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A patellar dislocation is a knee injury in which the
patella The patella (: patellae or patellas), also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in m ...
(kneecap) slips out of its normal position. Often the knee is partly bent, painful and swollen. The patella is also often felt and seen out of place. Complications may include a patella fracture or
arthritis Arthritis is a general medical term used to describe a disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, Joint effusion, swelling, and decreased range of motion of ...
. A patellar dislocation typically occurs when the knee is straight and the lower leg is bent outwards when twisting. Occasionally, it occurs when the knee is bent and the patella is struck directly. Commonly associated sports include
soccer Association football, more commonly known as football or soccer, is a team sport played between two teams of 11 Football player, players who almost exclusively use their feet to propel a Ball (association football), ball around a rectangular f ...
,
gymnastics Gymnastics is a group of sport that includes physical exercises requiring Balance (ability), balance, Strength training, strength, Flexibility (anatomy), flexibility, agility, Motor coordination, coordination, artistry and endurance. The movem ...
, and
ice hockey Ice hockey (or simply hockey in North America) is a team sport played on ice skates, usually on an Ice rink, ice skating rink with Ice hockey rink, lines and markings specific to the sport. It belongs to a family of sports called hockey. Tw ...
. Dislocations nearly always occur away from the midline. Diagnosis is typically based on symptoms and supported by
X-rays An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
. Reduction is generally done by pushing the patella towards the midline while straightening the knee. After reduction, the leg is generally splinted in a straight position for a few weeks. This is then followed by
physical therapy Physical therapy (PT), also known as physiotherapy, is a healthcare profession, as well as the care provided by physical therapists who promote, maintain, or restore health through patient education, physical intervention, disease preventio ...
. Surgery after a first dislocation is generally of unclear benefit. Surgery may be indicated in those cases where a fracture occurs within the joint or where the patella has repeatedly dislocated. Patellar dislocations occur in about 6 per 100,000 people per year. They make up about 2% of knee injuries. It is most common in those 10 to 17 years old. Rates in males and females are similar. Recurrence after an initial dislocation occurs in about 30% of people.


Signs and symptoms

People often describe pain as severe and being "inside the knee cap". The leg tends to flex even when relaxed. In some cases, the injured ligaments involved in patellar dislocation do not allow the leg to flex.


Risk factors

A predisposing factor is tightness in the
tensor fasciae latae muscle The tensor fasciae latae (or tensor fasciæ latæ or, formerly, tensor vaginae femoris) is a muscle of the thigh. Together with the gluteus maximus, it acts on and is continuous with the iliotibial band, which attaches to the tibia. The muscle a ...
and
iliotibial tract The iliotibial tract or iliotibial band (ITB; also known as Maissiat's band or the IT band) is a longitudinal fibrous reinforcement of the fascia lata. The action of the muscles associated with the ITB ( tensor fasciae latae and some fibers of g ...
in combination with a
quadriceps The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...
imbalance between the
vastus lateralis The vastus lateralis (), also called the vastus externus, is the largest and most powerful part of the quadriceps femoris, a muscle in the thigh. Together with other muscles of the quadriceps group, it serves to extend the knee joint, moving the ...
and
vastus medialis The vastus medialis (vastus internus or teardrop muscle) is an extensor muscle located medially in the thigh that extends the knee. The vastus medialis is part of the quadriceps muscle group. Structure The vastus medialis is a muscle presen ...
muscles can play a large role, found, mainly, in women involved in sports. Moreover, women with patellofemoral pain may show increased Q-angle compared with women without patellofemoral pain. Another cause of patellar symptoms is ''lateral patellar compression syndrome'', which can be caused from lack of balance or inflammation in the joints. The
pathophysiology Pathophysiology (or physiopathology) is a branch of study, at the intersection of pathology and physiology, concerning disordered physiological processes that cause, result from, or are otherwise associated with a disease or injury. Pathology is ...
of the kneecap is complex, and deals with the osseous soft tissue or abnormalities within the patellofemoral groove. The patellar symptoms cause knee extensor
dysplasia Dysplasia is any of various types of abnormal growth or development of cells (microscopic scale) or organs (macroscopic scale), and the abnormal histology or anatomical structure(s) resulting from such growth. Dysplasias on a mainly microscopic ...
, and sensitive small variations affect the muscular mechanism that controls the joint movements. 24% of people whose patellas have dislocated have relatives who have experienced patellar dislocations.


Athletic population

Patellar dislocation occurs mainly in youths (under age 20) engaged in sports that may involve accidental rotation of the knee while in
flexion Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, Limb (anatomy), limbs, and specific sections of the body. The terminology used de ...
, a movement clinically called '' valgus'', which is the cause of some 93% of patellar dislocation cases. It is more common in females than males and in young in-training military personnel who have a high incidence of patellar dislocation in relation to young athletes and the general population. Direct trauma to the knee displacing the patella is rare. Displacement of the patella laterally out of its groove strains the medial stabilizing connective tissues, particularly the
medial patellofemoral ligament The medial patellofemoral ligament (MPFL) is one of several ligaments on the medial aspect of the knee. It originates in the superomedial aspect of the patella and inserts in the space between the adductor tubercle and the medial femoral epicond ...
(supporting 50–80% of the knee mechanisms in lateral patellar glide), which is torn usually at its femoral attachment. Traumatic patellar dislocation may cause bleeding into the joint space, ligament and muscle attachment tearing, and fracture of the medial wing of the patella. Fracture of the weight-bearing portion of the lateral femoral condyle occurs in 25% of traumatic patellar dislocations. Surgical repair of the patellar stabilizing structures the medial patellofemoral ligament and
vastus medialis The vastus medialis (vastus internus or teardrop muscle) is an extensor muscle located medially in the thigh that extends the knee. The vastus medialis is part of the quadriceps muscle group. Structure The vastus medialis is a muscle presen ...
muscle may be needed for athletes.


Anatomical factors

People who have larger Q angles tend to be more prone to having knee injuries such as dislocations, due to the central line of pull found in the quadriceps muscles that run from the
anterior superior iliac spine The anterior superior iliac spine (ASIS) is a bony projection of the iliac bone, and an important landmark of surface anatomy. It refers to the anterior extremity of the iliac crest of the pelvis. It provides attachment for the inguinal ligament, ...
to the center of the patella. The range of a normal Q angle for men ranges from <15 degrees and for females <20 degrees, putting females at a higher risk for this injury. An angle greater than 25 degrees between the patellar tendon and quadriceps muscle can predispose a person to patellar dislocation. In patella alta, the patella sits higher on the knee than normal. Normal function of the VMO muscle (VMO) stabilizes the patella. Decreased VMO function results in instability of the patella.


Forces

When there is too much tension on the patella, the ligaments will be susceptible to tearing due to shear force or torsion force, which then displaces the patella from its groove. Patellar dislocation may also occur when the trochlear groove is shallow, a condition defined as trochlear
dysplasia Dysplasia is any of various types of abnormal growth or development of cells (microscopic scale) or organs (macroscopic scale), and the abnormal histology or anatomical structure(s) resulting from such growth. Dysplasias on a mainly microscopic ...
.


Mechanism of injury

Patellar dislocations occur by: * A direct impact that knocks the patella out of joint * A twisting motion of the knee, or ankle * A sudden lateral cut


Anatomy of the knee

The patella is a triangular sesamoid bone that is embedded in
tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
. It rests in the patellofemoral groove, an
articular cartilage Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...
-lined hollow at the end of the thigh bone (
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
) where the thigh bone meets the shin bone (
tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...
). Several
ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...
s and tendons hold the patella in place and allow it to move up and down the patellofemoral groove when the leg bends. The top of the patella attaches to the quadriceps muscle via the
quadriceps tendon In human anatomy Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and micros ...
, the middle to the vastus medialis obliquus and
vastus lateralis The vastus lateralis (), also called the vastus externus, is the largest and most powerful part of the quadriceps femoris, a muscle in the thigh. Together with other muscles of the quadriceps group, it serves to extend the knee joint, moving the ...
muscles, and the bottom to the head of the tibia (
tibial tuberosity The tuberosity of the tibia, tibial tuberosity or tibial tubercle is an elevation on the proximal, anterior aspect of the tibia, just below where the anterior surfaces of the lateral and medial tibial condyles end. Structure The tuberosity of ...
) via the
patellar tendon The patellar tendon is the distal portion of the common tendon of the quadriceps femoris, which is continued from the patella to the tibial tuberosity. It is also sometimes called the patellar ligament as it forms a bone to bone connection whe ...
, which is a continuation of the quadriceps femoris tendon. The
medial patellofemoral ligament The medial patellofemoral ligament (MPFL) is one of several ligaments on the medial aspect of the knee. It originates in the superomedial aspect of the patella and inserts in the space between the adductor tubercle and the medial femoral epicond ...
attaches horizontally in the inner knee to the
adductor magnus The adductor magnus is a large triangular muscle, situated on the medial side of the thigh. It consists of two parts. The portion which arises from the ischiopubic ramus (a small part of the inferior ramus of the pubis, and the inferior ramu ...
tendon and is the structure most often damaged during a patellar dislocation. Finally, the lateral collateral ligament and the
medial collateral ligament The medial collateral ligament (MCL), also called the superficial medial collateral ligament (sMCL) or tibial collateral ligament (TCL), is one of the major ligaments of the knee. It is on the medial (inner) side of the knee joint and occurs in ...
stabilize the patella on either side. Any of these structures can sustain damage during a patellar dislocation.


Diagnosis

To assess the knee, a clinician can perform the Patellar Aprehension Test by moving the patella back and forth while the people flexes the knee at approximately 30 degrees. The people can do the ''patella tracking assessment'' by making a single leg squat and standing, or by lying on his or her back with knee extended from flexed position. A patella that slips laterally on early flexion is called the , and indicates imbalance between the VMO and lateral structures. On
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
, with skyline projections, dislocations are readily diagnosed. In borderline cases of subluxation, the following measurements can be helpful: * The ''lateral patellofemoral angle'', formed by: :*A line connecting the most anterior points of the medial and lateral facets of the trochlea. :*A tangent to the lateral facet of the patella. :With the knee in 20° flexed, this angle should normally open laterally. * The ''patellofemoral index'' is the ratio between the thickness of the medial joint space and the lateral joint space (L). With the knee 20° flexed, it should measure 1.6 or less. File:Lateral patellofemoral angle.jpg File:Lateral and medial joint space of patella.jpg


Prevention

The
patella The patella (: patellae or patellas), also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in m ...
is a floating
sesamoid In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Greek word for 'sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be presen ...
bone held in place by the quadriceps muscle tendon and patellar tendon ligament. Exercises should strengthen quadriceps muscles such as
rectus femoris The rectus femoris muscle is one of the four quadriceps muscles of the human body. The others are the vastus medialis, the vastus intermedius (deep to the rectus femoris), and the vastus lateralis. All four parts of the quadriceps muscle attach ...
,
vastus intermedius The vastus intermedius () (Cruraeus) arises from the front and lateral surfaces of the body of the femur in its upper two-thirds, sitting under the rectus femoris muscle and from the lower part of the lateral intermuscular septum. Its fibers end ...
, and
vastus lateralis The vastus lateralis (), also called the vastus externus, is the largest and most powerful part of the quadriceps femoris, a muscle in the thigh. Together with other muscles of the quadriceps group, it serves to extend the knee joint, moving the ...
. However, tight and strong lateral quadriceps can be an underlying cause of patellar dislocation. If this is the case, it is advisable to strengthen the medial quadriceps, vastus medialis (VMO), and stretch the lateral muscles. Exercises to strengthen quadriceps muscles include, but are not limited to,
squats Squat, squatter or squatting may refer to: Body position * Squatting position, a sitting position where one's knees are folded with heels touching one's buttocks or back of the thighs * Squat (exercise), a lower-body exercise in strength and co ...
and lunges. Adding extra external support around the knee by using devices such as knee rthoticsor athletic tape can help to prevent patellar dislocation and other knee-related injuries. External supports, such as knee braces and athletic tape, work by providing movement in only the desired planes and help hinder movements that can cause abnormal movement and injuries. Women who wear
high heels High-heeled shoes, also known as high heels (colloquially shortened to heels), are a type of shoe with an upward-angled sole. The heel in such shoes is raised above the ball of the foot. High heels cause the legs to appear longer, make the we ...
tend to develop short calf muscles and tendons. Exercises to stretch and strengthen calf muscles are recommended on a daily basis.


Treatment

Two types of treatment options are typically available: * Surgery * Conservative treatment (rehabilitation and
physical therapy Physical therapy (PT), also known as physiotherapy, is a healthcare profession, as well as the care provided by physical therapists who promote, maintain, or restore health through patient education, physical intervention, disease preventio ...
) Surgery may impede normal growth of structures in the knee, so doctors generally do not recommend knee operations for young people who are still growing. There are also risks of complications, such as an adverse reaction to anesthesia or an infection. When designing a rehabilitation program, clinicians consider associated injuries such as chipped bones or soft tissue tears. Clinicians take into account the person's age, activity level, and time needed to return to work and/or athletics. Doctors generally only recommend surgery when other structures in the knee have sustained severe damage, or specifically when there is: * Concurrent osteochondral injury * Continued gross instability * Palpable disruption of the medial patellofemoral ligament and the vastus medialis obliquus * High-level athletic demands coupled with mechanical risk factors and an initial injury mechanism not related to contact Supplements like
glucosamine Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of two polysaccharides, chitosan and chitin. Glucosamine is one of the mo ...
and
NSAID Non-steroidal anti-inflammatory drugs (NSAID) are members of a therapeutic drug class which reduces pain, decreases inflammation, decreases fever, and prevents blood clots. Side effects depend on the specific drug, its dose and duration of ...
s can be used to minimize bothersome symptoms.


Rehabilitation

An effective rehabilitation program reduces the chances of re-injury and of other knee-related problems such as
patellofemoral pain syndrome Patellofemoral pain syndrome (PFPS; not to be confused with jumper's knee) is knee pain as a result of problems between the kneecap and the femur. The pain is generally in the front of the knee and comes on gradually. Pain may worsen with sitti ...
and
osteoarthritis Osteoarthritis is a type of degenerative joint disease that results from breakdown of articular cartilage, joint cartilage and underlying bone. A form of arthritis, it is believed to be the fourth leading cause of disability in the world, affect ...
. Most patella dislocations are initially immobilized for the first 2–3 weeks to allow the stretched structures to heal. Rehabilitation focuses on maintaining strength and range of motion to reduce pain and maintain the health of the muscles and tissues around the knee joint. The objective of any good rehabilitation program is to reduce pain, swelling and stiffness as well as increase range of motion. A common rehabilitation plan is to strengthen both the hip abductors, hip external rotators and the quadricep muscles. Commonly used exercises include isometric quadricep sets, side lying clamshells, leg dips with internal tibial rotation, etc. The idea is that because the medial side is most often stretched by the more common lateral dislocation, medial strengthening will add more stabilizing support. With progression more intense range of motion exercises are incorporated.


Epidemiology

Rate in the United States are estimated 2.3 per 100,000 per year. Rates for ages 10–17 were found to be about 29 per 100,000 persons per year, while the adult population average for this type of injury ranged between 5.8 and 7.0 per 100,000 persons per year. The highest rates of patellar dislocation were found in the youngest age groups, while the rates declined with increasing ages. Females are more susceptible to patellar dislocation. Race is a significant factor for this injury, where Hispanics, African-Americans and Caucasians had slightly higher rates of patellar dislocation due to the types of athletic activity involved in:
basketball Basketball is a team sport in which two teams, most commonly of five players each, opposing one another on a rectangular Basketball court, court, compete with the primary objective of #Shooting, shooting a basketball (ball), basketball (appro ...
(18.2%),
soccer Association football, more commonly known as football or soccer, is a team sport played between two teams of 11 Football player, players who almost exclusively use their feet to propel a Ball (association football), ball around a rectangular f ...
(6.9%), and
football Football is a family of team sports that involve, to varying degrees, kick (football), kicking a football (ball), ball to score a goal (sports), goal. Unqualified, football (word), the word ''football'' generally means the form of football t ...
(6.9%), according to Brian Waterman. Lateral Patellar dislocation is common among the child population. Some studies suggest that the annual patellar dislocation rate in children is 43/100,000. The treatment of the skeletally immature is controversial due to the fact that they are so young and are still growing. Surgery is recommended by some experts in order to repair the medial structures early, while others recommend treating it non operatively with physical therapy. If re-dislocation occurs then reconstruction of the medial patellofemoral ligament (MPFL) is the recommended surgical option. In animals, patellar luxation is a common condition in dogs, particularly small and miniature breeds.


References


External links

{{DEFAULTSORT:Patella Dislocation Sports medicine Dislocations, sprains and strains Patella Wikipedia medicine articles ready to translate