Options for Managing Traumatic Patellar Dislocation
Chun-Hao Tsai, Chin-Jung Hsu, Chih-Hung Hung, and Horng-Chaung Hsu
Acute traumatic patellar dislocation is the second most common cause of traumatic hemarthrosis of the knee, accounting for approximately 3% of all knee injuries [1,2].
It typically results from a sports injury, and it occurs about two thirds of the time in young, active patients under the age of 20. Most patellar dislocations are associated with participation in sports and physical activity [3-5]. The long-term impact of patellar dislocation and the resulting patellar instability can lead to considerable pain, recurrence, and even patellofemoral osteoarthritis .
Differentiating between acute primary patellar dislocation and habitual dislocation is important because the treatment approaches recommended in the literature vary. Primary (first-time) patellar dislocation is defined as a clinical entity that usually causes a traumatic disruption of the previously uninjured medial peripatellar structures [3,7,8].
One of the common findings related to acute, primary traumatic patellar dislocations is hemarthrosis of the knee, caused by rupture of the medial restraints of the patella.
Incidence and Risk Factors
The incidence rates of primary patellar dislocation in a population-based setting among adults have been reported [3-5,9]. The average annual incidence for patellar dislocation injuries ranges from 5.8 to 7.0 per 100,000 person-years in the civilian population, and up to 29 per 100,000 person-years in the 10- to 17-year-old age group. The incidence increased to 69 per 100,000 person-years in the military population, who needed to pass physical fitness tests and training requirements of military service . Women are more likely to sustain a patellar dislocation injury than men. Two trends – the highest rates of patellar dislocation injuries in the youngest age group and rates declining with increasing age – have been observed in military and civilian population-based studies [3-5,9]. These findings may be associated with increased activity levels in younger individuals and with a predisposition to anatomic features that render some youth more vulnerable.
Knee flexion and valgus have been noted as the leading mechanisms of injury associated with patellar dislocation, accounting for as many as 93% of all cases . Sillanpaa and colleagues reported that hemarthrosis, medial patellofemoral ligament injury, and medial retinacular disruption were present in most patients with acute traumatic primary patellar dislocation. Osteochondral fractures have also been noted in nearly 25% of acute patellar dislocations .
Results obtained after patellar dislocation have been reported in studies evaluating treatment approaches [10-14], but often no distinction has been made between acute traumatic and recurrent instability. The risk factors could not be adequately determined due to lack of consistent and qualified reports in many articles. Dislocation and recurrent instability occurred in up to 80% of cases and were attributable to predisposing factors of vastus medialis muscle hypoplasia, hyperlaxity of the ligaments [15,16], increased femoral anteversion with compensatory external tibial torsion , trochlear dysplasia [18,19], patella alta [20,21], dysplastic patella, and increased Q-angle with lateralized tibial tuberosity and genu valgum [22,23].
Examination and Diagnostic Study
The initial evaluation of a first-time traumatic patellar dislocation should include an appropriate patient history, family history of patellar dislocation and hyperlaxity, physical examination, and diagnostic studies. Sports (61%) and dance (9%) injuries are two common mechanisms of patellar dislocation. The risk of recurrence increased to six-fold for patients with a history of contralateral patellar dislocation, which was as much as a previous dislocation event on the index knee .
Diagnostic and therapeutic aspiration of the knee joint should be performed in patients with moderate to severe effusions. Joint aspiration performed with/without a local anesthetic can relieve joint depression to achieve patient comfort and improve clinical examination and radiographic assessment (such as 45° flexion Merchant view, 45° flexion weight-bearing view, and 30° lateral view, which are difficult to obtain in patients with an acute hemarthrosis). In addition, the presence of fatty globules may be indicative of an osteochondral fracture. In the acute setting, physical examination is important in making the diagnosis of acute lateral patellar dislocation and for noting any concurrent knee or lower extremity injury . Assessment for malalignment of lower extremities and hypermobility of the contralateral knee should also be included .
Patellar apprehension and mobility should be assessed by medial and lateral patellar translation. Palpation is important in detecting areas of retinacular tenderness and soft tissue injury. Palpable defects in the vastus medialis obliquus (VMO), adductor mechanism, and medial patellofemoral ligament (MPFL) and a grossly dislocatable patella are prognostic factors that may predict poor nonoperative outcomes .
Radiographic examination should include:
- AP extended knee weight-bearing view
- Mercer-Merchant view (45° flexion weight-bearing view)
- 30° flexion lateral view
A Merchant view in a “first-time” traumatic patellar dislocator will show an osteochondral fracture of the medial facet of the patella in a well-aligned patellofemoral joint with no lateral subluxation of the patella (Figure 1). Osteochondral fractures at the medal inferior edge of the patella are highly suggestive of injury pattern. However, reports of surgical and MRI studies indicate that this is missed in 30% to 40% of initial radiographs [26,27].
Figure 1. The plain Merchant axial radiograph demonstrates an osteochondral fracture of the medial facet of the patella in a first-time traumatic patellar dislocation, a secondary sign of an injury to the medial patellofemoral ligament, at the patellar attachment.
CT scan could be used to evaluate the bony predisposing risk factors for dislocation, including:
- Patellofemoral alignment
- Presence of osteochondral defects
- Patellar tilt
- Tibial tuberosity trochlear groove distance
- Trochlear dysplasia 
It is also helpful in evaluating long bone torsional deformities and determining the rotational relationship between the tibial tuberosity and femoral sulcus in varying degrees of knee flexion . The cartilaginous femoral sulcus contour is shallower than the underlying bony sulcus in patients younger than age 18 years. Therefore, measurement of the bony femoral sulcus angle on radiograph or CT scan is less important than measurement of the cartilaginous femoral sulcus angle using ultrasound or MRI . CT scan is also limited in looking at the location and extent of soft tissue defects of the medial patellar stabilizers.
Characteristic MRI findings of patellar dislocation include:
- Joint effusion
- Bruising of the medial patellar facet and lateral femoral condyle
- Osteochondral injury to the medial patella and anterolateral portion of the lateral femoral condyle
A concave impaction deformity of the inferomedial patella is a specific sign of lateral patellar dislocation [7,30]. But there is no correlation can be defined between initial size and size reduction in bone bruise volumetric and the presence/absence or type of associated injuries [31,32].
With the information available on utilizing newer types of magnetic resonance sequencing, MRI is becoming more specific in assisting the surgeon in deciding on nonoperative versus operative management and defining the specific injured structure for surgical repair. MRI assessment is important to evaluate not only the chondral surfaces of the patellofemoral joint, but also the location an extent of soft tissue damage to the medial patellar stabilizers (including medial retinaculum , medial patellofemoral ligament and the vastus medialis obliquus) [33-35]. MRI with the most specificity in visualizing the medial patellofemoral ligament is the primary restraint to lateral subluxation of the patella in early flexion (Figure 2) [29,31].
Figure 2. An axial T2-weighted fast-spin-echo magnetic resonance imaging scan illustrates a eighteen year-old female sustaining a primary traumatic lateral dislocation of the patella while jumping. Complete avulsion of the medial patellofemoral ligament from its femoral insertion can be seen (arrow) (Figure 2A). The bone contusions (stars) (Figure 2B) of the lateral femoral condyle and the medial patellar facet are noted.
The sequelae of a patient`s first patellar dislocation can be harmful and have been well described in the literatures [36,37]. More than 50% of patients have complaints after the first-time dislocation of the patella , and will likely develop some level of osteoarthrosis of the patellofemoral joint on long-term follow-up . Moreover, recurrence rates after primary dislocation can be relatively high, up to 40% .
Proper treatment is essential to minimize sequelae such as recurrent dislocation, painful subluxation, and osteoarthrosis. Whether initial stabilizing surgery for treatment of a primary traumatic patellar dislocation decreases the risk of further instability is still in debate. Results and recommendations on treatment of patellar dislocation vary widely and lack higher quality of evidence. The outcomes of conservative and operative management have been compared in limited studies [10,40]. To our knowledge, only two prospective randomized trials on treatment of acute patellar dislocations have been published in the English-language literatures [41,42]. The redislocation rates are generally high, varying between 10% and 30% for surgical treatment [1,14,23,40,43] and between 13% and 52% for conservative treatment [10-13,40].
Arthroscopy should be performed if chondral injury or osteochondral fracture is suspected. Whenever the osteochondral fracture is greater than 10% of the patella articular surface or part of the weight-bearing portion of the lateral femoral condyle, open repair is recommended, as long as the fragment is amendable to fixation.
Studies have shown a preference for nonoperative treatment of primary patellar dislocations [41,44,45]. Patients with habitual dislocations and patellofemoral symptoms seem to benefit from reconstructive surgery [46,47]. Stefancin and Parker recommended initial nonoperative treatment for first-time traumatic patellar dislocation in their systematic review of 70 articles, unless there are clinical, radiographic, CT, and/or MRI findings of chondral injury, osteochondral fractures, or large medial patellar stabilizer defects .
Few studies in the literature address the nonoperative treatment of the primary patellar dislocation. It is generally agreed that patients should be immobilized initially for comfort (3–4 weeks) to allow immediate weight-bearing as tolerated on crutches after closed reduction of the lateral dislocated patella. Early mobilization is important in maintaining articular cartilage health . Patella-stabilizing braces are used as soon as comfort permits, followed by resisted close-chain exercises and passive range of motion in the brace . The efficacy of physical therapy after the first patellar dislocation on, with either patellar braces or straps, has not been reported in any study.
There is some debate around the best model of knee immobilizers. The effects were compared in patients treated with in a posterior splint, cylinder cast, or patellar bandage/brace . Results showed that the posterior splint group had the lowest proportion of knee joint restriction and lowest frequency of redislocation per follow-up year. In a study using MRI to examine the effect of bracing on patella alignment and patellofemoral joint contact area in skeletally mature women with patellofemoral pain, the On-Track brace and the Patellar Tracking Orthosis (PTO) were shown to increase total patellofemoral joint contact area in comparison to the no-brace control group .
Surgical intervention for first-time traumatic patellar dislocation is indicated in the following situations:
- Evidence on imaging or clinical examination of osteochondral fracture or major chondral injury
- Palpable or MRI findings of substantial disruption of the MPFL-VMO-adductor mechanism
- Laterally subluxated patella seen on the plain Mercer-Merchant view, with normal alignment on the contralateral knee
- Patient fails to improve with nonoperative management, especially in the presence of one or more predisposing factors to patellar dislocation
- Subsequent redislocation 
In a prospective, randomized, controlled study, surgical stabilization significantly reduced the redislocation rate of primary traumatic patellar dislocation in a young adult population, compared with no surgical treatment .
A high prevalence of medial patellofemoral ligament injury is associated with acute patellar dislocation [42,49,50]. Injuries of the medial patellofemoral ligament and the medial retinaculum, as well as hemarthrosis, are the definite signs of an acute primary traumatic patellar dislocation . It is reasonable, and is becoming more accepted, to consider that large defects or avulsions are not going to heal or result in good functional outcome with closed treatment, especially in individuals with high-level athletic participation and those with evidence of one or more predisposing factors. MPFL injury has been demonstrated as the primary constraint in preventing lateralization of the patella in studies [26,50]. Immediate surgical repair of the injured medial patellar stabilizers, including the vastus medialis obliqus muscle and the medial patellofemoral ligament, is advocated in this situation. In our experience, surgical repair of medial stabilizer with medical plication and arthroscopic removal of osteochondral fragment produces satisfactory result and no recurrence of dislocation at 6-month follow-up.
Primary traumatic patellar dislocation is a common injury in the young active population. The importance of a detailed initial evaluation with MRI cannot be underestimated. Recent studies have proved that initial surgical stabilization of the medial patellofemoral avulsion is beneficial in acute primary traumatic patellar dislocation. Additional prospective randomized comparison studies with longer-term follow up are needed to identify risk factors, such as behavioral factors, strength, neuromuscular control, and postural stability, in high-risk populations.
Chun-Hao Tsai, Chin-Jung Hsu, Chih-Hung Hung, and Horng-Chaung Hsu are from the Department of Orthopaedic Surgery, China Medical University Hospital, School of Medicine, China Medical University, Taichung, Taiwan.
Tsai CH, Hsu CJ, Hung CH, Hsu HC. Primary traumatic patellar dislocation. Journal of Orthopaedic Surgery and Research, 2012;7:21 doi:10.1186/1749-799X-7-21. © 2012 Tsai et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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