Magnetic Resonance Imaging of
Articular Cartilage Defects and Repair

Dr. Vladimir Bobic
Consultant Orthopaedic Knee Surgeon
The Royal Liverpool University Hospitals
Broadgreen Hospital Knee Service, Liverpool
The Grosvenor Huffield Hospital Knee Clinic, Chester
UNITED KINGDOM

Introduction
It is well-known that the capacity of articular cartilage for repair is limited. Partial-thickness defects in articular cartilage do not heal spontaneously. Injuries of the articular cartilage that do not penetrate the subchondral bone do not heal and usually progress to the degeneration of the articular surface. Injuries that penetrate the subchondral bone undergo repair through the formation of fibrocartilage.

Recently, a great deal of interest has been focused on the repair of articular cartilage lesions. Early and accurate diagnosis of chondral injury has become more important than ever. However, non-invasive diagnosis of articular cartilage lesion in clinical practice is still difficult and unreliable.

Although the MR imaging of cartilage has been extensively researched and used in clinical practice, there is considerable disagreement with regard to the MR appearance of normal cartilage, the best technique for imaging cartilage abnormalities, and the accuracy of these techniques in the detection of abnormalities. In the CD textbook Magnetic Resonance Imaging in Orthopaedic & Sports Medicine, published in 1997, Kneeland states that "the cartilage has proven exceedingly difficult to evaluate accurately with MR imaging."3

In day-to-day practice, a routine clinical MRI scan has low sensitivity in diagnosing chondral damage when compared with arthroscopic findings. Levy et al4 reported in 1996 that preoperative MRI scans correctly identified only 21% of the chondral lesions seen at arthroscopic examination (five out of 23 knees in 15 high-calibre soccer players).

However, since 1996, the new awareness of the significance of chondral problems, an extensive laboratory and clinical research, and various attempts to repair hyaline articular cartilage, have resulted in increased interest in magnetic resonance imaging as a diagnostic and evaluation tool. Development of refined MRI techniques and recent advances in MRI technology appear to be very promising. Magnetic resonance imaging has potential to replace the more conventional invasive techniques, like arthroscopy and biopsy, in the evaluation of articular cartilage damage and repair.

New Developments in MRI Evaluation of Cartilage
MRI has unique capabilities to evaluate the cartilage non-invasively. Second-look arthroscopy and cartilage biopsy may soon become obsolete. In this respect, several areas of evaluation of the articular cartilage lesions, repair or transplanted osteochondral autografts1,6,7,8 are particularly interesting:

  • Accurate mapping of the entire femoral, tibial and patella normal articular cartilage thickness and shape.
  • Composite 3D rendering of individual plates segmented from magnetisation transfer subtraction images
  • Morphologic quantification of sites of cartilage lesion or repair
  • Comparative assessment of the transplanted osteochondral autografts and allografts (recipient vs. donor).
  • Differentiation of hyaline and fibrocartilage.
  • High resolution assessment of the tidemark.
  • High resolution assessment of the osteochondral autograft transplant integration, at various levels.
  • Longitudinal study of cartilage lesions and repair.
  • Evaluation of cartilage lesions and repair and comparison with adjacent normal cartilage.
  • Weight-bearing kinematic studies to assess normal hyaline cartilage pressure contact areas and deformation (iMRI).
  • Weight-bearing kinematic biomechanical evaluation of repaired articular cartilage (iMRI).

Magnetic Resonance Imaging Methods
Chondral injury such as fissures, erosions, fibrillation, and clefts, produce alteration in the morphology of the cartilage, which can be seen on MR scans as surface irregularities and focal defects filled with joint fluid. Numerous studies have been performed in an attempt to identify the optimum technique for the detection of these cartilage abnormalities.

The most commonly advocated MRI technique for showing articular cartilage is a fat suppressed three dimensional T1-weighted gradient echo technique. Reported sensitivities for the detection of chondral lesions range from 75% to 93%. However, this technique has several limitations: the long imaging time, inadequate visualisation of ligamentous and meniscal pathology, necessitating additional sequences. Gradient echo sequences are prone to magnetic susceptibility artefact, which is accentuated in the presence of orthopaedic instruments, including arthroscopic instruments, limiting evaluation of chondral defects and repaired cartilage following surgical intervention.1,4,6 The Department of Radiology at Stanford University is at the forefront of the development of MRI techniques to minimise the effects of metallic artefacts in cartilage imaging. Two techniques have been developed: view-angle tilting and spectroscopic imaging, which allow imaging of cartilage at short echo times while offering immunity to metallic fragments left in the joint. This is especially important in cases such as multiple osteochondral autograft transplantation, which leaves considerable metallic artefacts.1

Spectral-spatial three-dimensional magnetisation transfer was also developed at Stanford and provides high-resolution 3D imaging of the entire joint with excellent cartilage to bone and cartilage to fluid contrast. Use of the spectral-spatial pulse provides superior lipid suppression to other methods.1 This method is also useful in quantifying subchondral edema. The MT (magnetisation transfer) effect may be helpful in quantifying the amount of collagen present.

High-resolution short echo time spectroscopic imaging: Short echo times are essential to see zonal areas of cartilage and are more sensitive for cartilage pathology. This method provides ultra-short echo time, high resolution images of cartilage over a small area, in addition to spectroscopic data. It is possible that this method can be used to distinguish between hyaline and fibrocartilage. This technique is also unique to Stanford and includes immunity to metallic artefacts and the ability to examine spectra from specific areas within the transplant.1

MRI Evaluation of the Osteochondral Autograft Transplantation (OAT)

MRI evaluation of the osteochondral autograft transplantation, with the OATS technique2 and instrumentation (Arthrex, Inc.) has been used in Liverpool, UK, since early 1997 in 9 patients, at 3 to 12 months after transplantation. Appropriate high-resolution protocol for articular cartilage imaging has been used for both clinical and research MR imaging. MR image opposite: single 10 mm osteochondral autograft transplant to isolated medial femoral condylar cartilage defect, done simultaneously with the BPTB ACL reconstruction, after 6 months (good bone to bone integration, good cartilage cover, matching curvature and thickness, congruent articular surface).

Metallic artefacts: two 10 mm trochlear osteochondral graft transplants after 3 months. Metallic artefacts are visible as a cluster of black speckles within soft tissues, on the left side of the picture, close to the patella and medial femoral condyle.1,8

Technical Problems: the picture to the right and serial MR images to the far right clearly demonstrate that the wrong angle during harvesting and inserting the graft will result in incongruent transplant that is too proud on one side and sunk-in on the opposite side.9

OAT Transplant MRI Analysis: The orange pixels correspond to normal T2 values for bone. The blue and purple pixels are anomalous: the T2 relaxation times are elevated because the tissue is "wetter" than normal (fluid interface between recipient and donor bone).

OAT comparative analysis of normal and transplanted bone core: The mean T2 value of normal bone is 84.0 ms. In the region of the implant the T2 value is elevated to 116.3 ms. The synovial fluid has T2 values greater than 200ms.

The incorporation of the implant in the surrounding bone can be objectively followed and compared between patients, by pixel by pixel, histogram and profile analysis of T2 relaxation time maps.9

The variation in T2 values at the margin of the implant provides an objective measure of the magnitude of the discontinuity between the implant and surrounding bone (opposite picture).

Dynamic Studies in the iMRI: General Electric have developed an interventional MR system which was first installed in 1994. The machine relies on super-conductor technology and a cryogen-free open magnet with a 56 cm gap. General Electric identified 10 centres worldwide, including the Stanford University Department of Radiology, where these systems have been installed. The site in the United Kingdom is at Imperial College School of Medicine at St. Mary's Hospital in London.3

The interventional MRI system with the double donut design allows upright imaging during static weight-bearing conditions and during weight-bearing motion studies. This system may prove useful in a non-invasive biomechanical evaluation of repaired and transplanted articular cartilage.1 It has already provided interesting dynamic information on patellofemoral joint and of the anterior cruciate deficient knee during weight bearing.

Summary
In summary, all of the above techniques are promising in evaluating articular cartilage. MR imaging is already an effective method to diagnose chondral injury, to aid in the selection of therapeutic intervention and to assess the short-term and long-term outcome of repaired articular cartilage.1,4,6,7

References

  1. Bergman G, Lang P, Gold G. Articular Cartilage MRI Research. Department of Radiology, Musculoskeletal Section, Stanford University Medical Center, Stanford, California, USA. Unpublished data, 1997 (ongoing clinical research, personal communication).
  2. Bobic V. Arthroscopic osteochondral autograft transplantation in Anterior cruciate ligament reconstruction: A preliminary clinical study. Knee Surg Sports Traumatol Arthrosc 1996; 3:262-264.
  3. Hunt D, Gedroyc W. Interventional magnetic resonance imaging. British Orthopaedic News, Spring 1998, 24.
  4. Kneeland JB. MR imaging of articular cartilage and of cartilage degeneration. In: Stoller DW. Magnetic Resonance Imaging in Orthopaedic & Sports Medicine. CD-ROM. Lippincott-Raven Publishers, 1997.
  5. Levy AS, Lohnes J, Sculley S, LeCroy M, Garrett W. Chondral delamination of the knee in soccer players. Am J Sports Med 1996; 24(5):634-9.
  6. Linklater JM, Potter HG. Imaging of Chondral Defects. In: Miller M, Guest Ed. Treatment of Chondral Injuries. In: Fu F, ed. Operative Techniques in Orthopaedics. Philadelphia, PA: Saunders. 1997; 7(4):279-288.
  7. Peterfy CG, Howard DS. Imaging the Patellofemoral Joint: Current Status and Future Directions. Am J Knee Surgery, 1997; 2(10):109-120.
  8. Ritchie DA. Osteochondral Autograft Transplantation: Clinical MRI Research. The Royal Liverpool University Hospitals, Radiology Department, Liverpool, United Kingdom. Unpublished data, 1998 (clinical correspondence, personal communication).
  9. Whitehouse GH, Roberts N. Osteochondral Autograft Transplantation: Clinical MRI Research. University of Liverpool, Magnetic Resonance and Image Analysis Research Centre, Liverpool, United Kingdom. Unpublished data, 1998 (clinical correspondence, personal communication).