Knees do not necessarily wear out evenly, sometimes one part of the knee is perfectly fine while another part is completely destroyed. If only part of the knee joint is worn out, why replace all of it?
Red is the color. So much depends on red. Healthy red blood is the essence of all tissues and even the determinant of the health of bone. I look at life through this prism and notice the shades suggestive of this injury or that disease. I read those signals to predict outcomes.