Correlation of Articular Cartilage Thickness Measurements Made with Magnetic Resonance Imaging, Magnetic Resonance Arthrography, and Computed Tomographic Arthrography with Gross Articular Cartilage Thickness in the Equine Metacarpophalangeal Joint.

This study tested whether MRI, MR arthrography, or CT arthrography can accurately measure fetlock cartilage thickness in horses compared with direct (gross) measurements, and found that none consistently matched the true thickness. Even with joint contrast and maximal distension, cartilage surfaces that touched each other were hard to tell apart on images.

What problem the study addresses and why it matters

• Osteoarthritis in the equine metacarpophalangeal (fetlock) joint commonly causes lameness and is characterized by cartilage damage.
• Clinicians need a reliable, noninvasive way to measure articular cartilage thickness to detect early injury and guide timely intervention.
• This study evaluates whether three imaging options—MRI, magnetic resonance arthrography (MRA), and computed tomographic arthrography (CTA)—can provide measurements that agree with actual (gross) cartilage thickness.

Study design at a glance

• Type: Methods-comparison study correlating imaging-derived cartilage thickness with gross anatomic measurements.
• Specimens: 14 cadaveric Thoroughbred thoracic limbs (fetlock joints).
• Exclusion: Any limb showing metacarpophalangeal cartilage pathology on imaging was excluded (aiming to assess measurement accuracy in normal cartilage).
• Regions measured: Nine predefined regions across the distal third metacarpal bone (MC3) and proximal phalanx (P1).
• Imaging planes: Sagittal plane measurements for all modalities to standardize comparisons.
• Analysis: Mean ± SD reported; linear regression assessed correlation of imaging measurements with gross thickness.

Imaging modalities and measurement approach

• MRI: Baseline non-contrast sequences used to measure cartilage thickness in each of the nine regions.
• MRA: Repeated measurements after intra-articular contrast with the intent to improve visualization of cartilage–fluid interfaces.
• CTA: Sagittal reformations obtained after intra-articular iodinated contrast to capitalize on high spatial resolution and edge definition.
• Consistency: The same anatomic locations were measured across modalities to enable direct comparison to gross thickness.

Contrast strategy and rationale

• Initial contrast volume: 14.5 ml injected intra-articularly.
• Enhanced distension: For a second set of seven limbs, contrast volume was increased to maximal joint distension.
• Goal: Improve separation between opposing cartilage surfaces and enhance conspicuity of cartilage boundaries on both MRA and CTA.

Primary outcomes and statistical approach

• Outcome of interest: Agreement (correlation) between imaging-based thickness and gross anatomic thickness for each region.
• Statistics: Linear regression used to quantify correlations; summary descriptive statistics calculated for measurements.

Main findings

• No single imaging test (MRI, MRA, or CTA) produced cartilage thickness measurements that consistently correlated with gross measurements across regions.
• Even with intra-articular contrast and maximal joint distension, differentiating the apposed cartilage surfaces of MC3 and P1 was difficult in regions where they are in close contact.
• Increasing contrast volume alone did not reliably resolve the boundary between adjacent cartilage layers sufficiently to achieve consistent accuracy.

Why accurate measurement was difficult (technical interpretation)

• Close apposition of cartilage layers: In the normal fetlock, MC3 and P1 cartilage surfaces often coapt tightly, leaving little or no intervening fluid layer to outline boundaries, even after arthrographic distension.
• Partial volume effects: Thin equine articular cartilage is susceptible to blurring of edges when slice thickness or voxel size approaches cartilage thickness, reducing measurement precision.
• Limited cartilage–fluid contrast:
  • MRI without contrast may struggle to delineate the exact cartilage–cartilage interface.
  • MRA relies on contrast filling the joint space; coaptation can prevent contrast from interposing between surfaces where it is most needed.
  • CTA provides sharp edges but has inherently lower soft-tissue contrast for cartilage and similarly depends on contrast interposition.
• Curved joint surfaces and small target: Complex curvature and small regional thickness variations increase sensitivity to slight plane misalignment and registration errors across modalities.

Clinical implications

• Screening limitation: Neither MRI, MRA, nor CTA can be relied upon to yield universally accurate absolute cartilage thickness measurements in normal-appearing fetlock joints.
• Decision-making: Imaging evidence of focal thinning should be interpreted cautiously, especially in regions where opposing cartilage is closely apposed.
• Complementary assessment: Morphologic signs (surface irregularity, signal/attenuation changes, subchondral bone changes) and clinical findings may remain crucial alongside thickness estimates.
• Procedural expectations: Simply increasing arthrographic distension volume is unlikely to solve the boundary-delineation problem in tightly apposed regions.

Limitations to consider

• Cadaveric study: Postmortem tissue properties and absence of physiologic loading may affect joint space behavior and contrast distribution compared with live horses.
• Sample size: Fourteen limbs (with seven undergoing maximal distension) may limit power to detect modest modality-specific advantages across regions.
• Normal cartilage focus: Excluding joints with overt pathology avoids confounding but may not reflect clinical scenarios where cartilage injury creates separable surfaces or signal changes that could aid delineation.
• Sequence/protocol dependence: Specific MRI sequences, CTA acquisition parameters, and reconstruction settings can influence edge sharpness and may vary across institutions.

Practical takeaways for veterinarians and imagers

• Do not rely on a single modality for exact fetlock cartilage thickness; use multimodal assessment and clinical correlation.
• Be cautious interpreting small differences in measured thickness, particularly in regions of cartilage coaptation.
• When pursuing arthrography, recognize that more contrast or greater distension may not create a visible separation in normal, tightly apposed regions.
• Emphasize detection of ancillary indicators of cartilage disease (surface defects, fissures, subchondral bone changes) rather than absolute thickness alone.

Future directions

• Higher-resolution imaging: Exploration of ultra-high-resolution MRI, optimized 3D sequences, thinner slices/voxels, and advanced reconstruction to reduce partial volume effects.
• Contrast and sequence optimization: Use of cartilage-specific MR techniques (e.g., quantitative mapping) or dual-energy CT protocols to enhance tissue contrast, pending validation.
• Dynamic or positional imaging: Techniques that slightly distract the joint to separate surfaces safely, if feasible and ethical in vivo.
• In vivo validation: Studies in live horses, including cases with varying grades of cartilage pathology, to evaluate whether disease-induced separation or signal changes improve thickness accuracy and clinical utility.