Biomechanics of the Knee Extensor Mechanism and Its Relationship to Patella Tendinopathy: A Review Michael Dan, 1,2 William Parr, 1 David Broe, 1,2 Mervyn Cross, 3 William R. Walsh 1,2 1 Surgical and Orthopaedic Research Laboratory, Prince of Wales Clinical School University of New South Wales, Sydney 2052, Australia, 2 Prince of Wales Hospital, Barker St, Randwick, New South Wales 2031, Australia, 3 The Stadium Sports Medicine Clinic, Sydney 2012, Australia Received 29 May 2018; accepted 26 July 2018 Published online 3 August 2018 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jor.24120 ABSTRACT: The term jumpers knee for patella tendinitis, as coined by Dr. Martin Blazina, is now commonly referred to as tendinopathy. He believed it was associated with patella alta. Since then multiple studies have failed to reliably show an association between patella tendinopathy and associated intrinsic risk factors. There is, unfortunately, a well-established doctrine that the extensor mechanism is simply a pulley. The goal of the review is to examine the biomechanics of the extensor mechanism and apply this to studies investigating intrinsic risk factors for patella tendinopathy. A better understanding of the biomechanics of the extensor mechanism may stimulate the discovery of intrinsic risk factors for developing patella tendinopathy, and subsequent surgical options to address them. Clinical significance: The aim of this review is to direct future research into biomechanical risk factors for developing patella tendinopathy and subsequently, possible treatments. ß 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3105–3112, 2018. Keywords: patellar tendinopathy; jumper’s knee Patella tendinopathy is an overuse condition largely affecting high level athletes involved in jumping sports. 1 The relatively poor clinical outcomes are reflected by the fact there is a multitude of treatment options for patella tendinopathy, this intuitively means that one cannot be vastly superior to another nor can one be truly effective. 2 The aim of this review is to correlate the extensor mechanism biomechanics with patella tendinopathy in the hope of directing future research and treatments to improve clinical outcomes. ANATOMY The extensor mechanism is responsible for extending the tibiofemoral joint. The term refers to the common linkage of the four converging quadriceps muscles into the quadriceps tendon attaching proximally to the patella bone, continued distally by the patella liga- ment or tendon to the tibial tuberosity. 3 The patella is a sesamoid bone and articulates with the trochlea of the femur, forming the patellofemoral joint- capable of 6 degrees of movement. Confusion exists concerning nomenclature of the Patella tendon or patella ligament. Both have been used interchangeably as tendon and ligaments are thick closely packed collagenous bundles orientated parallel to the long axis of the structure. 4 Macroscop- ically, it can be argued to fit either definition. It connects the patella (bone) to the tibia (bone) which is the definition of a ligament (bone to bone connection). However, functionally the patella tendon connects the quadriceps muscle to the tibia bone, with the patella as a sesamoid bone within it. This is also the macroscopic definition of a tendon, which connects muscle to bone. From a histological and biochemical view it is a tendon as reported by Amiel and co-workers in 1983. When compared with the knee ligaments from a histological and biochemical view the connective tissue connecting the patella bone to the tibia more closely resembles tendon (Achilles) more so than ligament. Microscopi- cally it is more hypocellular, containing longer spindle- shaped fibrocytes which are more regularly arranged than ligaments (larger and rounder cells less regularly arranged). Biochemically the patella tendon had a higher collagen content, but a smaller percentage of type III collagen (5% for tendons vs. 10–12% for ligaments), less DNA, and glycosaminoglycan content, and a higher number of hydroxylysinonorleucine cross- links relative to dihydroxylysinonorleucine (reverse for ligaments). 4 Therefore we will refer to it as the patella tendon. The type I collagen forms the fascicles which are the subunits of tendon hierarchy. There are anatomi- cal differences within the human patella tendon. The posterior fascicles, when compared to the anterior fascicles, have been shown to be smaller in diameter, shorter and have higher concentration of cross linked hydroxylysylpyridinoline 5 which reflects mature colla- gen crosslinks and a robust mechanical construct. PATELLA TENDINOPATHY Patella tendinopathy continues to be an area of interest (Figure 1) for nearly 5 decades reflecting the search for better understanding of the pathology and need for improved clinical solutions. Patella tendinopathy is an overuse condition affect- ing mostly athletes. It was initially described by Dr. Martin Blazina in 1973 is his description of jumper’s knee. 6 While jumper’s knee includes pain at the quadriceps insertion (25% of cases), distal pole of patella (65%), and the tibial tubercle insertion (10%), 7 the enthesiopathy of the patella tendon at the distal pole of the patella is the definition of patella tendinop- athy (see  in Figure 2). Grant sponsor: The Australian Government. Correspondence to: Michael Dan (T: þ61428082964; F: 02 9382 2660; E-mail: michaeldan@hotmail.com) # 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. JOURNAL OF ORTHOPAEDIC RESEARCH 1 DECEMBER 2018 3105