Patellofemoral Instability Sachin Ramchandra Tapasvi
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The Epidemiology and Natural History of Patellar DislocationCHAPTER 1

Shantanu Sudhakar Patil
 
INTRODUCTION
Nearly 1 of every 4 painful knees seen by a treating physician has been reported to be of patello-femoral origin. While patellar injury and dislocation are more prevalent in individuals engaged in certain sports and activities, distal femoral developmental deficiencies also predispose to this condition. Here, we will focus mainly on the acute post-traumatic patellar dislocations. A great volume of techniques and procedures have been described to treat this condition surgically with varying results, hence it is necessary to understand the natural progression of this condition, especially in the skeletally-immature patient.
 
INCIDENCE
A high percentage of active sportspersons report knee pain at least once a month.1 Patellar injury and dislocation are commoner amongst certain sports and activities. These athletes initially present with anterior knee pain. In order of descending prevalence, soccer players, weightlifters, runners and swimmers are prone to this condition; while tennis players are not. The presence of knee pain is related to the type, amount and duration of the sports activity.
Gender variance is also seen in the presentation of these symptoms. In the general population, the female-to-male ratio for patellofemoral dysfunction is 2:1. However, in the athletic population, more men than women suffer from acute and recurrent dislocations.
Patellofemoral syndromes are most likely the result of biomechanical imbalances of the kinetic chain, with each individual having an optimal joint-loading limit that is dependent on his or her unique musculoskeletal anatomy, combined with unique neuromuscular patterning. It was initially postulated that genetic factors may be responsible for predisposition to patellar dislocations. Aoyagi and Zhang,2,3 in their studies of populations of similar genetic stock living in contrasting conditions, showed significant differences in the prevalence of anterior joint pain.
 
BIOMECHANICS OF THE PATELLOFEMORAL JOINT
A detailed anatomical description of the patellafemoral mechanism is not mentioned in this section. The soft tissues around the knee comprising of the fibrous capsule, synovial capsule 2and the collateral ligaments create a dynamic and static stabilizing system to keep the patella within the trochlear groove through knee joint motion. Three medial ligamentous structures medial patellofemoral ligament (MPFL), medial patellomeniscal ligament (MPML), and the medial patellotibial ligament (MPTL) provide medial stability to the patella. More often than not, one of these structures is injured or damaged after patellar dislocations.
The physiologic patellar position is maintained dynamically within the trochlea and extensor mechanism by a balance of various forces.
 
Static Stabilizers
The MPFL inhibits lateral patellar translations with the help of the MPML and the MPTL during the initial 20°–30° of knee flexion before the patella completely engages in the trochlear groove.
 
Dynamic Stabilizers
The quadriceps muscles provide the forces required to extend the knee. The vastus medialis obliquus (VMO) is responsible for the medially-directed dynamic stability. Weakness of the VMO allows lateral tracking and deviation of the patella. This in turn can lead to contracture of the lateral ligaments further contributing to greater lateral subluxation.
 
INCIDENCE IN THE ADOLESCENTS AND CHILDREN
Lateral patellar dislocation is the most common acute knee injury in the skeletally immature population and can cause recurrent dislocations and pain. The annual incidence rate of first- time (primary) dislocations in children has been estimated at 29/100000 as compared to 5.8 per 100000 in the general population.4 While a majority of these patients do not experience further instability, rates as high as 15–44% have been reported.5,6 A number of studies have shown the redislocation rates in patients aged less than 15 to be significantly high. Buchner found a 52% rate in children as compared to 26% for the entire population.7 Cash and Hughston's studies had the rates at 60% in the less than 14 years age population as compared to 33% in population older than 15 years.8
 
RISK FACTORS
The causes for primary patella dislocations include trochlear dysplasia, patella alta, increased tibial tubercle-trochlear groove (TT-TG) distance as measured on a CT scan, and patellar tilt. Risk factors for recurrent patella dislocation include a young age at primary dislocation, female, and dysplasia of the patellofemoral joint.
 
CLINICAL PRESENTATION AND PHYSICAL EXAMINATION
In primary patellar dislocations, hemarthrosis is nearly always present barring a case of severe trochlear dysplasia or hyperlaxity of tissues. Associated knee injuries, such as anterior cruciate or medial collateral ligament tears may be present depending on severity of the injuries. Recurrent dislocations may not cause extensive hemarthrosis if associated with low-energy trauma.
The apprehension test to lateral patella translation is positive, accompanied with pain and difficulty while performing any active quadriceps activity. Injuries or tears to other ligaments 3should be ruled out. If patella dislocates with every flexion arc, a significant injury to the medial restraints of the patella must be suspected.
 
INVESTIGATIONS
Plain radiographs including an anteroposterior, true lateral and an axial view should always be obtained. Patient is evaluated for the associated risk factors including patella alta, and dysplasia of the trochlea. Large osteochondral fractures can also be seen. In more severe injuries a MRI scan must be obtained to identify chondral damage, ligament tears, MPFL disruption.
 
NATURAL HISTORY
It is important to know how to treat the patient who presents for the first time with a traumatic patellar dislocation. Though multiple studies have been published comparing the results of operative versus nonoperative treatment, very few of them are randomized controlled trials (RCT). Buchner retrospectively reported on 126 patients at mean 8.1 years following primary dislocation. 37 of these had immediate surgical reconstructions. No significant differences were found in redislocation rates, activity levels or functional and subjective outcome measures. Sillanpää has also presented the results of treating patients with or without surgery at a 7-year follow-up.9 In this randomized prospective study, forty patients were randomized into initial surgical versus conservative care. The redislocation rate was 27% in the conservative group as compared to 0% in the surgical group. However, the subjective outcome scores and activity levels were the same in both groups. Lind published a RCT comparing nonoperative treatment and repair of MPFL in acute patellar dislocation. 80 patients at a mean of 50 days post primary dislocation were randomized to either bracing or surgery. The redislocation rates were 17% and 20% in the operative and conservative groups, respectively.10 Stefancin conducted a meta-analysis of over 70 papers to review the various treatment options and their success rates.11
Fithian also defined the epidemiology and the risk of subsequent patellar instability and other factors leading to chronic recurrent instability. They identified and prospectively followed 189 patients for 2–5 years to record and reveal potential risk factors for poor outcomes. The first group of 125 patients presented with primary dislocation, 61% following a sports activity. The second group of 64 patients had a prior history of subluxation or dislocation, 41% following sports activity. Both groups were investigated and evaluated with radiologic modalities including MRI to identify anatomical predisposition and chondral and ligamentous injuries. The first group had 17% redislocations compared to 49% in the second group. Patients with prior history had a 7 times higher odds of subsequent instability episodes during follow-up than first-time dislocators. Camanho also published the results of a RCT where the MPFL was repaired at the site of injury as seen on a MRI in the operative group.12 The nonoperative group had a 50% recurrence of redislocation at mean 40.4 months follow-up as compared to 0% in the operative group. These results represent the first published level 1 evidence showing improved outcomes for surgical intervention following primary patellar dislocation.
There must exist sufficient evidence to advocate that initial surgical management to stabilize a primary patellar dislocation will result in an improved outcome for the patient. Currently, no firm evidence is available that indicates that the natural history of the primary dislocator is improved by acute surgical intervention. After a second dislocation event a much higher risk of redislocation (49%) exists and surgical intervention may be considered.
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