Purpose: Patellar complications are among the most frequent after total knee arthroplasty. Encasing the patellar piece is one way of resisting the shear forces leading to loosening.

Material and Methods: We studied at more than five years the results obtained with a total knee prosthesis implanted with preservation of the posterior cruciate ligament (PCL). This prosthesis has an asymmetric encased patellar insert with a cemented central pivot. The instrumentation ensures patellar thickness. We reviewed 104 implants at more than five years. Six had been lost to follow-up. Ninety-eight implants were still in place.

Results: The following complications were observed: four fractures of the upper rim with little displacement (these fractures healed and pain regressed but the insert had moved); three vertical patellar fractures with little displacement (these fractures healed; two were symptomatic temporarily); one transverse fracture of the upper pole with displacement causing a defect in active extension; eight moderate asymptomatic impactions which were visible on the lateral x-ray (modified orientation of the insert with cement fracture). There was no significant difference for functional results (pain 40.9; movement 21.9; knee score 84.3) between patients with or without a patellar complication.

Discussion: Insertion of an asymmetric prosthesis increased the risk of an orientation error (two cases early in our experience). Encasing the patellar insert limits medialisation yet the centering was satisfactory (centred patella 95.2%, shift 3.6%, subluxation 1.2%). Encasing provides a peripheral wall protecting against transverse sheer forces. The lateral wall did not fracture, demonstrating its efficacy. The upper wall can fracture under the force of flexion without functional consequences. The other fractures, favoured by section of the lateral patellar wing (p< 0.05), were not treated. Moderate but certain impaction was noted in eight cases at a mean 3.5 years (1–6 years). It was due to failure of bony support under the effect of the compression forces applied on a small surface. The diameter of the encased patellar inserts was rarely more than 25 mm. Once the prosthesis is in place, the periphery of the patella is the only component articulating with the trochlea and its impaction does not cause further aggravation. This contact did not lead to pain in any patients.

Conclusions: Complications observed with encased patellar components differ from the better known apposed prostheses.

Purpose: Patellar complications are among the most frequent after total knee arthroplasty. Encasing the patellar piece is one way of resisting the shear forces leading to loosening.

Material and methods: We studied at more than five years the results obtained with a total knee prosthesis implanted with preservation of the posterior cruciate ligament (PCL). This prosthesis has an asymmetric encased patellar insert with a cemented central pivot. The instrumentation ensures patellar thickness. We reviewed 104 implants at more than five years. Six had been lost to follow-up. Ninety-eight implants were still in place.

Results: The following complications were observed: four fractures of the upper rim with little displacement (these fractures healed and pain regressed but the insert had moved); three vertical patellar fractures with little displacement (these fractures healed; two were symptomatic temporarily); one transverse fracture of the upper pole with displacement causing a defect in active extension; eight moderate asymptomatic impactions which were visible on the lateral x-ray (modified orientation of the insert with cement fracture). There was no significant difference for functional results (pain 40.9; movement 21.9; knee score 84.3) between patients with or without a patellar complication.

Discussion: Insertion of an asymmetric prosthesis increased the risk of an orientation error (two cases early in our experience). Encasing the patellar insert limits medialisation yet the centering was satisfactory (centred patella 95.2%, shift 3.6%, subluxation 1.2%). Encasing provides a peripheral wall protecting against transverse sheer forces. The lateral wall did not fracture, demonstrating its efficacy. The upper wall can fracture under the force of flexion without functional consequences. The other fractures, favoured by section of the lateral patellar wing (p< 0.05), were not treated. Moderate but certain impaction was noted in eight cases at a mean 3.5 years (1–6 years). It was due to failure of bony support under the effect of the compression forces applied on a small surface. The diameter of the encased patellar inserts was rarely more than 25 mm. Once the prosthesis is in place, the periphery of the patella is the only component articulating with the trochlea and its impaction does not cause further aggravation. This contact did not lead to pain in any patients.

Conclusions: Complications observed with encased patellar components differ from the better known apposed prostheses.