**2. Patellofemoral instability: acute dislocation of the patella**

Acute dislocation of the patella is an orthopedic disorder of the knee that frequently affects adolescent and young adult population (peak age of 10 to 20 years). Acute patellar dislocation accounts for 2–3% of all knee injuries. It has been reported that there is a 17–49% risk of redislocation following first-time, acute patellar dislocation [1]. The risk increases to 44–71% following a second-time dislocation [2]. Acute traumatic patellar dislocation (with or without associated osteochondral fracture) is the second most frequent cause of traumatic hemarthrosis of the knee, after anterior cruciate ligament tear. The patella usually dislocates laterally, causing ruptures of the MPFL in about 90% of the cases.

#### **2.1 Studies on the natural history of acute dislocation of the patella**

Hawkins et al. [1] have reported on the natural history of acute patellar dislocations. The authors of this study reviewed 27 patients who sustained primary dislocations of the patellae. Of these 27 patients, 20 were treated with immobilization and subsequent physical therapy (including nine patients who underwent arthroscopy) and seven with immediate surgical stabilization and lateral release. In this study, the patients with predisposing factors such as patellofemoral malalignment, abnormal patellar configuration, and a history of prior symptoms of patellofemoral instability were more prone to recurrent dislocation and may benefit from operative intervention. These authors noted that at least 30–50% of all patients having sustained a primary patellar dislocation will continue to have symptoms of instability and/or anterior knee pain.

Atkin et al. [3] prospectively studied the characteristics and early recovery of an unselected population of patients who had acute, first-time lateral dislocation of the patella. Seventy-four patients (average age 20 years) met the enrollment criteria. A standardized rehabilitation program was utilized, emphasizing range of motion, muscle strength, and return of function. Patients returned to stressful activities (including sports) as tolerated when they regained a full passive range of motion in the knee, had no joint effusion, and when quadriceps muscle strength was at least 80% as compared with the opposite, non-injured extremity. Sports participation remained significantly reduced throughout the first 6 months after injury, with the greatest limitations in kneeling and squatting. The patients who had acute primary patellar dislocation were young and active, and most injuries occurred during sports.

Fithian et al. [4] published a prospective cohort study to define the epidemiology and natural history of acute dislocation of the patella, and to identify risk factors for subsequent patellofemoral instability episodes. These authors prospectively followed 189 patients for a period of 2 to 5 years. The overall annual risk for a first-time patellar dislocation was 5.8 per 100,000 members, with 61% of injuries occurring during sports. There was an increasingly higher incidence of patellar dislocation in younger and female patients. The annual risk for patients with a previous history of patellar subluxation or dislocation was 3.8 per 100,000 members, with a statistically higher proportion of older and female patients.

#### **2.2 Anatomy**

Warren and Marshall [5] have delineated the anatomy of the medial aspect of the knee. These authors dissected 154 fresh human knee joints and found a consistent three-layered pattern with condensations between the tissue planes.

#### *Patellofemoral Instability DOI: http://dx.doi.org/10.5772/intechopen.99562*

The fibers of MPFL were transversely oriented within layer II, superficial to the joint capsule and deep to the vastus medialis. Since then, various studies have been reported on the anatomy of the MPFL [6–21]. Schottle and associates [22], in their landmark cadaveric study, defined a radiographic point representing the femoral attachment of the MPFL. This was described on a true lateral radiograph of the knee (with both posterior condyles projected in the same plane), as 2 mm anterior to the posterior cortex extension line, 2.5 mm distal to the posterior origin of the medial femoral condyle, and proximal to the level of the posterior-most point of the Blumensaat's line.

In earlier anatomical dissection studies, the MPFL has been defined as a pure ligament spanning from the medial femoral condyle to the medial border of the patella. However, recent advances in the surgical anatomy of the MPFL have revealed that there are fibers that insert onto the deep, undersurface of the quadriceps tendon as well as the patella, thus earning the name "medial patellofemoral complex" to allow for the variability in its anatomy [23]. The medial patellofemoral complex (MPFC) has been more recently identified as a broad, fan-shaped structure with both bony and soft tissue insertions [24, 25]. The MPFC origin is generally accepted to originate within a triangular saddle of bony landmarks on the medial condyle of the femur, formed by the medial gastrocnemius tubercle, the medial femoral epicondyle, and the adductor tubercle [17, 24, 25]. The insertion of the MPFC is more variable; about 57% of its fibers attach to the patella and the remaining 43% attach to the undersurface of the quadriceps tendon [26]. Fulkerson has described this quadriceps portion of the MPFC as the medial quadriceps tendonfemoral ligament (MQTFL) [27]. The length of the MPFL ranges from 45 mm to 64 mm, and its width is slightly greater at its patellar insertion than its femoral origin [11, 13]. The midpoint of the 30.4-mm-wide insertion of the MPFC has been reproducibly found at the junction of the medial border of the quadriceps tendon with the articular surface of the patella [24, 26].

Tanaka [26] undertook a cadaveric study to describe and quantify the variability of the attachments of the MPFL. In his study, 33 cadaveric knees were dissected, and the MPFL was identified from the articular side after anterior reflection of the extensor mechanism and removal of the synovium. The mean width of the MPFL was 10.7 ± 1.8 mm at the femoral origin and 30.4 ± 5.5 mm at the patellar attachment. Tanaka [26] concluded that MPFL fibers vary in their width and percentage of attachments to the patella and quadriceps tendon. Further research is required to identify the appropriate fixation points to recreate the anatomy and isometry of the MPFL during patellar stabilization surgery for patients with patellofemoral instability.

Aframian et al. [17] conducted a systematic review of anatomical dissections and imaging studies to identify the true anatomical origin and insertion of the MPFL. After screening and review of 2045 papers, a total of 67 studies investigating the relevant anatomy were included. The authors found that the origin of the MPFL appears to be from an area rather than a single point (as previously reported) on the medial femoral condyle. The weighted average length of the MPFL was 56 mm with an 'hourglass' shape, fanning out at both ends of the ligament. The MPFL is an hourglass-shaped structure running from a triangular space between the adductor tubercle, medial femoral epicondyle and gastrocnemius tubercle, and inserts onto the superomedial aspect of the patella. **Figure 1** shows the diagram summarizing the femoral and patellar attachment areas of the MPFL. Awareness of anatomy is essential for accurate placement of the graft while performing MPFL reconstruction for patellofemoral instability.

The MPFL has been regarded as the major medial soft tissue stabilizer of the patella (particularly in early knee flexion), originating from the medial femoral

#### **Figure 1.**

*Diagram summarizing the MPFL attachment areas. Darker shading represents study concordance. AT adductor tubercle; AMT - adductor magnus tendon; GT - gastrocnemius tubercle; mGT - medial gastrocnemius tendon; sMCL – superficial medial collateral ligament; MFE – medial femoral epicondyle. Reprinted with permission from: Aframian et al. [17]. Copyright © The Author(s) 2016. Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). No changes we made.*

condyle and inserting onto the proximal two-thirds of the medial border of the patella. The MPFL acts as a primary static checkrein to resist lateral translation of the patella, providing approximately 208 N of mean tensile strength before rupture [6, 28, 29]. Conlan et al. [6], based on their landmark cadaveric study of the knees, reported that the MPFL is the major medial soft tissue restraint that prevents lateral displacement of the distal knee-extensor mechanism, contributing an average of 53% of the total force. The patellomeniscal ligament and associated retinacular fibers in the deep capsular layer of the knee (which were previously thought to be functionally unimportant) in the stabilization of the patella, contributed an average of 22% of the total force. The patellotibial band and the medial patellotibial ligament are less important restraints to lateral translation of the patella. The quadriceps functions as a dynamic stabilizer of the patella.

A number of anatomic risk factors have been associated with acute dislocation of the patella (**Table 1**). These risk factors become increasingly important when evaluating patients with recurrent patellofemoral instability.

#### **2.3 Clinical presentation**

Most acute patellar dislocations occur during sport. Sporting injuries account for 61–72% of acute patellar dislocations [3, 4]. Acute dislocation of the patella can


#### **Table 1.**

*Anatomic Risk Factors Associated with Acute Dislocation of the Patella.*

occur either by a direct blow to the knee or indirectly, as the body rotates around a planted foot. The player may sense that "kneecap is out of place", but often the patella will dislocate and spontaneously reduce. If the patella remains dislocated, it may be palpable over the lateral aspect of the femur, and the medial femoral condyle appears prominent. The indirect mechanism of injury is more common than a direct blow. This mechanism is noncontact and occurs with the knee in slight flexion and valgus as the tibia externally rotates relative to the femur. It can occur on a planted foot as the femur and body rotate internally, such as the hind leg of a baseball player swinging hard at a pitch. Alternatively, the free foot can be forced into external rotation, such as a soccer player whose instep kick is met with excessive resistance, or a snow skier whose ski acts as an offending lever arm [30]. Patellar dislocation can occur in various sports, such as American football, soccer, baseball, basketball, ice hockey, gymnastics, wrestling, tennis and golf. **Figure 2** demonstrates the indirect (noncontact) and direct (contact) mechanisms of injury that can result in acute dislocation of the patella.
