Classifications in Brief: Kellgren Lawrence Classification of Osteoarthritis
Venue: Clinical orthopaedics and related research

Osteoarthritis (OA) ranks globally among the 50 most common sequelae of diseases and injuries, affecting over 250 million people or 4% of the world's population  Of the global disease burden for OA, knee OA constitutes 83% [39] A detailed analysis of Medicare beneficiaries reported the TKA annual utilization rate ranging from 287,006 in 2006 to 301,956 in 2010 [26] The demand for TKA is expected to grow exponentially over the coming decades with epidemiological data suggesting a 673% increase in the United States by 2030, representing 3.48 million procedures annually [22] As a polymorphic disease with a variety of clinical presentations, OA is challenging to rigorously define. A commonly encountered definition of OA describes .a heterogeneous group of conditions that leads to joint symptoms and signs which are associated with defective integrity of articular cartilage, in addition to related changes in the underlying bone and at the joint margins'  The pathogenesis of OA is poorly understood but is thought to include a complex interplay among mechanical, biochemical, cellular, genetic, and immunologic phenomena [7] Several attempts to develop diagnostic criteria for OA were previously undertaken and incorporate patientreported joint pain in conjunction with consistent radiographic findings [1, 6, 24] OA generally can be subcategorized into primary (idiopathic) and secondary OA [1, 25] Common causes of secondary OA include posttraumatic, dysplastic, infectious, inflammatory, or biochemical etiologies that are relatively well understood. Although the etiology of primary OA remains largely undefined, genetic factors, age related physiological changes, ethnicity, and biomechanical factors likely play an important role [16] Plain radiography remains a mainstay in the diagnosis of OA. The first formalized attempts at establishing a radiographic classification scheme for OA were described by Kellgren and Lawrence (KL) in 1957  After studying rheumatism in coal miners at the Bedford Colliery in North West England [18] Kellgren investigated the interand intraobserver reliability of radiographic changes of rheumatism observed in the hand  After concluding that there was wide disagreement among different observers, KL endeavored to establish a classification scheme with an associated set of standardized radiographs for OA of diarthrodial joints. They proposed a five grade classification scheme and examined plain radiographs of eight joints including the distal interphalangeal joint (DIP) metacarpophalangeal joint (MCP) first carpometacarpal joint (CMC) wrist, cervical spine, lumbar spine, hips, and knees to calculate the interand intraobserver reliability of each [19] They found that the tibiofemoral joint of the knee had the highest interobserver correlation coefficient of r 0.83 (range of all joints studied, 0.10 0.83) as well as the second highest intraobserver correlation coefficient of Each author certifies that he or she, or a member of his or her immediate family, has no funding or commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article. All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research editors and board members are on file with the publication and can be viewed on request.