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© 1997 The Journal of Bone and Joint Surgery, Inc. Assessment of Radiolucent Lines about the Glenoid: An In Vitro Radiographic Study*  , ATLANTA, GEORGIAInvestigation performed at Emory University School of Medicine, Atlanta
A study was undertaken to investigate the precision of plain radiographs in the assessment of the width of radiolucent lines and to define parameters for more accurate measurement. A metal-backed glenoid component was inserted into fourteen cadaveric scapulae; the component had a radiolucent spacer at the central post to provide a gap with a known width at the component-bone interface. The specimens were mounted in a custom-designed jig, and initial radiographs were made with the glenoid in neutral version; sequential radiographs then were made, at 5-degree intervals, with the glenoid in 0 to 40 degrees of anteversion and retroversion. Four independent observers with various levels of experience measured the width of the radiolucent lines with use of digital microcalipers. Osteometric analysis demonstrated that normal glenoid version ranged from 3 degrees of anteversion to 13 degrees of retroversion; these values were similar to those reported in previous studies. Radiographic analysis showed that accurate measurement of the width of the gap was dependent on the position of the glenoid. The measured widths of the radiolucent lines were significantly smaller than the known width of the gap when retroversion was 10 degrees or more and when anteversion was 15 degrees or more (p  0.05). Radiolucent lines were not consistently observed on radiographs that were made with the glenoid in more than 20 degrees of anteversion and retroversion. An analysis of interobserver error showed close agreement among the measurements made by the different observers when the glenoid was in 0 and 5 degrees of rotation, with decreased agreement when the glenoid was rotated more than 10 degrees from neutral.
CLINICAL RELEVANCE: Inaccurate positioning of the patient and anatomical variation in glenoid version may explain the variability in the reported onsets, progressions, and frequencies of radiographic loosening of glenoid components. The findings of the present study also may help to explain the poor association between clinical and radiographic findings reported for patients who have pain at the site of a total shoulder prosthesis. Radiographs made within 10 degrees of neutral should allow accurate assessment of radiolucent lines about the glenoid.
Many authors have concluded that total shoulder arthroplasty yields excellent relief of discomfort related to inflammatory and degenerative arthropathies3,7,8,13,16. In light of the increased number of total shoulder arthroplasties that are being performed and the inevitable long-term follow-up studies that will be conducted, a standard method for the analysis of failure will become increasingly important. Despite the importance of accurate evaluation of fixation of the component, a standard method of assessment has not been developed, to our knowledge. Historically, the primary method of evaluation has been plain radiography. Radiographic loosening has been arbitrarily defined as progressive radiolucency or a radiolucent line that is at least two millimeters wide at the bone-cement interface1,3,6,12. Interestingly, the reported prevalence of radiolucent lines about the glenoid has ranged from 22 to 95 per cent2,3,8,11,16. Many investigators have found no association between radiolucent lines and symptomatic clinical loosening3,4,6,7,10. This finding has been attributed to variations in radiographic technique and to non-standardized methods of measurement8,12,15,16,19. While it is common practice to use radiolucent lines to evaluate the fixation of an implanted glenoid component, little information is available regarding the errors introduced by radiographic projection and image magnification. The purpose of this study was to investigate the precision of plain radiographs in the assessment of radiolucent lines about the glenoid and to define parameters for more accurate evaluation.
Preparation of the Specimens
A standard-sized, metal-backed glenoid component (Cofield Total Shoulder System; Smith and Nephew Richards, Memphis, Tennessee) was implanted in each specimen with use of a standard operative technique9. A Teflon washer was inserted over the central post of each component during implantation to provide a known interval at the component-bone interface. The washer was 2.23 millimeters thick, 6.20 millimeters in inner diameter, and 12.00 millimeters in outer diameter. All components were well fixed into bone. The actual space between the edge of the glenoid and the component was measured with digital microcalipers (L. S. Starrett, Athol, Massachusetts). This space was difficult to measure consistently because the edge of the glenoid was rough, but it was essentially no different than the thickness of the washer. The width of the gap that was formed by the washer therefore was used to determine a single value with which the measured widths of the radiolucent lines could be compared. This comparison value, which was calculated by correction of the actual width of the washer (2.23 millimeters) for radiographic magnification (9.4 per cent), was 2.44 millimeters.
Radiographic Analysis
Radiolucent lines about the glenoid were measured with digital microcalipers by four independent observers who had various levels of experience; the observers included an attending orthopaedic surgeon, an attending musculoskeletal radiologist, an orthopaedic resident, and a senior medical student. The measured widths of the radiolucent lines were compared with the known width of the gap. Magnification at neutral rotation was determined with a standardized 100-millimeter radiographic marker that was mounted on the jig at the level of the glenoid. Changes in magnification due to rotation of the specimen were determined by comparison of the radiographically measured width of the central post at the neutral position with that at each anteverted or retroverted position.
Statistical Analysis
Anatomy of the Glenoid
Radiographic Assessment
The null hypothesis (that radiolucent lines measured at 0 through 40 degrees of rotation were equal) was rejected (p < 0.0001) (Table I). We could not demonstrate a significant difference between the measured widths of the radiolucent lines and the true width of the gap when the glenoid was in neutral version (p > 0.05) (Fig. 3). We also could not demonstrate a significant difference when the glenoid was in 5 or 10 degrees of anteversion (p > 0.05), although a strong trend toward smaller measurements was noted at 10 degrees. From 15 degrees of anteversion on, however, the measured widths of the radiolucent lines were significantly less than the true width of the gap (p
There has been considerable discussion regarding the importance of radiolucent lines about the glenoid. The results of previous studies3,4,6,7,10 have suggested that the onset, progression, and frequency of such lines are highly variable. This is thought to be secondary to variations in radiographic technique as well as interobserver error. Neer et al., in 1982, stated that there was "a need for more consistently reproducible roentgenographic techniques to make comparison studies valid."16 We know of no studies that have been performed to assess the accuracy of measurement of radiolucent lines or to define parameters for more precise radiographic evaluation of loosening of the glenoid component. Imaging of the glenohumeral joint is inherently difficult because of the anatomy of the scapula. Numerous projections have been proposed in an attempt to obtain accurate images of the joint18. None of these, however, account for individual variation in glenoid version. In the present study, version ranged from 3 degrees of anteversion to 13 degrees of retroversion, which is comparable with that in the studies by Mallon et al. (7 degrees of anteversion to 13 degrees of retroversion) and by Randelli and Gambrioli (2 to 15 degrees of retroversion). These ranges suggest that there is a potential error of as much as 15 degrees when a so-called standard anteroposterior radiograph is made with the glenoid in neutral version. Additional sources of error in the reported series have included operator-dependent error when making technically difficult radiographs of the shoulder as well as interobserver variability when measuring radiolucent lines. The findings of the present study indicate that the position of the scapula and the anatomy of the glenoid substantially affect the assessment of radiolucent lines about the glenoid. As the specimens were rotated away from neutral, the measured widths of the radiolucent lines tended to decrease. From 10 degrees of retroversion and 15 degrees of anteversion on, these differences were significant. Radiographs made with the glenoid in more than 20 degrees of rotation often were assessed as showing no visible radiolucent lines. For the measurement of radiolucent lines to be reliable, therefore, radiographs must be made with the glenohumeral joint within 10 degrees of neutral version. The anatomical variation in glenoid version, combined with operator-dependent error, can easily result in a projection that is more than 15 degrees from true neutral on a standard radiograph. Interestingly, in this study, observers who had various levels of expertise were able to measure the widths of the radiolucent lines at neutral version consistently and accurately. These findings suggest that the positioning of the patient for radiographic examination is more critical to obtaining accurate information than is the experience or training of the individual observer. Also of interest is the fact that radiographic loosening has been defined as a progressive increase in the width of a radiolucent line as measured on serial radiographs1,3,6,12. However, unless initial and subsequent radiographs are made with the glenoid in a true neutral position, at a constant magnification, apparent progression of the lines cannot be validated. A progressive increase in the width of a radiolucent line, therefore, probably is not a valid indicator of loosening of the glenoid component; this finding may help to explain why some patients who have apparently progressive radiolucent lines are asymptomatic. Other methods for the assessment of loosening of the glenoid component include arthrography and arthroscopy. Bonutti et al. compared the findings on arthroscopy, arthrography, and plain radiography with those noted intraoperatively. Those authors found arthroscopy to be the most accurate method of assessment, with both types of radiographic studies being poor predictors of loosening. This is not surprising in light of our findings. We believe that plain radiographs would be more accurate if they were made with the glenohumeral joint in true neutral version. The findings of the present study may help to clarify the variability in the reported onset, progression, and frequency of radiolucent lines as reported in previous series. On the basis of these data, we have proposed a more accurate method for the assessment of radiolucent lines. Our results also caution against the use of radiolucent lines in outcome studies and in the evaluation of component design, unless care is taken to make radiographs with the glenoid in true neutral version.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was Herrington Medical, Atlanta, Georgia.
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