Fracture of the unresurfaced patella after total knee arthroplasty: a report of two cases

Patellar component design influences size selection and coverage

BACKGROUND

Patellofemoral (PF) complications following total knee arthroplasty continue to occur. Outcomes are influenced by implant design, size and alignment in addition to patient factors. The objective of this study was to assess the effect of implant design, specifically round versus oval dome patellar components, on size selected and bony coverage in a population of 100 patients.

METHODS

Intraoperative assessments of patella component size were performed using surgical guides for round and oval designs. Digital images of the resected patellae with and without guides were calibrated and analyzed to measure bony coverage. Lastly, the medial-lateral location of the median ridge was assessed in the native patella and compared to the positioning of the apex of the patellar implants.

RESULTS

In 82% of subjects, a larger oval implant was selected compared to a round. Modest, but statistically significant, differences were observed in selected component coverage of the resected patella: 82.7% for oval versus 80.9% for round. Further, positioning of the apex of oval patellar components reproduced the median ridge of the native patella more consistently than for round components.

CONCLUSIONS

These findings characterized how implant design influenced size selection and coverage in a population of patients. The ability to "upsize" with oval dome components led to increases in bony coverage and better replication of the median ridge compared to round components. Quantifying the interactions between implant design, sizing and coverage for a current implant system in a population of patients supports surgical decision-making and informs the design of future implants.

Spontaneous patella fracture associated with anterior tibial tubercle pseudarthrosis in a revised knee replacement following knee arthrodesis

Background

Conversion of a knee arthrodesis to a Total Knee Arthroplasty is an uncommon procedure. Revision Total Knee Arthroplasty in this setting presents the surgeon with a number of challenges including the management of the extensor mechanism and patella.

Case presentation

We describe a unique case of a 69 years old Caucasian man who underwent a revision Total Knee Arthroplasty using a tibial tubercle osteotomy after a previous conversion of a knee arthrodesis without patella resurfacing. Unfortunately 9 months following surgery a tibial tubercle pseudarthrosis and spontaneous patella fracture occurred. Both were managed with open reduction and internal fixation. At 30 months follow-up the tibial tubercle osteotomy had completely consolidated while the patella fracture was still evident but with no signs of further displacement. The patient was completely satisfied with the outcome and had a painless range of knee flexion between 0-95°.

Conclusions

We believe that patients undergoing this type of surgery require careful counseling regarding the risk of complications both during and after surgery despite strong evidence supporting improved functional outcomes.

Influence of patellofemoral articular geometry and material on mechanics of the unresurfaced patella

Patellar resurfacing during knee replacement is still under debate, with several studies reporting higher incidence of anterior knee pain in unresurfaced patellae. Congruency between patella and femur impacts the mechanics of the patellar cartilage and strain in the underlying bone, with higher stresses and strains potentially contributing to cartilage wear and anterior knee pain. The material properties of the articulating surfaces will also affect load transfer between femur and patella. The purpose of this study was to evaluate the mechanics of the unresurfaced patella and compare with natural and resurfaced conditions in a series of finite element models of the patellofemoral joint. In the unresurfaced analyses, three commercially available implants were compared, in addition to an 'ideal' femoral component which replicated the geometry, but not the material properties, of the natural femur. Hence, the contribution of femoral component material properties could be assessed independently from geometry changes. The ideal component tracked the kinematics and patellar bone strain of the natural knee, but had consistently inferior contact mechanics. In later flexion, compressive patellar bone strain in unresurfaced conditions was substantially higher than in resurfaced conditions. Understanding how femoral component geometry and material properties in unresurfaced knee replacement alters cartilage contact mechanics and bone strain may aid in explaining why the incidence of anterior knee pain is higher in the unresurfaced population, and ultimately contribute to identifying criteria to pre-operatively predict which patients are suited to an unresurfaced procedure and reducing the incidence of anterior knee pain in the unresurfaced patient population.