• Factors Affecting ACL Injury

    Dr Donald Shelbourne pioneered the contralateral ACL reconstruction for torn anterior cruciate ligaments (using the Patella tendon of the ‘good’ knee as the ACL graft) and developed the accelerated ACL reconstruction rehabilitation protocol. Here he outlines the most recent data regarding ACL injuries to either knee (the ‘good’ and operated knees) after ACL reconstruction and explains the relationship between ACL size and ACL injury.

    Many theories exist as to how anterior cruciate ligament (ACL) injuries occur and as to why the incidence of ACL injury appears to be higher for women than for men, given equal exposure rates. Shelbourne et al. 1 published an article in 1998 that described the relationship of the intercondylar notch width of the femur (distance between the two ‘condyles’ of the thigh bone) and the incidence of ACL injuries after ACL reconstruction. This present article provides updated data that has been collected as part of an ongoing prospective analysis of ACL injuries.

    The size of the intercondylar notch has been studied by several investigators as a possible factor for ACL injury. The intercondylar notch size, however, is not the actual factor, but instead provides a way to identify the size of the cruciate ligaments that sit in the notch. It is the ACL size that is a factor for ACL injury, not the size of the notch.

    Davis et al. 2 performed MRI evaluations on 124 consecutive patients who were evaluated for knee pain but did not have an ACL or a posterior cruciate ligament (PCL) injury. One MRI view that was located at the mid line of the shin bone was measured for ACL and PCL width. The intercondylar notch width and the widths of the ACL and PCL were measured digitally on the physician-independent console.

    The results showed a positive correlation between notch width and the widths of the ACL and PCL. The mean ACL width was 5.7mm (± 1.1 mm) for women and 7.1mm (± 1.2 mm) for men, but the size of the ACL varied greatly for both men and women, from 3 to 10mm.

    This study showed that the measurement of the femoral intercondylar notch could be used as an indicator of ACL size and that the size of the ACL is not a standard size but varies tremendously. Shelbourne et al. 1 found that x-ray measurements of the intercondylar notch taken with a back to front view with the patient standing correlated with measurements of the notch that were made during knee surgery. Therefore, x-ray measurements can be used to estimate notch width and native ACL size.

    One might assume that larger people, both in height and stature, would have larger ACLs. Shelbourne and Kerr 3 conducted a study of 478 athletes to evaluate the relationship of the intercondylar notch to height, weight and sex. Like many other studies, women on average had significantly narrower intercondylar notches than men. However, there was no statistically significant correlation between height and intercondylar notch width for men or women. Futhermore, no significant correlation existed between weight and notch width for men or women. Therefore, body size is not a predictor of notch width or ACL size. Large men have the same size variation in ACL size as smaller men, and the same is true for women. However, for equal size men and women, whether at 6 feet or 5′ 6″ tall, men have larger ACLs than women in general.

    Given that the incidence rate for ACL injury is higher for women than men who have the same exposure rate (play and train for their sport over the same number of hours), ACL injury has been labeled as a female problem. However, it is important to remember that more men participate in athletics and have much more exposure to injury, and there are still many more ACL reconstructions performed on men than women. In my practice, only 35% of the ACL reconstructions are performed on women. I believe that men who have small ACLs are at equal risk for ACL injury as women who have small ACLs.

    When looking at subsequent ACL injuries after ACL reconstruction, it is important to consider not only ACL graft tears but also ACL tears to the opposite (normal) knee. I have heard many physicians boast an extremely low ACL graft rate by saying, “My ACL reconstruction was so strong that the patient tore the ACL in the other knee instead.”

    We have found that, when the ACL reconstructed leg is not rehabilitated to achieve strength and range of motion equal to the opposite normal knee, the risk of injury to the normal knee is increased. Therefore, it is important to consider injury to the normal knee after ACL reconstruction as part of the outcome from ACL surgery.

    This current data analysis reflects the results of ipsilateral (using a patella tendon graft from the same knee as ACL injury) ACL reconstructions performed between 1992 and 2003. The mean intercondylar notch width by sex and notch width group is shown in Table 1. Women have statistically significant narrower notches than men.

    Table 1 – Intercondylar Notch Width (Mean ± Standard Deviation (range))

    Group Women Men
    All patients 13.3 mm ± 1.4 mm (7-15) 15.2 mm ± 1.5 mm (8-24)
    Small notch group ( < 15 mm) 13.1 mm ± 1.5 mm (7-15) 13.6 mm ± 1.4 mm (8-15)
    Large notch group ( > 16 mm) 17.0 mm ± 1.0 mm (16-20) 17.9 mm ± 1.7 mm (16-24)

    Table 2 shows the ACL graft tear rate and the rate of ACL injury to the normal contralateral (other side) knee based on intercondylar notch width and sex. These numbers reflect the results of ACL reconstruction between 1992 and 2003. The data show that ACL graft tear rates after ACL reconstruction are similar for men and women. The ACL graft is 10 mm wide and approximately 5 mm thick for all patients. The factors present in women that would make them have a higher incidence of ACL still would be present after ACL reconstruction. One would assume that the ACL graft tear rate would be higher for women than men, but these data do not confirm this theory.

    Similar to our findings in 1998, men and women who have small intercondylar notches tend to have a subsequent injury to the contralateral normal knee. Patients who have a notch width of < 15 mm have a native ACL in the contralateral normal knee that is most likely smaller than the ACL graft (approximately 6 mm wide versus 10 mm wide for the ACL graft). Patients who have narrow intercondylar notch widths ( < 15 mm) had a 9.2% rate of injuring the contralateral (normal) ACL compared with a 2.5% ACL graft tear rate.

    This phenomenon appears to be more prominent in women, but within the small and large notch groups, women still have a much smaller intercondylar notch than men. Overall, only 17% of the men had a notch that was less than 14 mm; whereas, 46% of women did. Conversely, only 6.7% of the women had a notch wider than 17mm, where as 48% of men had a notch wider than 17 mm. Women in the large notch group have a contralateral ACL tear rate of 8.8% versus an ACL graft tear rate of 1.5%. This is probably due to the fact that the ACL graft, which measures 10 mm wide is still much larger than the ligament in a knee with an intercondylar notch of 16 or 17 mm wide.

    Men with narrow notches, like women with narrow notches, have a low incidence of ACL graft tears after ACL reconstruction and are more likely to tear the ACL in the normal contralateral knee that has the smaller ligament. Men in the large notch category have similar incident rates of tearing the ACL graft or the ACL in the normal contralateral knee. We suspect that this is because the ligament size is similar between knees, and once the patient returns to normal athletic activities, his knees are at equal risk for re-injury.

    Table 2 – Incident rate of injury to either knee after ACL reconstruction based on notch width and sex

    n (%)
    n (%)
    Total injuries/patients
    ACL-graft tears      
    < 15 mm notch 3/194 (1.5) 7/210 (3.3) 10/404 (2.5)
    > 16 mm notch 1/68 (1.5) 19/274 (6.9) 20/342 (5.8)
    Opposite knee ACL tears      
    < 15 mm notch 21/194 (10.8) 16/210 (7.6) 37/404 (9.2)
    > 16 mm notch 6/68 (8.8) 16/274 (5.8) 22/342 (6.4)
    Total injuries/patients 31/262 (11.9) 48/484 (9.9) 79/746 (10.6)

    The incidence of ACL tears is related to ACL size, which can be predicted by measuring the intercondylar notch width. On average, women have narrower intercondylar notches than men, even when they are of equal body size. People with narrow notches have a higher incidence of tearing the ACL in the contralateral normal knee after ACL reconstruction than people with wide notches.

    1. Shelbourne KD, Davis TJ, Klootwyk TE: The relationship between intercondylar notch width of the femur and the incidence of anterior cruciate ligament tears. A prospective study. Am J Sports Med 1998;26:402 – 408.
    2. Davis TJ, Shelbourne KD, Klootwyk TE: Correlation of the intercondylar notch width of the femur to the width of the anterior and posterior cruciate ligaments. Knee Surg Sports Traumatol Arthrosc 1999;7:209-214.
    3. Shelbourne KD, Kerr B. The relationship of femoral intercondylar notch width to height, weight, and sex in patients with intact anterior cruciate ligaments. Am J Knee Surg 2001;14:92- 96.

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