Did you know that bone can form outside of the boundaries of the skeleton? I am not talking about malignancies or cancer, but about masses of bone that appear in the soft-tissues surrounding the bones, and oftentimes end up blocking motion. In Medicine, the words “Heterotopic” and “Ectopic” basically mean “not in the right place”. Bone formation is termed “Ossification.” The terms heterotopic ossification and ectopic ossification are considered synonyms, and very commonly the abbreviation “HO” (for heterotopic ossification) is used. For reasons we do not fully understand, the elbow region is particularly prone to HO formation. If you want to know why HO matters, and how to deal with it, keep reading…
Shall I show you a few examples?
Heterotopic ossification is typically seen on radiographs or computed tomography (CT), except in the very early stages of bone formation when the new bone is barely mineralized. Below are examples of HO that occurred in different individuals after an injury or surgery.
When does elbow HO happen?
Elbow HO has been noted to form after elbow injuries, particularly high-energy fractures of the lower end of the bone in the arm (humerus) or the upper part of the forearm, or blast explosions at the time of war. Individuals that suffer extensive burns anywhere in the body (not just the elbow), or an injury to the brain or spinal cord are also known to have some predisposition to develop elbow HO, even in the absence of elbow injuries, although more so if the elbow has been injured as well. Elbow surgery can also be complicated by HO formation; the classic example is the formation of a block of bone connecting the two bones in the forearm after surgical reattachment of a torn biceps tendon. Finally, there is the genetic condition known as fibrodysplasia ossificans progressiva (FOP); affected individuals form HO in multiple locations with minor or no injuries.
But,… why does this happen?
Sadly, the short answer is that we do not know! Based on fundamental biologic principles, HO formation requires the unexpected and abnormal behavior of cells that are able to form bone. There has been a lot of research to try to understand why HO happens. If we knew, we might be able to predict who will get it, and prevent HO formation. Have you heard of Marshall Urist?
Marshall R, Urist MD (1914-2001) worked as an Orthopedic Surgeon at the University of California at Los Angeles (UCLA). In 1965, Dr. Urist published his landmark article “Bone: formation by autoinduction” in the prestigious Science journal. In his experiments, injection of demineralized bone matrix into muscle induced HO formation. It was later found that the bone formation effects of this matrix were mostly due to a family of proteins that were named bone morphogenetic proteins (BMPs). And guess what?: FOP, the genetic condition we mentioned before, is the consequence of an abnormality in a molecule called ACVR1, which happens to be used by BMPs to act on cells.
Thus, many believe that elbow injuries, elbow surgery, burns and injuries to the head or spinal cord lead to activation of biologic pathways involving BMPs to some extent. The question is how this activation happens in very different situations, from sustaining an elbow fracture to being in coma for a while. Some argue that HO formation may also be influenced by proteins released by peripheral nerves, or by regulatory abnormalities of the immune system. We do know that for patients undergoing surgery to fix an elbow fracture, the risk seems to be the highest when the individual sustains an injury to the head or chest at the same time, or when the elbow remains unstable for several days before it can finally be fixed.
Elbow HO is feared mostly because it limits motion
Some may wonder, “what is the big deal? Wouldn’t extra bone make my arm stronger?” The problem is that the areas of newly formed bone in the soft tissues may interfere with motion. In some instances, the limitation of motion is mild, but some patients may experience substantial restrictions in motion, and in some cases HO can cause complete loss of motion (this situation is called “ankylosis”). Depending on the location of growth and severity, HO may limit the ability to bend or extend the elbow and/or forearm rotation (with difficulty placing the hand palm up or down). HO may be only one of several contributing factors to elbow stiffness: in patients with prior injuries or surgery, excessive joint fibrosis or damage to the joint surface may play a central role as well. There is no perfect correlation between the size of HO and motion impairments. Occasionally, HO may form in locations that end up pinching or compressing one or more of the nerves located in the elbow region.
What is the treatment?
Once HO has formed, the only way to get rid of it is with surgery. Spontaneous dissolution of HO has only been reported during pregnancy and in very young kids, but these cases are rare. In the vast majority of individuals, HO may change in shape as it matures, but it does not magically disappear, which is frustrating for those suffering the consequences. As we mentioned in a previous post, many daily activities may be performed despite mild limitations of motion. Thus, surgical removal of HO is only considered when motion restrictions negatively affect performance and quality of life, or if there are other reasons for elbow surgery.
Even though arthroscopic elbow surgery has advanced substantially, only a few surgeons in the world feel that arthroscopic HO removal from the elbow is safe and effective. Arthroscopic removal could potentially be particularly effective in very early stages, when HO has not completely solidified. Most of the time, HO needs to be removed through an open incision. Surgery involves accessing and removing all HO bone that limits motion. Other factors contributing to elbow stiffness are address as well, particularly removal of scar tissue (fibrotic capsule) and release of scar and bone around nerves if needed.
Once HO has been removed, a good program of rehabilitation is paramount to maintain the motion gained in surgery. The program may involve exercises done on your own, working with a physical therapist, use of an automatic machine to passively move the elbow (continuous passive motion), splints, or braces. Talk to your elbow specialist to understand the potential complications of surgery and discuss details of the rehabilitation program.
Controversy remains regarding the ideal timing of surgery. Once HO has formed and is blocking motion, most individuals would prefer to have it removed as soon as possible. However, new HO will form again in some individuals despite adequate surgical removal. The risk of new HO formation after removal (recurrence) is perceived to be higher when HO is removed while it is still forming. In the past, surgeons would not consider removing HO for one or two years! Now, most agree that it is safe to remove HO as early as three months after the inciting event, provided HO looks mature on radiographs, and any associated fractures have healed.
Can HO formation be prevented?
Unfortunately, in some individuals HO will form again after removal. Predicting reoccurance can be difficult. Although new HO formation is relatively uncommon (the rate of recurrent is probably only 10-15%), reformation becomes a big deal, since all the benefits of surgery may be lost. Recurrence seems to be higher when HO is formed as a consequence of brain or spinal cord injuries. For these reasons, some elbow specialists will treat all or some of their patients with either a non-steroidal antiinflamatory drug (most commonly a medication called indomethacin) for 3 to 6 weeks, or alternatively they will recommend a single dose of radiation therapy just before or the day after surgery.
The question is whether these preventive modalities could also be used to prevent HO formation after an elbow injury or surgery, extensive burns, or a head or spinal cord injury. Radiation therapy is not favored after elbow injuries, since it may interfere with bone healing. Some elbow specialists do use indomethacin routinely after high-risk procedures, such as a repair of a torn distal biceps tendon.