Recent evidence suggests that the subchondral bone is important for both generation of pain and progression of osteoarthritis (OA) (1–3). Scintigraphic and magnetic resonance imaging studies have emphasized the importance of subchondral bone changes (2, 4, 5), but these methods cannot be used for routine assessments. Pathology studies of surgical specimens have shown that subchondral bone changes, including attrition or loss of bone, are common in persons with advanced OA (6). Attempts have been made to assess subchondral cysts and sclerosis as potential pathologic measures of the bone in conventional radiographs, but these are hampered by low intra- and interobserver reliability (7).
In 1968, the Swedish radiologist Ahlbäck introduced a scoring system (8) that could be used to grade the loss of subchondral bone in OA on conventional radiographs. This scoring system is based on the following criterion: the presence or absence of bone defects that give an impression of having been caused by attrition of the articular surfaces, with distinctions made between defects of <5 mm, between 5 and 10 mm, and >10 mm. Later, Ahlbäck and Rydberg recommended a 5-stage grading system (9), in which grade 1 = joint space narrowing, grade 2 = loss of joint space, grade 3 = loss of joint space with bone defects of <5 mm, grade 4 = loss of joint space with bone defects of 5–10 mm, and grade 5 = loss of joint space with bone defects of >10 mm. This system was never fully described or validated but has been widely used, particularly by orthopedic surgeons (10–12). In 2001, the International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine recommended that this system be used for selection of cases for total knee replacement (TKR) (13). A subsequent study found the interobserver reliability of the scoring system to be low, however, and concluded that the Ahlbäck 5-stage grading system cannot be relied on for TKR case selection (14).
Bone loss may be difficult to detect in the absence of clear defects of cortical integrity. We hypothesized that we could optimize intra- and interobserver reliability of the original Ahlbäck 3-grade approach through the use of templates designed to outline the hypothetical normal contours of the knee joint overlaid onto conventional knee radiographs (Figure 1). Using a random sample of radiographs from consecutive patients awaiting TKR for OA, we set out to determine the intra- and interobserver reliability of this approach, to explore whether bone attrition may be an additional dimension of severity in advanced OA, to determine whether scores for bone attrition are less likely to show a ceiling effect in advanced OA than would other scores of radiologic features, and to explore whether bone loss may be associated with the severity of pain and the occurrence of night pain. In subsequent random samples of radiographs from individuals with chronic knee pain who were not awaiting TKR and from asymptomatic control subjects, we explored distributions of scores and correlations between different features of OA.
Figure 1. Templates of the outline of the contours of the knee joint developed from weight-bearing anteroposterior radiographs of knees considered to be normal. Broken lines indicate the cutoff points for levels of bone loss. The templates were overlaid onto knee radiographs from patients with osteoarthritis. This film shows severe grade 3 bone attrition on the lateral tibial plateau.
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- PATIENTS AND METHODS
All selected radiographs were included in the analysis. The primary sample from patients awaiting TKR included 53 right knees and 47 left knees from 97 patients (1 man and 2 women had undergone bilateral knee replacements). The 97 patients, of whom 61 (63%) were female, had a mean ± SD age of 72 ± 8 years. Five radiographs had missing attrition scores, because of poor technical quality of 3 of the radiographs, and inability to use the templates due to gross varus deformities on 2 of the radiographs. Scores of joint space narrowing and K/L grades were unavailable for 1 film because the radiograph was not obtained with the knee in the weight-bearing position, and information on the intensity of global knee pain and the occurrence of night pain was missing for another knee. The sample from individuals with chronic knee pain who were not awaiting TKR included 40 right knees and 35 left knees from 75 participants who had a mean ± SD age of 67 ± 12 years, 42 (56%) of whom were women. Scores were available for all features in all participants. The sample from asymptomatic controls included 39 right knees and 36 left knees from 75 individuals who had a mean ± SD age of 60 ± 13 years, 51 (68%) of whom were women. One film could not be assigned scores because the knee was severely rotated.
Table 1 presents the intraclass correlation coefficients and kappa values for the intra- and interobserver reliabilities of the scores for individual radiologic features in patients about to undergo TKR. The reliability values for the presence or absence of attrition and for the degree of bone loss were high and comparable with those yielded for other scores of radiologic features. Interobserver reliabilities for determination of the presence of sclerosis and cysts were generally low.
Table 1. Intra- and interobserver reliability for scores of radiologic features on a random sample of 20 films from patients with knee osteoarthritis awaiting total knee replacement*
| ||Medial tibiofemoral joint||Lateral tibiofemoral joint||Tibiofemoral joint overall|
|Scores for severity of feature†|| || || || || || |
| Attrition||0.96 (0.92, 0.98)||0.81 (0.59, 0.92)||0.96 (0.92, 0.98)||0.90 (0.77, 0.96)||0.93 (0.87, 0.96)||0.79 (0.53, 0.91)|
| Osteophytes||0.83 (0.70, 0.91)||0.50 (0.09, 0.77)||0.93 (0.87, 0.96)||0.83 (0.62, 0.93)||0.89 (0.81, 0.94)||0.47 (0.05, 0.75)|
| Joint space narrowing||0.93 (0.86, 0.96)||0.66 (0.30, 0.85)||0.99 (0.98, 1.00)||0.91 (0.78, 0.96)||0.89 (0.80, 0.94)||0.49 (0.06, 0.77)|
| K/L grade||0.93 (0.86, 0.96)||0.61 (0.23, 0.83)||0.97 (0.94, 0.98)||0.94 (0.86, 0.98)||0.89 (0.80, 0.94)||0.49 (0.06, 0.77)|
|Presence of feature‡|| || || || || || |
| Attrition||0.95 (0.91, 0.97)||0.60 (0.22, 0.82)||0.79 (0.64, 0.88)||1.00 (0.99, 1.00)||0.90 (0.82, 0.95)||0.60 (0.23, 0.82)|
| Osteophytes||0.80 (0.66, 0.89)||0.62 (0.26, 0.83)||0.94 (0.88, 0.97)||0.74 (0.46, 0.89)||1.00 (0.99, 1.00)||0.78 (0.53, 0.91)|
| Joint space narrowing||1.00 (0.99, 1.00)||1.00 (0.99, 1.00)||0.93 (0.87, 0.96)||1.00 (0.99, 1.00)||1.00 (0.99, 1.00)||1.00 (0.99, 1.00)|
| Cysts||0.91 (0.84, 0.95)||0.00 (−0.43, 0.43)||0.66 (0.44, 0.81)||0.00 (−0.43, 0.43)||0.84 (0.72, 0.91)||0.00 (−0.43, 0.43)|
| Sclerosis||0.79 (0.64, 0.88)||−0.07 (−0.49, 0.37)||0.64 (0.41, 0.79)||0.65 (0.31, 0.85)||0.69 (0.48, 0.82)||0.24 (−0.22, 0.61)|
Some bone attrition (score ≥1) was present in 59 (62%) of 95 assessable radiographs from patients awaiting TKR, and definite attrition (score ≥2, corresponding to ≥5 mm bone loss) was present in 22 (23%) of the radiographs. The medial tibial plateau was affected most frequently (45%), followed by the medial femoral condyle (22%), the lateral tibial plateau (10%), and the lateral femoral condyle (7%). Twenty-five radiographs were scored as showing subchondral sclerosis, but only 2 radiographs were considered to show subchondral cysts.
In participants with chronic knee pain who were not awaiting TKR, some attrition was present on 7 films (9%) and definite attrition on 1 film (1%). Five films were scored as showing sclerosis (7%) and none was found to show subchondral cysts. Among asymptomatic controls, only 1 film had evidence of some attrition (1%) and 1 film had evidence of sclerosis (1%), but none of the control films were reported to show subchondral cysts.
With regard to the analysis of trends in scores of individual features across groups, we assessed a median bone attrition score of 1 in patients awaiting TKR (interquartile range [IQR] 0–3), a score of 0 in participants with chronic knee pain who were not awaiting TKR (IQR 0–1), and a score of 0 in asymptomatic controls (IQR 0–0) (P for trend < 0.001 for each). Similarly, median scores for the presence of osteophytes were 3 (IQR 1–3), 0 (IQR 0–2), and 0 (IQR 0–1), respectively, median scores for joint space narrowing were 3 (IQR 1–3), 0 (IQR 0–3), and 0 (IQR 0–1), respectively, and median K/L grades were 4 (IQR 2–4), 0 (IQR 0–4), and 0 (IQR 0–2) among patients awaiting TKR, patients with chronic knee pain who were not awaiting TKR, and asymptomatic controls, respectively (P < 0.001 for each).
In patients awaiting TKR, scores for joint space narrowing, scores for osteophytes, and K/L grades showed a ceiling effect, in which 36% of radiographs had grade 2 and 54% had grade 3 osteophytes, joint space narrowing was graded 2 in 41% of films and graded 3 in 52% of films, and 41% of radiographs were assigned a K/L grade 3 and 52% were assigned a K/L grade 4. No ceiling effect was found for scoring of attrition in this group of patients (39% of radiographs were scored 0, 38% were scored 1, 19% were scored 2, and only 4% were scored 3). Table 2 presents the correlation between scoring of the different radiographic features in the 3 samples. The correlation between attrition scores and scores of other features was consistently weak to moderate. Correlations were strongest between scores for joint space narrowing and K/L grades in patients awaiting TKR, and between scores for the presence of osteophytes and K/L grades in the other 2 samples of films.
Table 2. Correlation between scores of individual radiologic features in tibiofemoral joints on 100 films from patients about to undergo total knee replacement (TKR), 75 from individuals with chronic knee pain not awaiting TKR, and 75 from asymptomatic controls*
| ||Osteophytes||Joint space narrowing||Kellgren/Lawrence grade|
|Patients about to undergo TKR|| || || |
| Attrition||0.33 (0.14, 0.50)||0.37 (0.18, 0.54)||0.37 (0.18, 0.54)|
| Osteophytes||–||0.33 (0.15, 0.50)||0.33 (0.15, 0.50)|
| Joint space narrowing||–||–||1.00 (0.99, 1.00)|
|Individuals with chronic knee pain|| || || |
| Attrition||0.33 (0.11, 0.52)||0.37 (0.15, 0.55)||0.35 (0.14, 0.54)|
| Osteophytes||–||0.61 (0.44, 0.73)||0.92 (0.88, 0.95)|
| Joint space narrowing||–||–||0.80 (0.69, 0.87)|
|Asymptomatic controls|| || || |
| Attrition||0.26 (0.03, 0.46)||0.39 (0.18, 0.57)||0.25 (0.02, 0.45)|
| Osteophytes||–||0.39 (0.18, 0.57)||0.93 (0.88, 0.95)|
| Joint space narrowing||–||–||0.39 (0.18, 0.57)|
The mean reported intensity of global pain in patients about to undergo TKR was high, with a mean ± SD rating of 2.9 ± 0.6. In linear regression, definite attrition was not associated with pain intensity; the difference in global pain intensity scores between knees with and those without definite attrition was 0.1 (95% confidence interval [95% CI] −0.1, 0.4; P = 0.33). Night pain was reported to be present in 87 (88%) of 99 knees with available data. In univariable logistic regression, there was a nonsignificant, but potentially relevant association of night pain with the presence of definite bone attrition (odds ratio 4.2, 95% CI 0.5, 34.9; P = 0.18). We found no evidence of an association of night pain with definite osteophytes (odds ratio 0.7, 95% CI 0.1, 6.2) or definite joint space narrowing (odds ratio 0.7, 95% CI 0.1, 3.4). The association of night pain with definite attrition remained identical after adjusting for osteophytes and joint space narrowing in a multivariable model.
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- PATIENTS AND METHODS
Our novel approach for assessing bone attrition yielded substantial intra- and interobserver reliability for assessment of knee radiographs from patients awaiting TKR. A consistently low to moderate correlation of attrition with other features indicated that bone attrition is a distinct dimension of OA severity, different from osteophyte formation and joint space narrowing. Unlike other features of OA, bone attrition did not show a ceiling effect in advanced disease. Finally, exploratory analyses suggested that definite attrition may be associated with the occurrence of night pain.
Our study has several limitations. First, intraobserver reliability may have been overestimated due to the recall of certain OA patterns. Second, our study was based on the readings of 3 assessors only, and therefore our estimates of agreement may not be generalizable to other studies. Third, the analysis of the association of radiologic features with global pain intensity or night pain was based on individuals awaiting TKR only. Larger studies in unselected groups of OA patients are required to reliably determine the association of bone attrition with pain.
Several reports in orthopedic and rheumatology journals have described the use of Ahlbäck's system, in the absence of any clarity of how the method was applied or any attempt to assess reliability (10–12). One study compared Ahlbäck and Rydberg's 5-point system with K/L grades in a community-based sample of subjects with knee pain (21), and found a high concordance. In contrast to the 5-point scores (9), our approach allows the assessment of bone defects as a distinct entity, without assuming that they occur only when the joint space has been obliterated. Unsurprisingly, the correlation between K/L grades and our attrition scores was low to moderate.
Radiographic assessment of knee OA is widely used in clinical research and practice. Until recently, the K/L system was frequently used, particularly in epidemiologic studies. Over the last decade, more emphasis has been given to individual radiographic features (17, 22). However, subchondral bone changes remained difficult to assess because of low interobserver reliabilities (7) and low rates of recording of definite changes.
Ahlbäck (8) suggested that bone attrition is an important radiographic feature that could be measured relatively easily. We took up that suggestion and used templates to assess the extent of bone loss. As predicted by Ahlbäck, we found that bone attrition was common in advanced knee OA, but uncommon in individuals with less advanced disease and in asymptomatic controls. Our approach may therefore allow further classification of severe radiographic OA, when scores for other features cannot discriminate because of a ceiling effect. Since the clinical survival of prostheses may be related to the surrounding bone environment, this classification may be used, for example, to determine whether preoperative subchondral bone attrition is associated with an increased likelihood of prosthesis loosening. Subchondral bone changes may also be important in symptom generation and may correlate better with symptoms than would other radiographic features (2, 23). Thirty-seven years after the original concept was presented by Ahlbäck, we confirm that bone attrition is an additional radiographic dimension of knee OA, which can be measured reliably using templates that outline the hypothetical normal contours of the joint. The assessment of this dimension will be useful primarily in monitoring the advanced stages of knee OA.