The Journal of Biomechanical Engineering has been published since 1977. To honor papers published at least 30 years that have had a long-lasting impact on the field, the editors now recognize “Legacy Papers.” The journal is pleased to present the following paper as this year's Legacy Paper:
“In Vitro Measurement of Static Pressure Distribution in Synovial Joints – Part I: Tibial Surface of the Knee,” by A. M. Ahmed and D. L. Burke, ASME J. Biomech. Eng., 1983, 105(3): 216–225.
This paper has been cited more than 800 times (Google Scholar data; at the time this editorial was written). The impact of Ahmed and Burke's investigation on scientific understanding of knee biomechanics and on clinical implications of joint and tissue function has been multifaceted. Their direct quantification of the menisci's load distribution capacity and articular cartilage contact pressures, all as a function of joint loading, was immediately recognized by their contemporaries. Their findings have continued to guide many investigations of our times, i.e., since 2018, their work has been cited more than 70 times.
Explorations of knee biomechanics have been on the upswing during 1970s and early 1980s. Elaborate in vitro experiments emerged and documented knee joint mechanics and tissue function, primarily focusing on the stability of the joint. Laxity characteristics of the knee, under anterior–posterior forces, internal–external rotation torques, and varus–valgus torques, were quantified. For the time period, the increased level of interest for an in-depth understanding of natural knee joint stiffness and stability was understandable. This time period also corresponded to the emergence of articulating knee joint implants that aimed for reconstructing natural movement characteristics of the knee. The roles of individual tissue structures on stability, or on gross joint load–displacement behavior, were identified by performing experiments on tissue excised specimens following measurements on the intact knees. Nonetheless, direct measurements of functional tissue mechanics in relation to knee joint loading were scarce, if existing at all. Ahmed and Burke's study emerged during this important epoch in knee biomechanics and detailed the mechanical environment of cartilage and menisci by direct measurements of contact pressures on the tibial articulating surface.
The foundational contribution of the study by Ahmed and Burke was their clear response to the question: What is the mechanical role of menisci in tibiofemoral joint articulation? Based on the anatomical positioning of menisci and some basics in structural mechanics, a heuristic understanding of the potential role of menisci, for distribution and transmission of joint loading across the articulating surface, can be inferred. Indirect measurements, leading to the work by Ahmed and Burke, also demonstrated the early signs of a quantitative understanding of the menisci's mechanical role in articulation. Yet, until Ahmed and Burke's work, direct measurements of contact pressure, and therefore, comprehensive data on regional load transmission across the menisci and articular cartilage were not available. Suddenly, the biomechanics community was exposed to a plethora of quantitative knowledge to answer questions that many knee biomechanists were struggling to answer: How much of the compressive joint load is carried by menisci? Does the amount of coverage provided by each meniscus affect the amount of load transmitted through the tissue? Does the load sharing between menisci and exposed cartilage change by compressive loading magnitude, by flexion angle, and by the nature of combined loading? “From 30% to more than 50%. Yes. And, yes.” are the answers one can quickly infer from Ahmed and Burke's confident and well-supported statements and data. More importantly, their findings had direct relevance to clinics by providing a causal explanation of meniscal tears and by demonstrating the impact of meniscectomy (a routine clinical intervention) on cartilage loading. This unprecedented level of detail in quantitative description of functional menisci mechanics continues to serve as ground truth for recent studies. These not only include investigations of meniscal injury mechanisms and meniscectomy—as Ahmed and Burke anticipated, but also those on the design of meniscal implants, tissue engineering of meniscus, and the evaluation of meniscal interventions for the recovery of natural load distribution.
Another highly impactful contribution by Ahmed and Burke was the comprehensive documentation of cartilage contact mechanics as a function of joint loading. Contact pressure has become a routine biomechanical metric in knee biomechanics, as a potential risk indicator for injury, degeneration, and disease pathomechanics, e.g., that of osteoarthritis. Ahmed and Burke's demonstration of the change in cartilage contact stresses following meniscectomy, in particular, their localization and the increase in their magnitude, naturally supported the emergence of this school of thought. Ahmed and Burke's investigation established the first normative baseline for the expected range of cartilage contact stresses on the tibial surface, including peak pressures and regional distribution across medial and lateral compartments. This work, their companion study on contact pressures of patellar cartilage [1], and a synergistic study of femoral condyles by their peers [2], who were motivated by Ahmed and Burke's work, provided a full picture of cartilage contact stresses in the knee along with their variability. Since then, this knowledge has been used as reference for articular mechanics of healthy, diseased, and reconstructed knee. In regard to the latter, the mediolateral load balance and contact stress environment of the tibial articulating surface have informed safety and performance assessment of total knee replacements, a feature recognized early by another peer to Ahmed and Burke [3].
Ahmed and Burke's desire to diligently quantify the mechanical environment of the cartilage and menisci, especially at meaningful joint loads, necessitated technological innovations. These continue to inspire contemporary testing of musculoskeletal joints. Their multi-axis apparatus [4], which was capable of prescribing knee joint flexion and applying combined loads, was an important milestone for high fidelity testing of cadaver specimens. This setup enabled experimentation for in-depth explorations of a large variety of joint loads and to quantify corresponding tissue level mechanical response. In essence, their technical innovation set the stage for dynamic cadaver simulators and robotic testing of diarthrodial joints. Another innovation rested on Ahmed's development of a micro-indenter-based transducer for measurement of pressure distribution [5]. The device was relatively thin (0.285 mm), therefore minimizing disruption of in situ articular contact mechanics during tests. While it was designed for measurements under static loading, it featured the capacity to capture the nonuniform pressure distribution across the full articulating surface. At the time, sensors of this caliber, and the fidelity to insert in tight joint spaces, were scarce but essential for acquisition of high quality data.
The rigor in Ahmed and Burke's work cannot be emphasized enough. Their impeccable approach for experimentation, analysis, and reporting sets the bar to tackle challenges we face in our era, as we try to ensure scientific quality and to enhance reproducibility. Their desire to test a relatively large number of cadaver knee specimens (even by defacto standards of in vitro testing in our times), their care to apply and appropriately use the pressure sensor, the balance between managing understandable limitations of joint loading setup and the ease and quality of data acquisition, are all reminders of the necessary depth and grit to perform in vitro joint experimentation. Nowadays, access to robotics testing and dynamic contact pressure measurement is routine. Yet, Ahmed and Burke's study remains relevant by demonstration of good in vitro testing practices.
It is hard to imagine anyone in the world of computational biomechanics, not refer to Ahmed and Burke's study to ground simulation-based predictions of meniscus and articular contact mechanics. Their work mined a trove of information on contact pressures and articular load sharing for a sample knee population, at different loading conditions, and for scenarios following tissue resection. While in silico studies of knee biomechanics aspired for specimen-specific evaluation of predictive capacity, Ahmed and Burke's is an important resource to understand how well a virtual knee's response fits to known knee joint behavior. The utility of their study at this capacity started early on, in (now well recognized) models of tibiofemoral articulation [6], and continued for highly detailed finite element analysis of cartilage contact and meniscus behavior.
Ahmed and Burke's investigation on knee biomechanics has been an eminent guide to the biomechanics community. It will continue to inspire upcoming generations of biomechanical scientists and engineers, and musculoskeletal physicians. In the meanwhile, let's not underestimate the menisci—they carry a significant portion of knee joint loading.
