Speaking at the joint Specialty Day session of the Knee Society and the American Association of Hip and Knee Surgeons (AAHKS), Jess H. Lonner, MD, of the Rothman Institute, Philadelphia, and Douglas A. Dennis, MD, of the Colorado Joint Replacement Center, Denver, presented different perspectives on the subject of patellofemoral arthroplasty (PFA) (Fig. 1).

Many problems “virtually eliminated”
Speaking first, Dr. Lonner argued that the technology and techniques for performing PFAs have progressed since the early days of the procedure, leading to better patient outcomes and a continuing improvement trend.

“The design has changed,” he explained. “We’re better at selecting the appropriate patients. We have recognized the importance of appropriately rotating the trochlear component. Instrumentation has improved. All these issues are having a significant, positive impact on outcomes after PFA.”

Dr. Lonner supported his position by pointing to a series of generational changes in patellofemoral designs. Early models, he said, were short implants that did not extend proximally beyond the physeal scar. Modern implants, on the other hand, have longer designs that extend farther up the shaft of the femur, helping to engage the patella during the initial 60 degrees of flexion.

“In the earlier designs,” he explained, “the patella had to track from the native cortex anteriorly, up and onto the trochlear component, which predisposed the implant to patellar catching and instability problems.”

According to Dr. Lonner, inlay and onlay designs present different advantages and challenges that have yet to be fully assessed.

“Both designs are currently being used,” he noted. “Inlay designs are more conservative in terms of bone preparation. Onlay designs require a bit more anterior resection to accommodate the variety of morphologic patterns seen in knees.”

This wide variability in the shape and size of the natural knee is an issue with inlay designs. In addition, inlay designs tend not to track as well as onlay designs, given natural trochlear inclination, according to Dr. Lonner.

“If you measure the anterolateral and anteromedial femoral trochlear peaks and compare the inclination relative to the trans-epicondylar axis, all of these designs, even the plastic knees, have internal rotation of the trochlear piece,” explained Dr. Lonner. “So if you implant an inlay-style trochlear component, the component will be internally rotated, which compromises patellar tracking. An onlay design can be positioned perpendicular to Whiteside’s Line for good patellar tracking.”

Dr. Lonner also explained that the techniques for performing PFA have evolved.

“The initial techniques were all freehand,” he said. “By 1995, a hybrid technique had been developed, using an instrument for the anterior cut, but the intracondylar surfaces were still prepared freehand. Fully instrumented systems were introduced in 2007 and provide a much cleaner intracondylar preparation.”

According to Dr. Lonner, these evolutionary changes have had a significant impact on the outcomes of PFA.

“We’ve seen a reduction in the patellofemoral complications that plagued many of the earlier designs and that, frankly, continue to be a problem with some contemporary inlay designs. With inlay designs, the incidence of patellar instability is as much as 17 percent, and 84 percent of patients report good and excellent results. With onlay designs, on the other hand, the incidence of patellar instability is less than 1 percent, and 96 percent of patients report excellent results. Inlay designs tend to be revised earlier and with greater frequency, primarily due to patellar instability—a problem that has been virtually eliminated with many onlay-style implants.”

It’s too early to tell
When he stepped to the podium, Dr. Dennis asked, “Have the results improved? My answer is, ‘I don’t know,’ because data on modern designs and surgical techniques are limited both in numbers and in follow-up durations.”

According to Dr. Dennis, data show substantially lower survivorship with PFA, compared to alternative treatments such as total knee arthroplasty (TKA).

“PFA produces higher complication rates,” he explained. “Revision for unexplained pain is double that for TKA. Additional operative procedures are commonly required for a successful result, and the primary failure mode—disease progression—does not occur with knee replacement.”

Based on data from the Australian Orthopaedic Association National Joint Replacement Registry, PFA has a 30 percent failure rate at 10 years—a significant consideration, especially for younger patients, noted Dr. Dennis. In addition, of 17 studies with minimum 5-year follow-up, 14 displayed double-digit failure rates, ranging from 14 percent to 42 percent, with complications such as persistent anterior knee pain, arthrofibrosis, instability, tendon rupture, catching and snapping, and chronic infusions.

“In comparison, studies on TKA for isolated patellofemoral arthritis show failure rates of 0 percent to 3.3 percent, at longer than 5 years,” he said.

“Can tactile robotic technology improve many of the technical errors that have been associated with PFA?” he asked. “Again, I ask you to show me the data. It’s a very promising technology, but often a million-dollar investment for the hospital, which is certainly going to limit surgeon access to that technology.”

Dr. Dennis suggested that ongoing issues with PFA are rooted in the wide variation in patellofemoral anatomy, including trochlear groove angle and the height and shape of the patella. Patellofemoral systems typically have limited sizes and shapes, which may not accommodate the wide variances in human anatomy. In addition, sagittal plane kinematics are often not normalized following PFA; among the factors affecting patellofemoral kinematics are limb alignment, the presence of patellofemoral instability, neck shaft angle, and foot deformities.

“Dr. Lonner pointed out that the many different designs currently available have wide variations,” said Dr. Dennis. “I think that if manufacturers had found the right design, we wouldn’t see such wide variations.

“The clinical results for PFA for isolated patellofemoral arthritis have been far inferior to TKA. PFA has higher complication and reoperation rates and shorter survival times,” concluded Dr. Dennis. “I believe this is related, in part, to wide variations in patellofemoral anatomy and kinematic patterns. Although robotic implantation systems and custom implant designs may improve the results, current results are scant and have limited follow-up duration.”

Disclosure information: Dr. Lonner—Zimmer, Blue Belt Technologies, Mako Surgical, CD Diagnostics, Healthpoint Capital, Wolters Kluwer Health, Lippincott Williams & Wilkins, Saunders/Mosby-Elsevier, American Journal of Orthopedics, Journal of Arthroplasty (JOA), Knee Society, Philadelphia Orthopaedic Society. Dr. Dennis—DePuy, Innomed, Joint Vue, Porter Adventist Hospital, JOA, Journal of Bone and Joint Surgery, Clinical Orthopaedics and Related Research, Orthopedics Today, AAOS, Hip Society, International Congress for Joint Reconstruction, Operation Walk USA.

Peter Pollack is a staff writer for AAOS Now. He can be reached at ppollack@aaos.org

Bottom Line

• The debate over the use of patellofemoral arthroplasty (PFA) is ongoing.
• Although PFA has higher complication and reoperation rates and shorter survival times than TKA, new implant designs and improvements in technique are beginning to address some of these issues.
• The introduction of onlay designs has significantly reduced the incidence of patellofemoral instability and improved outcomes.
• However, long-term data are not yet available to support the efficacy of PFA.