Patients
Participants with knee OA were recruited to the Vitamin D Effects on Osteoarthritis (VIDEO) study, a multi-center, randomized, and double-blind clinical trial to evaluate the effects of vitamin D supplementation in patients with knee OA. Five knees lacked of readable MRI data and were excluded. Baseline data were collected for 408 participants (mean age: 63.2 years, 50% women, 149 from Melbourne and 259 from Tasmania), and 335 (82.1%) completed an approximately 2-years follow-up. In brief, eligible participants had symptomatic knee OA (assessed according to the American College of Rheumatology criteria) [22]. The exclusion criteria included grade 3 radiographic changes according to Altman and Gold’s atlas [23], severe knee pain on standing (> 80 mm on a 100-mm VAS), contraindication to MRI, rheumatoid or psoriatic arthritis, lupus, cancer, and history of significant knee trauma. The Tasmania Health and Human Medical Research Ethics Committee and Monash University Human Research Ethics Committee approved this study. Written informed consent was obtained from all participants. For the purpose of analysis, treatment and placebo groups were combined as a cohort.
MRI assessment
MRI scans of the knees were performed at baseline and follow-up. Image sequences included the following: (1) a coronal or sagittal T1-weighted, fat-saturated, three-dimensional (3D) spoiled gradient echo with flip angle 30°; repetition time: 40 ms; echo time: 7 ms; acquisition time: 5 min 58 s; slice number: 60; pixel matrix: 512 × 256; and slice thickness, 1.5 mm with no gap; and (2) a T2-weighted, fat-saturated, fast spin echo with flip angle 90°; repetition time: 3060 ms; echo time: 94 ms; slice number: 46; pixel matrix: 256 × 224; and slice thickness: 2 mm with no gap.
The morphological parameters of PTFJ
The morphological status of PTFJ was assessed as follows (Fig. 1), by using the software program Osiris. In coronal MRI, point c is the outermost point of the fibular articular surface, and a is the innermost point of the articular surface in PTFJ. e is the lateral vertex of the tibial plateau, and f is the medial vertex of the tibial plateau. Line oa is drawn through a, which is parallel to line ef. The length of ac and the angle oac (α) were measured at each slice of MRI. In sagittal MRI from the same participant, point b is the lowest point of the fibular articular surface, and c is the highest point of the fibular articular surface in PTFJ. e and f are points that connect the anterior and posterior horns of the lateral meniscus with the tibial plateau, respectively, and line ef runs parallel to the tibial plateau. Line ob is drawn through b and is parallel to ef. The length between bc and the angle of obc (β) was then measured at each slice of MRI. Next, we calculated the average angles of PTFJ in both coronal (Ave_COR_ang) and sagittal (Ave_SAG_ang) planes, contacting area of PTFJ based to the ideal model (marked as “S”), load-bearing area (“Sτ”), lateral stress-bolstering area (“Sφ”), and posterior stress-bolstering area (“Sυ”) of PTFJ (cm2). This method was valid and reliable for assessing the PTFJ and could be used to measure PTFJ morphology in knee OA. Intra-observer and inter-observer correlation coefficients were validated for the measurements of morphological parameters of PTFJ previously. PTFJ typing measures were repeated by one reader (JC) 3 months later or scored by two readers (JC and TM) independently in 50 randomly selected participants to calculate intra-reader and inter-reader reliabilities. The intraclass correlation coefficients (ICCs) were 0.90–0.95, and the inter-reader reliabilities were 0.90–0.94 [24].
Cartilage defects
The cartilage defects (0–4) were assessed at the medial tibial, medial femoral, lateral tibial, and lateral femoral sites using T1-weighted images as previously described and were further confirmed using T2-weighted images as follows [24]: grade 0, normal cartilage; grade 1, focal blistering and intra-cartilaginous low-signal intensity area with an intact surface and bottom; grade 2, irregularities on the surface or bottom and loss of thickness < 50%; grade 3, deep ulceration with loss of thickness > 50%; and grade 4, full-thickness chondral wear with exposure of subchondral bone. The intra- (ICCs: 0.89–0.94) and inter-observer reliabilities (ICC: 0.85–0.93) were excellent [25]. Increase in cartilage defects was calculated as follows: cartilage defects change = follow-up cartilage defects–baseline cartilage defects.
Bone marrow lesions (BMLs)
BMLs were defined as discrete areas of increased signal adjacent to the subcortical bone. The areas were measured semi-quantitatively using a modified Whole-Organ Magnetic Resonance Imaging Score (WORMS) method in 15 sub-regions [26]. The medial compartment was divided into three subregions (anterior, central, and posterior), and BMLs were scored categorically according to the maximal percentage of bone area that the lesion occupied within the total subregion. We scored grade 0 if there were no BMLs, grade 1 for lesions occupying < 25% of the subregion, grade 2 for lesions occupying between 25 and 50% of the subregion, and grade 3 for lesions occupying > 50% of the subregion. The final score was calculated as the total of subregional scores. The BMLs scores (0–9) were assessed at the medial tibial, medial femoral, lateral tibial, and lateral femoral sites. The intra- and inter-observer reliability of this BMLs scoring system has been shown to be excellent [27]. Increase in BMLs was calculated as follows: bone marrow lesions change = follow-up BMLs–baseline BMLs.
Cartilage volume
Knee cartilage volume was determined on T1-weighted MR images with image processing on an independent workstation, as previously described [25]. The tibial cartilage volumes were calculated by manually drawing disarticulation contours around the cartilage boundaries, section by section, which were then resampled for the final 3D rendering using OsiriX imaging software. The intra-class correlation coefficients ranged from 0.92 to 0.96 for intra-observer reliabilities. Changes in cartilage volume per annum in each site were calculated as shown: (follow-up cartilage volume–baseline cartilage volume)/(baseline cartilage volume × follow-up time); absolute change (mL) = follow-up volume–baseline volume.
Knee tibial plateau bone area
The areas of medial and lateral tibial plateau bone were measured manually on the three slices on axial T1-weighted MR images closest to the tibial cartilage. An average of these three areas was used as an estimate of the tibial plateau bone area.
Anthropometrics measures
Height (cm) was measured to the nearest 0.1 cm (with shoes removed) using a stadiometer (Leicester Height Measure, Invicta Plastics Ltd, Leicester, UK). Weight (kg) was measured to the nearest 0.1 kg using an electronic scale, with bulky clothing removed. Body mass index (BMI) [28] was calculated from these data as follows: weight (kg)/height2 (m2).
Statistical analyses
Baseline characteristics were compared between participants with high and low load-bearing area of PTFJ using Student’s t-tests for continuous data or Chi square tests for proportions.
Log binominal regression analyses were used to assess associations between PTFJ morphological parameters (independent variable) and increases in cartilage defects or BMLs (dependent variables) before and after adjustment for age, sex, height, weight, tibial plateau bone area, radiographic osteoarthritis (ROA), and intervention. Relative risks (RRs) were estimated using these analyses where the outcome was dichotomous.
The longitudinal associations between the morphological parameters of PTFJ and the change of tibial cartilage volume were analyzed using linear regression analyses with adjustment for age, sex, height, weight, ROA, tibial plateau bone area, and intervention. Scatter plots were used to examine the associations between baseline load-bearing area of PTFJ and changes in cartilage volume (including medial and lateral tibial) per annum.
All statistical analyses were performed on Stata V.13.0 (Stata Corp., College Station, TX, USA), and P < 0.05 was considered as statistical significance.