Superior dislocation of the patella is a rare diagnosis. A 72 year old woman attended the accident and emergency department of the hospital with a painful right knee after a knock to the knee. Clinical examination and radiographs confirmed a superior dislocation of the patella, which was reduced and closed with the aid of simple analgesia. The authors present the case report and discuss the relevant literature.
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A 72 year old woman attended the hospital with inability to bend her right knee associated with pain. She had been standing and reaching out to grab something three hours earlier, when she felt a sudden pain in her knee. The woman was initially seen in the accident and emergency department where a diagnosis of ruptured patellar tendon was made. When seen by us, she was indeed unable to perform active straight leg raise. However, examination of the knee showed that there was no palpable gap in the patellar tendon. The superior pole of the patella was projecting anteriorly and there was a prominent dimple below the patella.
Plain radiographs showed that the patella was superiorly dislocated with interlocking osteophytes at the inferior pole of the patella and anterior surface of femur (fig 1). After the administration of intramuscular analgesia, the patella was gently moved from side to side and a click was felt. After this the patient was able to actively straight leg raise and flex the knee through its full range. Reduction of the dislocation was confirmed by plain radiograph (fig 2).
The woman was mobilised fully weight bearing without any restriction and at review four weeks later was found to be asymptomatic.
Superior dislocation of the patella is a rare diagnosis. Excluding this case there have been 14 previous such cases reported in the literature.1–12 The average age of these patients is 58 years (range 43 to 81 years) and the ratio of male to female cases is equal. Two of these cases have been reported within the past year.1,2 As previously predicted, it may well be that the frequency of this condition is increasing given the degenerative nature of the underlying cause and the increasing elderly population in our society.3
The woman in our case reports that she was leaning over a chair to pick something from the floor when the chair slipped and pushed her patella up. This is in keeping with previous cases where the underlying mechanism has been reported as a low energy posteriorly directed force on the inferior pole of the patella with or without eccentric contraction of the quadriceps.4 Atraumatic cases in which the mechanism has been active quadriceps contraction and hyperextension of the knee have also been reported.1,3,5–7
Superior dislocation of the patella needs to be distinguished from patellar tendon rupture.8 Both conditions cause an inability to perform straight leg raise. However, in the case of superior dislocation, the patellar tendon is intact. Also the patella is invariably tilted anteriorly because of the locking osteophyte in superior dislocation of the patella with a characteristic dimple below the patella.
In all previous cases reduction was achieved closed with one exception, which required open reduction after failure of closed reduction.9 In the case presented reduction of the dislocation was achieved without the need for general anaesthesia or sedation. Previously, reduction required general anaesthesia in four cases. The rest were reduced with simple analgesia or sedation. Details of the reduction method are given in eight cases. This invariably entails gentle upward pressure on the inferior pole of the patella or medial-lateral pressure or a combination of both.
In summary this case highlights a rare case of superior dislocation of the patella. It is probable that this condition is increasing in frequency. It is important to distinguish this condition from patellar tendon rupture, which can be done by careful clinical assessment. After diagnosis, superior dislocation of the patella can be reduced closed with simple analgesia or sedation in most cases.
Rashpal Bassi initiated the writing of the report and the literature search. B A Kumar supervised the writing of the report.
Conflicts of interest: none.
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