It's a fact that hips generally heal themselves and that's because in hips, the surrounding tissue is a lot deeper, more flexible and more forgiving with more room to take up what swelling there is so it doesn't impact on the hip's activity. Only about 7%* of the tissue in the hip joint area is bone. Compared to the rest of the skeleton, the hip joint is actually quite small, much on a par with the shoulder joint. On the other hand the knee is the largest and most complex joint in the body, very superficial and with very little room to accommodate any swelling, so when swelling does occur, it locks the joint into a veritable plaster cast of tight, solid tissues which is extremely painful to work against. Hence the biggest hazard in the immediate post op period for any knee surgery - though not particularly common - including smaller procedures like arthroscopy, is compartment syndrome which is where when swelling becomes too much it can actually impede the circulation. The quantity of bone in the knee is about 90%* with only about 10%* soft tissue. *professional 'guesstimate'!