Fractures (breaks in bone) through the joint surface increase the risk of post-traumatic arthritis 20 times. In fact, three out of four people that sustain a break through the joint surface itself may develop arthritis in that joint eventually.
- Sudden mechanical damage to the cartilage at impact and break-point
- Biological response to injury
- Bleeding, inflammation, etc
- Chronic cartilage damage and overload due to uneven joint surfaces and
- misalignment after a fracture
- Instability
- Constant altered load on the damaged joint surface
Cartilage is the slick and firm surface of either side of a joint. A joint is two or more bones that move together; the elbow, knee, hip, ankle, wrist, etc are all joints. Cartilage is about 60-85% water by weight and is a matrix of collagen along with other biologic substances.
Cartilage is built to last. The function and structure of cartilage, as well as the body’s innate ability to adapt and heal, is such that it can handle a lifetime of repetitive use and demanding movements and even trauma. However, occasionally trauma is so severe that the cartilage has a difficult time recovering. This type of trauma involves such massive energy or force that the physiologic parameters of the bone and cartilage and ligaments are destroyed and the bone breaks along with the cartilage surface. Many other types of trauma the body can handle heal. If the body is incapable or recovery, then degradation ensues. Fractures through a joint are one cause of arthritis.
The massive energy of a fracture also impacts the cartilage before it too breaks. This causes cell death and dysfunction. The good news is that most of the cell damage has been shown to occur close to the actual fracture line itself. Cartilage and tissue away from that break-line does not show nearly as much damage. One study demonstrated about 1 in four cells were damaged if directly at or adjacent to the fracture line; further away, less than 10% were. The very high energy required to actually break a bone is necessary for post-traumatic arthritis. Impact injuries that simply pound the cartilage but are not associated with a bone breaking are much less damaging. Basically, the amount of force matters and how long that force is applied to the cartilage.
The biological response to the fracture or impact also plays a role. The initial phase of healing is inflammatory.
Inflammation is totally necessary for healing to occur. However, inflammation also involves the release of reactive oxygen species, or free radicals, that damage tissue. The pro-inflammatory substances that flow into an injury zone can often cause further damage by matrix (collagen) breakdown. Again, the larger the force load applied to the joint surface and underlying bone, the more inflammation and free radicals are produced. Tumor necrosis factor alpha, Interleukin-1, nitrous oxide, matrix metalloproteinase and others are all associated with ongoing damage. These are actually targets for therapies to treat arthritis. If post-traumatic arthritis occurs, it has been shown to occur rather quickly. Some studies demonstrate changes in the bone within 8 weeks. Others show transformation at the zone of injury in about 4 years. Either way, post-traumatic arthritis occurs earlier and more obviously than normal degenerative arthritis. Therefore, there should always be a difference in appearance of the affected joint relative to other joints in the body if this process happens.
When a joint has a fracture or a break in it, the joint surface changes. Joints are designed to be very smooth and congruent in most cases. A fracture will often leave a fissure or a fault line. Obviously, this will change how the joint moves and can change the pressures applied to the joint surface itself. An ankle fracture may cause changes of up to 300% in terms of pressure on the joint from a simple fracture line. This change in pressure on the joint over time will engender a sustained low-level impact to that particular part of the joint over time; therefore, that part of th joint is at risk of arthritis. One of the goals of fracture surgery is to restore the normal, smooth surface of a joint.
Misalignment and instability also occur after fractures. Typically, for the energy of an event to be great enough to actually break bone and cartilage, there are also torn ligaments and tendons.
Unless the proper soft-tissue balance is restored after such a break, chronic instability occurs. One of the most common reasons to develop ankle arthritis is to have untreated sprains that cause long-term instability. Instability changes where all the pressure on a joint goes and the new locations is likely not built to accept the weight of a person. The level of importance of alignment and instability varies depending upon the joint involved. Some joints can accept a large amount of variability (hip/shoulder) while others cannot (ankle/wrist).
Currently work is being done to further understand what happens to cartilage after it breaks along with a bone (articular fracture). The goals of treatment today are to restore the joint anatomically along with alignment, to produce stability of the ligaments, to limit the inflammation initially and to mitigate the damage at the time of the event.
