Arthrofibrosis ( excessive scar tissue) causes patella baja. For patella baja, scar tissue will form around the patella tendon and down in the anterior interval which causes the patella to be pulled down low and further contraction pulls the patella deep into the joint space. Everyone's symptoms are different, some can still have good ROM. It depends on how much scar tissue, where it is attached and how low the patella has been pulled down. Sometimes all it takes is to strengthen the quads to bring the patella back up.
With my patella baja , I had ROM problems. I had difficulty bending past 100 degrees.( extremely painful and tight) and I could not fully straighten my leg. I had to place a rolled up towel under my knee. I could not do stairs, walk down inclines, etc. My patella was also subluxating all the time..........sitting to stand, standing to sit, even when walking. I had to manually move the patella to uncatch. Sometimes my patella would uncatch by itself, in the form of a ripping sharp pain pop. My knee was swollen,hot, painful, with severe quad atrophy. My patella was tight and had little movement. It was intolerable to walk. On xrays, you could definately see the patella sitting much lower than the good knee. My OS made measurments on the xray. I had an MRI which showed permanent changes to my patella tendon, which had become shortened and thick. I was diagnosed with infrapatellar contracture syndrome and patella baja.
When I had surgery to correct it, my Dr said my patella had been pulled so low, the patella and patella tendon were completely adhesed down to the tibia. Patella baja caused severe damage to the articular cartilage of my patella , which led to a TKR 4 yrs later.
Here are some links to the information hub relating to patella baja. The first one is from Dr Noyes turtorial on arthrofibrosis, discussing patella bajahttp://www.kneeguru.co.uk/KNEEnotes/node/2063http://www.kneeguru.co.uk/KNEEnotes/node/220http://www.kneeguru.co.uk/KNEEnotes/node/1794http://www.kneeguru.co.uk/KNEEnotes/node/219