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Mechanisms of Anterior Cruciate Ligament Injury in Basketball: Video Analysis of 39 Cases

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The mechanisms of anterior cruciate ligament injury in basketball are not well defined. To describe the mechanisms of anterior cruciate ligament injury in basketball based on videos of injury situations. Case series; Level of evidence, 4. Six international experts performed visual inspection analyses of 39 videos (17 male and 22 female players) of anterior cruciate ligament injury situations from high school, college, and professional basketball games. Two predefined time points were analyzed: initial ground contact and 50 milliseconds later. The analysts were asked to assess the playing situation, player behavior, and joint kinematics. There was contact at the assumed time of injury in 11 of the 39 cases (5 male and 6 female players). Four of these cases were direct blows to the knee, all in men. Eleven of the 22 female cases were collisions, or the player was pushed by an opponent before the time of injury. The estimated time of injury, based on the group median, ranged from 17 to 50 milliseconds after initial ground contact. The mean knee flexion angle was higher in female than in male players, both at initial contact (15 degrees vs 9 degrees , P = .034) and at 50 milliseconds later (27 degrees vs 19 degrees , P = .042). Valgus knee collapse occurred more frequently in female players than in male players (relative risk, 5.3; P = .002). Female players landed with significantly more knee and hip flexion and had a 5.3 times higher relative risk of sustaining a valgus collapse than did male players. Movement patterns were frequently perturbed by opponents. Preventive programs to enhance knee control should focus on avoiding valgus motion and include distractions resembling those seen in match situations.
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Medicine
American Journal of Sports
DOI: 10.1177/0363546506293899
2007; 35; 359 originally published online Nov 7, 2006; Am. J. Sports Med.
E. Hewett and Roald Bahr
Tron Krosshaug, Atsuo Nakamae, Barry P. Boden, Lars Engebretsen, Gerald Smith, James R. Slauterbeck, Timothy
Mechanisms of Anterior Cruciate Ligament Injury in Basketball: Video Analysis of 39 Cases
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Although much attention has been focused on noncontact
ACL injuries in team sports, the exact mechanism of these
injuries remains unclear.
7,28
Understanding the joint kine-
matics and loading patterns that lead to injury is essential.
However, if the aim is to prevent injuries from occurring, it
may also prove useful to explore the nature of the injury sit-
uations in a wider context.
4
Important information would
include what kind of player actions are involved, whether
the joint kinematics is different during injury situations,
and, if so, what factors cause the abnormal behavior. One
such factor could be a perturbation occurring before the
injury, for example, by being pushed off balance. In other
words, it may be helpful to describe the injury mechanism
in terms of not only the involved biomechanics but also the
playing situation and player behavior.
4
Mechanisms of Anterior Cruciate
Ligament Injury in Basketball
Video Analysis of 39 Cases
Tron Krosshaug,*
PhD, Atsuo Nakamae,
Barry P. Boden,
MD, Lars Engebretsen,
MD, PhD,
Gerald Smith,
§
James R. Slauterbeck,
ll
MD, Timothy E. Hewett,
PhD, and
Roald Bahr,
MD, PhD
From the
Oslo Sports Trauma Research Center, Department of Sports Medicine, Norwegian
School of Sport Sciences, Oslo, Norway,
The Orthopaedic Center, Rockville, Maryland,
§
Department for Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway,
ll
Department of Orthopaedics and Rehabilitation, University of Vermont, Burlington, Vermont,
and
Sports Medicine Biodynamics Center and Human Performance Laboratory, Cincinnati
Children’s Hospital, Cincinnati, Ohio
Background: The mechanisms of anterior cruciate ligament injury in basketball are not well defined.
Purpose: To describe the mechanisms of anterior cruciate ligament injury in basketball based on videos of injury situations.
Study Design: Case series; Level of evidence, 4.
Methods: Six international experts performed visual inspection analyses of 39 videos (17 male and 22 female players) of ante-
rior cruciate ligament injury situations from high school, college, and professional basketball games. Two predefined time points
were analyzed: initial ground contact and 50 milliseconds later. The analysts were asked to assess the playing situation, player
behavior, and joint kinematics.
Results: There was contact at the assumed time of injury in 11 of the 39 cases (5 male and 6 female players). Four of these
cases were direct blows to the knee, all in men. Eleven of the 22 female cases were collisions, or the player was pushed by an
opponent before the time of injury. The estimated time of injury, based on the group median, ranged from 17 to 50 milliseconds
after initial ground contact. The mean knee flexion angle was higher in female than in male players, both at initial contact (15° vs
9°, P = .034) and at 50 milliseconds later (27° vs 19°, P = .042). Valgus knee collapse occurred more frequently in female play-
ers than in male players (relative risk, 5.3; P = .002).
Conclusion: Female players landed with significantly more knee and hip flexion and had a 5.3 times higher relative risk of sus-
taining a valgus collapse than did male players. Movement patterns were frequently perturbed by opponents.
Clinical Relevance: Preventive programs to enhance knee control should focus on avoiding valgus motion and include distrac-
tions resembling those seen in match situations.
Keywords: athletic injuries; anterior cruciate ligament (ACL); biomechanics; perception
359
*Address correspondence to Tron Krosshaug, PO Box 4014 Ullevaal
Stadion, Oslo, Norway 0806 (e-mail: tron.krosshaug@nih.no).
No potential conflict of interest declared.
The American Journal of Sports Medicine, Vol. 35, No. 3
DOI: 10.1177/0363546506293899
© 2007 American Orthopaedic Society for Sports Medicine
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at NORGES IDRETTSHOEYSKOLE on August 18, 2007 http://ajs.sagepub.comDownloaded from
360 Krosshaug et al The American Journal of Sports Medicine
Video footage of injury situations represents objective
sources of information on the kinematics involved in the
injury mechanism. The accuracy of this method ultimately
depends on the methods used to extract the data. In contrast,
athlete interviews are subjective reporting of the injury, are
laden with inaccuracies of remembering the event, and are
often biased by others’ input into what might have happened.
Mathematical, laboratory, or cadaveric simulation studies can
also provide accurate data, but they cannot provide informa-
tion from actual injuries. Hence, video analysis may represent
a valuable research tool to describe the mechanisms of injury.
Previous studies that used video analysis to describe the
mechanisms of noncontact ACL injuries seem to agree that in
most cases, the injury occurred early after initial contact (IC)
in landings or cutting maneuvers with the knee near full
extension.
5,34,40
Also, many situations resulted in a “valgus col-
lapse, that is, a situation in which the knee collapses medi-
ally from excessive valgus and/or internal-external rotation.
However, with the exception of the study of Olsen et al,
34
who attempted to quantify the joint kinematics, most descrip-
tions were qualitative, and the results are difficult to compare
across studies. Furthermore, apart from 1 study on team
handball,
34
previous video analyses have only investigated a
limited number of cases from mixed sports.
Therefore, the purpose of this study was to describe the
mechanisms of ACL injury in basketball in terms of the
playing situation, player behavior, and joint kinematics
based on 39 videotapes of real injury situations.
MATERIALS AND METHODS
Video Analysis
Six international experts, several with extensive experience in
visual video analysis of injury tapes, participated as analysts
in this study. A total of 39 videos of ACL injury situations were
analyzed (17 male and 22 female players). Twenty-eight of the
tapes were collected by sending out questionnaires to college
trainers and team doctors around the United States asking
for videotapes of ACL injuries. Of these 28 videos, 23 were
from the high school and college level (22 match and 1 train-
ing injury), and 5 were from National Basketball Association
(NBA)/Women’s National Basketball Association (WNBA)
games. The remaining 11 cases were match injuries obtained
from NBA Entertainment Inc (NBA and WNBA games). No
medical information was available other than the diagnoses.
Eight situations were filmed from 2 different views, and 2
situations were filmed from 4 different views. The remain-
der of the videotapes contained only 1 camera view. When
more than 1 camera recording was included, composite
videos were created by manual synchronization, using the
IC of the foot in each camera view as the synchronization
frame. If the foot could not be seen in both camera views, we
used another player’s foot contact instead.
The video quality, as judged by the picture quality, resolu-
tion of the subject, number of cameras, the camera angle(s),
and degree of occlusion, was excellent in 2 cases, good in 4
cases (see Figure 1 for an example), average in 16 cases, poor
in 11 cases (see Figure 2), and impossible to judge in 6 cases.
The cases that were impossible to judge were excluded for
the kinematic variables but were included in the analysis of
playing situation, although no consensus could be the result
for several variables. All the injuries occurred on finished
wood basketball court flooring.
The video recordings were processed using Final Cut Pro
HD (version 4.5, Apple, Cupertino, Calif), deinterlaced to
achieve a 60-Hz effective frame rate, and stored using either
the DV or the DVCPRO50 coded in NTSC format. Each video
was composed in 2 versions, 1 in real time and 1 in slow
motion (50% of normal speed). Each of the analysts used a
Macintosh computer (Apple) with a 20-in or 21-in LCD moni-
tor, and the analyses were done independently, with the ana-
lyst blinded to the results of the other analysts. QuickTime
(version 7.0, Apple) was used to play the videos, and the ana-
lysts could move the video sequence back and forth, frame by
frame, using the keyboard arrows.
For each situation, 2 distinct time points were analyzed:
IC and 50 milliseconds after IC. We decided to analyze a
predetermined time point instead of allowing for separate
time points to enable an intertester reliability analysis.
Previous video analysis studies,
34
as well as simulation
studies
29,32
and laboratory motion analysis studies,
31
have
indicated that such injuries are likely to occur immediately
after IC. Thus, we assumed that by analyzing these 2 time
points, it was probable that the injury would occur within
this narrow interval. However, the analysts were free to
suggest a point of rupture outside this interval.
In 4 of the 39 cases, it was not possible to deinterlace the
footage, thus making the effective frame rate only 30 Hz.
In these cases, we chose to do the analyses at IC and at
33 milliseconds, which corresponds to 1 frame after IC.
The analysts were asked to judge if there was contact at
and before the injury and to separate between situations in
which another player was close (within 1 m) or not. Contact
was classified into the following: direct blow to the knee,
collision of other kind, pushing, foot-foot contact, holding, or
other. Player action was categorized into the following: 1-
legged landing, 2-legged landing, 1-legged stopping, 2-legged
stopping, pivoting, cutting, and other. Player attention was
classified according to where the player appeared to have
directed his or her attention at the time of injury: the basket
rim, the player from whom he or she received the ball, the
player who received the ball, opponent, ball, and other. Ball
possession was classified as follows: yes, no, have passed, and
have shot. Game phase was classified as follows: offense,
defense, rebound, and turnover. Foot placement was classi-
fied as follows: narrow, normal, wide, and very wide.
The analysts were also asked to provide estimates of sev-
eral continuous variables. At the predefined frames marked
in each of the videos (IC and 50/33 milliseconds after IC),
they assessed knee flexion-extension, knee varus-valgus, hip
flexion-extension, hip adduction-abduction, approach veloc-
ity, and vertical velocity. No measurement tools were used to
aid the visual inspection estimates of the experts. Approach
speed was the instantaneous horizontal velocity of the center
of mass at IC. Similarly, vertical speed was the downward-
directed instantaneous velocity of the center of mass at IC.
In addition, 4 analysts assessed if the knee joint experi-
enced a “valgus collapse” in the injury.
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Vol. 35, No. 3, 2007 Video Analysis of Basketball ACL Injuries 361
Data Reporting and Statistical Methods
The data from the analysts’ forms were entered into a
custom-made database using Microsoft Access (version
2003, Microsoft Corporation, Redmond, Wash). Descriptive
statistics were calculated using SPSS (version 13, SPSS Inc,
Chicago, Ill). Knee flexion and valgus as well as hip flexion
and abduction angles are shown as positive values. For all
the continuous variables, except time of injury, we calculated
the mean value between analysts for each case. For the time
Figure 1. Synchronized video images from a 2-camera sequence with good quality. The injured player is seen in white shorts in
the middle of the images at initial contact (A); 33 milliseconds after initial contact, corresponding to the approximate estimated
time of rupture (B); and 133 milliseconds after initial contact (C). This situation was classified as a “valgus knee collapse.”
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362 Krosshaug et al The American Journal of Sports Medicine
of injury, we used the median instead of the mean because 1
analyst in several cases estimated the injury to occur much
later than did the rest of the group. Results are reported as
the means with SDs and ranges across cases. Finally, the
SDs across analysts are reported as a measure of intertester
reliability for each variable.
To obtain a consensus on each of the categorical vari-
ables, at least 3 of the analysts had to agree on the cate-
gory. If fewer than 3 analysts agreed on a category, or if the
analysts’ opinions were split in 2 groups of 3, the decision
was “no consensus.
We used an independent-samples t test to determine if
there were differences between genders. An independent-
samples t test was also used to determine if there were dif-
ferences in vertical speed between cutting maneuvers and
landings. Pearson χ
2
test was used to examine if there was
a difference in relative risk for sustaining a valgus collapse
between genders. For all analyses, an α level of <.05 was
used to denote statistical significance.
RESULTS
Playing Situation and Player Behavior
In 29 of the 39 cases, the injury occurred when attacking,
5 while defending, 2 after rebounds, and 1 injury occurred
during a turnover. In 2 cases, there was no consensus. In 28
cases (10 male and 18 female players), the injured player was
in possession of the ball when the injury occurred. In 3 cases,
the injured player had just shot, whereas in 7 cases, the player
did not have the ball at all. In 1 case, there was no consensus
on ball involvement. The attention of the injured player was
most commonly focused at the basket rim (15 cases), followed
by an opponent (11 cases). In 9 cases, analysts reported that
the focus was on the ball and in 1 case on the player who
received a pass. In 3 cases, there was no consensus.
There was contact at the assumed time of injury in 11 of
the 39 cases (5 male and 6 female players). Four of these
cases were direct blows to the knee, all in men. Three of the
11 contact cases were classified as “collision of other kind,”
all in female players. In the 4 remaining contact cases, there
was no consensus. However, in 22 of the remaining 28 cases
(9 male and 13 female players), another player was within
1 m at the assumed time of injury. Only 2 injuries occurred
with no other players within 1 m. In the 4 remaining cases,
there was no consensus. Although contact at the time of
injury was only registered in 6 cases in female players, in as
many as 11 of the 22 female cases, there was a collision or
the player was pushed by an opponent before the injury.
Only 1 male player was perturbed in this way before the
time of injury. Most of the player maneuvers were landings
(Table 1). There were no cases classified as 1-legged stop-
ping, 2-legged stopping, or other.
Knee and Hip Motion
For the joint motion description, we excluded direct blows
to the knee (4 cases). In addition, there were 5 cases in
which the video quality was too poor to allow further analy-
ses. This meant that 30 videos were available to assess
knee and hip motion during noncontact ACL injuries (13
male and 17 female players).
The mean knee flexion angle was higher in female players
than in male players at IC (15° vs 9°, P = .034) and at 50/33
milliseconds after IC (27° vs 19°, P = .042). This was the case
for hip flexion as well, both at IC (27° vs 19°, P = .043) and
at 50/33 milliseconds after IC (33° vs 22°, P = .020).
The estimated time of injury ranged between 17 and 50
milliseconds after IC (Table 2). At IC, the mean knee flex-
ion ranged between 8° and 15° across player actions and
genders (Table 3). At 50/33 milliseconds after IC, the knee
flexion angles were about twice as high as at IC.
No significant gender differences for female and male
players were found at IC for knee valgus (4° vs 3°, P = .071),
but at 50/33 milliseconds after IC, female players had larger
valgus angles (8° vs 4°, P = .018). Knee collapse occurred in
9 of the 17 cases in female players. All of the collapses were
cases that one could term valgus collapse; that is, the knee
collapsed medially, in what appeared to be a combination of
hip internal rotation, knee valgus, and external rotation of
the tibia. No collapse was found in 2 cases, and 4 situations
were impossible to judge. In addition, there were 2 cases of
no consensus. Knee collapse occurred in only 2 cases in male
players, whereas no collapse was found in 10 cases. One case
Figure 2. Video image from a single-camera sequence with
poor quality. The right leg (partly hidden) of the player to the
left, holding the ball, is injured. The video frame is captured
at initial contact of the right foot.
TABLE 1
Player Action at the Time of Injury (N = 39)
Action Male Players Female Players
1-legged landing 6 4
2-legged landing 4 9
Cutting 2 2
No consensus 1 2
Direct blow to the knee 4 0
Impossible to judge 0 5
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Vol. 35, No. 3, 2007 Video Analysis of Basketball ACL Injuries 363
was impossible to judge. The relative risk for sustaining a
valgus collapse was 5.3 times greater in female players than
in male players (P = .002).
Mean hip abduction angles were consistent across gen-
ders and player actions, ranging from 8° to 19° (Table 4) at
IC as well as at 50/33 milliseconds after IC. However, for
individual cases, hip abduction angles ranging from –7° (ie,
adduction) to 48° were reported.
Intertester Reliability
The mean SD for rupture time between analysts was 31
milliseconds, but values up to 105 milliseconds were seen
(Table 5). The mean SD between analysts was higher for
hip flexion compared with knee flexion, with a maximal SD
of 32° observed for hip flexion. Relatively low SDs were
observed for knee valgus angles, whereas for hip abduction
angles, they were substantially higher. There were only
small differences in SD between analysts between the
video sequences rated as excellent, good, average, or poor.
Similarly, there was no difference in intertester reliability
for knee and hip flexion estimates whether a sagittal-plane
view was present or not.
DISCUSSION
Playing Situation and Player Behavior
The results showed that 72% of the injuries did not involve
contact with other players at the assumed time of injury. This
agrees well with the findings of Boden et al,
5
in which 72%
noncontact injuries were registered among 100 cases. Arendt
and Dick
2
found 80% noncontact injuries in female players
and 65% in male players. Opponents were close by in nearly
all of the injury situations. This is not unexpected because a
competitive game like basketball implies close proximity
between players most of the time. However, as many as half
of the injured female players were pushed or collided before
the time of injury, which indicates that such perturbations
may have influenced the movement patterns. These findings
support statements by Boden et al,
5
Ebstrup and Bojsen-
Moller,
9
and Olsen et al,
34
who suggested that although there
was no body contact at the time of injury, the movement pat-
terns may have been perturbed by an opponent. This view is
supported by experimental studies, which show that the
introduction of a static defender in cutting maneuvers
31
or
using an overhead goal
12
in vertical jumps alters the knee
biomechanics significantly. Thus, it seems reasonable to sug-
gest that preventive programs
14,33
should include “distract-
ing elements” resembling those seen in match situations to
enhance knee control. Furthermore, there is an obvious need
to continue the development of laboratory protocols that
more effectively simulate actual game play by including ele-
ments forcing subjects to focus their attention elsewhere.
Joint Kinematics
Previous studies have concluded that ACL injuries occur
shortly after foot strike with the knee near full extension.
5,34,40
By interpolation between the estimates at IC and 50/33 mil-
liseconds after IC, the analysts’ knee flexion estimates at the
assumed time of injury were 18° and 24° in male and female
players, respectively. However, because of the systematic
underestimation discussed below, the true knee flexion angles
may be twice as high as the visual estimates.
22
Numerous recent studies have investigated if the gender
difference in ACL injury incidence is caused by differences
in knee and hip flexion in landings, with the rationale that
women are more extended during landing, perhaps because
of weaker musculature, than are men.
10,11,31,36,38
Boden
et al
5
hypothesized that a vigorous quadriceps contraction
on an extended knee was the main cause of the excessive
ACL force. However, although several laboratory studies
have supported this theory,
31,38
some studies also found no
differences,
36
and several studies have even reported larger
flexion angles in women.
10,11
Huston et al
18
showed signifi-
cantly less knee flexion at IC during a drop landing from a
60-cm height but not from a 20-cm height, indicating that
such a difference may be task specific if it exists at all. In the
present study, in which actual ACL injury situations were
analyzed, female players were found to have significantly
higher knee and hip flexion angles than men at IC, at 50/33
milliseconds after IC, and at the assumed point of injury.
These results suggest that women are likely not more prone
to the quadriceps drawer mechanism than are men. In the-
ory, it may even be possible that the larger knee flexion
angles will increase the risk of noncontact ACL injury.
Nevertheless, caution must be taken because, for example,
hamstrings muscle coactivation could be different.
The question still remains to what degree the quadriceps
drawer mechanism is a likely cause of injury in the situa-
tions studied here, considering that the knee flexion angles
at the time of injury may be substantially higher than what
TABLE 2
Mean Time Point of Rupture (ms) and SDs With Range (n = 27)
a
Male Players Female Players
Action Mean ± SD Range Mean ± SD Range
1-legged landing 37 ± 9 25-50 37 ± 5 33-42
2-legged landing 33 ± 7 25-42 39 ± 10 25-50
Cutting 46 ± 6 42-50 25 ± 12 17-33
a
The 3 cases of no consensus in player action are not included in the table.
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364 Krosshaug et al The American Journal of Sports Medicine
TABLE 3
Mean Knee and Hip Flexion (deg) and SDs With Range (n = 27)
a
Knee Flexion Hip Flexion
Men (n = 12) Women (n = 15) Men (n = 12) Women (n = 15)
IC 50/33 ms After IC IC 50/33 ms After IC IC 50/33 ms After IC IC 50/33 ms After IC
Action Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range
1-legged landing 8 ± 6 3-16 18 ± 6 10-28 10 ± 4 5-14 18 ± 4 13-23 16 ± 8 5-27 22 ± 7 17-31 20 ± 10 5-27 20 ± 7 14-30
(n = 10)
2-legged landing 9 ± 7 3-19 17 ± 6 11-23 15 ± 4 10-22 27 ± 7 18-40 18 ± 4 13-22 18 ± 5 14-26 25 ± 8 17-44 32 ± 11 21-54
(n = 13)
Cutting (n = 4) 12 ± 2 11-13 23 ± 7 18-28 14 ± 11 7-22 27 ± 4 24-29 29 ± 6 25-33 22 ± 1 21-22 37 ± 7 32-42 45 ± 6 41-49
a
IC, initial contact. The 3 cases of no consensus in player action are not included in the table.
TABLE 4
Mean Knee Valgus and Hip Abduction (deg) and SDs With Range (n = 27)
a
Knee Valgus Hip Abduction
Men (n = 12) Women (n = 15) Men (n = 12) Women (n = 15)
IC 50/33 ms After IC IC 50/33 ms After IC IC 50/33 ms After IC IC 50/33 ms After IC
Action Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range Mean ± SD Range
1-legged landing 3 ± 1 0-5 6 ± 3 2-10 4 ± 1 3-5 8 ± 0 8-9 12 ± 5 6-20 12 ± 3 9-16 15 ± 21 1-46 15 ± 22 –1-48
(n = 10)
2-legged landing 2 ± 1 1-4 5 ± 2 3-8 3 ± 2 1-7 7 ± 3 4-14 19 ± 13 9-38 17 ± 16 6-41 15 ± 9 6-33 14 ± 10 5-36
(n = 13)
Cutting (n = 4) 2 ± 2 1-4 –2 ± 12 –11-7 6 ± 0 5-6 10 ± 3 9-12 11 ± 20 –3-25 8 ± 21 –7-23 19 ± 6 14-23 14 ± 6 10-19
a
IC, initial contact. The 3 cases of no consensus in player action are not included in the table.
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Vol. 35, No. 3, 2007 Video Analysis of Basketball ACL Injuries 365
have been assumed previously. Although it has been shown
that isolated ACL rupture from the quadriceps mechanism
is possible,
7
the relationship between flexion angle and
ACL strain induced by quadriceps contraction, and how it is
affected by ground reaction forces, is still unclear.
8,17,26,37
Although some cadaveric studies have suggested that this
mechanism requires flexion angles of less than 30° to be
effective,
8,26
others have shown that quadriceps activity can
induce significant ACL strain
37
and anterior tibial transla-
tion
17
even at 45°. In a recent cadaveric study,
41
ACL strain
was reported to be proportional to the increase in quadri-
ceps force, with maximal strain occurring at approximately
30°. However, mathematical simulation studies have con-
cluded that sagittal-plane loading alone cannot produce
ACL ruptures,
29,35
although these authors acknowledged
that this mechanism may still contribute to the injury. In
light of these findings, it is difficult to interpret the flexion
angle results from the present study. If the strain produced
from the quadriceps drawer mechanism is not the primary
cause of the loading and injury, it is possible that an ante-
rior drawer before the time of rupture could place the knee
joint in a vulnerable position. If anterior drawer is combined
with valgus and rotational loading, these combined forces
may lead to ACL failure. However, investigation is required
to delineate the role of the quadriceps drawer in the ACL
injury mechanism.
Olsen et al
34
stated that the injury could be owing to valgus
loading in combination with external or internal knee rota-
tion. This would support the hypothesis of Ebstrup and
Bojsen-Moller,
9
who proposed notch impingement as the cause
of injury. Hewett et al
15
showed in a prospective study that a
landing pattern with valgus loading predicted ACL injury,
indicating that this was likely to be an important element in
the noncontact ACL mechanism. In support of this, Speer
et al
39
concluded that valgus loading must have been part of
the injury mechanism, based on the bone bruise pattern on
magnetic resonance imaging (MRI). Arnold et al
3
suggested
internal rotation as a probable injury mechanism from athlete
interviews. The internal rotation hypothesis is supported by
cadaveric studies that have shown that internal rotation will
put high stress on the ACL, especially at low flexion angles.
25
These hypotheses cannot be evaluated by video analysis
alone because it is not possible from such gross kinematic
estimates to determine the relationship between external
loads, muscle loads, and ACL force. For instance, as dis-
cussed, we know that a small anterior displacement of the
tibia may cause large ACL forces, but estimation of such
skeletal motions from ordinary television recordings is sim-
ply not possible. Still, the valgus collapses seen in many
cases indicate that valgus loading was likely present before
the rupture, although it is also possible that the valgus loads
lead to collapse after ACL failure. The noncontact valgus col-
lapses appeared strikingly similar to the collapses seen in
situations with direct blows to the lateral knee.
A possible implication of the higher proportion of valgus
collapses in female players is that the knee-loading patterns
in noncontact ACL injuries may be different between men
and women. This hypothesis is supported by laboratory
motion analysis studies in which men demonstrated internal
rotation combined with varus motion, whereas women
demonstrated the combination of valgus-external rotation.
6,31
It is generally agreed that lumbopelvic (or core) stability
plays an important role in controlling the knee.
24,27,30,33
Furthermore, Kraemer et al
19
found that the neuromuscular
performance was lower in women, and several studies have
shown a “ligament dominance” in women, implying that the
ligaments rather than muscles absorb the impact forces.
1,16
Insufficient lumbopelvic strength or lack of neuromuscular
control might therefore be the reason for the uncontrolled val-
gus collapses. Interestingly, Hewett et al
16
reported that this
unfortunate joint-loading pattern can be drastically reduced
through neuromuscular training. On the other hand, an
alternative explanation could be that the loading patterns
are not different between genders but that valgus collapses
are more apparent after injury in female players because
of reduced joint stiffness.
13,42,43
In support of this theory,
some degree of valgus was estimated in all cases in the
present study. However, the relatively poor accuracy and
precision of these estimates
22
do not allow firm conclusions
to be made. Likewise, the poor reliability in rotational vari-
ables
22
makes it difficult to assess the internal rotation
hypothesis, although some situations displayed rotational
motions similar to what was illustrated in Arnold et al.
3
Similar to what was reported in the study of Boden et al
5
and Olsen et al,
34
we did not find any injuries involving
hyperextension or varus.
TABLE 5
Intertester Reliability for Continuous Variables Reported as the SD Between Analysts
a
Intertester Reliability
Variable SD Minimum Value Maximum Value
Rupture time, ms 31 13 105
Knee flexion, deg 7 2 20
Hip flexion, deg 12 3 32
Valgus, deg 4 1 18
Hip abduction, deg 10 3 25
Approach speed, m/s 1.0 0.5 1.9
Vertical speed, m/s 1.1 0.4 2.0
Foot-pelvis rotation, deg 8 1 19
a
Data are shown as the mean SD across cases with minimum and maximum values.
© 2007 American Orthopaedic Society for Sports Medicine. All rights reserved. Not for commercial use or unauthorized distribution.
at NORGES IDRETTSHOEYSKOLE on August 18, 2007 http://ajs.sagepub.comDownloaded from
Study Limitations
The study was not based on a systematic, prospective collec-
tion of videos from a defined athlete population but repre-
sented a convenience sample obtained from athletic trainers,
team physicians, and the NBA. Thus, we do not know whether
these are representative. Also, because we do not have access
to the medical records, we do not know how the ACL tears
were confirmed, nor do we know if the athlete had a history
of previous knee injury. Nevertheless, it seems reasonable to
assume that the diagnoses are reliable because an ACL rup-
ture is a major injury usually requiring surgical intervention
in this population of athletes. Furthermore, it required an
effort to submit cases, which was not rewarded financially or
in other ways.
From the video analysis, it is not possible to verify the exact
moment when the ACL injury occurred. In fact, in many situ-
ations, it was not even easy to detect that an injury had
occurred until the athlete took his or her weight off the injured
leg. However, in other situations, obvious abnormal joint con-
figurations (Figure 1) were seen soon after IC. Although the
analysts most often agreed that the injury occurred within
50 milliseconds after IC, there were 6 situations in which 1 of
the analysts estimated the time of rupture to be more than
100 milliseconds after IC, demonstrating the difficulty in per-
forming such analyses. Still, the relatively consistent overall
judgment from the group does indicate that there is a high
probability that many of the injuries occurred shortly after IC,
a conclusion that also agrees with previous studies.
5,34,40
The reliability of the visual-inspection approach was
assessed in a recent study in which the same group of ana-
lysts examined a series of noninjury cutting and planting
video situations.
22
The results showed that the group con-
sistently underestimated knee and hip flexion, although the
differences were smaller near full extension. For instance, an
estimated flexion angle of 20° corresponded to a true angle of
approximately 40°. The results also showed that estimates
for other variables, such as knee valgus or rotation angles,
may not be reliable. Therefore, the results in the present
study must be interpreted with caution.
Future Perspectives
Future studies should aim to improve methods for analyzing
noncontact ACL injury situations from video, preferably
using more sophisticated methods that can produce continu-
ous estimates of the kinematics leading up to the point of
rupture, for example, model-based image-matching tech-
niques.
21,23
The introduction of high-definition television
broadcasts will be helpful for such analyses. In sports such as
basketball, in which play is confined in a relatively small cap-
ture volume, an increased number of camera views and pos-
sibly even introduction of high-speed imaging equipment
suitable for conducting 3D motion analysis can be consid-
ered. Even so, it will be necessary to combine different
research approaches to evaluate the hypotheses proposed in
the literature,
20
for example, to develop cadaveric models or
mathematical simulation models that will produce the kine-
matics and MRI results seen in real injury situations.
CONCLUSION
The knee flexion angles at the assumed point of injury were
higher in male players than in female players, and female
players had a 5.3 times higher relative risk of sustaining a
valgus collapse than did male players. The injuries occurred
predominantly during landing. Although the majority of the
injuries did not involve contact at the assumed point of
injury, the movement patterns were likely perturbed by an
opponent, for example, by pushing before the injury.
ACKNOWLEDGMENT
The Oslo Sports Trauma Research Center has been estab-
lished at the Norwegian School of Sport Sciences through
generous grants from the Norwegian Eastern Health
Corporate, the Royal Norwegian Ministry of Culture, the
Norwegian Olympic Committee & Confederation of Sport,
Norsk Tipping AS, and Pfizer AS. We thank Ingar Holme for
statistical advice and Paal Henrik Wagner for video editing
assistance. This work was supported in part by National
Institutes of Health grant R01-AR049735-01A1 (T.E.H.). We
also thank the National Basketball Association for gener-
ously providing several videotapes for this study.
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... In team sports, the majority of ACL injuries are non-contact in nature (1,2), with a subset of these occurring during cutting maneuvers (2). Young female handball players are at greater risk than their male counterparts (1,3,4). ...
... In team sports, the majority of ACL injuries are non-contact in nature (1,2), with a subset of these occurring during cutting maneuvers (2). Young female handball players are at greater risk than their male counterparts (1,3,4). ...
... In handball, video analysis suggests ACL injuries frequently occur during fake-and-cut situations (5,6). Biomechanical analysis from injury video sequences is limited to a joint kinematic description of injury situations (2,6). However, joint moments correspond more directly to ligament loading (7), which is currently only measurable in the biomechanics laboratory. ...
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Non-contact anterior cruciate ligament injuries typically occur during cutting maneuvers and are associated with high peak knee abduction moments (KAM) within early stance. To screen athletes for injury risk or quantify the efficacy of prevention programs, it may be necessary to design tasks that mimic game situations. Thus, this study compared KAMs and ranking consistency of female handball players in three sport-specific fake-and-cut tasks of increasing complexity. The biomechanics of female handball players ( n = 51, mean ± SD: 66.9 ± 7.8 kg, 1.74 ± 0.06 m, 19.2 ± 3.4 years) were recorded with a 3D motion capture system and force plates during three standardized fake-and-cut tasks. Task 1 was designed as a simple pre-planned cut, task 2 included catching a ball before a pre-planned cut in front of a static defender, and task 3 was designed as an unanticipated cut with three dynamic defenders involved. Inverse dynamics were used to calculate peak KAM within the first 100 ms of stance. KAM was decomposed into the frontal plane knee joint moment arm and resultant ground reaction force. RANOVAs (α ≤ 0.05) were used to reveal differences in the KAM magnitudes, moment arm, and resultant ground reaction force for the three tasks. Spearman's rank correlations were calculated to test the ranking consistency of the athletes' KAMs. There was a significant task main effect on KAM ( p = 0.02; η p 2 = 0.13). The KAM in the two complex tasks was significantly higher (task 2: 1.73 Nm/kg; task 3: 1.64 Nm/kg) than the KAM in the simplest task (task 1: 1.52 Nm/kg). The ranking of the peak KAM was consistent regardless of the task complexity. Comparing tasks 1 and 2, an increase in KAM resulted from an increased frontal plane moment arm. Comparing tasks 1 and 3, higher KAM in task 3 resulted from an interplay between both moment arm and the resultant ground reaction force. In contrast to previous studies, unanticipated cutting maneuvers did not produce the highest KAMs. These findings indicate that the players have developed an automated sport-specific cutting technique that is utilized in both pre-planned and unanticipated fake-and-cut tasks.
... Lack of knee joint muscles' ability to absorb force during landing may lead to changes in the kinematics of this joint, including increased knee valgus angle (KVA) and knee flexion (4,5). About 70% of knee injuries, especially anterior cruciate ligament (ACL), occur in non-contact in-juries (6,7). Increased KVA and decreased knee flexion, tibia spin, internal rotation, and hip adduction within cutting and landing maneuvers can usually be the mechanism of damage to ACL injuries (6)(7)(8)(9)(10). ...
... About 70% of knee injuries, especially anterior cruciate ligament (ACL), occur in non-contact in-juries (6,7). Increased KVA and decreased knee flexion, tibia spin, internal rotation, and hip adduction within cutting and landing maneuvers can usually be the mechanism of damage to ACL injuries (6)(7)(8)(9)(10). which can increase the strain on the ACL (11,12). ...
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Background: Non-contact anterior cruciate ligament (ACL) injuries commonly occur when athletes land in high risk positions such as knee valgus. The impact of foot orthoses during exercises that recreate a non-contact ACL harm system (i.e., landing) in any case will be obscured. Objectives: The purpose of the current study research was to evaluate the effect of two foot orthoses (semi-hard foot orthoses and hard foot orthoses) on knee valgus angle during single-leg drop landing. Methods: Twenty male leisure volleyball gamers performed landing in one-leg step from 30 cm height in 3 conditions (without foot orthoses, mid-hard foot orthoses, and hard foot orthoses). A motion capture system was used to measure lower extremity kinematics. Two risk factors of ACL injury, maximum knee valgus angle (KVA), and maximum knee flexion was measured. ANOVA was used for statistical analysis (P < 0.05). Results: With mid-hard foot, orthoses provide the maximum level of knee flexion and the minimum level of knee valgus during single-leg drop landing. Conclusions: It may be concluded showed that foot orthoses affect knee kinematics. More knee flexion and less knee valgus brought about by mid-hard foot orthoses can reduce injuries of the anterior cruciate ligament (ACL).
... The findings showed that the normal DKV group performed 45° SLS with significantly greater in hip adduction than the excessive DKV group. The DKV is defined as a combination of knee abduction, tibial internal rotation, hip adduction and internal rotation (26). Hip control loss was found to be related to knee valgus, as evidenced by increased hip adduction, external rotation and flexion (7). ...
... No statistically significant differences were observed in the lower limb joint kinematics of the sagittal plane during the 60° SLS test, which is similar to the findings from previous studies (20,31). This probably related with the mechanisms of DKV, which involved a blend of the frontal and transverse plane motions instead of sagittal motion only (26). Besides, we had fixed the tested squat depths to 45° and 60° of knee flexion during the SLS test, which may limit the kinematical differences in the plane. ...
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Introduction: Introduction: Knee valgus which also known as dynamic knee valgus (DKV), is frequently linked to non-contact lower-limb injuries, especially in females. This retrospective study aims to compare the kinematic variables of lower limb joints in physically active females with normal versus excessive DKV during single leg squat (SLS) at 45° and 60° knee flexion. Methods: Based on the outcomes of drop vertical jump screening test, 34 females were recruited and divided into two groups (i.e., normal and excessive DKV). Participants performed SLS at 45° and 60° knee flexion with three-dimensional motion capture and analysis. The kinematic variables of lower limb joints at both knee flexion of SLS were compared across groups using independent T-test. Results: During 45° SLS with the dominant limb, the normal DKV group performed significantly greater hip adduction angle (4.49±3.25°, t(32) = 2.371, p= 0.024) than the excessive DKV group (1.426±4.23°). During 60° SLS with the dominant limb, the normal DKV group showed knee adduction (0.223±0.07°, t(16.048) = 10.707, p=0.001) while the excessive DKV group showed knee abduction (-4.478±1.81°). Conclusion: Females with excessive DKV showed significantly different lower limb kinematics and motion control strategy compared to females with normal range of DKV. The findings highlighted the importance of DKV screening among physically active females, and the rationale for prescribing individualized exercise intervention to prevent lower limb non-contact injuries.
... ACL also provides proprioceptive feedbacks regarding its kinaesthetic status to the central nervous system (Biedert & Zwick, 1998;Friemert et al., 2005) and thus contributes to the dynamic neuromuscular stability of the knee (Solomonow et al., 1987). Unfortunately, ACL is frequently involved in injuries related to sporting motions such as landing, cutting and stoppings (Boden et al., 2000;Griffin et al., 2000;Ireland, 1999;Krosshaug et al., 2007). Female athletes have two to four times higher ACL injury risk than male counterparts (Arendt & Dick, 1995;Prodromos et al., 2007). ...
... In sports, up to 70% of ACL rupture typically occurs in the early deceleration phase of cutting or single-leg landing in a non-contact manner, without a direct blow to the knee from other players (Boden et al., 2000;Griffin et al., 2000;Ireland, 1999;Krosshaug et al., 2007). Videography of actual ACL injury has estimated that the time frame from initial foot contact (IC) to the ACL disruption is about 40 ms (Koga et al., 2011). ...
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... Recently, several studies have employed video-based analysis to analyze injury mechanisms in various sports. 2,3,13,14,19,20,27 Despite its use in other sports, no studies have assessed the validity of video analysis as a tool. Moreover, there is a paucity of studies utilizing video-based analysis among NBA athletes. ...
... As this was the first study to assess the validity of video analysis and it has been widely used in other sports, 2,3,13,14,19,20,27 future work assessing the efficacy of video analysis in other sports may prove useful. In doing so, we will gain a better understanding of the strengths and weaknesses of video analysis and how we can appropriately employ this methodology in professional athletics. ...
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Background In the National Basketball Association (NBA), lower extremity injuries account for over 70% of games missed, with ankle injuries being the most common. High-quality video analysis has been successful for studying injury mechanism. Purpose To (1) determine the validity of video-based analysis as a method to evaluate ankle injury mechanisms in NBA players and (2) analyze the circumstances associated with injury, games missed due to injury, and associated costs in player salary due to time missed. Study Design Case series; Level of evidence, 4. Methods Ankle injuries were identified using an injury report database, and corresponding videos were searched using YouTube.com to access high-quality video evidence of these injuries during the 2015-2020 NBA regular season. We reviewed 822 injuries, of which 93 had corresponding videos (video subset), in our final analysis. Variables including number of games missed, necessity for surgical treatment, and injury recurrence were reported for the entire cohort. In the video subset, the mechanism of injury and other corresponding situational data were evaluated. Results The most common mechanism of injury occurred via ankle inversion (83.9%; n = 78; P < .001). These injuries were significantly associated with indirect contact with the player’s ankle (79.6%; n = 74; P < .001). There were significant differences based on player position, within both the video subset ( P = .008) and the entire cohort ( P < .001), with guards being injured the most frequently. The average number of games missed due to injury was 7 games in the video subset and 5 games in the entire cohort ( P = .14). There were significant differences between the groups in average player salary per game ($133,878 [video subset] versus $87,577 [entire cohort]; P < .001). Conclusion Despite its low yield of 11.3%, video analysis proved to be a useful tool to determine ankle injury mechanisms as well as the distribution of injuries based on player position. However, this methodology was subject to selection bias, as evidenced by a $50,000 increase in player salary among the video cohort. These findings should be considered when using video analysis in future studies.
... 13 The mechanism of injury has also been studied using 2D video analysis, 14 which can be a valuable tool for in situ evaluation during athletic competitions. For example, previous literature has used this method in soccer, [16][17][18] basketball, 19 and alpine ski racing 20,21 for epidemiological purposes. Lemme et al 14 examined contributing factors involved with AT ruptures in the NBA and noted that a large angle of ankle dorsiflexion occurred at the time of AT rupture (see Figure 1). ...
... While this method has its limitations, it is an accepted practice for in situ competition analysis. [16][17][18][19] The AT ruptures can be deleterious to professional basketball athletes. Extreme ankle dorsiflexion was identified with outward turning of the foot following a "false step" as the predominant mechanism of AT injury in NBA players. ...
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A systematic search was performed of online databases for any Achilles tendon (AT) injuries occurring within the National Basketball Association (NBA). Video was obtained of injuries occurring during competition and downloaded for analysis in Dartfish. NBA athletes (n = 27) were identified with AT rupture over a 30-year period (1991–2021). Of the 27 NBA athletes found to have AT ruptures (mean age: 29.3 [3.3] y; average time in the NBA: 8.5 [3.8] y), 15 in-game videos were obtained for analysis. Noncontact rupture was presumed to have occurred in 12/13 cases. Eight of the 13 athletes had possession of the ball during time of injury. The ankle joint of the injured limb for all 13 athletes was in a dorsiflexed position during the time of injury (47.9° [6.5°]). All 13 athletes performed a false-step mechanism at time of injury where they initiated the movement by taking a rearward step posterior to their center of mass with the injured limb before translating forward. NBA basketball players that suffered AT ruptures appeared to present with a distinct sequence of events, including initiating a false step with ankle dorsiflexion of the injured limb at the time of injury.
... In addition to maximal muscle strength, the ability to generate muscle force rapidly (rate of force development-RFD), measured as the rate of torque development (RTD), may be important for dynamic knee joint stabilization 15,16 given that non-contact ACL injury in athletic movements like jump landings or side cutting occurs within 50 ms of ground contact. 17 While a divergent rate of recovery between maximal strength and RFD has been demonstrated after ACLR, 16,18 there is limited knowledge on the impact of the ST autograft procedure on knee flexor RTD and the RTD-joint angle relationship post-ACLR. Further, both neural and peripheral neuromuscular determinants of RFD exist. ...
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... It seems that most falls occur during 25-50 ms after initial contact with the ground, and most non-contact ACL injuries occur within 40 ms after initial contact. [23,24]. However, since the suggested latency of muscular reflexes elicited by electrical stimulations of ACL is 80-100 ms, it seems unlikely that there is time for mechanosensory feedback to directly protect against injury [25]. ...
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... The scoring criteria identify subjects with faulty landing biomechanics or high-risk movement patterns, such as increased knee valgus, hip internal rotation and adduction during initial contact, and maximal knee flexion. Moreover, the asymmetric landing of feet and side bending of the trunk in the frontal view and decreased sagittal plain joint flexion of the trunk, hip, and knee could be identified [18,19]. A higher LESS score indicates a more faulty jump-landing technique, which represents a higher risk of ACL injury. ...
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