Unstable Kneecap

Unstable Kneecap 2017-08-03T17:49:17+00:00

The information below was taken from our eBook, “The Unstable Patella (Kneecap): Everything You Need to Know to Make the Right Treatment Decision.” For more information on this and our other eBooks, see http://noyeskneebookseries.com/.

Kneecap Subluxation, Dislocation

Instability of the kneecap – or the patella – is one of the most common reasons patients seek medical treatment at the Noyes Knee Institute. Normally, the patella glides smoothly and stays within what is called the trochlear groove as the knee flexes (bends) and extends (straightens). An unstable kneecap comes either completely or partially out of the groove as the knee bends.

The term “patellar instability’ may indicate either a dislocation, where the kneecap comes completely out of its normal position, or a subluxation, where the kneecap only partially moves out and then goes back into its normal position.

There are many potential causes of patellar instability, ranging from a traumatic injury to inherent problems with the patient’s anatomy that predisposes them to this problem.

Acute patellar dislocation is a common traumatic knee injury in children and teenagers. At least one-half of patellar dislocations occur during sports such as basketball, football, and soccer. One study reported that approximately 43 per 100,000 children aged 16 years or younger sustained this injury. Several studies found the risk of patellar dislocation was highest in individuals 10-19 years of age. While some investigations reported that females had a higher risk compared to males, others have questioned this finding. Once this injury occurs, the chance of sustaining future dislocation injuries ranges from 14-57% in adult patients and 36-71% in children and teenagers

Chronic, or recurrent, patellar instability may occur for many reasons and may or may not happen as a result of a previous dislocation injury. There are many factors which affect patellar stability such as the angle of knee flexion, the shape (geometry) of the bones in the knee joint (trochlear groove and lateral femoral condyle), tissues referred to as static patella stabilizers, and tissues called dynamic patella stabilizers. An abnormality, weakness, or imbalance in these factors may cause recurrent patellar dislocation or subluxation episodes.

These persistent problems may result in kneecap pain, damage to the joint lining underneath the kneecap, loss of the ability to participate in sports activities, and even difficulty with activities such as squatting, kneeling, and going up and down stairs. If not treated appropriately, these repeat injuries may result in eventual arthritic damage to the undersurface of the kneecap and the trochlear groove.

What Causes Patellar Instability?

In order to understand how and why the patella may become unstable, a general understanding of the basic anatomy and mechanics of the patellofemoral joint is required.

The patellofemoral joint is the area of the knee where the patella and femur meet. There are many structures that affect the function of the patellofemoral joint in terms of how the patella tracks and absorbs the forces of weight bearing. These structures are collectively referred to as the “extensor mechanism”:

  • Quadriceps muscles (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius)
  • Quadriceps tendon
  • Patella
  • Patellar tendon
  • Medial retinaculum
  • Lateral retinaculum
  • Medial patellofemoral ligament (MPFL)
  • Tibial tubercle

Fig. 1


These structures propel the legs forward when you walk, run, or jump. An injury, deficiency, or anatomic abnormality of any of these structures may result in patellar instability, as we will discuss in detail next.


Factors That Influence Patellar Stability

Patellar stability, defined as the condition in which the kneecap glides normally and stays within the trochlear groove as the knee flexes and extends, is influenced by several factors:

1.  Angle of Knee Flexion and Dynamic Stabilizers

A quadriceps (thigh) muscle contraction straightens the knee by pulling at the patella, which in turn pulls on the tibial tubercle, causing the knee to extend. As the knee flexes, the patella acts like a pulley, causing the kneecap to be pressed into the trochlear groove.

The patella is most unstable in the range of 0-30 degrees of knee flexion (0 degrees indicates that the leg is straight). The quadriceps muscles are relaxed at this angle and the patella is not seated in the trochlear groove. Thus, it can easily be moved either towards the right or left.


Fig. 2

As the knee bends, patellar stability is normally increased due to the combined tensions of the quadriceps muscles and the patellar tendon that pull the patella into the trochlear groove. The vastus medialis oblique (VMO) muscle pulls the patella toward the inside (medial) of the knee joint. The MPFL acts as a check-rein to resist a patella dislocation toward the outside (lateral) portion of the knee joint. The vastus lateralis muscle tends to pull the patella toward the outside (lateral) aspect of the knee.

It is also important to understand the relationship between knee flexion and contact that occurs between the patella and trochlear groove. When the knee is full straight (0 degrees), there is very little contact between the undersurface of the patella and the trochlear groove. Then, from 0-90 degrees of knee flexion, increased contact occurs. At first, contact occurs between the inferior (distal) pole of the patella and the trochlea. Then, as flexion increases to about 45 degrees, the contact area moves toward the central portion of the patella. At 90 degrees of flexion, only the superior (top) region of the patella is in contact with the distal aspect of the trochlear groove. At approximately 120-135 degrees of flexion, only the most medial and lateral parts of the patella come into contact with the lateral and medial femoral condyles.


Fig. 3

Forces on the patella rise to about 3 times body weight when climbing up stairs, 5 times body weight when descending stairs, 7 times body weight during running, and up to 20 times body weight during deep squatting. The quadriceps muscles absorb energy during walking and running.

An alteration caused by excessive weakness, tightness, or imbalances of the dynamic stabilizers may result in patellar maltracking or instability problems, and increase the forces on the kneecap that lead to eventual deterioration of the joint lining.

2.  The Shape of the Trochlear Groove and Lateral Femoral Condyle

When looking at a leg from the front, the quadriceps muscles and tendon do not form a straight line. They point one way above the patella, and another way below the patella. The quadriceps angle (Q angle) is the angle formed by one line drawn from the pelvis anterior superior iliac spine to the middle of the patella, and a second line drawn from the middle of the patella to the tibial tubercle. The normal Q angle in men ranges from 8-14 degrees. In women, the normal Q angle ranges from 11-20 degrees.

Because of the Q angle, the patella has a tendency to be pulled toward the outside (lateral portion) of the knee when the quadriceps muscles contract (tighten). This is where the shape, height, and slope of the trochlear groove is important to help keep the patella in its proper position. Normally, the groove is higher on the outside (lateral side), which keeps the patella from sliding out of position as the knee bends and straightens.

  Fig. 4

There are different shapes, or variations of the trochlear groove. There is what is considered a normal appearance (just shown in the illustration above) and 4 variations called shallow, flat, convex, and cliff. These 4 variations are associated with what is termed trochlear dysplasia, or an abnormal shape of this area of the knee. A shallow or dysplastic trochlear groove that develops in some people allows the patella to move too far side-to-side and may be a source of chronic patellar instability problems.


Fig. 5



3.  The Medial Patellofemoral Ligament


The MPFL is a very important static stabilizer that helps prevent the patella from moving too far laterally out of the trochlear groove. Our understanding of the vital role this ligament plays in patellar stability is fairly recent (within the last 15 years or so). In knees with a large Q angle, the MPFL provides a balancing force that resists the increased lateral pull on the patella.


Fig. 6



The MPFL is always damaged in patellar dislocation injuries, although the extent of the damage varies and does not require surgery initially. Over time, the amount of damage to this ligament may become more of a problem if patellar subluxation or dislocation episodes continue. A second ligament located on the medial side of the knee called the medial patellomeniscal ligament is another important stabilizer.


Some people have excessively tight static stabilizers, such as the medial or lateral retinaculum. Excessive looseness of these structures may also cause problems with patellar stability.

4.  The Hip Muscles

Researchers have recently emphasized the role of the hip muscles in patients with patellar pain and instability. Realize that the hip shares a common bone with the knee – the femur. At the hip joint, the femur connects with the acetabulum of the pelvis and acts as a “ball-and-socket” joint that moves in all directions. At the knee joint, the femur is tightly connected to the tibia through ligaments, tendons, and the joint capsule.

Weakness of the hip muscles (hip abductors and external rotators) may directly impact the knee joint by causing the femur to internally rotate and the knee to go into a knock-knee (valgus) position and the foot to turn outward (pronate), as shown in the photograph below.


Fig. 7

A few studies have shown that patients with patellar pain and instability have weak hip muscles, but it is unclear if this problem was present before or as a result of the patellar pain.

5.  Extensor Mechanism Malalignment

“Extensor mechanism malalignment” is a commonly used phrase by orthopaedic surgeons to describe patellar tracking or instability problems. The term indicates an abnormal relationship between the patella and trochlear groove, which may be due to problems with the static and/or dynamic stabilizers we just discussed.

Some problems associated with soft tissue and muscle weakness or tightness may be resolved with muscle strengthening and flexibility exercises, shoe inserts, anti-inflammatory medications, and restriction from high-impact activities that overload the knee joint. However, there are cases where some of the stabilizers are completely deficient and unable to function properly that require surgery.



Common Risk Factors

The most common risk factors that may increase the risk of patellar instability are:

  • Traumatic patellar dislocation injury
  • Age less than 20
  • Excessive weakness, tightness or imbalance of the dynamic stabilizers
  • Abnormally shaped trochlear groove
  • Deficient MPFL from a prior injury or congenital laxity
  • Family history of patellar dislocation injuries
  • Patellar dislocation opposite knee

In addition, there are other more rare conditions that may influence patellar stability:

  • A rotational malalignment of the femur and tibia (femoral anteversion and external tibial torsion, known as “miserable malalignment”)
  • A patella alta position (where an abnormally long patellar tendon affects the position of the patella)
  • Problems from previous surgery, such as an overly extensive lateral release

The term “miserable malalignment” was first introduced in the medical literature in 1979 by orthopaedic surgeon Dr. Stan James. This type of malalignment, which is inherited, is referred to as a rotational malalignment because the femur at the hip joint is rotating inward while the tibia is rotating outward. These problems become quite pronounced during weight bearing activities. There exist two abnormalities related to the bony alignment: femoral anteversion and external tibial torsion. Femoral anteversion produces an inward pointing, or “squinting”, patella, shown here:


Fig. 8


Patients with this syndrome who have significant patellar pain and frequent dislocation or subluxation episodes may require an extensive operation known as a rotational osteotomy of the femur and tibia. Because this represents a very complex condition, we will not discuss this problem in detail here. It is important to understand that the orthopaedic surgeon must determine if this rotational malalignment is present because if it is, surgery at the knee joint is contraindicated. Instead, the rotation is corrected at the femur and tibia. If a surgeon suspects this problem may be present, a special diagnostic study (called a rotational magnetic resonance imaging

[MRI]) at the hip, knee, and ankle is obtained so that precise measurements may be made.

The Effects of Patellar Instability 

Patellar Dislocation Injuries

Patellar dislocations are categorized as either low-energy or high-energy injuries. In a low-energy injury, only a small amount of force is required to dislocate the patella, such as a sudden twisting injury in a knee with abnormally lax medial patellar ligaments and a dysplasic (flat) trochlea. Anatomic abnormalities that predispose a patient to a dislocation are a frequent finding in low-energy injuries.

On the other hand, a high-energy injury involves much greater force (usually in a knee with normal anatomy), such as that incurred by a direct blow to the patella during a sports activity. It is important to distinguish between low and high-energy dislocation injuries because the expected outcome, extent of damage sustained in the knee joint, and treatment recommendations are different.

Nearly all patellar dislocations occur in the lateral direction, meaning the patella slides out of the groove toward the outside of the knee. The patella may return to the normal position on its own (called a spontaneous reduction), or the patient or a medical professional may have to relocate it back into place. This painful injury is often accompanied by swelling and loss of normal knee motion and requires a medical evaluation

Studies have shown that 50-80% of patients who sustain a high-energy patellar dislocation also suffer an osteochondral fracture, whereby the articular cartilage lining and bone on the undersurface of the patella or femoral condyle are torn and fractured. Osteochondral fractures may require surgery to remove the damaged joint fragments.

The MPFL is always damaged in dislocation injuries and nearly all patients sustain a bone bruise to the patella and lateral femoral condyle. These problems occur much less frequently in patients with low-energy injuries.

Fortunately, most patients who suffer first-time patellar dislocations do not require surgery. However, there are some instances where an operation is necessary soon after the injury. In all patients, appropriate orthopaedic management of the injury is required, including a comprehensive examination, x-rays, and often MRI to determine the extent of the damage to the knee. A course of protection, partial weight bearing with crutches, bracing, and physical therapy is required to restore normal knee and lower extremity function.

In the first few months following a patellar dislocation injury, patients may experience pain, fear, and problems with kneeling, squatting and all sports activities. One study found that 58% of patients had significant problems with strenuous activities 6 months after their injury. While many patients will not experience further patellar instability problems following appropriate management, studies have reported that 14-57% of adult patients and 36-71% of children and teenagers will suffer recurrent dislocation injuries. These patients are at risk for incurring further damage to the stabilizing ligaments and soft tissue structures that surround the patella

Studies have reported that > 90% of patients who suffer from recurrent patellar dislocations develop damage to the articular cartilage joint lining in the patellofemoral joint. Therefore, all efforts should be followed to prevent a recurrent dislocation and further damage to the knee joint. Surgery usually becomes necessary in these cases to restore normal patellar stability and knee function, which should be done before significant joint damage occurs.

Patellar Subluxation Episodes

Patellar subluxation, when the kneecap moves partially out of the trochlear groove during knee flexion and then goes back to its normal position, is less painful but may eventually cause problems if it occurs frequently and is not treated effectively. This problem may happen suddenly without warning during sports or regular daily activities. It may be thought of as a temporary, partial dislocation of the kneecap. Patellar subluxation may happen due to malalignment (anatomical) problems we previously discussed, weak leg and hip muscles, or from damage to soft tissues from a previous patellar dislocation injury.

Recurrent patellar instability results in pain that may be felt under, around, or most commonly, in the front of the kneecap. Patients may experience knee giving-way or buckling (instability) during sports activities that involve twisting and turning, running on uneven surfaces, and even during daily activities such as squatting, kneeling, and going up and down stairs. These problems result in a decrease in physical activity due to lack of confidence in the knee joint.

Cases of patellar subluxation (without a history of a traumatic injury) need to be carefully evaluated to determine the causes of the problem which, in many patients, may be greatly helped with conservative treatment methods. Surgery is delayed to allow for muscle strengthening, balance training, and often a patellar support brace is used during sports. In some patients, the type of sport and/or the level of participation needs to be changed. For instance, sports that involve jumping, twisting, or turning activities may repeatedly aggravate the problem. If conservative methods fail to prevent recurrent subluxation episodes, surgery may become necessary to correct all anatomic abnormalities that exist.

It is important to note that many terms abound in the medical literature and on the Internet to describe patellar pain and instability. These include:

  • Patellofemoral pain syndrome
  • Painful patellar syndrome
  • Patellofemoral pain
  • Runner’s knee
  • Jumper’s knee
  • Housemaid’s knee
  • Anterior knee pain
  • Retropatellar pain
  • Chondromalacia patella
  • Patellofemoral chondromalacia
  • Extensor mechanism malalignment
  • Subluxation
  • Instability

There is considerable confusion about what all of these terms mean and if they represent an actual diagnosis or just a general description of what is believed to be going on with the knee. For instance, we previously discussed the term “extensor mechanism malalignment”, which is a frequent diagnosis made by physicians. We noted that malalignment may be influenced by many different parts of the knee, lower limb, and even hip, and the determination of the exact source(s) is very important to achieve a precise diagnosis and treatment plan.

This is why, in cases of patellar subluxation or recurrent dislocation, it is important for the physician to determine to the best of their ability exactly what is causing the patellar instability in order to treat the problem efficiently and correctly. A diagnosis should indicate all of the problems present, which can only be made after a thorough examination of the knee and entire lower limb that includes x-rays and a MRI. There frequently exists more than one source or cause of patellar instability that requires treatment. At the NKI, we spend a considerable amount of time and obtain all tests required to determine all of the factors that are causing your kneecap problems. Then, an accurate diagnosis will allow a treatment plan to be developed. Here are the possible diagnoses that may be made following the comprehensive examination.

  • Extensor mechanism malalignment: may or may not include excessive weakness, tightness or imbalance of the dynamic stabilizers; excessive Q angle; dysplasic trochlear groove; deficient or congenital laxity of the MPFL
  • Miserable malalignment syndrome (excessive femoral anteversion and external tibial torsion)
  • Patella alta (high riding patella, instability in knee extension)
  • Patella infera (low riding patella due to an abnormally short patellar tendon)
  • Bipartite patella (patella with two pieces, may be associated with tight lateral retinacular restraints)
  • Problems related to failed previous surgery, such as an excessive lateral retinacular release that causes medial patellar subluxation
  • Genu valgum (knock-kneed)
  • Genu varum (bowed leg)
  • Genu recurvatum (hyperextended knee)
  • Increased swelling in the fat pad, causing impingement into the knee joint (Hoffa’s disease)
  • Articular cartilage damage in the patellofemoral joint

Conservative Treatment, Physical Therapy

Nearly all patients who are treated at the NKI for patellar instability undergo conservative treatment consisting of a comprehensive, supervised physical therapy program. They may be prescribed a knee brace or sleeve, shoe inserts or orthotics, a weight loss program, and appropriate medications if swelling is a problem. The good news is that many patients are greatly helped with this program and surgery is not necessary or can be avoided for a long period of time. Our rehabilitation team has decades of experience in treating all of the possible causes of patellar instability and often, the problems of muscle weakness and imbalance are resolved with a dedicated patient and therapist team approach. However, if the patient has rehabilitated to the best of the medical team’s ability and problems persist, surgery may become necessary.

When Surgery is Necessary

The goals of surgery are to correct any malalignment issues, balance soft tissues, and reconstruct torn or deficient ligaments (especially the MPFL). Many different types of operations have been described in the medical literature for extensor mechanism malalignment. The decision of which procedure to perform is based on the number and extent of anatomic problems that exist. This is where the physical examination and either CT or MRI determine important factors such as the Q angle, patellar translation (glide or movement) and tilt, lower limb rotational alignment (femoral anteversion and external tibial torsion), shape of the trochlear groove, the tibial tubercle-trochlear groove (TT-TG) distance, and patellar height (to diagnose a high riding patella [patella alta]; positive J sign). Some of the common operative procedures include:

    • Lateral release
    • Proximal realignment
    • Proximal-distal realignment
    • Reconstruction of the MPFL and proximal realignment


In addition, several types of operations may be done for patients who have significant damage to the knee joint lining (articular cartilage). The goals of treatment are to restore function and allow the patient to resume normal activities, depending on the damage that is present. Another goal is to prevent further damage because this may lead to the necessity of a joint replacement years later. The articular cartilage procedures done at the NKI include:

  • Arthroscopic debridement, abrasion arthroplasty
  • Osteochondral autograft transfer
  • Autologous chondrocyte implantation

When the damage to the articular cartilage becomes severe and, over time, the normal amount of joint space is lost, a partial or total knee replacement may be required. Replacing just the patellofemoral joint is a valid treatment option, especially in patients 40-50 years of age (and sometimes younger) if other operations have failed to alleviate their kneecap pain and the medial and lateral compartments of the knee are not damaged. We have written a separate eBook, “Partial Knee Replacement: Everything You Need to Know to Make the Right Treatment Decision” which discusses the issues of replacing just a portion of the knee joint.

Postoperative Physical Therapy

All patients at the NKI who undergo surgery meet with the physical therapy team well before their operation and are educated regarding the postoperative rehabilitation process. We believe that preoperative counseling is absolutely necessary to ensure the patient understands what is required of them in order to have the best possible chance of achieving a successful outcome.

After surgery, all of our patients begin their rehabilitation immediately within a day or two after the operation. It is vital for all patients to begin working on regaining the ability to bend and straighten their knee and strengthen their leg and hip muscles. Our approach greatly lessens the chance of a complication that may occur if excessive scar tissue forms around and within the knee joint or if a great deal of muscle strength is lost.

In cases where large operations are performed, the patient will need to work for several months to restore normal knee function. Most of the exercises are performed at home or in a health club, but the therapist will guide the progress of the entire program.