760 ARTHRITIS & RHEUMATISM Volume 38 Numbmer 6, June 1995, pp 76&767 0 1995, American College of Rheumatology

SERUM HYALURONIC ACID LEVEL AS A PREDICTOR OF DISEASE PROGRESSION IN OSTEOARTHRITIS OF THE KNEE

MOHAMMED SHARIF, EMMANUEL GEORGE, LEE SHEPSTONE, WARREN KNUDSON, EUGENE J.-M. A. THONAR, JANET CUSHNAGHAN, and PAUL DIEPPE

Objective. To investigate the prognostic value of serum hyaluronic acid (HA) and keratan sulfate (KS) levels in relation to tibiofemoral osteoarthritis (OA) of the knee.

Methods. Clinical and demographic data were collected on 94 patients. Radiographs were obtained at study entry and at 5-year followup. Disease progression was defined as 2 mm of joint space narrowing of any tibiofemoral compartment, and/or knee joint surgery during the study period. Serum HA and KS were measured and levels were correlated with entry data and disease progression.

Results. At entry, HA levels were significantly related to disease duration (P = 0.036), minimum joint space (P = 0.049), and previous surgery (P = 0.001). After these variables were taken into account, patients whose disease had progressed were shown to have had significantly higher levels of HA at baseline compared with those whose disease had not progressed (P = 0.019). However, there were no significant differences in levels of serum KS between those with and those without disease progression, at entry (P = 0.779) or at subse- quent visits.

Conclusion. These results suggest that serum HA levels predict disease outcome in OA of the knee and confirm that a single measurement of the serum level of KS is not useful as a prognostic marker in OA.

Osteoarthritis (OA) is the most common cause of pain and disability in the elderly (1). Between 2%

Supported by the Arthritis and Rheumatism Council. Mohammed Sharif, PhD, Emmanuel George, PhD, Lee

Shepstone, MSc, Janet Cushnaghan, MSc, Paul Dieppe, MD: Bris- to1 Royal Infirmary, Bristol, England; Warren Knudson, PhD, Eugene J.-M. A. Thonar, PhD: Rush-Presbyterian-St. Luke's Med- ical Center, Chicago, Illinois.

Address reprint requests to Mohammed Sharif, University of Bristol Rheumatology Unit, Department of Medicine, Bristol Royal Infirmary, Marlborough Street, Bristol BS2 8HW, UK.

Submitted for publication May 31, 1994; accepted in revised form November 23, 1994.

and 10% of the adult popullation is affected by OA of the knee (2), and a small but significant proportion develop severe joint damage leading to surgery (3). Prognostic markers may help to define the processes that cause progression, identify patients likely to need joint replacement, and enable interventional therapy at earlier stages of the disease. Normal scintigraphy results have been shown to have negative predictive value in the progression of OA of the knee (3). However, scintigraphy is an invasive and expensive technique and not suitable for routine screening. Se- rum markers would be preferable.

Serum levels of hyaliironic acid (HA) and kera- tan sulfate (KS) have belen suggested as possible disease process markers in OA (43). We therefore used the well-defined group of OA patients included in our scintigraphic studies (3) to test the hypothesis that serum HA and KS levels may predict disease outcome in OA of the knee.

PATIENTS AND METHODS

Study population. Ninety-four patients referred to the Bristol Royal Infirmary Rheumatology Unit were recruited for the study. The diagnosis of knee OA was based on the combination of use-related pain and radiographic features of OA, including the presence of definite osteophytes or joint space narrowing (JSN) or both, and the absence of any other rheumatic disease. All patients were first examined in 1986, as described elsewhere (6); demographic and clinical data recorded included age, sex, disease duration (time since the patient first reported symptoms), height, weight, current therapy, clinical evidence of 0 . A of other joints, and clinical features of each knee joint, iincluding the presence of an effusion.

Blood samples collected from each patient at the time of entry into the study were sent for routine biochern- istry studies, and serum was separated and kept frozen at -70°C until analysis. In addition, standard anteroposterior and lateral radiographs of botlh knees were obtained. The patients were recontacted and reexamined at 1 , 2, and 5 years after entry into the study. Further blood samples and

SERUM HA LEVEL3 AND OA PROGRESSION 76 1

clinical data were obtained at each visit. A second set of radiographs was obtained at the 5-year review. Exactly the same patient positioning, camera-film distance, and radio- graphic apparatus were used for each set of radiographs.

None of the patients received oral or intraarticular steroids for the duration of the study and at least 3 months prior to entry. They were all receiving either no drug therapy, mild analgesics, or nonsteroidal antiinflammatory drugs (NSAIDs).

Assessment of radiographs. Radiographs were as- sessed and scored by a single observer (PD), who was blinded to the patient’s identity and the radiograph se- quence. The radiographic features that were recorded for each compartment of each knee included osteophytes (none, mild, or severe), subchondral sclerosis (present or absent), and interbone distance (measured in mm, at the midpoint of each tibiofemoral Compartment), and the Kellgren and Lawrence grade (7). Disease was considered to have pro- gressed if there was a decrease of 2 2 mm in the tibiofemoral interbone distance in any compartment over the 5-year period, or if the patient had undergone total knee replace- ment for tibiofemoral disease (3).

Laboratory methods. Measurement of hyaluronic acid. An enzyme-linked immunosorbent assay (ELISA) (8) was used to measure serum HA at baseline and 5 years from study entry. This method takes advantage of an anti-KS monoclonal antibody (1/20/5-D-4; a gift from Dr. Bruce Caterson, University of Wales of Cardif) to differentiate between the coated aggregating rat chondrosarcoma proteo- glycans that capture HA and the KS-bearing aggregating proteoglycans added subsequently. Absorbance in this assay is linear when plotted against the logarithmic concentration of HA (Healon; Pharmacia, Uppsala, Sweden) in the range of 15-1,000 ng/ml. Intraassay variation was <4%, and interassay variation was <6%. All determinations were made in triplicate, and investigators analyzing serum sam- ples were blinded to the clinical and radiographic data.

Measurement of keratan sulfate. Serum KS was measured at baseline and at I , 2, and 5 years from study entry, using an ELISA (9) that includes an inhibition step and utilizes the 1/20/5-D-4 anti-KS monoclonal antibody, which is specific for a highly sulfated sequence of several repeats of the disaccharide (6-sulfated N-acetylglucosamine linked pl-3 to 6-sulfated galactose); this epitope is present at the nonreducing end of long antigenic KS chains. The ELISA was performed at pH 5.3, a modification that yields steep inhibition curves for both standard and serum samples (10). All determinations were made in triplicate, and the analyses were performed under blinded conditions. Values are reported as equivalents of the International Standards of KS purified from human costal cartilage (a gift from Drs. M. B. Mathews and A. L. Horwitz, University of Chicago, Chicago, IL). Intraassay variation was <3%, and interassay variation was <4%.

Statistical analyses. Associations of sex and previous surgery in the group that had disease progression were tested using the chi-square test. Mann-Whitney tests were used to test for equality with respect to minimum joint space and number of joint sites involved in the 2 groups. The equality between the 2 groups with respect to the remaining demo- graphic variables was tested using a 2 sample t-test. A linear

model was constructed to test for a difference in HA levels between progression groups while allowing for any possible confounding variables. Simple contrasts were used to com- pare HA levels between sexes, those with and without previous surgery, and those with and without disease pro- gression. Backward deletion was used for the model selec- tion, beginning with all 8 background variables (Table 1) and progression group. In addition, a logistic regression model was constructed using progression as the dependent van- able. P values less than 0.05 were considered significant.

RESULTS

Of the original 94 patients recruited for the study, 19 (20%) had no radiographs available at the 5-year visit (10 had died and 9 were lost to followup). Of the remaining 75 patients, progression was identi- fied in 26 (34.7%), based on knee surgery alone in 1 1 , radiographic evidence of JSN at the 5-year visit in 12, and both in 3. Forty-nine patients (65.3%) had neither surgery nor JSN within the study period and were thus defined as not having disease progression.

Clinical and demographic data on all patients at study entry are shown in Table 1. There was radio- graphic evidence of OA (Kellgren and Lawrence grade 2 or above) in at least one compartment of one knee in all patients, 66% having medial compartment disease and 41% lateral compartment disease. No patient had an interbone distance of < 3 mm in all 4 tibiofemoral compartments at entry, so all could have potentially exhibited radiographic progression in the manner de- fined. There were significant differences at entry be- tween the patients whose disease progressed and those whose did not: the 2 groups were significantly different with respect to weight:height ratio (P = 0.004) and minimum joint space (P < 0.001). Patients whose disease progressed were, on average, heavier and had greater loss of joint space compared with those who did not have disease progression.

The relationship between minimum joint space, age, disease duration, and age at onset of disease was investigated using regression models, Minimum joint space was found to be negatively associated with age (P < 0.001). It was also found to be negatively associated with disease duration (P = 0.033), but after age was taken into account this was no longer signifi- cant ( P = 0.150). There was no significant relationship between minimum joint space and age at onset of disease (P = 0.515). Patients who died or were other- wise lost to followup were older and had a lower weight: height ratio than those available for followup, but there were no other differences between these 2

762 SHARIF ET AL

Table 1. Clinical and demographic data on the osteoarthritis patients studied

No disease Disease All patients progression progression Undetermined

(n = 94) (n = 49) (n = 26) (n = 19) P* ~ ~ ~~ ~ ~~

Sex Male Female

Yes No

0 1-2 3-4 5-8 9-12

Previous surgery

No. of other joint sites involved

Age, mean f SD Age at disease onset, mean t SD Disease duration, mean 2 SD Weight:height ratio, mean * SD Minimum joint space,

median (interquartile range)

29 65

15 79

12 21 30 25 6

64.2 t 11.6 48.2 f 14.9 16.1 2 12.9 0.44 2 0.09

3.0 2 3.0

16 33

6 43

4 13 17 11 4

61.1 t 11.1 46.3 2 15.9 15.0 f 13.2 0.42 f 0.08 4.0 f 2.0

12 14

6 20

6 4 6 8 2

63.9 * 11.0 46.6 2 14.0 17.0 f 13.1 0.49 t 0.08 2.0 f 2.0

1 18

3 16

2 4 7 6 0

72.6 4 9.9 55.1 t 12.0 17.6 f 12.6 0.40 t 0.07 3.0 t 3.0

0.250t

0.223i

0.969$

0.2940 0.9291 0.5321 0.0040

<o .oo 1 t

* Comparison between the group with and the group without disease progression. t Chi-square test. i Mann-Whitney U test 5 Two-sample t-test.

groups. At entry, clinical evidence of an effusion in one or both knee joints was found in 57 of the 94 patients, but HA levels were the same in those with and those without effusions. The number of affected joints (excluding the spine) was recorded only at entry, and did not correlate with baseline HA levels.

Hyaluronic acid levels. Baseline HA measure- ments were available for 60 patients, and all cross- sectional analyses were carried out on these patients (Table 2). The distribution of HA levels was found to be highly positively skewed (i.e., not distributed nor- mally) at baseline (skewness = 2.12) and at the 5-year visit (skewness = 3.33). Therefore, for further analy- sis, a logarithmic transformation (natural log [Ln]) was applied. The skewness of the transformed HA levels was 0.79 and -0.30 at baseline and at the 5-year visit, respectively. The Ln HA levels were compared with disease outcome, clinical and demographic features, and acute-phase response at presentation (Table 2). HA levels in patients with OA progression were sig- nificantly higher than those in patients without pro- gression ( P = 0.007). Levels in patients whose pro- gression was unknown were similar to those in patients who had progression. Patients who had had previous surgery had higher HA levels compared with those who had not had previous surgery (P = 0.013). However, no differences in HA levels were found with

regard to sex or use of NSAIDs. Of the continuous variables, age and minimum joint space were signifi- cantly associated with HA levels.

The differences between groups and the corre- lation of HA with age and joint space might have been due to confounding factors. In order to allow for this possibility a linear regression model was fitted to compare patients with and those without progression, with HA as the dependent variable. The baseline values of 2 continuous background variables were significantly related to the level of HA. These vari- ables (with parameter estimate and P value) were disease duration (-0.014, P = 0.036) and minimum joint space (-0.099, P = 0.049). Moreover, patients who had had previous surgery had significantly higher levels of HA compared with those who had not had surgery (0.372, P = 0.001). After these variables were taken into account, the patients whose OA progressed still had significantly higher levels of HA compared with those whose disease did not progress (0.203, P = 0.019). There was a significant interaction, suggesting that the relationship between HA and minimum joint space was different between those with progression and those without (-0.135 and 0.012, respectively, P = 0.006), although very few of the patients with progression had a large joint space (>5 mm). When

SERUM HA LEVELS AND OA PROGRESSION 763

Table 2. graphic, clinical, and hematologic variables at entry

Relationship between serum hyaluronic acid (HA) levels (transformed values) and demo-

HA level or correlation coefficient* P i

Discrete variables All patients (n = 60) Disease outcome

Progressors (n = 16) Nonprogressors (n = 31) Undetermined (n = 13)

Male (n = 17) Female (n = 43)

Previous surgery Yes (n == 9) No (n = 51)

Use of NSAIDs$ Yes (n == 41) No (n = 19)

Continuous variables

Sex

Age (n = 150) Age at disease onset (n = 58) Disease duration (n = 58) Weight:height ratio (n = 57) No. of joiint sites involved (n = 60) Minimum joint space (n = 60) Acute-pha.se response

Blood viscosity (n = 43) Plasma C-reactive protein (n = 46) Serum alkaline phosphatase (n = 46)

3.89 2 0.62

4.15 2 0.61 3.66 2 0.55 4.14 2 0.63

3.90 2 0.60 3.89 2 0.71

4.42 L 0.58 3.80 2 0.59

3.92 2 0.61 3.84 5 0.66

0.362 0.211

-0.095 -0.096 -0.106 -0.345

-0.224 0.131 0.108

0.007

0.945

0.013

0.670

0.005 0.112 0.478 0.478 0.475 0.007

0.149 0.386 0.475

* Values for discrete variables are the HA level (mean 2 SD units), values for continuous variables are the correlation coefficient (r), determined by Pearson’s product-moment correlation for age, age at disease onset, disease duration, weight:height ratio, and blood viscosity, and by Spearman’s rank correlation for no. ofjoint sites involved, minimum joint space, plasma C-reactive protein, and serum alkaline phosphatase. f By 2-sample t-test for discrete variables, and by test of zero correlation for continuous variables. t Nonsteroidal antiinflammatory drugs.

this interaction was added to the model, the disease duration effect was no longer significant (P = 0.238).

Logistic regression was performed with disease progression as the dependent variable. Seven indepen- dent variables were considered (age, sex, disease duration, minimum joint space, number of affected joints, weight:height ratio, and HA level), and a step- wise selection was used. The only variables found to be significant were HA (P = 0.007), weight:height ratio (P = 0.008), and number of affected joints (P = 0.029).

Only 46 patients (49%) had HA measured both at study entry and at 5-year followup. Accordingly, longitudinal analysis was carried out on these patients only. The mean HA levels of the patients whose dis- ease progressed (15 of thle 46) and those who did not have progression (3 I of the 46) are shown in Figure 1. The HA levels of the patients whose OA progressed were significantly higher at entry (P = 0.022 by t-test) and at 5-year followup (P < 0.001 by t-test). On

average, the HA levels in both groups decreased. The mean k SD change in HA level was -0.06 5 0.96 units and -0.69 k 0.88 units in the patients with and the patients without progression, respectively. The change in those without progression was significant (95% confidence interval [95% CI] - 1.01, -0.37), whereas the change in those with progression was not (95% CI -0.59, 0.47).

Changes in HA levels were further investigated by dividing the group of patients with disease progres- sion into 2 subgroups: those classified as having pro- gression based on JSN and those classified based on their having had surgery. One of these patients had both JSN and surgery; this patient was analyzed in the surgery group. The mean 2 SD HA level in the JSN group (n = 8) at study entry and at 5-year followup was 3.83 ? 0.15 and 4.27 k 0.24 units, respectively. For the surgery group (n = 7), the values were 4.40 -1- 0.24 and 3.78 k 0.27 units, respectively. All patients with-

764

J

600 -

500- L m al h

Q c)

h

400-

-2 v

3

SHARIF ET AL

p=0.022 m

0

0

0

0 0 0

a 3 5 1 00 0 0

00 0

0

' BASELINE FIVE YEAR Figure 1. Serum hyaluronic acid (HA) levels in osteoarthritis pa- tients at baseline and at 5 years, according to the presence (0) or absence (0) of disease progression at 5 years. Horizontal bars indicate the group means. Ln = natural log.

out progression had HA measured at entry (mean k SD 3.66 ? 0.55 units) and at 5-year followup (2.97 k 0.84 units). The mean increase in HA for the JSN group was not significantly different from the level at study entry (0.44 c -0.34, 95% entry CI -0.36, 1.23). However, the mean decrease in the HA level in the surgery group (-0.62 2 0.24) was significantly differ- ent from the entry level (95% CI -1.21, -0.04).

Keratan sulfate levels. Overall serum KS levels in the patients decreased significantly over the 5-year study period (mean difference -39.76; 95% CI -56.85, -22.67); however, the baseline levels correlated well with the levels at 5 years (r = 0.802) (Figure 2). Longitudinal analysis showed no differences in the change in KS levels between the patients with and those without progression ( P = 0.905, unpaired t-test). Unlike HA, the distribution of KS was not skewed (i.e., KS was normally distributed). Baseline KS levels in relation to disease outcome and clinical and demo- graphic data are shown in Table 3. KS levels were found to be higher in men compared with women (P = 0.005), but were not related to disease outcome,

previous surgery, or use of NSAIDs. None of the continuous variables, including acute-phase protein lev- els, were significantly related to KS levels (Table 3).

A general linear model was constructed at base- line, and, as with HA, each background variable was tested for a significant relationship with KS. Two background variables were found to be significantly related to KS. These variables (parameter estimate and P value) were sex (estiimate for males 44.8, P = 0.001) and number of invollved joints (estimate 11 .O, P = 0.015). After sex and number of joint sites were taken into account, there were no significant differ- ences in levels of serum KS between patients with and those without OA progression, at entry (P = 0.779) or at subsequent visits. A logistic model was constructed using a stepwise approach with progression as the dependent variable. Weight:height ratio was the only variable to emerge from the model, and there was no

700 1

m" /-. 'I /*

+B

*/

/=o * *

'I

+B

*/

,802

V * 1 0 0 - 1 ~ 1 1 I ' f

100 200 300 400 500 600 700

KS (nglml) at entry

Figure 2. Correlation between serum keratan sulfate (KS) levels at baseline and at 5 years, in 66 osteoarthritis patients for whom KS measurements were available at both points.

SERUM HA LEVELS AND OA PROGRESSION 765

Table 3. hematologic variables at entry

Relationship between serum keratan sulfate (KS) levels and demographic, clinical, and

Discrete variables All patients (n = 68) Disease outcome

Progressors (n = 23) Nonprogressors (n = 44) Undetermined (n = I )

Male (n = 24) Female (n = 44)

Previous surgery Yes (n == 9) No (n = 51)

Use of NSAIDsS Yes (n == 41) No (n = 19)

Continuous ,variables

Sex

Age (n = 68) Age at disease onset (n = 66) Disease duration (n = 66) Weight:height ratio (n = 64) No. of joiint sites involved (n = 68) Minimum joint space (n = 68) Acute-phase response

Blood viscosity (n = 48) Plasma C-reactive protein (n = 50) Serum alkaline phosphatase (n = 50)

KS level or correlation coefficient*

379.8 f 108.4

392.4 5 111.9 374.8 2 108.1

309.0

425.8 5 87.7 354.7 f I 1 1.2

358.9 2 96.8 383.8 f 110.8

361.9 f 101.3 389.5 f 112.0

0.147 -0.015

0.125 0.001 0.149

-0.047

-0.157 0.073 0.122

Pt

0.539

0.005

0.459

0.303

0.231 0.905 0.317 0.994 0.225 0.703

0.287 0.614 0.399

~~~~~ ~

* Values for discrete variables are the KS level (mean f SD ng/ml); values for continuous variables are the correlation coefficient (r), determined by Pearson’s product-moment correlation for age, age at disease onset, disease duration, weight:height ratio, and blood viscosity, and by Spearman’s rank correlation for no. ofjoint sites involved, minimum joint space, plasma C-reactive protein, and serum alkaline phosphatase. t By 2-sample t-test for discrete variables, and by test of zero correlation for continuous variables. f Nonsteroidal antiinflammatory drugs.

evidence of a relationshilp between KS and progres- sion of OA.

There was no significant correlation between KS and HA levels as a whole (r = 0.197, P = 0.194) or after the patients were grouped according to occur- rence or lack of occurrence of disease progression (r = 0.096, P = 0.793 and r = -0.011, P = 0.952, respec- tively).

DISCUSSION

In a study using the same group of patients as were used in the present investigation, we previously found that a normal (negative) bone scan result at entry correlated with a finding of little or no change in tibiofemoral JSN at 5-year followup (3). The present cross-sectional and longitudinal study shows that se- rum HA level is predictive of morphologic disease progression in the tibiofemoral joint. In contrast, se-

rum KS levels were not associated with disease pro- gression.

Obesity is a well-recognized contributory factor in OA of the tibiofemoral joint. Our finding that the group of patients with disease progression were on average heavier than those without progression is consistent with earlier reports ( 1 1,12). Moreover, our results showing no evidence of any association be- tween serum KS levels and disease progression are consistent with those of Spector et a1 (13), who found no evidence of an association between radiographic changes and serum KS levels. These results confirm that a single measurement of the serum level of KS is not useful as a prognostic marker in OA (5,14).

The significance of the correlation between KS levels and number of involved joints remains to be elucidated. Our finding that the level of KS does not rise with progression of knee OA is consistent with the

766 SHARIF ET AL

hypothesis that the elevation, often seen in patients with polyarticular OA, develops early and may even precede the onset of clinical symptoms (15). It is important, in this context, to distinguish between anatomic progression of the worst affected joints (the outcome measure used in this study) and the addition of newly affected joints over time. The number of joints affected by OA is difficult to assess without extensive radiography, and spinal disease presents special problems. By univariate analysis, there was no evidence of any relationship between disease progres- sion and number of joint sites involved at entry (Table 1). However, in the logistic regression model, after HA level and weight: height ratio were taken into account, a positive relationship between number of involved joints and the risk of progression was found. Also no relationship between joint space and risk of progression was found. This discrepancy is difficult to explain and may be due to missing values in the latter model, to relatively small numbers of patients studied, and/or to correlations between the explana- tory variables.

The patients whose OA progressed had signifi- cantly higher serum HA levels at entry than the group whose disease had not progressed after 5 years. The results of our modeling processes provided evidence suggesting that patients whose disease progressed and who had previous surgery on one knee had higher levels of HA than those who had disease progression but no previous surgery. The biggest difference in HA levels between patients with and those without disease progression was in those patients who had the smallest joint spaces at entry, although few patients with pro- gression had large joint spaces at entry. The 19 pa- tients lost to followup (I0 of whom had died) were an older group, who also had relatively high HA levels at entry. There were no data available on the progression of their disease; it is possible that their OA was static, in which case the strength of the reported association of HA with disease progression might be less clear. Campion et a1 (4) reported that in patients with OA, HA levels increase with age and radiologic score. We did not find a direct relationship between age and serum HA levels. However, since both HA levels and age are negatively associated with joint space, HA levels would appear to increase with age even if they are not directly related.

Serum HA levels were found to be significantly associated with several other variables, including min- imum joint space (P = 0.049) and previous surgery (P = 0.001). Even after these variables were taken into

account, patients whose 0’4 progressed still had sig- nificantly higher levels of ]HA at entry (P = 0.019). Moreover, the high HA 1e:vels in the patients with progression persisted throughout the study period, suggesting differences in the disease activity of the 2 groups over the 5-year period.

Several other factors that could affect serum HA levels have not been taken into account. For example, impaired renal or hepatic function could result in a reduction in the clearance of HA from serum. There was no evidence of hepatic or renal dysfunction at the 5-year observation point in any of the patients in our study, although we did not monitor liver and kidney function. Treatment with cortico- steroids reduces circulating levels of HA, but NSAIDs have been shown to have no effect (16). We found that HA levels were not different in patients who were versus those who were not laking NSAIDs, and none of the patients received steroids for at least 3 months prior to entering the study. Therefore, the differences in HA levels between the 2 groups of patients could not be explained by treatment.

There is a diurnal variation in HA levels, and the highest levels in normal individuals and in patients with RA have been reported to occur in the morning, 1 hour after arising and performing routine morning activities (17). Thus, the increased HA levels in the patients with disease progression could be accounted for by diurnal variation or exercise. However, since the serum samples from all our patients were obtained at late morning-early afternoon clinics ( 1 1 :00 AM to 1:OO PM), the high levels of HA in the group with OA progression are unlikely to be explained by these variables. Another explanation for the high serum HA levels in the group with progression may be that HA levels reflect transient synovitis in these patients, i.e., that serum samples from these patients were obtained during an episode of inflammation. If this is the case then perhaps the patients who have rapid deterioration and disease progression are the ones who go through such inflammatory phases. However, earlier studies in patients with OA yielded no evidence of a relationship between serum HA levels and blood markers of in- flammation (18,19). In addition, we did not find any evidence of elevated acute-phase protein levels in the patients whose OA progressed, or any association between clinically detectable knee effusions and base- line HA levels.

Minimum joint space was found to be associ- ated with age (P < 0.001) and disease duration ( P = 0.033). This is not surprising since both advancing

SERUM HA LEVELS AND OA PROGRESSION 767

age and duration of disease are likely to result in the destruction of more c,artilage, with a consequent decrease in the tibiofemoral interbone distance and possible increase in serum HA. Similarly, a drop in the serum HA level following surgery is not unex- pected since at least some of the serum HA is derived from the joint (15,20,21). 'The principal source of HA in a joint is the synovium, and since our results indicate that OA patients with high HA levels have more rapid deterioration, tlhe synovium may be at least partially responsible for driving the OA process in these patients.

In conclusion, levels of serum HA, which are thought to be a marker of synovial inflammation (21,22) and have been shown to correlate with ongoing joint damage in RA (23), also predict disease progres- sion in established tibiofemoral OA, a disease in which systemic markers of inflammation are usually within the normal range. The only other serum measure that has been reported to be of prognostic significance in OA is insulin-like growth factor 1 (24). Simple serum assays that predict disease progression in OA could be of immense clinical value, as well as providing further insight into the disease process.

ACKNOWLEDGMENT

We thank Ms Sara Browning for her secretarial assistance with data collection and entering of data onto spreadsheets.

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