Patellar instability can is a spectrum of conditions ranging from intermittent subluxation to dislocation. Patients typically present with obvious deformity and an inability to extend the knee. The recurrence rate following a first-time dislocation can be 15 to 60%. Generalized patellar instability is thought to represent up to 3% of clinical presentations involving the knee. In most cases, it can be managed conservatively with physiotherapy and bracing except in the presence of a fracture or recurrent episodes.
Acute patellar dislocations typically occur as a result of trauma, usually a direct blow, often to the medial aspect of the knee, or sudden change in direction. A common mechanism is external tibial rotation with the foot fixed on the ground. There is a group of patients that suffer from chronic laxity and recurrent subluxation of their patella.
Anatomic variation including patella alta and patellar and trochlear dysplasia can also predispose individuals to dislocation. Ligamentous laxity as a result of female gender or secondary to connective tissue disorders such as Marfan Syndrome or Ehlers-Danlos increases the risk of dislocation. Muscular imbalance, particularly weakness of the vastus medialis oblique also contributes.
Habitual dislocations are characterized by painless dislocation every time the knee flexes, which is usually as a result of abnormal tightness of vastus lateralis and the iliotibial band.
Congenital dislocations can occur, the most common association being Down syndrome; this is as a result of a small patella in combination with a hypoplastic condyle and typically requires surgical intervention to reduce it.
Patella dislocations account for approximately 2 to 3% of knee injuries. It tends to affect young, active individuals with adolescent females and athletes at higher risk. Incidence is reported as 5.8 per 100,000 but could be as high as 29 per 100,000 in the adolescent population. Acute dislocations tend to occur relatively equally in males and females. Patella dislocations are most common in the second and third decades of life.
Dislocations tend to occur in a lateral direction which is partly because the direction of pull of quadriceps muscle is slightly lateral to the mechanical axis of the limb. Medial instability is rare and more likely to result from congenital conditions, quadriceps atrophy, or iatrogenically. Intra-articular dislocation is also uncommon but can occur following trauma where the patella is avulsed from the quadriceps tendon and then rotated. Superior dislocations can occur in elderly patients where forced hyperextension causes the patella to lock on an anterior femoral osteophyte.
Individuals with an increased Q-angle are predisposed to dislocation. The Q angle forms from a line drawn from the anterior superior iliac spine (ASIS) through the center of the patella, then a second line drawn from the center of the patella to the tibial tubercle. In females, this measurement is typically higher than males, 18 degrees compared to 14 degrees. A high Q angle measures as 15 degrees in males and 20 degrees in females.
There are static and dynamic stabilizers of the patella. The dynamic stabilizers are the soft tissues. The vastus medialis obliquus (VMO) muscle is the most distal portion of the quadriceps muscle and exerts a medially directed pull which helps to maintain patella position. Weakness or dysplasia of this muscle makes dislocation more likely. The medial retinaculum of the knee joint capsule is reinforced by the medial patellofemoral ligament (MPFL) which prevents excessive lateral movement of the patella and usually suffers damage in acute dislocations.
The static stabilizers relate to the bone. Anatomical factors that predispose to dislocation include patella alta or high riding patella (as it does not articulate with the sulcus, the constraint is lost), trochlear dysplasia, excessive lateral patella tile, lateral femoral condyle hypoplasia — distortion of the normal anatomy results in loss of stability. “Miserable malalignment syndrome” also correlates; this includes femoral anteversion, genu valgum, external tibial torsion/pronated feet.
Patients typically describe pain and deformity of the knee following a direct blow or sudden change in direction in which the knee gives way. They may report feeling a ‘pop.’ For those with long-standing instability, pain is classically anteromedially. They may report giving way, clicking and catching. Symptoms are usually worse on flexion and kneeling.
On examination of an acute dislocation, a joint effusion or haemarthrosis is a typical finding but is less likely in the case of chronic dislocation. It is a frequent cause of haemarthrosis. On general inspection with the patient standing there may be signs of femoral anteversion, patella alta, tibial torsion, genu recurvatum, genu valgum or varum, pes planus and general ligamentous laxity. Next, the knee and surrounding structures should be palpated, covering the superior, inferior, medial and lateral poles of the patella. If pain permits, flexion, and extension should be checked, feeling for any crepitus or restriction of movement. Assessment of the collateral and cruciate ligaments in also necessary.
The J sign may be present; the patella tends to deviate laterally during knee extension. In a normal knee, the patella can be moved medially and laterally between twenty-five and fifty percent the width of the patella. In recurrent dislocators, it may move further (patellar glide). They may have a positive apprehension test whereby the knee is held relaxed in 20 to 30 degrees fo flexion and attempted lateral subluxation. They will be tender over the medial patellar retinaculum and will have a positive patellar apprehension test.
It is necessary to get anteroposterior (AP) and lateral radiographs of the affected knee, as well as axial (sunrise) views. These will help identify fracture, the presence of loose bodies, malalignment or arthritic changes. There can be associated avulsion fracture of the medial patellofemoral ligament (middle third of patella). It will also be possible to identify any potential risk factors for dislocation such as patella alta; there may be disruption of the Blumensaat line whereby the lower pole of the patella should lie on a line drawn anteriorly from the intercondylar notch with the knee flexed to 30 degrees. The Insall-Salvati ratio is the ratio measuring the length of the patella ligament (LL; from the inferior patellar pole to the tibial tubercle) and the patellar length (LP; the longest diagonal length of the patella). A normal ratio is 1.0; a ratio of 1.2 suggests patella alta and 0.8 patella baja. If there is acute patellar dislocation, then this may be evident on the lateral view.
The lateral view can also allow assessment of trochlear dysplasia. The 'crossing sign' is present when the trochlear groove is in the same plane as the anterior border of the lateral condyle, suggestive of flattening. The 'double contour sign' is present if the patient has a convex trochlear groove or underdeveloped medial condyle. It occurs when the anterior border of the lateral condyle is in front of the anterior border of the medial condyle. The Merchant view is for assessment of patellar tilt and is an axial view at 45 degrees of flexion; the sulcus and congruence angles can be examined. Weight-bearing views can be helpful to assess deformity and radiographs of the contralateral knee.
Computer Tomography Scans (CT)
CT scan allows measurement of the TT-TG distance; this is the distance between two perpendicular lines; one from the posterior cortex to the tibial tubercle and one from the posterior cortex to the trochlear groove and is normally less than 20mm. A measurement of 21-40mm is borderline and over 40mm abnormal. They can also show osteochondral fractures.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging is the diagnostic gold standard. If there has been a complete dislocation, there is typically a characteristic bruising pattern of the lateral femoral condyle and medial patella. There may be disruption of the MPFL (usually at the patellar insertion). It can be a helpful medium to diagnose articular cartilage damage on the medial patellar facet and is more sensitive in detecting osteochondral lesions. It can also aid the classification of trochlear dysplasia should it be present. This rating is done using Dejour's classification and has four grades; type A is flatter than normal with a sulcus angle greater than 145 degrees, type B which is flat, type C which is convex, and type D which is convex with a supratrochlear spur.
Acute management of an unreduced patella dislocation is to reduction. This procedure can be done in the emergency department with some sedation as needed. The reduction process involves flexing the hip, applying pressure in a medial direction to the patella while slowly extending the knee. It is also possible to perform with the patient sitting up with legs hanging off the side of the trolley.
The mainstay of treatment for first-time dislocators without evidence of loose bodies or intra-articular damage is analgesia, physiotherapy and activity modification. Bracing in a J brace or patellar stabilizing sleeve may be beneficial short term (3 to 4 weeks) to allow soft tissues to heal. SUbsequent management is by physiotherapy input with an emphasis on quadriceps and vastus medialis oblique strengthening, which is often curative. The patient can be allowed to weight bear as tolerated.
There remains debate on the role of surgical management of acute dislocations. A recent Cochrane review revealed that, although there was some evidence in support of surgical management, the quality of the evidence was insufficient to support a change in current practice. Some surgeons advocate performing arthroscopy if there is high suspicion of osteochondral fracture, with open repair of the fragment if necessary.
Surgical management can be a consideration in several situations:
There is evidence which suggests that early stabilization procedures can reduce the rate of subsequent dislocations but in the absence of clear subjective benefits at long term follow up.
Surgical management is usually via proximal and distal realignment. There are numerous surgical options available:
Arthroscopy +/- open debridement
Arthroscopic or open debridement with removal of any loose bodies may be necessary for displaced osteochondral fractures or loose bodies.
MPFL re-attachment or reconstruction (proximal realignment)
Proximal realignment constitutes reconstruction of the MPFL. In brief, to repair the ligament, a longitudinal incision is made at the border of the VMO, just anterior to the medial epicondyle. The ligament is usually re-attached to the femur using bone anchors. If the patient has had recurrent dislocations, then reconstruction may be necessary by harvesting gracilis or semitendinosus which are then attached to the patella and femur.
Isolated repair/reconstruction of the MPFL is not a recommendation in those with bony abnormalities including TT-TG distance greater than 20mm, convex trochlear dysplasia, severe patella alta, advanced cartilage degeneration or severe femoral anteversion.
Lateral release (distal realignment)
A lateral release cuts the retinaculum on the lateral aspect of the knee joint. The aim is to improve the alignment of the patella by reducing the lateral pull.
Osteotomy (distal realignment)
Where there is abnormal anatomy contributing to poor patella tracking and a high TT-TG distance, the alignment correction can be through an osteotomy. The most common procedure of this type is known as the Fulkerson-type osteotomy and involves an osteotomy as well as removing the small portion of bone to which the tendon attaches and repositioning it in a more anteromedial position on the tibia.
If the patient has patella alta, osteotomy also allows the surgeon to effectively lower the patella, thereby reducing the distance between it and the trochlea groove, which is achievable by moving the tibial tubercle to a more distal position.
Where there is a rotational deformity, derotational osteotomies of the femur may be a therapeutic consideration. These procedures are not appropriate in patients with open growth plates
Trochleoplasty is indicated in recurrent dislocators with a convex or flat trochlea. The trochlear groove is deepened to create a groove for the patella to glide through; this may take place alongside an MPFL reconstruction. Studies suggest it is not advisable in those with open growth plates or severely degenerative joint. This procedure is uncommon except in refractory cases.
Physiotherapy can be necessary for as long as two to three months following an initial dislocation. Physiotherapy is vital as if the medial retinaculum does not heal and the VMO does not adequately rehabilitate, recurrent dislocation will ensue. Studies suggest there is a 20 to 40% risk of dislocating again, with even higher rates following a second dislocation.
It is common for patients to remain symptomatic after the first dislocation. Atkin et al. found that at 6 months, 58% of patients experience limitation in strenuous activities. Although the range of motion was regained, participation in sports was reduced in the first 6 months following injury; kneeling and squatting were particularly challenging. Failure to return to sport occurred in 55%. Maenpaa et al. found that over 50 percent of patients have complications following a first-time dislocation; including redislocation and patellofemoral pain or subluxation.
Those patients with abnormal anatomy triggering dislocations may suffer dislocations on the contralateral side and require surgery there too. Stability, however, tends to increase with advancing age of the patient.
After a second dislocation, studies suggest the risk of dislocation goes up to just under fifty percent.
Complications of the acute dislocation include associated osteochondral fracture, the risk of recurrence, and degenerative arthritis.
General surgical complications include infection, neurovascular injury, deep vein thrombosis or pulmonary embolism.
Complications specific to MPFL surgery include saphenous nerve neuritis and rupture.
Following osteotomy, patients often complain of pain over the screw site. There may be a loss of ability to kneel comfortably. There is also a risk of proximal tibial fracture.
Even following surgery, there remains a risk of recurrent lateral patellar instability.
Postoperative rehab depends on the surgery completed. A lateral release is quickest to recover from with osteotomies taking considerably longer. Recovery from surgery can last from 6 months to a year and physiotherapy is vital after a period of immobilization.
Patients should be discouraged from returning too early to contact sports before they have undergone sufficient physiotherapy to aid the strengthening of surrounding muscles. Slip-on knee braces may be of benefit in the early rehabilitative phase. The importance of adherence to a physiotherapy plan after initial dislocation should be a point of emphasis as good compliance may minimize the risk of further dislocation/operative intervention.
Close liaison between emergency medicine doctors, orthopedic teams, radiology, and physiotherapy is vital to managing patellar dislocations safely. This is especially true in the early stages when additional CT/MRI scans may be required to rule out more serious injuries. Once the patient is on the non-operative management path, physiotherapists are crucial in ensuring patients strengthen muscles correctly as this may be all that is needed to manage an acute dislocation. Working within a multi-disciplinary team will enhance patient care and outcomes.
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