The anatomical basis for a novel classification of osteoarthritis and allied disorders

Abstract

Osteoarthritis (OA) has historically been classified as 'primary' where no discernible cause was evident and 'secondary' where a triggering factor was apparent. Irrespective of the triggering events, late-stage OA is usually characterized by articular cartilage attrition and consequently the anatomical basis for disease has been viewed in terms of cartilage. However, the widespread application of magnetic resonance imaging in early OA has confirmed several different anatomical abnormalities within diseased joints. A key observation has been that several types of primary or idiopathic OA show ligament-related pathology at the time of clinical presentation, so these categories of disease are no longer idiopathic - at least from the anatomical perspective. There is also ample evidence for OA initiation in other structures including menisci and bones in addition to articular cartilage. Therefore, a new classification for OA is proposed, which is based on the anatomical sites of earliest discernible joint structural involvement. The major proposed subgroups within this classification are ligament-, cartilage-, bone-, meniscal- and synovial-related, in addition to disease that is mixed pattern or multifocal in origin. We show how such a structural classification for OA provides a useful reference framework for staging the magnitude of disease. For late-stage or end-stage/whole organ disease, the final common pathway of these different scenarios, joint replacement strategies are likely to remain the only viable option. However, for younger subjects in particular, near the time of clinical disease onset, this scheme has implications for therapy targeted to specific anatomical locations. Thus, in the same way that tumours can be classified and staged according to their tissue of origin and extent of involvement, OA can likewise be anatomically classified and staged. This has implications for therapeutic strategies including regenerative medicine therapy development.

Fig. 1

Sagittal section of the knee joint showing the tissues on which the proposed classification of osteoarthritis is based: articular cartilage (AC), subchondral bone (SB), meniscus (M), synovium (S) and (inset) ligament (L). C, capsule; CB, cancellous bone; F, femur; H, Hoffa's fat pad; P, patella; PL, patellar ligament; QT, quadriceps tendon; T, tibia. Anatomical specimen kindly provided by Dr Stefan Milz.

Fig. 2

Primary ligamentogenic osteoarthritis (OA) of the distal interphalangeal joint. T1-weighted magnetic resonance imaging of the coronal image of a 51-year-old female with OA of the distal interphalangeal joint. The joint collateral ligaments are abnormal and completely disrupted (arrows). There is also complete loss of articular cartilage. In fact, age-related alternation of the ligaments is common and ligament changes predominate over cartilage changes in early hand OA. Therefore, it is proposed that any intervention solely aimed at cartilage repair in this setting is likely to fail as the primary discernible anatomical derangement is elsewhere within the joint.

Fig. 3

Early ligamentous osteoarthritis (OA)-based disease of the knee joint. Magnetic resonance imaging of a 38-year-old female with knee pain and clinical diagnosis of osteoarthritis, showing bone oedema at the anterior cruciate ligament insertion (asterisk). There is also meniscus extrusion (arrow) and peri-meniscal and high signal within and adjacent to the medial collateral ligament (arrowhead). Note that the articular cartilage is relatively normal. These changes point towards an OA disease process that evolves from the ligament.

Fig. 4

Osteogenic osteoarthritis (OA) of the foot. Fat-suppressed magnetic resonance imaging (MRI) of a 67-year-old patient with type 2 diabetes with neuropathic joint disease resulting in secondary OA affecting the mid foot, showing marked bone oedema affecting the mid foot region (asterisks) as well as adjacent soft tissue oedema. It is of note that there is strong evidence already that this type of proposed osteogenic OA responds well to bisphosphonate therapy (Jude et al. 2001). This raises the possibility that MRI selection of patients with marked bone oedema patterns of disease could help select cases for therapeutic agents that target the bone in osteogenic OA.