1. Gross Anatomy of the Knee Joint and
Common Acute Injuries
Andrew Bonett
University of South Florida
M.S.M.S. Anatomy
July 16, 2009

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No joint in the body endures as much constant force and mechanical stress as the
knee joint. And despite the strength required to support most of the weight of the body,
the knee is still one of the most flexible of joints. However, the skeletal framework
involved provides for a rather unstable joint and the knee is thus reliable on strong
support from several ligaments and muscles. Consequently, the knee joint is one of the
most common sites of orthopedic maladies, and is an especially vulnerable target for
sports-related injuries. There are many, many different variations of knee-related
injuries, but this paper will only discuss the acute and most common. These include
patellar dislocation, meniscal tears, and sprains of the four major ligaments. Finally,
current research in the orthopedic field, focusing on cruciate ligament reconstruction, will
be discussed briefly. In order to understand the mechanisms of these injuries, though, the
normal anatomy of the knee must first be appreciated.
Gross Anatomy
The knee is the largest synovial joint in the body and although it seems to act like
a simple hinge joint, it is actually a little more complex. The knee can be thought of as
two separate joints: the articulation that comprises the tibiofemoral condyloid joint and
that of the patellofemoral interaction. As can be inferred from the names of the
participating joints, the only bones included in the knee joint are the distal end of the
femur, the proximal end of the tibia, and the patella. The femur and tibia are long bones
that provide most of the structural support for the thigh and leg, respectively. The distal
end of the femur has a medial and a lateral condyle separated by the intercondylar fossa
posteriorly, and displays the patellar surface, sometimes called the trochlea, between the
condyles anteriorly. The palpable medial condyle is the larger of the two, having a

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greater area of articulation with the tibia. The lateral condyle is shorter when viewed
anteriorly and has a long axis that runs nearly parallel to the sagittal plane, whereas the
long axis of the medial condyle points about 22° anterolaterally from the sagittal plane.
The posterior surfaces of the condyles, which articulate with the tibia when the knee is
flexed, are round and allow smooth movement for varying degrees of flexion or
extension. The inferior surface of the distal femur, on the other hand, is rather flat and
contributes to the locking mechanism that alleviates some of the weight-bearing work
required from thigh muscles to keep the joint fully extended when standing. Hyaline
cartilage covers the patellar surface of the femur and continues inferiorly and posteriorly
to cover most of the femoral condyles for articulation with the patella and tibia. The
proximal end of the tibia is expanded as a “tibial plateau” that includes a medial and
lateral condyle and an intercondylar eminence. The medial tibial condyle is larger and
flatter than the lateral condyle, although both are slightly concave. Each condyle is
covered superiorly by a fibrocartilaginous C-shaped structure called the meniscus that
helps contour the rather incompatible joint structures. Both menisci have their
attachments in the intercondylar region of the tibia and a ligament connects their anterior
horns – the transverse ligament. The menisci are thicker around their peripheral borders,
giving them a triangular appearance in cross-section, such as seen in MRI. A clinically
significant characteristic of the menisci is that they only receive direct vascular
nourishment from the inferior geniculate arteries in their peripheral 2-3 mm. This
becomes considerably important when addressing tears of the meniscus and the approach
to treatment because the avascular central meniscus will not heal on its own. The medial
meniscus is more oval-shaped, whereas the lateral meniscus is more circular. Although

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the medial skeletal structures are larger, it is actually the lateral meniscus that covers a
larger portion of the articular surfaces. The lateral meniscus is also more mobile because
it is not attached to the fibrous capsule of the joint, unlike the medial meniscus. The
intercondylar eminence of the tibia has two tubercles that project superiorly between the
femoral condyles and simply help prevent side-to-side movement of the joint when it is
extended. The anterior face of the proximal tibia has a large protuberance called the
tibial tuberosity, which provides attachment for the patellar ligament. In reality, the
patellar ligament is simply an extension of the quadriceps femoris tendon in which a
sesamoid bone called the patella develops. The patella is a small, round bone with a
somewhat triangular shape that points inferiorly. Comprising half of the patellofemoral
joint, the patella is located anterior to the distal end of the femur and is covered with
hyaline cartilage on its posterior surface. Its posterior surface has a smaller medial facet
and a larger lateral facet that make for a fitting articulation with the patellar surface of the
femur. The patella changes the angle of the quadriceps tendon as it spans the knee joint
in such a way that the distance between the tendon and the center of rotation of the joint
is maximized. This allows for greater efficiency of the extensor muscles by the same
principles that govern the simple pulley system. It also decreases friction and prolongs
the effects of wear and tear on the tendon.
Like all synovial joints, the knee has a highly vascular synovial membrane, which
harbors synovial fluid that lubricates and nourishes the articulating surfaces. The
synovial membrane attaches to the borders of the articular surfaces and menisci, has a
posterior reflection to exclude the cruciate ligaments, and also has two extensions that act
as bursae beneath two associated tendons. The suprapatellar bursa is a superior

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expansion of the synovial cavity between the anterior surface of the distal femur and the
tendon of the quadriceps femoris. The subpopliteal recess is a small, lateral expansion of
the synovial cavity between the lateral meniscus and the intracapsular tendon of the
popliteus muscle. Other bursae that do not communicate with the synovial cavity include
the prepatellar bursa and the subcutaneous and deep infrapatellar bursae. The fibrous
capsule encompasses the synovial membrane and the intercondylar region of the femur.
It receives contributions from several muscles and ligaments, but it is outside the scope of
this discussion to explore its individual components.
The knee joint contains many strong ligaments that compensate for the lack of
stability provided by the bony framework. Because of the joint’s dependence on these
ligaments, they are commonly injured in athletes as a result of over-exertion or sudden
trauma. The most essential ligaments of the knee are the patellar ligament, the two
collateral ligaments and the two cruciate ligaments. The functional importance of the
patellar ligament was previously discussed, but it also provides anterior support as it
spans the joint and inserts on the tibial tuberosity. As in most hinge joints, collateral
ligaments provide lateral stabilization and are the most external of the ligaments. The
fibular (lateral) collateral ligament, or LCL, is attached to the lateral epicondyle of the
femur, runs inferiorly outside of the fibrous capsule, and attaches just below the head of
the fibula on its lateral surface. This ligament protects against adduction and
hyperextension injuries. The tibial (medial) collateral ligament, or MCL, is attached to
the medial epicondyle of the femur just inferior to the adductor tubercle, runs
anteroinferiorly, and attaches to the medial condyle of the tibia just superior and posterior
to the insertion of the pes anserinus. Because of its orientation, when the knee is

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extended the anterior fibers are relaxed while the posterior fibers are stretched. As the
knee flexes, the anterior fibers are stretched while the posterior fibers relax. A notable
characteristic of the MCL that is of clinical importance is its attachment to the fibrous
membrane and to the medial meniscus, which prevents posterior sliding of the medial
meniscus during flexion. This ligament also protects against abduction and
hyperextension injuries. The cruciate ligaments are intracapsular, yet extrasynovial, and
provide both anteroposterior and rotational stability. The anterior cruciate ligament,
commonly known as the ACL, runs posterolaterally from an anterior facet of the
intercondylar area of the tibia to the posterior medial aspect of the lateral femoral
condyle. It also shares some attachment with the medial meniscus, which is another
important clinical characteristic. The ACL prevents posterior displacement of the femur
relative to the tibia, excessive medial rotation of the tibia, and hyperextension. The
posterior cruciate ligament, or PCL, is a thicker and stronger ligament and crosses the
ACL medially. It attaches to the posterior part of the intercondylar area of the tibia and
runs anteromedially to attach to the lateral aspect of the medial femoral condyle. The
exact function of the PCL has long been debated, but it primarily prevents anterior
displacement of the femur relative to the tibia, especially when the knee is flexed. It also
checks excessive lateral rotation of the tibia and both hyperextension and hyperflexion of
the knee.
Numerous muscles cross the knee joint. These include most muscles from the
anterior compartment of the thigh, all from the posterior thigh, one from the medial
compartment of the thigh, three muscles from the posterior compartment of the leg, and
even one tendon from the gluteal region. These muscles are mainly involved in flexion

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and extension of the leg at the knee, while also providing stability and some rotational
function. The quadriceps femoris muscles (vastus medialis, vastus lateralis, vastus
intermedius, and rectus femoris) converge into the quadriceps femoris tendon that inserts
on the patella and becomes the patellar ligament. They are the main extensors of the
knee. The sartorius muscle crosses the knee medially, inserts just inferomedial to the
tibial tuberosity, and aids in flexion. The gracilis muscle is the only muscle from the
medial compartment of the thigh that is associated with the knee joint. It inserts into the
tibia immediately posterior to the sartorius tendon and also aids in flexion. The
hamstring muscles (biceps femoris, semimembranosus, and semitendinosus) are the main
flexors of the leg at the knee. Both heads of the biceps femoris converge into one tendon
that crosses the knee laterally, inserts into the head of the fibula, and contributes to the
LCL and other ligaments. The biceps femoris also laterally rotates the leg at the knee
when the knee is partly flexed. The other two hamstring muscles pass medially and thus
medially rotate the leg at the partly flexed knee joint. The semimembranosus contributes
greatly to the fibrous capsule and other ligaments, while the semitendinosus inserts into
the tibia just posterior to the insertion of the gracilis. The tendons of the sartorius,
gracilis, and semitendinosus muscles actually conjoin to form a common insertion known
as the pes anserinus. From the gluteal region, part of the gluteus maximus and tensor
fascia latae converge to form the iliotibial tract, which inserts into the lateral condyle of
the tibia, contributes to the fibrous capsule, and provides lateral stabilization for the
extended knee. The gastrocnemius and plantaris muscles are mainly involved in plantar
flexion of the foot, but because they have origins just superior to the femoral condyles,
they are also considered weak knee flexors. A deep muscle of the posterior leg, the

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popliteus is unique both anatomically and functionally. The popliteus muscle is one of
few in the entire body to penetrate the joint capsule. Inside the capsule, the tendon of the
popliteus originates from the lateral femoral condyle and separates the lateral meniscus
from the fibrous membrane before exiting and inserting into the tibia just superior to the
soleal line. Contraction of this muscle laterally rotates the femur on the tibia,
“unlocking” the knee and initiating flexion.
The posterior, diamond-shaped area formed by the hamstring tendons and by each
head of the gastrocnemius muscle is called the popliteal fossa. Its main contents are
essential to communication between the thigh and leg and are clinically relevant
regarding knee injuries. It contains the popliteal artery and vein and the primary
branches of the sciatic nerve – the tibial nerve and the common fibular (peroneal) nerve.
These structures give rise to the sural nerves, as well as the genicular, circumflex, and
tibial arteries and are of considerable importance when treating an injury to the knee.
One artery of particular significance is the middle genicular artery. Branching from the
popliteal artery, it descends anterolaterally before piercing the posterior fibers of the joint
capsule. Intra-articularly, it courses between the cruciate ligaments, running closely to
the distal end of the posterior cruciate, as it gives off multiple branches that supply these
two ligaments. Damage to these genicular and circumflex arteries, and especially to the
popliteal artery, during surgery can be devastating. Fortunately, vascular injuries overall
occur in less than 1% of arthroscopic procedures.

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The Injured Knee
The demanding forces exerted on the knee throughout life often lead to overuse
injuries such as stress fractures, arthritis (synovitis), tendinitis and bursitis. However,
acute injuries of the knee joint arising from sports or from an accident are often much
more painful and severe. These types of injuries include dislocations or subluxations,
sprains, strains, and fractures. A dislocation is complete displacement of the two articular
surfaces of a joint, whereas subluxation is a partial displacement. As far as the knee is
concerned, this type of injury is exclusive to the patellofemoral joint. The tearing of a
ligament is known as a sprain, while damage to a muscle or its tendon is called a strain.
Sprains and strains are both classified according to the severity of the injury. A minor
tear of only a few fibers constitutes a 1st degree injury, whereas 3rd degree sprains or
strains are characterized by a complete rupture. (These types of injuries can also be
called Grade I, II, or III). As if the knee isn’t under enough stress during daily activities,
participating in rigorous sports (especially football and soccer) is not the best way to
maintain a healthy knee. Of all the clinical visits from patients claiming to have incurred
a sports-related injury, almost 50% of them turn out to be related to the knee.
Furthermore, when taking into account all injuries from all sports, either acute or chronic,
both minor and severe, knee injuries represent 15%. Part of the reason for this high
incidence is the wide range of injuries that a joint as complex as the knee can sustain.
Patellar Dislocation
Patellar dislocation refers to the complete displacement of the patella from the
femoral trochlea to the lateral surface of the lateral femoral condyle. Individuals with

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genu valgum (knock knees), weak quadriceps femoris muscles, or patellar hypermobility
are predisposed to this injury. It commonly occurs in overweight individuals with genu
valgum, as well as in athletes as a result of direct contact to the medial side or from
external rotation of the tibia with forceful contraction of the quadriceps femoris, as in
cutting off of a planted foot. Normally, extension does in fact move the patella laterally,
but it is restrained by the lateral femoral condyle, the pull of the vastus medialis muscle,
the medial patellar retinaculum, and fibers of other nearby ligaments. Consequently,
patellar dislocations are almost always associated with a vastus medialis strain and
rupture of the medial patellar retinaculum. When this injury occurs, a distinctive popping
sound may be audible to other participants and the athlete is immediately disabled. The
dislocation can often be reduced simply by extending the knee, but hemarthrosis causes
major swelling within minutes. Diagnosis is obviously unmistakable if the patient
presents with a persistent dislocation, but that is rarely the case. The patellar
hypermobility/apprehension test is used to determine if a dislocated patella has been
reduced. Sitting on the examination table, the physician places the patient’s knee over his
thigh in a flexed position before placing both thumbs on the medial aspect of the patella.
He then applies a lateral force to the patella and notes the degree of laxity. If the patient
senses an impending subluxation of the patella, he or she shows extreme apprehension to
the maneuver, which is why the test was aptly named. It is important to note that when
performing specific tests such as this one, along with those yet to be discussed, they
should first be conducted on the unaffected knee so as to provide a reference for
comparison. After testing, X-rays may be taken to rule out an intra-articular fracture, as
that is the only associated complication in which surgery is indicated. Aspiration of the

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knee may temporarily relieve pain, but treatment is usually limited to ice, focal
compression, immobilization, and rest. Surgery is only necessary to repair intra-articular
fractures or tears of the medial patellar restraints. Rehabilitation exercises aimed at
strengthening the vastus medialis muscle help prevent recurrent subluxations.
Meniscus Injuries
Meniscal injuries are extremely common because the menisci are responsible for
much of the weight-bearing function of the knee and are regularly exposed to a great deal
of shear force. Because of the magnitude of stress placed on the menisci, and their semi-
vascular nature, the cartilage becomes increasingly brittle with age and the cause of
injury may be acute, degenerative, or a combination of both. Thus, the high incidence of
meniscal tears is widespread between athletes, overweight individuals, and the elderly.
Regardless of the health of the meniscus, the only motions that can cause a tear are
squatting or twisting of the knee joint. In acute injuries, especially in athletes, the flexed
knee is suddenly twisted (internal tibial rotation for a medial meniscus tear) while the
foot is fixed, and a multitude of injuries can occur. The “unhappy triad” is a term used to
refer to simultaneous injury to the medial meniscus, medial collateral ligament, and
anterior cruciate ligament as a result of sudden abduction and internal rotation of the
knee. Therefore, when diagnosing an injury to one of these three structures, the clinician
should be suspicious of the integrity of the other two. Tears of the menisci come in three
main varieties – longitudinal, horizontal, and radial. A horizontal tear is the most
common and is usually considered degenerative in nature, often being associated with
osteoarthritis. It can be thought of as a tear in the transverse plane of the meniscus. In
most cases, a horizontal tear is initially within the substance of the meniscus, not causing

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any mechanical disturbance, and is not apparent on the surface. However, it can easily
progress to reach the surface, more often forming an inferior flap that may move into the
intercondylar area and cause locking of the knee. Longitudinal and radial tears are more
associated with acute, traumatic injuries and are much more urgent and disabling.
Because of the twisting shear force applied by the femoral condyle during rotation of the
knee, longitudinal tears are vertically oriented but divide the meniscus obliquely
downwards and centrally in cross section. Initially, they are usually restricted to the
posterior horn of the meniscus as a partial tear. A complete longitudinal tear is one that
progresses to the anterior horn of the meniscus more anteriorly than the cruciate ligament.
If the inner segment of the torn meniscus becomes permanently displaced toward the
center of the joint, the condition is colloquially referred to as the bucket handle tear. This
type of tear is much more likely to cause locking and full extension can only be achieved
at the expense of the ACL. Radial tears are also vertical and trauma-related, but are
oriented in the coronal plane of the meniscus and arise from the internal, concave surface.
This type of injury immediately creates a free-moving flap that can fold upon itself and
cause locking and discomfort.
Medial Meniscus
As previously mentioned the medial meniscus is attached to the fibrous capsule
and MCL, is less mobile due to its further-apart attachments, and is slightly thinner in its
periphery as the lateral meniscus. All of these traits account for the higher incidence of
medial meniscus tears. Acute injury to the medial meniscus is caused by internal rotation
of the tibia while the knee is in the flexed position. In the clinic, there are many different
methods that can be utilized to produce an accurate diagnosis. Generally, if the patient

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presents with tenderness among the medial joint line upon palpation, it is more likely due
to a chronic lesion because of effusion. However, palpation is seldom sufficient for
diagnosing knee injuries. Specific tests provide the physician with valuable information
before imaging is necessary. The most popular test conducted to rule out meniscal tears
is called McMurray’s test. This test requires the patient to be lying down in the supine
position with the troublesome knee flexed but relaxed. With the knee maximally flexed,
the physician externally rotates the foot and extends the knee toward full extension while
palpating the medial joint line. A positive McMurray’s test is a painful clicking sound
produced during extension as the medial femoral condyle passes over the lesion. A
similar test is Apley’s compression test, or the Apley grind test, which can be helpful in
differentiating between meniscal tears and other internal knee injuries. Here, the patient
lies in a prone position with the knee flexed at 90 degrees. The physician again externally
rotates the tibia, but now applies axial pressure to the sole of the foot while flexing and
extending the knee. A positive Apley test produces the same results as McMurray’s test.
Although the physician can usually make a correct diagnosis with these tests, an MRI can
almost always confirm the lesion. A healthy meniscus shows up as a solid black triangle
in MRI imaging and any white interruption in that triangle depicts a meniscal tear. In
fact, MRI has a sensitivity of at least 94% in respect to medial meniscus tears. If for any
reason there is still doubt concerning the nature of the injury, diagnostic arthroscopy is
the definitive method of identifying a tear.
Small tears eliciting only minor pain and effusion may be treated with the basic
“PRICES” regimen (Protection, Rest, Ice, Compression, Elevation, and Support)
followed by rehabilitation, but only if there is no ligamentous instability. However,

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treatment usually includes an arthroscopic partial meniscectomy and is the preferred
approach. Longitudinal tears involving the peripheral vascular part of the meniscus used
to mean a complete excision of the meniscus, but are now treated with arthroscopic
meniscal repair. In fact, before modern imaging technology, total excision was the
preferred treatment for all meniscal tears because the menisci were thought of as mere
space fillers and diagnosis of multiple tears was nearly impossible. Partial meniscectomy
is most successfully performed on bucket handle tears and radial tears, with the goal in
both cases being to remove the locking agent that may also cause degeneration of the
articular surfaces if left untreated. For a bucket handle tear, the centrally displaced
portion of the meniscus is simply excised and what is left of the peripheral meniscus
remains. When treating a radial tear, the flaps are eliminated by shaving off a smooth,
rounded curve that includes the lesion. The post-operative meniscus should look similar
in shape, but maybe with a more pronounced recess on the concave surface.
Lateral Meniscus
Injury to the lateral meniscus has a lower incidence due to its greater mobility,
thicker and more uniform composition, and lack of attachment to the fibrous capsule.
The lateral meniscus is vulnerable to the same types of tears as those suffered by the
medial meniscus, but some conditions more typically involve the former and can
sometimes be more painful. Because the anterior horn of the lateral meniscus is more
posterior than that of the medial meniscus, it is not as rare for a tear to be limited to the
anterior horn. Also, the lateral meniscus is more prone to a rare, congenital variation
known as a discoid meniscus, in which the meniscus is a full disc of cartilage rather than
semilunar and there is no articulation between the lateral femoral condyle and the tibia.

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Because of its congenital nature, the discoid meniscus is present during childhood, but is
often asymptomatic until well into adulthood. Discoid lateral menisci only affect 1.5 –
3% of the population, while discoid medial menisci only develop in 0.1 – 0.3% of the
general population. A discoid meniscus is much more vulnerable to lesions and cystic
degeneration even despite the normal mechanism of a meniscal tear. Finally, the lateral
meniscus is more likely to incur a vertical radial, or parrot beak tear. This type of tear
can actually start out as two separate horizontal cleavages that connect and eventually
incorporate the superior and inferior surfaces of the meniscus. Therefore, it is essentially
a compound vertical/horizontal tear with a curved inner portion. The greater thickness of
the lateral meniscus facilitates this mechanism of rupture.
The same diagnostic procedures are used with the lateral meniscus as are with the
medial. The patient may experience tenderness along the lateral joint line upon
palpation, and the same two tests are used. However, when conducting McMurray’s test
and Apley’s compression test, this time the physician internally rotates the tibia during
flexion and extension. Unfortunately, MRI imaging is considerably less effective in
detecting lateral meniscus tears, with sensitivity being as low as 78%. There is no major
difference in the preferred treatment approach for an acute tear of the lateral meniscus,
with the exception of a discoid lateral meniscus. A discoid meniscus may be sculpted to
resemble a normal lateral meniscus or a complete meniscectomy may be indicated for a
hypermobile lateral meniscus. This subtype of discoid meniscus, also known as the
Wrisberg type, lacks a posterior attachment to the tibia (only to the Wrisberg ligament),
and is thus not effectively treated with a partial meniscectomy.

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Ligament Injuries
A simple balance of equilibrium between external and internal forces determines
the integrity of the knee joint. Integrity is maintained when internal forces prevail, while
exceeding external forces cause injury. Internal stabilizing forces of the knee joint come
from three sources: active stabilization from muscles that cross the joint; and passive
stabilization from the ligaments and the geometrical congruity of the joint itself. For
example, a posteriorly directed external force applied to the anterior aspect of the joint
would be countered by the flexors of the knee (along with the ligaments). Obviously the
passive forces are biologically and anatomically limited, but fitness and muscle strength
can contribute to a more stable joint. Nevertheless, sudden and unexpected external
forces, often encountered in sports such as football, skiing, soccer, or rugby, eliminate the
active component of stabilization. Since geometrical congruity of the joint is naturally
lacking, most of the burden is placed on the ligaments and they often succumb, leaving
the joint even more vulnerable to further injury. Diagnostic techniques are similar for
evaluating all ligaments of the knee. The first step is one of the most important.
Understanding the patient’s history and learning as much about the injury as can be
recalled can already give the physician an idea of where to direct his focus. This is why
it is crucial to have a strong appreciation for the mechanisms of these injuries. Specific
laxity tests also provide valuable information and are usually done next. The most
efficient imaging technique is MRI, but X-rays can sometimes supplement and help rule
out associated fractures. If all else fails, the definitive diagnostic technique is
arthroscopy, which is always the last resort.

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Medial Collateral Ligament
The medial collateral ligament, or MCL, is directly attached to the fibrous capsule
and medial meniscus and provides the most vital support against valgus stresses. Thus,
not only is it one of the most commonly sprained ligaments in athletics, but it also rarely
presents as an isolated injury. As previously mentioned, MCL sprains constitute one-
third of the “unhappy triad” seen by many a team physician. They can occur as a result
of a noncontact, external rotation of the tibia (skiing), but it is usually a direct valgus, or
abduction, force to the lateral knee that is the cause. Because of this, most injuries to the
MCL are suffered by football players. These sprains, along with those of the succeeding
ligaments, are classified as Grade I, Grade II, or Grade III according to their severity.
Pain associated with the injury is well localized to the medial joint line, but often
quickly subsides with a Grade III complete tear of the ligament. This is particularly
dangerous in athletics because it encourages the participant to continue with a poorly
stabilized knee. Often the most notable complaint by the patient is simply valgus laxity.
Specific tests to be performed by the physician include the abduction stress test,
performed with 30° of flexion and again with full extension, and the anterior drawer test
with external rotation of the tibia. The abduction stress test is performed with the patient
lying in the supine position and the physician supporting the thigh and knee so as to
create some flexion (about 30°) while keeping all of the muscles relaxed. This is
sometimes easier when the thigh is supported by the examination table and the leg
slightly hangs over the edge. With his elbow on the medial side of the patient’s foot, he
applies a valgus force to the knee with his opposite hand and palpates the medial joint
line for abnormal instability. Excessive flexibility or elicited pain are positive signs and

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indicate an MCL sprain. The test is then performed with the knee in full extension to rule
out an associated sprain of the posterior cruciate ligament. The anterior drawer test is
primarily used to check the integrity of the anterior cruciate ligament and will be
discussed in detail later with the corresponding injury. A variation of this test, however,
can help diagnose a tear of the medial collateral ligament when it is performed with the
tibia externally rotated 30°. Excessive anterior rotation of the medial tibial condyle
constitutes a positive sign for this test. X-rays are not too practical for diagnosing a
ligament tear, although they may be taken in the abduction stress position if the patient is
of young age to rule out an epiphyseal fracture. If the MCL is compromised, the X-ray
will show an enlarged medial joint cavity with excessive distance between the medial
condyles of the femur and tibia. An anteroposterior MRI taken in the coronal plane is the
best way to view the collateral ligaments. A normal MCL is depicted as a thin, taut, low-
signal (black) structure that is tightly adhered to the medial femoral epicondyle and
medial tibial metaphysis. Grade I tears show a less than well-defined MCL caused by
some surrounding intermediate signaling, which is indicative of edema. Grade II tears
show a seemingly thicker ligament with even more intermediate signaling, also due to
edema, but with possible accompanying hemorrhage. The ligament also appears “looser”
and is not as tightly bound to the bones. Grade III tears are quite remarkable with severe
edema and are difficult to misdiagnose.
Treatment of a torn medial collateral ligament is completely dependent on the
severity of the injury and whether it is isolated. Isolated injuries rarely call for surgical
intervention, except for some grade III tears, whereas compound injuries usually require
surgery. Isolated grade I and II tears are treated with the basic “PRICES” remedy that is

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also used for minor meniscal tears, along with the use of crutches for the first ten days.
Grade III tears are often repaired surgically, although immobilization with a knee brace
has proven to be sufficient in isolated cases. Unfortunately, a full recovery from a grade
III MCL sprain can sometimes take three to four months. Rehabilitation is a vital
component of the treatment plan for any ligament tear.
Lateral Collateral Ligament
Of the four major ligaments in the knee, the lateral collateral ligament (LCL) is
sprained least frequently, and even less so as an isolated incident. There are two reasons
for the rarity of this injury. First, the mechanism of injury is mainly adduction due to
contact to the medial knee joint while it is flexed. Needless to say, it is rare for a force
great enough to cause injury to be oriented in this direction. Other possible sources of
varus stress include noncontact incidences such as landing from a jump off balance or
unexpectedly stepping into a hole. Secondly, the tendons of the biceps femoris, popliteus
muscle, and iliotibial tract all aid the LCL in providing lateral stabilization. This
accounts for the greater resistance of the knee against varus stress than against valgus
stress, and is also why isolated LCL tears occur in the flexed knee. A violent blow to the
medial aspect of the extended knee would most likely affect all of the aforementioned
structures and, more devastatingly, may damage the common peroneal nerve. This nerve
gives branches that innervate the lateral and anterior compartments of the leg, as well as
some of the intrinsic muscles of the foot. Damage to it could result in permanent
disabilities such as foot drop and loss of sensation from parts of the leg and foot. More
commonly, the knee receives a violent blow from the anteromedial aspect, resulting in a
posterolateral corner injury.

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Patients may actually hear a popping sound during an acute injury and pain is well
localized over the lateral joint line. A feeling of varus laxity or general instability may
also be present, but not as apparently so as compared to a compromised MCL. The LCL
is more easily palpated in a flexed knee than the MCL, but similar tests are still carried
out during diagnosis. The adduction stress test parallels the abduction stress test that was
discussed earlier, but is performed in the opposite manner. The external rotation
recurvatum test is a rather simple test used to rule out a posterolateral corner injury. With
the patient lying relaxed and in a supine position, the physician lifts both legs by holding
on to only the great toes. Apparent recurvatum (hyperextension), external tibial rotation,
and an assumed varus of the knee constitute a positive sign. These features may even be
apparent upon standing when the tear is significant. The posterolateral drawer test is
similar to the corresponding test for the MCL. The tibia is still externally rotated 30° on
the knee while flexed at 90°, but this time posterior force is applied to the proximal tibia.
Excessive posterior rotation of the lateral tibial condyle is a positive sign and also
suggests a posterolateral corner injury. The fourth test, called the reverse pivot shift test,
involves temporary subluxation of the lateral tibial condyle and is preferably performed
under anesthesia for the sake of the patient. With the knee flexed to 90°, the tibia
externally rotated, and a slight valgus force applied by the physician, the leg is slowly
extended. For a positive test, the lateral tibial condyle subluxates posteriorly at around
30° of flexion and then forcibly reduces with further extension. MRI for diagnosing LCL
tears has an accuracy rate of 90%, but is a little more difficult than with the MCL. The
LCL courses slightly posterior on its way to the head of the fibula, making it impossible
to view in its entirety on a truly coronal scan. A coronal oblique scan may reveal the

21. 21
entire LCL, but lateral scans in the sagittal plane are necessary to accurately diagnose a
tear. A sagittal scan is also vital because it shows the fibular head and the common
insertion of the tendon of the biceps femoris. It is important not to neglect these
structures because fibular fractures and avulsions of the associated tendons frequently
occur in conjunction with LCL injuries. When examining the images, a torn LCL has
different characteristics than a torn MCL because of its completely extracapsular nature.
Never appearing to be enlarged, an acutely torn LCL appears to be less taut and often
discontinuous. Chronic tears do have a thickened appearance with more low weight
signaling.
Although the lateral collateral ligament does not heal as easily as the medial, there
are no remarkable differences in the treatment of grade I or II tears. Grade III sprains
almost always require surgery, especially because they are almost never isolated. In
severe cases, when tendons need to be reconstructed by using a graft from either the
quadriceps femoris or hamstring tendons, an open-knee procedure is necessary. Full
recovery time for a grade III repair can be up to eight weeks.
Anterior Cruciate Ligament
The anterior cruciate ligament, or ACL, is the smallest and shortest ligament in
the knee. However, what the ACL lacks in size it makes up for in function. It is the most
crucial stabilizer of the knee, checking hyperextension, medial rotation of the tibia,
posterior dislocation of the femur, and even hyperflexion in association with the PCL. It
is able to provide all of this stability because of its composition and helical orientation -
the anteromedial fibers are taut during flexion and mainly check anterior displacement,

22. 22
while the posterolateral fibers are taut during extension and mainly control rotational
stability. A sprain of the ACL may involve either or both of these fiber groups. Because
of its imperative and diverse function, it is the most frequently injured ligament for
athletes competing in basketball, football, soccer, skiing, gymnastics, etc. Another
reason that most athletes are so vulnerable to ACL injuries is that an isolated sprain is
seldom the result of contact. More commonly, sudden twisting movements, abrupt
deceleration, or landing awkwardly from a jump causes problems for the ACL. Collision
injuries usually affect the ACL in the event of an “unhappy triad” injury. An abduction
injury cannot tear the ACL unaccompanied by a tear of the MCL. External rotation of
the femur on a fixed tibia is the easiest way to cause an isolated tear. Posterior
displacement of the femur when the knee is flexed can also single out the ACL because
the collateral ligaments are more relaxed. Hyperextension is a threat to the posterolateral
fibers of the ACL, but would also bring the collateral ligaments back into consideration.
Depending on the mechanism of injury, a complete tear can occur at the tibial attachment
(3-10%), at the femoral attachment (7-20%), or in the middle of the ligament (70%). In
younger patients, the tibial spine may even be avulsed. The location of the lesion is
significant because the middle portion of the ACL is relatively avascular, while the
femoral end receives a branch from the middle geniculate artery, as does the synovial
membrane. Although the common superior lesion causes more complications, any type
of lesion is followed by relatively ineffective healing due to the extrasynovial nature of
the ACL.
When an ACL tear occurs, the individual can usually hear it pop or snap before
the knee buckles and gives out. Swelling will likely follow along with possible dizziness,

23. 23
sweating, or nausea. The intensity and duration of pain varies widely but is somewhat
localized to the anterior knee. Next to MRI, specific physical tests are the most effective
method for diagnosing ACL injuries. The classic ACL test is the anterior drawer test.
With the knee flexed at 90° and no tibial rotation, and the hamstrings relaxed by the
support of the physician, the proximal end of the tibia is gently pulled anteriorly. If there
is excessive anterior displacement or no definite end point when compared to the
uninjured knee, that is considered a positive test. However, severe ACL sprains
associated with a negative anterior drawer test have been well documented because this
technique does not eliminate the influence of other structures. Therefore, this test can be
contributory, but is not sufficient. The most accurate of physical tests for an ACL injury
is called the Lachman test. It is simply a variation of the anterior drawer test; the only
difference being that the knee is flexed no more than 30°. Similarly to with the LCL, the
pivot shift test or the jerk test may be conducted to supplement the Lachman test. The
pivot shift test is performed with the patient supine and relaxed, the tibia internally
rotated with the knee in full extension, and a slight valgus applied by abduction of the
foot. As the knee is gradually flexed, the physician feels for an anterior subluxation of
the lateral tibial condyle at around 30°. Upon further flexion, the subluxated tibia should
suddenly reduce. The jerk test is basically the same test but performed backwards. The
knee starts flexed at 90°, and the anterior subluxation and subsequent reduction should
occur with gradual extension. MRI provides an accurate diagnosis in as many as 95% of
ACL cases, but is not always necessary if the physician feels strongly enough about the
results of the Lachman test. Although the ACL is ideally evaluated in slight flexion,
imaging is done in full extension so as to include an accurate depiction of the menisci.

24. 24
Because the course of the ACL is in three dimensions (from its attachment to the tibia it
runs posteriorly, laterally, and superiorly), it is imaged in the coronal, sagittal, and axial
planes. Axial imaging is especially helpful for examining the proximal ACL, where it
normally shows up as a dark, low-signal eclipse on the medial side of the lateral femoral
condyle. One primary sign of an acute ACL tear is actually not being able to see the
ACL at all. In its place between the femoral condyles is a blurry, intermediate-signal
mass of edema and hemorrhage. Other primary indications of a tear include a high-signal
interruption, a nonlinear appearance, or an abnormal axis of the ligament in relation to the
bony structures. Secondary indications, including osteochondral fractures, anterior
displacement of the tibia, and Segond fractures, are not nearly as reliable. A Segond
fracture is a vertical, elliptical avulsion fracture of the lateral tibial condyle caused by a
combined varus force and internal tibial rotation. Segond fractures are accompanied by
an ACL tear 75-100% of the time. Once again, arthroscopy eliminates any doubt but is
preferably avoided.
The treatment approach heavily depends on the presence of compound injuries,
the patient’s activity level and age, and his willingness to pursue the proper physical
therapy if surgery is indicated. Regardless of the severity of the sprain, the patient should
use crutches and avoid all weight-bearing activities for the first week. For isolated partial
tears, the “PRICES” regimen, along with functional rehabilitation and bracing of the
slightly flexed knee, is often sufficient. Still, the ACL is extrasynovial and does not heal
as efficiently as other ligaments. A full recovery could take as long as six months. Most
acute tears suffered by active athletes cannot be repaired and require complete excision of
the ligament followed by reconstructive surgery. It is now recommended that the surgery

25. 25
be delayed about three weeks after the injury to allow the swelling to subside and a wider
range of motion to return. Studies have shown that late reconstruction is associated with
less post-operative stiffness and more favorable outcomes. A more in-depth look at
cruciate ligament reconstruction will be discussed later.
Posterior Cruciate Ligament
The posterior cruciate ligament is broader and much stronger than the ACL, and is
thus another one of the most important stabilizers of the knee joint. However, its precise
mechanism of stabilization carries a history of uncertainty with it due to its unique
structure. It has been demonstrated that the posterior fibers of the PCL are taut at both
full extension and deep flexion, while the anterior fibers are most stressed during mid-
flexion. While it is widely agreed upon that its primary function is to prevent posterior
displacement of the tibia, its secondary functions include prevention of hyperextension
and to a lesser extent lateral rotation and hyperflexion. Meniscofemoral ligaments (MFL)
originate at the same site as the proximal PCL and are thoroughly integrated with it.
They run parallel to the PCL and attach to the posterior horn of the lateral meniscus,
countering the action of the popliteus muscle. The MFL are the last restraint against
posterior displacement of the tibia when the PCL is completely ruptured. Injury to the
PCL occurs much less frequently than to the ACL, and rarely so in isolation. The
primary cause of an isolated PCL tear is a direct blow to the anterior face of the proximal
tibia when the knee is flexed. This commonly occurs in automobile collisions when the
knees strike the dashboard. In sports, hyperextension is the main source of PCL injury
(especially in combination with valgus/varus forces), although this mechanism also
renders other structures of the knee vulnerable. Another typical mechanism of injury

26. 26
involves falling on the knee with the foot in plantar flexion so that the tibia strikes the
ground first. Because PCL tears are so frequently associated with other injuries, minor
tears are often overlooked and the incidence is probably much higher than that observed
(anywhere between 3% and 20% of all knee injuries). Like the ACL, the proximal PCL
receives vasculature from the medial genicular artery and is extrasynovial, hindering the
natural healing process. However, the PCL does receive a better blood supply than the
ACL and minor tears can heal without surgical intervention. Severe tears can lead to
other associated injuries if not treated immediately. A grade III PCL tear may lead to a
posterior tibial dislocation, which could endanger the tibial and peroneal nerves, although
a posterolateral corner injury is more likely to damage the peroneal nerve. On occasion,
thrombosis or transection of the popliteal artery may also occur.
Symptoms associated with injury to the PCL parallel those for ACL injuries,
except, of course, pain is localized posteriorly. Rupture of the PCL usually elicits a
popping sound that is audible to the patient, followed by swelling, possible effusion or
hemarthrosis, and a sense of posterior instability. An accurate diagnosis is critical for
treating PCL tears because it is not uncommon for the patient to present with minimal
pain and full range of motion. An easy clue during initial observation is a contusion on
the anterior proximal tibia, suggesting the “dashboard injury” mechanism. The specific
physical tests for PCL assessment are simple and straightforward. The posterior drawer
test is much like the anterior drawer test, except that a posterior force is applied to the
proximal tibia. The physician still looks for abnormal posterior excursion and a sharp
endpoint to the displacement. It is crucial that the starting point of the tibia prior to
applying the posterior force is in fact the neutral position. Otherwise, returning the tibia

27. 27
to a neutral position from a posterior displacement may be misdiagnosed as a positive
anterior drawer test. This exemplifies the importance in assessing the healthy knee first.
Another simple test is called the posterior tibial sag test. With the patient in the same
position (supine, hips flexed at 45°, knees flexed at 90°), the physician simply observes,
from a lateral view, any posterior displacement of one tibial tuberosity in relation to the
other. This test is often repeated with the hips flexed to 90° while the physician supports
the legs by holding the ankles or feet. Positive signs for these tests are usually associated
with the primary mechanism of injury previously discussed. A positive sag test in full
extension indicates a compound injury. If the injury was sustained from a valgus or varus
force to the completely extended knee, a positive sign may be seen when conducting the
abduction or adduction stress test at full extension. Lateral view X-rays may reinforce
the sag test results or reveal an avulsion of the tibial attachment in a severe sprain. Once
again, the most effective method for diagnosing a PCL tear is MRI. Scans should be
taken on all three axes, but the most sensitive plane is a sagittal oblique one that parallels
the ligament. The PCL can be viewed in its entirety with one or two sagittal scans, but a
coronal scan will only show a portion of it. The intact PCL also shows lower-signal
intensity than the intact ACL because its fibers are more parallel and regularly organized.
Indications of a tear are the same as with the ACL and are characterized by high-signal
intensity interruptions.
Treatment of PCL tears is an issue of much debate. It is generally agreed that
isolated, partial tears can be treated with PRICES and rehabilitation, while avulsion
fractures and compound sprains require surgical reconstruction. However, controversy
exists for treating tears that fall within these two extremes. Surgery is avoided when at

28. 28
all possible because the position of the PCL makes it difficult to access and there are
additional risk factors associated with reconstruction, such as avascular necrosis of the
medial femoral condyle. It is seldom recommended for a tear that is less severe than
grade III. Regardless of the treatment approach taken, proper rehabilitation is crucial.
Exercises are usually aimed at strengthening the quadriceps muscles because they
provide an anterior drawer to the tibia and essentially compensate for a PCL of poor
integrity. Prognosis is rather favorable for isolated tears because the inferior genicular
arteries allow for successful revascularization.
Cruciate Ligament Reconstruction
Reconstruction of the cruciate ligaments involves complete excision of the injured
ligament followed by replacement with a graft. The graft is normally taken from a
tendon and inserted into holes drilled in the bones at the original insertion sites of the
ligament before being anchored with screws. Amazingly, this can all be carried out
arthroscopically. The ideal graft would have identical structural characteristics as the
ligament, heal and incorporate itself with the insertion sites quickly, and be simple to
harvest. However, the ideal graft does not exist and the source is therefore up to the
surgeon. The patellar tendon, quadriceps tendon, and calcaneus tendon are commonly
shaved to provide the graft for cruciate ligament reconstruction. There are also several
advantages and disadvantages to using an allograft versus an autograft. An allograft is a
tissue donated by another individual of the same species (usually a cadaver in this
instance); while an autograft is tissue taken from another part of the same individual’s
body. Allografts benefit the patient by requiring less operation time and causing less
stiffness since the harvest is performed on another specimen. This also equates to

29. 29
aesthetic advantages and no donor-site morbidity. On the other hand, using an allograft
always carries the risk of disease transmission and an unfavorable immune response to
the foreign tissue. An autograft might incorporate faster, but multiple studies indicate
that allografts have the potential to become more “ligament-like” and can be nearly
identical histologically after one year of incorporation. Once installed, the graft
undergoes major biological modifications for at least one month before it is fully
functional. At first the graft actually becomes inflamed and partially necrotic before
undergoing revascularization. It then becomes repopulated with extrinsic fibroblasts
while its collagenous structure is modified, and original donor fibroblasts are absent by
about six weeks post-op. The type of graft with the most favorable long-term effects is a
topic of many studies still today.
Reconstruction of the anterior cruciate ligament may be achieved with several
different types of grafts. Two widely used grafts that are commonly autogenic are the
quadruple hamstrings tendon graft and the B-PT-B (bone- patellar tendon-bone) graft.
The B-PT-B graft includes surgically avulsed fragments of the tibia and femur from the
donor, keeping the insertion sites of the patellar tendon intact. This allows for bone-to-
bone healing in contrast to tendon-to-bone healing and leads to a faster recovery. The
quadruple hamstrings graft is two parts semitendinosus tendon and two parts gracilis
tendon. Both of these reconstructive methods, however, utilize a “single-bundle”
approach and involve replacing two fiber bundles (anteromedial and posterolateral) with
only one (the tendinous graft essentially replaces only the AM bundle). Consequently,
anterior cruciate ligaments reconstructed with a single bundle do not provide sufficient
rotational stability to the knee joint and, in this aspect, are virtually ineffectual.

30. 30
Currently, double-bundle reconstructions are more frequently performed and are better
suited for full restoration of knee stability. In the double-bundle method, a combination
of the former two procedures, a B-T-B graft serves to function as the anteromedial bundle
and a semitendinosus graft (ST) mimics the posterolateral fibers of the ACL. The grafts
share a common tibial insertion at the isometric point, the B-T-B is inserted at the
femoral isometric point, and the ST is fed posteriorly and superiorly through the
intercondylar region to the posterior face of the lateral femoral condyle. It is vital to note
that in order to recreate the most physiologically accurate ACL, the B-T-B graft is fixed
with the knee at 20° of flexion, while the ST graft is fixed at 90° of flexion. A
retrospective study by Kubo et al. observed long-term integrity following full recovery of
this procedure. Their findings showed no signs of resulting pain, limited range of motion,
or instability in 14 out of 14 patients.
Posterior cruciate ligament reconstruction is less common because, as previously
mentioned, partial tears can heal without surgical intervention. When PCL reconstruction
is indicated, allografts are more useful, especially since isolated ruptures are rare and
there may be multiple ligaments to harvest. Also, because the PCL is a longer ligament
than the ACL, common autografts (B-T-B and ST) are sometimes too short to reach the
isometric points of PCL insertion. One major difference between ACL and PCL
reconstructions is that, as far as current studies suggest, full PCL function and stability
can actually be achieved by reconstructing only the anterolateral bundle. Lack of the
posteromedial fibers causes no observed deficiencies in stability, but our poor
understanding of the PCL’s exact functions might contribute to this conclusion. As with
the ACL, a chief area of concern and recent study is the long-term efficacy of these

31. 31
reconstructions and whether allografts or autografts should be the graft of choice. A
study published just this year by Miyamoto et al. is the first to report a long-term, in vivo
histological analysis of a PCL reconstruction 11 years after a calcaneus tendon allograft
was used. Their results show hypervascularity and increased cellularity at the periphery
of the graft, with hypovascularity and high collagen density in the core of the graft,
nearly indistinguishable from a normal cruciate ligament. They conclude that an allograft
used for PCL reconstruction has the ability to fully incorporate and withstand both
degeneration and host rejection over extended periods of time.
The future of knee ligament surgery surprisingly relies on advancements at the
cellular and molecular levels of biology, with the primary objective being to improve
healing and remodeling. There are several different methods for achieving this goal that
are being researched today. Gene therapy involves either a viral or non-viral vector
delivering a specific gene that encodes growth factors or antibiotics to a target tissue.
One advantage of this method is that, depending on which gene is delivered, it can be
used to target ligaments, cartilage, or bone. Another approach, called tissue engineering,
aims at using cells such as mesenchymal stem cells to deliver the therapeutic genes and
increase the production of growth factors. Regardless of which method is found to be the
best, the general idea is to improve healing time, remodeling of the ligament insertion
sites, and nerve and vasculature restoration via biological manufacture of growth factors,
antibiotics, other cytokines, etc.

32. 32
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