Classifications in Brief: Kellgren-Lawrence Classification of Osteoarthritis

Abstract

History 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 [39]. 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” [1]. 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 patient-reported 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 [19]. After studying rheumatism in coal miners at the Bedford Colliery in North West England [18], Kellgren investigated the inter- and intraobserver reliability of radiographic changes of rheumatism observed in the hand [17]. 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 inter- and 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 r = 0.83 (range of all joints studied, 0.42-0.88) among the diarthrodial joints they examined [19]. These early results would predict the future application of their classification scheme to the knee specifically. Currently, the KL classification is the most widely used clinical tool for the radiographic diagnosis of OA [5]. Purpose The KL classification has been commonly used as a research tool in epidemiological studies of OA, including landmark articles by Felson et al. [10] in the Framingham Osteoarthritis Study, and Bagge et al. [3] assessing osteoarthritis in European populations. The KL classification was also used in the development of atlases of radiographic features of OA, including the work done by Scott et al. [33]. The Kellgren and Lawrence classification may also assist healthcare providers with a treatment algorithm to guide clinical decision-making, specifically defining which patients may benefit most from surgical management. Furthermore, some insurers currently require providers to include documentation of the KL classification to receive approval for a TKA [37, 38]. Despite its common use, all research and clinical efforts using the KL classification depend critically on rigorous validation and continuous reevaluation of the schema's relevance to patient-centered outcomes. Description of the Kellgren-Lawrence Classification System Based on the data presented in their original work, the KL classification is typically applied specifically within the context of knee OA. The KL classification was originally described using AP knee radiographs. Each radiograph was assigned a grade from 0 to 4, which they correlated to increasing severity of OA, with Grade 0 signifying no presence of OA and Grade 4 signifying severe OA [19] (Fig. 1). Additionally, KL provided detailed radiographic descriptions of OA (Table 1). Although it is unclear from the original paper whether the radiographic descriptions were presented with the intent of demonstrating a linear disease progression of