**3. Patella clunk syndrome**

This condition was first described by Insall in 1982 who termed it as "peripatellar nodule" caused by peripatellar soft-tissue impingement against the anterior margin of the intercondylar box of the femoral component". The term "patellar clunk syndrome" however was coined by Hozack in 1989 who described the pathology as a prominent fibrous nodule at the junction of the proximal patellar pole and the quadriceps tendon which wedged into the inter-condylar notch during flexion and dislodged during extension, generating the symptoms. Thorpe and Bocell described a syndrome of similar presentation in 1990. The symptoms they described were "painful and usually visible popping, catching, or locking in the patello-femoral articulation as the knee was brought from flexion to extension." They used the term "tethered patella syndrome" to describe this condition. Condition described by Insall, Hozack, and Thorpe is within the spectrum of the same disease entity. It was caused by peripatellar fibrous hyperplasia, especially prominent in the suprapatellar region and the lateral parapatellar gutter. It was actually a spectrum of disease, which ranged from painful crepitation to full-blown patellar clunk syndrome.

### **3.1**

The exact cause of patellar clunk syndrome had not been identified. Most authors believed that it was multi-factorial. The design of prosthesis, extent of surgical trauma, change in joint line, patellar height, patellar thickness, and abnormal patellar tracking has been proposed as possible causes. The presence of unilateral patellar clunk syndrome in a patient with bilateral TKA of the same prosthesis provided a good model in examining this complex situation as some of the variables were controlled (i.e. same patient, same disease, and same prosthesis). The presence of excessive peri-patellar fibrosis is a prerequisite of this syndrome.
