Total knee arthroplasty (TKA) has shown excellent clinical outcomes even at long-term followup.3,14,20,22 Nevertheless, patellofemoral problems often cause unsatisfactory results after this procedure.5,6,18 Clinical outcome studies of TKA often show discrepancies between patellofemoral symptoms and results obtained by the scoring systems.1,23 Commonly utilized scores, such as the Knee Society Scoring System, do not take into account subjective symptoms or objective data specifically arising from the patellofemoral joint.
A paper by Bindelglass et al4 reported no relationship among patellar maltracking and pain, flexion, or poor score result. Shih et al23 showed Knee Society knee and functional scores were not adversely affected by patellofemoral radiographic abnormalities. The disparity in the reported results of TKA with and without resurfacing of the patella may be due not only to the implants and surgical technique utilized, but also to the lack of appropriate patellofemoral evaluation scores. Further, patellar tracking obtained with an unloaded radiographic view may be different from the typical in vivo activities which load the patellofemoral joint and cause symptoms.19
We presumed a dedicated patellofemoral scoring system would improve the accuracy of clinical outcome reports of TKA and would represent a complementary source of information to the existing Knee Society knee and function scores. We hypothesized the standard patellofemoral radiographic evaluation obtained with an unloaded axial view at 45° of knee flexion overemphasized patellar maltracking (tilt and subluxation).13,18,24 We further hypothesized a loaded patellar radiograph would increase the chance of obtaining correlations between clinical and radiographic assessments in TKA.
MATERIALS AND METHODS
To develop a specific scoring system to evaluate patellofemoral performance in TKA, 69 patients (100 knees) who underwent posterior-stabilized TKA between 1994 and 1997 were included for clinical and radiographic evaluation. At a minimum followup of 7 years (average 8.5, range 7.0-10.5 years) these patients were examined using a new scoring system which includes questions and tests specific to the anterior compartment of the knee. The same cohort underwent radiographic analysis using a recently developed weight bearing axial view. Correlations between different scoring system and loaded or unloaded axial views were investigated. Informed consent for participation in the study was obtained from all patients and we obtained institutional review board permission for this study.
All procedures were performed by the same surgeon (TPS) using the same technique. Anterior referenced intramedullary instruments were used for preparing the femur. Distal femoral resection was made at 5° alignment for varus knees and 2° alignment for valgus knees. Rotational alignment of the femoral component was referenced using the femoral condyles plus 3° of external rotation. Alignment of the tibial component was referenced using the medial ⅓ of the tibial tubercle. All patellae were resurfaced with an all-polyethylene, dome-shaped, three-pegged component to restore original thickness. Patella tracking was checked with the components in place using the no thumb rule.10 We recorded the incidence of lateral retinacular release at surgery and postoperative complications (eg, patellar clunk, dislocation, and patellar fracture) through a chart review. The implants were Insall-Burstein II Posterior Stabilized® (IB II PS®, Zimmer Inc, Warsaw, IN) and the Optetrak 913® prostheses (Exactech, Gainesville, FL).
Clinical evaluation was performed using the Knee Society knee and function scoring systems.9 We assessed knee function using a new scoring system developed at our institution (Table 1). The score was developed to be administered in less than 5 minutes. The scoring system ranged from 0 (worst case scenario) to 100 (best case scenario) points, and included ratings for subjective and objective clinical aspects of the knee after TKA. Subjective evaluation included anterior knee pain during a repetitive low chair-rise (three cycles) and questions regarding specific functional limitations. A maximum of 50 points was allocated for absence of anterior knee pain, as this was the most important clinical outcome. Anterior knee pain was evaluated during a simulated activity of daily living such as a low chair-rise. Pain intensity was rated using a visual analog scale (VAS) ranging from 0 to 10 points, with 0 being no pain and 10 being maximum pain. Each point then was converted in multiples of five. The patient was asked if they experienced functional limitations during activities that typically stress the patellofemoral joint (eg, stair climbing and/or descending, and sitting for prolonged time (≥ 30 minutes) with the knee bent in 90° flexion. Fifteen points were allotted for no limitations, 10 points for subjective limitation on one activity, five points for two activities, and no points for all three activities. Objective evaluation included: tenderness to patellar facets palpation, crepitus throughout active range of motion (ROM), and quadriceps strength. Palpable crepitus was categorized as mild (if present during limited ROM), moderate (if present throughout the entire ROM), and severe (if associated with a catching/clunking sensation). Quadriceps strength was evaluated.
Radiographic assessment was performed using standard and modified Merchant axial views. The Merchant axial view radiograph was modified by positioning the standing patient in a semisquatted position with the knees in 45° flexion. The relationship between the xray source, the joint angle, and the cassette position was kept unchanged from the original view. The standing position and consequent muscle involvement were the only differences (Fig 1). Measurements for patellar alignment (tilt and subluxation) were performed according to the method described by Gomes et al11 using standard and weightbearing axial-view radiographs.
All physical examinations and patient interviews were conducted separately by two authors (AB, JAA). The same authors completed the score sheet twice for each patient. Interobserver and intraobserver variability for physical examinations of the patients were calculated using the Student's t test (p ≤ 0.05). Intraclass correlation coefficient (ICC) for intraobserver and interobserver measurements were 0.992 (K = 0.92) and 0.985 (K = 0.87), respectively. Intraclass correlation coefficient scores greater than 0.750 were considered adequate for patients involved in clinical trials.21
Other data were analyzed with the two-tailed, unpaired Student's t test and the chi-square test. These tests were applied to the interval measures of the study. Each measure approximated a normal distribution (p < 0.05).
Preoperative Knee Society knee score was 43 ± 5 points. Postoperatively, the score increased to 93 ± 8. Preoperative function score improved from 39 ± 15 points to 82 ± 18 points. At followup, the patella score averaged 89 ± 8 points. Moderate anterior knee pain when rising from a chair (average VAS = 6) was present in two knees (two patients), and mild anterior knee pain (average VAS = 3) was present in 17 knees (12 patients). There were no cases of severe pain. The presence of anterior knee pain during low chair-rise did not correlate with a low result (< 80 points) in the knee or function scores, but correlated (p = 0.01) with a low result (< 80 points) at the patella score. Persistence of a tilt greater than 5° on the patellar dome, even when the load was applied on the weightbearing axial view, correlated positively (p = 0.045) with anterior knee pain and with a patellar score less than 80 points (p = 0.03). Radiographic patellar tilt did not correlate with a worse outcome on knee and function scores.
Postoperative lateral patellar tilt greater than 5° on the Merchant view was present in 17 knees (11 patients). Postoperative patellar tilt greater than 5° on the weightbearing axial radiographs was present in 10 knees (six patients). Postoperative lateral patellar subluxation greater than 5 mm on the Merchant view radiographs was present in 12 knees (eight patients). Postoperative lateral patellar subluxation greater than 5 mm on the weightbearing axial radiographs was present in one knee (Fig 2). None of the radiographic parameters obtained using the standard Merchant view radiographs correlated with pain and clinical scores. When patellar bone to femoral implant impingement was examined on the Merchant view, it did not correlate with the presence of pain. Extensive contact with bone sclerosis (radiodensity) or remodeling on the weight-bearing view showed a positive correlation (p = 0.03) with anterior knee pain. Another positive correlation between pain and a radiographic parameter was asymmetric patellar resection. When the patella was left asymmetric on the craniocaudal dimension, as observed on the lateral view of the knee (seven knees: four patients), it correlated (p = 0.001) with the presence of pain. When the patella was resected with residual major asymmetry (> 4 mm) on the mediolateral dimension, as observed on the axial view radiographs, it correlated (p = 0.02) with the presence of pain (Fig 3). Only 15% (12 of 84 knees) of knees with a symmetrical patellar profile (within 3 mm) experienced anterior knee pain at the low chair-rise test, compared with 44% (seven of 16 knees) of those with an asymmetric profile. Extensive patellar bone-implant contact and major patellar asymmetry correlated with an inferior result on the patellar score (p = 0.03 and p = 0.01, respectively). We were unable to find a correlation between these radiographic parameters and results on the knee or function scores.
We evaluated the clinical and radiographic patellofemoral results after posterior-stabilized TKA. We devised specific tools to evaluate knee performance and used existing clinical scoring systems to assess the TKA outcomes.2,7,9,12,23 These scores, developed decades ago, focused mainly on tibiofemoral aspects. They included only ratings for overall knee joint pain, stability, ROM, and alignment. A few attempts have been made to standardize scoring knee function results after TKA, but none have been validated or widely used.15,16 Further, they do not involve radiographic assessment, lack specific categories, and do not include a subjective symptom rating scale. We developed a simple, reproducible, and widely applicable scoring system to measure specific objective and subjective clinical and radiographic complications after TKA.
We note several limitations to our investigation, including the low rate of moderate to severe anterior knee pain and overall patellar problems. Our analysis was conducted to differentiate mainly between absent or mild anterior knee pain. Another limitation was the relatively low number (100 TKAs in 69 patients) enrolled for the followup evaluation. We found it complex to select which are the most specific tests to stress the anterior compartment of the knee. Some manual tests may elicit pain even in the absence of patellofemoral complications. Our comparison was limited to the Knee Society Scoring System and did not investigate other existing scores. The number of points allotted for each item represent a challenge for every scoring system. We allotted points giving priority to the subjective section and conceivability of final scores was selected creating different scenarios.
Radiographic abnormalities typically do not correlate with clinical symptoms when using standard patellar axial view radiographs.4 Weightbearing axial view radiographs with the patient standing in a semisquatted position demonstrated possible sources of symptoms. Many asymptomatic tilts or subluxations under load were neutralized, explaining their benign nature. The asymmetric patella bone resection, a technical imperfection, seemed to facilitate maltracking and bone impingements when the patella was under load. This may explain why Pagnano and Trousdale17 found a failure rate of approximately 50% at mid-term followup when the patella was asymmetrically resected. Bone impingements between the exposed (uncovered by the patella button), resected patellar surface, and the femoral component are frequently observed.8 This phenomenon was easier to observe under load, particularly when there was a retinacular imbalance and/or an asymmetrical bone resection. Some impingements can be painful, which may be also related to implant design features.
Similar to other reports,1,23 we found no correlation between the final Knee Society scores and functional status. By contrast, anterior knee problems were appropriately emphasized by a specific patellar scoring system. The scoring system showed satisfactory interobserver variability and was easy to use. However, the results did not always correspond to those obtained from the Knee Society scores. For example, a patient who had an excellent Knee Society score, no pain, good ROM, and good stability may have only scored a fair result with the patella-specific scoring system. This fair score could have been caused by mild discomfort the patient experienced during a cyclic low chair-rise, patellofemoral crepitus throughout ROM, or peripatellar tenderness. This patient scenario may represent a satisfactory overall result, but with some functional limitations due to symptoms not measured on the Knee Society score. Our new scoring system showed knee function may remain suboptimal even in the well-performing TKA population.
Minor prosthesis design changes produce substantial differences on outcomes and complications in TKA. A consistent scoring system may help determine these differences and reduce the risk of undetected symptoms. Loading the patella during radiographic analysis with the patient in a semisquatted position reproduced the typical scenario that provokes anterior knee pain. Our new scoring system was useful as an additional tool for determining patellofemoral function after TKA.
The authors thank Margaret Peterson, PhD, for her kind support in the statistical analysis. The authors also thank Mary Giesa and the Radiology Department staff of the Hospital for Special Surgery for their help in developing the patella weightbearing axial view.
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