Q&A: Rating Patellofemoral Arthroplasty

Abstract

QUESTION: A 33-year-old male sustained a right knee injury, which is diagnosed as patellar subluxation with an osteochondral lesion (on patellar articular surface but location and size are unspecified). He underwent arthroscopic “retropatellar chondroplasty.” Because of worsening symptoms, the patient had a second arthroscopy with a lateral release. He subsequently reinjured the same knee and underwent a patellofemoral arthroplasty for degenerative arthritis of the patellofemoral joint. Although preoperative X rays showed a 3-mm (normal) cartilage interval in the patellofemoral joint, the surgeon reported areas on the patella with no cartilage at the time of the arthroplasty. The patient had a fourth and final surgery of quadriceps tendon debridement and repair.During his evaluation 10 months after the last surgery, the patient reported mild knee pain with prolonged positioning, kneeling, and running. Apart from the surgical scars, the physical examination was normal, including normal gait, strength, motion, ligamentous stability, and patellar tracking with negative apprehension and grind. Radiographically, the tibiofemoral cartilage intervals were normal and the implant was in optimal position with no evidence of loosening.The patient does not appear to have any ratable impairment based on the examination findings, using the AMA Guides, Fifth Edition, which is used in my jurisdiction. With preserved cartilage intervals, he would not have had any ratable impairment before the patellofem-oral arthroplasty per Table 17-31, Arthritis Impairment Based on Roentgenographically Determined Cartilage Interviews (5th ed, 544). In addition, patellofem-oral arthroplasty (PFA) is not a named procedure in Table 17-33, Impairment Estimates for Certain Lower Extremity Impairments (5th ed, 546–547). Therefore, should the patient be rated as having a total knee replacement (TKR) with good result with a 37% lower extremity impairment (LEI)?ANSWER: PFA, also called partial (unicompartmental) knee replacement, ie, prosthetic replacement of the articular surface of the patella and trochlea, was first reported by McKeever in 1955; however, “the design was ultimately discontinued due to excessive trochlear wear.1 The first total PFA did not occur until 1979 following introduction of the Richards and Lubinus prostheses.1 (See Abstracts following this Q&A for a few sample abstracts for PFA.)PFA was not commonly performed when the AMA Guides, Fifth Edition, was published in 2000. Presumably, for this reason, PFA was not a named procedure in Table 17-33. However, unicondylar knee replacements were performed with greater frequency than PFA during those years and are, therefore, listed in Table 17-33. Short-term outcomes of PFA have been published in case series, but long-term outcomes and randomized controlled trials comparing PFA to non-operative treatment and TKR are needed.The AMA Guides states that “[i]n situations where impairment ratings are not provided, the Guides suggests that physicians use clinical judgment, comparing measurable impairment resulting from the unlisted condition to measurable impairment resulting from similar conditions with similar impairment of function in performing activities of daily living” (5th ed, 11; 6th ed, 499). The knee has three compartments: patellofemoral, medial tibiofemoral, and lateral tibiofemoral. From an impairment standpoint, a PFA can be considered as equivalent to the replacement of either the medial or lateral compartment because all three procedures are unicompartmental (partial) knee replacements. Hence, although the Table 17-33 listing for unicondylar replacement technically only applies to the medial or lateral femoral and tibial condyles, it would be rational to use the same ratings for a PFA.One might argue that there is less bone and cartilage loss with a unicompartmental than a tricompartmental (total) knee replacement; therefore, impairment should be less, perhaps one-third less. However, the listing in Table 17-33 of the fifth edition indicates that the ratings are the same regardless of the extent of replacement.About 8% to 12% of patients who have had a PFA will later undergo a revision to their TKR because of tibiofemoral degeneration.2–4 In the published series on isolated PFA, the mean age of patients is >50; therefore, a higher rate of conversion to TKR because of osteoarthritis in the other knee compartments would be expected in patients age 50 and older than it would be for the 33-year-old patient. Although the TKR would be more impairing than a PFA, assuming the same or similar number of points scored using the Table 17-35, Rating Knee Replacement Results (5th ed, 549), no additional impairment would be awarded following the TKR if impairment is rated using the fifth edition.While measurement of the cartilage interval is not applicable when both joint surfaces have been replaced, given the mild knee pain and normal physical examination, the patient scores at 95 points using the fifth edition's Table 17-35. Using Table 17-33, the “good result” of the surgery would warrant a 37% LEI.NOTE: In the original printing (2008) of the AMA Guides, Sixth Edition, Table 16-3 (6th ed, 509), only TKR was listed, which made it unclear how unicompartmental knee replacements, whether unicondylar or patellofemoral, should be rated. In contrast, using the 2014 printing of the AMA Guides, Sixth Edition, through the digital edition of the AMA Guides Sixth Edition 2021, partial knee replacement is in the same row as TKR (see Figure 1 for an excerpt of Table 16-3).Examiners using earlier printing(s) of the AMA Guides, Sixth Edition, should use the Patellectomy row and cite the 2014 printing or the new 2021 digital edition as a precedent for the same rating for unicompartmental knee replacements (unicondylar or patellofemoral) and TKRs. However, because PFAs and other “unis” generally have better early results than “totals,”5 Class 2 (14–25% LEI) is usually chosen for the former. Note that most of the possible ratings using the sixth edition are lower than the ratings using the fifth edition because of improvements in technology and surgical techniques, ie, the implants are better and surgical techniques are less invasive.