexion deformity of the distal femur by damaging the perichondral ring of LaCroix. Damage to the anterior tibial physis may result in recurvatum. In the worst-case scenario, Wester et al ²⁶ estimated that a 14-year-old boy with 2 cm of remaining distal femoral growth could develop a 14-degree valgus deformity with a lateral femoral epiphysiodesis or 11-degree recurvatum with partial tibial physeal arrest.

The results of these studies may be used to rank the potential consequences of an iatrogenic physeal injury. High-risk patients are prepubescent and in Tanner stages I and II of sexual maturation. Intermediate-risk patients are pubescent and in Tanner stage III. Low-risk patients with closing physes are in Tanner stage IV of sexual maturation.

Basic Science Research on Physeal Injury

Although a deficiency exists in the age-specific basic science on physeal injuries, research has demonstrated the effects of drill hole damage to the physis and the results of placing a soft tissue graft through a transphyseal hole in animal models. Makela et al ²⁷ in 1988 drilled 2- and 3.2-mm transphyseal femoral holes in a rabbit model. The 2-mm holes destroyed 3% of the cross-sectional area of the physis, and the 3.2-mm holes destroyed 7% of the cross-sectional area. The destruction of 7% of the cross-sectional area of the growth plate caused permanent growth disturbance.

Other researchers have evaluated the effect of placing a soft tissue graft across the physis. Guzzanti et al ²⁸ performed an ACL reconstruction with the semitendinosus tendon using 2-mm transphyseal femoral and tibial holes in immature rabbits. The femoral holes damaged 11% of the transverse diameter and 3% of the cross-sectional area of the physis, and the tibial holes damaged 12% of the transverse diameter and 4% of the cross-sectional area of the physis. Two of the 21 tibiae developed a valgus deformity, and one was shorter. The researchers recommended careful evaluation of the percentage of damage to the area of the physis before performing intraarticular methods of reconstruction of the ACL in adolescents.

Houle et al ²⁹ performed a transphyseal ACL reconstruction in a rabbit model using four tunnel diameters ranging from 1.95 to 3.97 mm. They found that the larger drill holes caused more marked deformity and the soft tissue graft did not offer protection to physeal arrest. They recommended that tunnels involve 1% or less of the area of the physis when reconstructing the ACL in children.

Janarv et al ³⁰ drilled 1.7-, 2.5-, and 3.4-mm holes in rabbit femurs. The hole in one femur was left empty, and the hole in the contralateral femur was filled with a soft tissue autograft. They found that growth retardation occurred when the drill injury destroyed 7% to 9% of the distal femoral physis, but no retardation was seen in injuries of 4% to 5% of the cross-sectional area of the physis. The soft tissue grafts prevented solid bone bridging, but a bone cylinder formed around the grafts. They also measured the tibial and femoral physis of a 12-year-old girl and estimated that an 8-mm drill hole would destroy 3% to 4% of the physis. In contrast to the results of Houle et al²⁹ and Guzzanti et al,²⁸ Stadelmaier et al³¹ found that a soft tissue graft placed in transphyseal drill holes of a canine model prevented formation of a bony bridge and subsequent growth disturbance.

The effect of tensioning a graft across open physes has also been evaluated. Edwards et al ³² found that tensioning a fascia lata autograft at 80N in a canine model caused valgus femoral and varus tibial deformities without radiographic or histological evidence of physeal bar formation, indicating the physes were responding to the Hueter-Volkmann principle, which states that application of compressive force perpendicular to the physes will inhibit longitudinal growth. This study illustrates the potential risks for ACL reconstruction in this age group, even with physeal-sparing procedures.

Risk Factors for Iatrogenic Growth Disturbance

The potential consequences of growth disturbance after ACL reconstruction in the skeletally immature knee have a major influence on decisions about surgical technique. Although the results of basic science studies on physeal injury in animals may not be entirely applicable to humans, several important risk factors for growth disturbance after physeal injury have been identified. The studies of Guzzanti et al ²⁸ and Houle et al²⁹ demonstrated that the proximal tibial physis seems to be more vulnerable than the femoral physis to growth arrest.

In general, the risk of growth disturbance is related to the extent of damage relative to the cross-sectional area of the physis. However, uncertainty still exists, even in animal models, regarding the size and orientation of the drill holes that can be made without causing growth disturbance. The drill hole size threshold for growth disturbance in animal models has been between 1% and 7% of the cross-sectional area of the physis.^27,²⁸^–³⁰ The holes should be drilled perpendicularly, rather than obliquely, to limit the area of damage to the physis. Although results have been mixed, placing a soft tissue graft across the physis appears to offer protection from bone bridging and growth arrest. Research also demonstrates that the physes are sensitive to compressive forces,³² and consequently ACL grafts, including those used in physeal sparing procedures, should not be overtensioned.

Significant leg length discrepancy or angular deformity, although rare, has been reported after ACL reconstruction in skeletally immature patients.^5,^6,¹⁴ Kocher et al⁵ surveyed Herodicus and the ACL study group. One hundred and forty respondents reported 15 cases of growth disturbance. We have also seen two cases of valgus knee deformity after ACL reconstruction in adolescents. One of these patients had a staple placed across the lateral femoral physis.⁶ The other patient was a 12-year-old boy who was recently seen 6 months after a transphyseal ACL reconstruction with an Achilles tendon allograft. The graft had failed, and the patient had a 3-degree valgus alignment of the normal knee and a 7-degree valgus alignment of the ACL deficient knee, without physeal arrest.

Treatment Options

Case reports and animal studies showing iatrogenic growth disturbance after intraarticular transphyseal replacement have prevented clinicians from routinely applying proven methods of ACL reconstruction for adults to skeletally immature patients. Nonoperative management of ACL tears in children and adolescents is an especially seductive approach. The advantages of delaying surgery include additional psychological maturation of the patient, which facilitates compliance with postoperative rehabilitation, and greater skeletal maturity, which allows for less risky and more familiar traditional procedures. For these reasons, some surgeons still advocate a nonoperative approach despite the poor results.^7,^12,¹⁴

Other surgeons have performed primary repair ^33,³⁴ or extraarticular replacement in this age group.^9,¹¹ Unfortunately, these procedures have been found to be no more successful in children than they are in adults.

Modified physeal-sparing intraarticular replacements have also been advocated to minimize the risk of physeal injury.¹⁹ Parker et al³⁵ reconstructed the ACL by passing the hamstring tendons through a groove in the anterior aspect of the tibia and over the top of the lateral femoral condyle. Kocher et al^13,¹⁸

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