injury Vol. 26, No. 5, pp. 347-348, 1995

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Acute traumatic patellar dislocation: the importance of skyline views

R. K. Nayak and D. R. Bickerstaff Department of Orthopaedics, Royal Hallamshire Hospital, Sheffield, UK

Injury, Vol. 26, No. 5, 347-348, 1995

Introduction

Acute traumatic dislocation of the patella occurs more commonly in young athletic individuals. These are often reduced at site by paramedical staff or in the Accident and Emergency (A&E) department with the patient under adequate sedation. Standard practice for these injuries include pre- and post-reduction anteroposterior and lateral X-rays of the knee. Following reduction, the knee is immobilized either in a plaster cylinder, brace or splint and a follow-up appointment is made for the next fracture clinic. Further treatment is usually non-operative in the absence of an osteochondral fracture.

We report two patients who were initially treated as having isolated patellar dislocations, but at subsequent review, skyline views’ revealed osteochondral fractures which required immediate operative treatment.

Case reports

Case 1

A 23-year-old man twisted his knee and fell while playing football. He was brought into the A&E department where clinical and radiological examination (AP and lateral X-rays of the knee) showed a lateral dislocation of the patella.

This was easily reduced under Entonox and he was placed in a back splint. When seen in the fracture clinic a week later, skyline views (at LO”, 40”, 60", of knee flexion) of the patella revealed a large osteochondral fracture which was not seen on the API lateral images (Figure I). This fragment, measuring 2 cm x 1.5 cm x 4 mm, was internally fixed the next day with three Herbert screws, the approach being through the torn medial parapatellar structures. (Figure 2 shows the intraoperative picture).

The patient started on continuous passive motion the next day and started non-weight bearing with crutches; 4 weeks later he started to bear full weight. This fracture healed uneventfully and he made a full and complete recovery.

Case 2 A Ii’-year-old boy twisted his knee while playing basketball. The paramedical staff attending the scene reduced the clinically laterally dislocated patella in the ambulance using Entonox. On his arrival in hospital, AP and lateral X-rays of the knee showed a

normal knee joint. The knee was immobilized in a plaster cylinder and he was seen in the fracture clinic a week later.

Examination of the knee after removal of the plaster revealed severe tenderness over the lateral femoral condyle and the presence of a haemarthrosis. After aspiration of the haemarth- rosis, skyline views revealed a fracture over the lateral femoral

Figure I. Skyline view which shows a large osteochondral fragment lying in the lateral parapatellar gutter.

Figure 2. Articular surface of the patella showing defect.

Injury: International Journal of the Care of the Injured Vol. 26, No. 5, 1995

Figure 3. Skyline view showing fracture over condyle.

condyle (Figure 3). Arthroscopy done the next day confirmed a large osteochondral fracture fragment, 2 cm x 2 cm x 2 mm,

lying in the lateral parapatellar gutter. This fragment had arisen from the lateral femoral condyle. This fragment, although large, was mostly cartilaginous with very little attached bone, making fixation impractical - it was therefore removed. The knee was protected with a plaster cylinder for 4 weeks during which time he did not bear weight. At long-term follow up (24 months) the knee function was normal.

Discussion Although it is a rare condition (5 per cent in one study2) all orthopaedic surgeons are aware of osteochondral fractures following acute traumatic dislocation of the patella. How- ever when we presented these two cases to 15 consultants and 20 higher surgical trainees none considered the need for further investigations (apart from AP/lateral views) to rule out an underlying fracture (though three suggested arthroscopy and washout). These are frequently over- looked in the acute stage because initially they may not be grossly disabling and partly because the diagnosis may not be considered.

The fracture runs through subchondral bone and the fragment therefore contains little radio-opaque material

which may not be readily seen on standard AP/lateral views. In the course of dislocation the quadriceps muscles contract in an attempt to restore the situation, exerting considerable force which shears off a portion of the articular cartilage together with underlying bone, as the patella passes over the margin of the lateral femoral condyle. This fragment is commonly from the median articular facet of the patella or from the antero-superior portion of the lateral femoral condyle. This fragment displaces into the joint. Early diagnosis and replacement of this fragment opposing the cancellous bone surfaces provides the best chance of retaining normal congruence of the affected articular surface.

It may be difficult to obtain skyline views in the acutely injured knee due to the effusion and pain; however our patients show that these views can be done early enough to allow for proper surgical treatment. Delay in diagnosis that leads to greater pain and delay in operation is ‘lost time’ from the patients’ point of view. It may also result in the need to dispense with the fragment more frequently than may be the case initially. When the delay is more than

a few weeks, further damage could be done by the ‘loose body’ moving within the joint.

In conclusion, we believe that our experience with the above cases illustrates the importance of ‘skyline’ views in acute traumatic dislocation of the patella.

References 1 Ficat P, Phillipe J and Bizou H. Le defile femoro-patellaire. Rev

Med Toulouse 1970; 6: 241. 2 Rorabeck CH and Bobechko WD. Acute dislocation of the

patella with osteochondral fracture.] Bone joint Surg [Br] 1976; 58B: 237.

Paper accepted 25 January 1995.

Requests for reprints should be addressed to: R. K. Nayak, Consultant Orthopaedic Surgeon, County Hospital, Louth LNIl OEU, UK.