Anatomy of the Knee Extensor Mechanism: Correlation With Patellofemoral Arthrosis

Amar D. Rajadhyaksha, MD, Kyle Low, MD, Dawn M. LaPorte, MD, David S. Hungerford, MD,


J South Orthop Assoc. 2001;10(1) 

In This Article

Abstract and Introduction

The patellofemoral articulation is a common and significant source of disability and discomfort in the aging population. This study examined the anatomy of the knee extensor mechanism in patients having primary total knee arthroplasties, characterized the anatomic variations of the extensor mechanism, and correlated these findings with the location and extent of osteoarthritic change of the patellar undersurface. Sixty-two knees (57 patients) were evaluated prospectively. Specific characteristics that were analyzed included the mean Outerbridge grade for rating patellar cartilage degeneration and anatomic patterns of the extensor mechanism. Knees with a quadriceps tendon width at 2 and 5 cm above the patella that differed by less than 1 cm had more statistically significant patellar degeneration in all patellar locations than knees with tendon width differences greater than 1 cm. Anatomic variations, such as tendons with minimal increments in width in the proximal-distal direction, may be associated with an increasing amount of patellar arthrosis at the lateral facet, central ridge, and, most significantly, medial facet.

Anatomic variations around the knee have been associated with clinical symptoms and conditions. Abnormal attachment of the iliotibial tract has been recognized as a cause of recurrent dislocation of the patella.[1,2,3,4,5] Subluxation of the patella has been associated with multiple anatomic findings, including dystrophy of the vastus medialis muscle, external tibial torsion, lateral insertion of the patellar tendon, genu valgum, and patella alta.[1,3] Patella alta has been associated with congenital anomalies, such as an abnormal trochlea, hypoplasia of the vastus medialis, and lateral position of the patella. The high-riding patella predisposes the patient to patellar subluxation, dislocation, and/or osteoarthritis. The development of osteoarthritis has been linked to ligamentous instability. Knees deficient in posterior cruciate ligament were found to have increased medial compartment and patellofemoral arthrosis.[6] Also, studies have documented changes in the force transferred to the knee and the development of arthrosis associated with variations in joint alignment. Varus alignment has been correlated with increased forces on the medial compartment and resulting acceleration of arthritis.[7]

Osteoarthritis of the patella is common in adults, with the incidence reported to range from 52% (89 of 172 patients older than 20 years of age at meniscectomy[8]) to 92% (97 of 106 patients older than 20 years at necropsy[9]). Initially, the etiology of this condition was attributed to injury,[10,11,12,13] while some authors suggested a constitutional disturbance[14] or a combination of the two.[15,16] Later, Chrisman[17] suggested a biochemical derangement as the underlying cause of osteoarthritis, and Shoji and Granda[18] believed that osteoarthritis was a continuum that began with proteolytic degeneration.

Osteoarthritis of the patella has been noted to start most frequently on the medial facet. Wiberg's report[19] on the congruity between the deep surface of the patella and the femoral condyle at different angles of flexion presented important new data. The data suggested that pressure or friction against the cartilage of the medial patellar facet as the patella rides over the ridge of the medial femoral condyle, in normal movement of the knee, might contribute to the onset of osteoarthritis. Outerbridge[8] supported this theory in his 1961 report on the etiology of osteoarthritis patellae, but he conceded that additional factors such as age, sex, occupation, and power of the quadriceps mechanism might also contribute to the condition. In his 1964 follow-up report, Outerbridge[20] reported more severe osteoarthritis in women (34%) versus men (10.4%) and in patients older than 50 years. While it was believed that the extensor mechanism likely played an important role in this condition, Outerbridge was unable to assess this in his study. Additional reports on the etiology of osteoarthritis patellae cite an association with patellar subluxation and dislocation,[21] abnormal patellar tracking,[22] and patella alta,[22,23] but they have not evaluated the role of the quadriceps mechanism.

No previous study, to our knowledge, has described the possible role of anatomic variations in the quadriceps mechanism in relation to patellofemoral arthrosis. The purposes of our study were to examine the anatomy of the knee extensor mechanism in patients having primary total knee arthroplasty and to correlate these findings with the observed osteoarthritis on the undersurface of the patella.