New Research: Knee and Hip Osteoarthritis Are Different
We all hear the term “arthritis,” and as a result we believe that knee arthritis and hip arthritis are the same disease, just located one joint apart. However, more than a decade of observation and now recent research has taught me that these are two different processes. Understanding this when using regenerative-medicine treatments, like PRP and stem cells, is critical for success.
What Is Arthritis?
There’s a 30,000-foot answer to this question that hasn’t changed much since the 1970s, and then there’s the real answer. From a big picture, disco standpoint, joint arthritis is loss of cartilage, the growth of bone spurs, and increased pain/stiffness. However, our understanding of this disease has been changing rapidly. For example, we now believe that arthritis involves a dance with the bone, which was not previously understood. Meaning what happens to the bone is often a harbinger of what will happen to the cartilage.
The Visual Differences Between Knee and Hip OA
Any physician who looks at the MRIs of patients with knee and hip osteoarthritis (OA) will soon conclude that these diseases are different. First, hips almost always progress toward a severe state at the speed of light compared to knees. For example, a knee may smolder for many years, slowly losing cartilage and growing bone spurs. However, many hips can go from mild to severe OA within one to two years.
There’s another curious difference between these two joints—cysts. Knee OA rarely presents with bone cysts, and hip OA almost always has them as the disease advances.
A bone cyst is a fluid-filled cavity that is a void of dead tissue. After a small area of the bone dies off, the body walls this area off almost like it does in an infection. It creates a hard shell around the dead area, which structurally shunts the forces that this part of the bone would normally handle around the dead spot.
The Research Showing That Knee and Hip OA Are Different Animals
A few years ago I blogged on a study that showed that the hip was less able to repair itself than the knee. This difference was believed to be due to the differences in the number or inert ability of the progenitor cells in the two joints (these are similar to stem cells but more specifically tasked with repairing the joint). I went searching for that reference last night and instead happened on a brand-new study by the same author that is basically a newer and reswizzled version of the same study.
The new study used an esoteric biomarker for the chronological age of cartilage to determine cartilage turnover. What’s that? Cartilage is constantly renewing itself, which is a process that involves stem cells and mature cartilage cells that produce the substance in which the mature cartilage cells reside. The composition of this substance (called ECM for extracellular matrix) is important, as too much or too little of a certain component will reduce its ability to protect the cartilage cells, leading to their untimely death and a hole in the cartilage surface.
What the authors found was that in knee OA, the cartilage is constantly repairing itself because the age of the cartilage ECM is younger than in hip OA. Basically, hip OA cartilage components were 30 years older than those found in knee OA! Meaning that cartilage in hip OA has very little ability to maintain itself by creating new cartilage ECM.
The upshot? New research confirms what we have observed for years: knee and hip OA are very different disease processes that have a similar endpoint—the joint is destroyed. This research also fits with our registry data (as well as that I’ve seen from others who have presented much smaller patient numbers), that hip OA results in patients with severe arthritis using a same-day stem cell procedure lag that of knee OA results. In addition, this new model of differences in cartilage ECM turnover may also explain why flooding the severe hip OA joint with many more stem cells (through a culture-expanded procedure) seems to give better results than a same day procedure—the hip needs those extra cells whereas the knee OA, in many circumstances, may not.