DOI: 10.1148/radiol.2473071097
(Radiology 2008;247:826-835.)
© RSNA, 2008
Anterior and Posterior Cruciate Ligaments at Different Patient Ages: MR Imaging Findings1
Hee Kyung Kim, MD,
Tal Laor, MD,
Norah J. Shire, PhD, MPH,
Judy A. Bean, PhD, and
Bernard J. Dardzinski, PhD
1 From the Department of Radiology and Imaging Research Center (H.K.K., T.L., B.J.D.) and Center for Epidemiology and Biostatistics (J.A.B.), Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, Cincinnati, OH 45229-3039; and Division of Epidemiology and Biostatistics, University of Cincinnati, Division of Digestive Diseases, Cincinnati, Ohio (N.J.S.). Received June 22, 2007; revision requested August 27; revision received October 29; accepted January 8, 2008; final version accepted January 16.
Address correspondence to T.L. (e-mail: laor@cchmc.org).
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ABSTRACT
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Purpose: To retrospectively compile normative data on the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) in children and young adults.
Materials and Methods: This HIPAA-compliant study was approved by the institutional review board. The requirement for informed patient consent was waived. Knee MR imaging examinations (n = 324) were performed in 168 female and 156 male patients (age range, 1–20 years) at 1.5 and 3.0 T, and the image findings were retrospectively evaluated by two blinded radiologists separately. One radiologist reviewed all images twice at two sessions, and the other reviewed a random subset of half the images during one session. Discordant assessments were resolved by consensus. The sagittal and coronal ACL-tibial angles, Blumensaat line–ACL angle, angle of inclination of the intercondylar roof, ACL-tibial insertion site, and PCL angle and horizontal component–to–vertical component ratio were measured. The associations between these values and patient age, patient sex, and physeal patency were assessed. Linear and fractional polynomial regression models were used to evaluate the relationships between measurements.
Results: ACL-tibial angles became significantly larger (P < .001) with increasing age during skeletal growth and approached adult values after physeal fusion. The Blumensaat line–ACL angle was constant after age 2 years. The inclination of intercondylar roof angle became significantly smaller (P < .001) with increasing age. The ACL-tibial insertion site was constant at the junction of the anterior and middle thirds of the tibial anteroposterior diameter and was not age dependent. The PCL angle became significantly larger (P < .001) with advancing age and in children who had fused as opposed to open physes. The horizontal component–to–vertical component PCL ratio became significantly smaller with advancing age (P < .001).
Conclusion: During growth, angulation of the ACL is age dependent. The angle and morphologic changes of the PCL are age dependent throughout skeletal maturation.
© RSNA, 2008
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INTRODUCTION
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Magnetic resonance (MR) imaging findings of normal anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) alignment and morphology in the knees of adults are well documented (1,2). Because the immature skeleton undergoes continuous growth and remodeling (3), the anatomic relationships between ligaments and bone that are well described in adults (4,5) may be different in children. This difference may affect the reliability of using alterations in the normal anatomy described in adults, such as buckling of the PCL as a secondary sign of ACL disruption (6), to evaluate the knees of children.
Study findings suggest that ACL injury is now more frequently recognized in children and adolescents (3,7–10). Several study investigators have advocated early ACL reconstruction in selected patients for prevention of additional injury and to allow early return to sports activities (7–10). Successful ACL reconstruction requires correct anatomic placement and angulation of a ligament graft with respect to the tibia and femur to avoid impingement and joint movement limitation (11,12). With the increase in reparative surgeries performed in younger children, similar anatomic data on the knees of younger patients are necessary. Although there have been reports of normal anatomy of the proximal tibia and tibial attachment sites of the ACL in children (13), to our knowledge, normative data on the MR imaging appearances of the ACL and the PCL in children have not been compiled. Thus, the purpose of our study was to retrospectively compile normative data on the ACL and the PCL in children and young adults.
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MATERIALS AND METHODS
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Our Health Insurance Portability and Accountability Act–compliant study was approved by the institutional review board of Cincinnati Children's Hospital Medical Center. The requirement for informed patient consent was waived.
MR Imaging Examinations
Data from picture archiving and communication systems computer records of all patients who underwent knee MR imaging examinations between January 2000 and August 2006 (n = 2519) were extracted. Patients were grouped in 1-year age intervals. Our goal was to retrospectively evaluate the findings of up to 25 examinations for each age group, when available. A patient's data were included in our study if the MR imaging examination was performed at our hospital and included, at minimum, a sagittal and coronal intermediate- or T2-weighted (conventional or fast spin-echo) MR imaging series or a fast inversion-recovery MR imaging series that adequately depicted the ACL and the PCL. These sequences enabled differentiation between the cruciate ligaments and nonossified epiphyseal cartilage. Patients were excluded if their imaging report included a diagnosis (where applicable) or history of partial or complete disruption of the ACL and/or PCL, previous reparative surgery, distal femoral or proximal tibial fractures, congenital structural anomalies, developmental delay, non–weight-bearing states, or syndromes. Because the normal morphologies of the ACL and PCL change with knee hyperextension or flexion (14,15), the MR images that depicted a femoral-tibial angle larger than 5° of hyperextension or larger than 15° of flexion were also excluded.
The findings of a total of 324 knee MR examinations performed in 313 children and young adults were included. Three patients underwent two imaging examinations performed 5, 14, and 27 months apart. The 324 examinations were performed in 168 knees of female patients and in 156 knees of male patients. The patients' ages ranged from 1 to 20 years (Table 1). In eight patients (three children aged 1 to <2 years and one patient each aged 2 to <3 years, 4 to <5 years, 7 to <8 years, 10 to <11 years, and 11 to <12 years), both knees were imaged on the same day. In four of these children, the knees were imaged together, and in four of them, the knees were imaged separately. For every patient in whom the data on both knees were included in our study, the values were sufficiently different and were measured during different sessions approximately 1 week apart to minimize intrapatient correlation bias.
In all patients, imaging was performed by using a 1.5-T (Signa Horizon LX Echospeed, Excite HD Echospeed, or Signa LX Echospeed Plus, GE Medical Systems, Milwaukee, Wis [137 patients]; Symphony or Magnetom Espree, Siemens Medical Solutions, Erlangen, Germany [151 patients]) or 3.0-T (Magnetom Trio; Siemens Medical Solutions [36 patients]) imaging system. The knee was positioned at full or nearly full extension. All examinations included, at minimum, the following sequences performed at 1.5 T: sagittal and coronal intermediate- or T2-weighted fast spin-echo or turbo spin-echo sequences (2500–4500/17–34 [repetition time msec/echo time msec] and/or 2500–4500/68–102 [effective], echo train length of six to eight, two signals acquired), conventional spin-echo intermediate- and T2-weighted sequences (2000–3000/12–15 and 2000–3000/64–80, one signal acquired), or a fast inversion-recovery sequence (3500–4500/34/155 [repetition time msec/echo time msec/inversion time msec], echo train length of six to eight, two signals acquired). The following parameters were used for 3.0-T MR imaging: 3000/8–18 and/or 3000–3500/56–64, a turbo factor of five to seven, and one signal acquired. For all sequences, the section thickness was 3–5 mm with an intersection gap of 0.5–1.5 mm and a matrix of (256–512) x (192–256) was used, depending on the patient's size. Sagittal and coronal sequences were prescribed from an axial sequence.
All knees were imaged by using a transmit-receive or receive-only extremity coil, torso or cardiac phased-array coil, neonatal body coil, or quadrature head coil. The choice of coil was based on the size of the patient, the desired field of view, and the need for unilateral versus bilateral knee imaging. When necessary, the patient was sedated according to radiology department guidelines.
MR Imaging Analysis of Physeal Patency
Patients were assigned (H.K.K., with musculoskeletal training and 5 years experience) to an open physes group or closed or closing physes group. Patients were considered to be skeletally immature or to have open physes when the signal intensity of the cartilage was seen in its entirety across both the distal femoral physis and the proximal tibial physis. Patients were considered to have closed or closing physes when the physeal cartilage signal intensity was discontinuous or absent across the physes.
All numeric measurements were performed by one author (H.K.K.). This author validated all physeal group assignments and measurements a second time. If the two measurements were different, the average value was used. A second author (T.L., pediatric musculoskeletal radiologist with 15 years experience) independently validated a random subset of half the measurements and the determination of physeal patency. All discordant assessments between authors were resolved by means of consensus measurement. During the image readings and measurements, the radiologists were blinded to the patients' sex and age.
The following data were then grouped according to 1-year age interval, patient sex, and whether the physes about the knee were open or closed. For each examination, the following values were measured by using clinical picture archiving and communication systems software (Centricity Enterprise PACS; GE Medical Systems).
MR Measurements of ACL
Sagittal ACL-tibial angle.—This angle was created between a line paralleling the midlateral tibial plateau (Fig 1, a, c) and a line demarcating the anterior-most margin of the ACL, drawn on the midline image best depicting the ACL (Fig 1b, 1d). Measurement of this angle is the technique used by Gentili et al (6).
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Figure 1a: Sagittal ACL-tibial angle. (a) Turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) of left knee of 3-year-old girl. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (b) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this girl, the angle is 38.5°. (c) Turbo spin-echo intermediate-weighted sagittal MR image (2000/16) of right knee of 19-year-old boy. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (d) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this boy, the angle is 53.9°.
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Figure 1b: Sagittal ACL-tibial angle. (a) Turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) of left knee of 3-year-old girl. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (b) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this girl, the angle is 38.5°. (c) Turbo spin-echo intermediate-weighted sagittal MR image (2000/16) of right knee of 19-year-old boy. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (d) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this boy, the angle is 53.9°.
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Figure 1c: Sagittal ACL-tibial angle. (a) Turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) of left knee of 3-year-old girl. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (b) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this girl, the angle is 38.5°. (c) Turbo spin-echo intermediate-weighted sagittal MR image (2000/16) of right knee of 19-year-old boy. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (d) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this boy, the angle is 53.9°.
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Figure 1d: Sagittal ACL-tibial angle. (a) Turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) of left knee of 3-year-old girl. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (b) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this girl, the angle is 38.5°. (c) Turbo spin-echo intermediate-weighted sagittal MR image (2000/16) of right knee of 19-year-old boy. The plane of the tibial plateau is demarcated (dotted line) on a lateral image. (d) On sagittal midline MR image obtained with the same sequence in same patient, the plane of the tibial plateau is superimposed, forming an angle with the most anterior margin of the ACL. In this boy, the angle is 53.9°.
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Coronal ACL-tibial angle.—This angle was created between a line demarcating the medial-most margin of the long axis of the ACL and a line connecting the medial- and lateral-most margins of the tibial plateau on the same section (Fig 2), as described by Sonin et al (16). This angle was drawn on the midcoronal image best depicting the ACL.
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Figure 2a: Coronal ACL-tibial angle. (a) Fast spin-echo intermediate-weighted coronal MR image (2800/34), with fat suppression, of right knee of 3-year-old boy. (b) Dotted lines on the same image mark the medial border of the ACL and the plane of the tibial plateau used to determine the coronal ACL-tibial angle of 41.7°. (c) Fast spin-echo intermediate-weighted MR image (2980/15), with fat suppression, of right knee of 19-year-old girl. (d) ACL-tibial angle on the same image (75.8°) is larger than that in b.
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Figure 2b: Coronal ACL-tibial angle. (a) Fast spin-echo intermediate-weighted coronal MR image (2800/34), with fat suppression, of right knee of 3-year-old boy. (b) Dotted lines on the same image mark the medial border of the ACL and the plane of the tibial plateau used to determine the coronal ACL-tibial angle of 41.7°. (c) Fast spin-echo intermediate-weighted MR image (2980/15), with fat suppression, of right knee of 19-year-old girl. (d) ACL-tibial angle on the same image (75.8°) is larger than that in b.
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Figure 2c: Coronal ACL-tibial angle. (a) Fast spin-echo intermediate-weighted coronal MR image (2800/34), with fat suppression, of right knee of 3-year-old boy. (b) Dotted lines on the same image mark the medial border of the ACL and the plane of the tibial plateau used to determine the coronal ACL-tibial angle of 41.7°. (c) Fast spin-echo intermediate-weighted MR image (2980/15), with fat suppression, of right knee of 19-year-old girl. (d) ACL-tibial angle on the same image (75.8°) is larger than that in b.
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Figure 2d: Coronal ACL-tibial angle. (a) Fast spin-echo intermediate-weighted coronal MR image (2800/34), with fat suppression, of right knee of 3-year-old boy. (b) Dotted lines on the same image mark the medial border of the ACL and the plane of the tibial plateau used to determine the coronal ACL-tibial angle of 41.7°. (c) Fast spin-echo intermediate-weighted MR image (2980/15), with fat suppression, of right knee of 19-year-old girl. (d) ACL-tibial angle on the same image (75.8°) is larger than that in b.
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Blumensaat line–ACL angle.—This angle was created between a line drawn along the edge of the intercondylar roof (ie, Blumensaat line) and the anterior-most margin of the ACL on the sagittal view best depicting this ligament (Fig 3).
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Figure 3: Blumensaat line–ACL angle and inclination of the intercondylar roof angle depicted on turbo spin-echo intermediate-weighted sagittal MR image (2000/16) of right knee of 19-year-old boy. Dotted line along edge of the intercondylar roof (Blumensaat line) and dotted line along most anterior border of the ACL form a 9.9° angle. Angle between the longitudinal femoral axis and the intercondylar roof is 44.7°.
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Angle of inclination of the intercondylar roof.—This angle was created between a line drawn along the edge of the intercondylar roof and the long axis of the femur (13) (Fig 3) on a midline sagittal image.
ACL-tibial insertion site.—On the sagittal image that best depicted the ACL, the distance from the anterior-most tibial border (at the level of the proximal physis) to the anterior edge of the ACL was measured and divided by the longest tibial anteroposterior distance measured on the same image, at the level of the proximal tibial physis (Fig 4). This is the method used by Shea et al (13).
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Figure 4: ACL-tibial insertion site depicted on turbo spin-echo intermediate-weighted sagittal MR image (2000/16) of right knee of 14-year-old boy. Distance between the anterior-most portion of the insertion of the ACL and the anterior aspect of the tibia (as determined at level of physis) (short dotted line) divided by the anteroposterior distance at the level of the proximal tibial physis (long dotted line) yields an insertion site of the ACL at 33% of the anteroposterior tibial diameter.
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MR Measurements of PCL
PCL angle.—This angle was created between a line connecting the origin site of the PCL on the femur to a point placed at the point of directional change of the PCL and a line drawn from the same point to the distal-most point of PCL insertion on the tibia, as described by Gentili et al (6) (Fig 5). A single image best depicting the PCL was used for this measurement.
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Figure 5a: PCL angle. (a) On turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) in 3-year-old girl, PCL angle is measured (dotted lines) between a line from the femoral origin site to the site of angular change and a line from this same site to the distal-most insertion site on the posterior tibia. The angle is 106.3°. The origin site is at the distal-most aspect of the intercondylar roof at the level of the joint surface. (b) Turbo spin-echo intermediate-weighted sagittal MR image (2000/16) in 19-year-old girl shows PCL angle of 122.8°. The origin of the PCL is now slightly more proximal on the intercondylar roof, a few millimeters above the joint surface.
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Figure 5b: PCL angle. (a) On turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) in 3-year-old girl, PCL angle is measured (dotted lines) between a line from the femoral origin site to the site of angular change and a line from this same site to the distal-most insertion site on the posterior tibia. The angle is 106.3°. The origin site is at the distal-most aspect of the intercondylar roof at the level of the joint surface. (b) Turbo spin-echo intermediate-weighted sagittal MR image (2000/16) in 19-year-old girl shows PCL angle of 122.8°. The origin of the PCL is now slightly more proximal on the intercondylar roof, a few millimeters above the joint surface.
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PCL horizontal component–to–vertical component ratio.—On the same image used to measure the PCL angle, we measured the ratio of the length of the horizontal component of the PCL from the origin site to the site of directional change, divided by the vertical length from this same site to the distal-most PCL attachment site on the posterior tibia (Fig 6).
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Figure 6a: PCL horizontal component–to–vertical component ratio. (a) Turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) in 3-year-old girl. Dividing the horizontal component length of the PCL by the vertical component length resulted in a ratio of 0.88. The knee is enlarged to approximate the size of the knee in b. (b) Fast spin-echo intermediate-weighted sagittal MR image (2000/16) in 19-year-old girl shows a horizontal component–to–vertical component ratio of 0.48. Note the larger posterior tibial protrusion compared with that in a.
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Figure 6b: PCL horizontal component–to–vertical component ratio. (a) Turbo spin-echo intermediate-weighted sagittal MR image (3000/8.8) in 3-year-old girl. Dividing the horizontal component length of the PCL by the vertical component length resulted in a ratio of 0.88. The knee is enlarged to approximate the size of the knee in b. (b) Fast spin-echo intermediate-weighted sagittal MR image (2000/16) in 19-year-old girl shows a horizontal component–to–vertical component ratio of 0.48. Note the larger posterior tibial protrusion compared with that in a.
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Statistical Analyses
Data normality was assessed by using the Shapiro-Wilk test. Nonnormal data were transformed when an appropriate transformation was possible; then normality was reassessed. Between-group analyses of patient sex– and physeal patency–based differences were conducted by using the rank sum test or the Student t test. Nonparametric analyses (eg, rank sum test) were used when no appropriate transformation was possible. Linear and fractional polynomial regression models were used to assess relationships between outcome variables (sagittal and coronal ACL-tibial angles, Blumensaat line–ACL angle, inclination of intercondylar roof angle, ACL-tibial insertion site, PCL angle, and PCL horizontal component–to–vertical component ratio) and predictor variables (patient sex, patient age, and physeal patency). Analysis results were stratified by sex, with regression functions fitted to the data for each sex. The goodness of fit for each model was assessed by using likelihood-ratio 
2 statistics and adjusted coefficients of determination for multivariate analysis (R2). Because interpretation of fractional polynomial regression functions is not always intuitive, we graphically represented those data with the best-fitting models. Changes with age are reported with linearity assumed, although changes over time may not progress in a linear fashion. All analyses were conducted by using software (Stata, version 8.2; Stata, College Station, Tex), and two-tailed P values less than or equal to .05 were considered to indicate statistical significance in all cases.
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RESULTS
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Open physes were identified at 237 examinations, while closed physes were identified at 87 examinations. Measurements and statistical significance analyzed according to physeal patency and patient sex are listed in Table 2.
ACL Measurements
Sagittal ACL-tibial angle.—This angle was significantly larger as a function of patient age (P < .001) and sex (P = .039) in the fractional polynomial regression model. The mean measurement was 55.5° ± 6.7 (standard deviation) for all female patients versus 53.9° ± 5.8 for all male patients. After controlling data for sex in a linear model, data showed that the angle became larger by a mean of 0.71° ± 0.05 with each additional year of age. In our patient cohort, the values continued to increase with increasing age in all groups, with a significant difference in angle between all patients with open physes (53.2° ± 6.2) and all patients with closed physes (58.8° ± 4.9) (P < .001) (Figs 1, 7). However, when we stratified the data according to open versus closed physes and made adjustments for sex, age was shown to be a significant predictor of sagittal ACL-tibial angle if the physes were open but not if they were closed.
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Figure 7: Sagittal ACL-tibial angle as a function of patient age and sex. Polynomial regression curves, with corresponding 95% confidence intervals (shaded), show that the angle became larger by a mean of 0.71° ± 0.05 with each additional year of age. Age was a significant predictor of the sagittal ACL-tibial angle when the physes were open but not when they were closed. The data obtained between the sexes were significantly different (P = .039).
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Coronal ACL-tibial angle.—This angle changed significantly with age in the fractional polynomial regression model (P < .001): It became a mean 1.3° ± 0.08 larger with each additional year of age after we controlled data for sex (Figs 2, 8). The difference in angle between the sexes was also significant: The mean angle measurement was 62.9° ± 10.0 for all female patients versus 60.7° ± 9.8 for all male patients (P = .04). The difference in mean coronal ACL-tibial angle between all the patients with open physes (59.2° ± 9.4) and all those with closed physes (69.1° ± 7.4) also was significant (P < .001). Again, after adjustments for sex, age was no longer a significant predictor of coronal ACL-tibial angle when the physes were closed.
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Figure 8: Coronal ACL-tibial angle as a function of patient age and sex. Polynomial regression curves, with corresponding 95% confidence intervals (shaded), show that the angle became larger by a mean of 1.3° ± 0.08 with each additional year of age. Age was a significant predictor of the coronal ACL-tibial angle when the physes were open but not when they were closed. The data obtained between sexes were significantly different (P = .04).
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Blumensaat line–ACL angle.—When the data of patients younger than 2 years were excluded, there was no significant effect of age, sex, or physeal patency on measurement of the Blumensaat line–ACL angle. When data from the youngest patients were included, this angle became larger in a curvilinear manner as a function of age.
Angle of inclination of the intercondylar roof.—A fractional polynomial model was fit to the angle between the longitudinal femoral axis and the anterior margin of the intercondylar roof (Blumensaat line). This angle became significantly smaller, by a mean of 0.42° ± 0.05, with each additional year of age (P < .001) after we controlled the data for sex, and it differed significantly according to sex (mean angle, 44.4° ± 5.1 for all female patients vs 46.5° ± 5.3 for all male patients; P < .001). There was a significant difference (P < .001) in this angle between all the patients with open physes (46.5° ± 4.8) and all those with closed physes (42.3° ± 5.3).
ACL-tibial insertion site.—The location of the insertion of the anterior border of the ACL with respect to the tibial plateau did not change significantly according to age, sex, or physeal patency. The mean insertion site percentage (of the anteroposterior tibial diameter) for both the open physes group and the closed physes group was 31% ± 4 (P = .59).
PCL Measurements
PCL angle.—This angle as measured on sagittal MR images was best described with a curvilinear model and was larger by a mean of 0.68° ± 0.09 with each additional year of age (P < .001). The PCL angle also differed according to physeal patency, with mean values of 113.9° ± 8.1 in the open physes group and 121.9° ± 8.7 in the closed physes group (P < .001). The difference in PCL angle between all the female patients and all the male patients was not significant (Figs 5, 9).
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Figure 9: PCL angle as a function of patient age and sex. Polynomial regression curves, with corresponding 95% confidence intervals (shaded), show the angle became larger by a mean of 0.68° ± 0.09 with each additional year of age. Age was a significant predictor of the PCL angle. The PCL angle also differed significantly between the open and closed physes groups. Data obtained between the sexes were not significantly different.
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PCL horizontal component–to–vertical component ratio.—This ratio diminished linearly and significantly, by a mean of 0.02 ± 0.01, with each additional year of age (P < .001), but it was unaffected by patient sex. There was a relatively shorter horizontal component and a longer vertical component with increasing age. There was a significant difference in mean horizontal component–to–vertical component ratio between the open physes group (0.67 ± 0.12) and the closed physes group (0.56 ± 0.18) (P < .001) (Fig 6).
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DISCUSSION
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In our study, we identified on both sagittal and coronal MR images a significantly larger angle that the ACL forms with the tibial plateau with advancing age; measurements of this angle approach adult values with skeletal maturity. Gentili et al (6) described a normal sagittal ACL-tibial angle of 55.6° in adults, with a cutoff angle smaller than 45° being suggestive of an ACL tear with 91% sensitivity and 97% specificity. The mean sagittal ACL-tibial angle in the knees of the youngest patients (aged 1 year, n = 10) in our study was 46.4° ± 4.9 versus a mean angle of 60.4° ± 3.1 in the knees of the oldest patients (aged 20 years, n = 11). The normal sagittal ACL-tibial angle in the youngest patients was similar to the cutoff angle used to suggest ACL tears in adults. On the coronal images, the mean ACL-tibial angle was 49.3° ± 6.8 in the youngest patients and 71.1° ± 5.9 in the oldest patients. Sonin et al (16) calculated a normal mean coronal ACL-tibial angle of 76° in adults. The differences between the angles measured in the patients in our study and those described in adults may be representative of normal growth.
We suggest that the larger ACL-tibial angle with advancing age in both planes is induced by an alteration in the morphology of the growing knee. Our study findings show that as a child matures, there is an increase in steepness of the intercondylar roof on sagittal images that reaches significance. This translates to a greater height of the intercondylar notch on coronal images. Because the ACL originates from the posterior part of the medial surface of the lateral femoral condyle (4), as this area elongates, it may "pull up" the ACL origin site.
The ACL-tibial insertion site remains proportionally stable relative to the anteroposterior diameter of the tibial plateau in children and adults (13). We found the insertion site to be located at a mean of 31% of the tibial diameter across the age ranges assessed in our study, similar to a published value of 28% in children aged 12–14 years (13). In addition, the Blumensaat line–ACL angle remained constant with growth after age 2 years. This was expected, because the smaller inclination of intercondylar roof angle complements the larger ACL-tibial sagittal angle, and, thus, the relationship between the distal femoral intercondylar roof and the ACL remains constant. In the groups with the youngest patients, there were a small number of imaging studies available for inclusion, and this possibly accounted for the variation in the Blumensaat line–ACL angle at ages younger than 2 years. The significant difference in sagittal and coronal ACL-tibial angles between the sexes likely reflected the difference in growth rates between male and female individuals (17).
We suggest that the measurement changes with growth of the PCL may reflect the relationship between the PCL and the morphology of the distal femur. The origin of the PCL on sagittal images of the knees of very young subjects appears to lie at the inferior-most aspect of the distal femoral epiphysis, along the articular surface. Our study results indicate that the angle of inclination of the intercondylar roof was smaller in the older patients, resulting in a steeper roof. We postulate that because the origin of the PCL is on the lateral surface of the medial femoral condyle in the intercondylar notch (4), the steeper roof at older ages might result in a more proximal translation of the origin, which is slightly cranial to the articular surface in older children. This relative upward migration of the origin site may be reflected in an increase in the PCL angle. The mean PCL angle in adults with an intact ACL is 123° (6), but it is substantially smaller (106°) in patients with an acutely torn ACL. However, in our younger patient population, the mean normal PCL angle was 113.9°, which is smaller than this angle in adults. This finding suggests that using a decrease in the PCL angle on the basis of values determined in adults as a secondary sign of ACL injury in the skeletally immature patient may not always be useful.
In addition to undergoing a gradual increase in the sagittal angle, the PCL also undergoes a configurational change with age. Early in life, the horizontal component of the PCL is just slightly shorter than the vertical component. However, the horizontal component–to–vertical component ratio becomes significantly smaller with advancing age because of a relative lengthening of the vertical component. The tibial insertion site in the more skeletally mature knee is at a prominent tibial protrusion posteriorly (4). Change in the shape of the posterior tibia may be responsible for the relative lengthening of the vertical component of the PCL in older children. Further study of the morphologic changes in the knee bones of pediatric subjects with age might be helpful to better understand the changes in the cruciate ligaments that we have observed.
The changes in the cruciate ligaments seen as children grow may have clinical importance because younger children are undergoing knee surgery more frequently. Although reconstructive treatment of ACL injuries in skeletally immature children is associated with a risk of physeal injury and subsequent growth disturbance, use of nonsurgical therapy can result in instability and meniscal and chondral injuries (7). ACL reconstruction is now advocated in selected patients for prevention of additional injury (7–10,18). When surgery is performed in younger patients, recognition of the immature skeletal anatomy may be important for successful ACL graft placement. To avoid graft laxity and loss of flexion in adults, an angle smaller than 75° in the coronal plane is optimal (19). To our knowledge, the optimal angle for graft placement in children is not known.
One limitation of our study was its retrospective design. In addition, the cases included were from a cross-sectional group and were not followed up in a longitudinal fashion. Because the examinations were performed for various indications, the imaging sequences and parameters used were not identical. We attempted to exclude knees that were positioned with exaggerated degrees of flexion or extension, but a dedicated knee holder was not used. We could not control for minor degrees of variation in section angulation. In addition, half the measurements were validated by one author only. Limited data for the younger children were available because fewer examinations had been performed in subjects in this age group. We used bilateral knee data in a small subset of cases, although measurements in the two knees were different. Although prior literature on adult findings describes an optimal graft location relative to the location of the posterior portion of the ACL footprint on the tibia to avoid graft impingement (20), we chose to measure the anterior-most margin on the sagittal images and the medial-most margin on the coronal images. These margins were identified most consistently and reliably in the children.
Further study is needed to determine if changes in the length and volume of the ACL occur with growth, because this additional information may prove useful when planning surgical repair. We did not directly assess the change in the width of the intercondylar notch of the distal femur, which possibly has an important role in cruciate ligament injury in children, or the change in the contour of the proximal tibia.
In summary, we found that during growth, there are significant angular and morphologic changes involving the ACL and the PCL. These changes may reflect normal developmental alterations in the osseous structures about the knee, such as steepening of the intercondylar roof. Once there is no potential for further growth in the knee, the appearance and position of the cruciate ligaments approach those described in adults and do not change significantly with age. The findings of our study may help to further elucidate the complex anatomy of the knee in children. Additional study is needed to determine whether the same parameters described as secondary signs of ACL injury described in the literature on adults are applicable to children.
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ADVANCES IN KNOWLEDGE
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- The angle between the anterior cruciate ligament (ACL) and the tibia as seen on sagittal and coronal MR images is significantly smaller in younger children than in older children.
- The angle of the posterior cruciate ligament (PCL) in younger children, as seen on sagittal MR images, is significantly smaller than that in older children.
- The morphology of the PCL changes during normal skeletal growth, with the horizontal component–to–vertical component ratio becoming relatively smaller.
- The attachment site of the ACL relative to the proximal tibia remains stable throughout all ages of childhood development.
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IMPLICATIONS FOR PATIENT CARE
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- Since ACL repair is now being performed in younger children, recognition of the normal developmental changes of the cruciate ligaments may be important for successful ACL graft placement.
- Use of the described PCL morphology in adults as a secondary sign of ACL injury in children may require further study, because the angle and configuration of the normal PCL change with skeletal growth.
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FOOTNOTES
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Abbreviations: ACL = anterior cruciate ligament • PCL = posterior cruciate ligament
Author contributions: Guarantors of integrity of entire study, H.K.K., T.L., B.J.D.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, H.K.K., T.L., B.J.D.; clinical studies, all authors; statistical analysis, all authors; and manuscript editing, all authors
Authors stated no financial relationship to disclose.
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References
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ABSTRACT
INTRODUCTION
