Reverse shoulder replacement is a great surgical procedure! Most patients experience pain relief and restoration of function after recovery from reverse shoulder replacement. However, as with any other surgery, complications can occur. The shoulder blade contains a thickening called “scapular spine”. Believe it or not, this bone can break spontaneously after otherwise successful reverse replacement surgery. Surgeons are still trying to figure out why is it that these fractures happen, and how to best manage them. Luckily, today we understand a little bit more about this unpleasant complication!
What are the spine of the scapula and the acromion?
The shoulder blade (shoulder surgeons call it scapula) is a triangle-shaped bone which rests on top of your upper ribs and serves as the main connection between the arm and the rest of your body. If you play the video below, you can identify its main parts: the body, the shoulder socket, and the spine-acromion complex. The socket (known medically as the glenoid) is where surgeons anchor the spherical prosthesis of a reverse replacement.
The spine of the scapula is thicker than the scapular body so that muscles can attach to it, the most important being the trapezius and the deltoid. The end of the scapular spine closest to the midline is called trigonum. The other outer end, called acromion, is shaped like a hockey stick. The scapular spine and acromion form the bony roof of the shoulder, extending over it like a diving board.
What is a stress fracture and why does it happen after reverse?
When we think of bone fractures, what probably comes to mind is a car accident or sport injury bad enough to break a bone. However, bones can also fracture slowly under excessive repetitive stress, like when you bend a paperclip back and forth until it finally breaks. Bones are flexible, and they bend under load. Engineers use the term fatigue to define weakening of any material due to repeatedly applied loads. Fatigue leads to micro-cracks. Because bones are alive, these micro-cracks can be repaired. However, if the rate of micro-crack formation is faster than the ability of the bone to heal, these micro-cracks start to link together and the whole bone eventually breaks.
But why would reverse replacement increase cyclic loading on the spine and acromion? Traditionally, this has been attributed to changes in the mechanics of the deltoid, which originates from the spine and acromion. Reverse replacement lengthens the deltoid and increases its moment arm. This combination leads to the application of high magnitude forces to the bones from which the deltoid takes off. However, this mechanism may be an oversimplification and may not explain every single stress fracture. It is also possible that this repetitive forth-and-back bending is due to the upper arm bone (greater tuberosity) constantly colliding with the acromion when the arm is used up in the air.
Who is at risk?
Fractures of the acromion or spine have been reported to affect between 1% and over 10% of shoulders after reverse replacement; the most quoted rate is 4%. Sometimes, patients may be seen by the surgeon before the fracture has completed; these impending fractures are oftentimes called “stress reactions”. Rarely, these fractures may occur before reverse!
Studies have identified several risk factors that seem to increase the chances of acromion and spine stress fractures. In Science, there is a statistical method to estimate the magnitude of risk factors, and it is called Odds Ratio (OR). OR compares de chances of a given outcome in two comparative groups that differ by a risk factor. The table below summarizes the most important patient risk factors for acromion and stress fractures after reverse with their reported ORs. For example, the OR for osteoporosis ranges between 2 and 2.6; that means that patients with osteoporosis who undergo reverse replacement are over 2 times more likely to suffer a stress fracture than individuals with no osteoporosis.
| Factor | Odds Ratio (range) |
| Female sex | 1.5 – 2.75 |
| Older age | 1.03 per year |
| Osteoporosis / low BMD | 2 – 2.6 |
| Dominant extremity | 2.2 |
| Prior shoulder surgery | 1.8 – 2.9 |
| Inflammatory arthritis / RA | 2 – 5 |
| Rotator cuff arthropathy / irreparable cuff tear | 1.7 – 2.3 |
| Fracture malunion/nonunion | 5.2 |
| Chronic dislocation | 3.7 |
There are many different implant configurations that can be used for reverse replacement. One major area of controversy is whether certain implant features are more likely to lead to stress fractures after reverse replacement. Depending on the size of the implants that the surgeon selects and how these are implanted, the overall geometry of the shoulder will be different. The table below summarizes odds ratios for a few implant and surgical factors.
| Factor | Odds Ratio (range) |
| Glenoid lateralization | 1.06 – 1.7 per unit |
| Global distalization | 1.04 – 2.2 per unit |
| Baseplate screw number & location | 1.5 per screw |
As you can see, the odds ratios of implant factors are generally smaller than the odds ratios of patient factors: modifying the lateralization of the glenoid will increase the odds only 1.06 times per millimeter of lateralization, whereas just suffering from rheumatoid arthritis will multiple the risk by 5. However, it makes sense to minimize glenoid lateralization and global distalization in patients at high risk due to osteoporosis, advanced age, and certain diagnosis.
Making the diagnosis
Acromion and spine fractures after reverse typically occur within the first year of surgery, with a mean time to diagnosis ranging between five months and one and a half years. Only one of every five patients recalls some sort of injury, typically minor.
Most patients feel that they were recovering well from surgery and making progress with physical therapy, and suddenly, when the fracture completes, they feel acute onset of severe pain over the shoulder blade and the can no longer raise their arm. Interestingly, plain radiographs do not always show the fracture, especially in the phase of “stress reaction” or with minimal displacement. Computed tomography is best to confirm the diagnosis and understand the location and extent of the fracture. Other tests, such as bone scans or positron emission tomography (PET), are used mostly for research purposes.
What are the treatment options?
Honestly,… this topic is constantly evolving. Traditionally, most of these fractures were managed without surgery, since bones can heal on their own. However, only about half of these fractures heal without surgery. Consequently, fracture fixation with plates and screws is being performed more often, but even after internal fixation some of these fractures will not heal… Surgeons decide on treatment options based on several factors, including location and displacement. These fractures cluster in four main areas: the acromion, the transition angle between the acromion and spine, the “diving board” portion of the spine, and the base of the spine.
Nonoperative treatment consists of immobilizing the shoulder for a minimum of six weeks with the arm supported on a pillow to relax the deltoid and take stress off the spine and acromion. Pain is well tolerated when the shoulder does not move, and can be helped with acetaminophen or over the counter non-steroidal anti-inflammatory such as ibuprofen an others. There is some interest in the potential value of medications that improve bone metabolism, such as bisphosphonates, teriparatide (recombinant PTH) and denosumab (monoclonal antibody against RANKL), but evidence on their value is limited.
Fractures limited to the acromion and those that are caught before displacement have the highest chance of success. On the contrary, displaced fractures, especially those involving the spine as opposed to the acromion, do not do well without surgery. To complicate things further, if one of these fractures is treated nonoperatively and it does not heal in a few months, by the time fracture fixation is attempted the fracture is in a state of “nonunion”. In Orthopedic Surgery, nonunions are more challenging to get to heal than fresh fractures. For those reasons, more surgeons are offering surgery as soon as possible for displaced fractures in bad locations.
When surgery is recommended, it is typically in the form of plate and screw fixation. We will discuss this procedure in detail below. Rarely, surgeons also may recommend changing the reverse itself to a different implant configuration (or even to a temporary partial replacement or hemiarthroplasty). Reasons to consider this additional step include concomitant dislocation of the reverse, implant loosening, and failure of prior fracture fixation attempts.
Lessons learned when these injuries need to be fixed
Initial attempts to fix acromion and spine fractures were completed with use of one single plate and three or four screws on each side of the fracture or nonunion. Unfortunately, our own research showed a failure rate over 20% with this fixation strategy. It was also puzzling that in some shoulders the fracture did heal with plates and screws, but then another fracture occurred later beyond the location of the plate. Our research efforts have led to five general principles we try to apply:
- Use two plates
- Span the whole acromion and spine (protect the whole length)
- Use plates with hooks at the end of the plate
- When in doubt (especially in nonunions) add bone graft
- Fix these fractures early when fixation is needed
The size and shape of the spine and acromion are very different from individual to individual. It is helpful to use plates that are precontoured to fit the unique shape and the many sizes of this bone. If you are interested in watching a surgical videorecording of the procedure, click below.
Surgical procedure
A word on prevention
As Benjamin Franklin wrote in 1735, “an ounce of prevention is worth a pound of cure” (he was taking about fire safety in Pennsylvania, but this quote obviously applies to health and life!). Prevention of acromion and stress fractures starts by understanding the risk factors we summarized above. The main modifiable patient factor is osteoporosis, so it makes sense to improve bone mineral density if possible, with calcium, vitamin D and maybe other medications. Surgeons also try to tailor implant selection and position for those patients with higher risks. There is some interest in considering prophylactic reinforcement of the acromion and spine at the time of reverse arthroplasty in high-risk patients (maybe those who already had one of this fractures complicate a reverse replacement of the opposite shoulder), but the true value of adding plate fixation of the spine to a reverse replacement remains controversial.
