OBJECTIVE To assess the correlation between radiographically diagnosed osteophytes in the axial and lateral view of the patellofemoral joint (PFJ) and (1) magnetic resonance (MR) detected cartilage defects in the same joint and (2) knee pain.
METHODS Fifty seven pepole with chronic knee pain, (aged 41–58 years, mean 50 years) were examined with axial and lateral radiograms when standing of the right and the left PFJ. The presence and grade of osteophytes was assessed. On the same day, a MR examination was performed of the signal knee with proton density and T2 weighted turbo spin-echo sequences in the sagittal and axial view on a 1.0 T imager. Cartilage defects in the PFJ were noted. The subjects were questioned for current knee pain for each knee.
RESULTS Osteophytes at the PFJ had a specificity varying between 59 and 100% and a positive predictive value between 74 and 100% for MR detected cartilage defects. The corresponding values for osteophytes at the lateral aspect of the femoral trochlea were both 100%. In PFJ with narrowing (<5 mm) osteophytes had a sensitivity and a positive predictive value of 90 and 95% respectively for MR detected cartilage defects, while in PFJ with non-narrowing (⩾5 mm) the corresponding values were 75 and 65% and the specificity was 50%. A correlation (p<0.05) between osteophytes at the inferior pole of the patella and knee pain was found.
CONCLUSIONS Osteophytes at the PFJ are associated with MR detected cartilage defects in the same joint. The relation was strong for osteophytes at the lateral femoral trochlea and in the PFJ with narrowing (<5 mm), but weak in the PFJ with non-narrowing (⩾5 mm).
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It is only recently osteoarthritis (OA) of the patellofemoral joint (PFJ) has been recognised as an important cause of pain and disability.1 Assessment of the PFJ in axial radiograms have been found more reproducible than grading of the joint in lateral radiograms.2 3 The axial view was found superior to the lateral view to detect progression of patellofemoral OA.4It has also been shown, that joint space narrowing (<5 mm) in the axial view of the PFJ has a high specificity for magnetic resonance (MR) detected cartilage defects in the same joint.5
In recent years several investigators have found a stronger association between knee pain and osteophytes than between knee pain and joint space narrowing.3 6 7 These results have been reported for the tibiofemoral joint (TFJ)6 as well as for the PFJ.3 7 In the TFJ the presence of osteophytes has been regarded as an early sign of OA since Kellgren and Lawrence developed a grading system in 1957.8
MR imaging with its good tissue contrast and anatomical resolution9 provides a non-invasive examination tool of the PFJ in early stages of OA, for detection of cartilage defects.
The aim of our study was to assess the correlation between the presence of osteophytes in axial and lateral radiograms of the PFJ and (1) MR detected cartilage defects in the same joint and (2) knee pain.
To create a cohort of people with chronic knee pain (duration >3 months at inclusion) for prospective follow up, an epidemiological survey among 2000 persons aged 35–54 years in a rural area in southern Sweden has been performed. Chronic knee pain was reported by 279 of 2000 and 204 of 279 accepted to be examined clinically, biochemically, and radiographically at baseline 1990–91.10 At a three year follow up a subgroup of 61 people was chosen as a random sample from the initial cohort of 204 after exclusion of those with joint disorders other than OA and those having more severe OA with obliteration of the joint space or bone attrition in the TFJ. All, except the first two subjects, had an axial radiogram of both PFJ. MR imaging of the signal knee (the most painful at inclusion) was performed in 60 of 61 patients. One patient could not take part in the MR study because of claustrophobia. The MR examination was diagnostic in all except one patient (59 of 60) because of trembling caused by Parkinson’s disease. Thus 57 of 61 patients had an axial radiogram of both PFJ and a diagnostic MR study and were included in the study.5 There were 29 women (aged 42–58, mean 50.8 years) and 28 men (aged 41–57, mean 49.5 years). Both examinations in each patient were performed on the same day.
At the follow up the subjects were questioned for current knee pain for each knee. The difference in the number of osteophytes between the signal knees and the contralateral knees was calculated in patients who had or had had pain (at inclusion in the study) in the signal knee only.
The axial view of the patella was obtained with the patient standing and with a vertical beam according to Ahlbäck,11 using a technique previously described.5 The aim was to get a tangential view of the dorsal aspect of the patella. The beam or the angulation of the femur or the tibia was adjusted blindly if this was not achieved. The flexion of the knee joint was in most examinations 40–60°. In 52 of 57 patients a lateral view of both knees also was obtained in weightbearing and semiflexion, and the degree of flexion was in most cases 30–50°. The focus was 0.6 mm and the film focus distance 150 cm in the axial and 115 cm in the lateral view. Kodak X-Omatic LW cassette with Lanex Screen 100 was used with Kodak TMS/RA 1 film.
The axial view was evaluated according to a radiographic atlas published by Altman12 for osteophytes at the medial and lateral aspect of the patella in four grades (0–3). A similar evaluation, but not published in any atlas, was used for the medial and lateral aspect of the femoral trochlea in the axial view. Figure 1 (A) and 2 (A) show osteophytes at the medial and lateral aspects of the patella as well as of the femoral trochlea. Osteophytes at the superior and inferior pole in the lateral view of the patella were registered if present (0) or absent (1) and are shown in figure 3 (A). All radiograms were evaluated by one observer (TB 1). The right knee in all patients was evaluated a second time by the same observer (TB 2) and once by another observer (OR) for calculating the κ values for intraobserver and interobserver variation and the percentage concordance within and between the observers. The radiograms were assessed without knowledge of the name and age of the patients or the results from previous examinations. The osteophytes were correlated to the minimal joint space width in axial radiograms in PFJ with narrowing (<5 mm) and in PFJ with non-narrowing (⩾5 mm). The joint space width measurements in the patients studied have been reported in a previous study5 and are shown in table 1. The bony margins used for measuring the minimal joint space width of the medial and lateral compartment of the PFJ were chosen according to Buckland-Wright.13 The minimal joint space width of the PFJ was measured in mm with a standard mm graded plastic ruler on the plain film across the joint space corresponding to the central 3/4 of each of the articular surfaces of the femoral trochlea. No correction for magnification was made.
MR imaging was performed in the signal knee with a 1.0 T imager (Impact, Siemens) with a circular polarised surface coil. The first five patients were examined with a coronal T1 weighted spin-echo sequence, a sagittal proton density and T2 weighted turbo spin-echo sequence (tSEPdT2) and a 3D-gradient echo sequence (Dess) obtained in the sagittal view. The remaining patients were examined with tSEPdT2 in the coronal, sagittal, and axial views. The sagittal sequence was perpendicular to a line connecting the dorsal aspects of the femoral condyles, the coronal sequence was parallel to that line and the axial sequence was perpendicular to the long axis of the patella. The sequence parameters for the tSEPdT2 were: TR/TE 4200/15–105 ms with two signals averaged, echo train length 7, FOV 145×145 mm, section thickness 3 mm with 0.3–0.6 mm intersection gap, matrix size 252×256 and acquisition time 5 min 8 s.
The MR examinations were assessed for cartilage defects in each of the articular surfaces of the PFJ.5 The defects were graded as absent (grade 0), ⩽50% reduction of the cartilage thickness (grade 1), >50% reduction of the cartilage thickness (grade 2), and as involving bone loss (grade 3). Signal changes in the cartilage with an intact surface were not registered. Figures 1 (B), 2 (B), 2 (C), and 3 (B) show the cartilage defects in Pd and T2 weighted MR images.
The MR studies were interpreted blind and separately by two of the authors (TB, OR) with experience in musculoskeletal MR imaging, who then reached a consensus. The MR detected cartilage defects in the patients studied have been presented in a previous study5and are shown in table 2.
The study was approved by the Committee on Ethics at the Faculty of Medicine, University of Lund.
The measure used to assess intraobserver and interobserver agreement was κ. A κ value of 1.00–0.81 was considered as very good, 0.80–0.61 as good, 0.60–0.41 as moderate, 0.40–0.21 as fair and 0.20–0.00 as poor.14 As the distribution of grades of the osteophytes was skewed, the percentage concordance within and between the observers was also calculated.
The McNemar test was used to compare the number of osteophytes between the signal knee and the contralateral knee. The test was also performed with 15 contingency tables in which the numbers of ostoephytes at the different aspects of the PFJ were compared with each other.
Bilateral knee pain or pain in the contralateral knee only was reported by 17 patients. In the remaining 40 subjects pain of the signal knee was unchanged or worse in 20 and was less or appeared only irregularly in 20 subjects during the three years of observation.
The lateral view of the patella was obtained in 52 of 57 patients and thus osteophytes at the superior and inferior pole of the patella could be assessed in 52 people only. In the other locations all 57 subjects could be assessed except for the femoral trochlea of the contralateral knee in one person. Tables 3 and 4 show the number of osteophytes in the different locations. In the signal knee, osteophytes were present in 49 patients and in the contralateral knee in 38. The number of osteophytes at the lateral femur in the signal knee was smaller (p<0.05–p<0.001) than at all other locations in the signal knee and also at the inferior pole of the patella (p<0.05) compared with the superior pole and the lateral patella.
In the 40 subjects who exclusively had or had had pain in the signal knee more osteophytes were found at the inferior pole of the patella of the signal knee compared with the contralateral knee (p<0.05).
Tables 5-7 show the correlation between osteophytes and MR detected cartilage defects. Osteophytes at the lateral aspect of the femoral trochlea had a specificity of 100% for MR detected cartilage defects in the PFJ. This means that MR detected cartilage defects could be predicted in 11 of 57 PFJ with osteophytes exclusively at this location. Osteophytes at the medial aspect and at the inferior pole of the patella had a specificity of 82% and 88% respectively. If osteophytes at these two locations were present in the same joint they also had a specificity of 100% for MR detected cartilage defects. Table 8 (A) and 8 (B) shows the correlation between osteophytes and MR detected cartilage defects in PFJ with narrowing (<5 mm) and in PFJ with non-narrowing (⩾5 mm).
The κ values for the intraobserver variation of osteophytes varied between 0.75 and 0.96 and the corresponding values for the interobserver variation between 0.53 and 0.92. The intraobserver percentage concordance of osteophytes varied between 88 and 98% and the interobserver percentage concordance between 81 and 96%.
OA is a multifactorial disease with a focal loss of articular cartilage, and with variable underlying bone reaction.15These lesions have a varying relation to radiographic findings (osteophytes, joint space narrowing, bone loss and subchondral sclerosis and subchondral cysts) and to clinical symptoms (articular pain and loss of function). Marginal osteophytes, whose relation to MR detected cartilage defects this study is about, arise in the synovium overlying bone at the junctional zone and is the result of metaplasia of the synovium into cartilage.16 17 “Vascularization of the subchondral bone marrow in this region produces calcification of the adjacent cartilage and stimulates endochondral ossification. The developing outgrowth extends into the “free” articular space, along the path of least resistance. Generally, it contains spongy trabeculae and fatty marrow and is covered with articular cartilage”.18
MR imaging is considered to be an accurate means for detecting and staging moderate and advanced cartilage lesions in the knee joint19-21 and is thus useful in the evaluation of knee OA. The fast spin echo 2D-sequence used in this study does not differ in this respect and has also been used by others.22 23The reason why we did not pursue with the 3D-gradient echo (Dess) was that reconstruction in coronal and axial planes were of inferior quality to evaluate hyaline cartilage. The examination time was considerably longer than the tSEPdT2 sequence which gives an increased risk of motion artefacts. The examination quality of the first five patients was considered equal and they have therefore been included in the study. According to recent results it appears as if high resolution 3D-gradient echo sequences with the addition of fat suppression or magnetisation transfer contrast are the best for depicting hyaline cartilage.24-26 As mentioned above the 3D-gradient echo sequences have a long aquisition time with potential diasadvantages.
This study was performed in middle aged people with comparatively mild OA of the PFJ, as only 21 of 57 had joint space narrowing (<5 mm), and no case with obliteration of the joint space or bone loss was found.5 In general a relation was found between osteophytes and MR detected cartilage defects and at the medial patella, at the lateral femoral trochlea and at the inferior pole of the patella the specificity and positive predictive value for cartilage defects was high and was 100% at the lateral femoral trochlea. However, the sensitivity for MR detected cartilage defects was in general low. The association between osteophytes and cartilage defects could be supported by studies on experimentally induced knee OA in dogs, in which osteophytes appear early in the course of the disease, being detectable within two weeks of surgery.17 27
In this study group joint space narrowing (<5 mm) in axial radiograms has been found to have a very high specificity (94%) and positive predictive value (95%) for MR detected cartilage defects while the sensitivity was low (50%).5 In this study the presence of osteophytes had a very high specificity and positive predictive value for MR detected cartilage defects in PFJ with narrowing (<5 mm). However, in the PFJ with non-narrowing (⩾5 mm) the values were considerably lower. Thus osteophytes seem not to be usable for clinical or epidemiological purposes in the PFJ with non-narrowing as indicators of MR detected cartilage defects at least in this age group.
The overall association between osteophytes and cartilage defects in our study was weak and several explanations are possible. Are the findings on experimentally induced OA of the knee (tibiofemoral joint) in dogs17 27 valid for the PFJ in humans? If osteophyte formation is focal and not general to a cartilage lesion the radiographic technique used is insufficient to show the entire boundary between cartilage and bone as this boundary is curved in various directions. Also, MR imaging of articular cartilage has in general low sensitivity for superficial cartilage lesions and this could also explain our results.19-21
The number of osteophytes in the contralateral knee without pain was generally smaller and the difference was statistically significant (p<0.05) at the inferior pole only. This finding is in agreement with previous studies.3 7
The κ values of intraobserver and interobserver variation for classifying osteophytes was with one exception in general good (>0.61) or very good (>0.81). This has also been found by others.3 However, the distribution of grades of osteophytes was skewed and under these circumstances the κ value tends to be low even if the percentage concordance between the gradings is high as in our study.
In the radiographic atlas used12 neither osteophytes at the medial and lateral aspect of the femoral trochlea in the axial view of the patella nor osteophytes at the superior and inferior pole of the patella in the lateral view are included. This is a drawback in this recently developed atlas.
In conclusion osteophytes at the PFJ are associated with MR detected cartilage defects in the same joint. The relation was strong for osteophytes at the lateral femoral trochlea and in PFJ with narrowing (<5 mm), but weak in PFJ with non-narrowing (⩾5 mm).
This study was supported by grants from the Thelma Zoégas Foundation, the Stig och Ragna Gorthon Foundation and Reumatikerförbundet. We are grateful to Göran Ejlertsson for statistical assistance.
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