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Does isolated patellofemoral osteoarthritis matter?

  • R. Duncan
    Correspondence
    Address correspondence and reprint requests to: Rachel Duncan, Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom. Tel: 44-1782-583905; Fax: 44-1782-583911.
    Affiliations
    Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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  • G. Peat
    Affiliations
    Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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  • E. Thomas
    Affiliations
    Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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  • L. Wood
    Affiliations
    Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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  • E. Hay
    Affiliations
    Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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  • P. Croft
    Affiliations
    Arthritis Research Campaign National Primary Care Centre, Primary Care Sciences, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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Open ArchivePublished:June 01, 2009DOI:https://doi.org/10.1016/j.joca.2009.03.016

      Summary

      Objectives

      To describe the structure-pain and structure-function associations in isolated patellofemoral osteoarthritis (PF OA).

      Design

      Population-based study of 819 adults aged ≥50 years with knee pain. The severity of knee pain, stiffness and disability were measured using the Western Ontario and McMaster Osteoarthritis Index (WOMAC). Three radiographic views of the knee were obtained.

      Results

      Isolated PF OA was mild in 142 participants and moderate/severe in 44. Mean WOMAC scores for pain, stiffness and function were associated with radiographic severity of PF OA (F2,389=4.7, P=0.01; F2,392=4.5, P=0.012 and F2,392=6.1, P=0.002, respectively, adjusted for age, gender, and body mass index (BMI)). Post-hoc tests demonstrated statistically significant differences for mean pain, stiffness and function score between those with mild PF OA and those with normal X-rays. In task-specific items there was evidence of a stepped response, the proportion of participants with moderate/severe/extreme pain or difficulty in performing everyday tasks increasing with the severity of PF OA. The strongest association was observed for pain going up and down stairs (age-gender-BMI adjusted odds ratio (OR) 3.0; 95% confidence interval (CI) 1.4,6.6. Functional tasks most strongly related to radiographic severity were: descending stairs (OR 3.2; (CI 1.5,6.5)), getting in/out of the bath (3.2; 1.5,6.6), getting in/out of a car (3.0; 1.4,6.1).

      Conclusions

      Mild isolated PF OA is significantly associated with symptoms of pain, stiffness and functional limitation. Further research on its recognition in clinical practice and the development of targeted treatments to prevent or slow progression are warranted.

      Key words

      Introduction

      Over 15 years ago McAlindon highlighted the importance of the patellofemoral (PF) joint
      • McAlindon T.E.
      • Snow S.
      • Cooper C.
      • Dieppe P.A.
      Radiographic patterns of OA of the knee joint in the community: the importance of the PF joint.
      in knee osteoarthritis (OA), a view reinforced in a recent review article
      • Hinman R.S.
      • Crossley K.M.
      Patellofemoral joint osteoarthritis: an important subgroup of knee osteoarthritis.
      . However, population studies investigating knee OA over the last two decades have often restricted imaging to the tibiofemoral (TF) joint. In those that have imaged the PF joint, the association between PF OA and symptoms and functional limitation remains unclear. While an association between the presence and severity of PF OA, particularly osteophytes, and the occurrence of knee pain
      • Cicuttini F.M.
      • Baker J.
      • Hart D.J.
      • Spector T.D.
      Choosing the best method for radiological assessment of PF OA.
      • Lanyon P.
      • O'Reilly S.
      • Jones A.
      • Doherty M.
      Radiographic assessment of symptomatic knee OA in the community: definitions and normal joint space.
      , and functional limitation
      • McAlindon T.E.
      • Snow S.
      • Cooper C.
      • Dieppe P.A.
      Radiographic patterns of OA of the knee joint in the community: the importance of the PF joint.
      has been reported in the general population, one recent study among adults with symptomatic knee OA found significantly higher pain severity and functional limitation only in those with combined PF and TF OA, not in those with isolated PF OA
      • Szebenyi B.
      • Hollander A.P.
      • Dieppe P.
      • Quilty B.
      • Duddy J.
      • Clarke S.
      • et al.
      Association between pain, function, and radiographic features in OA of the knee.
      .
      In our cohort of older people with knee pain drawn from the general population, we have previously found a consistent association between increasing symptom severity and the presence of radiographic knee OA
      • Duncan R.
      • Peat G.
      • Thomas E.
      • Hay E.M.
      • Mc Call I.
      • Croft P.
      Symptoms and radiographic oa: not as discordant as they are made out to be?.
      , more recently reporting severity of OA to be associated with increasing symptoms of pain and functional limitation
      • Duncan R.
      • Peat G.
      • Thomas E.
      • Hay E.
      • Croft P.
      How do pain and disability vary with compartmental distribution and severity of radiographic knee osteoarthritis?.
      .
      Although isolated PF OA was a common pattern within this sample, we did not specifically describe the association between symptom severity and radiographic severity in the PF joint after excluding co-occurring TF OA. This was the specific aim of the current paper. The association was investigated using (1) an overall grading score for symptoms and (2) individual task-specific questions.

      Methods

      The Clinical Assessment Study of the Knee (CAS-K) is a prospective observational cohort study of people with knee pain, sampled from the general population. All patients aged 50 years and over registered with three general practices in North Staffordshire, UK, were invited to take part in a two-stage postal survey. Almost all patients in the UK are registered with a general practitioner, and local registers provide a convenient sampling framework for the open population irrespective of any consultation they have had. Respondents to this survey phase who indicated that they had experienced knee pain within the previous 12 months were invited to attend a research clinic for a detailed assessment. This consisted of clinical interview, physical examination, digital photography, plain radiographs, anthropometric measurement and a brief self-complete questionnaire. Detailed descriptions of the study, and recruitment and retention of participants have been previously published
      • Peat G.
      • Thomas E.
      • Handy J.
      • Wood L.
      • Dziedzic K.
      • Myers H.
      • et al.
      The Knee Clinical Assessment Study – CAS(K) A prospective study of knee pain and knee OA in the general population.
      • Peat G.
      • Thomas E.
      • Handy J.
      • Wood L.
      • Dziedzic K.
      • Myers H.
      • et al.
      The Knee Clinical Assessment Study – CAS(K). A prospective study of knee pain and knee OA in the general population: recruitment and retention at 18 months.
      . Ethical approval was obtained for all phases of the study.

      Data collection

      Clinical data

      The 24-item Western Ontario and McMaster Universities (WOMAC) OA Index Likert version 3.0 was used to gather information on severity of knee pain, stiffness and function
      • Bellamy N.
      WOMAC Osteoarthritis index. A user's guide.
      • Jinks C.
      • Jordan K.
      • Croft P.
      Measuring the population impact of knee pain and function with the Western Ontario and McMaster Universities OA index (WOMAC).
      . Each item had five response options (none, mild, moderate, severe, extreme) and yields total subscale scores for pain (five items, total score 0–20), stiffness (two items, total score 0–8) and function (17 items, total score 0–68). Body mass index (BMI) was calculated from weight and height measured at the assessment clinic and categorised into under/desirable, overweight and obese (BMI24.9, 25–29.9 and ≥30 kg/m2, respectively).

      Radiographic views

      A weight bearing posteroanterior (PA) semiflexed view
      • Buckland-Wright C.
      • Wolfe F.
      • Ward R.J.
      • Flowers N.
      • Hayne C.
      Substantial superiority of semiflexed (MTP) views in knee osteoarthritis: a comparative radiographic study, without fluroscopy, of standing extended, semiflexed (MTP), and Schuss views.
      , a skyline and a lateral view were obtained of the knee. The skyline and lateral views were taken with the participants in a supine position, the knee flexed to 45° using a wedge for accuracy. Films were obtained in the radiology department at the University Hospital of North Staffordshire, Stoke on Trent, UK, by a team of six radiographers who had undergone training to standardise the X-rays and who met for regular quality control sessions.

      Radiographic scoring

      A single reader (RD) scored all study films blinded to all clinical and questionnaire data. The TF joint was assessed using the PA view and the posterior compartment of the lateral view. The PF joint was assessed using the skyline and lateral views. On the basis of the authors' original written description, a Kellgren and Lawrence (K&L) grade was assigned to the PA and skyline views (Table I)
      • Lawrence J.S.
      Rheumatism in Populations.
      . This description of the K&L scoring was used in preference to that published in 1957
      • Kellgren J.H.
      • Lawrence J.S.
      Radiological assessment of osteo-arthrosis.
      as it describes which features belong to each grade, which had previously been unreported.
      Table IK&L scores
      Grade 0NoneNo features of OA
      Grade 1DoubtfulMinute osteophyte, doubtful significance
      Grade 2MinimalDefinite osteophyte, unimpaired joint space
      Grade 3ModerateModerate diminution in joint space
      Grade 4SevereJoint space greatly impaired with sclerosis of subchondral bone
      In the lateral view, superior and inferior patellar osteophytes were scored using a standard atlas
      • Burnett S.
      • Hart D.
      • Cooper C.
      • Spector T.
      A Radiographic Atlas of OA.
      . Osteophytes on the posterior tibial surface do not appear in the atlas but were judged on a similar basis of severity as other osteophytes in the lateral view. The scoring methods used enabled a single score to be attributed to OA of the PF joint. The PF and TF joints were defined as normal, mild OA or moderate/severe OA (Table II).
      Table IIRadiographic severity of PF and TF joint OA
      Normal radiographsMild ROAModerate/severe ROA
      PF OASkyline K&L=0 or 1Skyline K&L=2Skyline K&L3
      ANDOROR
      Lateral osteophytes=0Lateral osteophytes=1 or 2Lateral osteophytes=3
      TF OAPA K&L=0 or 1PA K&L=2PA K&L3
      ANDOROR
      Posterior osteophytes=0Posterior osteophytes=1 or 2Posterior osteophytes=3
      Intraobserver and interobserver repeatability was assessed in 50 participants (100 knees); the second reader for the interobserver assessment (PC) had previous experience of grading knee radiographs. Unweighted kappa coefficients were calculated. Intrareader reliability scores for PA K&L score, skyline K&L score and lateral osteophytes were very good (unweighted κ=0.81–0.98); interreader scores were good (κ=0.49–0.76). More detailed results of the repeatability, radiographic scoring and definitions have been previously published
      • Duncan R.C.
      • Hay E.M.
      • Saklatvala J.
      • Croft P.R.
      Prevalence of radiographic OA – it all depends on your point of view.
      .

      Statistical analysis

      Participants with TF OA were excluded. Three mutually exclusive groups formed the basis for analysis (1) normal, (2) mild isolated PF OA and (3) moderate/severe isolated PF OA. Analysis of variance (ANOVA) was used to identify heterogeneity of the overall mean WOMAC pain, stiffness and function scores between the three radiographic groups described above; F- and P-values are presented. Analysis was repeated adjusting for age (50–59/60–69/70–79/80+ years), gender and BMI (≤24.9/25–29.9/≥30 kg/m2). Associations, either crude or adjusted that were statistically significant were examined using post-hoc pairwise comparisons to determine between group differences. WOMAC individual items were dichotomised into two categories: normal/mild and moderate/severe/extreme. The prevalence of “moderate/severe/extreme” pain, stiffness or difficulty for each individual item was described for each level of radiographic severity. The association between radiographic severity and moderate/severe/extreme pain, stiffness or difficulty for each item was expressed initially as a crude odds ratio (OR) with 95% confidence interval (CI). The associations were than adjusted for age (50–59/60–69/70–79/80+ years), gender and BMI (≤24.9/25–29.9/≥30 kg/m2) and adjusted ORs are presented.

      Results

      We have previously reported the recruitment and retention to the CAS-K study
      • Peat G.
      • Thomas E.
      • Handy J.
      • Wood L.
      • Dziedzic K.
      • Myers H.
      • et al.
      The Knee Clinical Assessment Study – CAS(K). A prospective study of knee pain and knee OA in the general population: recruitment and retention at 18 months.
      . Briefly, 819 people attended the research clinic, of which 777 with full radiographic data were potentially eligible for analysis (mean age (standard deviation) 65.5 (8.7) years; BMI 29.6 (5.2) kg/m2; 420 females). Reasons for ineligibility were: patient declined radiography (2), incomplete radiographic data (total knee replacement in index knee (15), unlabelled PA views (2), absent patella (2) and skyline views deemed uninterpretable (5)) and existing diagnosis of inflammatory arthritis (16). 246 (32%) had no evidence of radiographic OA in any compartment. 186 (24%) participants had isolated PF OA (142 mild, 44 moderate/severe). 345 participants had TF OA and were excluded from further analysis.

      Association of isolated patellofemoral OA and pain, stiffness and disability

      Mean WOMAC scores for pain, stiffness and function were associated with radiographic severity. Heterogeneity existed between normal, mild and moderate/severe PF OA for pain (F2,389=4.7, P=0.01), stiffness (F2,392=4.5, P=0.012) and function (F2,392=6.1, P=0.002); the trend being strongly linear in all three subgroups (Table III). Post-hoc tests for pairwise differences, adjusted for age, gender and BMI, demonstrated statistically significant differences for mean pain, stiffness and function scores between those with mild PF OA and those with normal X-rays. The magnitude of differences in mean pain and function scores between mild and moderate/severe PF OA were just as great as between normal and mild PF OA but were of borderline statistical significance due to the smaller numbers of participants with moderate/severe PF OA. There was no statistically significant difference in stiffness between those with mild PF OA and those with moderate/severe PF OA (P=0.284).
      Table IIIAdjusted overall mean WOMAC score for pain, stiffness and function and radiographic severity
      NormalMild ROAModerate/severe ROAANOVAPost-hoc pairwise comparisons
      WOMAC pain score
      N
      Due to missing WOMAC data N will vary.
      21813341
      Unadjusted WOMAC score5.076.107.47F2,389=7.3P=0.001Normal vs Mild: P=0.020 Normal vs Mod/sev: P<0.0001 Mild vs Mod/sev: P=0.056
      Age, gender, BMI adjusted WOMAC score4.945.896.87F2,389=4.7P=0.01Normal vs Mild: P=0.023 Normal vs Mod/sev: P<0.0001 Mild vs Mod/sev: P=0.049
      WOMAC stiffness score
      N22013441
      Unadjusted WOMAC score2.032.562.90F2,392=5.9P=0.004Normal vs Mild: P=0.010 Normal vs Mod/sev: P=0.006 Mild vs Mod/sev: P=0.304
      Age, gender, BMI adjusted WOMAC score1.882.422.61F2,392=4.5P=0.012Normal vs Mild: P=0.011 Normal vs Mod/sev: P=0.006 Mild vs Mod/sev: P=0.284
      WOMAC function score
      N22013441
      Unadjusted WOMAC score15.6820.2925.33F2,392=9.4P<0.0001Normal vs Mild: P=0.004 Normal vs Mod/sev: P<0.0001 Mild vs Mod/sev: P=0.056
      Age, gender, BMI adjusted WOMAC score15.5419.7623.28F2,392=6.1P=0.002Normal vs Mild: P=0.005 Normal vs Mod/sev: P<0.0001 Mild vs Mod/sev: P=0.049
      N=number in the group.
      Due to missing WOMAC data N will vary.

      Association of isolated patellofemoral OA and specific tasks (Table IV)

      Pain

      Across the individual WOMAC pain items, radiographic severity was associated with moderate/severe/extreme pain while performing tasks, but not all results were statistically significant. A stepped response occurred, the stronger associations being found for moderate/severe OA. Those individuals with moderate/severe PF OA had a 3.0 (1.4,6.6) times greater risk of reporting moderate/severe/extreme pain going up and down stairs than those with normal radiographs (age-gender-BMI adj OR, 95% CI).
      Table IVThe association between mild and moderate/severe isolated PF OA and moderate difficulty performing WOMAC tasks
      WOMAC taskMild isolated PF OA, unadjusted OR (95% CI)Mild isolated PF OA adjusted OR (95% CI)
      Adjusted for age, gender, BMI.
      Moderate/severe isolated PF OA, unadjusted OR (95% CI)Moderate/severe isolated PF OA, adjusted OR (95% CI)
      Adjusted for age, gender, BMI.
      Pain subscale
      Pain walking on flat1.3 (0.8,2.1)1.1 (0.7,1.8)2.3 (1.1,4.4)1.7 (0.8,3.5)
      Pain going up and down stairs1.6 (1.0,2.4)1.5 (0.9,2.4)3.8 (1.8,8.2)3.0 (1.4,6.6)
      Pain at night while in bed1.1 (0.7,1.8)1.3 (0.8,2.1)2.1 (1.0,4.1)1.7 (0.8,3.6)
      Pain on sitting or lying1.7 (1.1,2.8)1.7 (1.0,2.9)2.3 (1.1,4.6)1.9 (0.9,3.9)
      Pain on standing upright1.5 (0.9,2.4)1.4 (0.9,2.3)2.4 (1.2,4.8)2.0 (1.0,4.2)
      Stiffness subscale
      Stiffness after waking up in the morning1.6 (1.0,2.5)1.7 (1.0,2.7)2.0 (1.0,3.8)1.6 (0.8,3.4)
      Stiffness after sitting/lying or resting during the day1.5 (0.9,2.3)1.4 (0.9,2.3)2.7 (1.4,5.2)2.5 (1.2,5.0)
      Function subscale
      Difficulty descending stairs1.7 (1.1,2.7)1.5 (0.9,2.5)4.3 (2.1,8.5)3.2 (1.5,6.5)
      Difficulty ascending stairs1.5 (1.0,2.4)1.5 (0.9,2.4)3.7 (1.9,7.4)2.6 (1.2,5.3)
      Difficulty rising from sitting1.6 (1.0,2.4)1.5 (0.9,2.5)3.2 (1.6,6.4)2.6 (1.3,5.2)
      Difficulty standing1.5 (0.9,2.4)1.4 (0.9,2.4)2.1 (1.0,4.2)1.6 (0.7,3.3)
      Difficulty bending to the floor1.6 (1.1,2.5)1.6 (1.0,2.5)2.7 (1.3,5.3)2.2 (1.1,4.6)
      Difficulty walking on the flat1.5 (0.9,2.4)1.4 (0.8,2.3)2.6 (1.3,5.1)2.1 (1.0,4.4)
      Difficulty getting in/out of the car1.8 (1.1,2.7)1.7 (1.1,2.8)3.5 (1.7,6.9)3.0 (1.4,6.1)
      Difficulty going shopping1.5 (0.9,2.3)1.4 (0.8,2.2)2.9 (1.5,5.7)2.2 (1.1,4.6)
      Difficulty putting on socks/stockings1.8 (1.1,2.8)1.7 (1.1,2.9)2.8 (1.4,5.6)2.6 (1.3,5.5)
      Difficulty rising from bed1.6 (1.0,2.6)1.6 (1.0,2.5)3.1 (1.6,6.1)2.7 (1.3,5.6)
      Difficulty taking off socks/stockings2.1 (1.3,3.3)2.1 (1.2,3.4)3.0 (1.5,6.0)2.6 (1.3,5.5)
      Difficulty lying in bed1.8 (1.1,3.0)1.9 (1.1,3.3)2.7 (1.3,5.5)2.5 (1.2,5.4)
      Difficulty getting in/out of bath1.8 (1.2,2.8)2.2 (1.3,3.6)3.7 (1.8,7.4)3.2 (1.5,6.6)
      Difficulty sitting1.6 (1.0,2.7)1.8 (1.0,3.1)2.2 (1.0,4.6)2.1 (1.0,4.7)
      Difficulty getting on/off toilet1.8 (1.1,3.0)1.9 (1.1,3.4)3.1 (1.5,6.4)2.5 (1.2,5.3)
      Difficulty with heavy domestic duties2.0 (1.3,3.0)1.8 (1.2,2.9)2.7 (1.3,5.3)2.2 (1.1,4.6)
      Difficulty with light domestic duties2.3 (1.4,3.7)2.1 (1.3,3.7)3.0 (1.5,6.0)2.6 (1.2,5.4)
      Adjusted for age, gender, BMI.

      Stiffness

      The associations between isolated PF OA and stiffness were much weaker. The only significant association was in those individuals with moderate/severe PF OA who had a 2.5 (1.2,5.0) (age-gender-BMI adj OR 95% CI) times greater risk of moderate/severe/extreme stiffness after sitting/lying or resting during the day, compared to individuals with normal radiographs.

      Function

      The proportion of participants with moderate/severe/extreme difficulty in performing everyday tasks increased with radiographic severity. In 11 of the 17 individual function tasks, mild PF OA was significantly associated with difficulty in performing tasks. Those tasks most strongly related to the severity of radiographic OA were (age-gender-BMI adj OR CI for moderate/severe PF OA vs normal radiographs): descending stairs 3.2 (1.5,6.5), getting in/out of the bath 3.2 (1.5,6.6), getting in/out of a car 3.0 (1.4,6.1), rising from bed 2.7 (1.3,5.6), ascending stairs 2.6 (1.2,5.3), rising from sitting 2.6 (1.3,5.2).

      Discussion

      In our cohort of older persons with knee pain, our findings suggest that isolated PF OA does matter. An increase in severity of radiographic isolated PF OA is associated with greater levels of pain, stiffness and functional limitation, after adjusting for age, gender and BMI. Post-hoc analyses demonstrated moderate/severe isolated PF OA to be most strongly associated with symptoms. However, those with mild isolated PF OA also demonstrated significantly higher levels of pain and functional limitation when compared to those with normal radiographs. The levels of pain and functional limitation in those with moderate/severe isolated PF OA are of a similar magnitude (and not statistically significantly different) to those previously reported in moderate/severe isolated TF OA and moderate/severe combined TF/PF OA
      • Duncan R.
      • Peat G.
      • Thomas E.
      • Hay E.
      • Croft P.
      How do pain and disability vary with compartmental distribution and severity of radiographic knee osteoarthritis?.
      , reinforcing the importance of isolated PF OA. Our results are consistent with those of McAlindon et al.,
      • McAlindon T.E.
      • Snow S.
      • Cooper C.
      • Dieppe P.A.
      Radiographic patterns of OA of the knee joint in the community: the importance of the PF joint.
      who reported a two- to four-fold increased risk of disability in those with isolated symptomatic PF OA compared to age-gender matched controls. However, they conflict with those of Szebenyi et al.,
      • Szebenyi B.
      • Hollander A.P.
      • Dieppe P.
      • Quilty B.
      • Duddy J.
      • Clarke S.
      • et al.
      Association between pain, function, and radiographic features in OA of the knee.
      who reported no difference in pain severity or WOMAC function scores between individuals with isolated PF OA and those with normal X-rays. This may be due to differing methodology used between the studies, such as, radiographic views obtained, definition of radiographic OA and symptom evaluation. Our results also demonstrate an association between isolated PF OA and specific tasks encountered in everyday life. Function was affected even in the presence of mild PF OA, 11 of the 17 tasks evaluating function were associated with difficulty. Those tasks involving knee flexion whilst weight bearing, appear to be the most strongly associated with PF OA. Although the collection of detailed biomechanical measurements was beyond the remit of this large population study, our results are consistent with biomechanical studies that demonstrate a consistent relationship between increasing compression forces in the PF joint and increasing knee flexion
      • Mason J.J.
      • Leszko F.
      • Johnson T.
      • Komistek R.D.
      Patellofemoral joint forces.
      .
      Strengths of the current analysis include using two views to image the PF joint, the ability to restrict analyses to those with isolated PF OA (although insufficient numbers to separately examine severe isolated PF OA), and extending previous task-specific analyses in TF OA
      • Jordan J.
      • Luta G.
      • Renner J.
      • Dragomir A.
      • Hochberg M.
      • Fryer J.
      knee pain and knee osteoarthritis severity in self-reported task specific disability: the johnston county osteoarthritis project.
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      • Algra D.
      • Vandenouweland F.A.
      • Grobbee D.E.
      • Hofman A.
      Associations of radiological osteoarthritis of the hip and knee with locomotor disability in the Rotterdam Study.
      to PF OA. However, there are aspects that deserve detailed critical comment. Firstly, we relied on plain X-rays only and chose to apply a K&L score to the skyline view and not to investigate individual features. Secondly, we used WOMAC subscale scores for severity of pain, stiffness and function. Overlap in item content between the pain and function subscales and concerns over their factor structure have been highlighted
      • Stratford P.W.
      • Kennedy D.M.
      Does parallel item content on WOMAC's pain and function subscales limit its ability to detect change in functional status?.
      • Stratford P.W.
      • Kennedy D.M.
      • Woodhouse L.J.
      • Spadoni G.F.
      Measurement properties of the WOMAC LK 3.1 pain scale.
      • Hawker G.A.
      • Stewart L.
      • French M.R.
      • Cibere J.
      • Jordan J.M.
      • March L.
      • et al.
      Understanding the pain experience in hip and knee osteoarthritis - an OARSI/OMERACT initiative.
      . Although the association between severity of PF OA and function was seen for functional activities with no overlapping item content with the pain subscale, the strength of association between severity of PF OA and pain severity may still be over-estimated if the latter is “confounded by poor physical functioning”
      • Hawker G.A.
      • Stewart L.
      • French M.R.
      • Cibere J.
      • Jordan J.M.
      • March L.
      • et al.
      Understanding the pain experience in hip and knee osteoarthritis - an OARSI/OMERACT initiative.
      . This seems unlikely to fully explain our results as in a previous analysis the direction and strength of cross-sectional association between radiographic severity of whole-knee OA and pain intensity was generally the same whether one used the WOMAC Pain subscale or 11-point numerical rating scales that made no explicit mention of activities
      • Duncan R.
      • Peat G.
      • Thomas E.
      • Hay E.M.
      • Mc Call I.
      • Croft P.
      Symptoms and radiographic oa: not as discordant as they are made out to be?.
      .
      Isolated PF OA is common
      • Duncan R.
      • Peat G.
      • Thomas E.
      • Hay E.
      • Croft P.
      How do pain and disability vary with compartmental distribution and severity of radiographic knee osteoarthritis?.
      and appears to have different risk factors than TF OA
      • Cooper C.
      • McAlindon T.
      • Snow S.
      • Vines K.
      • Young P.
      • Kirwan J.
      • et al.
      Mechanical and constitutional risk factors for symptomatic knee osteoarthritis: differences between medial tibiofemoral and patellofemoral disease.
      • Cicuttini F.M.
      • Spector T.
      • Baker J.
      Risk factors for osteoarthritis in the tibiofemoral and patellofemoral joints of the knee.
      . If it is indeed independently associated with symptoms and function, and forms a relatively early stage in the process of knee OA, then further research on its recognition in clinical practice and the development of targeted treatments and evaluation of their ability to slow or prevent structural and clinical progression are needed, and indeed has begun
      • Crossley K.M.
      • Vicenzino B.
      • Pandy M.G.
      • Schache A.G.
      • Hinman R.S.
      Targeted physiotherapy for patellofemoral joint osteoarthritis: a protocol for a randomised, single-blind controlled trial.
      .

      Conflict of interest

      None of the authors have any conflict of interest or disclosures to report in relation to this work.

      Acknowledgements

      This study is supported financially by a Programme Grant awarded by the Medical Research Council, UK (Grant Code: G9900220) and by Support for Science funding secured by North Staffordshire Primary Care Research Consortium for NHS service support costs.
      The authors would like to acknowledge the contributions of Dr Krysia Dziedzic, June Handy, Charlotte Clements, Dr Jonathan Hill, Dr Helen Myers, Dr Ross Wilkie, and Dr Laurence Wood to aspects of the conception and design of the study and to the acquisition of data. Dr Jacqueline Saklatvala, Carole Jackson, Julia Myatt, Janet Wisher, Sue Stoker, Sandra Yates, Kath Hickson from the Department of Radiography, Haywood Hospital to the acquisition of radiographic data. Professor Chris Buckland-Wright for advice and training on the radiographic techniques and Professor Iain McCall for contribution to study concept and design. The authors would also like to thank the administrative and health informatics staff at the Arthritis Research Campaign National Primary Care Centre, Keele University and the staff and patients of the participating general practices.

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