The ACL is 1 of 4 major ligaments that provide stability to the
knee. The word "cruciate" is derived from the Latin word for
cross; the anterior and posterior cruciate ligaments cross each other in
the center of the knee. (See illustration of ACL anatomy in Figure
1).
The main function of the ACL is to prevent the tibia from sliding forward
relative to the femur; this ligament also prevents excessive extension, varus
and valgus positioning at the knee joint, and tibial rotation.[3,4] An intact
ACL protects the menisci, the cartilage shock absorbers of the knee, against
shearing forces with twisting and pivoting motions.
Who is at higher risk?
Although there have been no large-scale population studies in children,
epidemiologic studies have demonstrated that ACL tears are common in adults. An
examination of patient records for the national health care system in Norway
revealed that 85 reconstructions are performed there per 100,000 individuals in
the 16-39-year-old age group.[5] In college athletics, participants in men's
football and women's gymnastics, basketball and soccer have the highest rates of
ACL injury. Studies have consistently shown that female athletes, particularly
those who participate in sports involving twisting and pivoting, have
significantly more non-contact ACL tears than males in similar sports. An
examination of records from the National College Athletic Association (NCAA)
found that female soccer players have more than twice the risk and female
basketball players have 4 times the risk of noncontact ACL tears when compared
to their male counterparts.[6] ACL disruptions are uncommon in young children;
risk increases significantly at age 12-13 years in girls and age 14-15 years in
boys. At the age of 14 years, girls have 5 times higher rates of ACL tears than
boys.[7]
Diagnostic assessment
The diagnosis of an ACL tear can generally be made with a good history
and physical examination. Clinical history can be very useful in discerning the
pathology present after an acute knee injury. O'Donoghue described ACL injury as
being part of an "unhappy triad"; he provided the classic description of a
player who is hit in the lower leg from the lateral side and sustains a medial
meniscal injury, as well as ACL and medial collateral ligament (MCL) tears.[8]
However, the majority of ACL tears are noncontact injuries; athletes may recall
the injury occurring with a pivot or twist, landing from a jump, or a sudden
deceleration.[9,10] Over 70% of affected patients report hearing a "pop."[10]
Many patients are able to bear weight following an ACL disruption. Development
of hemarthrosis occurs rapidly; athletes often note that significant swelling
appears within several hours of their injury.
Several weeks after their injury, young athletes are often relatively
asymptomatic with activities of daily living. A sense of instability or feeling
that the knee is giving way with twisting or lateral motion is the hallmark of
ACL insufficiency. Clinicians should ask young patients about their recent
growth and pubertal stage (including age of menarche for females), as skeletal
maturity influences the treatment of these injuries.
In
the acute phase following injury, practitioners can often detect swelling and limited range
of-motion during the physical examination. Many patients note resolution of effusion and achieve full motion
within 1-2 weeks of their injury. The Lachman maneuver (Figure
2) assesses for anterior translation of the tibia; this
technique has 85% sensitivity for detecting ACL insufficiency.
The pivot shift maneuver is highly specific for ACL injury, but is
technically more difficult to perform and has a low sensitivity. Both tests are
more accurate when used with a patient who is able to relax the muscles around
the knee.[11] Physical examination should also include evaluation for associated
injuries, including fractures and meniscal tears, and determination of Tanner
stage in the skeletally immature patient.
Imaging is generally used
for confirmation of physical findings and to determine the presence of associated injuries.
Plain radiographs may detect associated injuries, such as tibial spine avulsion
fractures; they can also aid in determining skeletal maturity. In addition to
confirming the diagnosis of an ACL tear, magnetic resonance imaging (MRI)
helps evaluate for the presence of meniscal and articular cartilage lesions. (Figure 3 shows
a sagittal MR image illustrating an ACL injury).
Meniscal injuries are commonly associated with ACL tears; a study of adults
found meniscal tears in 73% of patients.[12] Millett et. al. found associated
injuries, including lateral and medial meniscal tears, MCL ligament injuries and
a femur fracture, in 66% of 10-14-year-old patients undergoing surgery for ACL
reconstruction.[13] MRI will detect bone bruises in over 70% of patients who
undergo imaging within 6 weeks of their injury, but this finding is generally of
little clinical significance.[14]
Management approaches
In the acute phase, athletes with ACL tears can be managed with "RICE" –
rest, ice, compression and elevation – with progression to range-of-motion
exercises and quadriceps and core abdominal/trunk strengthening. Surgical
reconstruction is uniformly recommended for skeletally mature athletes. The ACL
cannot be directly repaired; it must be replaced with a graft. A variety of
surgical techniques and graft choices have been employed with good
results.
Management of the skeletally immature athlete with an ACL
tear remains controversial. Surgical techniques that place the graft in an
anatomic position require drilling through the physis; this carries a risk of
growth arrest with possible limb-length inequality and angular deformity.
Certain drilling techniques appear to be quite safe, but given the potentially
devastating consequences, sports physicians often recommend against surgeries
that involve a trans-physeal approach for boys younger than 14 years and girls younger than 13 years. There are several reconstruction
procedures that avoid the growth plate; however, they place the graft
in a less anatomic position and this may result in the need for
future revision surgery.
In patients with a substantial amount of growth remaining, many
physicians recommend non-operative treatment with physical therapy, bracing and
activity modification until the patient approaches skeletal maturity. Graf found
that patients with ACL insufficiency treated non-operatively with bracing and
physical therapy had a high risk of sustaining a subsequent meniscal tear.[15]
Another group imposed more stringent activity restrictions along with
non-operative therapy and did not note a significant rate of ensuing meniscal
injuries.[16] Adults appear to do very well with activity modification and
bracing; however, the studies involving children and adolescents are small and
many clinicians are concerned that children will be less compliant with
restrictions to participation in athletics.[17]
Long-term follow-up of athletes who have sustained ACL tears has
consistently shown an increased risk of osteoarthritis. This is especially true
for patients with associated meniscus and articular cartilage lesions. However,
even individuals who have undergone surgical reconstruction for isolated ACL
tears appear to be at increased risk for degenerative joint
disease.[18]
Prevention programs
Given the concerns in managing ACL injuries in skeletally immature
athletes, prevention has been an area of active research over the last decade.
Most ACL prevention programs have focused on female athletes in an attempt to
mitigate their significantly increased risk of injury. Hewett, et al. noted
differences between female and male athletes in hamstring strength and mechanics
when landing from a jump.[19] Female athletes tend to land in an upright
position with increased dynamic internal rotation of the knees and decreased hip
and knee flexion. Hewett's group instructed coaches and trainers on how to
institute a neuromuscular training program emphasizing plyometrics, strength and
flexibility. They found that untrained athletes had a 3.6 times higher rate of
injury compared to trained athletes.[20]
Several centers have subsequently instituted successful neuromuscular
training programs. Cynthia LaBella, MD, medical director of Children's
Memorial
Hospital's Institute for
Sports Medicine, has established and continues to expand the Knee Injury
Prevention Program (KIPP) in Chicago Public High Schools (CPS). In one study she
found that female adolescent athletes who participated in KIPP during their
preseason training reported lower rates of sport-related knee pain and improved
self-rated athletic performance post-KIPP vs. pre-KIPP.[21] Data from the first
year of implementing KIPP at 46 Chicago public high schools revealed a 66%
reduction in knee ligament injuries and an 80% reduction in ACL tears among
participating vs. non-participating female soccer and basketball athletes
(written communication, March 2009; data presented at the American Medical
Society for Sports Medicine, April 2009).
Conclusion
ACL injuries are an increasing concern for young athletes, especially
girls, and their families. Care of the skeletally immature patient with an acute
knee injury requires a multidisciplinary team of physicians, physical therapists
and orthotists who are familiar with the particular needs of this population.
Because management of ACL tears in young patients involves unique challenges,
there is a greater impetus to reduce injuries in this population. Research shows
that participation in an appropriate neuromuscular training program can
significantly reduce the risk of ACL tears in female athletes.
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