The purposes of this article were identification (ie, verification and gradation) of anatomical risk factors that lead to anterior cruciate ligament (ACL) injury and determination of the probability of ACL injury among the population actively engaged in sports activities. We evaluated 66 patients divided into 2 groups: 33 patients in the examined group diagnosed with ACL lesion, and 33 patients in the control group diagnosed with patellofemoral pain. Patients were matched by age, sex, type of lesion, and whether the lesion was left or right sided. Measurements were carried out by radiography and magnetic resonance imaging. The study examined 32 anatomical factors. After identifying factors that lead to ACL injury, the following were determined: the coefficient of significance for each individual factor via the discriminant analysis and the canonical discriminant (ie, canonical correlation). Fifteen factors in men and 8 factors in women were differentiated as having influence on ACL injury. Based on these factors, it was determined whether the patients belonged to the examined or the control group with a success rate of 100% in men (100% sensitivity and specificity) and 91.7% in women (100% sensitivity and 83.3% specificity). The anatomy of the ACL prone to rupture and of the skeletal structures influencing it is significantly different from the anatomy of the ACL ligament resistant to injury. The probability of precise prognosis of ACL injury based on differentiated anatomical factors is 88.9% in men and 75.7% in women actively engaged in sports activities.
Numerous contemporary research papers describe different prevention programs aimed at decreasing the frequency of anterior cruciate ligament (ACL) rupture, as well as the rupture of other knee ligaments.1-3 Although risk factors for ACL rupture are numerous, these prevention programs target the biomechanical group of risk factors.4 Proper neuromuscular training can enhance the strength of the hamstrings (adductors), thereby decreasing the risk of ACL injury, especially in women.4 The same authors also state that adequate training can increase the level of knee fl exion during landing and thus eliminate 1 more ACL injury risk factor.
However, Karlson5 emphasizes that the key to normal ACL and knee joint functioning is not in surgical reconstruction techniques, but rather in the anatomy of the native anterior cruciate ligament. The same author also states that each patient is a unit in themselves whose ACL anatomy needs to be understood and studied in detail.
Bearing the above statement in mind, the question arises as to whether it is possible to influence the anatomical factors (and to what extent) with the aim of reducing the risk of ACL rupture. The most significant anatomical risk factors can be divided into 3 groups: the characteristics of the intercondylar notch, the tibial slope, and the morphometric characteristics of the ACL.6 In addition to these anatomical parameters, a special group of parameters related to the ACL position in the frontal and sagittal planes, as well as the relationship between the ACL on one hand and the femoral condyles and the tibial plateau on the other, has been added for the purpose of this study.
By monitoring all of the anatomical factors, those connected to ACL rupture would be identified as a separate group, and these factors could enable the identification of those knees with a potentially weak ACL, whose likelihood of rupture is high if the patient engages actively in sports activities for a longer period of time and is subject to sports knee injuries.
The purpose of this article is the identification, verification, and gradation of anatomical risk factors leading to ACL injury and determination of the probability of ACL injury in the population actively engaged in sports activities.
Materials and Methods
In this retrospective study, patients were accepted for knee interventions. None were diagnosed with gonarthrosis. The patients were divided into 2 groups. The examined group consisted of 205 patients with isolated ruptures of the ACL, and with no reported lesions of collateral ligaments, posterior cruciate ligaments, or other bone elements. The control group consisted of 258 patients who reported patellofemoral pain, but had no dysplastic change of the knee. After matching 33 pairs from the 2 groups, a total of 66 patients were considered. The patients were matched by their age at the time of the accident (a difference of up to 5 years was tolerated), sex, type of lesion (whether it was profession-related), and whether the lesion affected the left or right side of the knee. There were 21 male and 12 female pairs. Twenty pairs had the lesion on the right knee and 13 on the left.
The youngest patient was 15 years whereas the oldest patient in the examined group was 48 years and in the control group 52 years (Table 1). Measurement was performed on magnetic resonance images (MRIs) and on radiographs with the aid of a digital caliper and a goniometer. Anteroposterior and sagittal radiographs of the knee and the lower leg as well as magnetic resonance cross sections in all 3 planes were used. All MRIs were made using a 1.5T MRI scanner. This study covered 32 anatomical morphometric factors.
Methods of Measuring Morphometric Parameters of the Intercondylar Notch
Measurement of the dimensions of the intercondylar notch was performed on a horizontal magnetic resonance section cutting across the most posterior points of the medial and lateral condyles of the femur. Parameters were measured on each image at 2 levels: the level of the popliteal groove and at the points of maximal width for each parameter. These parameters were: the epicondylar width, the width of the medial and lateral condyles of the femur, and the width of the intercondylar notch. The height of the intercondylar notch was measured as the shortest distance between the highest point of the intercondylar notch and the line passing through the most posterior point of the medial and lateral condyles (Figure 1).
|Figure 1: The following were measured on the horizontal section: the width of the intercondylar notch (ICW), the width of the medial condyle (MW), the width of the lateral condyle (LW), the width of the distal end of the femur (ICW1LW1MW) at the level of the popliteal grove, the maximum width of the intercondylar notch (ICWmax), the maximum width of the medial condyle (MWmax), the maximum width of the lateral condyle (LWmax), the maximum width of the distal end of the femur (EWmax), and the height of the notch (ICH). The image in the top right corner shows the selected horizontal section (x) passing through the most posterior point of the femoral condyle (y). Abbreviations: EW, EWmax, epicondylar width; ICH, intercondylar notch height; ICW, ICWmax, intercondylar notch width; LW, LWmax, the width of the lateral condyle of the femur; MW, MWmax, the width of the medial condyle of the femur.
Based on the obtained measurements, the values of the notch width index were calculated as the ratio between the inter-condylar and epicondylar width, while the values of the notch shape index were calculated as the ratio between the height and width of the intercondylar notch. Both parameters were calculated at the level of the popliteal groove and in its largest segment (notch width index max and notch shape index max).
Method of Measuring the Morphometric Parameters of the Tibial Slope
To establish tibial slope, we used tibial proximal anatomic axis, which demonstrates the best correlation with tibial shaft anatomic axis.7,8
Methods from Stijak et al9 were used to determine the medial and lateral tibial slopes (Figure 2). Based on the tibial slope values of the medial and lateral condyles, the following were calculated: the mean value of the tibial slope, as the arithmetic mean of the said values, and the difference between the tibial slope of the lateral condyle and the tibial slope of the medial condyle.
|Figure 2: Determination of the tibial slope. The angle of the anterior tibial slope (a) was established on the radiograph using the tibial proximal anatomic axis and a line projecting from the anterior tibial plateau (line a). By transferring a to the line that follows the anterior tibial plateau on the sagittal MRI (line a), a line running parallel to the tibial proximal anatomic axis was obtained (line b). The obtained line was transferred to the sagittal MRI section of the medial (M) and lateral (L) tibial condyle. Using a line perpendicular to the tibial axis and a line running along the tibial slope of the medial (line c) and lateral (line d) condyle, the angle of the tibial slope (b and g) on the tibial condyle was obtained.
Method of Measuring the Morphometric Parameters of the ACL and Its Insertions
The frontal magnetic resonance section was used for the selection of the sagittal section that best covered the ACL (Figure 3). The borders of the tibial and femoral attachments were drawn on the selected section and the distance from the most anterior point on the tibia to the proximal point on the femur and the distance from the most posterior point on the tibia to the distal point on the femur of the ACL were measured. The same image was used to measure the sagittal diameter of the ACL represented as the length perpendicular to the direction of the ligament, at an equal distance from the tibial to the femoral attachment. The length of the tibial insertion was measured as the distance between the anterior and the posterior point of the insertion. The length of the femoral origin was measured as the distance between the most proximal to the most distal point of the origin. The width of the tibial insertion was measured on the frontal section running through the widest segment of the insertion.
|Figure 3: Method of measuring ACL dimensions and the dimensions of its attachments on the sagittal magnetic resonance section. (AC, anteromedial edge; BD, posterolateral edge; AB, length of tibial insertion; CD, length of femoral origin; EF, sagittal diameter or width). The image in the upper right corner shows a selected sagittal section (red section) in the frontal plane.
In case of central rupture, the ACL direction was determined according to the tibial and femoral insertions. In case of proximal rupture or distal avulsion, the ACL direction was determined based on one insertion, the median part of the ligament and the remains of the synovia.
Measuring the Position of the ACL and Its Relation to the Femoral Condyle and the Tibial Plateau
Measurement of the position of the ACL in the sagittal and frontal planes as well as measurement of its relation to the femoral condyles and the tibial plateau was performed on sagittal, frontal, and horizontal MRI sections. Methods from Stijak et al10 were used to determine: the ACL angle in the sagittal and frontal plane, the angle between the ACL and inner side of the lateral condyle, the angle of the inner side of the lateral condyle in the frontal and horizontal plane, the angle of the anterior tibial plateau and the angle between the ACL angle in the sagittal plane and the angle of the anterior tibial plateau (slope).
The differences between the examined and the control group regarding the above mentioned anatomical parameters were tested with the use of the Paired Sample t test in the SPSS 11.0 package. The significance level was set at 5% (P<.05). After identifying the anatomical factors leading to ACL injury, the significance coefficient for each individual anatomical factor was determined, with the use of the discriminant analysis test, as were the belonging and probability of belonging for each patient to the examined or control group, and the canonic discriminant (ie, canonic correlation).
On examination of 32 anatomical factors, the ones connected to ACL injury were differentiated, in men (15 factors) and in women (8 factors) (Table 2). With the use of the SPSS 11.0 program and the discriminant analysis method, the significance of each anatomical factor for ACL injury in men was determined (Table 3). The negative sign indicates that a smaller value is connected to ACL injury.
Based on 13 of 15 anatomical parameters, it was determined, for all patients, whether they belonged to the examined (with ACL rupture) or control (no rupture) group, with a 100% success rate (100% sensitivity and 100 % specificity), that is, without falsely positive or falsely negative cases, and with the centroid positioned at
2.776 for the examined group (Figure 4). Two parameters (the length of the posterior edge of the ACL and the angle between the ACL and the inner surface of the lateral condyle in the frontal plane) showed a significant correlation with some of the factors that were previously taken into consideration.
|Figure 4: The canonic discriminant of the examined male group (centriod 2.776).
Finally, with the aid of 13 anatomical risk factors, we obtained a canonic correlation coefficient of 0.934. Statistically speaking, if we take into account that the square of the coefficient of the canonic correlation represents the statistical probability of predicting a certain event, we can say that the probability of precise prognosis of ACL rupture in the population of men actively engaged in sports activities, based on these 13 anatomical factors, is 88.9%.
The same procedure was applied on the 8 anatomical factors which proved significant in the prediction of ACL injury in women. A smaller number of anatomical factors that lead to ACL injury in women confirm the theory that the following may play a more significant role in women: hormonal status, the elasticity of connective tissue, muscle strength, and the pattern of leaning against the surface when landing. Table 4 shows the significance of individual anatomical factors in ACL injury in women.
Based on 7 anatomical parameters (the angle between the ACL and the anterior tibial slope showed a significant correlation with some of the factors previously taken into consideration), it was determined whether these patients belonged to the examined (with ACL rupture) or the control (no rupture) group, with a success rate of 91.8%, with the centriod at 1.691 for the examined group (Figure 5). Namely, 2 patients from the control group were classified as patients with ACL rupture (2 falsely positive patients, 100% sensitivity and 83.3% specificity).
|Figure 5: The canonic discriminant of the examined female group (centriod 1.691).
Finally, with the help of 7 anatomical risk factors, we obtained a coefficient of canonic correlation of 0.870. In this case, we can say that the probability of accurate ACL rupture prognosis in the female population actively engaged in sports activities, based on these 7 anatomical factors, is 75.7%.
The Intercondylar Notch
None of the 13 anatomical parameters of the intercondylar notch proved to be significant for ACL injury in women, while 3 parameters of the intercondylar notch proved to be significant for ACL injury in men. In addition to the height of the intercondylar notch, whose higher values were observed in subjects with ACL rupture, the notch width index and the notch shape index, whose higher values were noted in the group of patients with no ACL rupture, proved to be significant. Data closest to the ones found in this study was 33.2 mm.11 They were obtained from measurements made on MRIs of male patients, while we found no data in reference literature related to the statistical significance of the height of the intercondylar notch for ACL injury.
Study performed on computed tomography scans showed that patients with a ruptured ACL had a statistically significant smaller value of notch width index and notch shape index than patients without ACL rupture.12
In a mixed population, patients with ACL insufficiency had a statistically significant smaller notch width index (0.23:0.25) than patients with an intact ACL.13 Smaller notch width index values were probably the result of a difference in methodology. The cited authors performed measurements on radiographic images that were made using the Holmblad method. The notch width index of patients with a bilateral rupture showed statistically significant smaller values than the index stated for patients with a unilateral rupture and the control group without ACL rupture.14 The study was performed on radiographic images and did not determine a statistically significant difference between the notch width index of patients without ACL and patients with unilateral ACL rupture. However, their results did not show statistical significance between the unilateral and the control group.
An 11-year prospective case-controlled study of 615 male track and field athletes found no statistically significant difference between the notch width index values in athletes with an intact ACL
(0.235) and those with an insufficient ACL (0.242).15 However, their measurements were performed on radiographic images. No statistically significant difference was found in the values of the notch width index between a group of patients with bilateral ACL injury and the matched, control group.16 Although their patients were matched according to age, sex, height, weight, and the type of physical activity, the patients from their control group did not have the knee injuries which were detected in patients in our control group.
The tibial slope proved to be a factor whose greater values corresponded with ACL rupture in both men and women, but only in the case of the slope on the lateral condyle. In addition, the difference between the slope of the lateral and medial condyle of the control group on one hand, and the examined group on the other, also proved to be significant (P<.01). This difference is positive in the examined and negative in the control group.
The tibial slope on the lateral plateau had a significantly higher value in the examined group than in the control group (P<.01). The anterior tibial translation during flexion was greater on the lateral tibial plateau.17-19 We believe that this can explain why the additional increase in the tibial slope imparted stress on the ACL that could result in its rupture.
Similar values of the tibial slope of the medial (4.80) and lateral (5.00) condyles were measured with the use of the Novel measurement technique in an MRI study of a mixed population of 100 patients.20 Patients with an intact ACL had a greater tibial slope on the medial (14.80) than on the lateral (11.80) condyle.21 The cited study was performed on photographs of tibias and the tibial slope was determined by means of the anterior tibial cortex line. However, the reasons for the higher values of the tibial slope on both the medial and lateral plateau together are multiple. One of the reasons was use of the line of the anterior tibial cortex as an axis in taking measurements (we used tibial proximal anatomic axis). The other reason refers to the authors’ statement that aging brings the increase in the tibial slope plateau. In that context, our population of the tested group was younger, 30 years of age on average, while in the Chiu et al21 study, the average age of the tested population was 68 years. A third reason may be to consider the difference between the 2 populations, Chinese and European.
When the differences between the lateral and medial condyles in the examined group were compared with the same differences in the control group, a significant difference (P<.01) was found. This finding confirmed the effect of the tibial slope on the lateral tibial plateau on an ACL lesion.
Our study has shown that the tibial slopes of the lateral and medial condyles are statistically and significantly different (P<.001) both in the examined and in the control group (men and women), which is why a comparison of their arithmetic means should not be performed. To be more precise, in the examined group, the tibial slope of the lateral condyle is greater than the slope of the medial condyle, while in the control group, the tibial slope of the lateral condyle is less than the slope of the medial condyle. By taking into consideration the arithmetic mean values (the average tibial slope), these differences are nullified, which leads to different results regarding their connection to ACL injury.
Morphometric Parameters of the ACL and Its Insertions
Measuring the width of ruptured ACL using a sagittal MR image posed a problem due to the broken continuity of the ruptured ligament. This is why, on MRIs of both groups, we measured the length of the ACL anterior edge as the distance from the most anterior point of the tibial insertion to the most proximal point of the femoral origin, which is the length of the posterior edge, as the distance between the most posterior point of the tibial insertion to the most distal point of the femoral origin. The width was also measured at an equal distance from the tibial and femoral attachments, perpendicular to the direction of the ACL, as the distance between the approximate anterior and posterior edges.
We feel that such measurement of a nonexistent ACL did not favor the examined group, as the result showed significance only between the male examined and the male control group, whereas this was not the case in women. Not one of the 7 morphometric parameters of the ACL itself showed a statistically significant difference between the female control and the female examined group (P<.05). In addition to the morphometric parameters of the intercondylar notch, the characteristics of the ACL also did not prove significant for ACL injury in women. However, sagittal MRIs of the male examined group showed a statistically significant greater length of the medial portion and the anterior and posterior edges of the ACL (P<.01 for all 3 dimensions) as well as smaller dimensions of the sagittal diameter as compared to the male control group (P<.01).
The lengths of the anteromedial and posterolateral parts of the ACL measured on 3-dimensional MRIs of 9 volunteers amounted to 34 mm and 27 mm, respecteromedial bundle was shorter, and the tively.22 In this case, the length of the an-length of the posterolateral bundle was greater than the lengths obtained in our study. The reason for greater values in our study can be explained by different places of measurement. Iwahashi et al22 measured the length of the anteromedial and posterolateral parts of the ACL while we measured its anterior and posterior edges.
The ACL width measured on oblique sagittal MRIs showed significantly smaller values in female basketball players (7.6 mm) than in male basketball players at the age of 16 years.2 Also, the cause of the lower ACL width values may be because body growth was not yet complete.
The Position of the ACL and Its Relation to the Femoral Condyle and the Tibial Plateau
Morphometric characteristics of the ACL position in the frontal and sagittal planes in the examined and the control group, as well as their relation to the femoral condyles and the tibial plateau, are statistically and significantly different in most cases. The male examined group showed a greater angle of the ACL in the frontal plane (P<.05) in comparison to its control group, while the female examined group showed a smaller angle in the sagittal plane (P<.05) as compared to its control group. The angle of the inner surface of the lateral condyle in the horizontal plane was steeper in the examined than in the control group, both in men (P<.01) and women (P<.05). However, the examined female subgroup displayed a steeper angle of the inner surface of the lateral condyle in the frontal plane than the control group (P<.01). This difference was not found in men, however, it transpired that the difference between the angles of the inner surface of the lateral condyle in the frontal and horizontal planes in the male control group was statistically significantly larger than the same difference within the male examined group (P<.01). In the male control group, during knee flexion, the angle of the inner surface of the lateral condyle decreased by 5.7º, providing more room for the ACL, which occurred neither in the male examined group nor in either of the female groups. The angle of the femoral insertion was sharper (small-er) in both examined groups (men, P<.01; women, P<.05) than in the control groups. The angle of the tibial insertion in the examined groups also showed smaller values than in the control group (men, P<.05; women, P<.01). In addition to these characteristics, both examined groups, male and female, showed a greater angle of the anterior tibial slope than the control groups (P<.05, in both groups).
In the MRI study of 96 patients divided into 2 groups (hamstring and bone-tendonbone graft groups) and 50 control patients (39 men and 11 women), the ACL angle in the frontal plane was 66º for the control group, 74º for the hamstring group, and 75º for the bone-tendon-bone graft group.23 If the ACL graft was performed at the ACL anatomical insertion points, the cited study speaks in favor of the thesis that patients prone to rupture have a vertically positioned ACL as compared to the control group.
Through intraoperative measurement of 13 male patients and 1 female patient, it has been determined that the position of the graft perpendicular to the tibial plateau is connected to abnormal graft tension, while a slanted position provides normal tension.24 The authors have shown that the angle of the tibial tunnel in the frontal plane of 75º±5º does not differ from the angle of the native ACL which is 76º±3º. In a later study, Howell et al25 found that the tibial tunnel being set in the frontal plane at an angle >75º connected with a greater loss of flexion and greater anterior laxity, which is why a position of the tibial tunnel in the frontal plane of 65º to 70º is recommended.
A lower angle of ACL in the sagittal plane was obtained on 6 cadaverous knees: 47.90 for the anteromedial part and 42.90 for the posterolateral part.26 The study on 20 cadaverous knees of patients under the age of 50 with an intact ACL, where the 3dimensional scanner was used to measure the angle between the 2 femoral condyles in the horizontal plane, with a poll on the roof of the intercondylar notch, the values of 48º for men and 42º for women were obtained.27 Considering that the lateral condyle makes up half of this angle, the value of the obtained angle would be approximately 24º for men and 21º for women. When reduced to the referential line running through the most posterior points of the femoral condyle, these angles would reach 66º for men and 69º for women, which is what differs from the values obtained in our study.
On the whole, out of 32 anatomical risk factors considered in this study, fifteen factors in men and 8 factors in women have proven significant to ACL injury. Due to mutual correlation of individual factors, statistics rely on 13 important anatomical factors in men and on 7 factors in women, providing us with a probability of prognosticating ACL rupture in the population actively engaged in sports activities with a reliability of 88.9% for men and 75.7% in women.
A case control study performed on 34 men compared the ACL volumes, the slopes of the tibial plateau and the dimensions of the intercondylar notch of patients with and without ACL rupture, and with the use of the discriminant analysis, the patients were properly grouped with a sensitivity of 70% and a specificity of 69%.28 We feel that greater values of sensitivity and specificity obtained in our study are primarily the result of the monitoring of a greater number of factors (15 in men and 8 in women). Our examined and control groups were also, in addition to sex and age, matched on the basis of the type of sports activity, which we feel is the most important for ACL rupture.
Since anatomical factors are not the only factors leading to ACL injury, it would be interesting to take into consideration the hormonal and neuromuscular factors so as to increase the percentage of reliability in predicting ACL rupture with the purpose of identifying patients actively engaged in sports activities who show a predisposition for this ligament rupture. Although it has covered a smaller number of female pairs (only 12), it has shown that women have fewer anatomical risk factors. As the frequency of ACL injury in the female population is high, we can assume that hormonal and neuromuscular factors play a more significant role in women than in men. We suppose that future studies with a greater number of matched pairs, monitoring not only anatomical, but also neuromuscular, hormonal, and external risk factors, would provide a prognosis of ACL rupture that would be 95% accurate as to probability.
The anatomy of the anterior cruciate ligament prone to rupture and the skeletal structures influencing it significantly differ from the anatomy of this ligament resistant to injury. The difference is greater within the male than within the female population. These anatomical risk factors and their values could, to a certain extent, play a role in identifying potentially weak ACL knees.
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Mr Stijak and Dr Filipović are from the Department of Anatomy, University of Belgrade, Belgrade, Dr Blagojević is from the Institute for Orthopedic Surgery Banjica, Belgrade, and Ms Santrač-Stijak is from the Health Center Novi Beograd, Belgrade, Serbia; Dr Spasojević is from the Department of Anatomy, School of Medicine, University of Banja Luka, Banja Luka, Bosnia and Herzegovina; and Dr Herzog is from Kantonales Spital Sursee-Wolhusen, Wolhusen, Switzerland.
Mr Stijak, Drs Blagojević, Spasojević, Herzog, and Filipović, and Ms Santrač-Stijak have no relevant financial relationships to disclose.
Correspondence should be addressed to: Lazar Stijak, MSc, Department of Anatomy, Faculty of Medicine, University of Belgrade, dr Subotica, 11000 Belgrade, Serbia (email@example.com).