Abstract
Introduction
The metal-backed patella was originally designed to address shortcomings found with cemented, all-polyethylene patellae. However, complications relating to an all-polyethylene patella were reported to account for up to half of all knee revisions. At the same time, good fixation with bone ingrowth was observed in both titanium and cobalt chromium porous-coated patellae. The advantages provided by using a metal-backed patella, such as uniform load sharing, decreased polyethylene deformation, and potential for biological fixation, may be unjustly outweighed by the fear of patellar component failure; high rates of failure have not been inherent to all metal-backed patella designs. Over the past decade, we have used a metal-backed patella design with excellent results that may be due largely to the design features of the component. Also, we believe there are certain selection criteria that should be strictly adhered to when implanting metal-backed patellae. Correct selection criteria and improved component design strongly indicate the use of press-fit metal-backed patellae.
Methods
This single-center study was designed to conduct clinical and independent radiographic review of primary metal-backed, press-fit patella patients with a minimum five-year follow-up. Potential patients were recruited from a group of existing metal-backed patella patients within the principal investigator's medical practice. All patients recruited for this study were required to have undergone primary knee replacement surgery at least five years prior to clinical and radiographic evaluation. Patients were included if they had a diagnosis of noninflammatory degenerative joint disease. Patients with a BMI >40 were excluded from this study. Radiographic analysis was conducted by an independent reviewer according to the current Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System. Any radiographs that the reviewer deemed questionable were shown to a second independent orthopaedic surgeon for review, comment, and validation of observations. Kaplan-Meier survivorship was determined for all metal-backed patellae. For survival analysis, only knees with radiographic data were included (74 knees). KSS, WOMAC, and SF-36 scores were calculated also.
Results
Seventy-four patients (88 knees) were enrolled in the study, 31 women (41.2%) and 43 men (58.1%). At the time of surgery, the average age was 59.7 years (range, 40–86 years), and the average BMI was 30.6 (range, 19.1–39.6). The breakdown of patients who completed the study and those who were lost to follow-up is shown in Table 1. One metal-backed patella was revised at 49 months for loosening at the bone/implant interface. Survivorship of the metal-backed patellae at minimum five-year follow-up was estimated to be 93.95% with bounds of 73.61% and 98.74%. No radiolucencies greater than 1 mm were observed in any radiographs (Fig. 1), with the exception of the one revision case.
Conclusion
Our experience with this metal-backed patella design has been excellent. Failure does not occur due to dissociation of the plastic. As the porous coating is almost under constant compression, biological fixation is assured in most instances, as confirmed by our minimum five-year radiological results. Improved component design and adherence to the correct patient selection criteria absolutely indicate the use of press-fit metal-backed patellae.
