Orthopedics

Irreducible Lateral Dislocation of the Patella: A Case Report

James Benjamin, MD; E C Percy, MD, FRCS(C)

Abstract

ABSTRACT: Acute lateral dislocation of the patella is a common clinical entity. The dislocation often reduces spontaneously or with the assistance of the patient or someone in attendance at the time of the injury. Very occasionally the patient presents to an emergency room with the patella still dislocated, and it is usually easily reduced at that time without an anesthetic by a physician. It is extremely rare to find an irreducible lateral dislocation of the patella, which cannot be reduced by manipulation even with a general anesthetic. A case is herein reported of an irreducible lateral dislocation of the patella that required open reduction followed by soft tissue reconstruction.

Abstract

ABSTRACT: Acute lateral dislocation of the patella is a common clinical entity. The dislocation often reduces spontaneously or with the assistance of the patient or someone in attendance at the time of the injury. Very occasionally the patient presents to an emergency room with the patella still dislocated, and it is usually easily reduced at that time without an anesthetic by a physician. It is extremely rare to find an irreducible lateral dislocation of the patella, which cannot be reduced by manipulation even with a general anesthetic. A case is herein reported of an irreducible lateral dislocation of the patella that required open reduction followed by soft tissue reconstruction.

Introduction

Reports of irreducible intraarticular,1-8 superior,9,10 and even intracondylar patellar dislocations11 are commonly reported in the orthopedic literature. Irreducible lateral dislocations have been reported, but with less frequency.11-'3 A case of an irreducible lateral dislocation of the patella is herein reported.

Case Report

The patient, a well-nourished 38-year-old woman, gave a history of having had two previous lateral dislocations of the left patella. The first had occurred some four years prior (o this admission, in a fall while hiking. The second occurred two years prior to admission, in a motor vehicle accident. Both reduced spontaneously at the time of the incident, neither episode having been treated other than with "physical therapy" following the second dislocation. The day of this admission the patient was climbing from a ladder onto the roof of her home . She swung her left leg in the fully extended position onto the roof while maintaining her body weight on her right foot on the ladder. In attempting to boost herself up onto the roof she noted the sudden onset of pain and inability to flex her left knee. The patient was brought to the hospital by paramedics who had had to lift her from the roof with a cherry picker.

Fig. 1: Photograph of clinical appearance of the knee following inability to reduce the dislocation.

Fig. 1: Photograph of clinical appearance of the knee following inability to reduce the dislocation.

Clinical assessment demonstrated a lateral dislocation of the left patella with the knee locked in about 10° of recurvatum (Fig. 1). The patella was displaced in the coronal plane with approximately 10° of external rotation on its longitudinal axis. The contralateral knee demonstrated genu valgum, recurvatum, and an increased Q angle with a small hypermobile patella. Radiographs confirmed the lateral displacement of the patella with no evidence of an osteochondral fragment in the joint (Fig. 2).

Several attempts at closed reduction of the patella both in the emergency room with analgesia and in the operating room under general anesthetic were unsuccessful. An arthrotomy was carried out using a medial parapatellar approach (Fig. 3). The medial capsule was incised from vastus medialis to the insertion of the patellar ligament. The patella reduced spontaneously when the inferior medial capsule was incised. At the time of arthrotomy there were no capsular rents, osteochondral fragments or hemarthrosis appreciated. The articular margin of the lateral femoral condyle was abraded, and there was a small notch under the lateral edge of the lateral femoral condyle (Fig. 4). The medial edge of the patella had been lodged in this notch. A lateral release and medial capsular reefing was performed, and the knee was immobilized in full extension in a Jones dressing for three weeks. During this time the patient performed isometric quadriceps and hamstring exercises.

Fig. 2: AP and lateral x-rays of the lateral dislocation of the patella in the irreducible position.

Fig. 2: AP and lateral x-rays of the lateral dislocation of the patella in the irreducible position.

When last seen, six months after surgery, the patient had full range of motion and had returned to her usual sedentary work.

Discussion

Lateral patellar dislocation is a common clinical entity. This type of dislocation of the patella generally reduces spontaneously following the traumatic episode as the patient or a bystander straightens the flexed knee.8,1318 The mechanism of dislocation is generally wim the foot planted and the femur internally rotated on the flexed knee. It may be associated with a blow on the medial aspect of the knee such as a direct contusion from a fall. As a general rule the dislocation occurs when the knee is slightly flexed and when the patella is just entering the intercondylar groove. In this position the vastus lateralis may play a role, exerting a dynamic displacing force on the lateral aspect of the patella.

In previous reports of irreducible lateral dislocation there has always been an associated rotation of me patella on its longitudinal axis.1113 This rotation resulted in the articular surface of the patella facing laterally with the anteromedial surface trapped behind the lateral femoral condyle. Closed reductions were obtained in several cases, but in those reduced surgically there was extensive capsular disruption along the medial patellar margin. In this case there was minimal patellar rotation, resulting in the medial edge being locked behind the lateral femoral condyle. At the time of arthrotomy the capsule was intact and not torn away from the medial edge of the patella; in fact, it formed a tethering band holding the patella in the dislocated position.

The patient's previous dislocations were no doubt precipitated by her congenital underlying patelJar malalignment syndrome. This consisted, as noted, of genu valgum, genu recurvatum, an increased Q angle, and a small high hypermobile patella. Apparently the increased valgus stress on the extended knee, associated with internal rotation of the femur on the tibia, led to this most recent dislocation. As the patella dislocated, its medial edge became incarcerated behind the lateral edge of the lateral femoral condyle. The medial capsule, which was under extreme tension, had not torn, and it tethered the patella in the coronal plane. The vastus lateralis and lateral capsule also probably contributed to holding the patella in this position. The patella was then effectively locked in behind the lateral femoral condyle as noted on the sketch (Rg. 5). On release of the medial tethering force at the time of arthrotomy, the patella easily reduced.

Fig. 3: Photograph of the patella locked in the lateral position, as viewed from the arthrotomy incision on the medial side of the patella.

Fig. 3: Photograph of the patella locked in the lateral position, as viewed from the arthrotomy incision on the medial side of the patella.

Fig. 4: Photograph of lateral recess of the knee joint showing area of impingement of the medial border of the patella.

Fig. 4: Photograph of lateral recess of the knee joint showing area of impingement of the medial border of the patella.

Fig. 5: Diagram of the patella locked in its irreducible position.

Fig. 5: Diagram of the patella locked in its irreducible position.

References

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10.3928/0147-7447-19841201-12

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