Although well described in case reports in the literature, intra-articular dislocation of the patella is an uncommon phenomenon. The dislocation can involve rotation about either the horizontal (as in this case) or vertical axis [1
]. The quadriceps mechanism may rupture completely or partially in order to facilitate the dislocation [2
]. The mechanism of injury is usually a direct blow onto the patella with the knee flexed. Two age groups are typically involved, adolescents and the elderly. In adolescents, it is important to be aware of the possibility of a concomitant sleeve fracture of the superior pole in association with the patellar dislocation [3
Recent case reports of inferior dislocation have mostly involved elderly females and closed reductions were possible [4
]. Choudhary reported a similar case in a 92-year-old lady, but following general anaesthesia, a closed reduction was possible [8
]. The authors suggested that if the patellar rotation was less than 90°, then a closed reduction was appropriate. In our case, chronicity of the event may have played a part in failure to achieve a closed reduction.
The quadriceps tendon is inserted into the superior pole of the patella and is composed of 3 distinct layers. The superficial lamina is derived from rectus femoris; the intermediate lamina is derived from vastus lateralis and vastus medialis; and the deep lamina is derived from vastus intermedius. The three laminae are however firmly fused through the interlinking of their tendinous fibres. An extension of the fibres continues over the anterior surface of the patella and blends distally with the patellar ligament.
A direct blow onto the proximal pole of the patella with the knee flexed can force the patella to rotate about a horizontal axis. The deep fibres of the quadriceps tendon are avulsed allowing the patella to hinge on the intact superficial fibres. The quadriceps then prevents spontaneous reduction, and, in the elderly, superior pole osteophyte can become locked in the intercondylar notch.
It is important to maintain a high index of suspicion for these uncommon injuries. Early specialist referral is advised to follow diagnosis. Reduction is usually possible by closed manipulation either under sedation or general anaesthesia. Prognosis is good and patients should return to previous mobility levels.