Background/Purpose: Knee osteoarthritis (OA) is the most common cause of functional limitation in older adults.  It is not known, however, what proportion progress to a trajectory of functional decline and to what extent worsening of structural damage is associated with decline, particularly among initially well-functioning adults.  We examined trajectories of physical function in adults with or at high risk of knee OA who started without functional limitation, and examined the relation of structural worsening to these trajectories.

Methods: We utilized data from the Osteoarthritis Initiative (OAI) and the Multicenter Osteoarthritis (MOST) to describe function trajectories over 48- and 84-months, respectively.  Physical function was measured with the WOMAC physical function (PF) subscale (0-68) for each knee in OAI and each person in MOST.  We included only participants with WOMAC-PF=0 (i.e., no functional limitation) at baseline.  We used a group-based method (Proc Traj) to identify homogeneous clusters of developmental trajectories in MOST and OAI.  We then examined the relation of worsening structural disease to the trajectory of subsequent functional decline. Since MOST had few high-functioning participants, we conducted this analysis in OAI only. Structural worsening was defined as any increase in Kellgren and Lawrence (KL) or joint space narrowing grade, or a new total knee replacement from 0- to 24-months.  We examined the association of worsening structural disease (0- to 24-months) with trajectories of subsequent physical function (24- to 72-months) using multinomial logistic regression, adjusting for age, sex, body mass index, and baseline KL grades in OAI.

Results: Our study sample comprised 3,320 knees in OAI (age 63.3 ± 9.2 yrs, 53.4% female, BMI 27.4  ± 4.4 at 24-month visit) and 336 people in MOST (age 61.3 ± 8.0 yrs, 53.9% female, BMI 29.6  ± 4.8 at the baseline visit).  We identified 3-trajectories in OAI and MOST (Figures).  For OAI, the 1^st trajectory, no limitation, had stable WOMAC-PF, with an average value < 1 at the last follow-up and included 65.6% of all knees.  The 2^nd trajectory, mild limitation, progressively worsened 1.0 unit/year to 4/68 at the last follow-up and included 24.8%.  The 3^rd trajectory, moderate limitation, progressively worsened 3.4 units/year to 13.7/68 at the last follow-up and included 6.3%.  We observed similar trajectories in the MOST study.  Worsening structural disease was associated with 1.6 to 1.8 times the odds of a trajectory of worsening function compared with those with no change in OAI (Table).    

Conclusion: In initially well-functioning people with or at high risk of knee OA, a minority has progressive worsening of physical function, with worsening structural disease contributing to risk of this decline.  Such findings highlight the importance of addressing methods to maintain and improve function even among those with knee OA who are well-functioning.