Cartilage thickening in early radiographic knee osteoarthritis: a within-person, between-knee comparison

Abstract

Objective: To determine whether the presence of definite osteophytes (in the absence of joint space narrowing [JSN]) on radiographs is associated with (subregional) increases in cartilage thickness in a within-person, between-knee cross-sectional comparison of participants in the Osteoarthritis Initiative. Based on previous results, the external weight-bearing medial femoral condyle (ecMF) and external weight-bearing lateral femoral condyle (ecLF) subregions were selected as primary end points.

Methods: Both knees of 61 Osteoarthritis Initiative participants (n = 4,796) displayed definite tibial or femoral marginal osteophytes and no JSN in 1 knee, and no signs of radiographic osteoarthritis (OA) in the contralateral knee; this was confirmed by an expert central reader. In these participants, cartilage thickness was measured in 16 femorotibial subregions of each knee, based on sagittal double-echo steady-state with water excitation magnetic resonance images. Location-specific joint space width from fixed-flexion radiographs was determined using dedicated software. Location-specific associations of osteophytes with cartilage thickness were evaluated using paired t-tests and mixed-effects models.

Results: Of the 61 participants, 48% had only medial osteophytes, 36% only lateral osteophytes, and 16% bicompartmental osteophytes. The knees with osteophytes had significantly thicker cartilage than contralateral knees without osteophytes in the ecMF (mean ± SD +71 ± 223 μmoles, equivalent to an increase of +5.5%; P = 0.015) and ecLF (mean ± SD +64 ± 195 μmoles, +4.1%; P = 0.013). No significant differences between knees were noted in other subregions or in joint space width. Cartilage thickness in the ecMF and ecLF was significantly associated with tibial osteophytes in the same (medial or lateral) compartment (P = 0.003).

Conclusion: The knees with early radiographic OA display thicker cartilage than (contralateral) knees without radiographic findings of OA, specifically in the external femoral subregions of compartments with marginal osteophytes.

Figure 1

Anterior-posterior fixed-flexion radiograph of a study participant. The right knee shows a grade 1 osteophyte on the lateral tibia (white arrow), but no medial or lateral joint space narrowing. In addition, there is a small osteophyte at the intercondylar notch that is not part of OARSI atlas-based grading (black arrow). The contra-lateral left knee shows neither osteophytes nor joint space narrowing.

Figure 2

Sagittal double echo steady state (DESS) MR images with water excitation. Image A shows a MR image of the medial femorotibial compartment without and image B with segmentation of the cartilage in the defined region of interest.

Figure 3

Graph showing the medial and lateral tibial, and the medial and lateral weight-bearing femoral cartilage plates. The 16 femorotibial subregions are shown. Percent differences between osteophyte versus contra-lateral non-osteophyte knees are given for each subregion. The values for the primary endpoints ecMF and ecLF are encircled and marked for significance (*). MT = medial tibia, cMF = weight-bearing medial femoral condyle, LT = lateral tibia, cLF = weight-bearing lateral femoral condyle; c= central, e=external, i=internal, a= anterior, p= posterior.