Fixed-flexion radiography of the knee provides reproducible joint space width measurements in osteoarthritis
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- Kothari, M., Guermazi, A., von Ingersleben, G. et al. Eur Radiol (2004) 14: 1568. doi:10.1007/s00330-004-2312-6
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The validity of a non-fluoroscopic fixed-flexion radiographic acquisition and analysis protocol for measurement of joint space width (JSW) in knee osteoarthritis is determined. A cross-sectional study of 165 patients with documented knee osteoarthritis participating in a multicenter, prospective study of chondroprotective agents was performed. All patients had posteroanterior, weight-bearing, fixed-flexion radiography with 10° caudal beam angulation. A specially designed frame (SynaFlexer) was used to standardize the positioning. Minimum medial and lateral JSW were measured manually and twice by an automated analysis system to determine inter-technique and intra-reader concordance and reliability. A random subsample of 30 patients had repeat knee radiographs 2 weeks apart to estimate short-term reproducibility using automated analysis. Concordance between manual and automated medial JSW measurements was high (ICC=0.90); lateral compartment measurements showed somewhat less concordance (ICC=0.72). There was excellent concordance between repeated automated JSW measurements performed 6 months apart for the medial (ICC=0.94) and lateral (ICC=0.86) compartments. Short-term reproducibility for the subsample of 30 cases with repeat acquisitions demonstrated an average SD of 0.14 mm for medial JSW (CV=4.3%) and 0.23 mm for lateral JSW (CV=4.0%). Fixed-flexion radiography of the knee using a positioning device provides consistent, reliable and reproducible measurement of minimum JSW in knee osteoarthritis without the need for concurrent fluoroscopic guidance.
KeywordsKneeOsteoarthritisRadiographyJoint space width
A broad-based research and development effort is underway to test and commercialize a variety of pharmacological agents aimed at inhibiting the pathogenetic processes related to the breakdown of articular cartilage and/or underlying subchondral bone associated with knee osteoarthritis [1–4]. Consequently, a critical need has emerged for accurate and precise radiographic outcome measures to evaluate the comparative effectiveness of these agents among large patient populations participating in multi-center clinical trials. A variety of methods have been proposed, but standard radiographic assessment of joint space width (JSW) is generally accepted as the most relevant . Indeed, Buckland-Wright et al.  demonstrated that JSW measurements made in the weight-bearing tunnel view reliably reflected medial compartment cartilage thinning as determined by double contrast macroarthography.
Longitudinal changes in JSW measured on serial films are small relative to inherent biological or treatment response variability. Thus, maintaining a high degree of JSW measurement precision is critical. A number of studies have verified that knee radiographs taken in the traditional weight-bearing position with full knee extension provide inadequate JSW reproducibility for the evaluation of chondroprotective agents in multi-center trials of knee osteoarthritis [6–11]. Most of this inherent measurement error can be attributed to irreproducible image acquisition, particularly with respect to consistent positioning of the knee .
Several authors have confirmed that even minor alterations in image acquisition such as the amount of knee flexion or foot rotation, non-standardized X-ray beam alignment and variable distance between the knee and the X-ray cassette can have a substantial influence on JSW measurement precision [7–9, 12–15]. Therefore, standardized image acquisition is essential, and some studies have shown that automated image analysis may provide additional improvements in reproducibility [7, 16, 17]. Importantly, imaging the knee in approximately 20–30° of flexion engages the central posterior aspect of the femoral articular cartilage, the region most markedly thinned in the arthritic disease process [18, 19]. However, achieving reproducible joint flexion even with the assistance of fluoroscopic guidance remains a formidable challenge in knee radiography.
The current paper introduces a technique for image acquisition and analysis in knee osteoarthritis. This method employs a specially designed positioning frame and calibration phantom that fixes knee flexion and positions the patient’s feet reproducibly. To assure that the X-ray beam is tangential with the floor of the medial tibial plateau, a 10° caudal X-ray beam angulation is used in all cases. These standardized radiographic acquisition parameters guarantee that the most affected region of articular cartilage is imaged reproducibly on initial and follow-up films. This study estimates the short-term JSW reproducibility of this technique and assesses whether the addition of an automated image analysis software method (SynaVu) that corrects for magnification and parallax effects offers further improvements in precision compared with manual JSW measurements.
Patients and methods
Chronic knee pain
An Osteoarthritis Research Society International (OARSI) grade of 1 or 2 
A minimum JSW measurement in the medial compartment of between 2.0 and 4.0 mm determined by manual measurement
Approximately 70% (165/238) of these patients from 47 centers elected to participate and were monitored prospectively with serial radiography of both knees and follow-up clinical assessment. The average age of these patients was 60.5 years (±8 years), and most patients were female (119/165, 72%). The baseline radiographs of these patients represent the study material for the current investigation and include a random sample of 30 knee radiographs (30 patients) obtained from 14 geographically dispersed centers where standardized knee radiography was repeated twice within a 2-week period to estimate short-term JSW measurement precision. The mean age of this patient subsample was 59.4 years (±6.8 years) and 53% (16/30) of the cases were female.
Utilizing the baseline films from the 165 patients participating in the prospective treatment study of knee osteoarthritis, the concordance and association between manual JSW measurements and the initial automated measurements with magnification correction were estimated using the intra-class correlation coefficient (ICC) [23, 24] and Pearson’s correlation coefficient , respectively, for medial and lateral compartments separately. Using the same study material, the initial and repeat automated measurements were compared using similar statistical procedures as above, and the reproducibility was estimated by calculating the corresponding coefficient of variation (CV). The short-term reproducibility of minimum JSW by the magnification-corrected automated analysis procedure was estimated by calculating the average standard deviation (SD) between measurements from the subset of paired radiographs (n=30) taken 2 weeks apart. The corresponding CV was calculated as an estimate of the magnitude of between-subject variability in JSW relative to the mean of JSW.
There was excellent concordance and association between automated JSW measurements repeated 6 months apart by the same reviewer using the SynaVu software system with magnification correction. For the medial compartment, the ICC and Pearson’s coefficient were 0.94 and 0.95, respectively. The corresponding CV was 3.9%. For the lateral compartment, the ICC and Pearson’s coefficient were 0.86 and 0.89, respectively, and the corresponding CV was 5.6%.
Among the subsample of 30 cases with fixed-flexion knee radiography repeated twice in a 2-week period, the short-term reproducibility in the medial compartment using automated analysis was estimated by an average SD of 0.14 mm and a corresponding CV of 4.3%. In the lateral compartment, differences measured between repeat radiographs resulted in an average SD of 0.23 mm and a corresponding CV of 4.0% using automated analysis.
The standardized radiographic technique described in the current study fixes the knee reproducibly in approximately 20° flexion, thereby engaging the articular aspect of the medial compartment most severely affected in knee osteoarthritis, the central posterior region [18, 19]. By standardizing radiographic acquisition parameters such as the amount of knee flexion, degree of foot rotation and X-ray beam angulation and by controlling evaluation factors including magnification errors, the current automated technique either corrects or attenuates the known variability associated with many of the factors identified to degrade measurement precision [9, 14]. The fixed-flexion technique is essentially similar to the Lyon Schuss view , but without the necessity of fluoroscopic guidance to assist in aligning the X-ray beam. Piperno et al.  reported a short-term JSW reproducibility with a corresponding CV of 3.5% for the Lyon Schuss view, an estimate that compares favorably with the precision demonstrated herein using the fixed-flexion method. However, the fixed-flexion method does not require a dedicated technologist trained in fluoroscopic examination and spares the patient the added cost and radiation exposure associated with fluoroscopy.
Other radiographic methods have been developed to measure minimum JSW and to allow for some degree of knee flexion. A semi-flexed (i.e., 7–10°) anteroposterior view with fluoroscopic guidance has demonstrated reasonable short-term reproducibility (CV=5.5%) in 25 arthritic cases imaged at a single center , but considerably worse measurement precision (CV=8.7%) when the same technique was employed across five clinical centers . Center-to-center variability was common with almost 30% of paired radiographs judged unsatisfactory with respect to the degree of knee flexion or foot rotation .
A non-fluoroscopically assisted protocol utilizing the same semi-flexed position with a PA horizontal beam has been proposed [8, 26]. In this method, the first metatarsophalangeal (MTP) joints are positioned directly beneath the front surface of the X-ray film cassette; patellae remain in contact with the cassette and aligned vertically with the first MTP joints. External foot rotation is standardized at 15° and maintained reproducibly with the aid of individual foot maps. Short-term reproducibility has been reported to be excellent (CV=1.6%) in a single center series of cases  and considerably worse (CV=8.0%) in a field test of the procedure . In both reports of this technique, parallel alignment of the tibial plateau and the X-ray beam occurred in only about 30% of cases [8, 26]. This problem, coupled with the fact that the slight degree of flexion achieved (i.e., 7–10°) fails to capture radiographically the most affected articular region, raises some concern about its use in the serial assessment of chondroprotective agents.
We noted somewhat worse inter-technique and intra-reader concordance and reliability for lateral compared to medial compartment JSW measurements. However, short-term reproducibility estimates for the subsample of repeat knee radiographs demonstrated similarly low imprecision for medial (CV=4.3%) and lateral (CV=4.0%) compartment measurements. Buckland-Wright et al.  likewise noted inconsistencies with respect to JSW measurement precision between the medial and lateral compartment, with the medial compartment generally offering somewhat better reproducibility overall. Nevertheless, minimum medial JSW is uniformly considered the standard measurement parameter for clinical study eligibility and for monitoring longitudinal articular changes .
Using 2.0-mm medial JSW as the lower cutoff eligibility criterion for study participation, we found that only four out of 165 patients that were eligible by manual measurement would have been excluded by the higher precision automated SynaVu analysis system with magnification correction. The results of our correlation analysis indicated that manual measurements of JSW, when performed by an experienced radiologist, were concordant and associated with automated measurement methods. In contrast to longitudinal assessments of joint space narrowing (i.e., change in JSW), in which measurement precision is critical, the rate of incorrect inclusion or exclusion in patient selection was not substantially impacted by the choice of measurement technique in this study.
The current study provides initial validation results for the fixed-flexion radiographic technique in determining JSW in knee osteoarthritis. A high degree of concordance was demonstrated between manual and automated measurement techniques as well as when the automated measurements were repeated by the same reader. Moreover, short-term measurement reproducibility was high. These findings were particularly impressive in that the study material of knee radiographs was obtained from 47 clinical centers overall and from 14 centers for the subsample of cases with repeat radiographs. This suggests that the SynaFlexer positioning frame aids in eliminating many of the image acquisition errors that plague non-standardized radiographic protocols used to evaluate JSW.
This image acquisition for this study was funded by Hoffman-La Roche. Authors thank Melissa M. Ta from Synarc for providing the drawings.