What Is Patellar Instability

Overview 

Your kneecap, also known as the patella, acts as a protective shield for your knee joint, usually staying in its place to maintain stability as you move. However, there are times when things don't go as planned, and the patella doesn't stay put. This is what we call patellar instability.

Think of your knee as a door, and the patella as the doorstop. Normally, the doorstop keeps the door stable, but if it's loose or the doorframe isn't stable, the door might unexpectedly swing open. Similarly, if the patella isn't securely in its groove, it can pop out or feel unstable, causing discomfort and difficulty in movement.

Patellar instability can have different causes. It might result from an injury, like a fall or a hit to the knee. Alternatively, it could be because the muscles and ligaments around the knee aren't doing their job correctly, allowing the patella to misbehave.

Symptoms may include pain, swelling, and a feeling that your knee might give way. It's akin to being uncertain about your footing, which can be quite unsettling.

Let's uncover more about patellar instability.

Introduction

Patellar instability is a common condition observed by orthopaedic surgeons and physiotherapists, with an overall frequency of five to seven per 100,000, but can be as high as 29 per 100,000 in the 10-17 year old age group. Following the first patellar dislocation, there is a 15-44% likelihood of recurrence and a 50%, if not higher, chance of continued recurrence following a second dislocation. Even in the absence of recurrent instability, there is a very high rate of ongoing mechanical complaints from the patellofemoral joint; nearly half of patients do not return to their previous level of athletic activity, and more than 70% have some type of limitation.¹

Definition of patellar instability

Patellar instability is defined as a condition in which the patella bone disarticulates from the patellofemoral joint, with either partial (subluxation) or total dislocation. Acute trauma, chronic ligamentous laxity, bone malalignment, or connective tissue dysfunction are all common contributors. Patellar instability can cause painful discomfort, difficulties in fundamental function, and long-term arthritis.²

Patellar instability can be summarised, and each entity will be discussed further below:

• Young patients (10 to 17 years old)
• Acute traumatic episode
• Anatomical pathology - trochlear dysplasia
• Eventual progression to pain, functional decline, and long-term arthritis

Risk factors & causes of patellar instability

Traumatic factors 

The most common mechanism underlying patellar dislocation is a flexion and valgus movement of the knee without direct contact, which accounts for 93% of traumatic patellar dislocations. The majority of patients report a sensation of slippage, acute pain, and subsequent fluid accumulation, which is usually indicative of a knee sprain. True traumatic dislocation occurs when a direct impact moves the patella to the side. The trauma mechanism can be detected by taking the patient’s medical history.³

Non-traumatic factors

Patients who have had a patellar dislocation are more likely to have trochlear dysplasia. Risk variables for recurrent displacement in teenage populations is higher. Age, sex, body mass index (BMI), and patellar alta were not found to be independent factors of repeated patella dislocation. The recurrent dislocation rate was 69% when trochlear dysplasia and open knee growth plates were present.⁴

Patella alta, an abnormally high patella in relation to the femur, appears to be associated with patellar instability and may be present in 50% to 60% of instances with a primary dislocation. The prevalence of patellofemoral height indexes reflects their inadequacies. On a simple radiograph, measuring patellar height in a child's more cartilaginous knee is challenging. This could explain why the study by Lewallen et al (2013), discovered that patellar alta assessed on a plain radiograph was not a risk factor for patella dislocation. Patellar alta is often associated with trochlear dysplasia.⁴

Patella alta, which causes soft tissue impingement/conflict of the lateral proximal patellar tendon and adjacent retinaculum/fat pad as these anatomical parts rub over the lateral side of the trochlea, has become increasingly recognised in recent years as a major factor in patients with patellar instability.

Patella alta causes articular cartilage wear in the lower region of the patella as a result of greater loading.⁵

Another factor is the medial retinaculum which is the next most essential element in preventing lateral translation of the patella. The medial retinaculum is made up of numerous fibre condensations, the most notable of which is the medial patella-femoral ligament (MPFL).

The transverse fibres of the medial retinaculum (TMR), the medial patellomeniscal ligament (MPML), and the medial patella tibial ligament (MPTL) are other components.⁵

Signs and symptoms

• Patients with partial or total patellar dislocation endure disabling discomfort during activity, limiting their recreational activities and quality of life 
• Anterior knee discomfort is frequently layered with sensations of instability
• Patellofemoral instability patients were previously lumped together with typical anterior knee pain patients under the umbrella name "patellofemoral pain syndrome"⁴

Diagnostic methods

Physical examination

A comprehensive musculoskeletal examination as well as a knee examination will be performed. In addition to general lower limb torsional and angular alignment, features of skeletal dysplasia, syndromes, and joint hypermobility are noted. A thorough examination of the knee should rule out any other possible causes of instability. Patellar height, tracking, and fear can all be used to determine the severity of instability.²

Imaging techniques (X-rays, MRI)

Plain radiographs of all patients with recurrent patellar instability should include an AP view, a true lateral in slight flexion with the posterior femoral condyles perfectly overlapping, and a single skyline view in 20-30 degrees of knee flexion with the beam as parallel to the femur as possible. Additional views of the skyline at 60 and 90 degrees of knee flexion are useless.³

Treatment options

Non-surgical approaches

Following completion of the appropriate imaging examinations, nonoperative treatment for a first-time dislocation consists of limiting movement with a hinged knee brace and subsequent rehabilitation. Patellar dislocations with osteochondral fractures, as well as cases of recurrent instability, are exceptions to this rule. Surgical options (covered more in this issue) are being examined for these patients.

Immobilisation in a cast or brace in extension with restricted weight-bearing has been used to treat acute patellar dislocation. Immobilisation causes quadriceps wasting and knee stiffness, which is followed by fear-avoidance for the patient, loss of confidence and function, and a potential recurrence of dislocation. These treatments are ineffective, and patients should be treated with early mobilisation and weight-bearing as tolerated, an elasticated bandage and ice therapy to minimise swelling, as well as rapid physiotherapy referral.

Reducing pain and edema following an injury is critical for quadriceps re-activation and return to function. In the event of a repeat dislocation, the patient is encouraged to stretch the leg to realign the patella and avoid unneeded surgery, in order to return to routine activities as soon as possible.²

Following an injury, progressive closed-chain eccentric exercises enhance the recovery of activity and strength. This should include gluteal and core muscle control, as well as quadriceps and hamstring strengthening. An elastic resistance band is a useful tool for strengthening the quadriceps in terminal extension. Recruitment of the vastus medialis oblique and quadriceps complex after injury is difficult in the presence of edema. It is also critical to recognise and treat any hip weakness.²

Surgical interventions

Two approaches to surgical therapy of patellar instability can be taken. One school of thought holds that patellar instability is a distinct issue that can be addressed with a single procedure. The other strategy entails identifying and fixing any individual anatomical causes that are causing the dislocation.² 

Three major surgical methods are used to treat patellar instability: MPFL reconstruction, tibial tubercle transfer, and trochleoplasty.Patients may require a mix of these techniques, and only in rare cases would osteotomy surgery be required to address aberrant valgus or rotation.³

Rehabilitation and recovery

Promoting vastus medialis oblique (VMO) and glute exercise is an important part of physical therapy for patellar instability. Exercises to strengthen the VMO and gluteal muscles are included in physical therapy and home exercise programs. VMO strengthening aids in the stabilisation of the patella and the reduction of swelling (oedema). During weight-bearing exercises, patients with weak gluteal muscles have increased adduction and internal rotation of the femur, which can exacerbate patellar instability. Strengthening the gluteal muscles can aid in increasing hip external rotation, which may reduce patellar instability. Closed-chain exercises have been proven to be more effective than open-chain workouts in activating all four quadriceps muscle segments, potentially leading to better outcomes in these patients.

Weight-bearing recommendations, quadriceps-strengthening exercises, range-of-motion targets, brace implementation, and formal physical therapy programs are all part of the rehabilitation process. Weight-bearing recommendations are by far the most important consideration, with 27 (69.2%) studies restricting postoperative weight-bearing.⁵ 

Summary

Patellar instability is strongly dependent on the relative architecture of the patellar components as well as the bony alignment of the knee joint and lower limb. While most patients with first-time dislocations may not require surgery, patellar instability frequently necessitates surgery in cases of repeated dislocation and lingering pain.⁵